1. Weetwood Surface Water Training
10 January 2014
Geoff Waite

2. Agenda TOPIC TIME STAFF Historical Background to Sewer Hydraulics
KB, AE, HE
Modified Rational Method
Micro Drainage – Basic Tank/Pond
KB, HE
Micro Drainage – Flow Controls
Micro Drainage – Complex Controls
KB, CC, AE, HE
Lunch
Micro Drainage – Other SUDs Components
Micro Drainage – Cascade
Interception Losses and Long Term Storage
Break
Drainage Strategy and Deliverability
15.15 – 17.00
KB, CC, AE, HE, RE
Close

3. Sewer Hydraulic Calculations
Method
Year
Lloyd Davies – Rational Method
1906
TRRL (Watkins) Method
1962
Wallingford Procedure
1981
InfoWorks
1996

4. 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)

5. 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)

6. 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

7. 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

8. Sewer Hydraulics - Background

9. 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

10. 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

11. 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)

12. 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

13. Sewer Hydraulics – Background (5)
Circa 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

14. 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

15. 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

16. MRM – Input Parameters Site Areas (Impermeable and Permeable)
Rainfall (M5-60, ratio and SAAR)
Soil Type/Soil Index
Time of Entry
Time of Flow

17. MRM – Derived Parameters
Time of Concentration = Time of Entry + Time of Flow
Urban Catchment Wetness Index (UCWI)
Percentage Impermeable Area (PIMP)
Percentage Runoff (Pr) Cv

18. MRM – Pr Equation Pr = 0.829*PIMP + 25*SOIL + 0.078*UCWI - 20.7
Cv = Pr / PIMP
Limitations of the Pr equation

19. 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

20. Example of using the Spreadsheet
MRM - Spreadsheet
Example of using the Spreadsheet

21. Micro Drainage – Basic Tank/Pond
Source Control Basic Principles
Quick Storage Estimate
Tank/Pond

22. 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
Infiltration
Flow Control (primarily controls the filling of the component)
Overflow (when the storage is full)
There must be at least one outflow
but all three can be used

23. Typical SUDs Component
Rainfall
Overflow
SUDs Component
Control
Component provides Storage
Infiltration (Sides/Base)

24. Source Control – Basic Principles (2)
For each SUDs component we need to define:
Global Variables – overview of inflow, component, controls, climate change
Rainfall - inflow
Area Time Diagram – impermeable areas
Details of the SUDs component itself
Details of Flow Controls/Overflows/Infiltration

25. 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

26. Basic Tank/Pond Freeboard Pipe Inflow above max storage level
Design Fill Level
Flow Control Outflow at Base

27. Basic Tank/Pond
Adam to Continue

28. 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

29. Flow Control - Orifice

30. Flow Control - Hydrobrake
Self activating vortex with air core
Type MD1 Hydrobrake 210mm diameter
Type MD12 Hydrobrake, 272mm diameter

31. Flow Control - Weir

32. Flow Control – Complex Control
Two controls at different levels to satisfy 2 year, 30 year and 100y+cc flow rates
Normally hydrobrake/orifice combination

33. 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)

34. Micro Drainage – SUDs Components
Soakaway - Lined and House
Permeable Paving
Cellular Storage
Swale
Pipe Storage

35. 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

36. SUDs Components – House Soakaway
Comprise a square excavation filled with porous stone
Typically one per house

37. SUDs Components – Permeable Paving
CBPP (Concrete Block Permeable Paving)
Clay pavers
Gravel
Permeable Resin-bound aggregate
Grass paving
Self-binding golden gravel
Permeable/No-fines concrete
Permeable Macadam

38. 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

39. 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

40. 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

41. SUDs Components - Swale
Overall Depth typically 500mm
Freeboard 150mm
Consider Longitudinal Gradient
Freeboard
Side Slopes 1 in 4
Base Width (variable)

42. 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

43. 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?

44. Cascade Example

45. Cascade Process Design Individual SUDs components
Downstream Components – take account of upstream inputs and/or rainfall
Link together in cascade facility
Analyse
Review results and adjust downstream components and re-analyse

46. 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

47. Long Term Storage Vol = RD.A.10[ (α 0.8) + 1- (β.SPR) – SPR] PIMP PIMP
Where:
Vol = the extra runoff volume (m3) of development runoff over greenfield runoff
RD = rainfall depth for 100 year, 6 hour event (mm)
PIMP = Impermeable area as a percentage of total area
A = area of site (ha)
SPR = 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
PIMP
PIMP
100
100

48. 100 year 6 hour Rainfall Depth

49. 100 year 6 hour Rainfall Depth

50. 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

51. 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

52. 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

53. 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

54. Levels/Watercourse Interaction
Claire

55. Pipe Free Conveyance Systems
Claire