The Importance of Pits in Overland Flow Modelling

Transcription

1 The Importance of Pits in Overland Flow Modelling PRESENTERS: David Whyte David Stone October 2018

2 Overview 01 Introduction 02 Pit Types and Data 03 Overland Flow Modelling Case Study and sensitivity of pit representation Outcomes Conclusion 1 Title of Presentation goes here

3 Acknowledgements > Inner West Council Ryan Hawken and James Ogg > Cardno Bala Kilaparty and Abhishek Gupta 2 Title of Presentation goes here

4 01 WHAT S THAT FLOW 3 Title of Presentation goes here

5 01 What is urban overland flow? Urban overland flow is runoff from a rainfall event that is flowing across the urban area in either a controlled or uncontrolled manner. > Shallow flow depths but can result in large ponding depth > Urban stormwater drainage networks: Minor System pit and pipe network for events up to 5% AEP (20 year ARI) Major System large stormwater channels and overland flow paths canals, roads, swales, large trunk systems > Most new design standards aim for a 10 year ARI or 20 year ARI minor system capacity > Legacy systems typically have a 5 year ARI or less variable throughout the network What doesn t fit in the stormwater network Urban Overland Flow 4 Title of Presentation goes here

6 01 Overland Flow Flood Studies > Overland flow studies in urban areas help to define: Flood levels and velocities flood risk and potential damages identify areas where risk mitigation measures are desired to manage or reduce the risk. > The definition of flood levels influences Flood planning levels for development Average Annual Damages estimates Council decisions on upgrades to the stormwater drainage systems. BUT But how accurate are these flood levels? AND How dependent are they on the accurate representation of pits? 5 How Important are Pits in

7 01 Pit and Pipe Networks > Urban catchments often include a significant number of pits and pipes as part of the stormwater network. Marrickville Valley 2200 pits and over 2000 pipes. 6 How Important are Pits in Source: Marrickville Valley Flood Study (WMAWater, 2011)

8 01 Overland Flow 7 How Important are Pits in

9 02 PIT TYPES AND PIT DATA 8 Title of Presentation goes here

10 02 A Pit is a Pit is a Pit? Many Different Pit Types > Grated pit varying sizes > Side entry pits/kerb inlets > Pits for different kerb types > Raised grates > Letterbox pits > Table drain pits > Depressed sump pits > Blind pits/junctions pits > Special/Custom Pits - On grade or at a sag 9 Title of Presentation goes here

11 02 Pit Types and Data > Various different pit types and dimensions across geographic regions or owning organisations Councils Road Authorities Different in different states / territories > Pit inlet capacities affected by: Dimensions pit / grate / side inlet / lintel Road or channel longitudinal grade Road cross fall Grate openings / bar design Blockage > Network capacities affected by: Pipe dimensions and blockage Pit inlet capacities, blockage and pit losses Not knowing any of the above can lead to inaccuracies and/or assumptions. 10 How Important are Pits in

12 03 OVERLAND FLOW MODELLING 11 Title of Presentation goes here

13 03 Overland Flow Modelling > 1D models - DRAINS, 12d, xp-swmm Good pit and pipe representation difficult to represent the overland flow paths accurately. > Two-dimensional models (TUFLOW, SOBEK, xp-2d, InfoWorks) simulate the overland flow paths well, but haven t always have the best one-dimensional network representation. When do we use each type: > Design 1d modelling to check minor system capacity or if simple flow > Flood assessment 2d for overland flows and feasibility 12 How Important are Pits in

14 03 Overland Flow Modelling > Advances in technology: more accurate representation of pits, and coupled with improved processing speeds smaller grid sizes a more complex and representative model set-up can be achieved. > Model Considerations: Available data on pit types and how to represent in model, pit inlet rating curves and pit losses, model grid size. 13 How Important are Pits in

15 04 CASE STUDY 14 Title of Presentation goes here

16 04 Marrickville Valley Floodplain Risk Management Study > Marrickville Inner West Council, Sydney 15 How Important are Pits in Source: Marrickville Valley Flood Study (WMAWater, 2011)

17 04 Marrickville Valley Floodplain Risk Management Study > Used this case study to examine the effects of the above considerations and their relative importance for determining flood risk and their influence on decision making with risk mitigation measures. > Different model parameters related to pit (and pipe) representation were examined to assess their impacts on model results: i. using pit inlet rating curves for a common pit type; ii. iii. iv. using unlimited capacity pits; pipe blockage as a surrogate for pit blockage impacts of pit blockage > Other considerations: i. using pit rating curves for actual pit types (if data available) ii. iii. model grid size; and special model features such as road cross fall tools. 16 How Important are Pits in

18 04 2 year and 100 year flow depths 17 How Important are Pits in

19 04 Inlet Rating versus Unlimited Capacity Pits 18 How Important are Pits in

20 04 Inlet Rating versus Inflows at Pit Inverts and Pipe Blockage 19 How Important are Pits in

21 04 Inlet Rating versus 50% Pit Blockage 20 How Important are Pits in

22 04 Pipe Capacity Assessment Inflow applied at pit inverts and pipe blockage FRMSP pit inlet rating curves 21 How Important are Pits in

23 04 Pipe Capacity Assessment Hydraulic Controls 22 How Important are Pits in

24 05 OUTCOMES AND CONCLUSIONS 23 Title of Presentation goes here

25 05 Summary of Outcomes 1. Stormwater Networks are complex 2. The representation of pits affects the amount of water that can enter the underground system and how much remains as overland flow 3. Older methods of representing pits with unlimited capacity tends to overestimate the amount of water entering the pipe system. Influences overland flow in different areas Does not identify deficiencies in pit inlet capacity to get stormwater into the pipe system. 4. Good data on pit types allows more accurate representation of the actual system performance 5. System capacity assessment can identify deficiencies as being due to insufficient pit capacity, undersized pipes or downstream influences. 6. Smaller model grid size and road cross-fall tools more accurate representation of flow depth at the pit and inlet flows 7. Blockage sensitivity is important 24 How Important are Pits in

26 05 So How Important are Pits? > The representation of pits can have a significant impact on the results of overland flow modelling > Highlights the importance in determining the best pit representation to match the quality of pit data available. > How important pits are depends on: the existing capacity of the stormwater system and influence on flood levels Catchment characteristics and features > The better the pit and pipe data, the more representative the model will be flood risk can be better approximated by more closely matching the system performance, more realistic surface flow rates and flood levels > A better model with more accurate flood levels leads to: improved diagnosis of system capacity issues and identification and assessment of suitable mitigation measures. > Therefore affects assessment of risk; and development of appropriate mitigation solutions. 25 How Important are Pits in

27 Thank you For more information David Whyte Manager Water Engineering Principal Hydrology and Civil Engineering Office: Title of Presentation goes here