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Chapter 2.17. Water Environment

SBL ES: 2.17 South Bristol Link: Environmental Statement Volume 2: Water Environment, July 2013

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17. Water Environment17.1. Introduction and Scope of Topic 17.1.1. This chapter provides an assessment of the water environment related to the scheme. The

assessment methodology followed is in accordance with the guidance provided in the Design Manual for Roads and Bridges (DMRB) HD45/09.

17.1.2. This chapter considers the existing water environment conditions for the scheme and the legislative context. It determines the likely significance of effects of the scheme on the quality of surface water and groundwater. Generic and specific impacts on the water environment during the construction phase and the operational phase are identified and assessed. Implications relating to abstraction and existing discharge of water have also been assessed.

17.1.3. Figure 17.1 (Volume 3 appendix 17.1) shows the principal features of the water environment that are referred to throughout this Chapter.

17.1.4. For ecological information related to the water environment the reader should refer to Chapter 13 – Ecology

17.1.5. A Water Framework Directive (WFD) assessment and a site-specific Flood Risk Assessment (FRA) have also been undertaken and are provided in Appendix 17.2 and Chapter 16 respectively.

17.1.6. A land contamination assessment has also been undertaken and is provided in Chapter 18.

17.2. Policy Context and Guidelines of Relevance 17.2.1. The potential impact of road construction and operation on the quality and quantity of water within

nearby surface and groundwater environments is considered in relation to both European and National legislation.

17.2.2. With regard to the protection of specific water resources, water quality standards and related policy relevant to the scheme are set out in the following European legislation: • The Dangerous Substances Directive* (76/464/EEC);• The European Community (EC) Shellfish Waters Directive* (2006/113/EEC);• The Freshwater Fish Directive* (2006/44/EC);• The Groundwater Directive (80/68/EEC to be repealed by 2006/118/EC in 2013);• The Habitats Directive (92/43/EEC);• The Nitrate Directive (91/676/EEC);• The Urban Waste Water Treatment Directive (91/271/EEC);• The Water Framework Directive (2000/60/EC);• The Floods Directive (2007/60/EC)* To be repealed by WFD in 2013.

17.2.3. The aim of water policy in England is to protect both public health and the environment by maintaining and improving the quality of natural waters. These include surface water bodies (e.g. rivers, streams, lakes, ponds) and groundwater. European legislation is implemented in the UK through specific sets of Regulations. The Department of the Environment, Food and Rural Affairs (DEFRA) is responsible for all aspects of water policy in England. Management and enforcement of water policy is the responsibility of the Environment Agency. The following National legislation is relevant: • Antipollution Works Regulations (1999);• Environment Act (1995);• Environmental Damage (Prevention and Remediation) Regulations (2009):;• Environmental Protection Act (1990);


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• Flood risk regulations (2009) Amended SI2011/2880 transpose directive 2007/60/EC • Flood and Water Management Act 2010 and Commencement Orders • Highways Act 1980 (HA 1980); • Groundwater (England and Wales) Regulations (2009); • Land Drainage Act 1991; • Water Act 2003 (WA 2003); • Water Environment (Water Framework Directive) (England and Wales) Regulations 2003: • Water Industry Act (1991) (Amendment) (England and Wales) Regulations (2009); • Water Resources Act 1991 (WRA 1991);

National Planning Policy Context

17.2.4. At the National Planning Policy level, there are two Planning Policy Statements that are directly relevant: • Planning Policy Statement 23: Planning and Pollution Control; and • Planning Policy Statement 25: Development and Flood Risk. Although now superseded by

the National Planning Policy Framework (NPPF), the policy principles remain unchanged and supporting Technical Guidance and associated Technical Guide remains in place. The Environment Agency’s Pollution Prevention Guidelines (PPGs) have also been considered.

Local Planning Policy

17.2.5. The route of the scheme is situated within the district of North Somerset and the city of Bristol and as such reference has been made to the Core Strategies commissioned by North Somerset Council (April 2012) and Bristol City Council (June,2011).

North Somerset Council Core Strategy

17.2.6. North Somerset Council Core Strategy was adopted in 2012 and identifies key long term development strategies, including policies regarding surface water management and flood risk. The report sets out the policies of sustainable development, considering climate change and flood risk. It states the need for developments to comply with the NPPF and site specific requirements (Halcrow, May 2013).

17.2.7. When considering proposals for development the council will require the application of best practice in Sustainable Drainage Systems (SUDs) to reduce the impact of additional surface water run-off from new development. The Core Strategy states such environmental infrastructure should be integrated into the design of a scheme and into landscaping features, and be easily maintained (North Somerset Council, April 2012).

Bristol City Council Core Strategy

17.2.8. Bristol City Council Core Strategy was adopted in 2011 and outlines the key planning and development plans for the region. Policy BCS16 states that developments need to be resilient to flooding through design and layout, and incorporate sensitively designed mitigation measures to ensure the development remains safe from flooding over its lifetime. A surface water management strategy should be incorporated to reduce surface water runoff and not increase flooding elsewhere. Under Policy BCS16 the requirement to incorporate SUDS into new development is highlighted. New development is expected to incorporate water management measures to reduce surface water run-off and ensure that it does not increase flood risks elsewhere. This should include the use of SUDS (Bristol City Council. June 2011).


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17.3. Assessment Methodology

Introduction

17.3.1. The assessment of the water environment has been based upon methodology detailed in the DMRB HD45/09 which deals with road drainage and the water environment and is described below. The significance of potential effects has been determined by: • Importance of the receptors; • Magnitude of the impacts; and • Influence of mitigation measures.

Data Sources

17.3.2. Ongoing correspondence and consultation with the Environment Agency, Natural England, English Heritage, Bristol City Council, North Somerset Council and Halcrow have been a key part of defining the scope of work for investigating water quality, drainage and hydrology, as well as guiding the production of the FRA. Data sources are detailed in Table 17.1 below.

Table 17.1: Datasets used within the water quality assessment Source Title Date on

dataset Description

Environment Agency

Severn River Basin Management Plan

December 2009 Annex B Severn River Basin District

Environment Agency

Data request CSC/6182 for South Bristol Link Scheme

September 2012 Data request for items including but not limited to: abstractions, discharges, environmental permits, location of any routine water quality monitoring points within 1km of the scheme

Flood Estimation Handbook (FEH)

Figure 11.6 – 1 hour rainfall (in mm) for return period = 100 years

1999 Rainfall data obtained for Method C Tests in the HD45/09

Government’s Multi-Agency Geographic Information for the Countryside (MAGIC) website

Interactive maps for rural designations – statutory

Accessed January 2013

Search on interactive maps for rural designations – statutory

Halcrow Drainage Strategy Document: CTRAEB 730 DOC DRA 020 Version: 2 South Bristol Link Road

May 2013 An outline of the proposed drainage strategy

Appendix F: Drainage Strategy Document: CTRAEB 730 DOC DRA 020 Version: 2 South Bristol Link Road: South Bristol Link

May 2013 Schedule of structures locations

Appendix G: Drainage Strategy Document: CTRAEB 730 DOC DRA 020 Version: 2 South Bristol Link Road: South Bristol Link CTRAEB_730_DRA_301 – 305

May 2013 Drainage catchments and route alignment with chainages

Metoffice.gov South West England: climate February 2013

Overview of rainfall volume in South West England


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Mott McDonald South Bristol Link Ground Investigation Report

February 2012

Geology data obtained for Method C Tests in the HD45/09

17.3.3. This assessment has been based on all currently available information. If new design information becomes available after or during finalisation of this report, and it is decided that the content will substantially alter the results of this assessment, the report will be updated. It is expected that this decision will be made by the Atkins Environmental Impact Assessment project manager in consultation with North Somerset Council and Bristol City Council.

17.4. Prediction and Evaluation of Effects

DMRB Methodology

17.4.1. Prediction and evaluation of effects follows the requirements of the DMRB detailed assessment process as set out in the DMRB HD45/09.

17.4.2. The DMRB methodology starts with identification of the importance of the environmental attribute. The magnitude of impact of the scheme on the attribute is then determined using historical records, calculations and tests from the DMRB, taking into consideration the influence of mitigation measures. The combination of the importance of an attribute and the magnitude of impact on that attribute gives a significance of potential effect on the water environment.

17.4.3. Importance of attributes is assessed in Section 17.5 and the magnitude of impacts and significance of effects are assessed in Section 17.6. Potential impacts of construction and operation have been investigated in accordance with DMRB HD45/09.

Water Quality

17.4.4. The DMRB guidance HD45/09 and Highways Agency Water Risk Assessment Tool (HAWRAT), provide the assessment methods for determining the impacts roads may have on the water environment. The HAWRAT tool has been used to make an assessment of the potential impacts of routine runoff on surface waters. It is designed to make an assessment of the short-term risks related to the intermittent nature of road run-off as well as the long-term risks. The toxicity thresholds used in HAWRAT have been derived from a collaborative research programme undertaken by the Highways Agency and the Environment Agency and are consistent with the requirements of the WFD. They have been designed to prevent adverse ecological effects in the receiving water.

Surface water

17.4.5. River catchments and low flow regimes known as Q95 (i.e. flows that are exceeded for 95% of the time) have been defined using a combination of the Flood Estimation Handbook (FEH) CD ROM version 3.0 (NERC – Centre for Ecology & Hydrology©) and Low Flows version 2.0 (Wallingford HydroSolutions Ltd ©) respectively. Low Flows enables river flows to be estimated for ungauged catchments in the United Kingdom. Low Flows has been adopted by most national agencies as the standard software system for the prediction of flow statistics within ungauged catchments.

Q95 Flow

17.4.6. Flow at low flow conditions has been used in the assessment to determine the in-channel concentration of pollutants once diluted by the river flows, as a worst case scenario (i.e. predicting impacts when there is least dilution in the environment). The low flow conditions are expressed in terms of the 95th percentile flow (Q95), which is defined as the flow exceeded in a watercourse for 95% of the time. The Q95 refers to a low flow scenario.

17.4.7. The Q95 flow is a key input parameter in HAWRAT, and is the flow rate of the river when exceedences of the ecological thresholds are more likely. This parameter is used by HAWRAT to


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calculate both dilution of soluble pollutants and the river velocity which, in turn, is used to estimate whether sediment is likely to accumulate.

17.4.8. Flows in the majority of water courses in the UK are affected by artificial influences: either abstractions, discharges or both, such that actual Q95 flows usually differ from natural Q95 flows. This assessment has used natural flows and has been estimated using Low Flows software package Wallingford HydroSolutions Ltd © (HD45/09).

HAWRAT Tests

17.4.9. Calculations and assessment relevant to determining effects on the water environment has been undertaken in accordance with DMRB HD45/09 methodology as follows: Method A – focus on dilution of routine runoff and pollutants. Method A – a simple assessment – includes the use of HAWRAT considering dilution of indicator metals (dissolved zinc and dissolved copper). Due to high levels of traffic and potentially sensitive watercourse receptors, all discharges have been tested using HAWRAT. The methodology for routine runoff involves tests to predict future concentrations of zinc and copper in receiving watercourses. This is based on Annual Average Daily Traffic (AADT) flows, catchment size for the road, dilution flows (Q95) and current water quality (hardness) for each watercourse. Method B – in the event that following the assessment using Method A application of mitigation still fails Environmental Quality Standards (EQS), Method B should be applied, although this is rare. This assesses the bioavailability of the soluble pollutants (dissolved zinc and dissolved copper). Method C – focuses on groundwater effects. This is the standard method for assessing the impact of a scheme on groundwater. Typically this considers the risk of pollution to groundwater of discharges from a scheme. Method D – focuses on serious spillage risk. The method provides the return period of a serious accident based on road length, road characteristics (e.g. presence of junctions, roundabouts, and crossroads) AADT, percentage of Heavy Good Vehicles (HGVs), spillage risk factors and emergency services response time (based on site environment – e.g. urban/rural). An assessment has been undertaken for each discharge to test whether the probability of an accidental spillage causing a pollution incident represents an unacceptable risk. The requirements of discharge from the scheme for meeting EQS in Method A (HAWRAT) and Method D are shown in Table 17.2.

