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Modelling of Rainwater Modelling of Rainwater Tanks and On-Site Tanks and On-Site Stormwater DetentionStormwater Detention
An overview of the project undertaken by An overview of the project undertaken by the University of Newcastlethe University of Newcastle
Geoffrey O’LoughlinGeoffrey O’Loughlin
Robinson GRC ConsultingRobinson GRC Consulting
In the early 1990s, the Upper Parramatta River Catchment Trust developed on-site stormwater detention (OSD) policies.
These required persons developing or re-developing sites within the Trust’s Area to provide storages on their properties to detain stormwater, to prevent any worsening of the risk of flooding.
Stormwater detentionStormwater detention
OSD regulationsOSD regulations
Outflows from sites in 100 year Outflows from sites in 100 year average recurrence interval storms average recurrence interval storms should not exceed 80 L/s/hashould not exceed 80 L/s/ha
Storages should be provided at the rate Storages should be provided at the rate of 470 mof 470 m33/ha/ha
Thus a re-development on a 600 m2 site would have to limit 100 year ARI outflows to 4.8 L/s and include a 28.2 m3 storage.
Typical OSD storagesTypical OSD storages
OSD policies, in the UPRCT Area and elsewhere, have always been controversial.
Opposition has been on grounds of cost, effectiveness and the difficulty of maintenance.
Nevertheless, OSD policies have been applied by about 65% of Sydney councils.
AcceptanceAcceptance
Rain tanksRain tanks
Recently there have been approaches to the Trust to encourage the use of rainwater tanks for the combined purposes of:
providing an alternative source of water providing an alternative source of water to householdersto householders
reducing flooding by storing stormwaterreducing flooding by storing stormwater reducing stormwater pollution by reducing stormwater pollution by
limiting the volumes of stormwater limiting the volumes of stormwater escaping from propertiesescaping from properties
To promote water sensitive urban design (WSUD), Bankstown City Council has allowed developers a 30% reduction in OSD requirements if a suitable raintank is provided.
Raintanks have a more positive image than OSD storages and are more likely to be properly maintained.
Other considerationsOther considerations
Trust actionsTrust actions
University of Newcastle researchers were University of Newcastle researchers were engaged to investigate the effectiveness of engaged to investigate the effectiveness of rainwater tanks for OSD, on individual sitesrainwater tanks for OSD, on individual sites
Geoffrey O’Loughlin was engaged as a Geoffrey O’Loughlin was engaged as a reviewerreviewer
Allan Goyen of Willing & Partners was Allan Goyen of Willing & Partners was commisisoned to investigate the cumulative commisisoned to investigate the cumulative effects of many raintank and OSD systemseffects of many raintank and OSD systems
ResultsResults
The University of Newcastle Report, by Peter Coombes, Andrew Frost and George Kuczera, can now be downloaded from the Trust’s website.
InterpretationInterpretation
The 64 page document is complex and would be difficult for a layperson to understand.
There are two main sections, dealing with:
generation of synthetic rainfallsgeneration of synthetic rainfalls modelling of a system including a modelling of a system including a
rainwater tank and an OSD storage in a rainwater tank and an OSD storage in a single site single site
Interpretation (continued)Interpretation (continued)
The study represents a major step The study represents a major step forward in stormwater and rain tank forward in stormwater and rain tank analysisanalysis
It builds on a substantial body of earlier It builds on a substantial body of earlier researchresearch
The results are tinged with uncertainty, The results are tinged with uncertainty, due to the lack of sufficient rainfall datadue to the lack of sufficient rainfall data
Rainfall dataRainfall data
Both rain tank and OSD systems store rainwater. To model these, the Newcastle researchers have set up a simulation model, that determines the state of the system and the inputs and outputs at a number of time steps.
This is like a financial accounting model, with a daily or monthly time step.
Pay
BankAccount
Expenditures
RainTank
Water Use
RainwaterOverflow
Time stepTime step
The time step is critical. There is plenty of daily rainfall data available, so it would be desirable to use a daily time step.
Unfortunately, this would be invalid, since most storms last for less than one day. An OSD tank could fill and empty more than once in a single day.
A much shorter time step is needed.
Available pluviograph dataAvailable pluviograph data
To obtain rainfall data with short time steps, we need to go to data collected by pluviometers operated by the Commonwealth Bureau of Meteorology and Sydney Water.
There are about 50 of these in Sydney, with the main site being at Observatory Hill, Sydney.
Rainfall generationRainfall generation
Due to the lack of suitable data, the University of Newcastle researchers generated synthetic rainfall sequences using a process named the Disaggregated Rectangular Intensity Pulse (DRIP) method, which uses pluviometer and daily rainfall records as a basis.
After many analyses, the Sydney Water record at Ryde Pumping Station was selected as a base.
Trust Area
Ryde P.S.
