Figure 4. St John's Dam, A week before removal, Nov 10 th, 2003 Figure 3. Breaching West Side of the St. John's Dam, March 18 th 2003 Figure 5. Removing St. John's Dam, Nov 17 th 2003, 8:00 Am Figure 6. Two hours later, Nov 17 th 2003, 10:00 Am Monitoring Sediment Transport and Geomorphology Change Associated with Dam Removal Fang Cheng, Timothy Granata Department of Civil and Environmental Engineering and Geodetic Science, The Ohio State University Introduction Sediment released after dam removal can cause significant changes in river morphology, flow, aquatic habitats and even in river ecosystem processes both downstream and upstream. However, quantitative study of sediment transport associated with dam removal is surprisingly insufficient. Knowing the dynamics of sediment transport in reservoirs is the key to understanding the influences of dam removal on river ecosystems. Site Description Sandusky River (Figure 1, 2), drainage area of 3637 km 2, flows north into Sandusky Bay at Lake Erie. St. John’s dam, 46 meter long and 2.2 meter high, was located on Sandusky River at river mile 50. Reservoir Length =10km, slope=0.01% Reservoir impoundment =0.59 km 2, and the storage is 561, 234 m 3 The sediment accumulated in the reservoir is composed of gravel, cobble, and sand On March 18 th, 2003, the west side breach was notched down to the bed (Figure 3,4) Dam was removed on Nov. 17 th, 2003 (Figure 5-6) For questions, please contact Fang Cheng, Methodology Transects were each surveyed at the reservoir, and 75 meters, 545 meters and 3km downstream from the dam. Each series of transects has at least five cross sections with about two meters apart. This provided a baseline for the changes caused by dam removal. Elevation data were collected at 1Hz using a Trimble® 5700 Receiver (Figure 7) Changes in river channel width and elevation resulting from dam removal is determined by differencing data of pre- and post- removal. For each series of cross sections, bed material distributions at the surface were collected using bulk sampling approach. Figure 2. Sandusky River WatershedFigure 1. Location of Sandusky River Watershed Downstream and upstream surface sediment size distribution are plotted in the figure above. We can see that the D 50 of downstream bed material is 1.2mm, 29mm, 50mm, 70mm and 82mm at stations of 24 km, 12.5 km upstream from the dam and 75 m, 545 m and 3 km downstream from the dam. It is interesting to notice the surface material size coarsening pattern. Most previous studies had shown downstream fining pattern. At least 60% of the transported particles are smaller than 2mm, which indicates that fine sands are mostly entrained rather than gravels. Figure 7. Surveying with Trimble 5700 receiver Time series of turbidity were measured at two locations, one at the west bank 100 meter upstream and the other one at the west bank 200 meter downstream from the dam using YSI 6600 Sonde® sensors Mean velocity was measured in shallow (< 0.6 m) depths using a handheld Sontek® FlowTracker ADV In deeper areas (depth >0.6 m) a BT-ADP (3 MHz) was used (Figure 9). Figure 8. Bedload Trap The pebble count method (Wolman, 1954) was employed for cross sections with coarse materials (diameter>128mm). Bedload traps were installed at downstream and upstream to measure bedload transport rates (Figure 8). Preliminary Results Processed cross sections are plotted as in Figure 10. The Purple dots are GPS points surveyed on Nov. 11 th, 2003, a week before the dam was removed, and the green dots are GPS points surveyed on Apr. 23 rd, The river bed surface is generated by creating TIN and Kriging interpolation. Immediately after dam removal, reservoir was dewatered and water depth drop about 1m (Figure 13-14), while downstream water level rose 0.6m. As we expected, downstream water turbidity suddenly increased 3 times within the first two hours after the removal and hit the peak 3 hours later (Figure 15). The base level of turbidity increased 15% after the dam was removed. The other peak as shown in Figure 15 on Nov. 21st was because of a small rain storm. Compared with the rainfall event, dam removal seems did not have significant short-term influence on downstream suspended sediment concentration. Conclusion Study sediment transport of St. John's dam removal helps us to fully understand how river channel response for disturbance. Future Work River cross sections will be surveyed at least twice at downstream and upstream to avoid survey error. Bed load and suspended load will be measured at the same locations for comparison. Same survey will be repeated at same sites after dam removal. Some empirical sediment transport model will be compared, and we will develop a new numerical model to simulate dam removal event. Simulation Sediment transport models, Van Rijn, and Meyer Peter and Muller, will be used to calculate bed load and suspended load. These models will be compared for the event of St. John's dam removal using software MIKE 11 (see right figure). A two-fraction model will be applied to calculate bed load as a comparison of the MIKE 11 simulation results. The two-fraction model, sand and gravel, considers sand and gravel as a mixture instead of individual and predicts more accurate results. Figure 10 River bed surface at downstream 75m from the dam Figure 9. BT-ADP YSI 6600 The size distribution of transported sediments at station 75m and 200m downstream and 12.5 km upstream was plotted in the figure above. Fig 13. Reservoir dewatering Fig 14. Reservoir dewatering (6km upstream) As we expected, reservoir water turbidity was lower than downstream turbidity within the first three days after complete notching west side of the dam. However, this relation dramatically changed three days later when a small rainfall event occurred. After a small rain storm, reservoir suspended sediment concentration increased faster than downstream and became 60% higher than downstream on the 5th day. Downstream and upstream water turbidity were measured from March 17, 2003 to March 28, (Figure 16). Notching the dam did not have influence on short-term turbidity. Acknowledgements We thank Matthew Nechvatal, Dan Gillenwater, Bryan Arvai, and Lauren Glockner at OSU and Ryan Murphy and Constance Livchak at the ODNR Lake Erie Geology Group for helping field work. We also appreciate Yudan Yi for helping GPS data processing. Reference Ferguson, R.T., and C. Paola, 1997, Bias and precision of percentiles of bulk grain size distributions, Earth Surface Processes and Landforms, 22: Wolman, M.G., 1954, A method of sampling coarse river-bed material, EOS Trans. AGU, 35(6): Wilcock, P.R., and Kenworthy, S.T., 2002, A two-fraction model for the transport of sand/gravel mixtures, Water Resources Research, 38(10): Objectives Quantitatively monitor geomorphologic change in river channel associated with dam removal Quantify bed load transport rate caused by dam removal Develop a numerical model to simulate graded sediment transport Dam Breach Mar., 03 Dam Removal Nov., 03 Figure 11 River bed surface change at downstream 75m from the dam DN3km DN545m DN200m DN75m UP50m UP 12.5km The elevation change at 75m downstream from the dam was calculated by differentiate the averaged elevation within 1 meter grid (Figure 11). The maximum, minimum elevation change is 0.155m, and -0.23m, respectively. The mean elevation change is -0.02m, and standard deviation is 0.1m It indicates elevation did not have significant change at 75m downstream of the dam within the half year. Detailed pre- and post-dam monitoring is necessary to determine the geomorphologic changes in river channel, and the consequential effects on river ecosystem. Quantitatively monitoring changes in river-bed elevation and sediment distribution provides foundation for numerical modeling channel response to dam removal. Figure 11 River bed surface change at downstream 75m from the dam