Table 17.2: Water quality thresholds

Method Test Maximum Limit

Method A EQS Downstream dissolved zinc concentrations 7.8µg/l*

Method A EQS Downstream dissolved copper concentrations

1µg/l

Method D Risk of serious spillage <100 years Key: EQS = Environmental Quality Standards; * The maximum limit for dissolved zinc is dependent on hardness of the receiving water. As hardness reduces so does the maximum allowable limit. Source: DMRB HD45/09 Table A1.

In addition to the tests outlined above, groundwater vulnerability and Source Protection Zone (SPZ) maps have been reviewed to determine vulnerability of the groundwater in the local area.

Abstractions and Discharges

17.4.10. Information from the Environment Agency has provided sources of existing licensed abstractions and discharges that are within 1km of the scheme.


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Assessment Criteria

17.4.11. The significance of effects on water environment attributes, such as watercourses, has been determined using the four tables in DMRB HD45/09 that are shown in Tables 17.3 to 17.6 below.

17.4.12. The assessment of significance of potential effects of the scheme has been made in three stages; firstly, evaluating the importance of a water attribute, secondly, assessing the potential magnitude of impact of the scheme, with and without mitigation and, thirdly, using a combination of the importance and the magnitude, the significance of potential effect on the water environment has been determined.

17.4.13. An evaluation has been made of the importance of the water environment by considering relevant features within 1km of the scheme and the importance of these features has then been graded using the examples given in Table 17.3. Examples for assessing the magnitude of impacts are shown in Table 17.4. A matrix for determining significance of effects is shown in Table 17.5 and examples of the potential significance of effects are shown in Table 17.6.

Table 17.3: Evaluating the importance of water environment attributes

Importance Criteria Typical Example

Very High

Attribute has a high quality and rarity on regional or national scale

Surface water: EC Designated Salmonid/ Cyprinid fishery WFD Class “High”. Site protected/designated under EC or UK wildlife legislation (SAC, SPA, SSSI, WPZ, Ramsar Site, Salmonid water)/ species protected by EC legislation.

Groundwater: Principal aquifer providing a regionally important resource or supporting site protected under EC and UK Habitat legislation. SPZ 1.

High

Attribute has a high quality and rarity on a local scale

Surface water: WFD Class “Good”. Major Cyprinid Fishery. Species protected under EU or UK habitat legislation.

Groundwater: Principal aquifer providing locally important resource or supporting river ecosystem, SPZ 2.

Medium

Attribute has a medium quality and rarity on a local scale

Surface water: WFD Class “Moderate”.

Groundwater: Aquifer providing water for agricultural or industrial use with limited connection to surface water, SPZ 3.

Low Attribute has a low quality and rarity on a local scale

Surface water: WFD Class “Poor”.

Groundwater: Unproductive strata.

Key; SAC = Special Conservation Area; SPA = Special Protection Area; SSSI = Site of Special Scientific Interest; SPZ= Source Protection Zone; WPZ = Water Protection Zone Source: Adapted from DMRB HD45/09 Table A4.3

Table 17.4: Assessing the magnitude of impact on water environment attributes

Magnitude Criteria Typical Example

Major Adverse

Results in loss of attribute and/or quality and integrity of the attribute

Surface Water: Failure of both soluble and sediment-bound pollutants in HAWRAT (Method A) and compliance failure with EQS values (Method B). Calculated risk of pollution from an accidental spillage > 2% annually (Method D). Loss or extensive change to a fishery. Loss or extensive change to a designated Nature Conservation Site.

Groundwater: Loss of or extensive change to an aquifer. Risk score >250 (Method C). Calculated risk from spillages > 2% annually (Method D). Loss or extensive change to groundwater designated wetland.

Moderate Results in effect on Surface Water: Failure of both soluble and sediment-bound


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Adverse

integrity of attribute or loss of part of attribute

pollutants in HAWRAT (Method A) but compliance with EQS values (Method B). Calculated risk of pollution from accidental spillages > 1 % annually and <2% annually (Method D). Partial loss in productivity of a fishery.

Groundwater: Partial loss or change to aquifer. Risk score 150-250 (Method C). Spillage risk >1% annually and <2%. Partial loss in integrity of groundwater supported designated wetland.

Minor Adverse

Results in some measurable change in attributes quality or vulnerability

Surface Water: Failure of either soluble or sediment-bound pollutants in HAWRAT. Calculated risk of pollution from accidental spillages > 0.5% annually and <1% annually.

Groundwater: Risk score <150 (Method C). Calculated risk of pollution from accidental spillages > 0.5% annually and <1% annually (Method D). Minor effects on groundwater supported wetlands.

Negligible Results in effect on attribute but of insufficient magnitude to affect the use or integrity

The scheme is unlikely to affect the integrity of the water environment. Surface Water: No risk identified by HAWRAT and risk of pollution from accidental spillages <0.5%.

Groundwater: No measurable impact upon an aquifer and risk of pollution from accidental spillages <0.5%.

Minor Beneficial

Results in some beneficial effect on attribute or a reduced risk of negative effect occurring

Surface Water: HAWRAT assessment of either soluble or sediment bound pollutants becomes Pass from existing site where the baseline was a Fail condition. Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk is <1% annually) (Method D).

Groundwater: Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk is <1% annually) (Method D).

Moderate Beneficial

Results in moderate improvement in attribute quality

Surface Water: HAWRAT assessment of both soluble and sediment-bound pollutants becomes "Pass" from an existing site where the baseline was a Fail condition. Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk > 1% annually) (Method D).

Groundwater: Calculated reduction in existing spillage risk by 50% or more (when existing spillage risk is > 1% annually).

Major Beneficial

Results in major improvement of attribute quality

Surface Water: Removal of existing polluting discharge, or removing the likelihood of polluting discharges occurring to a watercourse.

Groundwater: Removal of existing polluting discharge to an aquifer or removing the likelihood of polluting discharges occurring. Recharge of an aquifer.

Adapted from Source: Adapted from DMRB HD45/09 Table A4.4

Table 17.5: Determining the significance of effect on water environment attributes

Importance of Attribute

Magnitude of Impact

Negligible Minor Moderate Major

Very High Neutral Moderate or Large Large/Very Large Very large

High Neutral Slight or Moderate Moderate/Large Large/Very Large


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Medium Neutral Slight Moderate Large

Low Neutral Neutral Slight Slight Moderate Source: DMRB HD45/09 Table A4.5


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Table 17.6: Examples of overall significance of effect

Effect Description

Very Large Adverse

Where the proposal would result in degradation of the water environment, because it results in predicted very significant adverse impacts on at least one water attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Water Potential failure of both soluble or sediment bound pollutants in a High or Good watercourse OR in an EC Designated Salmonid fishery for both short term and long term assessments (Methods A and B); Loss or extensive change to a site/habitat protected under EC or UK legislation (SAC, SPA, Ramsar site, SSSI, WPZ, Salmonid waters); Calculated risk of pollution from an spillages is >2% when discharging into a Good watercourse> 1% for an High watercourse*; Groundwater Potential high risk (risk score > 250) of pollution in Method C (Annex I) to a principal aquifer providing a regionally important resource or supporting a site protected under habitat legislation OR a medium to high risk (score>150) to an SPZ1; Calculated risk of pollution from an accidental spillage is >1% when discharging into an SPZ 1 or principle aquifer; Potential loss or extensive change to a site/habitat protected under EC or UK legislation (SAC, SPA , Ramsar site, SSSI, WPZ, Salmonid water) through interception of groundwater flow or significant change to groundwater level.

Large Adverse

Where the proposal would result in a degradation of the water environment, because it results in predicted highly significant adverse impacts on a water attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Waters Potential short-term (HAWRAT) failure of both soluble and sediment-bound pollutants in a High or Good watercourse OR in an EC Designated Salmonid fishery. Calculated risk of pollution from spillages is > 1% for a Good watercourse (or one of lower ecological class) and >0.5% for a High watercourse; Loss or extensive change to a cyprinid fishery; Loss or extensive change to a Local Nature Reserve; Groundwater Potential high risk (risk score >250) of pollution to a secondary aquifer providing water for a small number of dwellings, agricultural or industrial use and/or supporting Local Nature Reserves aquifer OR medium risk (Score 150-250) of pollution of a principal aquifer providing a locally important resource or supporting or supporting a site protected under habitat legislation, OR medium to high risk (score>150) to a SPZ2, OR potential low risk (score <150) to a SPZ1, Calculated risk of pollution from spillages is >0.5% when discharging to a principle aquifer or SPZ 1 Loss or extensive change to a Local Nature Reserve through interception of groundwater flow or change to groundwater level.

Moderate Adverse

Where the proposal may result in the degradation of the water environment, because it results in predicted moderate adverse impacts on at least one attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Waters Potential short-term (HAWRAT) failure of either soluble or sediment-bound pollutants in a High


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or Good watercourse; Calculated risk of pollution from spillages is > 0.5% for a Good watercourse OR >1% for a Moderate or Poor watercourse; Partial loss or change to a fishery; Effect on the integrity of the existing flora and fauna Groundwater Potential medium risk (risk score 150-250) to a secondary aquifer providing water for a small number of dwellings, agricultural or industrial use and/or supporting Local Nature Reserves OR potential low risk (risk score < 150) of pollution to a principal aquifer providing a regionally important resource or supporting a river ecosystem OR medium to high risk score (>150) to a SPZ 3 , OR potential low risk (score<150) to a SPZ2, OR high risk (>250) for a discharge to unproductive strata Calculated risk of pollution from spillages is > 1% for an aquifer or SPZ3; Effect on the integrity of the existing flora and fauna through interception of groundwater flow or change to groundwater level

Slight Adverse

Where the proposal may result in a degradation of the water environment because it results in a predicted slight impact on one or more attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Waters Potential short term (HAWRAT) failure of either soluble or sediment-bound pollutants in a Moderate or Poor watercourse; Calculated risk of pollution from spillages is > 0.5% for an Moderate or Poor watercourse Temporary loss to, or loss in productivity of, a fishery; Groundwater Potential low risk of pollution (risk score < 150) to a secondary aquifer with limited agricultural use and connectivity to surface waters and local ecology OR low to medium risk (risk score <250) for a discharge to an unproductive strata, OR low risk (score<150) to a SPZ3

Neutral

Where the net impact of the proposals is neutral, because it results in no appreciable effect, either positive or negative, on the identified attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately For example: Surface Waters No risk identified by Method A or Method B assessment (Pass both soluble and sediment-bound pollutants) Calculated risk of pollution from accidental spillages < 0.5% annually; Groundwater No predicted change in quality of any type of aquifer and/or its use as a resource; Calculated risk of pollution from accidental spillages < 0.5% annually

Slight Beneficial

All other situations where the proposal provides an opportunity to enhance the water environment or provide an improved level of protection to an attribute. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Waters Method A assessment of either soluble or sediment-bound pollutants becomes a pass from previous Fail conditions for existing discharges. Reduction by 50% or more in existing pollution risk from accidental spillages into High to Poor watercourses (when existing spillage risk is < 1%); Groundwater Reduction by 50% or more in existing pollution risk from spillages into an aquifer (when existing spillage risk is <1%)

Moderate Where the proposal provides an opportunity to enhance the water environment, because it results in a moderate improvement for an attribute. More than one attribute may be affected by


