(Read part I First: https://deigmologyblog.wordpress.com/2016/05/06/river-mole-part-i/)
Factors affecting flood risk
- Most of bedrock is impermeable, largely Wealden clays and greensand; 60% is “low permeability”
- Basin shape- the river has two “sections” to it’s catchment basin. The upper river is mostly circular while the lower river is mostly long and thin. This shape is exacerbated by the Mole gap at Dorking, where the river has to pass through a narrow gap in chalk hills. During flooding, water can become trapped there, worsening the impacts of the flooding on the town. Circular basins result in higher and shorter flood peaks.
- The Mole has higher relief than most of Southern England, with maximum elevation of 265m at Leith Hill. The Downs from Ranmore to Box Hill are 100m taller than the valley below.
- Precipitation- the Mole has a fairly modest 750mm/pa.
- Gatwick airport- only a narrow channel is available for water to flow through
- Prevalence of urban areas. Urban areas mean more concrete, which is impermeable.
- Effluent discharge- The water is normally treated and fed into the Mole. Some effluence comes from other catchment areas.
Flood defense schemes:
- Upper Mole alleviation scheme: Protects urban areas n upper river- £15 million Environment Agency project
- Upgrading of flood retention at Clay lake
- Constructing a higher dam wall at Tilgate Lake (adding 2.5m onto the height)
- River Mole redirected at Gatwick Airport, running underneath it
- Flood ponds at Gatwick. Latest pond can store 180,000 m^3 of water, supposedly halving flood risk. Water is tested and has its quality improved to high standards before being allowed back into the river, to ensure heavy metals from the planes being washed off do not enter the river.
- Worth Farm
Worth Farm is located near Crawley and the M23 motorway. An embankment dam 6.5m tall has been built along the farm. No water is stored in the reservoir through most of the year, and the land can be used for farming, mostly of cattle.
During floods, the water in the Mole that would normally just flow under the embankment is captured and held, creating a reservoir. Over a few days, this water slowly drains out.
Water level can rise by 3.5m behind the embankment, which is predicted to occur 20% of years, while reaching peak capacity is predicted to happen once every 200 years on average.
- Weirs (normally natural-looking features involving using branches to redirect and limit the flow of water) and bank defenses at Molesey built in 1968.
- Barrage preventing discharge directly into the Thames, reducing any potential impacts (unlikely to be much!) upon London
- Redeveloping natural local wetlands
- Moors Project managed by Surrey Wildlife Trust
- Creating Water Meadows. Reinserting natural environments, and vegetation, which can absorb the impact of flooding and intercept some water to reduce the volume continuing to move downstream
- Flood warning scheme
Factors affecting management schemes
- If flooding worsened after installations of defenses elsewhere, tensions are created between towns
- Hard engineering requires frequent repairs
- Natural replacements of the environment can be expensive and take a while to become cost effective
- Hard engineering is expensive to build
- Locals in Leatherhead blame Molesey defenses for the flooding in their town
- Hard engineering can affect an area’s aesthetic appeal.