Reservoir Flood MappingBackground and progress

James Cheetham

Senior Analyst, JBA Consulting

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Overview

• What is RFM and what are the drivers for it?

• Background to the study

• Delivering the Reservoir Flood Mapping project (RFM)

• Improvements incorporated into the new RFM Specification

• Project deliverables

• Conclusions

Reservoir Flood Mapping

“Reservoir flood maps are used to inform people about areas at risk of flooding in the event of a dam or reservoir failure and sudden uncontrolled escape of water.” RFM Specification 2016

• Multiple formats

• Multiple uses

• Multiple applications

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Teton Dam, USA. Wikipedia

Drivers for RFM

• Flood Risk Regulations require a review of 2009/10 reservoir flood hazard mapping

• Changing requirements• Updated legislation requires risk categorisation of reservoirs

• Use by reservoir owners, undertakers and general public to assess risk

• Spatial planning purposes to identify appropriate development or mitigation downstream from a dam

• Not constrained to the listed uses

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Background to the RFM Project

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2008 2010 2014

2015

2016

2017

2018

2019

NRIM+NRIM

NRIM+

Lessons learned

exercise

RFM Tasks

1, 2, 3 & 4

All ~2,000 statutory

reservoirs in England

and Wales using the

2009 RIM spec

NRIM+ was a follow-on

commission for 225

reservoirs in England only –

newly constructed or

missed out of NRIM – also

uses 2009 RIM spec.

• Task 1 – Research in to breach model, reservoir data and 2-D routing model

• Task 2 – Development of an updated RFM specification

• Task 3 – Reservoir data collection exercise

• Task 4 – Reservoir Flood Mapping for large raised reservoirs in England

NRIM = National Reservoir Inundation Mapping

Reservoir Flood Mapping - Team

• Consortium of three consultants

• JBA Consulting• Continuity from NRIM+ and RFM Tasks 1, 2 & 3.

• Experience of large-scale GPU modelling

• NRIM involvement

• Mott MacDonald• Extensive in-house Reservoir Engineer expertise

• NRIM involvement

• BMT• TUFLOW development

• NRIM experience

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Reservoir Flood Mapping - Hardware

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• The Grid

• Massive Graphics Processing Unit (GPU) capacity

• Economics of scale

• Secure file access & sharing in a common data location

• Advanced scheduling

• Custom built

Reservoir Flood Mapping – Software

• TUFLOW HPC

• Custom TUFLOW development to deliver the RFM project

• Satisfies Environment Agency 2D hydraulic model benchmarking criteria

• Implements full Shallow Water Equations (SWE)

• Utilises GPU technology – vastly reduced simulation time

• Custom automation tools

• Model setup undertaken in ArcGIS

• Minimises manual interaction and increase standardisation

• Increases efficiency by speeding up repeated process

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RFM compared to NRIM

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Category NRIM RFM Improvements

Use of Reservoir

Flood Maps

Single use Multiple uses Expanded range of applications

Modelled

scenarios

Wet day Dry day, fluvial & wet day Far more comprehensive assessment.

3x as many simulations

Digital Terrain

Model (DTM)

5-20m cell size 2m cell size Significantly improved quality of outputs

~6x longer compute time

~6x larger output grids

Reservoir water

level

0.1 or 0.5m

increment

Accounts for catchment &

reservoir characteristics

Far more accurate estimations

Additional analysis effort

Impact

assessment

Likely Loss Of

Life (LLOL)

Detailed assessment Extended range of application of outputs

Additional analysis effort

Model quality SWE on Phase

2 reservoirs

only

SWE on all reservoirs Improved and consistent quality of outputs for

all reservoirs.

Additional effort

Transport

embankments &

large culverts

No specific

treatment

Flood backup and

subsequent breach of

TEs simulated

Improved quality of outputs

Additional analysis effort

Improved RFM Project Outputs – Spatial resolution & representation of hydraulic structures

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• Improved model spatial resolution

• (RFM 2m, NRIM 5-20m)

• Finer topographic detail

• Results in improved flow route definition

• Improved representation of hydraulic structures

• Greater precision in DTM cuts

• Inclusion of large culverts with representative losses and capacity

• Representation of transport embankment failure

Improved RFM Project Outputs – Representation of transport embankments

• New to RFM

• Road embankment attenuating flow

• Upstream pooling identified

• Profile shows impact of embankment

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Improved RFM Project Outputs – Wet Day scenario

• The Wet Day scenario represents a significant development in RFM over the NRIM approach

• Includes dynamic modelling of fluvial flooding downstream from a dam

• Intended to reduce storage prior to dam failure

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Dry Day Wet Day

Wet Day Flood Hydrograph

• The Wet day scenario simulates an event where the dam breaks during a 1 in 1000 year fluvial flooding event in the valley d/s of the dam

• The flood hydrograph (solid blue) is made up of two components:

• Dam break (dotted orange) – relatively short in duration, but with large peak flow and fast wave velocity

• Fluvial (dashed red) – relatively long in duration with smaller peak flow and slower wave velocity

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Wet Day scenario sensitivity testing

• Feedback from PSG raised concerns over the reliability of wet day maps leading to further sensitivity testing of the methods used• Flood risk potentially underestimated away from the location where

breach and fluvial hydrographs coincide

• Addressed through sensitivity testing that revealed:• Underestimation small for large proportion of reservoirs

• Tools developed to determine underestimation

• Tools run on 300 reservoir outputs to build evidence base

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Reservoir Flood Mapping – Project deliverables

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PDF Maps

Wet & Dry Day

Flood Extent

MapsWet & Dry Day

Flood Depth

MapsWet & Dry Day

Flood Velocity

Maps

Wet & Dry Day

Flood Hazard

MapsWet & Dry Day

Property Impact

MapsMultiple breach

composite maps

for all map

types

Flood

Modelling

Summary

Sheet

Digital Deliverables

TUFLOW model

data

GIS

Raster

data

GIS

Vector

data

Consequence

Metrics

RFM deliverables – Dry Day flood extent

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Reservoir Flood Mapping - Conclusions

• The RFM project is programmed to continue until 2020• Final product available in 2020

• RFM has resulted in revised deliverables that are of a higher quality compared to NRIM and available for a greater number of uses

• RFM is delivering reservoir flood mapping for both dry day and wet day scenarios

• The RFM has been through a comprehensive review process resulting in increased confidence in the wet day mapping

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