Natural Flood Management and the Catchment … Flood Management and the Catchment Based Approach Ian...

Natural Flood Management and

the Catchment Based Approach

Ian PattisonSchool of Civil and Building

Engineering, Loughborough University

Outline

• Background to the Catchment Scale flood

risk problem

• Sub-catchment interactions

• NFM and working with natural processes

- case study of soil compaction

• Therefore, this heterogeneity (Soil characteristics / land cover)

within catchments needs to be accounted for.

Context

• Impact of rural land management on flood risk is spatially and

temporally dependent

Pattison I, Lane SN. (2012). The link between land use management and flood

risk: a chaotic conception?, Progress in Physical Geography.

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

Doing Flood Risk Management

Research Differently

Field / Reach Sub-sub- Sub-catchment Catchment

catchment

• Upscaling to Downscaling

• Carlisle

Spatial Downscaling of Flood Risk

Petteril

Upper

EdenEamont

Caldew

?Carlisle

Irthing

Statistical Approach

• 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)

Hydraulic Modelling Approach

Explaining Flood Risk in Carlisle

• Eamont = 19.3%

Upper Eden = 18.7%

• Magnitude

= U. Eden

Timing (delay)

= Caldew

Carlisle peak flow magnitude = 67.4 PC1 - 45.2 PC2 + 497.3

• 84% downstream peak discharge predicted

• 50% Magnitude

34% Timing (21% Positive = increase lag = delay)

Hydraulic Modelling Results

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

0.26m respectively

• Petteril has no

effect

• Maximum reduction peak stage (0.331 m) caused by a 25%

reduction in the Upper Eden.

• Eamont = 0.22m, Irthing = 0.25m

• Caldew has little effect (especially <10%), Petteril = no effect

Magnitude

Earlier Delayed

Compaction

(adapted from O’Connell et al.,

2004)

Compaction degrades soil structure

- Decreased porosity

- Decreasing hydraulic conductivity

- Alters partitioning of precipitation

into overland and sub-surface

flow

Problem of Process Complexity

CompactionCompaction levels vary spatially and are caused by different

mechanisms

Gate

Feeding Trough

Tree

Shelter

Open

Field

Inter-

Field

Intra-

Field

Experimental Design

• Stratified Random sampling around

features of interest

• Combined Field and Laboratory

measurements

Hydrology

- Soil Moisture

- Saturated

Hydraulic

Conductivity

- Double Ring

Infiltrometer

Soil Properties

- Porosity

- Organic

Content

- Particle Size

- Cores

Compaction

- Dynamic

Cone

Penetrometer

- Pocket

penetrometer

(surface)

Dynamic Cone Penetrometer

Deeper =

less

compaction

Intra-Field

• Cattle Open Field = most variability (frequency of tread) and

statistically different to Gate (0.0001) and Tree (0.01)

• Sheep/Horse Tree Shelter = statistically different to all other

within-field sites

Inter-Field

• Open areas – Sheep statistically different to cattle/horses

• Trends between fields not as significant as intra-field variations.

Compaction Soil Properties Hydrolo

gy

Soil Porosity

• All sites statistically different to one another

• Largest difference is between Open field and Gate

Cattle

Compaction Soil Properties Hydrology

Soil Moisture

Intra-Field

• All sites statistically significantly different to one another for

Cattle and Horses fields

• Tree shelter site different to other parts of Sheep field

Inter-Field

• The Open and Gate areas are statistically different to one

another in each of the fields.

• No difference between feeding areas in fields with different

animals.

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.

Compaction Soil Properties Hydrology

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

Landscape Management Scenarios

Hydrological Model – CRUM 3

ID Process Representation2D Catchment Scale

Compaction

Winter 04-05

Light = 36.9 m3s-1

Moderate = 58.7 m3s-1

Heavy = 60.9 m3s-1

• Flashy response

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%

Soil Moisture Contents = 2 layer model

Main Soil

• Heavy compaction never below 0.95 (saturated)

• Moderate compaction reaches saturation in flood events

• Light compaction never reaches saturation

• M to H 3.7% increase in peak flows

• Soil Moisture drives flood generation

Dynamic Layer

• Heavy compaction at saturation for 60% of time

• Moderate compaction at saturation for 6.5% time (floods)

• Light compaction – maximum level of 0.84

• Continuum

• 2nd flood peak

- Light

compaction

produces

highest flow

= throughflow

= ppt intensity

/ total.

January 2005 Flood

Main Soil Dynamic Layer

• Fully saturated main soil for the whole period

• Dynamic layer has storage capacity (2%) during this

secondary flood event

Upscaling Effects to the Catchment

Scale

Spatially nested modelling approach

Upscaling effectsCompaction

• Compare L and H scenarios

• 24 m3s-1 (65%) Dacre

• 36 m3s-1 (16.4%) Udford (0.17m)(0.18%)

• 49.9 m3s-1 (3.5%) Eden (-14cm)

Conclusions• Landscape scale changes reduce flood risk at the sub-

catchment 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

i.pattison@lboro.ac.uk

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Questions ?