HYDRAULICS AND MORPHODYNAMICS OF MORSE RIVER INLET, MAINE

Morse River Inlet, in Phippsburg, Maine, provides tidal exchange between the ocean and Spirit Pond via a salt marsh and meandering tidal creek system. Bedrock outcrops constrict the entrance of Morse River such that ebb currents are directed eastward toward nearby Popham State Park. During the past few years, cut-bank erosion has removed > 50 m of tree-covered dunes and now is threatening Park facilities. Moreover, erosion has resulted in little high-tide beach for recreational use, and the strong tidal currents in the channel have led to Parks and Services rescuing numerous beach-goers who ventured out to the island spit just offshore of the beach. Measurements of channel cross section, coupled with Acoustic Doppler Current Profiler time series, are used to compute the tide curve, ebb and flood discharge and the tidal prism. Grain size trends from 29 sediment samples taken along the channel combined with bedform measurements indicate that flood currents and landward sand transport dominate the channel. This is supported anecdotally, by the presence of trees, eroded from the beach appearing within Spirit Pond. The ebb exceeds the flood durations by > 3 hrs leading to stronger flood currents (peak velocities of 0.9 m/s compared to peak ebbs of 0.45 m/s). Tidal prism calculations indicate that the inlet channel is in disequilibrium with its dimensions, which is likely due to its extended length and hydraulic inefficiency. Erosion of the State Park would be drastically reduced if the channel was relocated upstream and the present channel closed. This would shorten the Morse River tidal channel, increase the hydraulic gradient, and terminate tidal current induced erosion of the State Park shoreline.