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Beneficial a single project and each should be assessed and reported separately. For example: Surface Waters Method A assessment of both soluble and sediment-bound pollutants becomes Pass from previous Refer or Fail condition for existing discharges; Reduction by 50% or more in likelihood of pollution to watercourses from spillages from existing discharges through retrofitting of pollution control to outfalls into a High to Poor watercourse* (existing risk > 1%); Recharge of aquifer through provision of treated discharges to ground resulting in measurable improvements to a connected site/habitat of local nature conservation value i.e. Local Nature Reserve; Groundwater Reduction by 50% or more in existing likelihood of pollution arising from a spillage to an aquifer through retrofitting of pollution control (existing risk > 1%)

Large Beneficial

It is extremely unlikely that any proposal incorporating the construction of a new or improved trunk road would fit into this category. However, proposals could have a large positive impact if it is predicted that it will result in a ‘very’ or ‘highly’ significant improvement to a water attribute(s), with insignificant adverse impacts on other water attributes. More than one attribute may be affected by a single project and each should be assessed and reported separately. For example: Surface Water Removal of an existing polluting discharge through provision of pollution prevention measures, or any other measure, affecting a site/habitat protected under EC or UK legislation (SAC, SPA, Ramsar site, SSS, WPZ, Salmonid waters); Reduction by 50% or more in the existing likelihood of pollution arising from a spillage affecting a site/habitat protected under EC or UK legislation (SAC, SPA, Ramsar site, SSSI, WPZ, Salmonid water) where existing risk > 1%. Groundwater Removal of an existing polluting discharge within Zone 1 and 2 of a SPZ and/or a principal aquifer Reduction by 50% or more in the existing likelihood of pollution arising from a spillage at discharge points within Zone 1 or 2 of a SPZ, principal aquifer and/or a site supporting a habitat protected under habitat legislation (existing risk > 1%) Recharge of aquifer through provision of treated discharges to ground resulting in measurable improvements to a connected site/habitat protected under EC or UK legislation (SAC, SPA, Ramsar site , SSSI , WPZ, Salmonid water)

Source: Adapted from DMRB HD45/09 Table A4.6


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17.5. Baseline Conditions and Sensitivity

Baseline Conditions

17.5.1. The proposed route of the scheme crosses areas of previously developed and undeveloped land, as shown in Figure 17.1. Within the undeveloped land, the route passes through the Ashton Brook and New Colliters Brook flood plain and open farmland, crossing several streams and unnamed drains. These are shown in Table 17.7. The watercourses in the vicinity of the site have historically been subject to diversion and hydraulic improvement. The Malago, Ashton Brook, the New Colliters Brook and the Old Colliters Brook are all designated ‘main river’ watercourses under the jurisdiction of the Environment Agency. The watercourses directly affected by the proposed scheme are Ashton Brook, New Colliters Brook, Colliters Brook, The Malago and Pigeon House Stream. In addition to these ‘main rivers’ there are a series of ordinary watercourses and agricultural drainage ditches south of the A38 which will also be affected by the proposed scheme.

17.5.2. The site is influenced by the surface water runoff from a number of existing roads which merge with the proposed scheme; the drainage strategy has been developed to intercept the runoff from adjoining roads, in particular Queen’s Road where surface water runoff is believed to contribute to flooding.

17.5.3. There is little information with respect to the design and operation of the existing road drainage system. However, the limited drainage information available suggests a proportion of the route within the developed land is served by existing surface water sewer systems and drawings provided from Wessex Water suggest no form of existing mitigation is in place (Halcrow, May 2013).

17.5.4. There are also two surface water balancing ponds operated by Viridor Ltd through which the proposed route passes. It is envisaged that the current discharge arrangements will be retained, with the possible eventuality of relocating the discharge points slightly to align with the repositioning of the ponds. Viridor have advised that the site is supported by adequate drainage and thus it is anticipated that nominal cut-off drains will be required to protect the highway construction (Halcrow, May 2013).

17.5.5. A ground investigation along the proposed route was undertaken by Structural Soils Ltd in December 2011. The investigation established that the ground conditions generally comprised clay overlain by made ground to a depth of around 1m in some locations, it was concluded that there is limited opportunity to discharge surface water directly to the underlying groundwater regime due to the impermeable nature of the ground (Halcrow, May 2013).

Surface Water Quality

Water bodies

17.5.6. The water bodies in Table 17.7 would be receptors of surface water discharge from the scheme, and are shown on Figure 17.1. The assessment of the importance of these water bodies follows the definitions in Table 17.4 and is included in Table 17.7. Colliters Brook and The Malago are of ‘High’ importance, based on a ‘Good’ ecological quality in 2027. The current ecological quality of these water bodies is ‘Moderate’. New Colliters Brook is in direct hydraulic connectivity with Colliters Brook and therefore contributes to its overall quality; subsequently this has been assigned ‘High’ importance.

17.5.7. Although not classified under the WFD, Ashton Brook and Longmoor Brook both discharge into the Bristol Avon transitional water body (WFD ID: GB530905415405), and therefore contribute to its overall quality. The current ecological quality of this water body is ‘Good’. Subsequently these have been assigned ‘High’ importance.


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17.5.8. Similarly, although not classified under the WFD, Pigeonhouse Stream is in direct hydraulic connectivity with The Malago and therefore contributes to its overall quality; subsequently this has been assigned ‘High’ importance.

17.5.9. The proposed route would also encroach onto a drainage ditch situated within a county designated wildlife site – Highridge Common Site of Nature Conservation Importance (SNCI). Subsequently this has been assigned ‘High’ importance.

Table 17.7: Summary of water bodies and their importance

Watercourse Assessment of Importance

WFD Ecological Quality (in 2027)

Ashton Brook High Good

Longmoor Brook High Good

New Colliters Brook High Good

Colliters Brook (GB109053027360)

High Good

Tributary of Colliters Brook tributary

High Good

Drainage ditch on a SNCI High N.A

The Malago (GB109053021970) High Good

Pigeonhouse Stream High Good Key: Text denoted into brackets = WFD ID. N.A = Non Applicable. SNCI = Site of Nature Conservation Importance Source: Environment Agency. December 2009. Annex B Severn River Basin District

There are no relevant (current or historic) upstream or downstream WFD River or Groundwater monitoring locations and associated data within the 1 km of the scheme.

Lakes and Other Water bodies

17.5.10. Two streams pass under the existing road (Hengrove Way) and combine 10m north of the road in a small man-made lake, which becomes Pigeonhouse Stream Lake flowing to the north. Although by name this denotes a ‘Lake’, Pigeonhouse Stream is hydraulically connected and therefore flow is considered to pass through the lake as a contiguous water feature. Herein, reference to Pigeonhouse Stream Lake will be Pigeonhouse Stream. In the context of the scheme, discharge is proposed to Pigeonhouse Stream. No physical works are proposed to the “Lake”.

Groundwater Quality

17.5.11. There are no SPZs within 1 km of the scheme and there are no Environment Agency monitoring boreholes for groundwater levels within 1 km of the scheme (Environment, September 2012). A summary of the importance of groundwater quality is shown in Table 17.8. This is related to aquifer type which is indicative of the potential water resource within an aquifer.

17.5.12. Superficial Deposits along the route comprises Alluvial Deposits in the northern section (approximate chainage 0 to 350) with River Terrace Deposits present as the route heads south to the railway line and beyond to Colliters Brook (chainage 350 to 1700). Superficial Deposits beyond the Colliters Brook and the A38 are interpreted to be weathered bedrock of mudstone and limestone (chainage 1700 to 5300) (Atkins, March 2012).


482

Table 17.8: Summary of bedrock aquifer designations and groundwater quality and importance

Chainage (m) Aquifer type Aquifer designation

Importance

0 & 0 (B) – 900

Mudstone and Halite-Stone Secondary B Medium

900 – 2100 Mudstone and Halite-Stone Secondary B Medium

Mudstone and Limestone interbedded

Secondary A Medium

2100 – 3100 Mudstone Unproductive Low

Limestone and Mudstone interbedded

Secondary A Medium

3100 – 4500 Mudstone Unproductive Low

Limestone and Mudstone interbedded

Secondary A Medium

Source: Environment Agency. September 2012


17.5.13. No public abstractions have been identified within 1 km of the proposed scheme. Abstractions will not be considered further in this assessment (Environment Agency. September 2012).

17.5.14. There are 14 existing discharges identified within 1 km of the scheme. These are detailed in Table 17.9. The current WFD status includes the influence of existing discharges and it is recognised that they have the ability to affect this through change. However, in the context of this scheme, the discharges do not add to the value of any of the water features within the study area as they are already at High or Medium importance.

Table 17.9: Summary of existing consented discharges and their importance Number Consent Number Name Discharge Environment 1 103242 Deer Larder Soakaway 2 012600 David Lloyd Tennis Centre Longmoor Brook 3 012713 Park And Ride Scheme Longmoor Brook 4 103304 Kings Walk Ps The Malago 5 101818 Rear 9 Old Mead Walk Field Ditch 6 101825 Tugela Road CSO The Malago 7 101824 Queen's Road CSO The Malago 8 101822 Headley Road/Avon Road The Malago 9 101820 R/O 31 Derham Road CSO The Malago 10 101725 Manor Wood Open Space CSO The Malago 11 YNS421SW Yanley Landfill Site Colliters Brook 12 YNS422SW Yanley Landfill Site Surface discharge 13 YN501SW Yanley Landfill Site Leachate to the foul water system Water Discharge Activity Exemptions 14 EPR/GH0664AA/A001 Court Farm, Winford, Bristol Sewerage to ground Key: CSO = Combined Sewer Outfall Source: Environment Agency. September 2012


483

Ecology

17.5.15. Surveys reported in the Ecology Chapter 13 indicate otters occasionally use Colliters Brook and Longmoor Brook for commuting and potentially foraging and resting. No evidence of water voles were recorded in the surveys. It is concluded this species is unlikely to be present within the watercourses within the scheme area. As otters could be potentially present in the corridor of the scheme, the importance of the water environment attribute for ecology is High. However, the assessment for ecology has been undertaken in the ecology chapter and is not considered further in this chapter.

Designated sites

17.5.16. There are no designated sites within 1km of the scheme, in direct hydraulic connectivity with any of the water course receptors outlined in Table 17.5. Designated sites will not be considered further in this assessment. The importance of National and European designations and Local Non Statutory designated are considered further in Chapter 13: Ecology.

Water Framework Directive Compliance

17.5.17. A WFD compliance assessment has been undertaken in parallel to the Environmental Impact Assessment (EIA) using guidance supplied by the Environment Agency. The WFD is a European Directive which sets out a strategic planning process for the purposes of managing, protecting and improving the water environment. A compliance assessment is required by the Environment Agency who is the Competent Authority for the WFD in relation to impacts on main rivers.

17.5.18. There are four key objectives against which the impacts of proposed works on a water body need to be assessed to determine compliance with the overarching objectives of the WFD: • Objective 1: The scheme will not cause deterioration in any element of water body

classification • Objective 2: The scheme will not prevent the WFD status objectives from being reached

within the water body or other downstream water bodies • Objective 3: The scheme will not negatively impact critical or sensitive habitats within the

water body. • Objective 4: The scheme will contribute to the delivery of the Severn RBMP.

17.5.19. The first three obligations must be met to avoid infraction of the WFD. The delivery of the fourth objective is central to the Environment Agency’s implementation of the WFD, where it can be supported through its operational activities.

17.5.20. If it is considered that the scheme is likely to cause deterioration in water body status or prevent a water body from meeting its ecological objectives the scheme must not proceed as this would put the UK Government at risk of infraction from the European Union. Further assessment (known as the application of Article 4.7) would then need to be undertaken to justify overriding public interest; and/or that the scheme benefits outweigh the environmental and social benefits of achieving the WFD objectives; and demonstrate that there are no significantly better alternatives and that all possible mitigation has been identified. An initial screening assessment was undertaken during January – February 2013 using best available data to consider the scheme elements (e.g. river realignments, construction of culverts and bridges) that could impact on the hydromorphological, biological or physico-chemical elements of the three WFD assessed water body receptors of the scheme 1) Colliters Brook 2) The Malago and 3) the downstream transitional water body the Bristol Avon.

