Time of travel of water in the Potomac River, Cumberland to Washington

Abstract

This report introduces a graphical procedure for estimating the time required for water to travel down the Potomac River in the reach extending from Cumberland, Md., to Washington, D.C. The time of travel varies with the flow of the river; so the stage of the river at the lower end of the reach--the gaging station on the Potomac River near Washington, D.C.--is used as an index of flow. To develop the procedure, the reach between Cumberland and Washington was divided into five subreaches, delineated by six gaging stations. The average of the mean velocities of the river at adjacent gaging stations was used as the mean velocity in .the intervening subreach, and a unit mass of water was assumed to travel at a rate equal to the mean velocity of the river. A statistical analysis of possible variations in travel time between Cumberland and Washington indicated that the shortest travel time corresponding to a given stage near Washington would be about 80 percent of the most probable travel time. The report includes a flow-duration curve and a flow-frequency chart for use in estimating discharge at the gaging station near Washington and subsequently the travel time of Potomac River water without knowledge of stage. The flow-duration curve shows the percentage of time during which specified discharges were equaled or exceeded in the past, and it can be used to predict future flow in connection with long-range planning. The flow-frequency chart shows the time distribution of flow by months and can be used to make a more nearly accurate estimate of discharge in any given month than could be made from the flow-duration curve. The method used to develop the time-of-travel charts is described in sufficient detail to make it usable as a guide for similar studies on other rivers, where the velocity of flow is relatively unaffected by dams and pools in the reach being studied.