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Introduction to Drainage Design
Presented by Dominic Cronin
Surface Water
Foul Water
Surface Water
Session 1
Definition: Artificial removal of water
Before Step 6, Produce Levels Model before undertaking Drainage Strategy
Surface Water
Foul Water
Rainwater runoff
Waste Water from toilets, sinks, bin store, showers
Gully
Channel Drains
Rainwater Pipes
Floor Gully (FG)
Soil Vented Pipes (SVP)
Soil Stack (SS)
Gravel Trench
Permeable Paving
Manholes
Petrol Interceptor
Attenuation Tank
Pipes
Foul/Combined Sewer
Surface/Combined Sewer
Borehole
Standards
Building Regulations 2010 - Part H - Drainage and Waste Disposal
Sewers for Adoption 8th Edition
CIRIA SuDS Manual (C753) 2015
Guidance
Foul Guidance Document by Jean Benard
Two Day MicroDrainage Course Guidance
T&I Design flow Charts
Rainfall Distribution
Rainfall Profile
Difference in rainfall between NorthEast and SouthWest
15 minute, 1 year, Winter
High elevations on the West of the UK cause
the Westerly Atlantic Winds to form into frontal rainfall.
1 year
100 year
30 year
15
minute
The intensity of rain decreases as the winds travel East across the UK.
71.271 mm/hr
174.612 mm/hr
226.002 mm/hr
1440 minute
3.092 mm/hr
8.237 mm/hr
6.457 mm/hr
Rainfall Terminology
Rainfall Theories
Rainfall Event:
Storm Duration
Return Period
Season
Climate Change
15 minute,
1 in 100 year,
Winter,
+40%
FEH99 - Flood Estimation Handbook
FSR - Flood Studies Report
Climate Change
Season Variability
Storm Duration
Published in 1999
Published in 1975
Summer
Winter (critical)
Available online, charged per dataset
Embedded, Free, within MicroDrainage
15 minutes
30 minutes
10080 minutes (1 week)
Climate change allowances are predictions of anticipated change for:
-Peak river flow
-Peak rainfall intensity
-Sea level ridse
Focused on hourly and above durations
Focused on sub-hourly events, aswell as above 1 hour
Return Period
Example: Warwickshire
20% - Minimum requirement
40% - Senstiivty Analyis
Definition - The probability of an event, such as a flood, to occur
Design Consideration
FEH shows a conservative value comapred to FSR. Therefore, FSR is an economical option.
BUT Council's request FEH data as part of planning conditions.
Return Periods
1 in 1 year
1 in 30 year
1 in 100 year
Probability
100%
33.3%
1%
Design
No Flooding, No Surcharged Pipes
No Flooding, Surcharged Pipes Allowed
Flood Allowed, if contained within site
Existing Rates
Drainage Hierarchy
Brownfield
Greenfield
Undeveloped Land
Previously Developed Land
Restrict to Greenfield Runoff Rate; varies between councils
Non-Statutory Technical Standards state:
Proposed Runoff must be "as close as reasonably practicable" to greenfield but not higher than the rate of discharge prior to redevelopment
Planning Practice Guidance and SuDS proposed the following hierarchy for surface water runoff
- 5l/s
- Restrict to each Rainfall event
- Restrict to QBAR
1. into the ground (infiltration)
2. to a surface water body
Restricting Discharge
3. to a surface water sewer, highway drain, or another drainage system
4. to a combined sewer
HydroBrake
Orifice
Small Pipe
MicroDrainage
Why is Attenuation Required?
