USING GROUND PENETRATING RADAR TO DETERMINE THE QUANTITY OF IMPOUNDED SEDIMENT BEHIND A DAM

We investigate the viability of ground penetrating radar (GPR) as a method to estimate the quantity of sediment stored behind the Merrimack Village Dam (MVD) 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. Such estimates are necessary to plan sediment management after dam deconstruction. We surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR unit in May 2008. The instrument was deployed in a canoe and paddled across the reservoir, while a GPS unit placed on top of the GPR device recorded the location. We choose the MVD as the field site for this study for several reasons. First, the dam was removed in August 2008, allowing the GPR interpretations to be ground-truthed as the river incised into the reservoir deposit. Second, the sediment was known to be almost exclusively sand overlying a pre-dam riverbed of gravel, cobbles, and boulders. This difference in material creates a strong reflection of electromagnetic waves capable of being imaged by GPR. Also, the deposit included no ferromagnetic materials, so magnetic permeability will not affect the imaging, and the conductivity of fresh water is low, so the attenuation is low enough to survey several meters into the subsurface. Finally, the reservoir deposit could be at most ~6 m thick, the height of the dam, guaranteeing the pre-dam riverbed would be visible on the GPR image. 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 preliminary calibrated velocity of the impounded sand of 0.053 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 1.05 m and 4.10 m. We will present a comparison of the volume of impounded sediment calculated by interpretation of the GPR data with a 2004 estimate based on probing the sediment with a steel rod.