APPENDIX B
DREDGING
PURPOSE
a. Dredging is the removal of earthen materials, including rock or coral, from under water. It increases the size of the waterway and provides construction materials for land-based projects. It also improves logistics. The wider, deeper shipping channel provides easier passage of vessels with deep draft. The deeper the draft of a vessel, the more tonnage it can carry. If few ships are available for logistics, then dredging can make these ships more efficient.
b. Material removed during dredging is often a naturally-occurring construction material. In military operations, natural materials make up for scarce resources. Typically, sand is available along the coast and in most rivers. Care must be taken to avoid clay or fine-grained sediments.
TYPES OF DREDGING
Two primary types of dredging occur: mechanical and hydraulic.
a. Mechanical equipment dredges easily with a clam shell, shovel, backhoe, or other device that scoops the material up. A simple type of mechanical dredge is a land crane, equipped with a suitable bucket mounted on a barge. This dredge needs barges or scows to move the material from the dredging site to the disposal site.
b. The hydraulic dredge uses water to remove and transport the material. This system has a pump for moving the water. The pump creates a vacuum or a pressure head, which moves water rapidly through the pipe. This system always has at least three components: dredging device, pump, and discharge system. There are many common hydraulic dredging systems--hopper dredges, sidecast dredges, cutterhead dredges, and dustpan dredges.
DREDGING OPERATIONS
a. The dredging site must be surveyed prior to planning the operation. Engineers need detailed topographic and hydrographic surveys and data on tidal range, tidal prism, flood stages, velocity, and other hydrographic characteristics, including the status of siltation and scour. They also need data on bridges, breakwaters, jetties, piers, islands, overhead and submarine cables, and type and size of vessels scheduled to use the waterway.
b. Engineers should dredge depths two to five feet deeper than the desired channel depth. This extra depth permits shoaling without restricting the draft of vessels using the channel. The extra depth also permits vessels to operate in waves caused by the movement of other vessels in the water. Moving up and down at the surface is also moving up and down in relation to the bottom of the channel.
c. The alignment and location of the navigation channel are important considerations in port development. The channel should parallel the current flow as closely as possible, and engineers should align the channel so that wave direction will be along the channel axis. Other refinements can reduce future maintenance dredging without affecting channel use. Because the dredging discussed here applies to the TO, long-term factors are not considered.
d. Curved channels are undesirable because vessels navigate by sighting on front and back ranges and by checking channel buoys. Navigation in a curved path is difficult. The channels should be laid out as a succession of tangents connected by short transition curves. Tangents should be as long as possible and curves should not exceed 30 degrees. If larger curves are required, then the channel should be widened at the curve to two or more times the normal channel width.
e. The disposal site should be located as close to the dredging site as possible. This practice will reduce the time and cost needed to complete the dredging job. The disposal site should be selected to minimize the possibility of the material being carried back into the navigation channels by natural or ship-generated currents. A disposal area should use the dredged materials as fill for a revetment or dike.
f. Ingestion of live ammunition is a major threat to dredges. Ammunition placed or accidentally lost in the waterway can explode when it passes through the pump, such accidents can disable or sink the dredge.
DREDGING RESOURCES
a. Dredging is considered part of construction. Therefore, the construction agent for the theater is responsible for it, including furnishing the equipment or contracts. In the past, dredging in theaters was done by a combination of host country equipment, contractors from the region, contractors from the United States, and US Government forces.
b. The US Army Corps of Engineers maintains CONUS ports and waterways using contractors and government-owned and operated equipment. It maintains a small fleet of dredges suitable for defense and emergency needs of the country. These are supplemented by a reserve fleet of contractor-owned dredges. Historically, the construction agent, when not the US Army, has sought assistance from the Corps of Engineers. The construction agent usually establishes an in-the-theater dredging office that furnishes dredge support to the theater.
c. The one reserve dredging unit currently in the US is in the Texas National Guard. A small federally owned and operated dredge fleet that is supplemented by the reserve fleet of privately owned and operated dredges is the best solution for future dredging requirements.
OBTAINING DREDGING SUPPORT
A step-by-step procedure for obtaining dredging resources (including technical assistance) in the TO follows:
a. Request assistance from the local construction agent in the theater.
b. If help is not available from the construction agent, the agent must request dredging support through channels.
c. Both the US Air Force and US Navy have some dredging resources. Use them if the construction agent makes them available. Remember, technical assistance is a resource that must be requested.
d. If all else fails, contact the Office of the Chief of Engineers, Washington, DC.
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