COMBINING LEADLINE AND ECHO-SOUNDING 

 METHODS IN SURVEYS OF SUBMARINE CANYONS 



The Field Research Group of the Beach Erosion Board was required in 

 connection with littoral drift studies to make an accurate hydrographic 

 survey of the head of the submarine canyon off Redondo Beach; California. 

 The walls of the canyon are very steep, often exceeding slopes of 1:1, 

 •and the use of an echo sounder for determining submarine relief in this 

 case was not considered sufficiently accurate „ 



The usual survey procedure of the Field Research Group is to take echo 

 soundings on given ranges , starting from deep water and running shoreward „ 

 The echo-sounding equipment is operated from an amphibious truck (DUKW) 

 and horizontal control is maintained by two radio-equipped shore stations „ 

 It was desirable to continue utilizing this method as far as possible, 

 however the working schedule of the Group did not allow time for detailed 

 leadline sounding by this process „ A more rapid method was devised using 

 heavy duty stream gaging equipment in conjunction with the echo sounder „ 



A standard stream gaging unit was borrowed from the Los Angeles Office, 

 UoSo Geological Survey, consisting of a 100-pound streamlined lead weight 

 fastened to a stainless steel wire and supported by a collapsible boom, 

 see Figure 1„ The wire winding drum is equipped with a counting device 

 (Figure 2) that reads to tenths of feet„ This equipment is normally used 

 for stream gaging from bridges with either a 100- or 50-pound lead weight. 



Operational tests were conducted with both the 50- and 100- pound 

 weight, to determine the optimum operating speed of the DUKW, and the 

 feasibility of running ranges both seaward and landward „ It was found that 

 by reducing the speed of the DUKW from about 450 feet per minute (normal 

 operating speed) to 180 feet per minute, and using the 100-pound weight 

 the sounding line would not "bow" or "drag" appreciably in depths as great 

 as 150 fee to The characteristics of the depth counter are such that, if 

 the weight was allowed to run nearly free, the instant the lead weight 

 touched bottom and the line slacked the counter would stop long enough for 

 a reading to be taken before continuing to indicate Thus by running the 

 range seaward, or downslope, the leadline could be dropped and the brake 

 applied as soon as the indicator showed bottom, the forward speed of the 

 DUKW then would quickly drag the weight into deep water, the line would 

 resume vertical, and another sounding could be taken „ It was found after 

 a few hours training that the leadsman could take leadline soundings at 

 15 to 20 second intervals (4-5 to 60 feet sounding interval) in depths as 

 great as 150 feet„ 



While the echo-sounder recording does not show true vertical depths 

 it is useful to show the relative detail in the vicinity of the leadline 

 soundings 



The working procedure of the group was to start from the shore end 

 of the canyon and travel seaward at a rate of 180 feet per minute, using 

 both the echo sounder and the leadline. The leadsman would take soundings 



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