March 4, 2010
Determining The Size Of The Hydraulic Dredge
Needed For A Dredging Project.
By Don Searles
Dredging Specialists

1.  Determine the volume of material to be dredged in cubic yards or cubic meters.

1.1.   English System: To get cubic yards. Take the length in feet x the width in feet of the area to be dredged x the depth in feet of the material to be dredged, divided by 27 to get cubic yards.  If you know the number of acres to be dredged, take acres x 1613 x depth of material in feet to be dredged to get cubic yards. 

1.2.   Metric System: To get cubic meters.  Take length in meters x width in meters of the area to be dredged x depth in meters of the material to be dredged.  If you know the hectares of the area to be dredged, take the number of hectares to be dredged x the depth in meters, of the material to be dredged x 10,000 to get cubic meters.  

2.  Determine the time allowed to complete the project.  Days, weeks and months.

3.  Determine the hours that you will work.  Per day, per week.  Will you run 1 shift, 2 shifts or 3 shifts?  Could be 3 shifts 24/7.

4.  Determine the type of material to be dredged.  Fine sand, medium sand, large sand, small gravel, large gravel, silt which is mud, clay, cobbles 3 to 10 inches[76 to 254mm], boulders +10 inches [254mm].  When dredging a lake, the material will most likely be mud.

5.  Determine if you will be dredging sediments or undisturbed material.  Undisturbed material is much more difficult to dredge.  Stay away from dredging undisturbed material unless there is a valid reason like the depth needed for large ship passage.

6.  Determine the horizontal pumping distance.  Maximum and minimum.

7.  Determine the vertical pumping distance called static head.  This is the vertical distance from the water surface to the discharge point.

8.  Determine the altitude above sea level where the project is located. 

9.  Determine the maximum digging depth. This is the distance from the water surface to the lowest point of dredging. 

Only after the above 9 items have been determined is it possible to determine the size of the dredge, the inside diameter of the pipeline and the horsepower of the engines needed to do the job.

Dredge Sizes: The size of a hydraulic dredge is determined by the inside diameter of the discharge flange on the dredge pump.  Dredges come in sizes from 8 inch [203mm] to 36 inch [914mm].  An 8 inch [203mm] dredge will typically pump 2,500 gpm [gallons per minute] [9,464 lpm] with an engine horsepower of around 300 [223 kw] and a weight of 25 tons.  A 36 inch dredge [914 mm] will pump up to 70,000 gpm [265 cmm] with up to 10,000 horsepower [7,456 kw] and a weight of several thousand tons.  With a given size dredge the production can vary widely and is determined by the outside diameter of the impeller in the dredge pump, the speed of the impeller, the horsepower of the engine driving the dredge pump, and the friction and static head losses in the pipeline. 

Using booster pumps, pumping distances of 20 miles [57 km] have been attained.

When items 1 through 9 are known, Dredging Specialists can design the dredging system and recommend the dredge needed, determine if booster pumps are needed, the size of pipe and many other factors that are needed to complete a dredging project. 

The following chart will provide a ball park estimate of dredge production.

8 and 10 inch [203 to 254 mm] dredges can typically be hauled on one truck.  Larger dredges must be disassembled and hauled on several trucks.  For 8-10 and 12 inch [203-254-305 mm] dredges a large crane is needed to launch them.   Larger dredges are typically assembled at the dredge site on shipways made of timbers, and pushed into the water with bulldozers.  Most dredges, 20 inch [508 mm] and larger, are not portable and must be floated to the job.