Drainage Management (AQA A Level Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Issues of Catchment Management
Catchment management is an holistic way of looking at a whole river catchment, its interaction between water and land and how best to manage and improve drainage
The aim is to minimise flooding, drought, water pollution, channel erosion and improve ecosystems that rely on the river
Decisions are made by local authorities and private engineers, usually without consultation with local residents, which generates conflict and resentment
Catchment management schemes are disruptive and expensive
Ecological balance is disturbed through increased water temperatures and waste water discharge
Other issues include:
Concentrated water pollution and spread of viruses
Decreased water flow in dry periods impact aquatic ecosystems
Increased flooding and erosion
Management methods include:
Soft engineering - using knowledge of river basin processes to work with nature
Hard engineering - building structures to 'push back' against nature
Comparison of Soft and Hard Engineering Approaches to Urban Drainage Management
Soft Engineering | Hard Engineering |
|---|---|
Afforestation increases interception and reduces throughflow, along with surface runoff. EVT removes water that would eventually end up in the river channel. Afforestation reduces flood risk and water pollution | Dams, floodwalls and reservoirs are built to prevent flooding and ensure a regular supply of water |
Floodplain zoning - restriction on building on certain areas of a floodplain | River straightening increases the flow of water by increasing the gradient of a river's channel and removing natural meanders (bends) |
Riverbank conservation to reduce lateral bank erosion and collapse through planting to stabilise banks and reduce silting downstream | Levees - natural ones can be increased in height to increase bankfull capacity Embankments can be built or increased using concrete or sustainable materials (usually dredged from channel) |
River restoration aims to restore the river channel to its original course by removing and reversing past management strategies | Channelisation adds a liner to a straightened river channel (usually concrete) to reduce friction which aims to improve flow rate and therefore, reduce silting |
Restoring natural wetlands on floodplains | Diversion spillways also called flood relief channels bypass the main river's channel during periods of high flood risk. They can also be a permanent feature which act as a wetland, and improve the local environment |
For - overall a cheaper method of management compared to hard engineering and can improve the local environment Against - planning restrictions limit building of new homes and businesses and difficult to implement in already urbanised areas | For - prevents flooding of homes and businesses, thereby reducing costs of repairs and loss of revenue. Against - very expensive to build and maintain. They alter wildlife habitats and prevent aquatic migration. They affect river flow downstream - increased silting or flooding. |
SUDS
SuDS stands for Sustainable Drainage Systems and aims to mimic natural drainage regimes within the built environment, thereby reducing flooding, improving water quality and the value of the environment to people and biodiversity
SuDS achieve this through:
Storing runoff and releasing it slowly
Collecting and re-using surface water at source
Allowing water to infiltrate into the ground
Decrease transport of surface water flow rates
Filtering out pollutants
Allowing sediments to settle out by controlling the flow of the water
SuDS uses a sequence of techniques that together form a management train or sub-catchments
Natural drainage techniques are used in stages to change the characteristics of the flow and quality of the runoff
The management train starts with prevention - reducing impermeable areas and measures for reducing pollution
It progresses through local source controls to larger downstream site controls and finally regional controls
Runoff doesn't need to go through all the stages in the management train, it can flow straight to a site control
Managing and returning runoff to the natural drainage system near to source as soon as possible is preferable
If water can't be managed locally, it is slowly transferred elsewhere - the water might need further treatment or there is too much for the system to deal with - the excess flows are then routed off site
As surface water flows through the system, flow velocity is controlled and pollutants are removed
Source control methods include:
Interception methods such as diverting roof guttering for re-use or storage (e.g. for irrigation) or green roofs to increase evapotranspiration
Using pre-treatment steps, such as vegetated swales or filter trenches, which remove pollutants from surface water before discharge to watercourses or aquifers
Delaying discharge through retention systems of ponds, wetlands etc.
Retention systems that delay the discharge of surface water to watercourses by providing storage within ponds,
Infiltration trenches and soakaways, mimic natural recharge, which allows surface water to soak into the ground
SuDS impact on the flow of water during a storm event:
Surface water flows through swales and filter trenches removing pollutants
Peak river discharge is slowed and reduced through:
Capture and storage of water for re-use
Storage in ponds and wetlands
Infiltration of water to the ground through infiltration basins and soakaways
Water quality is improved and peak river discharge is decreased
Techniques, Purpose and Benefits of SuDS
Technique | Purpose | Benefit |
|---|---|---|
Swales | Wide shallow drainage channels, usually dry and can be landscaped as recreational spaces | Slows down surface runoff |
Green roofs | Grass and wildflower habitats on roofs with minimal runoff to gutters | Has insulating properties and increases evapotranspiration (EVT) |
Infiltration basins | Vegetated depressions that are are dry except during heavy rainfall | Stores surface runoff and allows infiltration into ground |
Rain gardens | Shallow depressions planted with flowers and shrubs | Creates a green space for recreation and increases EVT |
Detention basins | Excavated basins for temporary storage of water during flood events | Allows for infiltration and groundwater recharge |
Bioretention basins | Gravel and sand filtration layers beneath reed beds collect and filter dirty water | Wetland spaces provide wildlife habitats |
Permeable surfaces | Porous block paving, concrete and road surfaces | Reduces build up of surface runoff and flooding. Helps to reduce water pollution |
Retention ponds | Open areas of permanent shallow water, but are also designed to temporary store excess water from rainfall. | Can cater for all storms and are good at filtering urban pollutants |
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