Adjusting soil infiltration coefficients for groundwater level

Current UK guidance for the design of sustainable drainage systems recommends that infiltration devices, such as soakaways, permeable pavements and infiltration basins, should be able to operate during periods of extreme groundwater level. Furthermore, higher groundwater levels have recently been shown to cause a reduction in the empirical soil infiltration coefficient, as used in the design of infiltration devices. However, there is currently no simple method available to estimate the required reduction in the design infiltration coefficient to account for an extreme groundwater level. This paper uses exploratory numerical subsurface saturated–unsaturated hydrological modelling to quantify the effect of groundwater level on the infiltration coefficient for six typical soil types. The fixed resolution finite-element simulations are also benchmarked against a solution employing adaptive mesh refinement. The modelling results are distilled into charts and a simple equation to allow the calculation of adjustment factors, with which to reduce the design infiltration coefficient to account for a higher design groundwater level. Varying soil type sensitivity is highlighted. These factors could also be used to correct for soakage tests made during periods of lower groundwater level. Threshold depths to groundwater, below which no adjustment is required, are identified for each soil type.