natural flood management within the flood risk management ......5 12/09/12 neil nutt -natural flood...

1 Neil Nutt - Natural Flood Management12/09/12

Natural flood management within the

Flood Risk Management (Scotland) Act

Neil Nutt – Halcrow CH2M Hill (SEPA secondment project)

[email protected]

12/09/122 Neil Nutt - Natural Flood Management

Acknowledgements

Project Technical Advisory Group

• Andrea Johnstonova (Chair) – SEPA

• Drew Aitken – SEPA

• Heather Forbes – SEPA

• Julia Garritt – Forestry Commission Scotland

• Lorna Harris – SEPA

• Kirsty Jack – SEPA

• Richard Jefferies – SEPA

• Mark McLaughlin - SEPA

• Roy Richardson – SEPA

• David Scott – SEPA

• Nadeem Shah – Forest Research

• Mark Williams – SEPA

External reviewers via CREW

• Prof Alan Werritty (Dundee University)

• Dr Scott Arthur (Heriot-Watt University)

• Dr Tom Ball (Dundee University)


Introduction

• Key legislation relating to NFM

• Pilot catchments

• What is NFM

• NFM screening methodologies

• Fluvial NFM assessment

methodology


Natural Flood Management

Legislation


The Flood Risk Management (Scotland) Act 2009

Section 20 Identification of potential

Assess where the alteration or restoration of natural features

could contribute to the management of flood risk

Section 28 – Appraisals & Strategies

Consideration must be given to the Section 20 assessment when

setting Flood Risk Management objectives and measures

Section 34 - Local FRM Plans

Requirement to detail how implementing the plan may alter

(including enhance) or restore natural features and characteristics


The Flood Risk Management (Scotland) Act 2009

Section 20 Identification of potential

Running of Screening Tools

Output: Section 20 maps

Section 28 Appraisals & Strategies

Develop and refine options

Output: A list of preferred FRM measures for

inclusion in FRM Strategies

Section 34 Local FRM Plans

Further development of preferred option

Output: Local FRM Plans

2015 – 16

The relevant

‘Responsible

Authority’

2013 - 2014

SEPA /

‘Responsible

Authorities’

2012

SEPA



Pilot projects


Pilot NFM projects

• Eddleston Water

• Allan Water

• Upper Clyde

• River Devon

• Borthwick Water

• Forth FutureScape

• Scottish Government

considering funding 6 pilot

NFM/restoration catchments

Ordnance Survey, Crown Copyright


General synopsis of pilot findings

• Technical challenges (hydrology, hydromorphology, ecology)

• Does NFM work?

• What can be achieved?

• Where is NFM most effective?

• Land ownership/land manager issues

• Leadership/responsibility

• Funding

• Is it economic?

• Who should pay?

• Long timescales

• Maintenance

• Ambiguity over what NFM entails


… what is NFM?

“techniques that aim to work with natural hydrological and

morphological processes, features and characteristics to

manage the sources and pathways of flood waters. These

techniques include the restoration, enhancement and

alteration of natural features and characteristics, but

exclude traditional flood defence engineering that works

against or disrupts these natural processes.”

(SAIFF -The Scottish Advisory and Implementation Forum for Flooding,

2011)


…and what does that entail?

(SAIFF, 2011)



Screening for opportunities

Early identification of potential NFM locations


GIS screening methods

• Production of maps showing NFM potential for Section 20

• GIS toolboxes

• Nationally available data

• Nationally applicable

• Screenings looked at:

1. Runoff generation

2. Floodplain restoration

3. Sub-catchment desynchronisation

4. Hydraulic constrictions

5. Hydromorphology

6. Estuarine surge attenuation

7. Wave energy attenuation

Charlie Perfect, CRESS


Screening: Runoff generation

2

0460.01536.03062.8

1000

BFIHOSTSAARq ×=

weightlanduseweightslopeq BFIHOSTSAAR __0460.01536.03062.82

1000

×××=

Subjective scoring method (Environment Agency)

