A simplified model of soakaway infiltration interaction with a shallow groundwater table

This paper presents a new and simplified modeling concept for soakaway infiltration in the presence of a shallow groundwater table, including representation of the local groundwater mound and its effects on the infiltration rate. The soil moisture retention curve is used to represent the influence of the mound on infiltration rates.

The model is intended to be used in situations when distributed urban drainage models with soakaways or similar infiltration devices are coupled to distributed groundwater flow models. With this new modeling concept, the local mounding from small-scale infiltration systems, and its effects on the infiltration rate, can be represented even if the spatial resolution of the groundwater flow model is coarser than the extent of the mound.

The new model has been run for a number of scenarios and soil parameters, and the results compared to the output from a two-dimensional unsaturated/saturated flow model based on Richard’s equation. The comparison shows that soakaway emptying times calculated by the new model are on average 13% higher than the emptying times of the two-dimensional model.

The deviation is smaller for scenarios including a shallow groundwater table, only around 5% on average. Groundwater mounding depths are underestimated close to the soakaway and overestimated far from the soakaway. At 5 m distance from the soakaway center, the average deviation is 18% for the shallow groundwater scenarios.

Mass balance errors were checked and found to be below 1% for all scenarios at all times during the simulation period. The extra uncertainty introduced by this new model is compensated for by the reduction in runtime; it is on average 600 times faster than the two-dimensional model. Furthermore, the new model is based on the same input parameters as the two-dimensional model and thus requires no extra calibration.

The new modeling concept is therefore a useful tool for simulating small-scale stormwater infiltration in the presence of a shallow groundwater table with distributed models on larger scales.