PercPack for ICPR: A Groundwater - Surface Water Interface

A Groundwater - Surface Water Interface

In order to accurately model groundwater and surface water interactions, both vertical unsaturated flow and horizontal saturated flow must be considered. Vertical flow occurs when the soil column directly below the pond is unsaturated and the ambient water table is below the bottom of the pond as shown in the sketch below. As stormwater runoff enters the pond, it begins to soak into the soil column. If runoff enters the pond faster than the ability of the soil to accept it, water levels in the pond will rise and additional head becomes available for percolation.

Unsaturated Vertical Flow Vertical flow in PercPack^TM is based on a modified form of the Green-Ampt equation that tracks an advancing saturated wetting front in the soil column below the pond. In the standard Green-Ampt formula, the infiltration rate is a logarithmic decaying function dependent on the location of the wetting front among other factors. The standard Green-Ampt formula assumes that the soil surface is saturated, but the depth of water on the surface is negligible. This formula has been modified in PercPack^TM to include the additional driving head, H, due to fluctuating water levels in the pond. The water surface elevation in the pond is determined from the surface flow computations in ICPR and the modified Green-Ampt equation then incorporates the head, H, when computing infiltration rates.

In addition to including pond stages into the Green-Ampt equation, a mechanism has been included to recover soil storage so that multiple and independent storm events can be modeled. The recovery method depends on the location of the wetting front relative to the water table (which is actually moving with time) below the pond. This is particularly useful for calibration purposes with say 30-day rainfall periods.

Saturated Horizontal Flow The saturated horizontal flow algorithm is automatically triggered when the wetting front reaches the water table and vertical flow. PercPack^TM models the outward radial advancement of the groundwater mound away from the pond by segmenting the area beyond the edge of the pond into a number of finite difference cells. Water movement from one cell to the next is tracked based on continuity principles and Darcy’s Law.

Recovery of the Surfical Aquifer Leaky aquifers can also be modeled with the use of an annual recharge rate. Water is removed from the surficial aquifer at the user specified annual rate. This concept is illustrated in the figure below. The bottom of the pond is at elevation 101 feet and the ambient water table is at elevation 94 feet. The blue line represents water levels in the pond and the red line depicts the location of the wetting front. The black dashed line is the location of the water table directly below the pond. Notice how the wetting front (the red line) follows the pond bottom and then dips downward. As water enters the pond, the wetting front advances. If the potential infiltration rate exceeds the inflow rate to the pond, then all water percolates downward and the stages in the pond do not build up. However, when the inflow rates exceed the potential infiltration rates, stages increase in the pond as indicated by the blue line.

The pond eventually dries up after the first storm event due to percolation (i.e., when the blue line reaches the pond bottom). At this point, the water table begins to gradually subside creating an unsaturated zone directly below the bottom of the pond. The water table in this example is slightly below elevation 98 feet when the next storm event occurs. There is about a 3-foot unsaturated zone below the pond at this point, so vertical unsaturated flow can occur again and the process is repeated.

Groundwater Mounding As already mentioned, PercPack^TM models the outward radial advancement of the groundwater mound away from the pond by segmenting the area beyond the edge of the pond into a number of finite difference cells. Water levels are determined in each cell. Consequently, the groundwater mound as a function of both time and distance away from the edge of the pond is readily available as shown in the graphs below. Notice that the impact of the pond on the groundwater mound subsides with both distance and time, as would be expected.