Tony Clothier PhD, BSc Eng (Civil) CEng, CEnv, C.WEM, MCIWEM ICM - - PowerPoint PPT Presentation

Tony Clothier

PhD, BSc Eng (Civil) CEng, CEnv, C.WEM, MCIWEM

ICM Modelling Approach

Preliminary Assessment: Local Flood Defences

• Local defences remove flooding of

residential properties

• Increase in flood depths through Loxwood

by up to 0.9m

• Increased the risk of surface water flooding

behind defences

• Increase maintenance and risk of defence

failures to properties

• Increased flood risk in the event of
• vertopping

Existing Condition: 1 in 100 Fluvial Flood Event

• Flood risk shown to properties along:

– Guildford Road – Pond Close – Oak Grove – Burley Close

• Model shown fluvial flooding only
• Flooding of properties from surface water

flows prior to reaching the Loxwood stream not modelled (at this stage).

• All option models results presented

consider the 1 IN 100 Event

Option 1: Upstream Flood Storage Area

• Preliminary design:

– Excavating North Hall field to 23.75mAOD. – Spill from stream at 24.5mAOD – Flapped 450mm diameter pipe outfall

• Limited / negligible reduction in peak flood

water depths through Loxwood

• No properties removed from flood risk

Existing Condition: 1 in 100 Fluvial Flood Event

Option 2: Upstream Offline Flood Storage Area

• Preliminary design:

– Flood bund ties into high ground >27.0mAOD – Bund crest at 27.00mAOD – Bund spillway at 26.50mAOD – 900mm Diameter culvert flow control

• Reduction in peak flood water depths and

flood extents through Loxwood

• No properties removed from flood risk
• Significant increase in upstream flood

depths and extents

• Increased flood risk to 2 upstream

developments and flooding of arable land

Existing Condition: 1 in 100 Fluvial Flood Event

Option 3: Upstream Flood Bund and Offline Flood Storage Area

• Preliminary design:

– Flood bund at modelled for Option 2 – Offline flood storage area as for Option 1, but area reduced to area upstream of flood bund only

• Reduction in peak flood water depths and

flood extents through Loxwood

• No properties removed from flood risk
• Significant increase in upstream flood

depths and extents

• Increased flood risk to 2 upstream

developments and flooding of arable land

• Upstream marginally flooding reduced as

compared to Option 2

Existing Condition: 1 in 100 Fluvial Flood Event

Option 4: Channel Widening and Limited Culvert Improvements

• Preliminary design:

– Stream channel width doubled from upstream of Guildford Road to downstream of Station Road – Additional pipes / culverts added to existing culverts, where considered possible.

• Reduction in peak flood water depths and

flood extents through Loxwood

• Burley Close properties removed from flood

risk

• Reduction in flood depths to properties

upstream of Guildford Road

• Limited reduction in flood depths to

properties along Pond Close and Oak Grove

Existing Condition: 1 in 100 Fluvial Flood Event

Option 5: Channel Widening and Maximised Culvert Improvements

• Preliminary design:

– Stream channel width doubled from upstream of Guildford Road to downstream of Station Road – Culverts replaced with 3m wide open channels (maximising potential for culvert improvements).

• Significant reduction in peak flood water

depths and flood extents through Loxwood

• Properties removed from flood risk:

– Burley Close – Oak Grove – Pond Close

• Reduction in flood depths to properties on

either side of Guildford Road

Other Flooding Consideration: Surface Water Flooding Sources

Local Flood Defence Consideration for Remaining Properties

• Assumes channel and culvert capacity

improvements maximised

• Low flood defences in the form of

raised ground as far as possible

• Any defences will need to consider

surface water flooding sources to reduce the risk of water ponding behind defences

Assessment of the impact of the brick headwalls and other walls along the watercourse (Pond Close and Oak Grove) and their potential impact on peak flood depths

Impact of Existing Local Walls

1 in 100 Event (No Walls) 1 in 100 Event (Incl. Walls)

Brick Headwalls Brick walls restricting flow from re-joining the watercourse

Stream Channel Dimensions

Review (as example) of the variability in the capacity and potential efficiency of the stream channels for the Pond Close and Oak Grove reach

Culvert outfall headwalls in channel

Upstream Pond Close Upstream Oak Grove 1st Crossing Downstream Oak Grove 2nd Crossing Oak Grove 1st to 2nd Crossing Oak Grove 1st to 2nd Crossing Oak Grove 1st to 2nd Crossing

In-channel Weir