17.5.21. The initial screening assessment was shared with the Environment Agency and their comments and feedback taken onboard.

17.5.22. A detailed WFD compliance assessment then followed which included detailed baseline information for the physico-chemical, hydromorphological and ecological elements. Each scheme element was then assessed in terms of their impacts against the individual water body elements. Cumulative impacts were also considered. A draft report was issued to the Environment Agency for comment and approval in April 2013. Comments were received from the Environment Agency


484

on the 13th May 2013 stating that they were generally satisfied with the assessment and conclusions of the report. A small number of detailed comments relating to designated sites, and potential mitigations and enhancements were considered further and the report subsequently updated.

17.5.23. Due to a refinement in scheme designs an additional WFD compliance re-assessment (included within a technical memo) was undertaken for the elements that had subsequently changed. In this assessment Atkins has assessed the proposed works as compliant (assuming that a number of mitigation measures form part of the proposal). The technical memo was issued to the Environment Agency for comment and approval on the 13th May 2013. On the 28th May 2013 the Environment Agency confirmed they were satisfied with the memo and that the conclusions in terms of WFD compliance of the scheme are still valid..

17.5.24. Both sets of Environment Agency comments can be viewed within Appendix 17.3.

17.5.25. As part of the WFD compliance assessment a number of enhancements were also suggested and the Environment Agency is supportive of these. Some of these enhancements will be considered as part of the detailed design phase, and where possible the recommendations will be incorporated into the design. The mitigations and enhancements related to the WFD assessment are included within Section 17.7.3. The WFD compliance assessments are included within Appendix 17.2.

Flood Risk Assessment

17.5.26. The scope of the FRA of the existing flood risk features along the route of the scheme was agreed with the Environment Agency. Hydraulic assessments were undertaken specifically for the assessment of flood risk along the route of the scheme. All flood related information is referenced in the FRA Chapter 16.

Land Contamination

17.5.27. The proposed route of the scheme encroaches into a landfill site operated by Viridor Ltd and requires the repositioning of two of their surface water balancing ponds; the waste has been clarified as being inert (Halcrow, May 2013). Other landfills and land contamination are described and assessed further in the Ground Conditions and Land Contamination Chapter 18.

17.6. Identification and Assessment of Likely Significant Effects 17.6.1. This section will consider the proposed drainage of the scheme and the potential impacts and

effects of the scheme during construction and operation. The potential effects set out in this section do not take into account proposed mitigation.

Proposed Surface Water Drainage Strategy

Introduction

17.6.2. North Somerset Council commissioned Halcrow to compile a sustainable drainage strategy (Halcrow, May 2013). As outlined in the baseline, the route of the scheme passes through undeveloped farmland and south Bristol, and hence consideration has been given to assessing both Greenfield and Brownfield surface water runoff rates in the development of a sustainable drainage system.

17.6.3. Due to the extent of development south Bristol there has been limited opportunity to develop use a sustainable drainage with respect to both the collection of surface water runoff and its disposal in a sustainable manner. An element of the proposed surface water drainage in the suburb of Bishopsworth is to discharge into watercourses via surface water sewers owned and managed by Wessex Water. In principle there are two drainage catchments where it is deemed necessary to use Wessex Water’s surface water sewers to aid in the conveyance of highway drainage to downstream watercourses. The catchments in question are detailed below and within Sections 17.6.3 and 17.6.4.


485

17.6.4. The drainage strategy has been developed to intercept the runoff from adjoining roads, in particular Queen’s Road where surface water runoff is believed to contribute to flooding (Halcrow, May 2013). To control run-off in the areas of open farmland consideration has been given to the incorporation of a number of cut-off drains to be constructed within the highway boundary (Halcrow, May 2013).

17.6.5. The Factual Report on Ground Investigation for south Bristol (Structural Soils Ltd, 2011) established that the ground conditions generally comprised clay overlain by made ground to a depth of around 1m (Halcrow, May 2013). Subsequently, it is considered there is no opportunity to discharge to surface water directly to the underlying groundwater regime due to the impermeable nature of the sub-strata. The investigation also informed that the groundwater level was typically high and in some instances was perched and believed to have a potential artesian head (Halcrow, May 2013).

Consultation

17.6.6. The Environment Agency, together with North Somerset Council, Bristol City Council, Viridor Ltd and Wessex Water has been consulted during the development of the drainage strategy.

Drainage Catchments

17.6.7. The route of the scheme has been assessed in terms of drainage catchments based on the vertical alignment of the proposed highway (Halcrow, May 2013).

17.6.8. Ground investigations identified that it is not possible to discharge to ground for this scheme therefore, the next preferred method of discharge will be to surface water courses. Where it is not possible to discharge to surface water courses discharge will be considered to the storm water sewer network.

17.6.9. Suitable discharge points have been determined for each catchment; where viable surface water is discharged directly to watercourses located in close proximity to the network’s attenuation facility (Halcrow, May 2013).

17.6.10. It has not been possible to provide a direct discharge for all catchments, particularly in the urban section of the scheme where open land is at a premium. Discharge to surface water sewers is proposed in these locations. (Halcrow, May 2013).

17.6.11. Details of the catchments are shown in Table 17.10 below and Figure 17.1 (Volume 3, Appendix 17.1) illustrates the locations.

Outfalls

17.6.12. All outfalls are proposed to discharge to watercourses; however some outfalls are proposed to discharge to watercourses via existing surface water sewers owned and managed by Wessex Water. Table 17.10 below provides a summary of the proposed outfalls.

Table 17.10: Catchment summary Catchment Chainage (m) Proposed outfall Existing

conditions Importance of attribute Start Finish

A 0 750 Ashton Brook Greenfield High B 0 (B) 0 (B) New Colliters Brook Greenfield High C 250 (B) 580 (B) New Colliters Brook Greenfield High D 750 1100 New Colliters Brook Greenfield High E 1100 1990 Colliters Brook Greenfield High F 1990 2090 Colliters Brook Greenfield High G 2090 2603 Tributary of Colliters Brook Greenfield High H 2603 2973 Drainage Ditch through a SNCI Greenfield High


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J 2973 3666 The Malago Brownfield High K 3666 4014 The Malago Brownfield High L 4014 4197 The Malago Brownfield High M 4197 4448 Pigeon House Stream Brownfield High Source: Halcrow, May 2013 Key: (B) = Bus Link; SNCI = Site of Nature Conservation Importance

Proposed schedule of works and structure locations The effects on the water environment of in-channel works such as water course diversions, construction of culverts and bridges with mitigation are considered within 17.6.6. In-channel work requires particular care to prevent pollution or damage to the bed and banks of the watercourse. Disturbance of silt within the channel is the main issue, in addition to the polluting material that may be present as part of works. Water body receptors affected by such works are listed in Table 17.11 below.

Table 17.11: Surface water receptors at risk of water pollution from schedule of works

Catchment Water Feature Chainage (m) Importance

New culvert (with mammal underpass)

A Cut-off ditch in hydraulic connectivity to Ashton Brook 137 High

528

768

E Colliters Brook 1346 High

1515

H Drainage Ditch through a SNCI 2762 High

New Bridge

A Longmoor Brook 296 High

D Colliters Brook 936 High

E 1515 High

New culvert

A A370 embankment toe drain (in connectivity with Ashton Brook)

9 High

B New Colliters Brook 75 (B) High

158 (B)

C 315 (B)

F Tributary of Colliters Brook 2030 High

G 2350

H Drainage ditch through a SNCI 2760 High

New culvert & channel works

D Colliters Brook & Relocated Viridor pond

978 High

E 1295

1640 – 2000


487

G Tributary of Colliters Brook 2141 High

Retaining Wall

E Colliters Brook 1420 – 1490 High

1520 – 1690

Channel works – watercourse realignment

E Colliters Brook

1435 – 1455 High 1555 – 1580

Channel works – de-culverting

E & F Tributary of Colliters Brook 1640 – 2000 High

F 2002

2045 – 2075

Channel works – Fill and Watercourse diversion

G Tributary of Colliters Brook 2250 High

2408

2448

2573

H Field ditch draining north through a SNCI 2667 High

2716 Source: Halcrow, May 2013. Appendix F – Schedule of Structure Locations Key: (B) = Bus Link; SNCI = Site of Nature Conservation Importance

Construction Phase

17.6.13. The proposed construction works for the scheme, have the potential to impact water quality in any of the receiving surface water receptors due to the deposition or spillage of soils, sediment, fuels or other construction materials, or through mobilisation of contamination following disturbance of contaminated ground or groundwater, or through uncontrolled site runoff.

17.6.14. Prior to construction a Construction Environmental Management Plan (CEMP) would be compiled to provide targeted guidance throughout the construction period. This would detail both generic and specific instruction to enable construction to be undertaken with minimal impact on the water environment and ensure appropriate consents are obtained prior to works commencing.

17.6.15. Guidance has been collated from DMRB Volume 11 Section 3 Part 10, the Construction Industry Research and Information Association (CIRIA c648) and the Environment Agency PPGs. Generic methods to prevent pollution during construction are presented in the mitigation section.

17.6.16. In addition to best practice guidance, effects of construction have been assessed for particular construction activities and how they may vary according to different watercourses and their sensitivity.

17.6.17. Significance of effects of construction impacts on surface water quality, groundwater quality, lakes and other water bodies have been considered. The importance of the receiving water feature, magnitude of impact of construction activity including mitigation and the significance to the water environment is reported.


488


17.6.18. A detailed sediment and erosion plan to limit silt entering the watercourses would be prepared as part of the detailed CEMP prior to construction. Daily visual inspections of the local watercourses and ground for evidence of contamination e.g. sheens and increases in turbidity, would be undertaken.

17.6.19. At the time of reporting, the main site compound is likely to be located near the Great Western railway bridge (catchment D). It is thought a secondary site compound will be set up within or closer to the urban areas within the vicinity of the scheme. The effects and risks on the water environment of temporary site work compounds and main construction activities with mitigation are considered within Table 17.12 below.

Potential Effects of Lakes and other Water bodies

17.6.20. No discharge would be made to lakes without consent of the relevant drainage authorities or landowners.

Groundwater Quality

17.6.21. Without appropriate mitigation, construction work could affect groundwater quality particularly through discharge of site compound waste waters. Table 17.13 summarises the magnitude of impacts and significance of effects on groundwater quality during construction with mitigation.


