Insights from editing the ARR Urban Book. Peter J Coombes

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1 Insights from editing the ARR Urban Book Peter J Coombes

2 Motivation for revision of ARR Many practices and data in ARR 1987 are outdated More rainfall and streamflow data Improved understanding of climate and runoff processes Additional science and capability Changes in professional and community aspirations Environmental flows Climate change Water resources

3 Knowledge Gaps addressed in Revision Insufficient and incomplete rainfall and streamflow data Inadequate advice on modelling and simulation techniques for catchment systems Narrow focus on major and extreme peak floods Need guidance on a complete spectrum of flood flows Climate change Volumes Patterns and variability Recent research dominated by regional scale flooding and water quality

4 Changes since 1987 Changes in computing power and emergence of internet Transition from hand calculations (1987) to complex analysis using computers Closer alignment to actual physical processes Integration of hydrology and hydraulics (and WQ) GIS and internet inputs Multiple objectives Water cycle systems

5 Urban Book: Runoff in Urban Areas

6 Urban is different to Rural Ch. 2, 3 and 6 A large variety of land uses, impervious surfaces and connectivity urban form and continuous change Conveyance, storage and water balance infrastructure Focus on smaller scales which drives variable runoff processes New estates, local flooding, impacts on urban waterways and amenity Rainfall runoff responds to full spectrum of rainfall Regional assumptions local urban processes

7 Volume (ML) Stormwater is part of the urban water balance and cycle (Ballarat Example) Dependent on water collected from local rivers (Moorabool and Yarrowee) Reduced flows in local rivers supplemented by groundwater and importing water from dryer region Halved water demand using water efficiency and rainwater harvesting Cumulative impacts on surrounding communities Local stormwater and wastewater available 25,000 20,000 15,000 10,000 5,000 0 Ch. 2 Surface Water (local) Surface Water (imported) Stormwater Run off Ground Water Water use (metred) Wastewater discharge Calendar year

8 Water Balance (kl/household/year Quantum of the Resource Ch. 2 Average annual volumes of stormwater runoff from properties similar or greater than water use Reticulated water supply Wastewater Stormwater Similar for wastewater 150 The local water resource is greater than water extracted from surrounding catchments City

9 Relative Efficiency of Urban Catchments: Ch. 2 & 3 Urban catchments more efficient at generating stormwater runoff than natural catchments Ability to supplement regional water supplies Allows banking water in regional storages and increased environmental flows in regional rivers Increased runoff volumes are challenges for flooding

10 Full Spectrum of Runoff Ch. 3 WSUD

11 Traditional scale issues Ch. 3 Water demanded, stormwater and wastewater generated, hydrology altered at local scale Urban areas alter hydrology and water quality: Water demands (B) Within urban areas (a-h) Downstream of urban areas (C) Flow management or analysis at bottom of urban catchment (D) Does not account for changes in catchments (connectivity, density, aging infrastructure, climate issues) Water, wastewater and stormwater impacts are linked but analysed separately Parameters derived at bottom of catchment (D) unlikely to describe within catchment processes

12 Scale and Cumulative Systems: Modern reality of Integrated Solutions Ch. 3 Multiple physical and ecosystem responses in catchments Solutions at multiple scales Distributed within catchment processes and connectivity Cannot be realised by analysis at bottom of catchment (D) and engineering judgement about runoff coefficients Need to deconstruct peak flows into volumes, patterns and timing. Local connectivity, land uses and solutions change catchment responses

13 Stormwater Drainage Ch. 3 Minor/major system Minor system designed to minimise nuisance Pipes and pits provided to discharge stormwater runoff from smaller rain events Major system designed to prevent property damage and loss of life Roads and overland flow paths convey stormwater runoff from larger rain events Infrastructure designed using event based storm burst methods

14 Design Storms (1987) versus Real Rainfall (2016 +) Ch. 3 & 6 Analysis using real rainfall accounts for flow volumes and variability Computer age allows use of multiple rainfall patterns Statistical design storm bursts Rational Method was best known = scale peak runoff (no volume) Hand calculations, programmable calculators and computers (highest complexity) But most rainfall results in runoff in urban areas

15 Urban Runoff Processes Ch. 2, 3 & 6 Majority of rainfall generates urban runoff Pre-burst rainfall can generate significant runoff Changes performance of infrastructure

16 Volume Management Ch. 4 Detention Storage solutions at multiple scales Flow and volume management Retention

17 Conveyance networks and Modelling Ch. 5 & 6

18 Urban networks Ch 5 & 6 Urban stormwater networks include volume and flow management at multiple scales Local source controls Neighbourhood controls Regional controls Linked by conveyance networks Overland flows Conduit flows

19 Urban flood objectives Minor system processes Defined and managed overland flowpaths Acceptable overland flow depths and widths Major system processes Controlled overland flow paths Avoiding hazard

20 Scales and models Scales Lot (up to 1,000 m 2 ) Site (up to 1 ha) Neighbourhood (up to 10 ha) Precinct (up to 100 ha) Region Peak flow, model and DCI performance varies with scale Need models and methods that are internally consistent Models Approximate methods such as deemed to comply or peak rainfall (AS/NZS 3500) Runoff routing models with simple conduit hydraulics Rainfall runoff with linked 2D surface and conduit networks Direct rainfall on 2D grids Continuous simulation

21 Lot scale Building block of urban catchments Least research effort but complex processes Modern research reveals short flow times to street gutter is less than 2 minutes Roof, impervious and pervious area losses different to regional defaults Design using deemed to comply (AS/NZ3500) or approximate methods (Rational Method peak flows Will need more detailed methods for storages or WSUD or WQ measures

22 Site scale Complex interactions within the site. May be suitable for approximate methods Will need more detailed analysis to determine impacts on site and street storm burst patterns + preburst rainfall Volumes Detailed runoff routing Conduit and storage network

23 Sub-catchment Connectivity: Total impervious area versus directly connected impervious area Order of actions and different flow paths to inlet Lumped catchment assumptions versus catchment detail

24 New tools Ch. 6 Ensembles of rainfall and temporal patterns Storm burst rainfall Pre-burst rainfall Complete storms Based on local reality New IFDs RFFE Monte Carlo testing Continuous simulation and rainfall

25 Research results: Peak rainfall, ensembles and real rainfall Ensemble of peak bursts Ensemble of full storms Designs challenged by different patterns and volumes in bursts Higher peak discharges one failure of drainage network Full volume storms create higher peak discharges from different storms 40% failure of drainage network (30% using continuous simulation)

26 For Example New Estate near Drysdale 5.5 ha 75% impervious Discharge to detention basin and then into waterway 10% AEP

27 Regional Flood Frequency Analysis

28 Regional Flood Frequency Analysis Best estimate of natural flows Improved by use of nearby gauges

29 Rainfall Ensembles prior to basin Pre-burst rainfall 1.5 mm for 10% AEP & 60 min storm 10% AEP ensembles Note wide range of storm patterns and volumes

30 Selection of design ensemble Box and whisker plot of each duration Selected 25 minute storm with highest average and medium peak flows Variability revealed overland flow basin

31 Insights Should overcome scale and connectivity issues using internally consistent hydrology/hydraulics models Calibration at bottom of catchment does not mean that within catchment processes are captured Lumped catchment assumptions cannot capture within catchment processes

32 Insights and aspirations ARR2016 provides the industry with latest methods and data A data driven and evidence based process test and evaluate solutions Design of stormwater management based on realistic inputs and variability Urban flood frequency analysis Embrace whole of urban systems processes Account for volumes, accumulative effects, scales and variability Climate change Scales important