Impact of Compaction Depth on Urban Floding

Urban development relies on soil compaction in order to provide structural stability for infrastructure (buildings, transportation, pipes, etc.). Research has shown that, without substantial intentional soil restoration, it may take up to 40 years to naturally recover reasonable soil infiltration rates in compacted soils (e.g., 1 -2 inches per hour). However, total infiltration volume in urban soils is controlled not just by the surface infiltration rate but also by the depth to a compaction layer where the resistance is greater than 300 psi. A compaction layer acts as a restrictive barrier, preventing infiltrating water from reaching the groundwater, and when the soil above the compaction layer is saturated, the remaining rainfall in a storm cannot infiltrate. This project is investigating 3 periods of development in Middletown, PA, to improve understanding of the impact of development age/type on infiltration rate and compaction depth. Current testing in the older areas of town where houses date to the late 1700s and early 1800s still have compaction depths ranging from 2 to 6 inches, whereas testing in a park developed during the 1950s has compaction depths of 1.25 to 3 inches. SWMM modeling in a nearby watershed highlighted the impact of shallow soil storage on flooding during high-intensity rains, with the flooding not seen when the same watershed is modeled using the SCS Type II design storm.