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Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
Project Dorothy House Date 30th November 2016
Note Drainage Strategy Technical Note Ref A096365/161128
Author Matt Mercer Rev -
1.0 Introduction
WYG have been appointed by Dorothy House Hospice Care to produce a drainage strategy for an extension
to the existing car park at the care home located at Bradford Road, Winsley. As the site is less than 1
hectare in size, and lies solely within Flood Zone 1 (as designated by Environment Agency flood maps), a
full Flood Risk Assessment is not required. A copy of the Environment Agency Flood Maps confirming that
the site is within Flood Zone 1 is provided as Appendix A. This Technical Note aims to demonstrate that the
construction of the car park can proceed without any increase of flood risk downstream as a result of the
development. Infiltration has been undertaken at the site to inform the drainage strategy and ensure that
mitigation follows the surface water drainage hierarchy by utilising an infiltration system when viable.
2.0 Existing Site
The site is located to the south of Bradford Road, Winsley comprising a care home and car park to the
west. The current drainage regime the existing car park utilises a soakaway for surface water runoff. The
Topographical Survey for the site is provided as Appendix B with an existing plan of the site provided as
Appendix C.
The site is bounded to the north by Bradford Road and residential dwellings; to the west by Murhill and to
the east and south by agricultural land. There are no watercourses on the site.
3.0 Proposed Site
The proposals for the site involve amendments to the existing car park to create an additional 14 parkings
spaces, extending the existing car park to the south to create 30 new spaces and creating an overflow car
park for a further 79 spaces. It is currently intended that the new areas of parking will be in the form of
Grasscrete or a similar material to minimise the visual impact. The proposed layout is shown in Appendix D.
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
4.0 Flood Risk
From the Environment Agency Flood Map and Government Long-term Flood Risk Maps it can be seen that
the entire site is within Flood Zone 1, defined as “land assessed as having a less than 1 in 1,000 annual
probability of river or sea flooding (<0.1%).” Thus, Main River flooding would not be an issue to the site.
The Surface Water Flood Map indicates one localised area at the north west of the site that is at low risk of
surface water flooding, expected to be below 300mm in depth during the most severe storms. From review
of the topographical survey this appears to be a localised low point on site that abuts the building.
The main issue to the site in relation to flood risk will be the potential increase to flood risk further
downstream of the site that may arise if the car park was constructed without mitigation. The Drainage
Strategy set out below has been designed to mitigate against a potential increase in flood risk that may
otherwise arise.
5.0 Drainage Strategy
As previously mentioned, existing surface water drainage discharges to a soakaway on site. In order to
develop an appropriate drainage strategy, infiltration testing to the requirements of BRE 265 was
undertaken on the site of the proposed car park on the 23rd November 2016 . The results of this testing are
included in Appendix E and show that infiltration rates encountered are very good, in the region of 1x10-3
ms-1 to 2.5x10-3 ms-1. As a result of this observed rate, it is proposed that infiltration is utilised for the mode
of discharge.
Current proposals for the car park are to construct the extension from a permeable surface such as
Grasscrete (or similar) in order to limit the visual impact of the car park. If the car park is constructed using
permeable materials, it is proposed that providing best practices are followed in the construction this
material will allow for natural drainage within the site, allowing surface water to infiltrate close to the
source.
In the event of impermeable surfacing being used in the construction of the car park, preliminary
calculations show that a trench soakaway 50m long, 1.8m deep and 1.2m wide with a 600mm perforated
pipe will allow for the required soakage. If proposals do change to include impermeable surfacing for some
or all of the car park including the aisles, detailed design of the soakaway will be required.
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
6.0 Conclusion and Summary
WYG have produced this drainage strategy on behalf of Dorothy House Hospice Care in order to identify an
appropriate drainage strategy for the construction of a new car park without increasing the existing risk of
flooding within or outside the development boundary. As infiltration rates are high it is proposed that
infiltration in the form of either natural infiltration features of construction materials such as Grasscrete or a
trench soakaway at the low point of the car park will be the mode of discharge for surface water up to and
including the Q100 rainfall event. This will therefore provide for the appropriate level of mitigation required
to ensure the proposed car park does not increase the existing risk of flooding either with or outside the
development boundary.
