A simplified analysis was completed by the DNR Division of Waters to estimate the amount of upstream storage that would have been needed to reduce the 1997 flood waters to 89,000 cubic feet per second (cfs) in the Grand Forks area. The 89,000 figure represents the 100 year or 1% chance flood established by FEMA and is currently used for administering the local floodplain management ordinance.
Preliminary U.S. Geological Survey daily flow data indicate that the total volume of water that flowed past Grand Forks/East Grand Forks in April and May 1997 was approximately 5.1 million acre-feet. Water in the snowpack and rainfall during this period was about 8-10 inches on average over the Red River basin upstream of Grand Forks, or approximately 13-16 million acre-feet. Since only about five million acre-feet of river volume was measured, approximately 8-11 million acre-feet were still stored in upstream areas or were dissipated naturally through soil moisture, evaporation or percolation to ground water.
The actual daily flow data for April and May were entered into the HEC-1 computer model. The water volume was routed through simulated reservoirs of various sizes via a trial and error method until the predicted peak discharge was reduced to 89,000 cfs, the 100-year flood mark. (In the modeling exercise, a single simulated reservoir was located on the Red River immediately upstream of Grand Forks.)
The analysis showed that approximately 1.3 million acre-feet of floodwater storage would have been required to reduce the peak flow of 136,900 cfs to 89,000 cfs. The simulated reservoir would have had to contain 1.3 million acre-feet of water, representing the equivalent of a 10-foot deep Mille Lacs Lake - a 132,516 acre lake.
This modeling exercise represents an idealized ungated reservoir located immediately upstream of Grand Forks. In actuality, a single reservoir holding 1.3 million acre-feet of water storage could not be provided. Creation of this amount of storage volume would require storage sites throughout the basin. Storage sites located farther upstream of the Grand Forks area would provide fewer flood control benefits. Dispersed sites do not capture and contain all runoff from the contributing basin.