BEsT MANAGEMENT PRACTICEs HANDbooK - Tahoe BMP

There are various types of dredges. The selection of a specific type of dredgewill be specific to each project’s needs and goals. First, mechanical dredges havea longer cycle time, which reduces the velocity of the ascending loaded bucketthrough the water column. This can reduce the potential for incidentalfallback, a process that washes sediment from the bucket back into the water.However, limiting the velocity of the descending bucket reduces the volume ofsediment that is picked up and often results in the operation requiring moreshovel bites to remove the targeted volume of material. The majority ofsediment re-suspension for a clamshell excavator occurs when the bucket hitsthe substrate bottom. 48Mechanical dredges come in a number of forms and designs, from basic augersand hoes to cutterheads and clamshell drop buckets. BMPs encourage theoperator to minimize multiple bites when possible. For example, when aclamshell bucket hits the bottom of a lakebed, an impact wave of suspendedsediment travels along the bottom away from the bucket. When the clamshellbucket takes multiple bites, the bucket loses sediment as it is re-opened forsubsequent bites. Sediment is also released higher in the water column as thebucket is raised, opened, and lowered. When using a mechanical dredge, theoperator should eliminate bottom stockpiling when possible. Bottomstockpiling of the dredged sediment in a silty substrate has a similar effect asmultiple bite dredging; an increased volume of sediment is released into thewater column from the operation. 49Hydraulic dredges, such as a low horsepower suction auger, are powerful piecesof excavating technology. They are very effective at disrupting the waterbed.When the operator uses care and manages the cutter-head rotation speed, thenthis approach can be well suited to still water settings where waves and othermitigating forces are less likely to be a factor (e.g. marinas). Reducing cutterheadrotation speed reduces the potential for side casting the excavatedsediment away from the suction entrance and re-suspending the sediment. Thismeasure is typically effective during maintenance dredging in relatively loose,fine grain sediments. The operator should avoid bank undercutting. As anoperational goal or BMP, dredge operators should strive to remove thesediment in maximum lifts equal to 80 percent or less of the cutter-headdiameter. Furthermore, application of BMPs encourages the operator to bothmonitor and effectively manage the swing speed. Reducing the swing speedensures that the dredge head does not move through the cut faster than it canhydraulically pump the sediment. Therefore, reducing the swing speed reducesthe volume of re-suspended sediment. The goal is to swing the dredge head at aspeed that allows as much of the disturbed sediment as possible to be removedwith the hydraulic intake. 5048USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal Geology49USACE, 2008, Coastal Engineering Manual - Parts I-650USACE, 1995, Engineer Manual EM 1110-2-1810, Engineering and Design: Coastal GeologyCHAPTER 8: Shorezone Protective Structures and BMPsTRPA BMP Handbook8.9 Dredging 2012Page 8-60