Infiltration Basin 1

[ngneer]

In designing an infiltration basin (40% voids, assume size is 20'L x 6'w x 3'd), how should I determine the hydraulic gradient? If i = h/L, is h the depth of water in the basin (3'). Is L also the depth of the basin (3'), thus i = 3/3 = 1. Unless stormwater is ponding above the basin, is there any situation in which i would not equal 1. Thanks in advance.

 

[Hambley]

The rate of infiltration into the infiltration trench can be calculated using the Dupuit-Forchheimer dischaarge formula for a seepage face. To use it, you need to know the head in a well upgradient from the trench, the expected height of water in the trench, and the hydraulic conductivity of the material upgradient of the trench:

Q' = k (ho^2-hL^2)/(2*L)

where Q' is the seepage rate per unit width of trench (L^2/T units), k is the hydraulic conductivity (L/T units), ho is the hydraulic head in the upstream well (L units), hL is the hydraulic head in the trench (L units), and L is the distance from the upgradient well to the trench in the direction of flow (L units). If we consider that Q'=k i ho, then by inspection the hydraulic gradient becomes:

i = (ho^2-hL^2)/(2*ho*L).

Hope this helps. By the way, the Dupuit-Forchheimer equation is given in most groundwater texts, for example p. 78 of Hydraulics of Groundwater by Bear (1979). Also remember that the head in the trench and in the upgradient well must be referenced to a common datum.

Doug Hambley, Ph.D., P.E., P.G.