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===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 reduced and should be recalculated.
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