Addendum to Flood Risk Assessment and Drainage Strategy
Dunholme Road - Scothern
1.0 Introduction
1.1 This addendum has been prepared to take account of issues raised by the Multi
Agency Group (MAG), at a meeting held at West Lindsey District Council Offices on
18th February 2015, and the comments made subsequently by individual members of
the group.
1.2 The basic assessment of flood risk remains the same as that in the original
document. Also, the need to consider drainage of the site in 2 sub-catchments is still
valid.
1.3 Changes are proposed to the conveyance system for roof water and surface water
from private driveways, to the hydrobrake control systems, to the attenuation ponds
and to the layout plan (only for Plot No. 1). A revised outfall arrangement is also
suggested for the Eastern Pond, and overflows are introduced from both ponds, to
be used in extreme conditions.
2.0 Conveyance
2.1 All surface water is now proposed to flow through the road sub-bases. Interception
by means of rain water harvesting is retained, as is the use of permeable paving. A
revised Drainage Plan is attached to this addendum – Fig 8AD Revised Drainage.
Swales are not now proposed, but a grassed area will be provided where the swales
were originally located. As the water table is approximately 1m below the road subbase, the roads will not be “tanked” and some infiltration to the sub-soil will take
place.
3.0 Attenuation Ponds
3.1 Ponds will be “wet” with some water permanently in them. Sizes have been
calculated for each sub-catchment by using a control figure of 3l/s for the hydrobrake
discharge rates and taking into account storage volumes in the road sub-bases
(taken as 0.3m thick, with a 30% voids ratio). The basic standard of a 6 hour 1 in 100
year storm, plus an allowance for the impact of climate change, has been used and
revised calculations are provided, including the consideration of pond sizes at
various average depths.
3.2 Detailed pond designs will be agreed with Anglian Water, and, if required, will include
for “toddler fencing” and/or other safety features. Access will be directly from main
roads and sufficient space will be provided to enable maintenance activities to be
carried out.
3.3 Plot 1, in the Eastern sub-catchment has been altered to allow more space for the
attenuation pond. The outfall will be piped from the Hydrobrake to connect into the
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existing surface water system in Dunholme Road. This currently takes water from the
ditch on the northern boundary, and from roadside inlet gullies. This is the system for
which the client has offered to make a contribution, if improvements are required.
3.4 Access arrangements for maintenance of the watercourses on the southern
boundary will be agreed with Witham 3rd IDB, and a commuted sum negotiated for
future maintenance.
4.0 Extreme Conditions
4.1 High level overflows are suggested for each pond, discharging to adjacent
watercourses. This should direct some flows away from properties. In very extreme
conditions which exceed the design criteria, overland flows will be in the direction of
the natural contours i.e. towards the ditch on the north-east boundary from the
eastern sub-catchment, and towards the riparian watercourse on the south-west
boundary from the western sub catchment. If systems become full, and water can no
longer permeate into the sub-bases of the roads, flows over the road surfaces will
probably be contained within the kerbs (depending on the severity of the event).
5.0 Revised Calculations
5.1 The basic hydrological data, and design standards are as per those in the original
FRA. However, the limits of discharge from each Hydrobrake has been set at 3l/s.
Storage requirements in the 2 sub-catchments have been calculated:Western Sub-Catchment
Total area = 11,493m2
Hard surfaces = 4,115m2
Road Area = 1,221m2
For a 1 hour storm, 20mm depth, run off = 4,115 x 20 x 1000 = 22.86l/s
1000 3600
If balanced to 3l/s, volume required = (22.86 – 3) x 3.6 = 71.5m3
Now consider a 1 hour storm, 1 in 100 years = run off = 22.86 x 3.56 = 81.38l/s
If balanced to 3l/s, volume required = (81.38 – 3) x 3.6 = 282m3
To allow for climate change with 6 hour storm, 1 in 100 years,
Discharge rate = 115 x 4115 x 1000 = 21.9l/s
1000
21,600
Allowing a factor of 1.3 for climate change – rate = 150 x 4115 x 1000 = 28.6l/s
1000
21,600
Extra volume required = (28.6 – 21.9) x 21.6 = 72m3
2
Total storage volume required = (71.5 + 210.5 +72) = 354m3
The available volume in the sub-base = 1221 x 0.3 = 366m3 x 0.3 = 110m3
Volume of storage required in pond = (354 -110) = 244m3
Area of pond at various depths =
Average depth for storage 0.5m
0.6m
0.7m
0.8m
Area of pond
488m2
406m2
348m2
305m2
Eastern Sub-Catchment:
A similar calculation for the eastern pond produces results:Average depth for storage 0.5m
0.6m
0.7m
2
2
Area of pond
188m
156m
134m2
2
0.8m
117m2
Sufficient space has been allowed to create ponds with average storage depths
between 0.6m and 0.7m. These depths would comply with typical values acceptable
to Anglian Water.
6.0 Conclusion
6.1 The revised strategy takes into consideration all issues raised by MAG members.
6.2 The strategy involves the use of “interception” by rain water harvesting, some degree
of “infiltration” balancing of flows to greenfield rates, environmental enhancements,
and discharge of surface water to existing drainage systems. It will lead to a
controlled system which will have fewer and reduced discharges of surface water.
6.3 There should be no adverse effect on adjacent properties, and no significant hazards
created on site.
6.4 Roads will be designed to standards acceptable to Lincs CC, and ponds to
specifications agreed with Anglian Water.
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