17.6.22. Consent would be obtained for abstractions and discharges where required for construction.


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Table 17.12: Potential effects of construction on surface water quality with mitigation

Water feature Chainage Importance Potential effects Magnitude of impact

Example of proposed mitigation

Significance

New culvert (with mammal underpass)

Cut-off ditches in hydraulic connectivity to Ashton Brook

137 High Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Reduction in channel capacity Inhibited flow conveyance Release of cement into surface water bodies Damage to the bed and banks of the watercourse

Negligible Sediment Erosion Management Plan controls Use precast units where practicable, to minimise the use of wet cement. Machines would be protected by drip trays with emergency booms nearby. All plant would be regularly inspected for leaks and refuelling would take place on sand mats to prevent discharge to watercourses All water upstream of a culvert would be over pumped into a manhole allowing settlement of silt before discharge downstream

Neutral 528 768

Negligible Colliters Brook 1346 High Neutral

1515

Drainage Ditch through a SNCI

2762 High Negligible Neutral

New Bridge

Longmoor Brook 296 High Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Reduction in channel capacity Inhibited flow conveyance Release of cement into surface water bodies Damage to the bed and banks of the watercourse

Negligible Sediment Erosion Management Plan control Use precast units where practicable, to minimise the use of wet cement. Machines would be protected by drip trays with emergency booms nearby. All plant would be regularly inspected for leaks and refuelling would take place on sand mats to prevent discharge to watercourses

Neutral

Colliters Brook 936 High Negligible Neutral

1515


490

New culvert

A370 embankment toe drain (in connectivity with Ashton Brook)


Negligible Sediment Erosion Management Plan controls Use precast units where practicable, to minimise the use of wet cement. Machines would be protected by drip trays with emergency booms nearby. All plant would be regularly inspected for leaks and refuelling would take place on sand mats to prevent discharge to watercourses All water upstream of a culvert would be over pumped into a manhole allowing settlement of silt before discharge downstream

Neutral

New Colliters Brook 75 (B) High Negligible Neutral

158 (B)

315 (B)

Tributary of Colliters Brook


2350

Drainage ditch through a SNCI


New culvert & channel works

Colliters Brook & Relocated Viridor pond culvert


Negligible Sediment Erosion Management Plan controls Use precast units where practicable, to minimise the use of wet cement. Machines would be protected by drip trays with emergency booms nearby. All plant would be regularly inspected for leaks and refuelling would take place on sand mats to prevent discharge to watercourses All water upstream of a culvert would be over pumped into a

Neutral

1295

1640 – 2000




491

manhole allowing settlement of silt before discharge downstream

Retaining wall

Colliters Brook 1420 – 1490 1520 – 1690

High Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Reduction in channel capacity Inhibited flow conveyance Release of cement into surface water bodies Damage to the bed and banks of the watercourse

Negligible Sediment Erosion Management Plan controls Morphological study to advise on required extent of retaining wall due to mobile channel

Neutral

Channel works – watercourse realignment

Colliters Brook 1435 – 1455

High Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Reduction in channel capacity Inhibited flow conveyance Release of cement into surface water bodies Damage to the bed and banks of the watercourse

Negligible Sediment Erosion Management Plan controls Morphological study to advise on required extent of retaining wall due to mobile channel

Neutral

1555 – 1580

Channel works – de-culverting


1640 – 2000

High Dewatering Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Reduction in channel capacity Inhibited flow conveyance Damage to the bed and banks of the

Negligible Sediment Erosion Management Plan controls

Neutral

2002

2045 – 2075


492

watercourse

Channel works – Fill and watercourse diversion



Negligible Sediment Erosion Management Plan controls

Neutral

2408

248

2573 Negligible Neutral

Field ditch draining north through a SNCI

2667 Low

2716

Construction of temporary storage

All rivers and ditches All High Increased suspended sediment affecting surface water bodies Release of hydrocarbons to surface water Release of cement into surface water bodies

Negligible Sediment Erosion Management Plan controls Temporary storage would be positioned to intercept runoff and allow settlement, such that there should be no silt-laden water discharged to watercourses during storms. Hardcore surface and materials storage, including topsoil, aggregates, cement and fuels Materials storage (topsoil, aggregates)

Neutral

Disposal of foul water from main compounds

All rivers and ditches All High Changes in chemical balance of water quality

Negligible First choice would be discharge to foul water connections to local authority sewers. Second option would be an onsite water treatment plant with treated

Neutral


493

effluent discharged to a watercourse meeting discharge consent

Disposal of surface water runoff from compounds


Negligible The principal contractor will provide a method statement of control of contamination and pollutants

Neutral

Dust caused by construction works


Negligible Suppression of dust using water sprinklers, from waters gained from abstraction and bowsers. Use of permanent storage area for water storage where required.

Neutral

Key: (B) = Bus Link; SNCI = Site of Nature Conservation Importance

Table 17.13: Potential effects of construction on groundwater quality

Water Feature Importance Magnitude of impact

Mitigation Significance

Temporary Storage

Groundwater Medium/Low Negligible Areas which may generate contaminated water would be bunded and have water discharged to self contained units with treatment facilities. There would be no discharge to groundwater.

Neutral

Groundwater Abstraction Groundwater Medium/Low Negligible If insufficient water is available to supply requirements at site compounds any

abstractions from groundwater would first require licence from the Environment Agency.

Neutral

Storage of material from construction excavations Groundwater Medium/Low Negligible Tests would be undertaken to ensure contaminated material is identified, isolated

and reworked or removed to special landfill to avoid any leachate problems. Neutral


494

Cutting below groundwater table Groundwater near headwall installation or other structures

Medium/Low Negligible Consultation with the Environment Agency would determine if an abstraction licence is required for dewatering.

Neutral


495

Operational Phase


17.6.23. Water quality tests have been undertaken in accordance with Methods A and D in Annex I of DMRB HD45/09. A summary of the results scheme is shown in Table 17.14. Water hardness has a significant effect on the short-term toxicity of pollutants, in particular zinc, such that toxicity values decrease with increasing water hardness (Johnson & Crabtree, 2007). In the absence of hardness data a conservative approach was adopted and hardness was set to the lowest hardness default value in HAWRAT (<50mg CaCO3/l) for all receiving water bodies.

17.6.24. Initially, tests were undertaken for catchments H, J and M only using baseline, existing conditions in order to allow comparison of pre and post development surface water quality associated with the urban elements of the scheme. Although within the urban element, catchments K and L are new sections of highway, which are to be constructed within a protected corridor and thus no existing baseline conditions are available.

17.6.25. The remaining catchments were only assessed on the proposed scheme as these catchments will be within previously undeveloped areas. Table 17.15 provides a summary of existing catchment details. Catchments A – H inclusive can be categorised as being Greenfield catchments as they currently comprise open farmland/fields. Catchments J – M inclusive can be categorised as being Brownfield catchments, as pre- development these catchments comprise urban development and lie within the bounds of Bristol City (Halcrow, May 2013).

17.6.26. Tests undertaken using Method A show that, with the exception of catchment C, without mitigation all individual discharges would meet and pass the intermittent highway run-off 6 hour and 24 hour ecologically-derived Runoff Specific Thresholds (RSTs). Catchment C would fail the RST for zinc. RSTs assess the acute impacts caused by road pollutants and identify the risk of short-term exposure of organisms to them.

17.6.27. Following this failure, and in the absence of water quality data for New Colliters Brook, water quality data for watercourses within the Avon catchment was sourced from the Environment Agency and water quality tests for catchment C were refined. Specifically the hardness band was set to the medium value in HAWRAT (50 – 200 mg CaCO3/l). Tests show that with this parameter change and without mitigation catchment C would pass the RSTs.

17.6.28. Tests also show that, without mitigation all discharges would meet the EQS for both copper and zinc, as the dilution provided by the water bodies (Ashton Brook, New Colliters Brook, Colliters Brook, tributary of Colliters Brook, drainage ditch encroaching on an SNCI and The Malago and Pigeon House Stream) is adequate.

17.6.29. The difference in the magnitude of impact between the existing and proposed conditions for catchments H, J and M is negligible as the traffic values (ADDT and HGVs) for each scenario (existing and proposed) are within the minimum AADT thresholds band (>10,000 and <50,000) specified in HAWRAT.

17.6.30. Three Method C tests were undertaken as site ground conditions (e.g. superficial and solid geology) change along the length of the scheme (Mott McDonald, February 2012). Test one was for catchments A – E (0 – 1990m chainage), test two was for catchments F+G (1990 – 2603m chainage) and test three was for catchments H – M (2603 – 4448m chainage). Tests show there would be a medium risk to groundwater.

17.6.31. However, as the existing ground has a low permeability and the risk from all run-off discharges is low, it is considered additional mitigation is not required. As highlighted, the route will encroach onto a landfill site operated by Viridor and requires the repositioning of two of their surface water balancing ponds; the waste has been clarified as being inert, however as a precaution the new detention basins and cut-off ditches in these areas (catchments E+F) are to be fully lined with an impermeable membrane (Halcrow, May 2013).

17.6.32. Method D tests undertaken show there would be no discharge with a serious spillage risk of less than the threshold of 1 in 100 years, as shown by the Method D results in Table 17.14.


496

17.6.33. However, the scheme will discharge to watercourses Ashton Brook, Longmoor Brook, New Colliters Brook, Colliters Brook and The Malago Brook, all of which are designated ‘main river’ watercourses under the jurisdiction of the Environment Agency and the runoff from a number of catchments is to be conveyed to the watercourse via existing surface water sewers owned and maintained by Wessex Water.

17.6.34. Furthermore, the Environment Agency has requested the use of SUDS to reduce surface water runoff and aid pollution control to provide maximum benefit, even though the risks are low. The Environment Agency has requested that two levels of SUDS treatment is provided at each outfall in line with PPG3. Therefore, mitigation has been proposed for all sections of the scheme to ensure appropriate mitigation is provided.

Lakes and Other Water Bodies

17.6.35. No direct discharges are proposed to lakes or other water bodies during operation giving a negligible magnitude of impact and a neutral significance of effects.

Maintenance

17.6.36. Maintenance of the drainage system would be undertaken by the relevant highways authority, in accordance with their normal practices.

17.6.37. Attenuation facilities have been located outside of the 1% annual probability, plus an allowance for climate change, flood level to ensure the integrity of the functional flood plain and associated watercourses (Halcrow, May 2013).


497

Table 17.14: Summary of effects of operation on watercourses (water quality) Catchment reference

Receiving watercourse

RST no mitigation

EQS limit (mg/l) no mitigation no mitigation

Method D serious spillage risk no mitigation (threshold <100yrs)

Importance Magnitude of impact

Significance of effect

HAWRAT copper

HAWRAT zinc

HAWRAT copper concentration

HAWRAT zinc concentration

A Ashton Brook Pass Pass 0.07 0.26 6237 High Negligible Neutral B New Colliters

Brook Pass Pass 0.16 0.6 525995 High Negligible Neutral

C New Colliters Brook

Pass Pass 0.39 1.42 343040 High Negligible Neutral

D New Colliters Brook


E Colliters Brook Pass Pass 0.14 0.53 13144 High Negligible Neutral F Colliters Brook Pass Pass 0.07 0.26 9128 High Negligible Neutral G Tributary of

Colliters Brook Pass Pass 0.05 0.19 7435 High Negligible Neutral

H Drainage ditch through a SNCI


J The Malago Pass Pass 0.12 0.45 18892 High Negligible Neutral K The Malago Pass Pass 0.04 0.14 206256 High Negligible Neutral L The Malago Pass Pass 0.03 0.12 61081 High Negligible Neutral M Pigeon House

Stream Pass Pass 0.05 0.19 28696 High Negligible Neutral

Key: SNCI = Site of Nature Conservation Importance


498

17.7. Mitigation Measures 17.7.1. The proposals for the scheme drainage have been summarised in the Proposed Surface Water

Drainage Strategy section above and directed through Method A and Method D tests. The influence of the mitigation measures for the water environment is summarised below.

17.7.2. Mitigation measures for drainage and the water environment during operation are required for several reasons: • To treat contaminants in normal road run-off; • To deal with any accidental spillages occurring on the carriageway; • To attenuate flows from additional impermeable areas of road; • To prevent any increase to flood risk in the area; and • To protect and enhance wildlife corridors near watercourses.

17.7.3. The following sections look at mitigation specifically relating to surface water quality, lakes and other water bodies, groundwater quality and abstractions and discharges, for both construction and operation.

17.7.4. Mitigation measures that are required to ensure WFD compliance have also been summarised as part of this chapter.

Construction Phase


17.7.5. The risk of pollution during construction can be reduced by the adoption of good working practices and strict adherence to the Environment Agency PPGs. The key guidelines are: • PPG 1 General Guide to the Prevention of Water Pollution; • PPG 2 Above ground oil storage tanks; • PPG 3 Use and design of oil separators in surface water systems; • PPG 4 Treatment and disposal of sewage where no foul sewer is found; • PPG 5 Work in, near or liable to affect a Watercourse; • PPG 6 Working at Demolition and Construction Sites; • PPG 22 Dealing with Spillages on Highways; and • PPG 23 Maintenance of Structures over Water.

