The Bay System
San Francisco Bay is an estuary where the fresh water of the Central Valley meets and mixes with the salt water of the Pacific Ocean. The formation of the Bay basin originated with the vertical warping of the Continental Plate as it collided with the Pacific Ocean Plate. With the rise of sea level, the basin was flooded with sea water. The forces of the ocean, freshwater flow, and man continue to reshape the Bay. Seventy percent of the Bay is 12 feet deep or less, while only twenty percent is greater than 30 feet deep. Fed form the converging river systems which from the Sacramento/ San Joaquin Delta, over forty percent of all California’s freshwater runoff enters the Bay system annually. Each year, this runoff carries with it about 10 million cubic yards of new sediment in the Bay and a large residue left as a result of earlier Gold Rush hydraulic mining activity. Wind generated waves over vast shallow areas and tidal currents circulate this sediment around the Bay. Scientists estimate that about 170 million cubic yards of sediment circulates within the Bay each year being redistributed by the currents. Some of this settles within the navigation channels, partially filling them.
Need for Dredging
The natural shallowness of the San Francisco Bay, and the large amount of sediment that pours into it annually, creates a need to dredge. There are several reasons for dredging in the Bay, but the primary reason is to allow for navigation.
The San Francisco Bay, and the maritime traffic it produces are key to the economy of the Bay Area. A diverse array of navel, commercial, industrial and recreational vessels use the Bay. Some of the larger vessels, such as container ships, tankers and passenger liners, need up to –45 feet for safe navigation. This makes dredging essential to maintain the depth of the channels. Some estimate that if dredging were to be stopped in the Bay, the accumulation sediment would close the ports within a year.
Testing of Dredged Material
The Corps, as a regulatory and construction agency, must comply with and enforce environmental standards set by law. To do this, all dredging material must be tested before it is disposed of in an aquatic environment. Several types of tests are run, including physical and chemical analyses of the sediment. Sometimes biossay tests are completed to determine the biological response to dredging and the disposal process. Scientists use their best professional judgment when evaluating dredging material for aquatic disposal. If there is an unacceptable test evaluation, the material must be disposed of upland or by special handling in order to minimize the environmental impact.
In order to dredge, there must be a place to put the dredged material. In the past, most of the material was put back into the Bay in places where the current would sweep a portion of it out through the Golden Gate. Many environmental and economic factors must be considered in finding a disposal solution. At present, three options are being considered: in bay, upland, and ocean.
In The Bay
The majority of all dredged material is disposed of in the Bay, approximately six million cubic yards annually. There are three sites right now (Alcatraz Island, San Pablo Bay and Carquinez Strait) and all are located in areas of strong currents.
This is an environmentally attractive option, especially if it can be combined with wetlands creation. Unfortunately, it tends to be expensive and great care must be taken to avoid adverse impacts on existing wetlands. There are long-term management responsibilities which must be addressed.
There is no approved ocean site available at present, this puts the burden of the disposal on the “in bay” sites. An ocean site is being studied but it is a long, expensive process.
There is no perfect solution. Each option has good and bad points; therefore, each one must be considered carefully.
Types of Dredging Equipment
The Hopper Dredge - or “Trailing Suction Dredge”, works like a water vacuum cleaner. It lifts the sediment, mixed with water, through suction pipes and deposits it into hoppers on board. When the hoppers are filled, the dredge proceeds to the disposal area and opens the bottom bins, releasing the sediment.
Clamsell Dredge – This dredge is especially useful for confined spaces, such as around piers, and can be at almost any depth. It has a bucket, or clamshell, that is raised and lowered by a cable. The sediment is emptied into a scow and transported to a site by tugboat. The sediment is then released.
Pipeline Dredge – The pipeline dredge may have a sharp rotating cutter head that loosens the sediment, which is then sucked through a pipe into a scow, or it is routed by pipeline to a disposal site. This method is often used when an upland disposal site is available.