Civil Engineering Project: Storm Sewer Flood Risk Management Design

This project focuses on the design and analysis of a storm sewer system and flood risk management for a commercial site in Wolverhampton, UK. The student begins with a flood risk assessment, determining the site's flood zone and the need for a detailed assessment, considering both river/sea and surface water flooding risks. The assessment includes site descriptions, existing flood alleviation measures, and potential flood sources. The project then delves into the current drainage system, including sustainable drainage systems (SUDS), and topographical features. Surface water runoff is assessed using the Modified Rational Method, including calculations for rainfall intensity, catchment area, and peak discharge, ensuring compliance with building regulations. The design phase covers pipe sizing, locations of manholes and gullies, and calculations for velocity and maximum discharge. The project includes considerations for water storage systems to manage excess water and concludes with a bill of quantity and discussion on innovative methods to minimize peak flow rates. Finally, the project addresses uncertainties in peak flow and their potential impact.

Design and analysis of storm sewer
Flood risk managment
3/3/2020
Student name -
Roll no.

Table of content
Task 1
1.1 Flood zone ………………………………………………………………………………3
1.2 Flood risk assessment report…………………………………………………………….3
1.2.1 Introduction……………………………………………………………………………3
1.2.2 Requirement for flood risk assessment………………………………………………..4
1.2.3 Site description and proposed development program…………………………………4
1.2.4 Existing flood alleviation measures …………………………………………………..5
1.2.5 Sources of potential flood……………………………………………………………..5
1.2.6 Assessment of flooding……………………………………………………………….5
1.2.7 Rate of flooding in case of storm……………………………………………………..5
1.2.8 Hydraulics of drainage on site………………………………………………………..5
1.2.9 Consideration of future climate change effects………………………………………5
1.2.10 Conclusions………………………………………………………………………….6
1.3 Current drainage system, flood defence system………………………………………..6
1.3.1 Sustainable drainage system (SUDS)………………………………………………..6
1.4 Topographical features of site………………………………………………………….6
Task 2
2.1 Area of catchment……………………………………………………………………….6
2.2 Return period of site…………………………………………………………………….6
2.3 Maximum rainfall intensity …………………………………………………………….6
2.4 According to Modified rational method………………………………………………..6
2.5 According to Modified rational method……………………………………………….7
2.6 Compliance with Building regulations act 2020………………………………………7
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Task 3
3.1 Signing authority for drainage system ……………………………………………………7
3.2 Design of drainage pipe…………………………………………………………………..8
3.3 Pipe locations, flow direction, diameter and slope……………………………………….11
3.4 Location of Inspection hole/Man holes/Gullies………………………………………….11
3.5 Existing ground level and final ground level…………………………………………….12
3.6 Calculation of velocity ………………………………………………………………….12
3.7 Calculation of maximum discharge……………………………………………………..13
3.8 Water storage system to manage excess water from site……………………………….13
3.8.1 Location……………………………………………………………………………….13
3.8.2 Shape and size…………………………………………………………………………13
3.9 BILL OF QUANTITY…………………………………………………………………..14
3.9.1 Redundant drainage runs………………………………………………………………14
3.9.2 Storm drainage below ground…………………………………………………………14
3.9.3 Storm drainage below ground for average depth 1750 – 2000………………………..14
3.9.4 Plain in-situ concrete; mix 1:12 all in aggregate………………………………………14
3.9.5 Imported free draining limestone………………………………………………………14
3.9.6 Brick manhole………………………………………………………………………….15
3.9.7Testing and commissioning……………………………………………………………..15
3.10 Innovative ways to minimize peak flow rate……………………………………………15
Task 4
4.1 Uncertainty in peak flow ……………………………………………………………….15
4.2 Impact on costumer who shops…………………………………………………………16
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Task 1
1.1 Flood zone
The site comes under flood zone 1. Means this site has low risk of flooding. Low risk means
a chance of flooding is less than 0.1% in a year. No flood risk assessment is required for
flood zone 1 but if the site area is greater than one hectare in flood zone 1 flood risk
assessment required.
Since the area is around 5 hectares. So flood risk assessment is required for this site.
1.2 Flood risk assessment report
1.2.1 Introduction
Wolverhampton is a commercial complex having an area of 5 hectare. Whose location is
shown in map shown below.
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Figure 1: Asda, Molineux Way, Waterloo Rd, Wolverhampton, UK, Postcode: WV1 4DE
This report is prepared for Flood Risk Assessment to make full planning application for
construction at Asda, Molineux Way, Waterloo Rd, Wolverhampton, UK, Postcode: WV1
4DE. The report aims to address the statutory requirements of the National Planning Policy
Framework relating to flooding.
The objective of this report is to provide the Planning Authority and Environment Agency
(EA) with sufficient information to consider flood risk in the planning process.
1.2.2 Requirement for flood risk assessment
The environmental agency (EA) flood map Figure 2 have shown that site falls in flood zone
1. When site falls in flood zone 1 and have area less than one hectare then no flood risk
assessment is required but if site area is more than one hectare then flood risk assessment is
required. Our site area is 5 hectare hence flood risk assessment is required.
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Flood map for planning (Environmental agency 2020)
1.2.3 Site description and proposed development program
Site has large catchment area it consist of superstore, a pharmacy, Charge master charging
station and parking area. Site has no risk by river and sea but in case of heavy storm flood
may occur at this site.
Site has good road connectivity Waterloo road, Oxley street, Molinuex street and Jack
Hayward are along the periphery of this site
1.2.4 Existing flood alleviation measures
We are not aware of any flood alleviation measures to protect this site.
1.2.5 Sources of potential flood
Since the site is location in inland therefore there is no risk from river and sea.
Potential source of flooding will be from heavy storm events.
