Drainage Impact Assessment - Renfrewshire Council

26 March 2013
GL 121050
Woolgar Hunter Consulting Engineers 100 West Regent Street, Glasgow, G2 2QD
tel: 0141 332 0471 fax: 0141 332 6246 e-mail: info@woolgarhunter.com
Woolgar Hunter Limited Registered in Scotland No: SC 142064
Sportstock Limited
Proposed Residential & Retail
Development
Old Govan Road
Braehead, Glasgow
Drainage Impact Assessment

Contents
1.0 Introduction
2.0 Site Location and Description
3.0 Development Proposals
3.1 General
3.2 Drainage Requirements
3.2.1 Existing Scottish Water Sewers Layout
3.2.2 Drainage Strategy
3.3 Foul Water Drainage
3.4 Surface Water Drainage
3.4.1 Surface Water Management Strategy
3.4.2 Roof Run-off
3.4.3 Car Park Run-off
3.4.4 External Hard Standing
4.0 Greenfield Run-off
5.0 Attenuation Volumes
6.0 Flood Risk Assessment
6.1 General
6.2 Fluvial Flooding
6.3 Pluvial Flooding
6.4 Site Access and Egress
7.0 Conclusions
Appendices
Appendix A – Co-ordinated Existing Services Layout

1.0 INTRODUCTION
Woolgar Hunter have been appointed by Sportstock Limited to prepare a Drainage
Impact Assessment to support an application for Outline Planning Permission, for a
proposed retail and residential development at Old Govan Road, Braehead, Renfrew.
The purpose of this report is to demonstrate to Renfrewshire Council, as Planning
Authority, and the relevant Planning Consultees, these being Scottish Water and the
Scottish Environmental Protection Agency (SEPA) that the proposed development will
not be at an unacceptable level of Flood Risk, can be drained in a sustainable
manner, and will not increase the risk of flooding elsewhere.
2.0 SITE LOCATION AND DESCRIPTION
The proposed site is located to the south of Old Govan Road and west of Kings Inch
Road, Braehead, Renfrew at OS Grid Reference NS 518669.
The south and west boundaries are formed by existing industrial sites.
The site is L shaped and is approximately 6.26ha, as shown in Figure 1 below.
Historically the site has been occupied by factory units and hardstanding and as such
is considered as brownfield. The site is level with a fall of approximately 1m from the
southeast corner to the northwest corner.
With regards to existing sewers serving the site there is a 525mm diameter vitrified clay
combined sewer which runs along Govan Road and a 300mm diameter vitrified clay
combined sewer running along the east boundary of the site and to the west of Kings
Inch Road.
The only other services noted to be present on site are British Telecom underground
plant and telegraph poles and Scottish Power cables which served the existing
buildings on the site. These are noted as Presumed out of use.
Figure 1 – Site Location Plan

3.0 DEVELOPMENT PROPOSALS
3.1 General
The proposed development will comprise of the following;
i. A Class 1 Retail Store incorporating external play areas, associated carparking
and service yard.
ii. The neighbouring site land will comprise of 150 residential units. The layout and
formation of the dwelling units is unknown but it will likely form a mix of single
dwelling and flatted accommodation, accessed by a 5.5m housing road.
The approximate summary of the site areas is given in Section 3.4. It is not proposed
that any major remodelling is required but there is a re-grading exercise to provide
suitable platforms on which the development can take place.
3.2 Drainage Requirements
3.2.1 Existing Scottish Water Sewers Layout
The Scottish Water sewer records indicate there are combined sewers present in Old
Govan Road and off King’s Inch Road, which it is proposed we would utilise to serve
the site. Refer to Appendix A for a copy of the Coordinated Existing Site Services
Drawing No 12001GA-20-005.
3.2.2 Drainage Strategy
The strategy will ensure that flow of surface water from the development into the public
system is minimised and will be limited to the pre-development 2 year greenfield runoff
with all design parameters agreed with Scottish Water and Renfrewshire Council.
3.3 Foul Water Drainage
Foul water generated from the development will be directed into the existing Scottish
Water combined sewers which as noted above are present in Old Govan Road and off
King’s Inch Road. There will be 1 connection, for the retail unit development site with
likely 2 connections for the residential site.
Reference was made to the Scottish Water Asset Capacity Search facility. This
indicates that there is current capacity within the existing sewer network. From this, it is
not expected that there should be any foul drainage constraint, as the foul discharge
from the development will remain relatively small.
A Drainage Impact Assessment form has been submitted to Scottish Water for
confirmation of this position.
Foul flow rates from the proposed development have been calculated using the
Probability Method to Annex B of BS EN 752 : 2008 as shown in Figure 2 below for the
retail unit. The foul flow rates from the proposed residential units have been based on
the guidance provided in Sewers for Scotland, 2nd Edition, Clause 2.20. The design
flows for foul gravity sewers should be 4000 litres per unit per 24 hours.

