1. Dam Removal as a Solution to Increase Water Quality Matthew Nechvatal, Tim Granata Department of Civil and Environmental Engineering and Geodetic Science The Ohio State University Background St. John’s Dam (Figure 1) was a concrete low-head dam located on the Sandusky River in north central Ohio. The river drains approximately 3,680 km 2 of the Sandusky River watershed (Figure 2) and flows north to Lake Erie. Eighty-nine percent of the watershed is located in four counties: Sandusky, Seneca, Wyandot, and Crawford, which consist of 27 major cities or villages. Table 1 shows the different types of land use designated in the watershed. Along with the St. John’s Dam, the Sandusky River has four other dams. The St. John’s dam was constructed in the 1930s to serve as a water supply for the city of Tiffin. It was originally 2.2 m high, 46 m long, and was located at River Mile (RM) 50. It was breached on March 18, 2003, by notching the dam on the west bank to 0.5 m. The dam was then removed on November 17, 2003. Figure 1 – Breaching of St. John’s Dam (Photo by Fang Cheng) Figure 2 – Sandusky River Basin (Courtesy of Fang Cheng) TotalUrbanAg.ShrubsWoodsWater WetlandsBarren Km 2 368043309018465134110 % cover1001.2840.512.60.41.10.2 Table 1 – Land use and area in the Sandusky River Watershed (Courtesy of the Sandusky River Watershed Coalition) Objectives Statistically determine water quality changes upstream and downstream of St. John’s Dam after removal Changes in denitrification potential in the former impoundment will be studied to assess if it is a source or sink of nitrogen Model dam removal with a hydrodynamic model (Mike 11) to determine water quality parameters Methodology Nutrients (NO 3 -, PO 4 3-, NH 4 + ) in the river were sampled monthly at 4 locations upstream and 4 locations downstream of the dam before and after removal (Figure 3) Water quality YSI sonde was deployed from a canoe measuring DO, pH, temperature, ORP, specific conductivity, and turbidity at 5 Hz from 17.7 km upstream of the dam down to the dam Vertical profiles were taken with the YSI water quality sonde directly behind the dam measuring temperature and DO Preliminary Results Pre-removal nutrient data In September and November nutrient concentrations increased from above the dam down to the dam then leveled off below the dam, however, in October data fluctuated Post-removal nutrient data In November nutrient concentrations increased above the former dam site and then leveled off below the former dam. This occurred approximately 8 days after removal. Nutrient data had not yet leveled off In December and April nutrient concentrations leveled off to similar concentrations above and below the former dam site Figure 3 – Sample sites upstream and downstream of the dam Pre-removal temperature data In October temperature increased as you moved down the reservoir toward the dam, however, in November data fluctuated Post-removal temperature data In December temperature leveled off. There was no increase or decrease in temperature as you moved down the former impoundment toward the dam Figure 5 – Spatial temperature profile upstream of the dam (mean subtracted from each point) Figure 4 – Spatial nitrate distribution at eight different locations before and after dam removal upstream and downstream of St. John’s Dam Figure 7- Sandusky River gaging station flow data before, during, and after removal Figure 6 - Turbidity, ORP, and temperature downstream of St. John’s Dam before, during, and after removal YSI water quality sonde was place 200 m downstream of the dam Dam was removed November 17, 2003 Turbidity increased from 32.2 NTU to 120.0 NTU in two hours following dam removal ORP decreased from 129 mV to 46 mV in a 70 minute duration following removal Temperature increased 3.93 o C over the next 2.3 days as the water from the reservoir drained downstream There are two gaging stations upstream of the dam One gaging station is approximately 33 river miles downstream from the dam One day after dam removal an increase of 5.7 m 3 /s was detected Three days after dam removal, a large flooding event increased the discharge by 38 m 3 /s Removing the dam was similar to a small flooding event Acknowledgements We would like to thank Fang Cheng and Dan Gillenwater at OSU and Bob Vargo at the Ohio DNR for the field work they have contributed to this project.