An Analysis of Trends in the Magnitude of Floods in Urbanized Watersheds on Long Island, New York

Summary:

This study aims to do a thorough analysis of trends in peak streamflows on Long Island. Reliable information about the magnitude and frequency of floods is essential for flood insurance studies, flood-plain management, and the design of transportation and water-conveyance infrastructure, such as roads, bridges, culverts, dams, and levees. Federal, State, regional, and local officials need peak flow information to effectively plan and manage land use and water resources, protect lives and property in flood-prone areas, and determine flood-insurance rates, particularly in densely populated urban areas. Long Island is a regional economic driver which has experienced public health and safety issues due to extreme and frequent flooding, which also threatens highly valued public and private infrastructure.  Peak discharges of a stream can increase markedly as a result of urbanization within the drainage basin. The paving of open areas for roads and parking lots, and construction of buildings prevent storm water from infiltrating into the ground and thus increases overland runoff to stream channels either directly or through storm-sewer systems. This rapid discharge to streams can increase the magnitudes of peak flows and lead to local flooding of streams and storm sewers (Stedfast, 1986; Sauer and others, 1983). There has been limited study regarding urban flood frequency in New York, particularly on Long Island. The applicability of existing methods that have been developed with data from throughout the Nation have not been tested on Long Island. Due to the hydrology of Long Island, and its complex and changing land use, it is likely that existing methods will prove insufficient for current and future hydrologic conditions on Long Island. The results of this study will help determine if a flood frequency model for ungaged locations on Long Island would be practical and reliable.

Problem:  

Are there statistically significant trends in peak flows at USGS streamgages on Long Island? What are the main drivers of those trends (urbanization, climate change, storm-water management etc.)? The current USGS New York Water Science Center regionalized flood-frequency equations in StreamStats do not include Long Island. Long Island has more complex surface-water/ groundwater interactions and greater urbanization than most of New York State. The effects of urbanization on Long Island hydrology may include an increase in runoff from impervious areas and consequent increase in peak-flow magnitude and flashiness. In many near shore areas storm runoff is directed into streams, and subsequently, the ocean, whereas inland areas often divert storm runoff into engineered stormwater infiltration structures, such as recharge basins (Seaburn and Aronson, 1973). In areas where infiltration structures capture storm runoff, an increase in recharge and subsequent increase in base flows is likely to occur. A decrease in vegetated areas within a basin may reduce evapotranspiration and result in higher groundwater levels as well as higher and sustained base flows in streams. Ku and others (1992) showed that the proportion of groundwater recharge on Long Island during the growing season increased significantly with urban development and the use of recharge basins. Additionally, they showed that the net annual recharge increased about 12 percent in areas where recharge basins are used and decreased about 10 percent where storm runoff is routed into streams and coastal waters. Before considering the development of a regionalized flood-frequency model for Long Island, these fundamental questions must be addressed. To test the feasibility of such a model, two basins on Long Island will be selected as pilot cases and investigated to identify explanatory variables and to develop site-specific models that can be used to estimate peak flows for the selected basins.