1. introduction · report (reference: imse100328-vba-xx-re-rp-hy-2003-s8-p1-l0600-ea3-lod3-redhill...

RBBC proposed development locations rev1.docx of 11

Project: KSL 105 Appraisal Package: Redhill

Subject: Redhill proposed development locations Consultant: VBA

Date: September 2018 Version: 1

QA

Version Originated Checked Reviewed Authorised Date

1 BW WR RM MV 28/09/2018

1. Introduction

As part of the Redhill Flood Alleviation Study (FAS), an integrated catchment model (ICM) was built by VolkerStevin, Boskalis Westminster and Atkins (VBA). The output of this modelling shows the areas at risk from flooding in Redhill. A range of annual probability storm events have been modelled.

An objective of the Redhill FAS was to develop a long list of options to alleviate flood risk in Redhill. This long list of options was refined to a short-list and five of these options, alongside the Do Nothing and Maintain scenarios have been modelled using the Redhill ICM.

The area under investigation in the Redhill FAS is also subject to Reigate and Banstead Borough Council (RBBC)’s Local Plan, that highlights the future development planned in the borough. This document both highlights the current flood risk to these development sites and identifies flood alleviation efficiencies that may be gained through incorporating drainage improvements in the areas of designated development.

VBA have compared the modelled flood risk in Redhill to the location of proposed development in Redhill. RBBC provided three datasets representing areas of future development within their Local Plan: potential town centre development sites; potential urban development sites; and sustainable urban extension.

2. Modelled options

The modelled options are described below in Table 1. Figure 1 shows the modelled options’ locations in relation to the proposed development sites in Redhill.

Table 1 Modelled options.

Option Description

Do Nothing • Economic baseline.

Maintain • Existing situation.

BB2 and BB3 • Flood storage on the Battlebridge Brook upstream of the A23.

BB7 • Conveyance improvements on the Battlebridge Brook (Subrosa Park).

LD1 • Flood storage on the Lynwood Ditch at St. Bede’s School playing

fields.

LD3 • Underground flood storage on the Lynwood Ditch (Memorial Park).

R6 • Conveyance improvements on the Redhill Brook.


Figure 1 - The proposed development areas and the Redhill FAS modelled option locations.


3. Proposed development sites allocated by RBBC

The proposed development sites that have been allocated by RBBC, shown in Figure 1, have been compared to the modelling outputs from the Redhill ICM.

Table 2 List of all the proposed development sites in Redhill and their flood risk and

proximity the Redhill FAS proposed options.

Development type

Development site

Development reference

Existing flood risk to proposed site in the existing situation (maintain scenario).

Proximity to proposed option and resulting benefit.

Potential town centre development site

Marketfield Way/High Street, Redhill

RTC1 Modelled to be at risk of flooding from a 1.33% (1 in 75) AP flood event.

The modelling indicates that the development site may benefit from the following options:

• LD1 – Coles Mead storage

• LD3 – Memorial Park

• R6 – Culvert upsizing

Royal Mail Sorting Office, Redstone Hill, Redhill

RTC3 Modelled flood risk of 5% (1 in 20) AP flood event.

The modelling indicates that it is unlikely that the development site would benefit from any of the modelled options.

Potential urban development sites

Church of the Epiphany, Mansfield Drive, Merstham

RED4 Modelled flood risk of 5% (1 in 20) AP flood event.


Colebrook, Noke Drive, Redhill

RTC4 Modelled flood risk of 5% (1 in 20) AP flood event.


Cromwell Road, Redhill

RTC2 Not modelled to be at risk of flooding.

Not applicable.

Depot & Bellway House, Station Road, Merstham and adjacent land

RED2 The Depot & Bellway House, Station Road, Merstham is not modelled to be at risk of flooding. However, the adjacent land is modelled to be at risk of flooding from 2% (1 in 50) AP flood event.


Former Longmead Centre,

RTC5 Modelled to be at risk of flooding from a 0.2% (1 in 200) AP flood event.

The modelling indicates that it is unlikely that the development site would


Development type

Development site




Holland Close, Redhill

benefit from any of the modelled options.

Former Oakley Centre, Radstock Way, Merstham

RED6 RED2 Not modelled to be at risk of flooding.

Not applicable.

Gloucester Road Car Park, Redhill

RTC6 Modelled flood risk of 5% (1 in 20) AP flood event in the east of the site.


• BB7 – Conveyance improvements on the Battlebridge Brook

Hockley Business Centre, Hooley Lane, Redhill

RED3 Not modelled to be at risk of flooding.

Not applicable.

