Drainage Impact Assessment - Renfrewshire Council

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.