(2) Ultrasonic sampler 



These samplers measure sediment concentration using high- 

 frequency ultrasonic sound waves. A transmitter and 

 receiver of sound waves are located some distance above the 

 bed such that the material moving as bed load passes 

 between them. Different amounts of ultrasonic wave energy 

 are absorbed, depending on the concentration of sediment. 

 Graf (1984) reports that reliable results have been 

 produced with these types of samplers. However, several 

 assumptions are inherent when using the method: (a) the 

 size distribution of the bed load is identical to the bed 

 material; (b) the bed load is traveling at the velocity of 

 flow (this assumption does not hold for regions near the 

 bed); (c) the bed-load transport rate is directly propor- 

 tional to the concentration of sediment and the fluid 

 velocity; (d) suspended sediment entering the sampling 

 region has the same ultrasonic absorption properties as the 

 bed load; and (e) the hydraulic and sediment- trapping 

 efficiencies of the sampler are close to 1.0. 



(3) Tiltmeter 



Tiltmeter samplers measure the ground tilt resulting from 

 the passage of water and sediment, thereby estimating the 

 total sediment load when the water-stage data are removed. 

 Hubbell (1964) describes limited use of a tiltmeter 

 sampler. He also discusses several questionable assump- 

 tions inherent for operation of the tiltmeter method. 



11. Other methods used in estimating the rate of riverine bed-load 

 transport include: using flow velocity, bed-load grain size, and other 

 parameters in analytical formulae; tracking the size and movement of bedforms 

 through time; and documenting grain movement with high-speed photography. 



Nearshore Zone Measurement Apparatus 



12. Instruments designed to measure nearshore and surf zone sediment 

 transport have only recently been developed and used in the field. Probably 

 the earliest documented use of a sediment -measurement apparatus in the surf 

 zone was by the U.S. Army Corps of Engineers Beach Erosion Board (1933); brass 

 cylinders of known volume were opened at varying depths along a pier to 

 collect a water -sediment mixture. More recently, a multitude of coastal 

 apparatus have been developed, from simple devices very similar to early 

 riverine samplers, to indirect devices that measure the transmission of light, 

 absorption or backscatter of radiation, and absorption of sound. This section 



A20 



