approxijnation of the current velocities at the depth of the vanes. Floats 

 of too great length will drag on the bottom and give improper readings. 

 Floats of comparatively short length record only surface currents. The 

 float lengths should be varied by regular increments to permit selection 

 of the proper length float for the varying depths along a profile. This 

 selection of proper float lengths is based on experience at the site after 

 a trial run. One design for vaned float is shown in Figure 5l<. Pegram 

 current meters are used on extensive studies to measure bottom currents. 

 These are not generally used in the determination of direction of littoral 

 drift because the information gained is seldom commensurate with the cost 

 of the operation. In some instances balls of slightly greater density than 

 the water have been used to roll along the bottom dragging a very light 

 line and float to mark their location. 



87, Rate of Littoral Drift . - The rate of littoral drift is as 

 important as the direction of littoral drift in the functional and structural 

 design of shore protective structures. The rate of littoral drift can only 

 be measured accurately at a substantially complete artificial littoral 

 barrier. At such barriers this rate can be computed by measuring either 



the accretion at the updrift side of the barrier or the erosion at the 

 downdrift side. Accretions can be measured at partial barriers, but no 

 methods have been devised to determine what proportion of the total littoral 

 drift is trapped by each partial barrier. Until some such method has been 

 devised, the measurement of material trapped by groins or short jetties is 

 a most inadequate way of determining the rate of drift. Natural littoral 

 barriers are of little use in determining the rate of littoral drift 

 because over geologic time the beaches either updrift or downdrift from these 

 barriers tend to reach a condition of stability where the sand supply equals 

 the sand losses. 



88, Typical examples of essentially complete j substantial, and 

 temporary artificial barriers are shown in Figures 52, $3, and 5Uo In 

 these examples, and in all similar cases, the rate of littoral drift is 

 determined by measuring the amount of accretion or erosion occurring 

 during a known period of time. To compensate for seasonal changes, surveys 

 should be taken at about the same time each year. To compensate for annual 

 fluctuations, the period of time between surveys shoiold be extended as 

 conditions permit. The rate of drift should be expressed in amoimt of 

 drift per unit time, usually a year, 



89, Where the rate of littoral drift is to be established at a 

 littoral barrier, the base surveys should be extended a vsufficient 

 distance updrift and downdrift from the barrier to include the entire 

 accretion and erosion zones at the end of the study period. Where erosion 

 is anticipated, the base line should be referenced to points at a con- 

 siderable distance from the ocean as recessiorPof the shore line of 1,000 

 feet or more are not uncommon. Profiles are run from the base line seaward 

 at least to the 30-foot depth contour, although extension of the profiles 

 to greater depths may be required in areas of severe exposure or in the 

 vicinity of submarine canyons. Profiles are seldom spaced in conformity 



59 



