| Popis: |
We investigate the viability of ground penetrating radar (GPR) as a method to estimate the quantity of sediment stored behind the Merrimack Village Dam on the Souhegan River in southeastern New Hampshire. If the pre-dam riverbed can be imaged, the thickness and volume of the reservoir deposit can be calculated without sampling. We surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR unit in May 2008. In a related study, topographic and bathymetric surveys were conducted in 2007, 2008, and 2009 to monitor the sediment flux associated with the removal. By 2009, these surveys measured the pre-dam riverbed in the uppermost cross sections from the dam. We compare these surveys to the interpreted GPR images to calculate a calibrated velocity through the impounded sand of 0.043 m/ns. Using this velocity, the average thickness of the sediment above the interpreted pre-dam riverbed calculated for each of the six cross sections varies between 0.81 m and 3.11 m. The estimate of the quantity of sediment stored behind the dam is ~67,000 m3 compared to a 2004 estimate of ~62,000 m3 based on probing the sediment with a steel rod to the point of refusal. |
