Presented By

Alex Nwoko

Understanding who is at Risk Modelling Flood ExtentRiver Wansbeck – Morpeth

Outline• Introduction• Overview of HEC-RAS Model• Methodology• Results• Discussion/limitation• Summary

Introduction• Floods in the United Kingdom (UK) are one of the

most notable climate-related disasters and also the costliest natural disaster.

• In 2009, 5.2 million residential and commercial properties in England were identified as being in areas at risk of flooding from rivers, the sea and surface water (Defra and EA, 2012).

• According to EA (2009), there is an estimate 2.4 million properties already built in the floodplain.

• Increase in Population and Urbanization = Increase exposure to flood risk.

• Since Risk = Hazard * Vulnerability * Exposure

• Morpeth is located on the down stream section of the River Wansbeck catchment.

• Catchment area = 287.3 km2

• Elevation: Lowest - 31.4m , Highest - 440

Overview of the HEC-RAS

• HEC-RAS is hydrological modelling software developed by the U.S. Army Corps of Engineers (USACE)• The HEC-RAS system contains four one-dimensional river analysis components

for:• Steady flow water surface profile computations; • Unsteady flow simulation; • Movable boundary sediment transport computations; and • Water quality analysis.(Horritt and Bates, 2002)

Why model?• Flood modelling makes it possible to analyse the behaviour of rivers • Extrapolated extreme (peak) flow behaviour of the river and its impact

on the flood plain.• To understand risk you now can MODEL it, NOT experience it

The 2008 Morpeth Flood• Caused by heavy sustained rainfall in the

preceding 24 hours. • The River Wansbeck burst its banks and

inundated the town’s flood defences, causing damage to 995 properties, 906 of which were residential.

• 56% of rain fall was converted into surface runoff (increase in urbanisation since 1963!)

• 400 residents were evacuated. • Total losses estimated at £40 million.• 6 September 2008 : Largest event on record.• Other recent Severe Events• 25 September 2012 : 10’s of properties

affected

Flood Hydrograph

Methodology

Manning's Selection for Channels

- Irregularity in the Channel – Width and Depth

- Variations in channel cross section

- Obstruction to flow (i.e. Boulders, debris and bridges)- Shape and Size

- Vegetation in channel (depends on depth of flow and vegetation density)

- Meandering

This could increase the “n” value by 30% but mostly in confined channels.

In open channel with floodplains like Morpeth, effect of meanders is reduced (Chow, 1959)

Manning's Selection for flood plains• Vegetation Density

• Vegetation type

• Obstruction to flow - Isolated boulders

• Surface Irregularity

• Nature of Bedrock- E.g urban areas.

• "Roughness" is represented in using flow velocity equations such as the Manning's equation

Manning’s Values

Winding and gravelly 0.045

winding 0.040

Earth channel 0.030

Earth channel - weedy 0.035

Earth channel - stony, cobbles 0.040

Floodplains - pasture, farmland 0.035

Floodplains - light brush 0.060

Floodplains - heavy brush 0.075

Floodplains - trees 0.15

Outputs from HEC-RAS

X Y Z Perspective Plot

Profile Plot

Sensitivity Analysis

• Effect of Cross section

Cross Sections Flood Extent25 599662.3

50 666208

100 672194.7150 684806.5175 684634.8200 683453.6

0 50 100 150 200 250590000.0600000.0610000.0620000.0630000.0640000.0650000.0660000.0670000.0680000.0690000.0700000.0

Flood Extent

Cross section

Area

(m2)

Effect of cross sections

reach1640463066208

61696071 59365837

5721

5674 55825541

5458

5374

52865165

50654947

4870481847504686

45834449

4376

4303

4222

4139

4062

3970 3818

36573610

3399

3083

3020

29362859 2682

2514

2394

2300

2195

2095

2025

1915

1752

16541548

1431 13131218

1123

1015

908

781

663

560 449

341

93

0

175 cross sections

75 cross sections

Effect of Manning's

• Using 24 cross sections

Return Period Discharge

Default Manning's value

Calibrated Manning’s n

1000 857582.5 884588.07

100 599662.33 628136.34

50 487977.57 525548.02

7.7 % change in flood extent

Effect of Levees

Without levees Effect of Levees

Flood Extent Result using optimized cross sections

Optimized cross section outputFlood event Flood extent

2008 flood 408536.21

50 Year 530788.07

100 Year 684392.54

1000 Year 963599.04

Definition Appropriate uses Policy AimsZone 1 (Low Probability): Land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%).

All uses of land are appropriate in this zone. In this zone, developers and local authorities should seek opportunities to reduce the overall level of food risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Zone 2 (Medium Probability):

Land having between 1 in 100 and 1 in 1000 annual probability of river flooding (1%- 0.1%) or between a 1 in 200 and 1 in1000 annual probability of sea flooding (0.5% - 0.1%)In any year.

