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Bay-Delta Office
Department of Water Resources

1416 9th Street,
Sacramento, Ca 95814

Mailing Address:
P. O. Box 942836,
Sacramento, Ca 94236-0001

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 Dredging Cont'd
More on Dredging...

Sealed Clamshell Dredging Method
Conveyance Dredging
Hydraulic Dredging Method
West Canal
Middle River
Old River
Spot Dredging for Agricultural Diversions in Old River, Middle River, and Victoria, North, and Grant Line Canals
Maintenance Dredging
Disposal of Dredged Materials

Sealed Clamshell Dredging Method
Clamshell dredging could occur from either a barge within the river channel or from the top of a levee, depending on restrictions caused by vegetation on channel banks or the width of a channel. Barge clamshell dredges are not self-propelled and would therefore need a small tugboat to maneuver within the channel. From a barge, the operation would begin when the bucket assembly, attached by a long arm (up to 100 feet) is lowered, in the open position, into the water and allowed to settle into the channel bottom to collect sediments. As the dredge operator pulls the bucket up it closes on the sediment, pulling out up to 5 cubic yards of water-sediment mix. The dredge operator can then deposit the water-sediment mix in either a runoff management basin constructed on the landside of the levee adjacent to the channel, or onto a barge that would move it to a runoff management basin in a different location. The clamshell dredge may also sit atop the levee and scoop up to 5 cubic yards of water-sediment mix from the channel bottom, using the same method as from a barge, and deposit the dredged material into a runoff management basin.

A runoff management basin is typically rectangular in shape and uses the levee as one of its walls. The remaining three walls are constructed of compacted local soil. The three constructed walls would not exceed 6 feet in height. Runoff management basins are necessary to contain the 50% moisture sediment-mixture and prevent drainage into agricultural ditches and channels. The mixture would reach 25% moisture content in 2 to 6 weeks, depending on the climate and the thickness of the spread. Once the material loses half of its original water content, it may be used beneficially for levee reinforcement or for agricultural soil supplement.

The clamshell dredging method can be more cost efficient than the hydraulic method, when sediment spoil areas are immediately adjacent to the dredged area. However, it can cause greater disruption to channel vegetation when the bucket scrapes layers of sediments from the channel bottom. This method would likely be used in situations where there is limited space for settling ponds, the likelihood of major disruption to vegetation and other organisms in the channel bottom is minimal, the area to be dredged is small, there are channel islands, or when there are no issues concerning temporary turbidity and sedimentation in the water. It is possible, however, to reduce turbidity generated by this method through the implementation of dredging practices such as lowering and raising the clamshell bucket slowly, or using a closed bucket as proposed.

Conveyance Dredging
In addition to the dredging required to construct the gates, portions of West Canal, Middle River, and Old River would be dredged to improve conveyance and the operation of private agricultural siphons and pumps. . Approximately 250,000 cubic yards of material would be dredged and spoiled within the south Delta. Dredging would be conducted in the center of the channel to avoid tidal emergent wetlands and riparian habitat. Conveyance dredging would be conducted using either a sealed clamshell dredge or hydraulic dredge, as described below. A decision on which method to use would be made prior to commencing work, based on access, sediment composition, and potential impacts on vegetation and other organisms. Conveyance dredging in Middle River, West Canal, and Old River to the east of the CVP intake would occur between August 1 and November 30 to minimize effects on delta smelt and juvenile salmonids.

Hydraulic Dredging Method
The hydraulic dredging method siphons a water-sediment mix (four parts water for every one part sediment) from the bottom of a channel and deposits it into a settling pond to dry. A dredging barge within the channel lowers a pipe into the bottom sediment. The pipe siphons the sediment-water mix into a flexible pipe that may be inexpensively extended up to 1,000 feet up or down the channel. This pipe may be weighted down to avoid interfering with boat navigation adjacent to the project site. The flexible pipe is attached to a semi-permanent, stationary pipe that is braced to the waterside of the levee, extends across the top, and down the landside of the levee into the primary basin of a settling pond. The flexible pipe would allow the barge to move relatively far from the stationary pipe, allowing more dredged area per settling pond. The stationary pipe would range from 8 to 18 inches in diameter and would require that gravel be placed on either side to create a ramp over the pipe for vehicles and agricultural equipment. The direct deposition of the material into settling ponds on adjacent lands allows uninterrupted dredging up to the capacity of the settling pond. Up to 5,000 cubic yards of material may also be transported to settling ponds by barges. The settling ponds would be constructed on the landside of the levee adjacent to the channels, and would be used for the decanting process, effectively separating the sediment from the water and allowing dried material to be put to beneficial use. The ponds would be constructed of local compacted soils to avoid toxicity and erosion of their sides. Settling ponds are typically composed of three basins: primary, secondary, and return basins. The primary and secondary basins serve to settle sediments out of the dredged mix. When water reaches the return basin, most suspended sediment has settled out of it, and the water is then pumped back into the channel from which it was taken. The sediment would take between 24 and 36 days to settle out of the water.

