A channel is normally dredged wider than the final planned width so there is room for sediment to flow back in on the sides. The end result is a channel whose bottom is the planned width, sloped sides, and a wider channel top. The width you actually dredge is determined by an engineering firm and is based on several factors: final channel width, channel depth, natural slope of the channel, and sediment thickness. The WWMD hired Graef to complete a Sediment Investigation Report in April, 2013, the section related to this question is included below.
Sediment Investigation Report Waterford Impoundment Dredging
Graef (professional engineer firm), April, 2013.
5.1 Dredging Project Scope, in reference to dredged channel width
As noted in Section 1, the total volume including the main navigational channel, the access channels in the bays and the sediment trap was estimated at 486,550 yd3.
The locations of the navigational and access channels are shown in Figure 1. Generally, there will be three dredged channel widths: 100’, 50’ and 25’. For a 100’ channel, an approximately 170 foot wide channel would be dredged to the 5 foot depth. This would allow the sediments remaining in the river initially outside the dredged area to flow back into the dredged area over time a distance of roughly 35 feet to reach their natural slope, leaving an approximately 100 foot channel with a 5 foot depth. This is based on a natural slope for the material in the main channel of 20:1 to 25:1 and an average sediment thickness of 3 feet. The before and after surveys used to calculate volumes for payment would require a total width of 190 to 200 feet perpendicular to the centerline of the dredged channel in order to account for material that flows back into the dredged area from outside the dredged area. A typical 100’ dredge section is illustrated below.