Due to discharge rates being restricted, attenuation will be required
Attenuation (Storage)
Attenuation Pond
Attenuation Tank
Permeable Paving
To understand the basic principles of Drainage
To understand the BWB process for Drainage Design
To understand Discharge Rates
To calculate and apply attenuation to a Drainage system
the artificial removal of water
Building Regulations 2010 - Part H - Drainage and Waste Disposal
Sewers for Adoption 7th Edition
CIRIA SuDS Manual (C753) 2015
Foul Guidance Document by Jean Benard
Two Day MicroDrainage Course Guidance
Probability
Sewers for Adoption
Return Periods
1 in 1 year
1 in 30 year
1 in 100 year
100%
3.33%
1%
No Surcharge
No Flooding
Contain Flooding within site
Developed site
Undeveloped site
Rainwater Pipes
Channel Drains
Gully
Start
Permeable Paving
Gravel Trench
Middle
Petrol Interceptor
Pipes
Attenuation Tank
Manholes
Outfall
Outfall Manholes
Boreholes
PCC Manhole
PCC - Pre-Cast Concrete Chamber
Type 1 (3-6m deep) and Type 2 (<3m deep) Manholes
Sizes: 900mm, 1200mm, 1500mm, 2100mm dia.
Typically serves Surface Water
PPIC Manhole
PPIC - Polypropelyne inspection chambers
Type 3 (<3m deep) and Type 4 (<2m deep) Non-entry Manholes
Sizes: 450mm dia and 600mm dia
Typically serves Foul Water
CL - Cover Level
Finished ground level
SL - Soffitt Level
Top of pipe
IL - Invert Level
Bottom of pipe
What
Depth between the CL and the SL
Why
- Ensure pipe isn't crushed by surface loading
- Ensure pipe doesnt clash with services
Minimum Cover Depths
Part H
Landscaping/Footway- 0.6m
Road - 1.2m
Sewers for Adoption
Landscping - 0.35m
Domestic driveway - 0.5m - 0.9m
Highawys - 1.2m
Lower IL = Upper IL +/- Length/Gradient
Gradient = (Lower IL +/- Upper IL) / Length
Minimum velocity that ensures non-settlement of suspended mater in sewers
Self-Cleansing Velocity
1 m/s - Surface
0.7 m/s - Foul
100mm - 1 in 60
150mm - 1 in 100
225mm - 1 in 170
300mm - 1 in 240
375mm - 1 in 320
450mm - 1 in 400
600mm - 1 in 580
Q (l/s) = 2.78 x C x I (mm/hr) x A (ha)
Discharge
Colebrook-White
Pipe Diameters
150mm
225mm
300mm
375mm
450mm
525mm
600mm
Q (l/s) = 2.78 x C x I (mm/hr) x A (ha)
Q = Discharge (l/s)
"proportion of water that falls on the site entering the drainage system"
Cv = Volume Runoff Coefficient = 0.75
C = Cv x Cr
Cr = Routing Factor = 1.3
"Cr is used to factor the shape of the time-area diagram, and the peakedness of the rainfall profile"
C = 0.75 x 1.3 = 0.975 = 1
50 (mm/hr)
I = Rainfall Intensity (mm/hr)
BS8005
A = Impermeable area (ha)
Q (l/s) = 0.014 x A (m^2)
Q (m3/s) = C x I (m/s) x A (m2)
Q 0.001 (l/s) = C x I 0.001/3600(mm/hr) x 10000 A (ha)
Q (l/s) = 2.78 x C x I (mm/hr) x A (ha)
Storm Durations
15
60
10080
Foul - 1.5
Surface Water - 0.6
Example
Area of Car Park = 1000m2
Q = 0.014 x 1,000m2
Q= 14 l/s
100mm
150mm
225mm
300mm
375mm
Q = 0.014 x A (m2)
100mm - 1 in 60 (Q = 7.809)
150mm - 1 in 100 (Q = 17.723)
225mm - 1 in 170 (Q = 39.680)
300mm - 1 in 240 (Q = 71.294)
375mm - 1 in 320 (Q = 111.056)
450mm - 1 in 400 (Q = 160.387)
600mm - 1 in 580 (Q = 283.326)
Pipe Length
Reference
Type 1
Type 2
Type 4
Type 3
Manhole Diameter