OHIO RIVER BASIN REPORT
Spring 2007
Submitted by John Fulton,
Basin Director, PA-AWRA, Ohio River Basin
Forecasting streamflow and stage during extreme hydrologic events
Forecasting streamflow and stage during extreme hydrologic events such as floods can be problematic. During the July 2006 flooding in the Susquehanna and Delaware River basins in Pennsylvania, 11 forecast points exceeded the upper end of the uniform rating curve. As a result, routing predictions were compromised, because the stage values associated with the modeled flows and the streamflows associated with the observations could not be accurately forecasted.
The National Weather Service (NWS), Middle Atlantic River Forecast Center (MARFC) and the U.S. Geological Survey (USGS) have partnered to evaluate alternative methods for measuring water velocity and computing streamflow dominated by unsteady flow events such as floods. The objective of the project is to pilot the use of radar and computational methods that support real-time, noncontact streamflow measurement. Two types of instruments are being considered. Hand-held radar (see Figure 1) is operational for both low- and high-streamflow extremes; however, the unit is not capable of storing or transmitting real-time surface-water velocities and cannot be deployed in a permanent, standalone mode. A fixed, continuous wave microwave unit (see Figure2) offers a more robust method for measuring surface-water velocity; however, conditions dominated by low flows and relatively fl at water surfaces may limit the use of the instrument.
Two existing USGS streamflowgaging stations (Susquehanna River at Bloomsburg, PA – 01538700 and Chartiers Creek at Carnegie, PA– 03085500) were selected for the trial, because they represent a range of hydraulic extremes and drainage basin areas. The Susquehanna River at Bloomsburg drains a relatively large area, has experienced a wide range of streamflow conditions, is hydraulically influenced by instream structures used to regulate streamflow and stage, requires that measurements be made by boat or at a bridge, and has a relatively wide channel with a stable bed. Chartiers Creek at Carnegie drains a small urban area, is hydrologically flashy, allows measurements to be made by wading or bridge, and has a shallow channel width with steep walls.
Streamflow was computed using data collected from the radar units and corroborated using conventional methods such as hydroacoustics (acoustic Doppler velocimeters and acoustic Doppler current profilers), current meters, and rating curves. Percentage differences between the radar-derived streamflow and conventional techniques at Chartiers Creek at Carnegie, PA ranged from 0 to 11 percent with an average percent difference of 5 percent and standard deviation of 10 feet per second (fps). Percentage differences between conventional and radar-derived discharges at the Susquehanna River at Bloomsburg, PA, ranged from 0 to 8 percent with an average percent difference of 4 percent and standard deviation was 310 fps.
Based on these preliminary findings, the radar-derived streamflow is capable of providing accurate and defensible measures of surfacewater velocity and streamflow, when compared to conventional methods. Work with MARFC is ongoing. The results of this effort will be used to support an operational mode, where real-time data is delivered to MARFC forecasters to provide the necessary feedback that can be used to adjust model parameters and deliver more accurate stage forecasts.
Photographs courtesy John Fulton