. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography ®. © FIGURE 14. Model for littoral sediment transport. (A) Wave refraction pattern, with wave approach normal to coast. (B) Re- sulting curves for energy density at the breaker E (dimensions AIT-2)/ longshore component of littoral wave power PL {dimen- sions MLT~3); and the littoral discharge gradient dq/dx (dimen- sions L2T~l). (C) Advanced state of coastal evolution. After May and Tanner (1973). angle at c. The maximum is attained, however, closer
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography ®. © FIGURE 14. Model for littoral sediment transport. (A) Wave refraction pattern, with wave approach normal to coast. (B) Re- sulting curves for energy density at the breaker E (dimensions AIT-2)/ longshore component of littoral wave power PL {dimen- sions MLT~3); and the littoral discharge gradient dq/dx (dimen- sions L2T~l). (C) Advanced state of coastal evolution. After May and Tanner (1973). angle at c. The maximum is attained, however, closer to point c, as the gradient of wave energy density on this subdued model coast is relatively flat. Both the sand transport rate //. (see p. 247) and the discharge of sand (q) are proportional to Pi and vary with it. Therefore, the longshore discharge gradient dq/ varies as the derivative of Pi (Fig. 14). The sediment continuity equation (p. 190) states that the time rate of change of seafloor elevation along a streamline in the littoral current is proportional to the littoral discharge gradient under conditions of steady flow. In other words, if more sand is moving into a given section of shoreface than is moving out (negative dq/dx), then the seafloor of that section must aggrade. If, on the other hand, more sand is being exported than imported (positive dq/dx), the seafloor of that sector must erode (see the discussion on p. 190). In general, erosion occurs along a positive discharge gradient, and deposition occurs along a nega- tive discharge gradient. In the model of Fig. 14, this relationship means that the shoulder of the headland, from a to c, should erode, with the material being transported into the bay, to fill sections c through e. The same sort of process should occur on the other side of the bay (not shown) and the other side of the headland (not shown). If the direction of wave approach were held constant and normal to the regional trend of the coast, then
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