Land at Reading Arch Road/Brighton Road, Redhill

RED8 Modelled to be at risk of flooding from a 5% (1 in 20) AP flood event. The Redhill Brook flows in open channel and culverted through the site.

Option R6 – Culvert upsizing is within this development site.


Merstham Library, Weldon Way, Merstham

RED5 Not modelled to be at risk of flooding.

Not applicable.

Quarryside Business Park, Thornton Side, Redhill

RED1 Modelled flood risk of 5% (1 in 20) AP flood event.


• BB7 – Conveyance improvements on the Battlebridge Brook

Redhill Law Courts,

RED7 Modelled to be at risk of flooding from a 5% (1 in 20) AP flood event.

The modelling indicates that it is unlikely that the development site would


Development type

Development site




Hatchlands Road, Redhill

benefit from any of the modelled options.

Sustainable urban extension

Former Copyhold Works, Redhill

ERM3 Modelled to be at risk of flooding from a 5% (1 in 20) AP flood event.


Land at Hillsbrow, Redhill

ERM1 Not modelled to be at risk of flooding.

Not applicable.

Land south of Bletchingley Road

ERM4a Not modelled to be at risk of flooding.

Not applicable.

Land South of Bletchingley Road 2

ERM4b Not modelled to be at risk of flooding.

Not applicable.

Oakley Farm, off Bletchingley Road, Merstham

ERM5 Not modelled to be at risk of flooding.

Not applicable.


4. Modelled improvements to select development sites.

The Redhill ICM has been used to assess the potential benefit to flood risk resulting from local interventions (e.g. improving the local drainage by pipe upsizing or relocating) near the proposed development areas. This has been tested using the Redhill ICM by assessing the impact of 5 high-level conceptual options against a 5% (1 in 20) AP rainfall event. These high-level conceptual options consist of the following:

• Drainage improvement: whereby improvement has been modelled by allowing water within or passing a development site is drained into the local piped network. Note: headlosses for gulley arrangements have not been considered due to the high-level approach required.

• Storage: whereby water from existing surface water networks can drain into a nominal storage introduced to the model.

It is important to note that both modelled conceptual arrangements provide a conservative maximum benefit that could be gained from interventions at each site. As such, these should be investigated in more detail before any schemes are taken forward. The location of these five interventions were selected from the list of development sites where there was minimal downstream surcharging within the drainage network, as such surcharge would impair any improvement implemented. No small interventions were tested where the development site was not at risk of flooding. Similarly, no small interventions were tested where there was a lack of drainage information near the proposed development site or where small interventions were deemed not viable. A depth difference map has been produced for each identified development site to demonstrate the difference in flood depth to the existing situation (Maintain Scenario). Please refer to the model build report (reference: IMSE100328-VBA-XX-RE-RP-HY-2003-S8-P1-L0600-EA3-LOD3-Redhill model build report) for any information regarding the model build or the Maintain scenario. These drainage interventions have been tested at a high-level, as such, any conclusion should be investigated in more detail. Conceptual interventions were modelled at the following five proposed development sites:

• Marketfield Way/High Street (RTC1);

• Church of the Epiphany, Mansfield Drive Merstham (RED4);

• Gloucester Road car park (RTC6);

• Land at Reading Arch Road/Brighton Road (RED8); and

• Redhill Law Courts (RED7). The Marketfield Way/High Street intervention was modelled by applying a storage area, connecting into the existing draining network, to store surface water runoff from the site. The remaining four interventions were modelled by applying drainage improvements within the site, to convey more surface water into the local surface water network. The modelling demonstrated that local interventions at these development site may improve flood risk. Except for Gloucester Road car park and Land at Reading Arch Road/Brighton Road, where a small increase in flood risk to the surrounding area was shown. The benefits of the modelled high-level conceptual options are shown below in Figures 2 to 6.


Figure 2 - Small intervention modelling at the Marketfield Way/High Street (RTC1) proposed development site.

Marketfield Way/High Street (RTC1)


Figure 3 - Small intervention modelling at the Church of the Epiphany, Mansfield Drive Merstham (RED4) proposed development site.

Church of the Epiphany, Mansfield Drive Merstham (RED4)


Figure 4 - Small intervention modelling at the Gloucester Road Car Park (RTC6) proposed development site.

Gloucester Road Car Park (RTC6)


Figure 5 - Small intervention modelling at the Land at Reading Arch Road/Brighton Road (RED8) proposed development site.


Figure 6 - Small intervention modelling at the Redhill Law Courts (RED7) proposed development site.

Redhill Law Courts (RED7).

1. introduction · report (reference: imse100328-vba-xx-re-rp-hy-2003-s8-p1-l0600-ea3-lod3-redhill...

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