Weetwood
Surface Water Training
10 January 2014
Geoff Waite
Agenda
TOPIC
TIME
STAFF
Historical Background to Sewer Hydraulics
09.00 - 09.30
KB, AE, HE
Modified Rational Method
09.30 - 10.30
KB, AE, HE
Micro Drainage – Basic Tank/Pond
10.30 - 11.00
KB, HE
Micro Drainage – Flow Controls
11.00 - 11.30
KB, HE
Micro Drainage – Complex Controls
11.30 - 12.00
KB, CC, AE, HE
Lunch
Micro Drainage – Other SUDs Components
12.45 - 13.45
KB, CC, AE, HE
Micro Drainage – Cascade
13.45 - 14.15
KB, CC, AE, HE
Interception Losses and Long Term
Storage
14.15 - 15.00
KB, CC, AE, HE
Break
Drainage Strategy and Deliverability
15.15 – 17.00
Close
KB, CC, AE, HE, RE
Sewer Hydraulic
Calculations
Method
Year
Lloyd Davies – Rational Method
1906
TRRL (Watkins) Method
1962
Wallingford Procedure
1981
InfoWorks
1996
Definitions
• Types of Sewerage System
• Separate Sewerage Systems
• Combined Sewerage Systems
• Partially Separate Sewerage Systems
• Ancillaries – overflows, pumping stations, attenuation
tanks, dual manhole
• Standard Technical Committee Report 25 (STC25)
Lloyd Davies – Rational
Method
• Put simply: flow = impermeable area multiplied by
rainfall intensity
• Method originally developed to size pipes rather than
to calculate flow rates
• Main difficulty is how to calculate the correct rainfall
intensity to use in the analysis
• Provides only a peak flow (not a hydrograph)
Rational Method - Assumptions
• Rainfall - uniform intensity over whole catchment
being analysed
• Pipes run full (i.e. storage, surcharge and flooding is
ignored)
• The whole area upstream contributes to runoff at
the location being analysed
Rational Method
Q = 2.78 * imp area * rainfall intensity
• 2.78 is a factor to calculate flows in litres/sec if the
impermeable area is in hectares and the rainfall
intensity is in mm/hr
• Flow is at a maximum when the whole area upstream
just starts to contribute runoff at the point of
consideration
• The time at which this occurs (which is different for
every pipe in the network) is known as the Time of
Concentration
Sewer Hydraulics Background
Sewer Hydraulics – Background (1)
• Rational Method known to over estimate flows
• Key weaknesses – uniform rainfall intensity, using the
whole catchment upstream of the point of analysis and
ignoring storage available in the pipe system
• Other weaknesses include catchment shapes, unable to
deal with surcharge and effect of any ancillaries
Be wary of carrier pipes with no
impermeable areas
Sewer Hydraulics – Background (2)
• TRRL – introduced for design of pipes on motorways
• Computer Method running on mainframe computers
• Incorporated variable rainfall intensities, area/time
diagrams and allowed for pipe storage
• Produces Hydrographs
• Shortcomings – could not analyse
surcharge/flooding and no ancillary modelling
available
Sewer Hydraulics – Background (3)
1981 – Wallingford Procedure (Four Main Volumes)
• Software – WASSP running on mainframes
• Utilised rainfall data from the 1975 Flood Studies Report and
rainfall/runoff research for urban areas
• Separate inputs (RED, SSD) and models for rainfall, urban
rainfall/runoff process, pipe analysis and ancillaries
• Pipe analysis included surcharge and flooding (although
to a limited extent)
Sewer Hydraulics – Background (4)
• Modified Rational Method is Volume 4
– utilised the findings of the Wallingford Procedure
• Rainfall data and urban rainfall/runoff models were
incorporated into the procedure
Sewer Hydraulics – Background (5)
• Circa 1990 - Wallingford software circa converted to
run on a pc and called WALLRUS
• Weaknesses - pipe analysis was limited to dendritic
systems and could not deal with reverse flows
• Intermediate software introduced circa 1994 to handle
reverse flows and backwater effects – SPIDA
• Infoworks introduced circa 1996 – fully pc based
Sewer Hydraulics – Background (6)
•
Infoworks is GIS Based information using STC25 Referencing
•
Separate databases for nodes, links and area information
•
New pipe analysis software based upon St Venant open
channel flow equations (deal with reverse flows)
•
Internationally applicable with different runoff models for
different countries/catchments
•
Dry Weather Flow (foul sewage) generator and Water Quality
analysis
Modified Rational Method
