Directly Connected Impervious Area in Residential ... · Impervious area that drains directly to the stormwater drainage network via pipes and/or gutters. DCIA includes > Roads

Directly Connected Impervious Area in Residential Subdivisions in Western Sydney

PRESENTER:

Stephen Gribble

Stormwater 2018


Stormwater 20181

INTRODUCTION01

2 Directly Connected Impervious Area in Residential Subdivisions in Western Sydney

Stormwater 2018

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


Stormwater 2018

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


Stormwater 2018

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


Stormwater 2018

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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Stormwater 2018

>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


Stormwater 2018

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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Stormwater 2018

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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Stormwater 2018

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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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


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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.

02


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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%


Stormwater 201822

Discussion03


Stormwater 2018

> 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?


Stormwater 2018

> 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


Stormwater 201826

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.

04


Stormwater 201828

References05


Stormwater 2018

> 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

http://www.udia.com.au

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Directly Connected Impervious Area in Residential ... · Impervious area that drains directly to the stormwater drainage network via pipes and/or gutters. DCIA includes > Roads