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
APPENDIX A
ENVIRONMENT AGENCY & GOVERNMENT LONG TERM
FLOOD MAPS
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
Environment Agency Flood Map For Planning
Government Long Term Flood Risk Map – Risk From Surface Water
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
APPENDIX B
TOPOGRAPHICAL SURVEY
S6
379742.382mE
160786.428mN
121.713m
S4
379714.593mE
160807.171mN
121.685m
S1
379787.753mE
160822.464mN
122.449m
S3
379771.286mE
160803.431mN
122.306m
S5
379758.539mE
160807.129mN
122.071m
S7
379697.828mE
160721.375mN
115.726m
S8
379685.444mE
160697.239mN
112.006m
S9
379657.164mE
160676.866mN
105.398m
S10
379721.373mE
160743.799mN
119.552m
S6A
379750.572mE
160779.694mN
121.441m
S11
379772.329mE
160712.785mN
117.612m
S12
379730.464mE
160730.248mN
118.355m
S13
379730.077mE
160693.660mN
115.011m
S12A
379714.222mE
160737.771mN
119.924m
S17
379611.569mE
160673.793mN
98.802m
S2
379840.155mE
160807.149mN
122.562m
S14
122.615m
S15
379847.498mE
160829.678mN
123.667m
S16
379832.367mE
160797.471mN
122.275m
S40
379701.873mE
160718.922mN
116.223m
S41
379674.078mE
160658.371mN
105.587m
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
APPENDIX C
EXISTING SITE PLAN
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
APPENDIX D
PROPOSED SITE PLAN
Technical Note
WYG 5th Floor, Longcross Court, 47 Newport Road, Cardiff, CF24 0AD
T+44 (0)29 2082 9200 E [email protected] W www.wyg.com creative minds safe hands
APPENDIX E
INFILTRATION TEST RESULTS
PROJECT No.:
5th Floor, Longcross Court, 47 Newport Road, Cardiff
Tel: 02920 829200
Fax: 02920 455321
Environmental Consultancy
Ground Engineering Services
TRIALPIT No.: SA01
TEST No.: 1
DATE: 23/11/2016
0 0.00 0.880 INPUT PARAMETERS:
30 0.50 0.950 Effective depth of Pit (m) 0.09
60 1.00 0.970 Depth of Pit (m) 0.97
90 1.50 0.980 Length of Pit (m) 1.10
120 2.00 Width of Pit (m) 0.60
150 2.50 Total volume of pit (m3) 0.06
180 3.00 Total volume of Water (m3) 0.06
210 3.50 Level of water in pit at 75% effective depth (p75) (m) 0.02
240 4.00 Level of water in pit at 25% effective depth (p25) (m) 0.07
270 4.50 Surface area of pit up to 50% effective depth (Ap50) (m2) 0.813
300 5.00 Time at 75% effective depth (p75) (s) 10
360 6.00 Time at 25% effective depth (p25) (s) 35
420 7.00 Volume of outflow between 75% and 25%
480 8.00 effective depth (Vp75 - 25 )
540 9.00 Time taken for the outflow between 75% and 25%
600 10.00 effective depth (Tp75-25)
720 12.00
840 14.00
960 16.00 WATER INPUT: 350 litres in 80 seconds
1080 18.00 GEOLOGY OF TEST SECTION:
1200 20.00
1320 22.00
1440 24.00 Compiled by: SF
Checked by: PV
After BRE Digest 365, Soakaway Design, September 1991
25
Too many points to display
White and light orangish brown weak heavily fractured LIMESTONE. Recovered as
angular cobbles and boulders with some clay.
Page 1 of 1
SOIL INFILTRATION RATE (f)Vp75 - 25
(m/s) 1.46E-03Ap50 x Tp75 - 25
FIGURE No.: A096365-SA01/1
SOAKAWAY TEST - SOIL INFILTRATION RATE/PERMEABILITY CALCULATION
WYG EnvironmentA096365
PROJECT NAME: Dorothy House
CLIENT: Dorothy House
(s)
Time
Elapsed
(mins)
Time
Elapsed
(s)
(m3) 0.03
Depth of
water below
ground level
(m)
0.9
0.9
0.9
0.9
0.9
1.0
1.0
1.0
0 200 400 600 800 1000 1200 1400 1600
De
pth
of
wa
ter
be
low
gro
un
d l
eve
l (m
)
Time from filling to maximum effective depth (seconds)
PROJECT No.:
5th Floor, Longcross Court, 47 Newport Road, Cardiff
Tel: 02920 829200
Fax: 02920 455321
Environmental Consultancy
Ground Engineering Services
TRIALPIT No.: SA01
TEST No.: 2
DATE: 23/11/2016
0 0.00 0.860 INPUT PARAMETERS:
30 0.50 0.920 Effective depth of Pit (m) 0.11
60 1.00 0.940 Depth of Pit (m) 0.97
90 1.50 0.960 Length of Pit (m) 1.10
120 2.00 0.980 Width of Pit (m) 0.60
150 2.50 Total volume of pit (m3) 0.07
180 3.00 Total volume of Water (m3) 0.07
210 3.50 Level of water in pit at 75% effective depth (p75) (m) 0.03
240 4.00 Level of water in pit at 25% effective depth (p25) (m) 0.08
270 4.50 Surface area of pit up to 50% effective depth (Ap50) (m2) 0.847
300 5.00 Time at 75% effective depth (p75) (s) 15
360 6.00 Time at 25% effective depth (p25) (s) 35
420 7.00 Volume of outflow between 75% and 25%
480 8.00 effective depth (Vp75 - 25 )
540 9.00 Time taken for the outflow between 75% and 25%
600 10.00 effective depth (Tp75-25)
720 12.00
840 14.00
960 16.00 WATER INPUT: 350 litres in 80 seconds
1080 18.00 GEOLOGY OF TEST SECTION:
1200 20.00
1320 22.00
1440 24.00 Compiled by: SF
Checked by: PV
After BRE Digest 365, Soakaway Design, September 1991
Too many points to display
White and light orangish brown weak heavily fractured LIMESTONE. Recovered as
angular cobbles and boulders with some clay.
