Natural Flood Management and the Catchment Based Approach

Transcription

1 Natural Flood Management and the Catchment Based Approach Ian Pattison School of Civil and Building Engineering, Loughborough University

2 Outline Background to the Catchment Scale flood risk problem Sub-catchment interactions NFM and working with natural processes - case study of soil compaction

3 Context Impact of rural land management on flood risk is spatially and temporally dependent Therefore, this heterogeneity (Soil characteristics / land cover) within catchments needs to be accounted for. Pattison I, Lane SN. (2012). The link between land use management and flood risk: a chaotic conception?, Progress in Physical Geography.

4 Impact of Land Use on Flows Spatial Scale = Plot/Field (10m²) Vs Catchment (2000m²) - Individual practices - Diffuse (combine) - Different practices (amplify/balance out) Bloschl et al., 2007

5 Doing Flood Risk Management Research Differently Carlisle Upscaling to Downscaling Field / Reach Sub-sub- Sub-catchment Catchment catchment

6 Spatial Downscaling of Flood Risk Statistical Approach Hydraulic Modelling Approach?Carlisle Irthing Caldew Eamont Petteril Upper Eden Uses widely available gauged data Sub-catchment Peak flow magnitude and Timing Multivariate Statistics PCA and Stepwise Regression isis model calibration Sensitivity of downstream hydrograph to sub-catchment flow magnitudes and timing. Scenario testing approach (Single and Multiple)

7 Explaining Flood Risk in Carlisle Carlisle peak flow magnitude = 67.4 PC PC % downstream peak discharge predicted 50% Magnitude 34% Timing (21% Positive = increase lag = delay) Eamont = 19.3% Upper Eden = 18.7% Magnitude = U. Eden Timing (delay) = Caldew

8 Hydraulic Modelling Results Magnitude Timing Delaying U.Eden and Eamont reduce peak stage by 0.32 m and 0.27 m respectively Speeding up Caldew and Irthing reduces peak stage by 0.33m and Maximum reduction peak stage (0.331 m) caused 0.26m by respectively a 25% reduction in the Upper Eden. Petteril has no Eamont = 0.22m, Irthing = 0.25m Earlier Delayed effect Caldew has little effect (especially <10%), Petteril = no effect

9 Problem of Process Complexity Compaction Compaction degrades soil structure - Decreased porosity - Decreasing hydraulic conductivity - Alters partitioning of precipitation into overland and sub-surface flow (adapted from O Connell et al.,

10 Compaction Compaction levels vary spatially and are caused by different mechanisms Inter- Field Gate Tree Shelter Intra- Field Feeding Trough Open Field

11 Experimental Design Stratified Random sampling around features of interest Combined Field and Laboratory measurements Compaction - Dynamic Cone Penetrometer - Pocket penetrometer (surface) Soil Properties - Porosity - Organic Content - Particle Size - Cores Hydrology - Soil Moisture - Saturated Hydraulic Conductivity - Double Ring Infiltrometer

12 Compaction Soil Properties Hydrolo gy Dynamic Cone Penetrometer Deeper = less compaction Intra-Field Inter-Field Cattle Open Field = most variability (frequency of tread) and statistically Open areas different Sheep to statistically Gate (0.0001) different and Tree to cattle/horses (0.01) Sheep/Horse Trends between Tree fields Shelter not as = statistically significant different as intra-field to all variations. other within-field sites

13 Compaction Soil Properties Hydrology Soil Porosity Cattle All sites statistically different to one another Largest difference is between Open field and Gate

14 Compaction Soil Properties Hydrology Soil Moisture Inter-Field Intra-Field The All sites Open statistically and Gate significantly areas are statistically different to different one another to one for another Cattle and in each Horses of the fields fields. No Tree difference shelter site between different feeding to other areas parts in of fields Sheep with field different

15 Compaction Soil Properties Hydrology Saturated Hydraulic Conductivity Cattle No statistically significant results at 0.05 level Largest difference between Open field and Feeding and Shelter areas.

16 Conclusions Inter-Field Variation = Significant differences between fields with different types of animals in. Intra-Field Variation = Significant differences between different areas of the same field (Open and Features) Implications of small scale variability (sub-field/grid scale) for Catchment Scale Flood Risk

17 Landscape Management Scenarios Hydrological Model CRUM 3 2D Catchment Scale ID Process Representation

18 Compaction Winter Light = 36.9 m 3 s -1 Moderate = 58.7 m 3 s -1 Heavy = 60.9 m 3 s -1 Flashy response

19 Compaction Effect on Hydrological Processes Runoff decreases from 77% to 65% with compaction Proportion as throughflow decreases from 56% to 1% Storage increases from 3.2% to 16%

20 Soil Moisture Contents = 2 layer model Main Soil Dynamic Heavy compaction Layer never below 0.95 (saturated) Moderate Heavy compaction at saturation reaches saturation for 60% of in time flood events Light Moderate compaction compaction never at reaches saturation saturation for 6.5% time (floods) Light compaction maximum level of 0.84 M to H 3.7% increase in peak flows Soil Moisture drives flood generation

21 January 2005 Flood Continuum 2 nd flood peak - Light Main Soil Dynamic Layer compaction produces Fully saturated main soil for the whole period highest flow Dynamic layer has storage capacity (2%) during = this throughflow secondary flood event = ppt intensity / total.

22 Upscaling Effects to the Catchment Scale Spatially nested modelling approach

23 Upscaling effects Compaction Compare L and H scenarios 24 m 3 s -1 (65%) Dacre 36 m 3 s -1 (16.4%) Udford (0.17m)(0.18%) 49.9 m 3 s -1 (3.5%) Eden (-14cm)

24 Conclusions Landscape scale changes reduce flood risk at the subcatchment and catchment scale (whole sub-catchment managed) Importance of WHERE management is implemented on its impact. Questions of where to focus on? and what to do there? need to answered simultaneously = Where to focus and what to do there? Impact of land management scales up to the catchment scale even for extreme floods

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