water clarity conditions at the time of the survey. Other bathymetric features 

 observed in Figure 5 are a series of shoals north of the boat basin running in an 

 almost north-south direction and a very steep descent into the river channel from 

 terraces on either side. Based on sediment sampling (discussed below), this area 

 is primarily characterized by sand. These types of linear shoals are typical for 

 riverine (steady, but high, flow) type environments. The navigation channel is 

 also observable in the north-central portion of Figure 5 and appears to lead 

 directly into the deep area beyond measurable depths. A second deep area (depths 

 of 12.2 m (40 ft)), located just offshore of the boat basin, is separated from the 

 1 8+ m (60+ ft) area by higher relief (approximately 8.5 to 11 .0 m (28 to 36 ft) 

 deep). 



Current Measurements 



During the September/October field-data collection effort, currents were 

 measured to identity the magnitude of flows in the vicinity of the boat basin. 

 Depth-averaged currents were used to calibrate/verify the physical model, and 

 bottom currents extracted from the data sets were used to compute theoretical 

 potential sediment-transport rates. 



Currents were measured with an ADCP along transects shown in Figure 7. 

 An ADCP transmits acoustic pulses from a transducer assembly into the water 

 column. ADCPs can either be bottom mounted "looking" up or deployed from a 

 floating vessel "looking" down. The transducers receive backscattered signals 

 from particles that move with the water currents. Velocities are derived by the 

 Doppler effect whereby the transmitted signal changes frequency with respect to 

 the ADCP and particle movement. These frequency changes are directly 

 proportional to current velocities providing a two-dimensional image of the 

 current field. 



During this field study, the ADCP was deployed from a 6. 1 -m- (20-ft-) long 

 boat provided by the Detroit District. Surveys were conducted along the transects 

 shown in Figure 7 on 29 and 30 September and 8 October. Only Transect CI was 

 surveyed on 29 September; all transects were surveyed on 30 September and/or 

 8 October. Figures 8 and 9 show depth-averaged current vectors (direction and 

 magnitude) for each survey on 30 September and 8 October, respectively. The 

 absence of data on any one transect indicates that the fransect was not surveyed on 

 that day. Because data from multiple transect surveys were similar and for clarity, 

 only one survey is shown per day on Figures 8 and 9. No currents were measured 

 on 1 October because of adverse weather. 



Current measurements showed that the strongest flows south of the boat basin 

 were at the head of the St. Clair River (Transects CI and C2). These depth- 

 averaged flows near the center of the channel exceeded 1 .8 mps (6 fps) on the day 

 of the measurements. Velocities near the boat basin ranged from nearly 1.8 mps 

 (6 fps) at Transect 3 just south of the basin to around 1 .2 mps (4 fps) at 

 Transect C4 north of the basin. Velocities continued to decrease with distance 

 north having magnitudes on the order of 0.6 mps (2 fps). 



12 Chapters Field-Data Collection 



