I am working on a redevelopment / retrofit project for a small residential site. I need to eliminate the open discharge from an existing catch basin discharging directly to a wetland and reduce the increased runoff from the redevelopment. I thought I would cut back the pipe, cap and drill holes in the sides and construct an exfiltration trench to contain the pipe. The exfiltration trench will overflow and flow to the wetland as a shallow channel or swale (which is acceptable).

The site constraints and seasonally high ground water elevation are the limiting factors which will allow only a 0.75m deep x 1.5m wide exfiltration trench at about 0.1m above water table. I have measured the saturated hydraulic conductivity of the soil in the area.

My question is related to determining the exfiltration rate I can expect from the trench when ground water is near the bottom of trench elevation and therefore determine the drawdown period.

If I model the exfiltration trench as a pond in HydroCAD with an exfiltration outlet using the measured conductivity I see higher exfiltration rates and volumes as I increase the ground water elevation below the trench. HydroCAD applies Darcy's Law to calculate the exfiltration and as the distance between the bottom of trench and ground water decreases the hydraulic gradient increases so I see why. I have concerns the model is not valid with the the ground water elevation so near the exfiltration trench.

Is anyone aware of a reference where the performance of exfiltration trenches was studied with small media thickness between the bottom of trench and ground water? I would like to figure out how best to model this situation with HydroCAD.

Thanks!