# Weetwood Surface Water Training

## Presentation on theme: "Weetwood Surface Water Training"— Presentation transcript:

Weetwood Surface Water Training
10 January 2014 Geoff Waite

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

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 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 Site Areas (Impermeable and Permeable)
Rainfall (M5-60, ratio and SAAR) Soil Type/Soil Index Time of Entry Time of Flow

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

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

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 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

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: 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

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

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

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

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
Soakaway - Lined and House Permeable Paving Cellular Storage Swale 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
CBPP (Concrete Block Permeable Paving) Clay pavers Gravel Permeable Resin-bound aggregate Grass paving Self-binding golden gravel Permeable/No-fines concrete Permeable Macadam

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

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?

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

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[ (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

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