4.713 Sediment Budget Boundaries . A sediment budget is used to identify 

 and quantify the sources and sinks that are active in a specified area. 

 By so doing, erosion or deposition rates are determined as the balance 

 of known sinks and sources. Boundaries for the sediment budget are deter- 

 mined by the area imder study, the time scale of interest, and study pur- 

 poses. In a given study area, adjacent sand budget compartments (control 

 volumes) may be needed with shore-perpendicular boundaries at significant 

 changes in the littoral system. For example, compartment boundaries may 

 be needed at inlets, between eroding and stable beach segments, and between 

 stable and accreting beach segments. Shore-parallel boundaries are needed 

 on both the seaward and landward sides of the control volumes. They may be 

 established wherever needed, but the seaward boundary is usually established 

 at or beyond the limit of active sediment movement, and the landward bound- 

 ary beyond the erosion limit anticipated for the life of the study. The 

 bottom surface of a control volume should pass below the sediment layer 

 that is actively moving, and the top boundary should include the highest 

 surface elevation in the control volume. Thus, the budget of a particular 

 beach and nearshore zone would have shore parallel boundaries landward of 

 the line of expected erosion and at or beyond the seaward limit of signifi- 

 cant transport. A budget for barrier island sand dunes might have a bound- 

 ary at the bay side of the island and the landward edge of the backshore. 



A sediment budget example and analysis are shown in Figure 4-43. 

 This example considers a shoreline segment along which the incident wave 

 climate can transport more material entering from updrift. Therefore 

 the longshore transport in the segment is being fed by a continuously 

 eroding sea cliff. The cliff is composed of 50 percent sand and 50 per- 

 cent clay. The clay fraction is assumed to be lost offshore while the 

 sand fraction feeds into the longshore transport. The budget balances 

 the sources and sinks using the following continuity equation: 



Sum of Known Sources - Sum of Known Sinks = Difference 



An example calculation is shown in Figure 4-43. 



4.72 SOURCES OF LITTORAL MATERIAL 



4.721 Rivers . It is estimated that rivers of the world bring about 3.4 

 cubic miles or 18.5 billion cubic yards of sediment to the coast each year 

 (voliime of solids without voids). (Stoddard, 1969; from Strakhov, 1967.) 

 Only a small percentage of this sediment is in the sand size range that is 

 common on beaches. The large rivers which account for most of the volume 

 of sediment carry relatively little sand. For example, it is estimated 

 (Scruton, 1960) that the sediment load brought to the Gulf of Mexico each 

 year by the Mississippi River consists of 50 percent clay, 48 percent silt, 

 and only 2 percent sand. Even lower percentages of sand seem probable for 

 other large river discharges. (See Gibbs, 1967, p. 1218, for information 

 on the Amazon River.) But smaller rivers flowing through sandy drainage 

 areas may carry 50 percent or more of sand. (Chow, 1964, p. 17-20.) In 

 southern California, sand brought to the coast by the floods of small 

 rivers is a significant source of littoral material. (Handin, 1951; and 

 Norris, 1964.) 



4-119 



