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Redwood Shores Lagoon Improvement - Dredging
LATEST UPDATES:
Phase 1 dredging will be complete May
14 and equipment removed by May 21
Phase 2 dredging will be complete by mid-June
 Why Dredging is Needed
There is no doubt that the lagoons in Redwood Shores are a source of beauty, pride, and quality of life for the residents - in many ways, they help to define Redwood Shores living.
Over the decades, a great deal of silt has been accumulating in the lagoons, and as a result the lagoons are not in the prime condition that is desired. The silt build-up on the floor of the lagoon is a significant factor preventing the water from "flushing" - being replaced through tidal action.The lagoon water needs to be flushed at least once a week to avoid potential problems.
Some parts of the lagoon meet that standard, but there are others that are flushed much less-frequently. In the areas where natural flushing doesn’t occur, the results can include:
- An overgrowth of bacteria and algae
- An increase in widgeon grass (which further reduces the natural flow of water and leaves an accumulation of debris when it dies out)
- Cloudy and unclean water
- An unpleasant odor due to higher levels ofbacteria and algae
- Difficulty in navigation of the small boats in the lagoon, due to built-up high spots on the floor of the lagoon
These conditions do not reflect the value and pleasant appearance of the surrounding Redwood Shores homes - the solution is to remove portions of that silt from the floor of the lagoon to allow for proper tidal flushing.
The Project
The City hired a consultant who performed a hydrographic survey to map the lagoon’s underwater contour. From that, we calculated the quantity and location of excessive silt and developed a program for removal by dredging. This does NOT involve draining or otherwise removing water from the lagoon. Approximately 21,000 cubic yards of silt is expected to be removed.
The dredging will be performed from a small barge which will slowly make its way through the specified areas of the lagoon. The barge will hydraulically pump the silt from the lagoon floor, transmitting it through a pipeline running along the edge of the lagoon, to be deposited into Area H. There, the silt will contribute to the wetland restoration taking place there as part of a development now under construction.
On September 10, 2009, the bids were opened for the contract to do the dredging. Staff is recommending that the City Council approve a contract with Aquatic Environments. This is on the Council agenda for October 14, 2009. Barring unforeseen circumstances, we expect the project to commence in earlyNovember. The project should be largely complete by the end of January, 2010.
Location
There are two areas that are scheduled for hydraulic dredging (click on map for larger view; map is for general location purposes only - dredge locations shown are approximate only and not to scale):
- The lagoon surrounding Shorebird Island. For the dredging project, this site is referred to as Zones 3, 4, and 5.
- The lagoon near Area H between Laguna Pointe and California Harbor Court. For the dredging project, this site is referred to as Zones 8, 9, and 9A (we will be dredging portions of zones 8 and 9, and all of zone 9A).
The silt from these lagoons will be pumped through a pipeline to Area H.
See a larger map of the lagoon areas to be dredged, and the approximate path of the pipeline.
It should be noted that certain areas within Zone 9 have been found to contain a compound, PAH (polycyclic aromatic hydrocarbons) in levels higher than is allowable for use as wetland cover material at Area H. These specific areas within Zone 9 will not be dredged since it's not cost-effective at this time to have it hauled to an upland disposal site.
The source of PAH is primarily runoff from streets and cars, and human exposures to PAH happens every day from combustion, such as diesel exhaust. The levels of PAH found at this site allow it to be used for other kinds of fill – and would be suitable for Area H if three feet of other fill was placed on top of it. This silt is acceptable for use at Bair Island for "foundation fill," for example, and that is being considered. These levels of PAH do not pose any risk to humans.
Please note that all silt in all dredging locations has been thoroughly tested, and the other areas of dredging in the lagoons have only very low, acceptable levels of PAH, or none at all.
Phase 2 Location
This dredging is taking place in the lagoon between Sandpiper Lagoon Subdivision No. 1 and Bridgeport Subdivision, and will be completed by June 25. This completion date is later than originally anticipated, and the City thanks the community for their patience and understanding.
A community information meeting took place on Thursday, March 18th to provide neighbors with information about the Phase 2 dredging project.
Here are some details about this Phase 2 dredging project:
- A total of 3,500 cubic yards of sediment is anticipated to be removed from this lagoon.
- A small barge (Aquamog) will be in the lagoon to scoop up the sediment and put it into another small barge. When full, that barge will off-load the sediment onto a flat area on shore at the northern end of the lagoon (adjacent to Redwood Shores Parkway). The sediment will be loaded into a waiting truck and be hauled out for re-use or disposal.
- The outer eastbound lane of Redwood Shores Parkway will be closed to traffic from Avocet Drive to Egret Lane between 9 am and 2:30 pm daily; access to Waterside Circle and Bay Harbour Drive will be maintained.
- There is the potential for an estimated 15 – 20 truck trips per day.
- Work will take place on weekdays only, March 29 to June 25; the Aquamog will be operating in the lagoon between 8 am and 4:30 pm.
