VII. SEDIMENT BUDGET 

 1. Introduction . 



a. Sediment Budget . A sediment budget is a sediment transport volume 

 balance for a selected segment of the coast. It is based on quantification of 

 sediment transportation, erosion, and deposition for a given control volume. 

 Usually, the sediment quantities are listed according to the sources, sinks, 

 and processes causing the additions and subtractions. In this chapter, the 

 sediment discussed is usually sand and the processes are either littoral 

 processes or the changes made by man. 



The purpose of a sediment budget is to assist the coastal engineer by (1) 

 identifying relevant processes, (2) estimating volume rates required for 

 design purposes, (3) singling out significant processes for special attention, 

 and, on occasion, (4) through balancing sand gains against losses, checking 

 the accuracy and completness of the design budget. 



Sediment budget studies have been presented by Johnson (1959), Bowen and 

 Inman (1966), Vallianos (1970), Pierce (1969), Caldwell (1966), and Jarrett 

 (1977). 



b. Elements of Sediment Budget . Any process that increases the quantity 

 of sand in a defined control volume is called a source . Any process that 

 decreases the quantity of sand in the control volume is called a sink. 

 Usually, sources are identified as positive and sinks as negative. Some 

 processes (longshore transport is the most important) function both as source 

 and sink for the control volume. 



Point sources or point sinks are sources or sinks that add or subtract 

 sand across a limited part of a control volume boundary. A tidal inlet often 

 functions as a point sink. Point sources or sinks are generally measured in 

 units of volume per year. 



Line sources or line sinks are sources or sinks that add or subtract sand 

 across an extended segment of a control volume boundary. Wind transport 

 landward from the beaches of a low barrier island is a line sink for the ocean 

 beach. Line sources or sinks are generally measured in units of volume per 

 year per unit length of shoreline. To compute the total effect of a line 

 source or sink, it is necessary to multiply this quantity by the total length 

 of shoreline over which the line source or sink operates. 



The following conventions are used for elements of the sediment budget: 



(a) Q . is a point source 



^ 



(b) Q . is a point sink 



t 



(c) q . is a line source 



(d) q . is a line sink 



x 



These subscripted elements of the sediment budget are identified by name in 

 Table 4-15 according to whether the element makes a point or line contribution 



4-113 



