- Hayley Meibach,
- Samuel Moeller (Team Leader),
- Jonathan Ruff
Advisors: M. Horst, V. Krstic
As many areas of The College of New Jersey campus are prone to flooding, the project purpose was to evaluate the hydrological and geotechnical conditions surrounding the site of the proposed STEM building for flood susceptibility. Additionally, design solutions which mitigate the potential flooding are presented.
A hydrologic and geotechnical analysis was completed for the site. This analysis included the following: determination of the watershed area, design storm, soil properties, and soil classifications following the standards set by ASTM and the New Jersey Best Management Practices. After a topographic map of the campus was obtained the watershed was measured to be 0.0054 square miles with 41.22% found to be impervious. The 24-hour precipitation for a 100 year design storm was 8.14 inches. The average slope of the watershed was 4%. The longest hydraulic length was found to be 649 feet. A HEC-HMS analysis found the peak flow and the volume of the design storm to be 16.7 cfs and 84300 CF. Based on data retrieved from Web Soil Survey the soil was found to be 11.2% silt loam and 88.8% clayey substratum. Field soil samples were obtained up to a depth of five feet in one foot increments. An observation well was to be installed to monitor fluctuations in the groundwater table. However, due to the depth of the ground water table the well was rendered unnecessary. A sieve analysis was performed on a sample from each of the five feet and based on observation the soil was between 30% and 90% clayey. A field permeability test showed the soil to have a hydraulic conductivity of 1.388 x 10-3 ft/min.
The peak runoff on the existing site, defined as 100% pervious, was 14.5 cfs. The impervious area of the STEM Building and walkways increased the runoff to 16.7 cfs. Therefore, using this and additional data, three separate designs were developed which reduced runoff from a corresponding design storm. The walkways were redesigned with permeable pavers and incorporated a 3 foot deep, under-ground storage bed with a 1 foot diameter, culvert overflow outlet. The outlet was located with its centroid at 0.8 feet from the surface. The porosity of the bed was designed to have 40% void spaces with a geo-textile lining on the floor of the bed. With this design the peak runoff was reduced to 13.0 cfs. Because the runoff was lower than the existing site, this design was adequate.
The second design was for a stormwater drainage pipe to handle the peak flow from the 100 year design storm. A concrete pipe was selected based on its durability compared to other common materials. A 1.5 foot diameter, 50 foot long pipe located underneath Metzger drive with a change in elevation of 8 feet was enough to handle the peak flow of 16.7 cfs.
The final design also included a green roof system which employed a 2.36 in (60mm) sedum carpet substrate layer with a 1.18 (30mm) drainage layer. The drainage system contained a geo-textile, geo-synthetic drainage layer, root barrier, and water-proof sealant. The expected dead load of the system was found to be 19.5 lb/ft^2 (95 kg/m^2). Hydraulically, the green roof was unable to withstand the 100 year storm and therefore should be used in combination with one of the other 2 designs mentioned previously. However, the green roof was able to control up to 1.4 cfs of runoff produced from a 2 year design storm event. This flow constituted approximately 90% of all rain events that were expected to occur within any given year.