Urban flooding in coastal areas: A case study of runoff management and storm runoff and tidal flooding

Traditionally, engineers use floodplain studies performed by the Federal Emergency Management Agency (FEMA) in cooperation with local authorities to establish hydraulic criteria for designing structures in floodplains. However, a case study performed for design of a light rail transit (LRT) system in a coastal watershed in New Jersey indicated that additional detailed hydrologic/hydraulic studies were required to adequately estimate flooding frequency and elevations to enable the development of safe design criteria and efficient drainage system design. In this watershed, which is politically divided among several municipalities, both fluvial and tidal flows cause flooding events. This paper focuses on a case study performed for the Bellmans Creek watershed extending over three municipalities located near Jersey City, New Jersey. The approach taken in this study accounted far the entire watershed as one integrated unit, regardless of political boundaries, and it also accounted for tidal and fluvial flows. This approach resulted in a more realistic designation of the floodplain and flooding levels and frequencies than available through traditional studies. Whereas traditional studies interpreted Bellmans Creek to be dominated by tides, the case study found that fluvial flooding in Bellmans Creek resulting from storm runoff is dominant on most of its length and that the corresponding design flood level exceeds estimates of previous studies by approximately 4 feet. The case study results were confirmed by field investigations. This paper reviews a set of symptoms that may indicate the need for additional detailed watershed studies in the design of structures in floodplains. It also reviews planning, management, hydrologic, hydraulic, and field investigation procedures and techniques used in this case study, which provided more realistic results than traditional studies. The experience gained and conclusions obtained from the case study are also presented.