Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
PRESENTER:
Stephen Gribble
Stormwater 2018
Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
Stormwater 20181
INTRODUCTION01
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OLD APPROACH
> Objective was conveying peak flow;
> Rational Method the method for calculating the peak flow
> Detention only for reducing the 1:100 AEP peak flow
CURRENT APPROACH
> Multiple objectives including water quality, impact on the receiving
environment as well as peak flow conveyance
> Detention considers full range of AEPs, not just the 1:100 AEP
> New objectives require the calculation of runoff volume
IMPERVIOUS AREA
> Important parameter for estimating runoff volume
> Impervious Area values that are being used are still tied to the old
objectives & methods.
Introduction01
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Directly Connected Impervious Area (DCIA)
Impervious area that drains directly to the stormwater drainage
network via pipes and/or gutters.
DCIA includes
> Roads
> Roads
Research indicates that urban runoff quantity and quality is
correlated with directly connected impervious area rather than total
impervious area (TIA).
Types of Impervious Area01
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Indirectly Connected Impervious Area (ICIA)
> Impervious area that runs off onto pervious surfaces before it flows into the
stormwater system.
> ICIA may include the following:
▪ footpaths,
▪ Paving,
▪ garden sheds etc.
> Some or all of the runoff is lost before it is collected by the Stormwater
system. Therefore its contribution to the runoff volume is negligible in small
frequent events.
Types of Impervious Area01
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Types of Impervious Area
Effective Impervious Area (EIA)
> Effective Impervious Area (EIA) is the area that generates a rapid response in rainfall events.
> Research shows that this is usually less than the DCIA.
> DCIA could be ineffective in the following situations:
▪ Roof draining to absorption pit;
▪ Roof draining to a rainwater tank
▪ Gutter blocked by debris
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>Australian Rainfall and Runoff 2016
> ARR 2016 devotes a whole section to effective impervious area
(Book 5 Section 3.4)
> Recommends that the Effective Impervious Area (EIA) be used for
runoff estimation rather than Total Impervious Area (TIA)
> The use of TIA “can result in the overestimation of urban runoff
volumes and peak flows”
> Includes:
> An overview of research dating back to the 1970s
> Guidance for estimating EIA/DCIA
Industry Guidelines01
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Industry Guidelines
MUSIC Modelling Guidelines for NSW (2010)
> Effective impervious area rather than TIA should be used for MUSIC modeling.
> EIA/TIA factor of 0.55 is recommended, (this is at the lower end of the range estimated by
Phillips et.al).
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Research
Dayaratne (2000)
> Referenced in the DRAINS User Manual (O’Loughlin & Stack 2014).
> Estimated DCIA and ICIA by calibrating ILSAX models on several catchments in Victoria
> Derived relationships between Impervious Area and Dwelling Density
> Unfortunately DCIA decreases when these relationships are extrapolated beyond 15 houses/ha
> Not suitable for the design of the higher density subdivisions
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Research
Phillips et.al. (2014)
> One of the research projects undertaken for Australian Rainfall & Runoff 2016
> 8 catchments, one catchment in each State and Territory of Australia
> Measured DCIA & IDCIA were measured for sample areas in each catchment.
> DCIA was found to be 81% to 95% of the TIA
> EIA calculated by a regression analysis of rainfall versus runoff volume
> EIA found to be 59% to 66% of the Total Impervious Area,
> EIA found to be 70% to 85% of the DCIA
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Increasing Urban Density
> Lot Density is increasing
> New lot sizes in Sydney decreased by 24% between 2010 and 2016 (Harley ed. 2017)
> Does this change Impervious Area?
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Objectives
> Investigate the impact of increasing lot density on directly impervious area.
> To determine values of DCIA that are appropriate for the design of stormwater
infrastructure in the contemporary, higher density residential subdivisions;
> Determine relationships for estimating DCIA as a function of lot size and lot
density.
