The U.S. Coast Guard (USCG) maintains a base of operations and boat mooring area for their Marine Safety Unit on Lake Erie in downtown Cleveland, Ohio. In April of 2011, BMJ Engineers & Surveyors, Inc. partnered with a consulting firm to provide bathymetric survey services in support of their design for the dredging of sediment that had accumulated within the mooring basin.

 

A bathymetric survey is an accurate model of the bed of a body of water. Typical “off-the-shelf” depth finders do not have the precision needed to create a model of sufficient quality or calculate accurate quantities. Traditional techniques for creating subsurface models, including using a lead-line for depths and a projected grid for horizontal positioning, are time-consuming and labor-intensive. Over the past 10-15 years, as GPS and echo sounding technology have progressed, BMJ has developed a system that produces accurate field data very quickly. With the combination of AutoCAD® Civil 3D® and the BMJ system of data collection, a final product can be delivered within a few days.

 

The echo sounder utilized, operates at a frequency of 200 kHz, has a beam width of 9 degrees, a ping rate of 6 Hz, and is accurate to 1cm in 60m. The horizontal position of each sounding is determined using survey grade GPS system linked via radio to a base station located nearby.

 

The Cleveland site required soundings on a 15’ by 15’ grid from the seawall to 75’ into the lake. BMJ covered the area with a set of data three times as dense, and extended the grid to 125’ into the lake. The collection of this data set took three hours on board the boat. Project set-up, other site work, and preparation for the soundings were completed in 2.5 days, and the product was delivered to the client for design in 3 days after the boat work was completed.