DE Streamstats

Objectives

This proposed study is designed to improve the peak-flow frequency information on which DelDOT's hydrologicmodeling approach is based. The study will:

1. Compute peak-flow frequencies (2-, 5-, 10-, 25-, 50-, 100-, 200-, and 500-year floods) for USGS streamflowgaging stations and peak-flow partial-record stations with 10 or more years of peak-flow data (through September 2003) in Delaware and nearby areas in adjacent States;

2. Use GIS technology to determine basin and climatic characteristics for the stations analyzed in the previous step;

3. Develop a new method for estimating the generalized-skew values needed to obtain the peakflow frequency estimates for streamflow-gaging stations;

4. Produce a new set of equations for estimating peak-flow frequencies on ungaged streams in Delaware; and

5. Place the revised station peak-flow frequency estimates and the new equations on the Web through a single interface that will allow users to obtain estimates for any gaged or ungaged stream not affected by flow regulations or diversions.

Statement of Problem

The mission of the Delaware Department of Transportation (DelDOT) is to provide a safe, efficient, and environmentally sensitive transportation network in the State of Delaware. Some tasks that are carried out as a part of this mission include effective flood-plain management and the efficient and cost-effective design of bridges, culverts, and roadway embankments. To carry out these aspects of its mission DelDOT needs reliable techniques for making accurate peakflow frequency estimates for various sites on streams and rivers in Delaware.

Peak-flow frequency estimates for streamflow-gaging stations currently maintained and operated in Delaware by the U.S. Geological Survey (USGS) in cooperation with Delaware Geological Survey are used to predict the frequency of occurrence of floods of various magnitudes and identify areas of probable flood inundation. In most locations USGS stations are not available for use in this process, making it necessary to use equations developed by the USGS to estimate peak-flow frequencies for ungaged sites. These equations relate peak-flow frequencies at the ungaged sites to physical and climatic characteristics, including, but not limited to, drainage area, mean basin slope, mean basin elevation, mean stream slope, population density, reservoir area and capacity, percentage of wetlands and water bodies, percentage of urban land use, percentage of impervious land cover, mean annual rainfall, and 2-year, 24-hour rainfall, that can be measured from maps or determined from Geographic Information System (GIS) databases.

Several factors limit the use and accuracy of the information that the current USGS peak-flow frequency estimation techniques for Delaware provide, thus diminishing the accuracy of any hydrologic or hydraulic analyses based on this information. These problems include:

1. The last USGS study to estimate peak-flow frequencies for streamflow-gaging stations and to develop new equations for estimating peak-flow frequencies at ungaged sites in Delaware is based on data that are now 13 or more years old, and do not include significant floods from 1999 and 2003. These recent floods have had a substantial impact on calculated flood frequencies at gaging stations, and are likely to have a similar effect on regional flood-frequency relations;

2. The national map used to determine generalized skew in the Bulletin 17B method (Interagency Advisory Committee on Water Data, 1982) for estimating peak-flow frequencies at streamflow-gaging stations is over 30 years old;

3. The equations used for estimating peak-flow frequencies for ungaged streams in Delaware are not valid for the most highly developed stream basins (those having Basin Development Factor (BDF) values greater than 10); and

4. Determining the basin characteristics used as independent variables in the current equations for estimating peak-flow frequencies for ungaged sites is often time-consuming, resulting in high costs, and results may not easily be reproduced and verified.

Strategy and Approach

USGS personnel from the Baltimore office of the MD-DE-DC Water Science Center, and from the MA-RI District will conduct the proposed study. A GIS specialist in the Baltimore office will use GIS to provide the most current data available for a number of climatic and basin characteristics that have predictive potential for peak-flow frequency estimation. A GIS specialist from the MA-RI District will assist the local GIS specialist with preparation of the GIS data. Concurrently, a team of hydrologists from the Baltimore office experienced in regional peak-flow studies will update the peak-flow statistics to include data through September 2003 for all USGS streamflow gages in Delaware. Subsequently, this team will develop the most appropriate techniques for peak-flow frequency estimation in Delaware. These techniques will address the issue of generalized-skew determination for Delaware as well as the overall method of equation development (fixedregion or region-of-influence). Baltimore office personnel familiar with Web-interface technology will incorporate all new data, estimation techniques, and equations generated by the study into the USGS National Flood Frequency (NFF) and StreamStats programs.