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Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
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Methodology &
Results
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Methodology
1. A total of 7 sample areas were selected:
▪ Four (4) low density residential subdivision in 4 local government areas in Western Sydney
(Blacktown, Camden, Campbelltown and Penrith)
▪ 3 Medium density areas. Two of the medium density areas were situated in 2 of the above
subdivisions while the 3rd was located in separate subdivision.
2. Impervious Areas were classified as DCIA or ICIA following the classification adopted by
Phillip (2014)
* In some medium density lots, hardstand areas are DCIA
02
Area DCIA ICIA
Residential Lots Roof, drivewayHardstand areas*, paving,
garden sheds
Road ReserveRoad carriageway, driveway,
kerb rampsFootpath, shared path
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Lots
1. Medium density lots, roof areas, driveway areas and hardstand areas could be measured
directly from the DA drawings.
2. For low density residential lots, data was collected from House and Land Package
advertisements.
• QA checking reduced the sample size from about 90 lots to 73 lots
• Roof area was estimated by ensuring garages, patios etc. were included as well as 450mm
allowance for eaves.
3. A regression analysis was undertaken to derive a relationship between lot size and DCIA.
02
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Roof Area vs Lot Area02
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0 100 200 300 400 500 600 700 800 900 1000
RO
OF
AR
EA
/LO
T A
RE
A
LOT SIZE (m2)
House & Land Packages
Intergrated Development
Phillips et.al (2014)
Linear fit
95% Confidence Interval
16 Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
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Lot DCIA vs Lot Area02
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0 100 200 300 400 500 600 700 800 900 1000
DC
IA /
LO
T A
RE
A
LOT SIZE (m2)
House & Land Packages
Intergrated Development
Phillips et.al (2014)
ICIA
ICIA Phillips et.al. (2014)
Linear Fit
Upper 95% Confidence Interval
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Lots
Equations of Best Fit:
• % Roof area = 74% - 5.4x10-4 x Lot Size (95% confidence interval +/- 3%)
• % Lot DCIA = 82% - 5.7x10-4 x Lot Size (95% confidence interval +/- 11%)
ICIA
> Highly variable, does not seem to vary with Lot size.
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Road Reserve Impervious Area
> Low residential sample areas were selected in each subdivision
> Road reserve DCIA and ICIA for a sample area in each subdivision were measured from design
CAD drawings
02
Subdivision Lot Density DCIA ICIA Total
CP 17.3 51% 14% 65%
GH 16.6 65% 11% 77%
WD 18.2 67% 10% 77%
WD_MD 37.1 68% 17% 84%
EL 16.6 69% 13% 81%
R_MD 37.6 78% 14% 92%
EL_MD 44.0 99% 0 99%
MEDIAN 68% 13%
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Combined (Lots+Roads) DCIA
> For each low density residential sample area
DCIA was calculated using the road reserve
area plus the relationship derived for Lot DCIA
> % DCIA = 50% + 0.01 x (No. of Lots/ha) 95% confidence interval: +/-9%
> The overall DCIA results were compared with
results from Phillips et.al (2014) and
Dayarante (2000)
02
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Overall DCIA (Lots + Road)02
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0
% D
CIA
DWELLINGS per HECTARE
Total DCIA
Phillips et.al. (2014)
Dayarante (2000)
Linear Fit
Upper 95% Confidence Interval
* Results from Dayarante (2000) are measured from Aerial Photos rather than published results which are based on hydrograph analysis
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Overall DCIA Table02
Code Sample Area (ha) Density (Lots/ha) Total DCIA (Lots+Road)
E 1.7 16.6 59%
GH 6.4 16.6 59%
CP 24.8 17.3 56%
W 0.9 18.2 63%
W_MD 2.0 37.1 64%
R_MD 0.2 37.6 71%
E_MD 1.2 44.0 75%
Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
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Discussion03
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> Use of Effective Impervious Area (EIA) rather than DCIA for design and analysis is a more
hydrological sound approach. It will give the more realistic estimate of runoff volume.
> the most accurate method for quantifying EIA is the statistical analysis of observed rainfall-
runoff data.
