Effects of urbanization on streamflow, sediment loads, and channel morphology in Pheasant Branch basin near Middleton, Wisconsin

A 5-year, data-collection and modeling study was conducted on Pheasant Branch basin in and near Middleton, Wisconsin. The objectives of the study were to: (1) describe the streamflow characteristics, sediment transport, and stream-channel morphology in the Pheasant Branch basin; and (2) relate the above factors to changes caused by urbanization and project the effect of urbanization on the hydrology and channel morphology of the study area.

Streamflow data were collected at five sites for 4 years in the basin to defme present streamflow conditions. Suspended-sediment data also were collected at these sites. In addition, periodic surveys of monumented channel cross sections were made and compared with a survey from an earlier study to document changes in the channel over a period of 10 years.

The suspended-sediment data show a decrease in suspended-sediment load through the fully urbanized reach of the stream in 3 of the 4 years studied. This corresponds with the slight net decrease in cross-section area in this reach for the same period. Possible explanations for the decrease in suspended-sediment loads through this reach include (1) sediment being trapped at the five drop structures and the Park Street erosion-control structure and (2) sediment being deposited in overbank areas throughout the reach. Farther downstream, the suspended-sediment load decreased through the Pheasant Branch marsh during a year of high flow but increased through the marsh in 2 of the 3 years for which complete data are available. The marsh is not acting as a net sediment trap in some years.

A rainfall-runoff model was calibrated and verified for the basin upstream from U.S. Highway 12. This model was used to simulate 68 years of summer flood hydrographs for three conditions: Current land use, projected urban development, and complete urban development of all lands in the basin. Analysis of simulated flood flows indicates that projected urban development would double the mean annual flood peaks at U.S. Highway 12. Complete development of the basin would increase the mean annual flood peak by a factor of 2.4.

From 1971 to 1977, the mean streambed elevation lowered by almost 2 feet, and the mean channel width increased by more than 35 percent in the reach downstream from the fully urbanized part of the basin. In other reaches, the mean streambed elevation lowered by more than a foot. Changes in channel cross sections after 1977 were smaller.

Increases in flood flow would tend to enlarge the channel. An increase in the mean annual flood by a factor of 2. 0 to 2.4 will cause a 40 to 50 percent increase in channel width and a 30 to 40 percent increase in channel depth.