Q = 2.78 * Cv * CR * i * A
• Cv is the volumetric runoff coefficient (proportion of
rainfall which enters the pipe network)
• CR is a routing coefficient = 1.3
• i is rainfall intensity
• A is imp area
MRM – Input Parameters
1. Site Areas (Impermeable and Permeable)
2. Rainfall (M5-60, ratio and SAAR)
3. Soil Type/Soil Index
4. Time of Entry
5. Time of Flow
MRM – Derived Parameters
1. Time of Concentration = Time of Entry + Time of
Flow
2. Urban Catchment Wetness Index (UCWI)
3. Percentage Impermeable Area (PIMP)
4. Percentage Runoff (Pr)
5. Cv
MRM – Pr Equation
Pr = 0.829*PIMP + 25*SOIL + 0.078*UCWI - 20.7
• Cv = Pr / PIMP
• Limitations of the Pr equation
MRM – Rainfall Calculation
• Rainfall intensity based upon the time of concentration
• Obtain the M5- 60 and rainfall ratio values
• Calculate M5-D (where D is the time of concentration)
– using the Z1 coefficient
• Calculate MT-D rainfall depths (where T is return
period)
• Determine appropriate rainfall intensities
MRM - Spreadsheet
Example of using the Spreadsheet
Micro Drainage – Basic
Tank/Pond
• Source Control Basic Principles
• Quick Storage Estimate
• Tank/Pond
Source Control – Basic Principles (1)
• Firstly, always sketch out the layout of the
component including levels
• For any SUDs component there is:
– Inflow – principally rainfall
– One, two or three outflows
1. Infiltration
2. Flow Control (primarily controls the filling of the
component)
3. Overflow (when the storage is full)
There must be at least one outflow
but all three can be used
Typical SUDs Component
Rainfall
Overflow
SUDs
Component
Control
Component provides Storage
Infiltration
(Sides/Base)
Source Control – Basic Principles (2)
For each SUDs component we need to define:
1. Global Variables – overview of inflow, component,
controls, climate change
2. Rainfall - inflow
3. Area Time Diagram – impermeable areas
4. Details of the SUDs component itself
5. Details of Flow Controls/Overflows/Infiltration
Quick Storage Estimate
• Useful tool for quick calculation
• Provides estimated storage requirements with or
without infiltration
• Provides results for a range of infiltration components
• Useful way to input data required for analysis
Basic Tank/Pond
Freeboard
Pipe Inflow
above max
storage level
Design Fill Level
Flow Control Outflow at Base
Basic Tank/Pond
• Adam to Continue
Micro Drainage – Flow Controls
• Orifice
• HydroBrake
• Weir
• Complex Control – more than one control
• ALL flow controls use a head/discharge relationship
• Can be lower than the base of the SUDs component
Flow Control - Orifice
1.6
1.4
Head in m
1.2
1
0.8
0.6
0.4
0.2
0
0.0
10.0
20.0
30.0
40.0
Flow in litres/sec
50.0
60.0
70.0
Flow Control - Hydrobrake
Self activating vortex with air core
Type MD1 Hydrobrake 210mm diameter
Type MD12 Hydrobrake, 272mm diameter
•
•
•
1.8
1.6
1.4
Head in m
1.2
1
Orifice
0.8
Hydrobrake MD12
0.6
Hydrobrake MD1
0.4
0.2
0
0.0
10.0
20.0
30.0
40.0
Flow in litres/sec
50.0
60.0
70.0
Flow Control - Weir
Flow Control – Complex Control
• Two controls at different levels to satisfy 2 year, 30
year and 100y+cc flow rates
• Normally hydrobrake/orifice combination
Flow Control – Complex Control
•
Lower control – hydrobrake for 2 year flow
•
Higher control – normally orifice with soffit set at 2 year water
level
•
Hydrobrake and orifice pass forward 100y+cc flow when
component is full
•
Complex control builder – trial and error
•
Analyse with SUDs component and check results for 2 year, 30
year and 100 year+cc flow rates
•
Adjust as necessary (may need to adjust control and component)
Micro Drainage – SUDs
Components
1. Soakaway - Lined and House
2. Permeable Paving
3. Cellular Storage
4. Swale
5. Pipe Storage
SUDs Components – Lined Soakaway
•
•
•
Comprises circular concrete manhole rings with holes
Constructed within a square excavation filled with porous stone
Infiltration at base and sides
SUDs Components – House Soakaway
•
•
Comprise a square excavation filled with porous stone
Typically one per house
SUDs Components – Permeable Paving
Clay pavers
CBPP (Concrete Block
Permeable Paving)
Gravel
Grass paving
Permeable
Resin-bound
aggregate
Self-binding
golden gravel
Permeable
Macadam
Permeable/
No-fines
concrete
SUDs Components – Permeable Paving
•
•
•
•
