Using ground-penetrating radar to determine the quantity of sediment stored behind the Merrimack Village Dam, Souhegan River, New Hampshire

We investigated 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 southern New Hampshire. If the predam 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. In May 2008, we surveyed six cross sections with a Mala Geosciences ProEx 100 MHz GPR. In a related study, topographic surveys were conducted in 2008-2009 to monitor the sediment flux associated with the removal of the Merrimack Village Dam in August 2008. Within a month of the removal, these surveys mapped the predam riverbed in the uppermost cross sections in the former impoundment. We compared these surveys to our interpreted GPR images for one cross section to determine a calibrated velocity for the impounded sand of 0.043+ or -0.020 m/ns. We also estimated the radar velocity of the deposit by analyzing hyperbolic reflections in the GPR images, and found a similar result (0.039 m/ns). Using the calibrated velocity, we estimated a total volume of sediment stored behind the Merrimack Village Dam of 66,900+ or -9900 m (super 3) , which compares well to a previous estimate (62,000 m (super 3) ) based on a depth-to-refusal survey. Our findings indicate that GPR is a useful technique for quantifying impounded sediment prior to dam removal in reservoirs containing 1-10 m of sand overlying a coarser predam riverbed, but it may be less effective in settings with finer and/or thicker impounded sediment.