Entirely subjective scores (1-4) given to land cover, soil, slope and rainfall

Score =

Restatement of the QMed by catchment descriptors

Dropping FARL & AREA

Score (runoff per unit area) =

Hybrid method

Weight QMed based runoff per unit area with

scores for slope and

Score (weighed runoff per unit area) =


Screening: Floodplain restoration

• Calculation based on Manning’s equation

• Gives an indication of the increase in water level due to floodplain

roughening (Increased Storage Potential)

• High scores (red above) indicate greater potential

• Morphological Pressures Database and Wetland Inventory used to give

an indication of floodplain connectivity

• Need to consider backwater extents

−= 2

3

2

3

max

4

3

2

3

existingnn

S

vISP

v = flow velocity

s = hydraulic gradient

n = Manning's roughnessOrdnance Survey, Crown Copyright


Screening: Sub-catchment desynchronisation

• Potential to desynchronise sub-catchment hydrographs

• Based on similarity of FEH Time to Peak for the two

sub-catchments (Time to Peak Similarity)

• Controversial due to uncertainty (storm movement

and quality of Tp estimate)

• Requires difficult to access data

5

max,

min,

=

p

p

T

TTS




Screening: Hydraulic constrictions

• Identification of artificially constricted flow paths

• National fluvial flood hazard mapping

(Early draft of national fluvial hazard mapping, SEPA/Halcrow - ongoing)



Screening: Areas of

heightened morphological

activity

• Sediment budget modelling using

ST:REAM (Parker in press)

• Overview of probable areas of:

• Source

• Transport

• Deposition


Screening: Estuarine surge attenuation potential

• Latest national coastal maps

• Projection of estimated

extreme water levels

• Simple process of overlaying

flood maps with known

defence locations

• SFDAD & coastal

hydromorphology database

• Direction on where to

consider in more detail


(For illustration only, not produced as part of national

coastal mapping project)


Screening: Wave

attenuation potential

• Identifying high energy coastlines

which potentially have adequate

space to permit (semi) natural

wave energy dissipation measures

• Poor data

• Incident wave power

• Shore width/slope

• Shore substrateOrdnance Survey, Crown Copyright



Quantifying the hydrological effects


Quantifying the effects

• Adequate tools for assessing coastal, groundwater

and urban NFM

• not to say the tools are always used

• Large gap for assessing fluvial (& pluvial) NFM


What do we need to do to assess fluvial NFM?

“The modelling should be distributed and be capable of running

continuous simulations. It should also be partially or wholly

physically based so that the physical properties of local landscapes,

soils and vegetation can be represented, and it should include detailed

modelling of surface water flow so that the effects of changes can be

tracked downstream.”

O’Connell et al. (2004) Review of impacts of rural land use and management on flood generation: Impact study report. DEFRA R&D Technical Report FD2114/TR. 2004.

Charlie Perfect, CRESS


What are the issues with current hydrological

methods when it comes to fluvial NFM?

• Statistical methods can’t be used to quantify minor

‘tweaks’ to catchments ->

• Hydrological models less accurate

• Current practice to use lumped hydrological models

• no representation of land use

• unit hydrographs (not possible to track effects

through catchment)


What do we need to do to assess NFM?

• Distributed model;

• Capable of running continuous simulation;

• Partly or wholly physically based so that landscape, soil

and vegetation can be represented; and

• Detailed modelling of surface flow so that effects can be

tracked downstream.


What does the model we need look like?


Making a few simplifications

Rain storms assumed to

be standard FSR storm

profile and instantaneously

over the entire catchment

Interception capacity

used up at a constant

rate at start of event

and no evaporation

during event

Constant base flow

throughout event

Array of PDM based

moisture stores

No return of water to

the floodplain

Runoff velocity is

constant throughout

the event


Process overview

Runoff generation

Interception

Flow routing model

Baseflow estimate

Antecedent

moisture

Rainfall

Land use

Soil type

Drainage network

OUTFLOW

Potential

Evaporation

Runoff model

Flow routing model


Generating runoff from rainfall via

PDM as used by ReFH & G2G

Soil moisture capacity

depends on soils

Initial soil moisture

(antecedent condition)

depends on catchment

wetness

Flow routed to outlet

using Unit

Hydrograph

Rainfall

Catchment assumed to be

homogeneous (average

values taken to be

representative)


Generating runoff from rainfall using a PDM with some

minor additions to enable land use change to be

represented

Soil moisture capacity based

on soil type.

