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Southwell Flood Modelling & Feasibility Study Integrated Catchment Modelling using ISIS-TUFLOW-ESTRY Richard James

11th November 2015

Presentation Overview

1. Background

2. Project Scope

3. Known Issues

4. Modelling Approach

5. Challenges

6. Limitations and Recommendations

Southwell Flood Modelling

Project Context

• Southwell is a town in east Nottinghamshire

with a history of flooding from ordinary

watercourses, surface water and sewers

• Southwell experienced severe flooding on 23

July 2013.

• Nottinghamshire County Council (NCC) is the

LLFA responsible for managing local flood risk

sources in Southwell.


Project Scope

Construction of a flood model considering key

sources of flooding. The model had to:

• Be compatible with existing Environment

Agency modelling and data.

• Match known flood mechanisms based on

available hydrometric data and anecdotal

evidence.

• Be suitable for strategic options appraisal.



Known Issues

23rd July 2013 Flood Event

• 93mm of rainfall fell in just 75 minutes.

• Rapid onset of flooding with little warning for

residents.

• Approximately 250 properties subject to

internal flooding.

• Significant volume of qualitative data collated

post-event but limited quantitative data.


The 23rd July 2013 storm was described by the MET Office as:

“At the very least, one of the largest hourly rainfall events in the UK”


Modelling Approach

• Conventional methods of design flow estimation were considered unsuitable.

• Client requirement to understand surface water flood mechanisms.

• Discrete surface water and fluvial models were considered but were not

thought to be representative of the key flooding mechanisms in isolation.

• An integrated catchment approach created one holistic model.

• Integrated modelling allowed for appraisal of options on a number of sources

simultaneously.

Integrated Catchment Approach: ISIS-TUFLOW-ESTRY


Key Feature Software Description

Boundary Conditions ISIS / TUFLOW

RF grid used to apply design rainfall to the 2D TUFLOW DTM. Catchment response

moderated by runoff coefficients, infiltration losses and Manning’s ‘n’ roughness values.

ReFH baseflows equations used in the ISIS model.

River Channels ISIS / TUFLOW

River channels in detailed study area represented using conventional ISIS-TUFLOW

techniques. HX lines used to control the passage of surface water between ISIS and TUFLOW.

2D DTM used to route surface water flows in the upper catchment towards the detailed river

reach. 2D SX lines capture flow and link to the ISIS model

Floodplain TUFLOW Two TUFLOW model domains were created. A 5m2 cell size was selected for the urban core

and a 10m2 cell size for the rural catchment.

Surface Water Network ESTRY

Severn Trent Water sewer model used to refine the representation of sewers, manholes and

gullies in the ESTRY sewer model. A significant culverted reach of the Halam Hill watercourse

was also included in the ESTRY model.

ESTRY model linked to the ISIS river model via X1DQ and X1DH links. Manhole/ gully pits

used to control the movement of flow with the 2D domain.

Combined Sewers TUFLOW Assumed infiltration loss off 11mm/hr applied to areas served by combined sewers.


ISIS Model


TUFLOW Model


ESTRY Model


23rd July 2013 flood

Model Verification


Positive Outcomes

• A holistic representation of flood risk sources.

• Well suited to the urban environment where water is

exchanged between the floodplain, watercourses and

sewers.

• Reproduced observed flooding unlike fluvial models

informed by standardised hydrological techniques.

• The model helped the LLFA understand local sources of

flooding and meet its objectives as a Risk Management

Authority.


Challenges

• Model performance and stability.

• Uncertainty – particularly with design flow estimation.

• Few industry standards for integrated approaches.

• Verification predominantly based on qualitative data.

• Communication of flood risk to local stakeholders and

decision-makers.


Limitations & Recommendations

• Most applicable for small, flashy catchments where

baseflow or variable catchment wetness not a factor in

catchment response.

• Well suited for catchments where conventional techniques

of design flow estimation are not applicable.

• Significant verification and analysis required to have

confidence in design flow estimates.

• Long model run times – can have significant implications

for programme when options appraisal is required.


Questions?


Southwell Flood Study and modelling reports are available online at:

http://www.nottinghamshire.gov.uk/planning-and-environment/flooding/the-councils-role

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