the importance of pits in overland flow modelling · 2018. 10. 23. · 1. stormwater networks are...

The Importance of Pits in Overland Flow Modelling

PRESENTERS:

David Whyte

David Stone

October 2018

1 Title of Presentation goes here

01 Introduction

02 Pit Types and Data

03 Overland Flow Modelling

04Case Study and sensitivity of pit

representation

05

06

Outcomes

Conclusion

Overview


Acknowledgements

> Inner West Council – Ryan Hawken and James Ogg

> Cardno – Bala Kilaparty and Abhishek Gupta


WHAT’S THAT FLOW01


What is urban overland flow?

> 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

01

Urban overland flow is runoff from a rainfall event that is flowing across the urban area in either a controlled or uncontrolled manner.

Urban Overland FlowWhat doesn’t fit

in the stormwater

network

5 How Important are Pits in

Overland Flow Modelling?

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?

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.

01

Source: Marrickville Valley Flood Study (WMAWater, 2011)



Overland Flow01


PIT TYPES AND PIT

DATA

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

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.

02


OVERLAND FLOW

MODELLING

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

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.

03


CASE STUDY04



Marrickville Valley Floodplain Risk Management Study

> Marrickville – Inner West Council, Sydney

04

Source: Marrickville Valley Flood Study (WMAWater, 2011)



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. using unlimited capacity pits;

iii. pipe blockage as a surrogate for pit blockage

iv. impacts of pit blockage

> Other considerations:

i. using pit rating curves for actual pit types (if data available)

ii. model grid size; and

iii. special model features such as road cross fall tools.

04



2 year and 100 year flow depths04



Inlet Rating versus Unlimited Capacity Pits04



Inlet Rating versus Inflows at Pit Inverts and Pipe Blockage04



Inlet Rating versus 50% Pit Blockage04



Pipe Capacity Assessment04

Inflow applied at pit inverts and pipe blockage FRMSP – pit inlet rating curves



Pipe Capacity Assessment – Hydraulic Controls04


OUTCOMES AND

CONCLUSIONS

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

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.

05

Thank you


Thank you

For more information

David Whyte

Manager – Water Engineering

Principal – Hydrology and Civil

Engineering

Office: +61 2 9024 7034

www.cardno.com

the importance of pits in overland flow modelling · 2018. 10. 23. · 1. stormwater networks are...

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