The water-compatible, less vulnerable and more vulnerable uses of land and essential infrastructure in Table D.2 are appropriate in this zone. Subject to the Sequential Test being applied, the highly vulnerable uses in Table d.2 are only appropriate in this zone if the Exception Test is passed.

In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

Zone 3a (High Probability):

Land assessed as having a 1 in 100 year or greater annual probability of river flooding (>1%) or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

The water-compatible and less vulnerable uses of land in Table D.2 are appropriate in this zone. The highly vulnerable uses in Table D.2 should not be permitted in this zone.

In this zone, developers and local authorities should seek opportunities to:

Reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques.

Zone 3b (The Functional Floodplain):

Land with a an annual flood probability of 1 in 20 (5%) or greater in any year, or is designed to flood in an extreme (0.1%) flood, should provide a starting point for consideration and discussions to identify the functional floodplain.

Only the water-compatible uses and the essential infrastructure listed in Table D.2 that has to be there should be permitted in this zone

In this zone, developers and local authorities should seek opportunities to:

Reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques

Planning Policy - The Sequential Test (PPS 25) (Pardoe et al. 2011; DCLG 2006)

Flood Alleviation Scheme (FAS)

• Combination of : – Upstream storage and Town defences (new and upgraded)

• Flood Management Strategies and implications• StructuralCost- benefit-basedHigh maintenance cost• Non structuralLand-demanding Costly over large area.

• 50 year return period flood defences • 1m -1.5m flood defences given our results

Climate Change Effect • The effects of climate change are expected to

increase the frequency and intensity of flooding in Morpeth. • Current defences and culverts were not designed

to accommodate increased river flows and therefore cannot deal with the effects of climate change. • The catchment maybe sensitive to climate change

and due to the relatively quick response to rainfall and little natural attenuation in the catchment. • Hence, any increases in rainfall would be reflected

in immediate increased river flow. • 20% increase in river flow as a result of climate

change (Defra’s climate change guidance ) • Increase the flood risk to properties in Morpeth.

Environmental Considerations• Flood defences and channel dredging

pose threat to biodiversity.• Crayfish and Fisheries – River

Wansbeck is the most important water course in North East for these species. • High priority –Bed diversity

important for sustainability of channel for crayfish

Flood Risk Assessment Limitations• Errors in digitizing river geometry• Number of Cross-Sections• Errors in Data acquisition (data accuracy/ missing data)• Effect of tributaries on flow energy assumption• Flood defence.

Summary• Flood extent modelling involves a lot of parameterization to improve accuracy of

abstraction – Hence, Expert judgement is key.• Every catchment (channel & floodplain) is complex – Spatial and temporal variability in manning’s

R estimation.

• Spatial and time resolution of data affects our understanding of flood risk• - DEM resolution would affect flood inundation output (Tate et al, 2002; Haile and Rientjes, 2005; Sanders,

2007)- Return period and predicted discharge calculation- Flood modelling parameters have serious land use planning and insurance policy implications (de MOEL and

Aerts, 2011; Pardoe et al., 2011)

- Flood alleviation Scheme usually involve ethical and cost benefit considerations in flood protection and mitigation.- Environmental Considerations

References• de MOEL, H. and Aerts, J.C.J.H., 2011. Effect of uncertainty in land use, damage models and inundation depth on flood

damage estimates. Natural Hazards, 58(1), pp.407-425.

• DEFRA & EA, 2012. Understanding the risks , empowering communities , building resilience : the national flood and coastal erosion risk management strategy for England Unnumbered Act paper Correction required for the map on page 8 , figure 3 - Main urban areas at risk of su. Water Management, pp.2010–2012.

• Department for Communities and Local Government (CLG): Plan- ning Policy Statement 25: Development and Flood Risk, CLG, London, 2006a

• Environmental Agency (EA), 2009. Flooding in England. Environment, p.36.

• Haile, A.T. and Rientjes, T.H.M., 2005. Effects of LiDAR DEM resolution in flood modelling: a model sensitivity study for the city of Tegucigalpa, Honduras. ISPRS WG III/3, III/4, 3, pp.12-14.

• Horritt, M.S. and Bates, P.D., 2002. Evaluation of 1D and 2D numerical models for predicting river flood inundation. Journal of hydrology, 268(1), pp.87-99.

• Pardoe, J., Penning-Rowsell, E. and Tunstall, S., 2011. Floodplain conflicts: regulation and negotiation. Natural Hazards and Earth System Sciences,11(10), pp.2889-2902.

• Sanders, B.F., 2007. Evaluation of on-line DEMs for flood inundation modeling. Advances in Water Resources, 30(8), pp.1831-1843.

• Tate, E.C., Maidment, D.R., Olivera, F. and Anderson, D.J., 2002. Creating a terrain model for floodplain mapping. Journal of Hydrologic Engineering, 7(2), pp.100-108.