Observatory Hill
Accuracy of rainfall synthesisAccuracy of rainfall synthesis
1000 sequences of rainfalls were generated in tests, usually for about 50 years, in 2 minute time steps. Statistics were then calculated. Results were compared with: Statistics of observed rainfalls for various Statistics of observed rainfalls for various
durations and frequenciesdurations and frequencies Estimates of rainfall intensities from the Estimates of rainfall intensities from the
standard design manual standard design manual Australian Rainfall Australian Rainfall and Runoffand Runoff..
1
10
100
.01 .1 1 5 10 20 30 50 70 80 90 95 99 99.9 99.99
Ryde AR&R IFDs vs DRIP
0.50hrs Obs1.00hrs Obs3.00hrs Obs12.00hrs Obs24.00hrs ObsMedian DRIP SimulationAR&R Estimate
Inte
nsity
(m
m/h
r)
Percent
200yrs100yrs
50yrs
20yrs10yrs
5yrs
500yrs
2yrs
1yr = ARI
Ryde P.S. Results
Comparison of Ryde P.S. and Parramatta Rainfall Curves
Conclusions of the rainfall studyConclusions of the rainfall study
No suitable base pluviograph exists within the Trust AreaNo suitable base pluviograph exists within the Trust Area The Observatory Hill record matches Parramatta data The Observatory Hill record matches Parramatta data
poorly poorly The 53 year Ryde Pumping Station record has similar The 53 year Ryde Pumping Station record has similar
statistics to Parramatta and is the best base recordstatistics to Parramatta and is the best base record Australian Rainfall and RunoffAustralian Rainfall and Runoff appears to contain errors appears to contain errors
Recommended generation Recommended generation processprocess
It was recommended that the Trust should use Ryde P.S. as a base and transfer information from this to various parts of the Trust’s Area by adjusting the base numbers or parameters used in the DRIP Model.
1. Calibrate DRIP Parameters
2. Calibrate scaling parameters
Master Site
Long Pluvio
Target Site
Short Pluvio or
Long Daily
Rain tank modellingRain tank modelling
Modelling involved a complex system including a rain tank that overflowed into an OSD tank.
Simulations were performed at 2 minute time steps using a 1000 year synthetic rainfall record.
Allotment water balance model
Model componentsModel components
Synthetic rainfall dataSynthetic rainfall data Roof catchment and first flush deviceRoof catchment and first flush device Rain tank characteristics - 5 kL to 15 kLRain tank characteristics - 5 kL to 15 kL Outdoor Water UseOutdoor Water Use Indoor Water UseIndoor Water Use Pervious and impervious catchment Pervious and impervious catchment
areas on siteareas on site OSD tank characteristicsOSD tank characteristics Infiltration trench characteristicsInfiltration trench characteristics
Allotment typesAllotment types
Single dwellingSingle dwelling
Duplex developmentDuplex development
Allotment typesAllotment types
Townhouse Townhouse developmentdevelopment
three-storey, walk-three-storey, walk-up apartmentsup apartments
Results for single dwellingResults for single dwelling
Surface discharge from the siteSurface discharge from the site
Implications for OSDImplications for OSD
As part of the studies, the numbers of times that OSD storages would fail by overflowing during a 1000 year period was assessed. It was found that using the Trust’s storage requirement of 470 m3/ha, failures occurred more frequently than the required level of once in 100 years.
However, this assessment of the required storage was heavily dependent on model assumptions such as the time step.
ResultsResults
Allotmentscenario
Imperv-ious
fraction(%)
Totalarea(m2)
UPRCTOSD
storage(m3)
PSDL/s
ARI atwhichPSD is
exceeded
OSD storagefor PSD to
be exceededat 100 yearARI (m3)
Single dwelling 58 600 28.2 4.8 63 55Duplex 83 600 28.2 4.8 12 67
Townhouse 75 1858 87.3 14.9 22 165Apartment 67 1200 56.3 9.6 15 119
CommentsComments
These results reflect a number of factors:
how much stormwater goes to the rain tank and how how much stormwater goes to the rain tank and how much to the OSD storage, depending on areas of roof, much to the OSD storage, depending on areas of roof, impervious and pervious areasimpervious and pervious areas
how much water is drawn from the tank for water supply how much water is drawn from the tank for water supply (no. of persons)(no. of persons)
sizes of storagessizes of storages
Amount of raintank storage that Amount of raintank storage that can be counted as OSDcan be counted as OSD
Volume of rainwater tankcounting as OSD storage (%)
Scenario No airspace intank
50% airspacein tank
Allotment 42 65Duplex 50 72Townhouses 40 53Walk upapartments
32 51
The Bottom LineThe Bottom Line
It appears that about 40% of the volume of a rain tank can be credited to OSD, and if extra storage (airspace) is added, this increases to 60%.
For a 600 m2 site, requiring 28.2 m3 of OSD storage under Trust requirements, a 10 m3 rain tank would allow the storage to be reduced from 28 to 24 m3.
ConclusionsConclusions
There is still a way to go, with research by Allan Goyen still to be completed. However, it appears that the deduction on OSD storages is not as great as has been hoped, but is still significant.
The analysis that has produced this result, despite some areas of vagueness, is a landmark study, and will be highly influential in future design processes for stormwater management systems.