Waterborne Electrical Resistivity and Land-Based GPR Surveys for Characterization of Potential River Erosion in Larvik, Norway

An increased risk of erosion of riverbanks and coastal zones are among the expected effects of the present climate change. Erosion can be caused by several natural processes including more intense precipitation. Investigations of the subsurface structures of deltas, riverbanks and coastal areas can give better knowledge of the subsequent erosion risk. Our aim is to estimate the amount of erodible material per unit distance along a channel before an event or the eroded material per unit distance after an event. We present a case-study using near-surface geophysical investigations to image the sedimentary structures in a river mouth near Larvik, Southern Norway. Waterborne electrical resistivity tomography (ERT) measurements were conducted in brackish water above the delta and GPR profiles were collected on the river banks. This case-study shows that imaging the stratigraphy at a land/river boundary can be challenging. The waterborne ERT survey provided limited results for the purpose of the project which was to estimate the amount of erodible material. Due to the limited maximum penetration depth, the bedrock was not imaged in the whole area of interest. On the contrary, the fast-deployed GPR survey provided useful information on the geometry of the different layers.