Natural flood management within the Flood Risk Management (Scotland) Act

Transcription

1 Natural flood management within the Flood Risk Management (Scotland) Act Neil Nutt Halcrow CH2M Hill (SEPA secondment project) 1 12/09/12

2 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) 2 12/09/12

3 Introduction Key legislation relating to NFM Pilot catchments What is NFM NFM screening methodologies Fluvial NFM assessment methodology 3 12/09/12

4 Natural Flood Management Legislation 4 12/09/12

5 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 5 12/09/12

6 The Flood Risk Management (Scotland) Act 2009 Section 20 Identification of potential Running of Screening Tools 2012 SEPA Output: Section 20 maps Section 28 Appraisals & Strategies Develop and refine options Output: A list of preferred FRM measures for inclusion in FRM Strategies SEPA / Responsible Authorities Section 34 Local FRM Plans Further development of preferred option Output: Local FRM Plans The relevant Responsible Authority 6 12/09/12

7 Natural Flood Management Pilot projects 7 12/09/12

8 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 8 12/09/12

9 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 9 12/09/12

10 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) 10 12/09/12

11 and what does that entail? (SAIFF, 2011) 11 12/09/12

12 Natural Flood Management Screening for opportunities Early identification of potential NFM locations 12 12/09/12

13 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 13 12/09/12

14 Screening: Runoff generation 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) = q = SAAR BFIHOST 2 Hybrid method Weight QMed based runoff per unit area with scores for slope and Score (weighed runoff per unit area) = q= SAAR BFIHOST 2 slope _ weight landuse _ weight 14 12/09/12

15 Screening: Floodplain restoration ISP v = 2 2 nmax 3 4 S n existing Ordnance Survey, Crown Copyright 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 15 12/09/12 v = flow velocity s = hydraulic gradient n = Manning's roughness

16 Screening: Sub-catchment desynchronisation TS T = T p,min p,max 5 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 Ordnance Survey, Crown Copyright Ordnance Survey, Crown Copyright 16 12/09/12

17 Screening: Hydraulic constrictions Identification of artificially constricted flow paths National fluvial flood hazard mapping Ordnance Survey, Crown Copyright (Early draft of national fluvial hazard mapping, SEPA/Halcrow - ongoing) 17 12/09/12

18 Screening: Areas of heightened morphological activity Sediment budget modelling using ST:REAM (Parker in press) Overview of probable areas of: Source Transport Deposition 18 12/09/12

19 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 Ordnance Survey, Crown Copyright (For illustration only, not produced as part of national coastal mapping project) 19 12/09/12

20 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 Ordnance Survey, Crown Copyright Shore substrate 20 12/09/12

21 Natural Flood Management Quantifying the hydrological effects 21 12/09/12

22 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 22 12/09/12

23 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 Charlie Perfect, CRESS 23 12/09/12

24 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) 24 12/09/12

25 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 /09/12

26 What does the model we need look like? 26 12/09/12

27 Making a few simplifications Runoff velocity is constant throughout the event No return of water to the floodplain 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 Array of PDM based moisture stores Constant base flow throughout event 27 12/09/12

28 Process overview Runoff model Potential Evaporation Land use Rainfall Interception Flow routing model Drainage network Antecedent moisture Runoff generation Flow routing model Baseflow estimate Soil type OUTFLOW 28 12/09/12

29 Generating runoff from rainfall via PDM as used by ReFH & G2G Rainfall Flow routed to outlet using Unit Hydrograph Initial soil moisture (antecedent condition) depends on catchment wetness Catchment assumed to be homogeneous (average values taken to be representative) Soil moisture capacity depends on soils 29 12/09/12

30 Generating runoff from rainfall using a PDM with some minor additions to enable land use change to be represented Rainfall Interception via canopy storage via leaf area index Runoff routed to catchment outlet using physically based Time to Outlet grid Initial soil moisture (antecedent condition) depends on catchment wetness which in turn varies with land cover water usage Spatial variation allowed for by splitting catchment into large number of moisture stores, cell average now used Soil moisture capacity based on soil type. -Degraded via Packman et al (2004). -Effects of steep slopes can be included as incorporated in G2G 30 12/09/12

31 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 Petr Kratochvil /09/12

32 Modifying PROPWET with landuse PROPWET Regression Selected HiFlows Stations Regression PROPWET y = x - 5E-09 R 2 = Ordnance Survey, Crown Copyright FEH PROPWET PROPWET = Log R 2 = 0.828, n= [ SAAR ( PE LAI r 2.003E 5) ] Climate: SAAR standard average annual rainfall PE potential evaporation Landuse: LAI leaf area index r bulk canopy resistance 32 12/09/12

33 Landuse impact on runoff generation Modification of PROPWET with landuse [ SAAR ( PE LAI r ) ] PROPWET= Log E 10 Introduction of canopy interception I = 0. 2LAI (Hough and Jones, 1997) (upper limit ~2mm) I Max = ( LAI ) ( LAI (Hoyningen-Huene, 1981) (upper limit ~7mm) 2 ) Variation of BFIHOST with soil degradation Using analogue degraded HOST presented by Packman et al (2004) Geoland /09/12

34 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 34 12/09/12

35 Flow routing 2 Percentage runoff approximation (i.e. SPRHOST) Rainfall intensity Flow->Velocity Time to outlet Isochrones 35 12/09/12

36 Generating the hydrograph Calculate runoff using array of PDMs Flow (cuemcs) Rainfall depth (mm) Band 4 Band 3 Band 2 Band 1 Band 0 Baseflow Rainfall Time step 36 12/09/12 Route runoff to outlet using time to outlet grid

37 Method summary Runoff model Potential evaporation MORECS Land use Rainfall FEH DDF Interception Leaf area index Flow routing model Drainage network Antecedent moisture Modified PROPWET Runoff generation PDM Flow routing model Time to Outlet Grid Baseflow model Constant Soil type HOST (and degraded HOST) OUTFLOW 37 12/09/12

38 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 38 12/09/12