17.7.6. Generic mitigation measures that would be applied prior to and during construction include the following. Specific examples are detailed in Table 17.12 above. • Provision of adequate temporary storage to contain surface run-off during the construction

period, particularly when there are large areas of exposed earthworks or cutting as these lead to substantial increases in surface flows during intense rainstorms and can carry silt through to receiving watercourses;

• On-site availability of oil spill clean-up equipment including absorbent material and inflatable booms for use in the event of an oil spill or leak;

• Use of drip trays under mobile plant; • Sediment-trapping matting installed downstream of any construction activities adjacent to or

over watercourses; • Preparation of incident response plans, prior to construction, and present on site throughout

construction to inform sub-contractors of required actions in the event of a pollution incident; • Timing of works close to watercourses so that they do not interfere with spawning fish; • The use of construction materials on site free from contaminated material, so as to avoid any

potential contamination of the watercourse; • Ensuring that wet cement does not come into contact with river or groundwater; and • Testing of made soils and soils that would be reworked to identify any soil contamination.


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17.7.7. During construction an incident response plan would be in place to deal with any issues as soon as they occur for a particular site and to ensure that works are undertaken with the utmost care where they have potential to lead to contamination of any watercourse. Emergency action planning would include measures to be taken to prevent pollution caused by severe weather.

17.7.8. Water management would be an important part of the earthworks operation. During wet periods, storage of surface run-off would be undertaken to assist in dust suppression during dry periods. Prior to the commencement of any major soil stripping activities, initial water management systems would be installed.

17.7.9. Temporary site drainage would be designed where practicable to retain surface run-off within the site boundary. Where possible the permanent drainage arrangements would be utilised in the temporary management system. Prior to starting the main earthworks, the permanent cut-off ditches would be constructed at the tops of batters and toes of embankments. These ditches would be used to direct the surface water to the storage lagoons and hence prevent the run-off from entering the earthworks areas. Where required, additional settling areas would be constructed to allow for settling of sediment before discharge or water to watercourses.

17.7.10. Consideration would be given to existing drainage when storing materials and plant on the existing carriageways (catchments H, J and M). Where bunds and large amounts of soil are to be stored for extended periods of time they would be damped down with water and seeded where possible to suppress dust.

17.7.11. Wherever practicable, grey water systems would be used at site compounds to reduce run-off from site, improve water efficiency and lessen the potential for polluting discharges to surface watercourses.

17.7.12. Uses that could generate contaminated water, such as refuelling areas, would themselves be bunded and would drain to lagoons that would have oil interception facilities. The water would be discharged to ditches or streams through balancing ponds that would also have oil retention measures. Any oils or contaminants retained would be pumped into tankers at regular intervals for disposal off site.

17.7.13. Foul water drainage would be piped to an on-site wastewater treatment plant, or contained within the facilities for removal during regular maintenance.

Lakes and Other Water bodies

17.7.14. If evidence of connectivity from surface water discharge to lakes and other water bodies is found at time of construction and it is not possible to mitigate the impact, alternative discharge pathways and receptors would be identified and used, where other appropriate mitigation cannot be provided.

Groundwater Quality

17.7.15. No controlled discharges would be made to ground or groundwater during construction without Environment Agency consent and the permission of landowners. Measures to prevent and mitigate accidental pollution of groundwater would be the same as those already described under surface water above.

17.7.16. Where any excavation is to be undertaken below the water table during the installation of headwalls or other structures, consultation with the Environment Agency would determine if an abstraction licence for dewatering would be required.


17.7.17. Any abstractions required for provision of water for dust suppression or site works is required to be consented by the Environment Agency.

17.7.18. All temporary discharges (either directly or via attenuation features) would be reported to and required consent by the Environment Agency and would be to surface waters where practicable.


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Where discharges are to groundwater they would only be undertaken with the consent of the Environment Agency or the landowner.

Operational Phase


17.7.19. Water quality tests have been undertaken in accordance with Methods A of HD45/09 have shown that without mitigation all discharges would meet EQS and RSTs for both copper and zinc, as the dilution provided by the water bodies is adequate. However, a number of different treatment options are available for road run-off for further enhancement in pollutant loading to water courses.

17.7.20. DMRB Volume 4 Section 2 Part 1 Vegetative Treatment Systems for Highway Run-off, National Planning Policy Framework guidance on flood risk and other industry standard guidance recommend the use of sustainable drainage systems (SUDS)

17.7.21. The DMRB considers how SUDS may be used to treat run-off. This includes a range of SUDS that could provide mitigation for both quality and attenuation of water. The choice of system is dependent on the physical environment of the scheme and needs to consider availability of land, climate and rainfall characteristics, soil permeability, topography and spillage risk.

17.7.22. The baseline assessment has shown that: • The selection of an outfall has been based on the following order of preference: 1) infiltration;

2) discharge to watercourse; and 3) discharge to existing surface water sewers. As described earlier, infiltration drainage is not an option therefore outfalls will either be existing watercourses where practicable, or existing surface water sewers.

• Groundwater quality is not thought to be sensitive but soil permeability for the majority of the proposed route is generally low particularly as a large proportion of the area is clay which has a high impermeable nature.

• All outfalls are proposed to be to watercourses; however some outfalls are proposed to discharge to watercourses via existing surface water sewers owned and managed by Wessex Water. Table 17.19 provides a summary of the proposed outfalls.

• The receiving surface waters of the scheme have a High sensitivity to water quality. • The site covers areas of previously developed and undeveloped land. There is opportunity in

the undeveloped areas for detention basins, however due to insufficient available space and/or undesirable topography for an adequate attenuation facility in the developed areas runoff is to be conveyed to the watercourse via existing surface water sewers owned and maintained by Wessex Water.

• The volume of traffic and the type of goods that would be transported by the scheme indicate that serious spillage risk is low.

Vegetative treatment options, described in HA103/06 available to mitigate potential effects on water quality include:

• Swales and grassed surface run-off channels; • Infiltration basins; • Sedimentation ponds; • Balancing Ponds; and • Wetlands (sub surface and surface).

Applicable mitigation options for SBL Swales and grassed surface run-off channels

17.7.23. North Somerset Council has stated that the maximum verge width on an embankment is 1m, meaning that it would be impossible to construct a sufficiently large swale next to the road, at a depth sufficient to receive flow from a gully outlet pipe. Swales on embankment could be lined to avoid any negative effects during operation on the stability of the earthworks, however such


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construction would be deemed a maintenance issue, as during maintenance, the impermeable layer could be penetrated leaving a weepage path of the embankment, and therefore, the use of swales has been discounted.

Detention Basins

17.7.24. Detention basins are at-surface storage basins that provide attenuation of storm water runoff to aid in the control of surface water discharge. Detention basins allow for some water quality treatment, primarily settlement of solids, and with it heavy metals. These facilities are predominantly dry.

Attenuation Tanks

17.7.25. Underground tanks can take various forms but commonly these are geocellular modular systems or box culverts. geocellular modular systems provide high void capacities that allow for large storage volumes over a minimum footprint. The majority of systems provide no water quality treatment, furthermore the implementation of such systems are subject to site conditions and anticipated loadings. In order to limit maintenance requirements, a catchpit should be located upstream of the underground attenuation tank to remove sediment from the runoff before it is able to settle inside the tank itself (Halcrow, May 2013).

17.7.26. Catchpits have a grill that would prevent larger sediment from being washed into the drainage system. Piped drainage would provide an effective form of transferring road drainage without any risk of discharge to sensitive groundwater. Incorporating bypass interceptors into the treatment sequence would provide mitigation by allowing improved retention of hydrocarbons from daily run-off and from lay-bys, which are deemed to be high risk areas.

Tank Sewers

17.7.27. Online oversized pipes can be implemented as on-line or off-line storage. These systems allow for potentially high storage volumes dependent upon the restrictions of the site and restrictions in the hydraulic performance of the overall drainage network. These systems do not provide water quality treatment. In order to limit maintenance requirements, a catchpit should be located upstream of the tank sewer to remove sediment from the runoff before it is able to settle inside the tank sewer itself.

Preferred mitigation options for SBL

17.7.28. As discussed, water quality tests undertaken using HD45/09 show that without mitigation all individual discharges with the exception of catchment C would meet and pass the intermittent highway RSTs thresholds and would meet the EQS for both copper and zinc, as the dilution provided by the water body receptors (Ashton Brook, Longmoor Brook, New Colliters Brook, Colliters Brook, The Malago and Pigeon House Stream) is adequate.

17.7.29. However, the Environment Agency has requested the use of SUDS to reduce surface water runoff and aid pollution control to provide maximum benefit, even though the risks are low.

17.7.30. Limitations in available space adjacent to carriageways due to highway boundaries or surrounding development restrict the implementation of many SUDS features; particularly along the urban section of the scheme. Furthermore, the permeability of the ground is found to be negligible, and as such infiltration SUDS are deemed inappropriate (Halcrow, May 2013).

17.7.31. Although all of the catchments are proposed to discharge to a watercourse, there are two catchments (H & M) whose low points are reasonably distant to the nearest watercourse. Due to their urban locations, it would be impractical to connect directly to the watercourse from the proposed catchment. In these catchments, an existing connection to the watercourse has been found, namely existing Wessex Water surface water sewers. These are outlined in Table 17.15. The principle of discharge has been agreed, and the use of sewers to transfer flows to watercourses has been proposed (Halcrow, May 2013).

17.7.32. Following a high level review of the proposed scheme (by Halcrow), available space and the vertical alignment of the highway, it is determined that the applicable SUDS measures for the


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collection, conveyance and attenuation of anticipated surface water flows are detention basins and underground attenuation tanks (Halcrow, May 2013).

17.7.33. Additionally, the proposed cross-section pavement detail of the scheme is a kerbed road and thus a positive drainage network comprising gullies will be required to intercept road runoff. This entails that ‘over the edge’ drainage is not possible (Halcrow, May 2013). A summary of the attenuation facilities proposed is provided in Table 17.15.

Detention Basins

17.7.34. In order to improve the basin's water quality performance and reduce the long-term maintenance requirements, it is suggested to incorporate a sediment forebay at the upstream end of the basins. Erosion control should also be considered at the inlet to the basin (Halcrow, May 2013).

17.7.35. Detention basins with a flow control type structures are proposed for seven (A, C, D, E, F, G and M) out of the twelve catchments. For detention basins A-G a permanent area of ponded water shall be provided below the level of the outlet. This ponded area of water is intended to retain and treat the most polluted water from the rainfall runoff from all events (Halcrow, May 2013). For detention basin M, this will discharge to a lake, which forms part of Pigeon House Stream watercourse

17.7.36. Due to high ground water levels observed and detention basins (specifically for catchment E and F) will be located in areas of historical landfill, it is proposed to line the permanent ponds in all detention basins with an impermeable membrane such as Bentonite Geotextile or similar approved (Halcrow, May 2013).

17.7.37. The proposed system within the detention basin would therefore include three main stages that link in a treatment process: 1. Sediment retention; 2. Areas of ponded water; and a 3. Flow control structure.

Attenuation tanks

17.7.38. It is proposed to use attenuation tanks (H, K & L) with complex flow control device for three out of the twelve catchments. It should be noted that Bristol City Council and the Environment Agency agreed that oil separators would not be required due to maintenance requirements, and that other treatment means should be considered over separators (Halcrow, May 2013).

17.7.39. Serious accidental spillage could lead to a serious pollution incident in one of the local watercourses and contamination of the SUDs, affecting their operation. Particular high risk sites for spillage include roads with very high traffic flows, outfalls draining to sensitive watercourses, or roundabouts where the risk of accidents is higher.

17.7.40. The scheme has traffic flow of around 37,000 vehicles a day and a typical HGV percentage of <20%. The Environment Agency has agreed that anti-pollution penstocks may be deemed appropriate during the detailed design phase (pers comm. Halcrow, April 2013).

