Project: Laches Wood Outdoor Education Centre Job No: CS/65629

Subject: Foul and Surface Water Drainage Strategy Date: 5th July

Doc No. CS/65629-001

Castleway House, 17 Preston New Road, Blackburn BB2 1AU

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The purpose of this document is to describe and detail the design parameters and drainage

strategy used in the development of the proposed surface water and foul water drainage networks

that will serve the Laches Wood Outdoor Education Centre in Staffordshire.

The scheme is to develop the existing buildings and infrastructure with the addition of a new

dormitory to the main building, provision of the existing access is to be maintained off the

surrounding highway network and sports facilities within the existing site boundary.

In constructing the new dormitory and landscape features, a minor reallocation of the existing

drainage network, catchments and discharge is proposed.

This document describes the method and parameters to be used in the development of the new

drainage networks and utilisation of the existing infrastructure.

1 Existing Drainage Summary

1.1 Foul and surface water drainage Laches Wood has a network of privately owned and maintained foul and surface water sewers on

site. The private sewers outfall into the existing evaporation trenches, balancing and infiltration

lagoons/ponds network adjacent to the site. A number of existing foul and surface drainage pipes

can be found within the site which following assessment by means of drainage survey are

considered suitable to utilise with minor like for like replacement due to condition.

The current foul network utilises an existing Klargester BIODISC Sewerage Treatment plant, to

treat and improving the water quality prior to outfall.

The existing surface water drainage infrastructure can be split into three types; rainwater,

impermeable hardstanding and permeable landscaping.

1.2 Foul Drainage

Currently, foul water from site is drained through a private sewer system. From current survey and

historical record information it is considered that the system discharges in to the lagoon/pond

network north of the main buildings via the existing Klargester BIODISC Sewerage Treatment plant

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(Noted as Outfall 3 in sketch LW-CSS-SK007). Intermediate pumping is currently used to allow

flow from the south western extent of the site to tie in with the treatment station.

As the proposed development is small in comparison to the existing site infrastructure it is not

anticipated that foul flows will increase significantly overall as a result. It is proposed that the

existing and additional foul discharge is collected and transported via existing infrastructure to

discharge via a new replacement Klargester Envirosafe 55HEQ Sewerage Treatment plant.

Foul water drains are to be sized using the discharge unit method contained within BS EN 12056

Part 2 and the current building regulation requirements. The system will be designed to flow not

more than three-quarters full and will be laid at gradients that allow self-cleansing velocities to be

achieved. Maximum velocity within the system will be 3m/s.

1.3 Surface Water Drainage PPS25 and Part H of the Building Regulations state a hierarchy of where surface water should

discharge to.

This hierarchy should be followed were practicable, and is listed below:

(i) Infiltration (ii) Watercourses (iii) Public Sewers

As part of the site drainage strategy the above parameters have been considered in order to

possibly discharge surface water firstly via existing evaporation trenches (Noted as Outfall 1 in

sketch LW-CSS-SK007) with remaining the flows via existing infiltration ponds/lagoons in

accordance with the existing SuDs infrastructure.

It is not proposed at this stage to differ from the existing SuDs principles which are used on the site and that the discharge will be limited to match existing by provision of a small volume of additional attenuation storage (approx 32m3 of shallow crated modular system) to ensure the existing ecology and habitats are subject to no detrimental effect. An assessment of the drainage discharge has been completed by Ecological specialists to ensure the existing conditions are not negatively impacted ( refer to Laches Wood Great Crested Newt Survey Report July 2013).

2 Design Parameters and Methodology

2.2 Contributing Areas The development of the site has a minor impact on the proportion of impermeable and permeable

areas.

The foul drainage network discharge is estimated to have a small increase but similar flow rates

taking into account existing development sizes. The existing foul drainage connections are to be

maintained where possible.

2.4 Microdrainage Simulation Software Variables In accordance with guidance from the Highways and Environment Agency (EA) for future design

scenarios, no increase to the overall discharge rate for the 30 year return is proposed. The surface

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water network model will be modelled for a 1 in 30 year return period in accordance with Planning

Policy Statement 25, NPPF Guidance and 1:100 period taking into account Building Regulation

Part H requirements.