Typically 600mm deep
400mm porous stone, 200mm blocks
With or without infiltration
Consider longitudinal gradient
Block Paving
Design Fill Level
Porous Stone
SUDs Components – Permeable Paving
•
•
•
•
Typically 600mm deep
400mm porous stone, 200mm blocks / laying course
With or without infiltration
Consider longitudinal gradient
Grit not sand
Course
graded
aggregate
SUDs Components – Cellular Storage
•
•
•
•
Cellular Units typically 350mm or 660mm deep
Cover required – 500mm landscaped areas and
600mm to 1m depending on traffic loads
With or without infiltration
Consider longitudinal levels
Cover to Units
Units
Design Fill Level
Sound undisturbed earth
or prepared subgrade
Coarse Sand/Gravel
SUDs Components - Swale
• Overall Depth typically 500mm
• Freeboard 150mm
• Consider Longitudinal Gradient
Freeboard
Side Slopes 1 in 4
Base Width (variable)
SUDs Components – Pipe Storage
• Generally Source Control only applicable for simple
pipe systems
• Manholes provide large storage volumes of storage
(not accounted for in Source Control)
• Consider Longitudinal Gradient
Micro Drainage - Cascade
•
Joining together one or more SUDs Components
•
Components can be linked in a chain
•
Multiple components can be connected to a single component
•
The flow control and the overflow can link to a downstream Component
•
Some components can have infiltration
•
Non upstream components need not receive runoff from rainfall
•
Relative levels between components are not considered (beware! because
Source Control only analyses levels for individual components)
•
Must be one outfall?
Cascade Example
Cascade Process
1. Design Individual SUDs components
2. Downstream Components – take account of
upstream inputs and/or rainfall
3. Link together in cascade facility
4. Analyse
5. Review results and adjust downstream components
and re-analyse
Interception Storage
•  50% of rainfall events < 5mm
• No measurable runoff from greenfield areas
• Runoff from a development takes place for virtually every
rainfall event - frequent discharges with polluted runoff
• Interception storage - prevent any runoff from rainfall depths
up to 5mm.
• Certain SuDS features such as swales and pervious pavements
provide runoff characteristics that reflect this behaviour
Long Term Storage
Vol = RD.A.10[
Where:
Vol
RD
PIMP
A
SPR


PIMP
100
(α 0.8) + 1-
PIMP
100
(β.SPR) – SPR]
= the extra runoff volume (m3) of development runoff over greenfield runoff
= rainfall depth for 100 year, 6 hour event (mm)
= Impermeable area as a percentage of total area
= area of site (ha)
= standard percentage runoff index for the soil type
= proportion of impermeable surface draining to network / receiving waterbody
= proportion of permeable surface draining to network / receiving waterbody
100 year
6 hour
Rainfall Depth
100 year 6 hour Rainfall Depth
Drainage Strategy Detail
• Pre-planning
•
•
•
Informing the masterplan - Opportunities & Constraints
Deliverability
SuDS & land take
• Planning
•
•
•
•
NPPF
Demonstrating a feasible solution
EA, LLFA / LPA and IDB requirements
Deliverability & flexibility for the detailed design
Detailed Design/Planning Conditions
• By this stage the development proposals (layout/site
levels) are finalised
• The foul and SW pipe systems can be designed
• Use Micro Drainage Simulation to analyse the SW pipe
network including controls
• Design Outputs – plans, long section, manhole
drawings (with emphasis on the flow control manhole)
• Brief report
Storage – 1 in 30 year and 1 in 100
year plus climate change
• Is the pipe system to be adopted by the water
company under a Section 24 agreement?
• If so, the pipe system must:
– Run free (no surcharge) in the 1 in 2 year event
– No flooding in the 1 in 30 year event
• Still cater for 100 year+cc flows on site
• Take account of flow control and/or restrictions
Pipe Storage/Conveyance
• Do pipes store the 30 year event?
• Flow control and/or restriction required to do this –
some flooding may result
• Pipes can still contain the 30 year event even if rates
are restricted to greenfield
• Need to consider the interaction between the pipe
network and the SUDs components (s) which requires
a Simulation model
Levels/Watercourse Interaction
• Claire
Pipe Free Conveyance Systems
• Claire