Watershed modeling using HEC-RAS, HEC-HMS, and GIS models – A case study of the Wreck Pond Brook Watershed in Monmouth County, New Jersey
Managing stormwater on a watershed basis is considered the best strategy to address flooding. A watershed model should provide temporal and spatial distribution of runoff response for a given storm. GIS applicability in watershed modeling is increasing due to the availability of spatial information, fast processors and interfaces such as ArcHydro, HEC-GeoHMS, and HEC-GeoRAS linking hydrologic and hydraulic models to the ArcGIS environment.
Soil Conservation Service methods are used widely in hydrologic models. Several parameters inherent to these empirical methods are average values derived from various watershed conditions. These average values overestimate peak flows for flat, low-lying coastal terrains. The design of flood control structures based on these flow values allow more post-development discharge, make the system more hydraulically efficient, increase project costs, and cause flooding for areas downstream. In this study, Wreck Pond Brook Watershed (WPBW), a coastal New Jersey area was used for sensitivity studies of the initial abstraction ratio and peak rate factor. The HEC-HMS modeling results indicated use of a lower peak rate factor (e.g. 284) and 5% initial abstraction ratio provided better characterization of stream response. These updated parameters provide new technical information for improving stormwater management in coastal areas.
An important limitation in hydraulic modeling is the economic constraint on cross-section spacing for surveying channels and floodplains. Applying GIS techniques in hydraulic modeling eliminated this constraint. Floodplain analysis was done using ArcGIS, HEC-GeoRAS and HEC-RAS. Detailed elevation data (LIDAR information from Monmouth County) was incorporated into the HEC-RAS using GIS models. This innovation was important for improving model efficiency. The modeled floodplain demonstrated close agreement to the observed floodplain for the October 2005 storm and showed greater accuracy compared to the FEMA floodplain for the 100-year storm. This study validated use of LIDAR elevation data in floodplain analysis for the second-order streams in coastal NJ.
Finally, an approach was demonstrated using modeled floodplain and HEC-HMS for flood control analysis. This study presents an innovative watershed modeling approach using GIS models while addressing the limitations of traditional hydrologic and hydraulic methods using WPBW as an example.
0543: Civil engineering
0775: Environmental engineering