According to PPS25 (Table 2), rainfall intensity is estimated to increase by 20% until 2085 to allow

for climate change. Beyond this it is expected there will be up to 30% increase in rainfall intensity.

In accordance with EA policy and the expected lifetime of the proposed development, the

developments drainage design network will be designed to account for 20% increase in rainfall

intensity, over its lifetime for the change in runoff and flow.

2.5 Foul Drainage

An assessment of likely foul discharge from the proposed development anticipated foul discharge

has been completed based on the additional development indicating a small increase in additional

flow in accordance with British Water Code of Practice; Flows and Loads -2; 2005.

3 Network Modelling

3.1 Network Model Criteria The proposed surface water network will be developed using WinDES software with the following parameters considered:-

• Return Periods up to 1in 100 year (including 1in1, 1in2, 1in5, 1in 30 and 1in 50)

• M5-60 = 20

• Ratio – r = 0.4,

• Global Time of Entry = 4min,

• Max Rainfall of 50mm/hr and

• Volumetric Run Off Co-efficient = 0.75 summer & 0.84 winter. 3.2 Discharge and Outfalls The existing outfalls (as shown in sketch LW-CSS-SK007) are to be utilised by both the surface water and foul systems. Where feasible the rate and flow of all discharge will be matched by the proposals by means of introducing attenuation and storage within the proposed drainage system. In accordance with CIRIA guidance and PPG25, the current SuDs systems will be utilised and an assessment of the network used to establish the current parameters which will be met. Additional measures employed within the SuDs system will combine the existing treatment and infiltration system with new potential filter trench and cellular storage options locally to the new development. Further detailed review is proposed to assess the opportunity for localised soakaways for the surface water and

3.3 Network Analysis

Using the inputs and design parameters detailed two network scenarios are developed for surface water:

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(i) Proposed Network with climate change allowance for a 1 in 30 year storm period;

(ii) Proposed Network with climate change allowance for a 1 in 100 year storm period

Climate change allowance of 20% will be factored into the proposed network model and storm analysis.

Proposals

4.1 Foul Drainage The proposed foul network for the proposed dormitory building is developed with the building

extent accommodated by means of discharge to proposed foul network.

The foul network runs by means of carriers with the head being at the northern extent of the site in

to the proposed replacement Klargester Envirosafe 55HEQ Sewerage Treatment plant. The

proposed Klargester has been sized to internally attenuate and store the additional effluent

flows/volumes discharging at rates and volumes matching the outflow from the existing Treatment

plant (Max Daily Flow of 30m3/day and Peak Flow Rate of 3.7m3/hr) which will be abandoned and

maintain the with the existing Environment Agency consent to discharge limit of 5.7m3/day.

The proposed below ground foul water drainage system proposals will comply with: - BS EN 752 (2008) - BS EN 12056 Part 2 (2000) - Local Authority Requirements - Building Regulations (2000) Part H

Minimum pipe sizing will be taken as 100mm with minimum longitudinal falls of 1:40 as necessary.

All connections from internal pipes are to be via individual inspections chambers (minimum 450mm

square or diameter) to allow for ease of rodding.

It is considered that the proposed replacement Klargester Envirosafe 55HEQ Sewerage Treatment

plant is more efficient and effective than the existing treatment infrastructure and would therefore

be expected to improve the water quality, therefore further supporting the proposal to have no

detrimental impact on the current ecological conditions at the outfall. This also supports the

existing and proposed SuDs infrastructure strategy.

4.2 Surface Water Drainage

In order to achieve the existing outfall levels and flows/volumes a shallow modular attenuation

structure of 32m3 is proposed, this will discharge to the existing storm water drain that ultimately

discharges to the existing evaporation trenches.

In order to restrict to outflow in to and out of the attenuation features (tank modular/ stone trench network) in accordance with the existing combined development discharge rate calculated (11.9l/s including climate change allowance of 20%) and a flow control device within the network modelling. Minimum pipe sizing of 150mm will be implemented for all connections and carrier drains with gradients in accordance with achieving self cleansing.