Infiltration Trench
Content Table
General Description
Infiltration trenches or infiltration ditches (in Germany called Rigoles) are devices used to infiltrate clean and pollutant free stormwater runoff into the ground. They are built as excavations filled with porous material to ensure most of the stormwater can be stored during precipitation periods.
Runoff is stored in the void space between the porous material and infiltrates through the bottom and into the soil matrix over a few days. As porous materials rock or gravel are commonly used, for underground infiltration trenches there are porous bodies made from plastic material available as well.
The size of the excavation has to be calculated to meet the impervious area connected to the infiltration trench, the local precipitation preconditions and as well as soil characteristics. In germany they are usually calculated to hold back precipitation events that occurs one time in a five years. Pretreatment measures, such as buffer strips, swales, or detention basins (sediment basin) that limit the amounts of coarse sediment entering the trench are important to avoid clogging (see Performance Drawbacks) and for the removal of pollutants.
Infiltration trenches are one alternative of the measures classified as sustainable drainage systems (SUDS) and can help to keep or restore the potentially natural water balance of a settlement area. DWA (2006, p. 21) gives a value of 200 m3/ha, that is 20.000 m3/km2 decentralised retention volume that can typically be retrofitted in most existing settlement areas with sustainable drainage systems. This is an equivalent of 20 mm of precipitation.
To reach significant effect as a flood risk mitigation measure infiltration trenches or other sustainable drainage systems have to be realised on many sites in the catchment. Usually this is not easy due to the ownership structure in settlements. But there are administrative, economic and informative instruments that can be taken by communities / municipalities to facilitate implementation.
Pictures
Schematical drawing of a subterrenean infiltration element located under a swale used for the infiltration of the run-off from a roof. (HCU, Jeannette Thamm)
Infiltration trench beside a cross-country road on the island of Usedom, Germany (Coordinates: 53.960595,14.063442, Picture taken August 2009 by Elke Kruse).
Inlets to the infiltration trench (Island of Usedom, Germany, Coordinates: 53.960595,14.063442, Picture taken August 2009 by Elke Kruse).
Inlet details of the shaped concrete bricks (Island of Usedom, Germany, Coordinates: 53.960595,14.063442, Picture taken August 2009 by Elke Kruse).
Infiltration trench built to drain a small parking lot in Braunschweig (Germany, Coordinates: 52.2986,10.516367, Picture taken in Summer 2005 by Office Prof. Nagel, Schonhoff + Partner
Infiltration trench built to drain a small parking lot in Braunschweig (Germany, Coordinates: 52.2986,10.516367, Picture taken in Summer 2005 by Office Prof. Nagel, Schonhoff + Partner
Flood Risk Mitigation Strategy
This flood risk mitigation measure is usually aming at
- flooding risk from flash floods in urban catchment areas
- flooding risk along the watercourses (in or downstream urban areas)
This measure is predominantly aiming to reduce flooding risks
- in the catchment.
It is building up decentralised retention volume and thus reducing the risks of flash flooding. Depending on catchment size and steepness it can reduce discharge peaks causing flooding along the watercourses.
Measures which build decentralised retention volume usually have most effect on flood peaks in small catchments (<100 km2) with little steepness (<2 o/oo).
Time to take Effect
Once build, the run-off reducing effects of this measure are immediate.
Due to clogging performance can decrease during the lifetime, especially when it's not well maintained.
Pro Con to use this measure
Pro:
- good solution in location with limited space for other types of sustainable drainage systems (like shallow swales, raingardens)
Contra:
- limited by soil permeability: hi permeability (hydraulic conductivity kf >> 10-3) leads to insufficient cleaning effects, low permeabilty (kf << 10-6)leads to inappropriately large necessary infiltration areas [DWA 2005]
- limited functionality when the ground freezes
- maintenance issues not yet perfectly established
- needs sustainable drainage systems implementation program to take effect on catchment scale
Knock-Out Criteria:
- rainwater / run-off contaminated (metal roofs, main roads, industrial sites, etc.), in this case has to be combined with pretreatment / filtering technique (see filtering and retention structures)
Performance Drawbacks
If there is sediment transport into the infiltration trench, sediment accumulates inside and retention volume is lost. Fine particles can clog the soil matrix so infiltration rate decreases. A solution to this issue is that the water to be infiltrated in the infiltration trench is cleaned of its sediment by a filtering technique. This goes along with the advantage of reduced pollutant freight depending on the filtering technique chosen.
References
- DWA-Themenheft (2006): Dezentrale Maßnahmen zur Hochwasserminderung. S. 22f., DWA, Hennef
- Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL) (2005): Empfehlung zur Versickerung und Wasserrückhaltung.
Technical Standards in Germany:
- DWA (2005): A-138 Planung, Bau und Betrieb von Anlagen zur Versickerung von Niederschlagswasser, Arbeitsblatt, DWA, Hennef.
Links
- Sustainable Urban Drianage: The Stormwater Managers Resource Center: http://www.stormwatercenter.net/
- U.S. Environmental Agency - Municipial Technology - Technology Fact Sheets concerning Stormwater treatment: http://www.epa.gov/OWM/mtb/mtbfact.htm
- http://www.ciria.org.uk/suds/publications.htm
