Baba urban precipitation

What are the characteristics of urban drainage?

Urban areas experience higher rates of precipitation compared to rural regions because of:

· Pollution - urban air holds more condensation nuclei in the form of dust and dirt particulates

· Warmer air - the warmer the air, the more moisture it holds

Surface runoff (overland flow) is dominant as much of the surfaces are impermeable or designed to shed water - the camber of a road, funnels water to drains. There is also less vegetation to intercept precipitation, which means less evapotranspiration (EVT) rates to cool the air and reduce humidity. As a result, more drainage is needed to remove surface water quickly.

How does urbanisation impact the drainage system?

The urban form impacts the drainage system in the following ways:

Inputs are increased: Precipitation is increased due to pollution and temperature. Extra water is artificially imported into the system.

Outputs are unbalanced:

· Less vegetation reduces EVT

· Impermeable surfaces increases runoff

· Wastewater discharge is an adjusted output into a river or sea (sewage discharge is treated but there will still be liquid waste added to the system)

Transfers are increased:

· Runoff is contaminated through dust and pollutants from industry and transport

· Artificial drainage systems are added above and below ground (e.g. guttering and storm drains etc.)

Stores are decreased overall:

· Impermeable surfaces reduce ground infiltration and stores are reduced

· Rivers are diverted, dredged or channelised which increases capacity

· Reservoirs and ponds are permanent stores but vulnerable to evaporation

· Depression (puddles) stores are temporary

· Lakes and ponds in parks and homes are often drained

These changes become more pronounced across the urban pathway, with the urban CBD system seeing the greatest impacts, compared to the rural-urban fringe system.

What is the urban water cycle like and how are flash floods increased?

Urban hydrographs are 'flashy'. They show a rapid rise in discharge over a short period of time:

· Short lag time

· High peak discharge

· Fast return to base flow

Caused by:

· Low infiltration rates

· High surface runoff

Flash floods in urban areas are increased through:

· Heavier rates of precipitation

· Increased thunderstorms

· High rates of impermeable surfaces

· Channelling of surface runoff to drains

· Building on floodplains to meet demand for housing

· Culverts, rivers and drains blocked with debris

· Narrow channels under bridges constricting flow

Examiner Tip: Practice drawing and annotating or labelling a typical urban hydrograph to help explain how urban areas modify the water cycle.

What is catchment management and what issues are associated with it?

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

Compare soft and hard engineering approaches to urban drainage management. (Afforestation, dams and floodplain zoning)

Soft 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

· Floodplain zoning - restriction on building on certain areas of a floodplain

· Riverbank conservation to reduce lateral bank erosion and collapse through planting to stabilise banks and reduce silting downstream

· River restoration aims to restore the river channel to its original course by removing and reversing past management strategies

· Restoring natural wetlands on floodplains

Hard Engineering:

· Dams, floodwalls and reservoirs are built to prevent flooding and ensure a regular supply of water

· River straightening increases the flow of water by increasing the gradient of a river's channel and removing natural meanders (bends)

· 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)

· Channelisation adds a liner to a straightened river channel (usually concrete) to reduce friction which aims to improve flow rate and therefore, reduce silting

· 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.

What are SUDS and what do they aim to do?

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.

How does the SUDS management train work?

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

What are the 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 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

What is river restoration and conservation?

Human activity on rivers has often been far from effective. In the past, rivers have been straightened, deepened, diverted, dammed, altered by steel and concrete, and adapted to reduce flooding and benefit people. However, many of these changes had negative impacts and unforeseen consequences.

E.g. dams have reduced flooding, improved navigation and allowed year round irrigation. But they also increase rates of evaporation and river erosion below the dam. Meander straightening on the Mississippi was only short-term as the river began meandering again and abandoned the new channels that engineers had created.

River restoration and conservation is the protection, preservation, management, or restoration of wildlife and natural resources (forests, soil, and water etc.). Achieved through the removal of hard-engineered adaptations to meanders, wetlands and floodplains, in order to return the river to natural flood management. It is often difficult and impractical to do, as the built environment has be developed in response to the original changes. As a result, most river restoration projects are only partial, but they can be successful conservation projects.

Examiner Tip: You do not have to 'stick' to provided case studies, you can use your own. You are more likely to remember local details and argue the finer points in your exam. Find a river near where you live that has been 'restored'. Visit the River Restoration Centre for details on projects around the UK.