KAPLAN, WELKER, and KRAUS: EFFECTS OF DREDGING 



This factor almost certainly reduced available 

 light to the plants despite the relative clarity of 

 the water. 



Current Velocity 



Readings of current velocities were taken on 

 9 June 1967, before dredging, and on 19 July 1968, 

 after the new channel was completed. Attempts 

 were made to match the tide height and wind 

 direction and velocity on both occasions so as to 

 minimize variables related to natural fluctuations 

 of water level and current velocity. During both 

 readings the wind was from the southwest and 

 differences in wind velocity between the two days 

 were not greater than 10 mph. The wind velocity 

 was slightly higher during the post-dredging 

 series (7 mph vs 15 mph) as was the tidal 

 range (70 cm vs 73 cm). These factors would tend 

 to increase the velocity of the flood tide rather 

 than decrease it. Since current velocities de- 

 creased, this effect cannot be attributed to the 

 influences of wind and/or tide. 



The bridge readings were made with an Ekman 

 current meter at 20 min intervals, 0.5 m beneath 

 the surface and 0.5 m above the bottom of the 

 channel. The meter was allowed to run for 120 

 sec and the readings were converted into centi- 

 meters per second according to the standard 

 formula. 



The readings in the channels were taken with 

 Price meters on hand-held rods. The meters were 

 set at 0.5 m above the bottom. Maximum depth 

 of water at any station was approximately 1 .2 m so 

 that lamination or stratification according to 

 current velocity was minimized. Maximum inter- 

 vals between readings at the same station were 

 less than 30 min. 



The data consisted of the number of revolu- 

 tions of the wheel during a 70 sec interval con- 

 verted into centimeters per second from a nomo- 

 graph calibrated to each meter. One replication 

 of each measurement was taken as a check on 

 the accuracy of the meters. 



Table 2 and Figure 4b compare current veloci- 

 ties before and after dredging. Figure 4a indicates 

 the current velocity sampling stations. 



Maximum current velocity before dredging was 

 through channel A. After dredging, the most 

 westerly portion of channel A still has the maxi- 

 mum current velocity, but approximately 100 m 

 east of the point of trifurcation at stations 4 and 



Table 2. — A comparison of current velocities at flood tide in 

 Goose Creek, before and after dredging. 



'Station 3 was a sand bar with a thin, rapid flow. The water was 

 never more than 30 cm deep over this bar. It was removed by the 

 dredging operation and replaced by a 2.1 m deep channel. 



^f^aximum surface velocity. 



^Maximum bottom velocity. 



GOOSE CREEK, N.y: 



Figure 4 a. — Map of current velocity sampling stations. 



Figure 4 b. — Map of current velocity differences before (open 

 arrows) and after (solid arrows) dredging. Each millimeter 

 represents 4 cm/sec current velocity. 



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