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|Title:||Sand movement by waves|
|Authors:||University of California, Berkeley.|
|Publisher:||United States. Beach Erosion Board.|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical memorandum (United States. Beach Erosion Board) ; no. 48.|
Abstract: A series of two dimensional experiments were carried out in a wave channel using waves having both high and low steepness ratios to investigate the movement of sand along the bottom. Waves having steepness ratios above 0.03 produced storm profiles, eroded the foreshore, and built up longshore bars, whereas those having steepness ratios below about 0.02 built up the foreshore and produced ordinary or summer profiles lacking longshore bars. Previous experiments have indicated that waves having steepness ratios from 0.03 to 0.025 can produce either storm or ordinary profiles. One of the present experiments using waves having a steepness ratio of 0.019 resulted in a storm profile and thus extends this range. The greater thickness of the turbulent zone developed along the bottom by the high, steep waves caused a general lowering of the nearshore profile. The relative amplitudes of the vertical and horizontal components of orbital wave motion, however, appear to be of greater importance than the amounts of turbulence in causing variations of depth along the profile. The rate of profile change, and thus of sand movement, was found to be greatest immediately following the change from one wave steepness to the other, i.e., when the profile was furthest from equilibrium with the waves acting upon it. As this greatest disequilibrium may be expected to occur in nature when waves are changing from steep to gentle or vice versa, any longshore transport taking place would also be expected to be at a maximum at this time, as has been found in previous three dimensional wave-tank experiments. The large steep waves moved an appreciable quantity of sand offshore to a point where it could not be returned to the nearshore area by the waves having a low steepness ratio. As a result, the nearshore profile was lowered during a cycle from low to high waves and back to low waves. This suggests that there will be an annual loss of sand offshore along coastlines where deep water is found close to shore. Ripples were present on the bottom during all of the experiments. They moved shoreward in the nearshore area and offshore along the farthest offshore portion of the profile. Ripple movement was caused by the differences between the onshore and offshore orbital water accelerations and/or velocities The shape of the ripples was found to be related to their velocity of movement, being more skewed with higher velocities. The ripple wave length was found to be proportional to the horizontal component of orbital water motion. A consideration of the mechanism of onshore ripple movement in shallow water shows that when sand is being transported in the direction of ripple movement, the net transport will be less than that found by measuring ripple volume and velocity, while the total transport will be less than twice the measured values. At equilibrium, when there is no net transport of sand, the total amount of sand being shifted will be twice that indicated by ripple measurements. When the net transport is in the opposite direction from ripple movement, no limiting factors are inherent from the mechanism of ripple movement. The net transport will, of course, be less than the total transport by the amount measured in the ripple movement. The experiments were carried out using a well sorted, medium grain sized sand. Despite the initially low sorting coefficient, a marked range in grain size was developed along the profile. In general, the larger grains moved beachward while the smaller grains moved offshore. The median diameters along the profiles at equilibrium show a fairly good correlation with the horizontal component of orbital water motion. The structure of the sediments deposited varied with the different conditions of wave action at the points of deposition. The tightness of packing in the various lamina was a fuhction of the anount of reworking the surface of the deposit underwent during the time of deposition. The beds formed by grains slumping or rolling down a slope were loosely packed, whereas those formed by slow deposition accompanied with reworking of the depositional surface were tightly packed. The various lamina and cross-bedding developed in the beach and along the bottom are illustrated.
|Appears in Collections:||Technical Memorandum|