Combined impacts of compaction and microburst storms on runoff from an area of the Penn State Harrisburg Campus

As part of the modeling of the watershed that contains Penn State Harrisburg, field measured infiltration capacities and depths to a confining layer/compaction layer, as well as differences in rainfall discretization, were explored. This project focused specifically on a small area of the campus where green infrastructure design guidelines from the early 2000s were used to address soil compaction/remediation. The campus has a 300-year history of land manipulation due to agriculture and military operations prior to its use as a college campus. Field measurements of infiltration and compaction were collected at 10 locations around campus and compared to the results from WebSoil Survey, which classified most of the campus in Hydrologic Soil Group A. The watershed was modeled under four scenarios: ARM with a curve number of 78, and Horton’s infiltration equation using field data from a compacted location, a soil restoration location, and an area of new construction. The three rain gages were based on the SCS Type II 10-year storm event (the common design event storm for this area) and the rainfall from a July, 2017 storm event where the campus received 4.71 inches in 87 minutes discretized into hourly increments based on National Weather Service hourly data and into one minute increments based on NOAA 1-minute data. These results highlight the need to incorporate the shorter, more intense rain events that we are now seeing into the evaluation of any stormwater design. It is these storms that exceed the system and result in flooding. It also highlights the impact of that the discretization increment of rainfall can have on the results. Finally, these results highlight the impacts of compaction and identifying the location of the compaction layer, as compared to not including the compaction depth as a limit to infiltration capacity in modeling when using the Horton equation.

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