Anticipated Foul Drainage Flows
The following is an assessment of the foul flow rates generated from the respective
parts of the development.
Retail Development Flow Rates
The total Discharge Units from the anticipated appliances within the unit ∑DU = 16.7
Applying a Frequency Factor (constant) for frequent use kDU = 0.7
Flow Rate Q = kDU [∑DU] 0.5
Q = 0.7 [16.7] 0.5
Q = 2.9 l/s
Estimated Peak Flow Rate for Building = 2.9 litres per second.
Residential Development Flow Rates
The total Flow Rate Q = 4000 litres/unit x 150 dwelling units.
Q = 600000 litres per day. Equating this to a rate per second.
Q = 6.9 l/s
Estimated Peak Flow Rate for Residential Portion of the Site = 6.9 litres per second.
Combined Total Site Development Flow Rates
Combining the total foul flow rates generated from the site, the Estimated Peak Flow
Rate = 9.8 litres per second.
3.4 Surface Water Drainage
3.4.1 Surface Water Management Strategy
The site is brownfield and is currently occupied by buildings, hardstanding and
landscape areas. As such, surface water falling onto the site will dissipate by a
combination of overland flow, infiltration, evaporation and evapotranspiration.
Surface water generated from the development will be controlled in accordance with
the “Sustainable Urban Drainage Systems, Design Manual for Scotland and Northern
Ireland, issued by CIRIA”, in the following manner;
Surface water from the proposed development will be attenuated to a runoff
flow equivalent to the 2 year Greenfield, for up to a 1:30 year storm event with all
storage below ground. The 1:200 year storm plus climate change will be assessed
and if practicable stored below ground. If not the risk of flooding of the building
or impact on the safe access and egress from the building will be assessed in
accordance with Renfrewshire Councils Drainage Assessment – Notes for
Guidance document.

Surface water runoff from the roof areas will undergo one level of treatment
Surface water runoff from the car parks will undergo two levels of treatment.
Surface water runoff from the service yard will undergo two levels of treatment.
Surface water runoff from the access roads will undergo two levels of treatment.
This will be achieved as follows:
3.4.2 Retail Roof Runoff (Area 3537m 2 )
Surface water generated from the building roofs will be directed into filter trenches
located in the landscape areas, this will provide the necessary treatment and
attenuation, prior to discharge into the combined sewer.
3.4.3 Residential Roof Runoff (Area 19091m 2 )
Surface water generated from the building roofs will be directed into the porous
make ups located beneath the car park running surfaces, which will provide the
necessary treatment and attenuation, prior to discharge into the combined sewer.
As the formation of the development is unknown at this stage, it is suggested that
70% of the site area is taken as contributory to the run off generated. This will be
divided into 50% as roof run off and 20% as roads and car parking.
3.4.4 Retail Car Park Runoff (Area 10355m 2 )
Surface water generated from the carpark and access road will be subjected to two
levels of treatment.
The surface of the car parks will comprise permeable paviours/porous asphalt with
a stone reservoir below, this will provide the two levels of treatment.
The water will then be discharged via an attenuation manhole to the combined sewer.
3.4.5 Residential Car Park and Access Road Runoff (Area 7636m 2 )
Surface water generated from the carpark and access road will serving the residential
development will be subjected to two levels of treatment.
The surface of the car parks will comprise permeable paviours/porous asphalt with
a stone reservoir below, this will provide the necessary two levels of treatment.
The access road will comprise of a perimeter swale with a wrapped stone filter
trench below, which will provide the necessary two levels of treatment.
The combined water generated will then be discharged via an attenuation manhole
to the combined sewer.

