© nerc all rights reserved groundwater flooding flood1 – interreg iiia project (bgs, bgrm,...

© NERC All rights reserved

Groundwater floodingFLOOD1 – Interreg IIIA project (BGS, BGRM, Brighton Uni.)Aim to understand processes leading to groundwater flooding in Chalk and

produce predictive models for Somme and Brighton

Somme valley at Abbeville, April 2001(Ministère de l’Ecologie et du Développement Durable)

Flooding of the Patcham area, Brighton November-December 2000

( John Ellis, EA)

www.flood1.info


Outline

• What specific questions are we trying to answer using models?

• How do we represent the processes of interest?

• With current commonly applied techniques, what can we do well, and where is there room for improvement?


Groundwater flooding

• Definition:

”A groundwater flood event results from a rise in groundwater level sufficient for the water table to intersect the ground surface and inundate low lying areas”

Flooding from other sources (HA4a), 2006, jba Consulting


What do we want models to do?

• Explain – why and how

• Predict – where and when

Constraint – need a methodology that can be applied in other regions


Processes of interest

• Response of water table to rainfall

• Effect of “micro” geology on water table


What have we tried

• Using DTM and water table contours

• Distributed recharge model

• Regional groundwater model with simplified geology

• Unsaturated zone model


What have we tried






Average recharge (1990-2003)

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65

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85

1990

1991

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2001

2002

Gro

un

dw

ater

lev

el (

m a

OD

)

0

5

10

15

20

Mean

mo

nth

ly recharg

e (mm

/day)

Pyecombe Old Rec Mean monthly recharge (mm/day)


What have we tried






Average groundwater levels


Transmissivity distribution


Modelled water levels


Difference between model and observed water levels


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BRIGHTON PRESTON PARK

Model


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BRIGHTON ST PETERS CHURCH

Model


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LOWER STANDEAN Model


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NORTH BOTTOM Model


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FAULKNERS BOTTOM Model


What have we tried





Data


Envirosmart Probe

0

5

10

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30-Jul 11-Aug 23-Aug 04-Sep 16-Sep 28-Sep 10-Oct 22-Oct 03-Nov 15-Nov 27-Nov 09-Dec 21-Dec 02-Jan 14-Jan 26-Jan

Date

Wat

er c

on

ten

t

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1

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Rai

nfa

ll (m

m)

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0.6 m

0.8 m

1.0 m

1.2 m

1.5 m

Rainfall


What have we tried





Data

Boundary conditions


The way forward for prediction

• Analysis of hydrographs


The way forward for prediction

• Analysis of hydrographs

Interested in GWL maxima

Water level related to rainfall

Want to predict how high water level may rise during next recharge season


Predicting groundwater floods1. Calibrate and validate model using historic GWL dataMultiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm)


2. Predictive modelUse Monte Carlo simulation to predict a range of annual maximum groundwater levels, based on a range of possible winter rainfall scenarios, up to nine months ahead

1. Calibrate and validate model using historic GWL dataMultiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm)

Predicting groundwater floods


1. Calibrate and validate model using historic GWL dataMultiple linear regression model where annual maximum groundwater level (m OD) is a function of previous annual minimum and winter rainfall (mm)

Predicting groundwater floods

2. Predictive modelUse Monte Carlo simulation to predict a range of annual maximum groundwater levels, based on a range of possible winter rainfall scenarios, up to nine months ahead

Frequency Chart

(m OD)

.000

.006

.011

.017

.022

0

138.2

276.5

414.7

553

9.30 10.33 11.37 12.40 13.43

25,000 Trials 24,733 Displayed

Forecast: Predicted Max (m OD)

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