The site is located at large distance from canals and reservoirs therefore there is no need to
consider flooding from these sources.
Flooding events at this site:
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Flooding events at this site are not known.
Description of any man made structure that may cause flooding at this site:
There is no such structure nearby this site.
1.2.6 Assessment of flooding
The sites are shown to be within the flood zone 1 as detailed on the Environment Agency’s
Indicative Floodplain Maps.
Planning Policy Statement 25: Development and Flood Risk (PPS 25) which came into
existence on December 2006, mentions that:
The site which exists in Zone 1 have chances of flooding less than 0.01%.
Since our site lies in Zone 1 therefore all kinds of land use is allowed. Proposed development
of this allowed if constructed in accordance with Planning Policy Statement 25.
1.2.7 Rate of flooding in case of storm
Adjacent sites which are at higher elevation will be the cause of excess runoff water at this
site. There is no remedy known to deal with this kind of situation but rate of accumulation of
water will be so slow that people affected have enough time to move at higher elevation.
1.2.8 Hydraulics of drainage on site
The site mostly consists of impermeable paved surface to prevent excess water runoff petrol
interceptor need to be incorporated in the proposed site.
1.2.9 Consideration of future climate change effects
Environmental agency takes future climate change into consideration. Therefore we don’t
need to consider future climate change effects.
1.2.10 Conclusions
i) The proposed site has no flooding history
ii) According to Environmental agency the proposed site falls in Zone 1.
iii) There is no risk of flooding from rivers, canals or any other artificial source as the site
falls in zone 1.
iv) Potential risk of flooding is from storm water.
1.3 Current drainage system, flood defence system
We are not aware of any flood defence system. Excess water will flow from high lying
area to the low lying area but people will get enough time to escape because time of
concentration of flood from storm water will be high.
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1.4 Sustainable drainage system (SUDS)
90% of the site is covered with concrete and other impermeable surface. So, we need to
provide infiltration galleries but there is road along periphery of site so we cannot provide
infiltration galleries or any other sustainable drainage system nearby site.
Task 2
2.1 Topographical features of site:
The site is located at inland there is no water body like river, sea, or any other artificial water
in proximity to the site. The site is free from risk of fluvial flooding but there is risk of
flooding from surface water due to storm. The average annual rainfall is 650mm per year.
For Wolverhampton maximum rainfall intensity of five year return period is 20mm according
to Wallingford map.
2.2 Area of catchment:
Area of catchment is approximately 5 hectares. Catchment area is calculated from Google
maps provided in the assignment.
2.3 Return period of site:
Maximum rainfall intensity for 60 minutes of a return period of 5 years for the concerned
location is 20mm.
2.4 Maximum rainfall intensity
According to Wallingford procedure
M5-60 Min = 20mm (Metrological office for working party on the hydralic design of strom
sewer2011)
And r = 0.42
Z1 = 0.79
Therefore M 5- 30 Min = 0.79 X 20 = 15.8mm
Z2 = 0.8
Therefore M 2-30Min = 0.8 X 15.8 = 12.64mm
Average point intensity =
12.64
30
60
= 25.28mm
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2.5 According to Modified rational method
Qp = CiA (Wallingford 2012)
90% of the catchment area of concrete whose C = 0.95 and 10% is soil whose C value is 0.6
Average C of the site = 0.95 X 0.9 + 0.1 X 0.6 = 0.915
Qp = 0.915 X 20
1000 X 5 X 104 = 915m3/hr = 0.254m3/s
Compliance with Building regulations act 2020
Diameter of pipe is taken 250mm which is adequate to carry the max discharge of 0.0617m3/s
Peak discharge calculated above is 0.254m3/s which can carried by 250mm pipe in adequate
time.
Task 3
3.1 Signing authority for drainage system
Propose of drainage work is to carry storm water from building in a way that manages
minimizes flood. Local authority approves the design arrangement prepared by engineer there
are three main local authorities who approve the drainage design which is mentioned below:
i) Local planning authority: New development project approval is given by this
authority in accordance with National policy framework.
ii) Lead local flood authority: This is a statutory body who recommends design of all
the major projects.
iii) Local land drainage authority: This authority approves drainage development
nearby rivers
We are dealing with new development project so, we need approval from local planning
authority. We need to provide separate foul system and surface water system for this
project.
3.2 Design of drainage pipe
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Pipes should be laid underground at a slope of 1% and the diameter should be such that
problem of slit accumulation in the pipe can be controlled. For prevention of slit
accumulation minimum scoring velocity should be greater than 0.45m/s. Our site’s
catchment area is equal to 5 hectare so the pipe diameter of 250mm will be sufficient for
carrying excess runoff during storm.
Drawing of outfall pipe
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Section A-A of outfall pipe
Front elevation of section A-A
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Safety grillage to prevent entry of large substance into pipe
3.3 Pipe locations, flow direction, diameter and slope
A separate drainage system is incorporated in the design for drainage if rainwater. Rainwater
is diverted to west park Victorian. Flow direction is from the concerned site location to west
park Victorian. Diameter of pipe is assumed to be 250mm and slope is assumed 0.28%.
3.4 Location of Inspection hole/Man holes/Gullies
Man holes are required at location where there are sharp changes in the directions are
required, change in elevation and where there is change in in diameter of pipe.
In our case pipe diameter is uniform throughout the course and change in elevation is also
uniform. Only location where there is sharp changes in direction man hole/inspection
chamber is required.
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