Groundwater Flow in a Tidally Influenced Gravel Beach in Prince William Sound, Alaska: Fid-Bau Portal

Groundwater Flow in a Tidally Influenced Gravel Beach in Prince William Sound, Alaska

Bobo, Adane M. / Khoury, Naji / Li, Hailong / Boufadel, Michel C.

Abstract

The hydraulics of a gravel beach on Eleanor Island in Prince William Sound (PWS), Alaska, was investigated by measurements of the water table and by numerical simulations using the software SUTRA. Sediment samples were obtained from six wells at the approximate three depths: 0.30, 0.60, and 0.90 m, and they were sieved in the lab to generate the grain size distribution, and subsequently the saturated hydraulic conductivity, $K_{0}$ . The results indicated that the beach consisted of two layers: an upper layer with a high $K_{0}$ and a lower layer with a low $K_{0}$ . In the landward portion of the intertidal zone, $K_{0}$ was around $10^{- 2} m / s$ and $10^{- 5} m / s$ for the upper and lower layers, respectively. In the seaward portion of the intertidal zone, the $K_{0}$ values were $10^{- 4} m / s$ and $10^{- 6} m / s$ in the upper and lower layers, respectively. The agreement of the model with the observed water table data was very good. A decrease in the slope of the beach in the seaward direction caused water to flow out from the lower layer and propagate seaward within the upper layer. This ensured that the water table seaward of the exit point was replenished and remained above the interface of the layers. The results suggest that the porosity of the beach decreases from 0.33 at the surface to around 0.05 at 1.0 m deep. The absence of oil on this beach is probably because the beach water table remained above the layers’ interface during low tides. Using the calibrated model, it was found that the flow from land into the beach is around $6 m^{3} / day$ (per meter parallel to the shoreline) and that the flow out of the lower layer varied with time reaching a maximum of $13 m^{3} / (day \cdot m)$ at low tide. The beach exchanges up to $7 m^{3} / (day \cdot m)$ with the sea.