Floodplain hydraulics in a glaciated bedrock river valley

Abstract

Data and one dimensional, unsteady theory document the average, seasonal, and diurnal hydraulics of an unconfined aquifer in the surficial floodplain deposit of the Neponset River, which flows through a glaciated bedrock valley in eastern Massachusetts. The 20 m thick silty sand deposit has a permeability of 1.4 × 10−11 m2, a porosity of 0.37, a 600 m halfwidth, and an infiltration coefficient of 0.39. The steady water table is parabolic with a 0.60 m value at the valley wall that implies an average 33-year travel time across the floodplain in response to an average recharge rate of 7.0 × 10−9 m/s. Seasonal hydraulics are governed by the floodplain porosity and marked by periodicity of the river (0.48 m amplitude) and recharge (1.9 × 10−8 m/s amplitude), which maintain flow from the floodplain into the river year round. Attenuation of the diurnal fluctuations suggests that the specific yield ranges from 0.05 to 0.14, and yields horizontal flow excursions of 1 m scale near the riverbank.