Combined Impact of Inland and Coastal Floods: Mapping Knowledge Base for Development of Planning Strategies

Abstract

Concurrent incidence of heavy rainfall events and coastal storm surges can cause damages of exponential proportion to the coastal habitants and infrastructures. Super Storm Sandy and hurricane Irene were wake up calls that the floodplains and evacuation zones in New York City need major revision. Federal and State emergency agencies have been working on revising the appropriate maps and spatial tools for planning and management purposes. This paper presents an integrated scheme for floodplain delineation in coastal areas, considering the combined effect of inland and coastal flooding. The proposed scheme is applied to the Bronx River watershed in the Bronx section of New York City. Consideration of inland and coastal flooding is provided by developing a conceptual rainfall-runoff model and a sea level simulation model. Climate change has intensified the intensity and frequency of extreme rainfall events and storm surges in the region. In order to evaluate climate change impacts on floodplain, projections from two GCMs (General Circulation Models) for A1B (balanced) and A2 (medium-high emissions) climate change scenarios are used. Change factor methodology is employed to downscale the GCMs’ outputs. Flooding scenarios are defined based on the frequency analysis of the projected runoff and water level for different recurrence intervals. This is a key issue in planning and management of coastal region considering different levels of acceptable risk. Inundation maps of the coastal floodplain are created for each flooding scenario using a geographic information system (GIS) platform. Result shows that long-term average of runoff and water level in the region in a 30-year period will be increased by 42% and 0.46 m respectively. The study finds that the effect of sea level rises on floodplain extent is greater than changes in runoff. In order to plan for effective flood risk management strategies in urban areas, the spatial analysis and mapping of flood inundation should be implemented. This study emphasizes the importance of updating the delineation of coastal floodplains as a part of planning and management of land use zoning and future developments in coastal regions.