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.

Figure 1. Annual peak flows at four U.S. Geological Survey streamgages on Long Island.

 

 

Objectives: 

1. To investigate the trends in peak flows that have occurred at selected sites on Long Island and to identify, where possible, the influences that changes in the degrees of development over time and changes in the intensities of precipitation events have had or are having on peak flows. 

2. To evaluate existing regional and urbanized peak flow equations as to their applicability in estimating flood frequencies given the unique hydrologic characteristics of Long Island (Moglen and Shiver, 2006; Steadfast, 1986). Identify streamgages (with at least 30 years of peak-flow data) that either exist in a mostly rural basin, or in a developed basin that during some definable period exhibited peak-flow records that indicate pre-development or static-development conditions from which peak-flow frequencies could be computed. 

3. To assess available hydrologic data and Geographic Information Systems (GIS) layers that could be used to create a new set of regional equations, a model, or tool for estimation of flood frequencies at ungaged locations on Long Island.

References:

DeGaetano, Arthur T., 2009: Time-Dependent Changes in Extreme-Precipitation Return-Period Amounts in the Continental United States. Journal of Applied Meteorology and Climatology, 48, pp. 2086–2099, https://blogs.cornell.edu/cicca/files/2015/03/Time-Dependent-Changes-in-Extreme-Precipitation-Return-Period-Amounts-in-the-Continental-United-States-2lwexp1.pdf

Feaster, T.D., Gotvald, A.J., and Weaver, J.C., 2014, Methods for estimating the magnitude and frequency of floods for urban and small, rural streams in Georgia, South Carolina, and North Carolina, 2011 (ver. 1.1, March 2014): U.S. Geological Survey Scientific Investigations Report 2014–5030, 104 p., http://dx.doi.org/10.3133/sir20145030

Hayes D.C., Young, R.L, 2006, Comparison of peak discharge and runoff characteristic estimates from the rational method to field observations for small basins in central Virginia. U.S. Geological Survey Scientific Investigations Report 2005–5254, 38 p., at https://pubs.usgs.gov/sir/2005/5254/sir05_5254.pdf.

Hodgkins, G.A., 2010, Historical changes in annual peak flows in Maine and implications for flood-frequency analyses: U.S. Geological Survey Scientific Investigations Report 2010–5094, 38 p., at http://pubs.usgs.gov/sir/2010/5094/.

Konrad, C.P, 2003, Effects of Urban Development on Floods. U.S. Geological Survey Fact Sheet FS-076-03, 4 p., https://pubs.usgs.gov/fs/fs07603/

Ku, H.F., Hagelin, N.W., Buxton, H.T., 1992; Effects of Urban Storm-Runoff Control on Ground-Water Recharge in Nassau County, NY, Ground Water, v. 30, no. 4, pp. 507  514

Melillo, Jerry M., Richmond, Terese and Yohe, G.W. Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 p. doi:10.7930/J0Z31WJ2, http://s3.amazonaws.com/nca2014/high/NCA3_Climate_Change_Impacts_in_the_United%20States_HighRes.pdf

Moglen, G.E., and Shivers, D.E., 2006; Method for adjusting USGS rural regression peak discharges in an urban setting: U.S. Geological Survey Scientific Investigation Report 2006-5270, 65 p.

Sauer, V.B., Thomas Jr., W.O., Stricker, V.A., Wilson, K.V., 1983, Flood Characteristics of Urban   Watersheds in the United States, U.S. Geological Survey Water-Supply Paper 2207.

Seaburn, G.E., and Aronson, D.A., 1973, Catalog of recharge basins on Long Island, New York, in 1969, New York State Department of Environmental Conservation Bulletin 70, 80 p.

Seaburn, G.E., and Aronson, D.A., 1974, Influence of recharge basins on the hydrology of Nassau and Suffolk Counties, Long Island, New York, U.S. Geological Survey Water Supply Paper 2031, 66 p.

Stedfast, David A., 1986, Evaluation of six methods for estimating magnitude and frequency of peak discharges on urban streams in New York. Water Resources Investigations Report 84-4350.

Project Location by County

Nassau County, NY Suffolk County, NY