2D Flood Modelling in Urban Areas

Australian Rainfall and Runoff A guide to runoff estimation

2D Flood Modelling in Urban Areas

WRL Research Supporting

Project 15, Chapter 5


2D Numerical Models

A FPM standard?


Reality


The Issues

Is there a preferred method(s) to represent buildings in a 2D Numerical

Model?

Do the houses need to be physically in the model topography?

Does the volume of flow inside a building matter?


Syme (2008)


Upstream Walls


Downstream Walls


Syme (2008)


Research Objectives

To develop a detailed base dataset of flood behaviour information (flow levels and depth, flow

distributions and flow velocities) for an urban

floodplain; and

To test various methods for representing buildings in numerical models with the aim of determining a

preferred method(s) for representing buildings in a

2D numerical model.


Literature Review

Plenty of water level datasets BUT no flow gauging between buildings in a real

flood

Some physical model data sets, but mostly hypothetical cases i.e. no calibration

! Build our own calibrated physical model!


Pasher Bulker Storm

June 2007

1500+ Floodmarks

Flash Flooding

Numerous Buildings flood affected


Merewether Site

Small urban catchment

Fierce flows

Structural damage

No building collapse


Merewether Catchment

Area 84 Ha


Merewether Site


Rainfall Data


Topography

High Quality Photogrammetry

: +/- 0.1m vertical


Peak Water Levels


Physical Model Build


Physical Model Build

30:1 horizontal

9:1 vertical

12.5 m long

5 m wide

59 Houses


Physical Model Calibration

Flood flow estimated by:

Friction slope and Flow area

Hydrological model

Validated catchment wide

Contributing catchment at site:

Peak Flow = 19.7 m3/s

0.5% - 1% AEP c.f. Flood Study


Peak Water Levels

6 high quality peak flood levels in model domain

Matched in various locations down the model

Matched to within 100mm prototype (9 mm model)


Physical Model Parameters

Topography

Building form loss

Bed roughness

Impo

rtan

ce

Model Parameter


Measured Flow Velocities


Measured Flow Distributions


Flow Analysis


Supercritical Flow


Flow Volume in Buildings

Volume in Hydrograph

124,015 m3


Flow Volume in Buildings

Volume in all buildings

2,790 m3

= 2% of hydrograph volume

Total Volume in Houses passes

onto floodplain every 2 minutes


Numerical Modelling


Numerical Calibration

1m grid

Levels

Point ID

Recorded Flood

Elevation

1m Buildings as

Obstacles diff

MWR_0042 23.36 23.57 0.21

MWR_0043 23.14 23.10 -0.04

MWR_0044 23.01 22.79 -0.22

MWR_0031 19.98 20.09 0.11

MWR_0032 18.38 18.34 -0.04

MWR_0033 18.65 18.67 0.02


Velocities - Control Section


Velocities


Flow Directions?


Numerical Calibration

1m grid


Sensitivity to Grid Resolution

2 m grid

Point ID 2m Buildings as Obstacles Drift MWR_0042 23.58 0.01

MWR_0043 23.11 0.01

MWR_0044 22.77 -0.02

MWR_0031 20.09 0.00

MWR_0032 18.20 -0.14

MWR_0033 18.67 0.00



5 m grid


MWR_0043 23.14 0.10

MWR_0044 23.01 0.16

MWR_0031 19.98 0.13

MWR_0032 18.38 -0.25

MWR_0033 18.65 -0.01



10 m grid


MWR_0043 23.13 0.03

MWR_0044 22.85 0.06

MWR_0031 20.30 0.21

MWR_0032 18.67 0.33

MWR_0033 19.46 0.79


Buildings as Roughness

1m grid

Point ID

1m Buildings

as Roughness Drift

MWR_0042 23.57 0.00 MWR_0043 23.10 0.00 MWR_0044 22.77 -0.02 MWR_0031 19.91 -0.18 MWR_0032 18.28 -0.06 MWR_0033 18.69 0.02


Buildings as Roughness

1m grid


1m grid

Building Walls U/S Only

Point ID 1m Buildings

as US Walls Drift

MWR_0042 23.56 -0.01

MWR_0043 23.10 0.00

MWR_0044 22.79 0.00 MWR_0031 20.07 -0.02

MWR_0032 18.33 -0.01

MWR_0033 18.70 0.03


1m grid

Building Walls U/S Only


Building Walls Downstream Side Only

1m grid

Point ID 1m Buildings as DS Walls Drift

MWR_0042 23.57 0.00

MWR_0043 23.10 0.00

MWR_0044 22.81 0.02

MWR_0031 20.08 -0.01

MWR_0032 18.37 0.03

MWR_0033 18.68 0.01


1m grid

Building Walls Downstream Side Only


90% Porous Buildings

1m grid


Form Loss Buildings

0.5 Energy Loss (v2/2g)

1m grid


Flood Hazard

1m grid


Flood Hazard

2m grid


Flood Hazard

5m grid


Flood Hazard

10m grid


Conclusions

Applied correctly - 2D Numerical models are not too bad! Model interpretation is still required

Model Spatial resolution is important for flood velocities and hazard definition

Building representation method is important to match flow patterns

Buildings need to deflect flow in numerical models if flow is deflected in prototype

Flow volume in buildings is a secondary consideration


Acknowledgements

DCCEE for funding

UNSW WRL for funding

Newcastle City Council for data

2D Flood Modelling in Urban Areas

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