which permit collection and analysis of over 12,000 bits of information per 

 hour over a survey track about 5,500 meters (18,000 feet) long. 



These developments in radioactive tracing permit in situ observations and 

 faster data collection over much larger areas (Duane, 1970b) than has been 

 possible using fluorescent or stable isotope tracers. Hovever, operational and 

 equipment costs of radionuclide tracer programs can be high. 



Accurate determination of long-term sediment transport volume is not yet 

 possible from a tracer study, but qualitative data on sediment movement useful 

 for engineering purposes can be obtained. 



Experience has shown that tracer tests can give information on direction 

 of movement, dispersion, shoaling sources, relative velocity and movement in 

 various areas of the littoral zone, means of natural bypassing, and structure 

 efficiency. Reasonably quantitative data on movement or shoaling rates can be 

 obtained for short time intervals. It should be emphasized that this type of 

 information must be interpreted with care, since the data are generally 

 determined by short-term littoral transport phenomena. Hovever, tracer 

 studies conducted repeatedly over several years at the same location could 

 result in estimates of longer term littoral transport. 



3. Sediment Transport Calculations . 



a. Longshore Transport Rate . The example calculation of a sediment 

 budget in Section VII, 6 is typical in that the magnitude of the longshore 

 transport rate exceeds by a considerable margin any other element in the 

 budget. For this reason, it is essential to have a good estimate of the 

 longshore transport rate in an engineering study of littoral processes. 



A complete description of the longshore transport rate requires knowledge 

 of two of the five variables 



[%^y Qpt' \' Qn' ^^ 



defined by equations 4-31, 4-32, and 4-33. If any tvo are known, the 

 remaining three can be obtained from the three equations. 



Section V, 3,a describes four methods for estimating longshore transport 

 rate, and Sections V,3,b through V,3,f describe in detail how to use two of 

 these four methods, (see methods 3 and 4). 



One approach to estimating longshore transport rate is to adopt a proven 

 estimate from a nearby locality, after making allowances for local conditions 

 (see method 1). It requires considerable engineering judgment to determine 

 whether the rate given for the nearby locality is a reliable estimate and, if 

 reliable, how the rate needs to be adjusted to meet the changed conditions at 

 the new locality. 



Method 2 is an analysis of historical data. Such data may be found in 

 charts, maps, aerial photography, dredging records, beach fill records, and 

 related information. Section VIII, 1, a describes some of these sources. 



4-146 



