Suspended sediment measurements near the seabed. A newly developed 

 FOBS was also deployed at Colorado River during die three shoit-term 

 experiments, as shown in Figure 7. This sensor package contained a stack of 

 miniaturized OBS's which could be deployed at the seabed level to measure 

 bedload or near-bedload sediment concentrations, as described in Beach, 

 Sternberg, and Johnson (1992). The sensors measured sediment concentrations 

 at five elevations within the bottom 6 cm of the water column, and were 

 deployed on each of two small, mobile tripods (Kraus, Gingerich, and 

 Rosati 1989). These tripods also held a standard OBS higher in the water 

 column, a 4-cm near-bed ducted-impeller current meter, and one tripod also 

 had a strain gage pressure transducer. 



Prior to deployment, the sensors on the tripod were tested for functionality. 

 The tripods were carried out to the designated position in the surf and the legs 

 were forced into the sand to anchor the tripod. Sensor output was transmitted 

 through a shore cable to a data logger located inside the WES instnmient van, 

 and was recorded on a hard disk for periods up to 130 min duration. The shore 

 cable was attached to a vertical pole located approximately 5 m inshore as a 

 strain relief. The tripods were frequently inspected to confirm and maintain 

 proper sensor orientations and elevation above the bed. Data runs coincided 

 with WES sled and platform runs. Figure 8 shows the deployed tripods along 

 with the WES platform (which is mostly underwater) in the surf zone. 

 Typically, tripod locations were not changed during a day. More information 

 on the deployment of this instrumentation is found in Beach (1995). 



Table 5 gives deployment dates and locations. The depth column gives the 

 depth of the bottom of the FOBS sensor, which was essentially the sediment 

 depth below the mean water surface, as determined by the pressure transducer. 

 Since only tripod A had a pressure transducer, only some estimated depth 

 values are given for tripod B. Each tripod also held only one ducted-impeller 

 current meter; thus each could only measure one component (cross-shore or 

 longshore) of velocity. 



Other measurements. Sediment samples (grab samples) were obtained at 

 the site on October 18, 1989 and February 11, 1994. Aerial photographs were 

 obtained during the first experiment on 3 and 5 May, 1990. Other aerial 

 photographs were obtained from SWG files and from the National Aeronautic 

 and Space Administration. During the first experiment, a surf zone pressure 

 cable was deployed, as shown in Figure 3. This new instnmient consisted of 

 10 strain gage pressure sensors imbedded within an approximately 23-m-long 

 cable which was stretched across the surf zone. The instrumented cable 

 performed satisfactorily when transmitting power and collecting data. 

 However, it was determined that pressure sensors mounted on the platform and 

 sleds supplied sufficient surf zone wave information to satisfy the mission 

 requirements, so the cable was not used during the later experiments. 



20 Chapter 3 MCCP Monitoring Plan and Implementation 