Page 1 of 1
SOIL INFILTRATION RATE (f)Vp75 - 25
(m/s) 2.14E-03Ap50 x Tp75 - 25
(m3) 0.04
(s) 20
FIGURE No.: A096365-SA01/2
SOAKAWAY TEST - SOIL INFILTRATION RATE/PERMEABILITY CALCULATION
Time
Elapsed
(s)
Time
Elapsed
(mins)
Depth of
water below
ground level
(m)
WYG EnvironmentA096365
PROJECT NAME: Dorothy House
CLIENT: Dorothy House
0.8
0.9
0.9
0.9
0.9
0.9
1.0
1.0
1.0
0 200 400 600 800 1000 1200 1400 1600
De
pth
of
wa
ter
be
low
gro
un
d l
eve
l (m
)
Time from filling to maximum effective depth (seconds)
PROJECT No.:
5th Floor, Longcross Court, 47 Newport Road, Cardiff
Tel: 02920 829200
Fax: 02920 455321
Environmental Consultancy
Ground Engineering Services
TRIALPIT No.: SA01
TEST No.: 3
DATE: 23/11/2016
0 0.00 0.850 INPUT PARAMETERS:
30 0.50 0.910 Effective depth of Pit (m) 0.12
60 1.00 0.940 Depth of Pit (m) 0.97
90 1.50 0.960 Length of Pit (m) 1.10
120 2.00 0.980 Width of Pit (m) 0.60
150 2.50 Total volume of pit (m3) 0.08
180 3.00 Total volume of Water (m3) 0.08
210 3.50 Level of water in pit at 75% effective depth (p75) (m) 0.03
240 4.00 Level of water in pit at 25% effective depth (p25) (m) 0.09
270 4.50 Surface area of pit up to 50% effective depth (Ap50) (m2) 0.864
300 5.00 Time at 75% effective depth (p75) (s) 18
360 6.00 Time at 25% effective depth (p25) (s) 36
420 7.00 Volume of outflow between 75% and 25%
480 8.00 effective depth (Vp75 - 25 )
540 9.00 Time taken for the outflow between 75% and 25%
600 10.00 effective depth (Tp75-25)
720 12.00
840 14.00
960 16.00 WATER INPUT: 350 in 80 seconds
1080 18.00 GEOLOGY OF TEST SECTION:
1200 20.00
1320 22.00
1440 24.00 Compiled by: SF
Checked by: PV
After BRE Digest 365, Soakaway Design, September 1991
Too many points to display
SOIL INFILTRATION RATE (f)Vp75 - 25
Ap50 x Tp75 - 252.55E-03
White and light orangish brown weak heavily fractured LIMESTONE. Recovered as
angular cobbles and boulders with some clay.
Time
Elapsed
(s)
Time
Elapsed
(mins)
Depth of
water below
ground level
(m)
PROJECT NAME:
CLIENT:
FIGURE No.:
SOAKAWAY TEST - SOIL INFILTRATION RATE/PERMEABILITY CALCULATION
WYG Environment
Page 1 of 1
A096365
Dorothy House
Dorothy House
A096365-SA01/3
(m/s)
(m3) 0.04
(s) 18
0.8
0.9
0.9
0.9
0.9
0.9
1.0
1.0
1.0
0 200 400 600 800 1000 1200 1400 1600
De
pth
of
wa
ter
be
low
gro
un
d l
eve
l (m
)
Time from filling to maximum effective depth (seconds)