- Contact information for Aquatic Environments: daytime phone: 925-521-0400; after-hours phone: 510-406-3473; email gforni@aquamog.com
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Lagoon to be dredged - click on map for more detail |
Potential impacts to the neighborhood
The engines on board the water craft utilize mufflers and are enclosed to minimize noise – however some level of noise is unavoidable and will be heard by residents directly adjacent to the lagoon. The trucks on Redwood Shores Parkway will also cause noise, and the daily temporary lane closures will be an inconvenience to motorists. The trucks will be lined with plastic in order to minimize any spillage, and the contractor will handle any street clean up that may be required resulting from the trucks. Your patience throughout this project is appreciated
Schedule - Phase 1
The contractor's work hours are limited to 7:30 am - 4:30 pm, weekdays only (should a critical situation occur, the City may give the contractor permission to work past 4:30, on a rare basis and only if needed). The contractor will generally not work on weekends or holidays (special exceptions can be made if circumstances require).
- October 14, 2009 - Council considers approval of the recommended dredging contractor
- November 1, 2009 - City gives the contractor official "Notice to Proceed" (the contractor has 180 days from the Notice to Proceed in which to complete the project)
- Early November - Portions of Laguna Pointe/California Harbour Court lagoon dredging begins (this portion of the project is expected to last for approximately 15 - 20 working days)
- Mid-to-Late December - Shorebird Island lagoon dredging begins (this portion of the project is expected to last for approximately 20 - 30 working days)
- May 14, 2010 - Project expected to be complete - note: unfortunately, the contractor has uncovered some unforeseen conditions that have slowed the work progress. Old rebar (metal), pieces of concrete, and hundreds of golf balls in the lagoon require the contractor to stop work and remove this debris - which has slowed down their operation.
The City has received approval from the U.S. Army Corps of Engineers and the Regional Water Quality Control Board to continue the work past the original end date, subject to certain restrictions (avoiding “ground disturbing activities”). Assuming that the contractor does not run into any further significant unforeseen debris in the lagoons, they now anticipate the work to be completed by May 14, 2010 (equipment removed by May 21). This work will be focused on the lagoon around Shorebird Island (dredging zones 3 and 4). We apologize for this delay in completion of the dredging and appreciate the community's patience.
What to Expect- Phase 1
Please understand that in order to complete this project, which is for the benefit of the residents of Redwood Shores, there will be some impacts and inconveniences as a result of the work taking place. These are unavoidable; however the contractor will make every reasonable effort to reduce the impacts and inconveniences to residents.
For hydraulic dredging, a small barge will be deployed into the lagoons, which includes a hydraulic dredge. The hydraulic dredge uses water to remove and transport the silt, with a pump for moving the water. The pump creates a vacuum or a pressure head, which moves water and silt rapidly through the pipe. This system has three primary components: dredging device, pump, and discharge system.
The barge has a generator-powered pump on board that is used to vacuum up the watery silt and transmit it through the pipeline to Area H. When in narrower parts of the lagoon, the barge will potentially be within 20 feet of the shore and homes. The areas within which the dredge will be operating (away from the shoreline) will be well-marked with poles or other markers to ensure accurate dredging.
Possible impacts to nearby residents include:
- Noise from the on-board generator and pump during dredging, and from the "booster" pump that will be located along the path of the pipeline.
- Boat navigation impediment - the pipeline in the water from the dredge location to Area H will be very visibly marked, however it will impact boating: those residents “inside” the area of the pipeline may not be able to launch boats at times when the pipeline is in place. Other boaters should be aware of and avoid the pipeline.
- Temporary water discoloration (resulting from stirring up the sediment).
Community Meeting Held on October 22 (Phase 1)
The Redwood Shores Community Association and Redwood City hosted a project information community meeting on Thursday, October 22, 2009. Approximately 15 residents attended in the Community Room of the Redwood Shores Library, in addition to five staff, contractors, and consultants.
An overview of the project was provided, including an outline ofthe scope of work and the schedule, maps were shown, and City staff, the contractor, and consultants werein attendance to answer questions. Click here to view the Power Point presentation slides from the October 22, 2009 meeting.
The key issues raised by attendees at the meeting were:
- The expected noise levels of the dredge machinery and booster pump.
- Unfortunately, some level of noise is unavoidable; the contractor likened this machinery to a Ford F250 truck engine. Expected noise level is approximately 80-85 decibels at 20 feet distance from the machinery. The machinery utilizes mufflers and is enclosed to minimize noise. We ask for residents' patience throughout this project.
- Access for boats while the temporary pipeline is in place.
- While the pipeline is in place, it will not be possible for boats to cross over the pipeline; this is particularly an issue for parts of Shorebird Island, and along the northern side of the main lagoon for about a month while the Shorebird Island area is being dredged.The contractor will have floating buoys and markers attached to the pipeline to indicate its location.
- The fact that "Zone 9" (between Laguna Pointe and California Harbor Court) will be only partially dredged.
- Due to the presence of the PAH compound, it is not financially feasible for the City to completely dredge this lagoon, since the sediment cannot be disposed of in Area H, and would have to be trucked to an "upland" location. The City's intent is to determine a way to finance the dredging of this area at a later date, once funds are fully available.
Sign up for Email Updates
Please click here if you would like to receive periodic updates on the project via email.
Contacts
For overall project information:
Brian Lee, Supervisory Engineer
Redwood City
(650) 780-7391
blee@redwoodcity.org
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