FLOOD CRESTS ON THE OHIO AND MISSISSIPPI, AND THEIR MOVEMENT

Abstract

SYNOPSIS This study is based on the results of daily gagings of the Ohio and Mississippi Rivers under the varying conditions of stream flow that may arise in the come of a year. The actual velocity of flood flow for a number of freshets and flood flows have also been considered. Owing to the difficulty in evaluating the influence of some of the important factors that determine the velocity of flow of water in open channels, no simple formula of general application has yet been developed. While the velocity of flow of water in an open channel is greater the higher the stage and is greater in a rising than in a falling river, yet the movement of crest stages—not being dependent wholly upon the velocity of flow—appears to be more rapid at comparatively low stages when the water is well within the banks of the stream than when the banks are overtopped as at very high stages. The reasons are fairly obvious although local conditions and the tributary effect may be the dominating influence as at Cairo, Ill., where, at times, the balance between inflow and outflow is disturbed by the channel capacity below Cairo. Overflow of lowlands on the left bank opposite and immediately below Cairo at stages above 40 feet results in more or less retardation in the velocity of the outflow and crest stages on the Cairo gage may be due to that cause alone. The average rate of flood flow in the upper Ohio is about 5 miles per hour and on the Mississippi below Cairo very close to 4 miles per hour, although crest stages move at a slower rate.