The Nearshore Morphology of Sand Beaches on the Great Lakes Shoreline of Southern Ontario

Abstract

<p> The nearshore morphology of sand beaches in the Great Lakes usually consists of a ridge and runnel unit in the shallow, inner portion of the nearshore zone and one or more longshore bar and trough units in the deeper, outer portion. These two sets of features are morphologically and genetically different and exhibit spatial and temporal variations indicating the relative importance of environmental factors in controlling the form of each. </p> <p> Ridges and runnels form and evolve in response to a combination of the controlling factors of wave and water level variations. Ridge formation occurs in response to a rapid decrease in wave energy and lowering of the water level during the subsiding period of a storm. Ridges, with heights of 0.04-0.40 m, take 5-10 days to migrate 10-30 m across an inner nearshore terrace toward the shore. Ridge migration produces laminations dipping at 20°-30° toward the shore. Ridge attachment to the shore produces an accretional berm composed of laminations dipping at about 5° toward the lake. </p> <p> An annual cycle of erosion and deposition occurs in the inner nearshore and foreshore zones and is due to seasonal wave energy and water level variations. Erosion occurs in spring and summer because of rising water levels while the deposition of planar laminations dipping at less than 5° toward the lake is caused by lower water levels and higher energy waves in autumn. </p> <p> Based on the analysis of over 100 echo sounder profiles of the outer nearshore zone of 7 beaches, longshore bar and trough morphologic properties (maximum depth of bar formation, number of bars, and bar amplitudes), are controlled by three main environmental factors (wave energy or surf base, nearshore slope, and sediment size). Bars are present at depths of 3.5 m and at distances of 500 m from the shore, the number of bars ranges up to 6, and bar amplitudes range up to 2 m. Surf base ranges from 3-6 m, slopes from 0.009-0.02, and mean sediment size from 1.85 Ø - 3.30 Ø. There is a good correlation between the slope and sediment size. Temporal variation in bar morphology during the summer period of study is small or insignificant. </p> <p> Individual longshore bars probably only approach, rather than attain, equilibrium forms because of the temporal flucuations in the environmental controls of wave energy and water level. The geometrical trend in all longshore bar systems for the bar amplitude and spacing between bars to increase with increasing depth and distance from the shoreline, suggests a wave energy dissipation process which is somewhat analogous to dampened oscillation. </p>