# Hooghoudt

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### Spacing drainage pipes to reduce groundwater mounding

The yellow box represents the recommended hydraulic conductivity of bioretention filter media

In most LID underdrain applications, lateral drains should be spaced between 5 - 6 m apart.

This recommendation is supported by an analysis of Hooghoudt's equation [1][2][3] in relation to loamy or clayey native soils, where Kmedia>>Ksoil, finds the first term of the numerator negligible, so that the original equation$Drain\ spacing=\sqrt{\frac{8K_{soil}H\left(D_{i}-D_{d}\right)\left(D_{d}-D_{w}\right)+4K_{soil}\left(D_{d}-D_{w}\right)^{2}}{q}}$ may be simplified to$Drain\ spacing=\sqrt{\frac{4K_{media}\left(D_{d}-D_{w}\right)^{2}}{q}}$

Where:

• Kmedia is expressed in m/day
• Dd is the depth to the drain pipe (m)
• Dw 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)
1. 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
2. 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
3. van Beers, W.F.J. 1976, COMPUTING DRAIN SPACINGS: A generalized method with special reference to sensitivity analysis and geo-hydrological investigations, International Institute for Land Reclamation and Improvement (ILRI) Wageningen, The Netherlands