West Canal
West Canal is a major artery carrying water to the SWP and CVP. When exports are high and San Joaquin River flow is low, the velocities in the channel become high enough to cause scouring and erosion of the channel bottom. Data collected from a DWR scour monitoring project at two locations in West Canal indicate erosion of approximately 5 feet of channel bottom within 4 years (reference 1997 to 2001). To reduce these velocities that cause scouring, West Canal would be enlarged by dredging 3 feet from the channel bottom between the CCF intake, north to the point where Victoria Canal meets West Canal. Approximately 40,000 cubic yards of material would be removed. Dredged spoils would be dried in either settling ponds or runoff management basins at Widdows Island, depending on the method used. The hydraulic method would require an area of approximately 40 acres for spoils ponds, assuming they can be reused during each dredging phase. No more than one pond would be necessary to spoil all the dredged water-sediment mix. The clamshell method would require no more than 7 acres if runoff management basins were filled to a depth of 4 feet. Dredging estimates are based on a dredging efficiency of the 20% solids removal commonly achieved by hydraulic dredging, and the 50% solids for clamshell dredging.

Middle River
Middle River would be dredged from the head of Middle River (at Old River) to approximately 5.3 miles west to an elevation of –8 msl to accommodate agricultural siphons and pumps. Dredging would likely be done hydraulically from a barge. Approximately 200,000 cubic yards of dredged material would be dredged and spoiled at one or more drying areas on Union or Roberts Islands, or on Stewarts Tract. It is estimated that the dredged material would occupy a total area of up to 165 acres for spoiling ponds, assuming they can be reused during each dredging phase. Dredging estimates are based on a dredging efficiency of the 20% solids removal commonly achieved by hydraulic dredging.

Old River
There are several agricultural siphons and pumps on the Old River that provide water for agriculture in the south Delta. Near the area where Old River, Paradise Cut, and Tom Paine Slough meet, sedimentation has accumulated near these siphons and pumps and is affecting the ability of these siphons and pumps to provide water for agricultural uses. Dredging in this area would be conducted to improve siphon and pump operation. The dredging method may be hydraulic or clamshell dredging by barge, depending on the areas that are in need of dredging. Approximately 10,000 cubic yards of dredged material would be removed from the channel and either placed in settling ponds (hydraulic) or placed in runoff management basins (clamshell) to dry to an approximate moisture content of 25%. Ponds and/or basins would be placed on Stewart Tract. It is estimated that all of the dredged material would occupy an area of less than 10 acres if done hydraulically and less than 3 acres if the clamshell method is used. Hydraulic dredging estimates are based on a dredging efficiency of the 20% solids removal commonly achieved by hydraulic dredging, and the 50% solids removed for clamshell.

Spot Dredging for Agricultural Diversions in Old River, Middle River, and Victoria, North, and Grant Line Canals
Dredging in specific areas around siphons in the south Delta may also be performed as part of the dredging operation. Siphons that have sediment blockage that prohibits the effective diversion of water would have an area up to 100 square feet cleared around them. Siphons that are –2 msl or shallower would be extended and dredged around to ensure effective and operable diversion.
These siphons are located primarily in Old River and Middle River (outside the areas mentioned above to be dredged to improve conveyance and remove accumulated sediment) and in Victoria, North, and Grant Line Canals. Refer to the Extension of Agricultural Diversions discussion below for more detail regarding the location of potential spot dredging. All spot dredging activities would occur within the channel and would not affect the adjacent land or levees. A total of up to 44,000 cubic yards of sediments would be removed from these areas.

Maintenance Dredging
Maintenance dredging may be necessary to remove collected sediment in channels as well as near agricultural pumps and siphons. Depending on the rate of sedimentation, DWR would perform one round of maintenance dredging up to 5 years after initial dredging. It is estimated that up to 25% of the original dredge removal amount would be necessary for maintenance purposes within this timeframe. Maintenance dredging and spoiling methods would be similar to those described above. A formal dredging plan with additional details on specific maintenance dredging activities will be developed prior to dredging activities.

Disposal of Dredged Materials
Each method of dredging (sealed clamshell and hydraulic) would effectively dry the material so that it could be beneficially used in the south Delta. Approximately 5% of all the spoils would be used for levee reinforcement. The semi-dry material would be placed approximately 1 foot deep on the landside of existing levees. This would cover any holes or cracks and would therefore improve the stability of the levee system in the south Delta. To avoid any impacts on sensitive vegetation and wildlife, areas of levees with vegetation would not be reinforced. The other 95% of the material would be spread over agricultural land at an approximate depth of 1 foot and could improve the quality of the existing soil. According to the Section 404 permit issued for previous dredging in the Grant Line Canal, The results of the physical measurements, chemical analyses, and other tests on these sediments indicate that they would be suitable for most reasonable uses, which may include levee stabilization, upland, or agricultural applications. Dredged material would be tested prior to dredging, during dredging and prior to disposal of the dredge material for reuse.

 


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