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M M M source. In the case of Indian River County, the only additional sand source is the beaches themselves. Sand is removed from the beach to make up the deficit in the sand transport system. The observed effect is accelerated erosion of the beaches downdrift (south) of the inlet. The sand losses into lagoon and offshore deposits at the inlet directly translate into erosion losses of about 61,700 cubic yards.per_,year from Indian River County's shoreline. Offshore Reef/Outcrop System An offshore reef/rock outcrop system is known to generally lie offshore of the County in a fairly broad shore -parallel band. The general location of the system in the southern half of the County is well defined from aerial photography. In this area, the reef/rock outcrop lies within a few hundred feet of the shore. The system extends out in a band of about 2,000 feet wide. In other portions of the County, the location and general shape of the reef system is not as well known. It is generally believed to extend over the length of the County in the same shore—parallel configuration. The reef/rock outcrop is reported to lie at water depths of 10 to 15 feet. The system consists of limestone rock outcrops covered with marine life consisting of sponges, marine algaes, and soft and hard corals. Worm reef structures also exist. Evaluation of the offshore profile changes suggests that most of the sand losses are occurring at depths greater than 10 feet. This is approximately the water depth in which the reef occurs. It would therefore appear that the offshore reef system may be a factor in the observed profile changes. The reef outcrop may be acting as a shallow sill which supports the nearshore beach profile. Areas offshore of the reef would lack this protection and would therefore be subjected to greater erosion. Offshore Shoal Area A large shoal area is located offshore of the central and southern portions of the County's coast. Hydrographic charts suggest that the shoal extends up to 20,000 ft offshore (distance to the 30 ft contour). This large submerged feature has a significant influence on the Indian River County shore. For the areas immediately onshore of the shoal, it represents a measure of protection against storm wave damage. Being a relatively shallow feature, it will cause large storm waves to break further offshore that they would along other portions of the County's coast where the shoal does not exist. It also reduces the overall wave energy which reaches the coast. As a result, sand in transport by wave energy will then to be deposited in this area. The beaches in the lee of the shoal do show such accretional trends. The shoal also has an influence upon the beaches which lie up and down the coast from it. Since water depth affects the speed at which wave waves move, areas where the water depths change in the longshore direction can change the direction of wave fronts, much the - same way as a prism bends or refracts light waves. The shoal acts to bend wave fronts around it so that they move towards the interior of the shoal. Coastal areas on the fringes of the shoal area therefore likely to have more of the incoming waves impact the beach at angles to the shore. When waves approach beaches at angles they are able to impart more energy in the longshore direction. Beaches erode faster in these areas where longshore wave energy is locally increased. Vero Beach is just upcoast of the shoal. As experience has shown, it has suffered more serious erosion damage than most other sections of the County's shore except for the Sebastian Inlet area. The down coast limits of the shoal include the southern 3.8 miles of the County. Similar erosion effects are also noted there. 57 AUG 16 1988 800F 13 r-a,E X87