Lower Platte Sedimentation Study

In Fall 2008 UW-Madison graduate students—Benjamin Lee, Cory Anderson, Megan Corrado and Hoi Lai Tseung—taking a course in Sediment Processes and Engineering in Natural Channels under the direction of Professor John Hoopes undertook the problem of understanding the sedimentation in the Lower Platte River (LPR) and developing solutions to restore a stable channel in the lower Platte. This fall they completed their report with a historical account of the cause of excess sedimentation, an analysis of present sedimentation and flow rates and recommendations for establishing a stable channel in the lower Platte River. Important features of the LPR have been identified and used to propose a stable channel from the Hwy 61/35 Bridge to the Mississippi River:

• Instability in the Lower Platte River (LPR) was the result of a base level change introduced by Lock and Dam 11 in 1937;
• Low velocities in and excessive sediment supply to the LPR continue to cause sediment deposition and channel alteration principally downstream of the Banfield Bridge and probably upstream, though vertical accretion was not apparent in the field inspection;
• The channel width through the LPR appears relatively stable upstream of the Banfield Bridge and unstable downstream of the bridge, although the original LPR thalweg is evident downstream;
• The channel between the Banfield Bridge and the Hwy 61/35 Bridge cut itself off after the opening of Lock and Dam 11 to increase the bed slope and transport the sediment supplied to it;
• The slope of the LPR is about 0.000306;
• The low flow channel width at the Hwy 61/35 Bridge was 110 ft and 210 ft at the Banfield Bridge;
• The measured flow rate on November 15, 2008 was similar to the value obtained by scaling the flow at the Rockville gauge to the LPR by the ratio of drainage areas;
• The 100-year flood discharge is approximately 36,500 cfs;
• Suspended sediment is the dominant mode of transport, and medium silt is the median grain size;
• A yearly sediment supply of 80,759 tons is expected at the Rockville flow gauge, and a yearly supply of 196,211 tons is predicted to the LPR by scaling of drainage areas;
• The dominant discharge is approximately 5,500 cfs;
• A bankfull depth and width of approximately 7 ft and 140 ft, respectively, are expected for a stable channel cross section.

These data will be used to explore methods of attaining a stable channel (through methods such as parallel embankment or porous sediment fence)