Using Flood Modeller for real-time

Flood Forecasting in Wales

Samantha MitchellFlood Forecasting Technical Specialist, Natural Resources Wales

Using Flood Modeller for real-time

Flood Forecasting in Wales

• NRW

• Flood Forecasting in Wales

• Mapping vs Forecasting Models

• Methods of updating

• Gauge updating unit– Reservoir – Upper Taff model

– River – Usk model

• Our Purpose

– To ensure that the environment and natural resources of Wales are

sustainably maintained, sustainably enhanced and sustainably used, now

and in the future

Natural Resources Wales

• Created in April 2013 from 3 main

organisations:

– Environment Agency Wales

– Forestry Commission Wales

– Countryside Council for Wales

– Also some Welsh government

functions

Flood Forecasting Team

• New FEWS-Wales

– Latest version of FEWS

– Cloud hosted

– 12 forecasting shell servers available

– New Forecast Web Service with

improved functionality

– Due to go live early December

NRW Flood Forecasting System

• Currently NFFS (shared with EA)

– FEWS based

– Server hosted

– 3 forecasting shell servers available

– Basic NFFS Webserver for Area duty

officers to view outputs

Flood Forecasting in

Wales – 2006

• 10 catchments across SW Wales:

• 29 Forecasting Locations

– 25 Hyades (simple unit hydrograph)

– 2 Fluvial Correlations

– 2 Estuary Correlations

Flood Forecasting in

Wales – 2016

• 23 catchments across Wales:

• 94 Forecasting Locations

– 23 Hyades (simple unit hydrograph)

– 5 Fluvial Correlations

– 3 Estuary Correlations

– 2 MCRM

– 8 DODO

– 21 PDM

– 32 Flood Modeller

Flood Forecasting in

Wales – The Future

• ~33 catchments across Wales:

• ~135 Forecasting Locations

• Model Conversions

– Hyades converted to PDM/Flood

Modeller

– Vyrnwy, Severn and Teme converted to

PDM/Flood Modeller

• New Models

– Wye – PDM/Flood Modeller

– Neath – PDM/Flood Modeller

– Mawddach/Wnion – PDMs

– Rhymney, Ely and Ebbw – PDM/Flood

Modeller

– Clwyd and Dee – PDM/Flood Modeller

• Forecasting models

– Only 1D

– Simple – just need to forecast

onset of flooding accurately

– If not much floodplain, can get

away with just routing (VPMC)

– Needs to be stable throughout

whole flow range

– Ideally needs to be calibrated

throughout whole flow range

– Run using observed and forecast

data in real-time – outputs are

often updated with observed

level/flow data to further improve

the forecast

Mapping vs Forecasting Models

• Mapping models

– Usually 1D-2D

– Complex – need to model whole

flood extent accurately

– Hydrodynamic – cross sections

and structures needed

– Only needs to be stable for

med-high flows

– Only needs to be calibrated for

med-high flows

– Often run using historical events

or estimated “design events”

Tawe Models

• Tawe S105 Mapping model

– 1D-2D

– >1300 nodes for main channel

– Many lateral FEH inflows

– Run time – ?

• Tawe Forecasting model

– 1D – VPMC routing only

– 39 nodes for main channel

– 4 main PDM inflows

– Run time approx. 5 seconds

(for a 48 hr run)

Model Updating

• State Updating

– For rainfall runoff models, adjusts the

internal stores

• ARMA Error Correction

– Can be applied to any model type, looks

at model error over last few timesteps

and applies an error correction to the

forecast

• Flood Modeller Gauge Unit

– Alters model parameters to bring

simulated results in line with observed

– Can be applied to Reservoir units or

River units

– For River units, applies a phase as well

as an amplitude correction

Gauge Updating

Upper Taff Reservoirs

• Problem:

– PDM error corrected with flow from

spillway

– Updated PDM fed back into Flood

Modeller u/s of reservoir

– Assumed reservoir was always full and

spilling

• Solution:

– Telemetered reservoir levels measured to

1 m below spill level

– Reservoir Gauge unit added for Llwynon

and Pontsticill

– Volume deficit or excess correction

– Forecasts always begin (at time now) with

the correct reservoir level

Gauge Updating

Usk Levels

• Two flow stations on main Usk –

Llandetty and Chainbridge

• River Gauge Unit for these two locations

– First simulation up to T0 to derive relevant

updating parameters and determine

additional flow

– Second simulation uses additional flow

and projects this into forecast period

– Two river sites – model updates each site

simultaneously and runs a number of

iterations

– Updates on water level – uses rating to

convert flows to levels

– Applies phase correction first, then

amplitude correction

Gauge Updating Performance

Usk – Llandetty

Before Gauge Updating After Gauge Updating

Gauge Updating Performance

Usk – Crickhowell

Before Gauge Updating After Gauge Updating

Gauge Updating

Instead of ARMA?

• ARMA can produce strange results if model has a timing difference

• Gauge Updating applies phase correction before amplitude correction so

could potentially solve this problem

• Data File

– One file per location

– Info on stage/flow, updating limits,

missing data, timeseries, rating

Gauge Updating

Configuration Files

• Gauge Control File

– One file per model

– Analysis period, number of iterations,

phase and amplitude decay, forecast

decay period

• Cons

– Relies on good gauge

observations – can be a problem

at spillways

– Can sometimes make forecasts

downstream worse

– Can’t see a true simulated output

– would need to run model again

in normal mode

– Increases run time

Gauge Updating

• Pros

– Relatively straightforward to

configure

– Improves timing and amplitude

of forecast

– Improves forecasts downstream

(mostly!)

– Can potentially solve the

unrealistic ARMA forecast

problem

Thank you