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Groundwater

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Reducing groundwater mounding with underdrains
===Reducing groundwater mounding with underdrains===
[[File:Drain spacing.jpg|thumb|The yellow box represents the recommended hydraulic conductivity of bioretention filter media]]
Mounding of groundwater can be mitigated by correctly sizing and spacing of the pipes within the underdrain structure. In most large infiltrating LID systems(e.g. [[Bioretention]] or [[detention basins]], lateral drains should be spaced between 5 - 6 m apart. But when groundwater mounding must be minimized, this distance will be reducedand should be recalculated.
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This recommendation is supported by an analysis of Hooghoudt's equation <ref>H.P.Ritzema, 1994, Subsurface flow to drains. Chapter 8 in: H.P.Ritzema (ed.), Drainage Principles and Applications, Publ. 16, pp. 236-304, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. ISBN 90-70754-33-9</ref><ref>W.H. van der Molen en J.Wesseling, 1991. A solution in closed form and a series solution to replace the tables for the thickness of the equivalent layer in Hooghoudt's drain spacing equation. Agricultural Water Management 19, pp.1-16</ref><ref>van Beers, W.F.J. 1976, COMPUTING DRAIN SPACINGS: A generalized method with special reference to sensitivity analysis
*''D<sub>w</sub>'' is the minimum acceptable depth to the water table during infiltration event
*''q'' is the inflow volume expressed as a depth over the entire surface (m)}}
 
===Preventing groundwater interaction===
Many LID systems rely upon reuse, or evaporation and transpiration instead of infiltration to the ground. If the site cannot support any infiltration, consider [[Rainwater harvesting]], [[Flow-through planters]], [[Green roofs]], or [[Blue roofs]].
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