> Need a means to collect rainfall & runoff data. i.e. a gauging station with suitable length of
record (10-20 years).
> Such data is not available for new subdivisions
> Therefore this assessment focuses on DCIA.
DCIA or EIA?
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> Data shows significant variability. 95% confidence intervals of 11% and 9% for
lots and overall DCIA respectively.
> Might be tempting to adopt upper 95% confident interval
> Phillips (2014) found that EIA was 70% to 80% of the DCIA.
> 80% EIA/DCIA ratio is similar to 95% confidence interval of 11%
> Use of DCIA rather EIA is already conservative.
Variability
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> Western Sydney (i.e. No infiltration)
> Lot sizes between 120m2 and 700m2. Correlates with Philips’ et.al. (2014)
results for lots up to 1000m2
> Lot density from 16 lots/ha to 44 lots/ha. May overestimate impervious area for
smaller lot densities
Applicability of Relationships
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Conclusions04
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Conclusions
1. Directly Connected Impervious Area increases both with the decreasing lot size and
increasing lot density.
2. Relationships have been derived relating lot size and lot density to Directly Connected
Impervious Area.
3. Need to keep in mind that Directly Connected Impervious Area is more conservative than
Effective Impervious Area.
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Directly Connected Impervious Area in Residential Subdivisions in Western Sydney
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References05
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> Beard, L.R. and Shin Chang. (1979), “Urbanization impact on streamflow”,
Journal of the Hydraulics Division ASCE, Volume 105, pp.647-59.
> BMT WBM (2015), NSW MUSIC Modelling Guidelines, prepared for Greater
Sydney Local Land Services.
> Cherkaver, D.S. (1975), “The hydrologic response of small watersheds to
suburban development: Observations and modeling in urbanization and
water quality control”, in Whiple Jr. (Eds), Proceedings of the American
Water Resources Association (pp. 110-119), Herndon,VA
> Coombes, P. (2017), Personal Communication.
> Dayaratne, S.T. (2000), Modelling of Urban Stormwater Drainage Systems
Using ILSAX, PhD Thesis, School of the Built Environment, Victoria
University of Technology, Melbourne.
> Gribble, S. (2017)., “Assessing Directly Connected Impervious Areas in
Residential Subdivisions in Western Sydney,” Paper presented to 2017
Stormwater NSW Conference, Newcastle, NSW.
> Gulliver J. S., Ebrahimian, A., Wilson B. N. (2015), Determination of
Effective Impervious Area in Urban Watersheds, Minnesota Department of
Transportation, Report MN/RC 2015-41.
> Harley, R. ed. (2017), State of the Land 2017, Urban Development Institute
of Australia, Online version at www.udia.com.au, accessed 21-09-18.
> Hill, P. and Thomson, R. (2016), “Estimation of Effective Impervious Area”
Section 3.4, Book 8 in Australian Rainfall and Runoff - A Guide to Flood
Estimation, Commonwealth of Australia.
> O’Loughlin G. G. and Stack, R. (2018), DRAINS User Manual, available at:
http://www.watercom.com.au/DRAINS%20Manual.pdf, accessed
20/07/2018.”
> Phillips, B.C., Thomson, R.S., Goyen, A.G., Pathiraja, S. and Ball, J.E.
(2013), “Estimating Effective Impervious Area and its Implications for
WSUD”, Proceedings, Stormwater NSW Conference, 17-19 September
2013, Blue Mountains,
> Phillips, B., Goyen, A., Thomson, R., Pathiraja, S. and Pomeroy, L. (2014),
Australian Rainfall and Runoff Revision Project 6: Loss models for
catchment simulation - Urban Losses Stage 2 Report, February.
> Thomson, R.S, Phillips, B.C., Goyen, A.G., Pathiraja, S. and Griffin, M.
(2012), “How Connected are Urban Catchments? Estimating the Effective
Impervious Area”, Proceedings, 34th Hydrology & Water Resources
Symposium, 19 - 22 November, Sydney.
References