-Degraded via Packman et

al (2004).

-Effects of steep slopes can

be included as incorporated

in G2G

Initial soil moisture

(antecedent condition)

depends on catchment

wetness which in turn varies

with land cover water usage

Runoff routed to catchment

outlet using physically based

Time to Outlet grid

Interception via canopy

storage via leaf area index

Rainfall

Spatial variation allowed for

by splitting catchment into

large number of moisture

stores, cell average now

used


How antecedent moisture varies with landuse

• PROPWET (FEH descriptor) indicates likely

antecedent conditions

• The soil moisture capacity and initial soil

moisture both depend on PROPWET

• PROPWET comes from MORECS (Met Office)

• proportion of days when the soil moisture

deficit < 6mm

• MORECS provided 40km tile estimates of

soil moisture for selected land use types

• i.e. pasture, forest, arable …

• PROPWET within the FEH CD is resampled

to a local 1km grid based on the dominant

land use

• Unused potential to vary PROPWET with

land use

PetrPetr KratochvilKratochvil www.publicdomainpictures.netwww.publicdomainpictures.net


Modifying PROPWET with landuse

PROPWET Regression

Selected HiFlows Stations

y = x - 5E-09

R2 = 0.8276

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

FEH PROPWET

Regression PROPWET

( )[ ]43,828.0

998.05003.25365.0

2

79.070.4

10

==

−−×××−=

nR

ErLAIPESAARLogPROPWET

Landuse:

LAI – leaf area index

r – bulk canopy resistance

Climate:

SAAR – standard average annual rainfall

PE – potential evaporation



Landuse impact on runoff generation

• Modification of PROPWET with landuse

• Introduction of canopy interception

(upper limit ~2mm)

• Variation of BFIHOST with soil degradation

( )[ ] 998.05003.25365.0 79.070.4

10 −−×××−= ErLAIPESAARLogPROPWET

LAII 2.0=

2)(00575.0)(498.0935.0 LAILAIIMax −+=

(Hough and Jones, 1997)

(Hoyningen-Huene, 1981) (upper limit ~7mm)

Using analogue degraded HOST presented by Packman et al (2004) Geoland2


Flow routing

• Current practice based on “Unit hydrograph”

• Do not allow the effects to be tracked

downstream

• Propose to replace the “Unit hydrograph”

with a “Time to Outlet” grid

• “Time to Outlet” grid based on a

representative bankfull flood event

• Accounting for rapid flow mechanisms (land

and channel flow) via simple normal depth

(“kinematic wave”)

• Full hydrodynamic models could be used to

model floodplain interventions


Flow routing 2

Flow->Velocity

Time to outlet Isochrones

Rainfall intensity

Percentage

runoff

approximation

(i.e. SPRHOST)


Generating the hydrograph

0

100

200

300

400

500

600

700

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Time step

Flow (cuemcs)

0

2

4

6

8

10

12

14

16

18

Rainfall depth (mm) Band 4

Band 3

Band 2

Band 1

Band 0

Baseflow

Rainfall

Calculate runoff

using array of

PDMs

Route runoff to

outlet using time

to outlet grid


Method summary

Runoff generation

PDM

Interception

Leaf area index

Flow routing model

Time to Outlet Grid

Baseflow model

Constant

Antecedent

moisture

Modified

PROPWET

Rainfall

FEH DDF

Land use

Soil type

HOST (and

degraded HOST)

Drainage network

OUTFLOW

Potential

evaporation

MORECS

Runoff model

Flow routing model


Summary

• Overview of legislation

• Overview of pilot projects

• Two levels of tools are required to facilitate NFM within FRM Act

• GIS screening tools have been presented which could allow the rapid

national identification of NFM opportunities Assessment

• Fluvial NFM measures could be quantified using a spatially distributed

semi-physical model

• Aligned with current methods

• Additional components to address DEFRA requirements

natural flood management within the flood risk management ......5 12/09/12 neil nutt -natural flood...

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