booklet 11 - urban drainage

• Why is urban drainage an issue

  Urban drainage is an issue for many reasons -

  Water doesn't absorb into the ground very well due to the surface being impermeable

  Drains and sewers are often blocked and not properly maintained, water pools on the streets - causes flooding
• Why does it rain more in urban areas

  Rains more in urban areas because cars, buildings and industrial processes give off heat, the heat then gets evaporated, the hot air then cools and condenses to form rain. Rain happens everywhere but it is more frequent in urban areas
• How does urbanisation affect rates of infiltration, run off and evapotranspiration?

  Urbanisation means run off, infiltration and evapotranspiration levels are much different to levels in rural areas.

  In urban areas run off is 55%, 45% higher then in rural areas, evapotranspiration levels are 30% in urban areas and 40% in rural areas. Infiltration levels in urban areas are 15% compared to 30% in rural areas
• How do urban areas become polluted from urban runoff?

  impervious urban surfaces prevent run off absorbing in the soil. The water sits on the tarmac surface and this water picks up pollutants on the way such as motor oil, fertilisers, metals, dirt, pesticides
• What impact might this pollution have?

  This pollutants can cause issues - like problems to health, road conditions can get worse, urban areas can become contaminated etc
• What are storm hydrographs?

  type of graph that shows the changes in river discharge in the lead up to and following the start of a storm
• What do they show and why are they useful in managing urban drainage?

  They show the occurrence of a short period of rain (a heavy shower or storm) over a drainage basin and the subsequent discharge of a river.
• Lag time


  Time between peak rainfall and peak discharge
• Rising Limb


  The increase in river discharge as rainwater flows into the river
• Normal base flow


  The discharge which is added by the ground and through the soil. This is much slower and shows small changes in reaction to rainfall events.
• Peak discharge


  the time when the river reaches its highest flow
• Why are storm hydrographs in urban areas described as 'flashy'?

  Short lag time, high peak, steep rising limb
• What are the hard and soft engineering strategies used in urban areas?

  Hard - River straightening, natural levees, diversion slipways, river channelisation

  Soft - river restoration, flood plain zoning, river bank conservation, afforestation - planting trees and forests
• Costs of using them

  Hard - can be expensive to put in and maintain as well as being intrusive to the natural environment

  Soft - May have to maintained regularly and can mean recreation areas have to be given up for river restoration
• What are SuDs systems?

  Sustainable Drainage Systems
• Aims of SUDS


  drainage solutions that provide an alternative to direct channelling of surface water through networks of pipes and sewers to nearby watercourses
• Techniques used in suds

  - swales - wide drainage channels

  - permeable roads

  - infiltration trenches

  - bio retention basins

  - rain gardens and green roofs
• Benefits of these techniques

  Slowing down surface run off

  reducing flood risk

  preventing water pollution
• Mini case study - Lamb Drove SuDS - were the aims of the project?

  - showcase practical and innovative sustainable water managements techniques within residential areas

  - demonstrates that SUDS are viable and attractive alternatives to more traditional forms

  - to deliver practical solutions for new housing areas
• what techniques have they used?

  - swales

  - retention ponds

  - water butt

  - green roof

  - detention basins

  - filter strip
• Impacts of Lamb Drove

  reduces risk of flooding

  great ecosystems

  area looks more attractive

  very effective
• how successful was lamb droves?

  Plan was a big success -

  very cost effective - 10% les then original forms

  water quality is not hazardous and safe to drink

  substantial improvements in ecosystems and biodiversity