Building in what the Federal Emergency Management Agency (FEMA) classifies as flood hazard areas can be problematic. FEMA categorizes flood area into zones based upon risk factors, depending on the chance and frequency of flooding events. Zone designations typically include Floodway Areas (stream channel and area to be kept free of encroachment), Special Flood Areas Subject to inundation by the 1% Annual Chance Flood (100 Year Frequency Flood), and Other Flood Areas, which include those areas subject to a 0.2% annual flood (500 year frequency flood). Municipal Zoning Ordinances typically contain a Flood Hazard District, which often are divided into several districts to correlate directly with the above referenced FEMA Floodplain zones. Each district then references uses either permitted by right, special exception, or that are prohibited.
Several municipalities within Chester County have large tracts of land which fall into some type of Flood Hazard District. While building and structure construction is typically prohibited in these areas, certain types of development may often occur, ranging from athletic fields, passive recreation areas, and parking lots. The development of parking lots in particular creates challenges to the due to the physical constraints that typically occur within the Flood Hazard District. These constraints include flat topography, limiting the ability of the site to drain positively, and the high water table / soil limiting zone that occurs within a flood hazard area.
D.L. Howell has recently completed design of such a project located within the Borough of Downing town. This project involved the expansion of an existing church parking lot within the Borough’s Flood Fringe District, which included area within the 100 Year Frequency Flood Area, but outside of the Floodway Area. The site’s topography could not accommodate traditional storm drainage and stormwater management, due to the lack of elevation drop from the site the East Branch of the Brandywine Creek, which formed the church’s southern property boundary line. In addition, based upon stormwater infiltration testing performed by D.L. Howell, the minimal depth to limiting zone did not allow for a typical stone stormwater infiltration bed or trench.
As a result, a parking expansion consisting of porous asphalt paving was proposed for this site. This paving consisted of a subbase consisting of clean crushed aggregate, a choker course of fine graded aggregate placed between the surface course and aggregate base course, and a surface course consisting of open graded porous asphalt. The bottom of the aggregate base course is to be placed on pervious geotextile on uncompacted sub-grade, and was specified to be installed at an elevation where sufficient infiltration was present based upon field infiltration testing. The bottom of this paving course was “stepped”, to allow a continuous flat bottom, while maintaining an elevation that would accommodate percolation based upon the soils testing results. This base course served two purposes: to serve as part of the structural paving section, and to provide stormwater management by allowing the paving to infiltrate at a much shallower elevation than a typical stone infiltration bed would require. The design of this pavement, along with the specifications for operation and maintenance, was proposed to comply with the applicable guidelines set forth in the Pennsylvania Best Management Practices Manual.
It shall be noted that, based upon construction cost estimates received for this project, estimated costs ran approximately two to three times that of a standard commercial paving specification. However, this design includes costs for stormwater management / best management practices facilities that would typically be incurred separately with a standard paving specification, therefore the overall costs may be considered competitive, based on factors such area of proposed paving, site topography, etc.