Tank Sewers

17.7.41. Two tank sewers are proposed. These are for catchments B and J for attenuation purposes, with vortex separators. Additional mitigation has been proposed for catchment B in the form of a pollution device similar to a downstream defender, to ensure some removal of some sediments and related metals.

Proposed discharge rates

17.7.42. The attenuation facilities promoted in the design for catchments A-G, and K and L, have been sized on the storage required to control 1 in 100 year site discharge rates to less than or equal to 1 in 100 year Greenfield peak rates, subject to a practicable minimum limit of 5 l/s. Catchments J and M have been sized to control post-development discharge to a 30% betterment over pre-


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development site discharge. Catchment H has been sized to control 1 in 100 year site discharge rates to no greater than 5 l/s, as requested by Wessex Water (Halcrow, May 2013).

17.7.43. A summary of the proposed discharge rates is provided in Table 17.15.


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Table 17.15: Drainage Philosophy Catchment Receiving

watercourse Existing runoff conditions

Proposed post-development runoff conditions

Proposed allowable discharge rate 1 in 100 year (l/s)

Attenuation device

Attenuation philosophy

A Ashton Brook

Greenfield Greenfield 28.24 Detention basin with complex vortex-type flow control device

The low point of the road is at chainage 295m. The detention basin has been located to the east of Ashton Brook, adjacent to the low point of the road. This will limit the depth of excavation for the pipework and basin. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, to Ashton Brook. The basin is located in Flood Zone 2. Due to the location of the catchment's low point, it would be impractical to locate the basin outside of this zone.

B New Colliters Brook

Greenfield Greenfield 2.81 Tank Sewer + complex vortex-type separator device (similar to a downstream defender)

The low point of the road is at chainage 81m of the bus link road. The basin is located in Flood Zone 3. Due to the location of the catchment's low point, it would be impractical to locate the basin outside of this zone. The most appropriate attenuation device is a tank sewer as it can be installed beneath the highway if necessary and its footprint lends itself to this situation. The tank sewer and vortex separator will be located within the highway embankment. Under design conditions the tank sewer will discharge through a complex vortex-type flow control device, to mimic Greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, directly into New Colliters Brook. In order to provide some treatment to the runoff it is proposed to install a vortex separator downstream of the tank sewer. The separator would then need to treat a maximum of 5 l/s.


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C New Colliters Brook


The low point of the road is at chainage 250m of the bus link road, at the most northerly point of this catchment. It is necessary to split the bus link road into two catchments in order to limit the size of the tank sewer (in catchment B) required at the bus link road's low point. There is plenty of space available for a detention basin in this area. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic Greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, directly into New Colliters Brook. The basin is located in Flood Zone 2. Due to the location of the catchment's low point, it would be impractical to locate the basin outside of this zone.

D New Colliters Brook


The low point of the road is under the rail crossing structure. The detention basin has been located to the north of this location as the low point occurs as the road passes under the proposed railway structure, where there is no space for an attenuation device. The location selected is the nearest available appropriate location for a detention basin. It should be noted that there are several buried services in this area; however this would need to be confirmed during detailed design. The ponds are designed so that the final geometry can be determined when the location of services is known. It is proposed to use a linear drainage system, such as combined kerb drainage, where the road passes beneath the railway, due to the longitudinal grade and limited space in the underbridge for a piped system. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, to a cut-off ditch which feeds a new pipe underneath the Brook Gate connection, to the cut-off ditch to the west of the


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bus link road, which in turn discharges to New Colliters Brook. The basin is located in Flood Zone 1.

E Colliters Brook

Greenfield Greenfield 34.35 Detention basin with flow control device

The basin has been located adjacent to the low point of the catchment as this is the most practical available location. The basin cannot be located to the east of the road as there is insufficient space. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, directly into Colliters Brook via a new culvert under the SBLR. The basin cannot discharge to the nearby cut-off ditches as they will be at a higher level that the outlet. The basin is located in an area of historical landfill and as such will need to be completely lined with an appropriate impermeable membrane, along with the cut-off ditches within the extents of this historical landfill area. The basin is located in Flood Zone 1.

F Colliters Brook


The low point of the catchment occurs just northwest of the A38 roundabout. The basin has been located downstream of the low point of the catchment as this is the most practical available location. The basin is located in an area of historical landfill and as such will need to be completely lined with an appropriate impermeable membrane, along with the cut-off ditches within the extents of this historical landfill area. It is not practical to locate the basin outside of the landfill area due to limited space, the location of Hanging Hill Wood and extremely steep slopes. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, into Colliters Brook via the deculverted section of the Colliters Brook tributary.


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G Tributary of Colliters Brook


The low point of the catchment occurs just southeast of the proposed A38 roundabout. The road continues to fall north of this point towards catchment F. The basin has been located adjacent to the low point of the catchment as this is the most practical available location. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, directly into the Colliters Brook tributary.


Greenfield Greenfield 5.0 Attenuation tank with flow control device

This catchment is located within a county designated wildlife site and within an area of common land (Highridge Common SNCI) and therefore a pond is considered an inappropriate feature to locate on the common. It is thus proposed to construct an online attenuation tank. The depth of the attention tank precludes the option to discharge to the watercourse in the local vicinity and hence discussions with Wessex Water have been undertaken to obtain their consent to discharge into their surface water system in Sandburrows Road to aid in the conveyance of surface water to a watercourse. The permitted rate of discharge to Wessex Water's sewerage network should be limited to 5 l/s to ensure the network is not overwhelmed.

J The Malago Brownfield 30% betterment on existing runoff

118.22 Tank Sewer + complex vortex-type

The proposal will involve replacing the existing drainage system on King George's Road. The low point of the road in the catchment is a sag in the highway's vertical alignment, where the proposed route crosses Queen's Road. It is proposed to locate an attenuation tank in the playing fields of St. Pius Xth Roman Catholic VA Primary School. There are some initial health and safety concerns relating to construction and maintenance access. It may be possible to site associated manholes and catchpits


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outside of the school grounds, meaning most maintenance access would not need to be through the school. Under design conditions the tank sewer will discharge through a complex vortex- type flow control device, directly into a manhole on the culverted watercourse The Malago.

K The Malago Brownfield Greenfield runoff 22.38 Underground attenuation tank with flow control device

Since there is limited space in this corridor for attenuation devices, the catchment has been split into two sections, K and L so that two underground attenuation tanks can be used. It should be noted that there is insufficient space for a detention basin. There are two low points of the road in the catchment and the attenuation tanks have been located as close as possible to these. Under design conditions the basin will discharge through a complex vortex-type flow control device, to mimic greenfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events, into the culverted Malago watercourse via a new section of pipework.

L The Malago Brownfield Greenfield runoff 11.07 Underground attenuation tank with flow control device

As catchment K – see row above

M Pigeon House Stream

Brownfield 30% betterment on existing runoff

32.38 Detention basin with flow control device

This catchment takes runoff predominately from the modified Whitchurch Road. The low point of the road in the catchment is at the Cater Road Roundabout end of the catchment at chainage 4448m. The location selected is near this low point and is the most appropriate location for a detention basin. Two detention basins have been selected in order to avoid existing services crossing the area and to avoid unnecessarily deep excavations. It is proposed to have a section of the catchment draining to the western basin and the remainder to the eastern basin. The eastern basin's discharge would be controlled to a certain rate before


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discharging to the eastern basin. Under design conditions the basin will discharge through a complex vortex-type flow control device, to achieve a 30% betterment on existing brownfield runoff during 1 in 1, 1 in 30 and 1 in 100 year design storm events (with the 1 in 100 year storms restricted to 1 in 30 year discharge rates), to approximately 330m of existing Wessex Water sewer, eventually discharging to a lake on the 'Pigeonhouse Stream' watercourse. It is proposed to connect to an existing Wessex Water manhole at chainage 4490m; however the invert levels of this manhole will need to be determined. The proposal has been discussed with Wessex Water, who has agreed to a betterment over the existing flows to this manhole. From record drawings it would appear that the existing arrangement of Whitchurch Lane drains to the lake on the Pigeon House Stream via the same existing Wessex Water infrastructure. It is unclear if there are any existing attenuation devices for this system, but it is considered unlikely, meaning the proposal would be likely to represent an improvement on the existing drainage arrangement.

Source: Halcrow. May 2013 Key: SNCI = Site of Nature Conservation Importance .


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No further mitigation is considered for Lakes and other water bodies, groundwater, abstraction, discharge and ecology as these have been scoped out of this assessment.

Water Framework Directive compliance mitigation measures

17.7.44. Compliance against the WFD has been considered within this EIA. In order to be compliant a number of mitigation measures have been recommended. For ease of WFD assessment, these have been based on scheme elements outlined in Table 17.16. Specific mitigation measures for each element are outlined in Table 17.17. This table also contains some of the enhancement options recommended within the WFD compliance assessment. These will be considered at the detailed design phase and where possible they will be incorporated into the design.

Table 17.16: WFD Scheme elements considered in WFD compliance

Scheme element

Description

1 Drainage solutions for the scheme (The Malago)

2 Drainage solutions for the scheme (Colliters)

3 Colliters Brook upstream tributary field drain realignment, culverting and infilling

4 Colliters Brook upstream tributary field drain culvert extension/de-culverting of reach

5 Colliters Brook wide span bridge crossing (with excavation works on bank top) and channel realignment at adjacent locations

6 New Colliters Brook tributary to be culverted under the road

7 Colliters Brook flood compensation storage areas

8 Ashton Brook tributary diversion


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Table 17.17: WFD Mitigation Measures Scheme Element

Mitigation

Detailed Design Construction Operational

1 Mitigation measures relevant here are listed within section 1.7.2.3 – which are the recommended mitigation measures outlined in the Surface Water Drainage Strategy undertaken by Halcrow (Halcrow, April 2013).

2 Mitigation measures relevant here are listed within section 1.7.2.3 – which are the recommended mitigation measures outlined in the Surface Water Drainage Strategy undertaken by Halcrow (Halcrow, April 2013).

3 Culverts within this element include a depressed invert to allow fish passage within existing designs.

None identified. None identified.

4 Morphological improvements to enhance the reach such as increasing the sinuosity and installing fencing to help vegetation establish. (This enhancement is required to mitigate for impacts to scheme element 3). Hydromorphological advice required to assist with the alignment of the de-culverted channel. A field access culvert to be inserted along the de-culverted channel length and a depressed invert required to mitigate impacts to fish.


5 A hydromorphological study of Colliters Brook is required to assess the active nature of the channel at the bridge location and up and downstream of this point. The study should determine the risk of channel erosion and potential future channel morphology. For example erosion could lead to the channel becoming constrained against structures. The study is necessary to inform detailed design and should seek to prevent the requirement of retrospective bank protection at a later date. Bank excavation needs to allow a gradual slope from the abutment to the river channel to allow drainage of surface runoff into the channel. The public right of way to be positioned as close to the abutment as possible to mitigate

Care should be taken not to interfere with the channel or bank top during construction of the fluvial flood mitigation retaining wall and the maintenance roads upstream and downstream of Colliters Brook bridge. Wetland creation enhancement works within existing meander bends to provide additional aquatic habitat to be incorporated within Colliters Brook realignment works. Stabilise exposed surfaces following channel realignments the banks.

None identified.


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impacts to the channel banks. Other mitigation measures include planting of the exposed soil surface to stabilise the bank and inhibit erosion. Obtain topographic survey data to correct channel planform in order to accurately consider locations of structures close to watercourses. Hydromorphological advice required when considering realignment options and hydromorphological risks. Realignment options need to allow for no loss of channel length along the impacted reach (es). If this is not possible additional mitigation works should be agreed with the Environment Agency.