3.4.4 Retail External Hardstandings (Area 10608m 2 )
The surface water from these areas will be treated by a combination of bioretention
features and filter trenches before discharging to the combined sewer. This will provide
the required two levels of treatment.
The proposed SUDS measures will accommodate storm events up to a 200 year return
period including 20% increase for climate change.
Due to the site topography, being relatively flat, will be retaining the 1 in 200 year storm
within the site, prior to discharge at a controlled rate to the combined sewers.
4.0 Greenfield Runoff
An estimation of the Greenfield Run - off rate has been carried out using Micro
Drainage QBAR software as indicated in Figure 3a & 3B below.
The discharge rate in each case, is to be agreed by Scottish Water and Renfrewshire
Council.
Figure 3a – Retail Greenfield Run – off.
Based on a total site area of 24493m 2 this has been calculated at 20.2l/s, equivalent to
8.2l/s/Ha.

Fig 3b – Residential Greenfield Run – off
Based on a total site area of 38182m 2 this has been calculated at 31.5l/s, equivalent to
8.2l/s/Ha.
This provides a combined greenfield run – off of 51.7l/s for the full development site.

5.0 Attenuation Volumes
We have carried out an assessment of the attenuation volume required for various return
periods and storm durations and these are shown in Figure 4a and 4b below. This has
been divided between the retail and residential portions of the site.
Figure 4a – Retail Attenuation Calculation
This revealed for a 1:200 year storm the critical storm duration is 540minutes and requires
attenuation of 1229m 3 .

Figure 4b – Residential Attenuation Calculation
This revealed for a 1:200 year storm the critical storm duration is 540minutes and requires
attenuation of 1439m 3 .

6.0 Flood Risk Assessment
6.1 General
An extract from the SEPA Flood Map is shown in Figure 5 below. The map indicates that
the site is not at risk from flooding under a 1;200 year (0.5% AER) storm event.
Figure 5
Notwithstanding this we have also considered the following;
Risks to the development and risks arising from the development.
Considerations of the vulnerability of the end users and arrangements for the safe
access and egress to/from the development.
The primary risks of flooding to the development are from fluvial and pluvial flooding,
which are assessed below.
6.2 Fluvial Flooding
The only possible source of flooding in the vicinity of the site is from the River Clyde. The
development is relatively flat with an average ground level of +7.2m AOD, which is
some 2.2m above the land adjacent to the River Clyde level.
We therefore consider that there is adequate distance from the River Clyde and
freeboard that fluvial flooding does not pose a threat to the development.

6.3 Pluvial Flooding
There are two instances when pluvial flooding may be a problem;
During the initial soil strip, when rainwater has a reduced ability to percolate
through the exposed sub soil. At present the site is covered with roof or
hardstanding, ie impermeable and as such we do not consider this condition to
be a concern.
Upon completion of the development when the surface water can drain across
impermeable surfaces without being contained on site. The surface water system
will be designed to store all rainwater on site.
6.4 Site Access and Egress
As the site will be designed for a 1:200 year plus climate change storm, stored below
ground, and fluvial flooding is not considered to be a risk, there is no risk to safe access
and egress.
7.0 Conclusions
The proposed drainage design satisfies all elements of the current guidelines with
regards to impacting on the existing sewers network and surrounding areas.
Prepared By ………………………………. Checked By ……………………………………….
Jim sharp Sandy Porter
BSc (Hons) CEng MICE BSc CEng MICE

Appendix A
Co-ordinated Existing Services Layout

Co-ordinated Services Layout

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