Traveltime and longitudinal dispersion in Illinois streams

Abstract

Twenty-seven measurements of traveltime and longitudinal dispersion in 10 Illinois streams made from 1975 to 1982 provide data needed for estimating traveltime of peak concentration of a conservative solute, traveltime of the leading edge of a solute cloud, peak concentration resulting from injection of a given quantity of solute, and passage time of solute past a given point on a stream. These four variables can be estimated graphically for each stream from distance of travel and either discharge at the downstream end of the reach or flow-duration frequency. From equations developed from field measurements, the traveltime and dispersion characteristics also can be estimated for other unregulated streams in Illinois that have drainage areas less than about 1,500 square miles. For unmeasured streams, traveltime of peak concentration and of the leading edge of the cloud are related to discharge at the downstream end of the reach and to distance of travel. For both measured and unmeasured streams, peak concentration and passage time are best estimated from the relation of each to traveltime. In measured streams, dispersion efficiency is greater than that predicted by Fickian diffusion theory. The rate of decrease in peak concentration with traveltime is about equal to the rate of increase in passage time. Average velocity in a stream reach, given by the velocity of the center of solute mass in that reach, can be estimated from an equation developed from measured values. The equation relates average reach velocity to discharge at the downstream end of the reach. Average reach velocities computed for 9 of the 10 streams from available equations that are based on hydraulic-geometry relations are high relative to measured values. The estimating equation developed from measured velocities provides estimates of average reach velocity that are closer to measured velocities than are those computed using equations developed from hydraulic-geometry relations. INTRODUCTION Traveltime and mixing of water within a stream reach are basic streamflow characteristics that water-resources managers and planners must know to predict the rate of movement and dilution of contaminants that may be introduced into streams. They also are necessary for most waterquality models used in water-resources planning. A study designed to measure traveltime and longitudinal mixing in selected reaches of Illinois streams under a range of steady flow conditions was carried out from 1975 through 1982 in cooperation with the Illinois Environmental Protection Agency. Purpose and Scope The purposes of this report are to summarize the results of measurements that were used to develop relations for estimating traveltime and mixing characteristics for unregulated streams in Illinois and to present the estimating techniques. Ten streams were selected for measurement of traveltime and mixing characteristics (fig. 1). They include a range in drainage area and conditions expected to govern velocity and mixing. Measured reaches range from 5.2 to 40.3 mi in length, average channel slopes range from 0.98 to 7.97 ft/mi, and drainage areas above sampling sites range from 12.4 to 1,516 mi2 . Most of the studied streams, like the majority of streams in Illinois, have sand beds with sand and gravel riffles. Beds of the Apple and Vermilion Rivers are coarser than those of the other eight streams and are composed primarily of gravel and bedrock. Although nine of the streams have predominantly naturally formed meandering channels, the Kaskaskia River has a straightened, dredged channel in the reach measured. Flow is unregulated in all reaches measured, but some artificial structures or modifications are present. Riffle-pool sequences are apparent on the streambed at low flow, and these sequences cause local variations in channel geometry, velocity, and slope. Stream gages are located on each stream (fig. 1, table 1), and the gaging records provided the data base used to relate measured variables to the long-term streamflow regime. Details of the measurements and graphical relations for estimation of traveltime and mixing characteristics are presented for each measured stream. Equations for estimating traveltime and mixing for unmeasured streams are derived, and examples of the application of techniques to Introduction 1