6 None identified at the outline design stage. None identified. None identified. 7 A hydromorphological study to be undertaken to consider the

risk of small scale erosion of the New Colliters Brook banks (from the change in hydrology during the operation of the flood culverts). If the risk is deemed significant it would be recommended that the banks of New Colliters Brook be stabilised with soft engineering techniques


8 Culvert extension should allow continuity of flow between it and the existing culvert. A depressed invert to allow fish passage should be included in the design (assuming the existing culvert has this functionality).


Other All bridges proposed as part of the scheme to have otter ledges/separate dry otter culverts/mammal culverts suitable for aquatic species.



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17.8. Residual Effects 17.8.1. Due to the low risks from discharges and the additional mitigation proposed there would be no

residual impacts from the scheme. It is envisaged the current discharge arrangements for the two surface water balancing ponds operated by Viridor Ltd will be retained. Ponds are designed with sufficient flexibility to allow the concise extent to be determined at the detailed design stage

17.8.2. Water quality tests have been undertaken in accordance with the Methods in the DMRB HD45/09 and show that, with mitigation there is a reduction in concentrations for both copper and zinc for seven out of the 12 catchments (A,C,D,E,F,G and M) – where detention basins are proposed. Attenuation tanks and tank sewers provide no treatment for soluble pollutants such as copper and zinc and therefore, tests show no change in the concentrations for both copper and zinc. Table 17.18 provides a summary of the pre and post mitigation concentrations for both copper and zinc.

17.8.3. Baseline investigations of the ground conditions indicate there will be no risk to groundwater quality from the proposed discharges and mitigation features.

17.8.4. Method D spillage risk tests undertaken with mitigation also show there would be a neutral significance of effects in the pollution risk from accidental spillages relative to the existing conditions. Table 17.18 provides a summary of the pre and post mitigation spillage risk return periods.

17.8.5. The majority of WFD mitigation measures relate to the detailed design stage and are necessary to mitigate local impacts so there are no residual impacts that will cause deterioration of the water environment at the overall water body scale.


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Table 17.18: Summary of residual effects of operation on watercourses (water quality) with mitigation Catchment reference

Receiving Watercourse HAWRAT copper concentration (EQS limit mg/l)

HAWRAT Zinc Concentration (EQS limit mg/l)

Method D Serious Spillage Risk – return period in years (threshold <100yrs)

Importance Magnitude of Impact

Significance of Effect

No Mitigation

With Mitigation

No Mitigation

With Mitigation

No Mitigation

With Mitigation

A Ashton Brook 0.07 0.04 0.26 0.16 6237 11878 High Negligible Neutral B New Colliters Brook 0.16* 0.16* 0.6* 0.6* 525995 1051991 High Negligible Neutral C New Colliters Brook 0.39 0.24 1.42 0.87 343040 61531 High Negligible Neutral D New Colliters Brook 0.36 0.22 1.31 0.8 18331 30551 High Negligible Neutral E Colliters Brook 0.14 0.09 0.53 0.32 13144 11216 High Negligible Neutral F Colliters Brook 0.07 0.04 0.26 0.16 9128 15213 High Negligible Neutral G Tributary of Colliters Brook 0.05 0.03 0.19 0.12 7435 12392 High Negligible Neutral H Drainage ditch through a SNCI 0.04 0.03 0.13 0.13 69475 69475 High Negligible Neutral J The Malago 0.12* 0.12* 0.45* 0.45* 18892 18892 High Negligible Neutral K The Malago 0.04* 0.04* 0.14* 0.14* 206256 206256 High Negligible Neutral L The Malago 0.03* 0.03* 0.12* 0.12* 61081 61081 High Negligible Neutral M Pigeon House Stream 0.05 0.03 0.19 0.12 28696 47827 High Negligible Neutral Key: * No change in concentration as attenuation and sewer tanks provide no water quality treatment; SNCI = Site of Nature Conservation Importance


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17.9. Consideration of Likely Combined Effects 17.9.1. For assessment of impacts associated with soluble pollutants (specifically zinc and copper),

where outfalls for catchments are within 1km of each other and discharge to the same water body receptor these were aggregated for assessment following guidance for the use of HAWRAT in HD45/09. Table 17.19 below details those outfalls which were aggregated for assessment.

Combined effects on Surface Water Quality from aggregated outfalls

17.9.2. Water quality tests have been undertaken in accordance with Method A in HD45/09. A summary of the results is shown in Table 17.20. Initially, tests were undertaken individually for each catchment (A – M inclusive) and then aggregated for assessment.

17.9.3. Tests undertaken using Method A show that, without mitigation all aggregated discharges would meet EQS concentrations for both copper and zinc, as the dilution provided by the water bodies (New Colliters Brook, Colliters Brook, and The Malago) is adequate.

17.9.4. It should be noted however, that without mitigation the combined runoff discharging to New Colliters Brook from catchments B+C+D fails the intermittent highway run-off 6 hour and 24 hour ecologically-derived RSTs.

17.9.5. In the absence of water quality data for New Colliters Brook, water quality data for watercourses within the Avon catchment was sourced from the Environment Agency and water quality tests for the aggregated catchments B+C+D were refined. Specifically the hardness band was set to the medium value in HAWRAT (50 – 200 mg CaCO3/l). Tests show that with this parameter change and with mitigation, aggregated catchments B+C+D would pass the RSTs.

17.9.6. Water quality tests have also been undertaken for the aggregated outfalls/catchments using the mitigation proposed.

17.9.7. Tests show that with mitigation all aggregated catchments would pass the EQS for both copper and zinc.

17.9.8. A summary of the results is with mitigation is shown in Table 17.20.

Combined effects on Surface Water Quality from future schemes

17.9.9. The Ashton Vale to Temple Meads (AVTM) scheme, which is part of the wider West of England transport package will be a guided bus-way linked to the Long Ashton Park and Ride site directly to Temple Meads. This scheme will connect just east of the Long Ashton Park and Ride site.

17.9.10. It is assumed there will be no cumulative impacts assuming the AVTM scheme is designed to best practise as required by DMRB. Should the AVTM scheme be commissioned after this scheme, detailed water quality tests should be undertaken as part of its planning application in accordance with Methods A and D in HD45/09 to determine if there is a cumulative risk assessment.


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Table 17.19: Summary of outfalls aggregated for assessment

Catchment reference

Water Feature Individual assessment

Combined assessment

Comment

A Ashton Brook x Individual assessment only. No other outfalls/catchments contribute to this water body.

B New Colliters Brook Individual assessment plus combined assessment with C+D. Outfalls into the same water body and within 1km.

C New Colliters Brook Individual assessment plus combined assessment with B+D. Outfalls into the same water body and within 1km.

D New Colliters Brook Individual assessment plus combined assessment with B+C. Outfalls into the same water body and within 1km.

E Colliters Brook Individual assessment plus combined assessment with F. Outfalls into the same water body and within 1km.

F Colliters Brook Individual assessment plus combined assessment with F. Outfalls into the same water body and within 1km.

G Tributary of Colliters Brook x Individual assessment only. No other outfalls/catchments contribute to this water body.


x Individual assessment only. No other outfalls/catchments contribute to this water body.

J The Malago Individual assessment plus combined assessment with K+L. Outfalls into the same water body and within 1km.

K The Malago Individual assessment plus combined assessment with J+L. Outfalls into the same water body and within 1km.

L The Malago x Individual assessment plus combined assessment with J+K. Outfalls into the same water body and within 1km.

M Pigeon House Stream x Individual assessment only. No other outfalls/catchments contribute to this water body. Key: SNCI = Site of Nature Conservation Importance; = Applicable; x = Non Applicable


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Table 17.20: Summary of effects of operation on watercourses (water quality) for aggregated outfalls

Catchment Receiving Watercourse

No Mitigation

With Mitigation No Mitigation

With Mitigation Importance Magnitude of Impact

Significance of Effect

HAWRAT Copper (EQS limit mg/l)

HAWRAT Zinc (EQS limit mg/l)

B+C+D New Colliters Brook

0.73 0.45 2.62 1.6 High Negligible Neutral

E+F Colliters Brook 0.2 0.12 0.74 0.45 High Negligible Neutral

J+K+L The Malago 0.17 0.17 0.65 0.65 High Negligible Neutral


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17.10. Summary and Conclusions 17.10.1. The existing baseline information demonstrates High importance for water quality, groundwater

quality and groundwater attributes and other water bodies within the water environment.

17.10.2. DMRB tests have shown that EQS for copper and zinc would be met for both individually and aggregated outfalls. With mitigation there would a negligible effect on the RSTs for both individually and aggregated outfalls. There would also be an unquantifiable improvement in copper and zinc concentrations for catchment M with the proposed mitigation compared with the existing concentrations.

17.10.3. Further, there would also be a slight beneficial improvements in the spillage risk for catchment M as Method D tests (spillage risk) shows a reduction of >50% compared to the existing pollution risk

17.10.4. Groundwater quality would be protected by ensuring that no discharge would be allowed to groundwater and by specifying piped drainage from the road, so avoiding or reducing diffuse or point source pollution pathways.

17.10.5. The proposed works are WFD compliant (assuming that a number of mitigation measures form part of the proposal).

17.10.6. The significance of effect on each attribute of the water environment, based on environmental importance and magnitude of impact both during construction and operation is neutral.


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17.11. References and Sources 17.11.1. Atkins. March 2012. South Bristol Link. Environmental Scoping Report

17.11.2. Bristol City Council. June 2011. Core Strategy. Accessed and downloaded at: http://www.bristol.gov.uk/sites/default/files/documents/planning_and_building_regulations/planning_policy/local_development_framework/Bristol%20Development%20Framework%20Core%20Strategy%20June%202011.pdf

17.11.3. Environment Agency website. Accessed January – March 2013. http://www.environment-agency.gov.uk/

17.11.4. Environment Agency. September 2012. CSC/6153 – South Bristol Link Scheme Data Request

17.11.5. Environment Agency. December 2009. River Basin Management Plan. Severn River Basin District Annex B: Water body status objectives

17.11.6. Government’s Multi-Agency Geographic Information for the Countryside (MAGIC) website. Accessed January – March 2013. http://magic.defra.gov.uk/

17.11.7. Halcrow. May 2013. Drainage Strategy. Document: CTRAEB 730 DRA DOC 020 Version: 2. South Bristol Link Road.

17.11.8. Halcrow. April 2013. Pers comm. 01/05/2013 – Email correspondence from Halcrow Urban Water. SBL Existing Mitigation

17.11.9. HA 103/06. May 2006 Vegetative Treatment Systems for Highway Runoff. Volume 2. Section 2, Part 1

17.11.10. HD 45/09. November 2009. Design Manual for Roads and Bridges, Road Drainage and the Water Environment. Volume 11. Section 3, Part 10

17.11.11. Johnson I and Crabtree, R.W. (2007). Effects of Soluble Pollutants on the Ecology of Receiving Waters, WRc Plc, Report No.: UC 7486/1, UK Highways Agency. Cited in HD 45/09 (2009)

17.11.12. Metoffice.gov. February 2013. South West England: climate. http://www.metoffice.gov.uk/climate/uk/regional-climates/sw

17.11.13. Natural England’s website. Accessed January – March 2013. http://www.naturalengland.org.uk/

17.11.14. North Somerset Council. April 2012. Core Strategy. Accessed and downloaded at: http://www.n-somerset.gov.uk/NR/rdonlyres/71796935-6330-4D00-9232-8ACD7E776D0A/0/CoreStrategyFinal2012.pdf

17.11.15. Structural Soils Ltd. December 2011. As cited in Halcrow, May 2013. Drainage Strategy. Document: CTRAEB 730 DRA DOC 020 Version:1. South Bristol Link Road

17.11.16. Reed, D. et al.1999. Flood Estimation Handbook, Institute of Hydrology, Wallingford UK.

http://www.environment-agency.gov.uk/

http://www.environment-agency.gov.uk/

http://www.naturalengland.org.uk/


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chapter 2.17. water environment - wordpress.com · 2/1/2014 · south bristol link: environmental...

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