Sample Site #1: A project with no natural outlets
The site shown below was previously an airfield. It had well drained soils with no streams or natural channels draining the site. While the soil provided sufficient drainage for the preexisting condition, the proposed development would increase the runoff while decreasing the pervious area.
The solution lay in the design of several on-site infiltration areas, which were essentially gravel-filled dry wells. Combined with suitable storage and detention areas, this enhanced infiltration was able to handle all runoff from the site, meeting the requirement for "no net increase in runoff."
When designing this system, the engineers performed full scale tests on one of the proposed dry-wells, thus providing a reliable infiltration rating that could be used in the design of the entire site. Since many factors can influence infiltration, actual testing provides valuable confirmation of the proposed design.
Utilizing infiltration is complicated by the fact that there are no widely accepted design standards. Despite these complications, infiltration must often be incorporated into a watershed model. HydroCAD provides a number of techniques that can be used to model infiltration/exfiltration. These range from the use of a constant CFS rating (as in the above project) to an exfiltration velocity applied over the wetted area of a gravel-filled dry well. By providing a range of flexible exfiltration techniques, designers can handle most situations they are likely to encounter.