Dewatering Dewatering may include damming, stream bypass, pumping, and/or draining the site. Dewatering is the removal of water from the work area. The purpose is to prevent water from interfering with the work, and to prevent the discharge of contaminants such as suspended sediment and concrete. The dewatering of an adromous fish stream must be conducted in consultation with the Department of Fish and Game and NOAA Fisheries.
Dewatering:Removal of water from the work site • Permitting – California Dept. of Fish & Game requires a Streambed Alteration Agreement • Consult with Fish/Aquatic Biologist. • Intakes and outlets designed to minimize turbidity. • Intakes should be screened with wire mesh not larger than 5 millimeters to prevent amphibians from entering pump system. • Filtration/settling system must be included to reduce downstream turbidity.
Dewatering Options Cofferdams – Sand Bag and Gravel Berm Gravel Berm Cofferdam Cofferdams may be made of rock, sandbags, wood, or aqua barrier. A cofferdam is a temporary structure built into a waterway to enclose a construction area and reduce sediment pollution from construction work in, or adjacent to water. The most common method used in small streams is gravel berms made from river run rock (uniformly graded – no fines) wrapped in plastic or sandbags. More information on cofferdams is located in FishNet Roads ManualAppendix A-BMPs Toolbox – Water Management BMPs (page A-151). Illustrations courtesy of FishNet 4C Sand Bag Cofferdam
Sand Bag Cofferdams Examples of cofferdams made of sandbags with plastic sheeting. The top photo is from the mainstem Trinity River while the lower photo is a small creek in coastal Humboldt County. Water collected behind the dam is pumped around the construction site, filtered for sediment, and putback into the river/stream. Photo courtesy of Caltrans Photo by Michael Love
Dewatering Options Aqua Barrier • Applications & Benefits • Used for water damming, water diversion, and silt containment • Portable, re-usable, and comes in a variety of sizes • Limitations • May need permit • Need abundant water • Not for steep gradients Illustration courtesy of FishNet 4C Aqua barriers are temporary, re-usable water-filled plastic tubes installed as dams. They come in a variety of sizes and are easily transported when drained. More information on Aqua Barriers is located in FishNet Roads ManuaLAppendix A-BMPs Toolbox – Water Management BMPs (page A-149).
Examples of Shallow Trench Dewatering System Here are simple dewatering systems for shallow trenching. The drawing at left shows the use of a perforated drum and pipe dropped at the low spot in the trench to dewater. The fabric around the pipe will act as filtration. The drawing on right shows perforated pipe wrapped with fabric and surrounded by rock aggregate about a half inch in diameter. The rocks and fabric will act as filtration to remove fine particles.
Examples of Large Scale Dewatering System For large dewatering of groundwater, the dewatering system will consist of above ground tanks to settle sediment before it is discharged into an authorized drain or creek.
Media Filtration Units Here is a two-step filtration unit which uses both a sand filter and a finish filter to remove sediment from water before releasing it to the environment. Sand filter For fine sediment removal from dewatering operations, advanced treatment will be required. Media (e.g., sand, flocculants and filter cloth) Filtration Units are used for treating small particulates such as silt. Some are able to treat various bacteria, oils, etc.
Dirt bag may not be effective filtration initially because it takes time for the water to swell the fibers of the bag enough for the filtering of small particles to begin.
Water Diversion Options Diversion Berm • Applications & Benefits • Berm used to direct water away from unstable slope • Made from sandbags or aggregate material/plastic • Temporary use • Inexpensive BERM Unstable Slope • Limitations • Not for fast flowing water • Not for replacement of failing roadway or shoulder ROAD Illustration by Tyler Ledwith
Water Diversion Options Slope Drain (Temporary, Overside, Swale) • Applications & Benefits • Used to direct water away from unstable slope • Made from PVC, ABS, or corrugated metal. • Temporary use • Limitations • Gully erosion if fails • Could be tricky to install Illustration courtesy of FishNet 4C
Temporary Run-on Diversion • Divert upland drainage flow from work site • Stabilize all disturbed areas prior to rainy season with spray-on application. Mulch, tackifier, Bonded Fiber Matrix (BFM) etc. • Control site drainage by installing appropriate combination of BMPs. • Good house keeping and maintenance of BMPs. • Inspect weekly and document all observations and corrective measures.
Temporary Diversion of Run-on using plastic pipe
Example of Diversionof Run-on Erosion Control Blankets are keyed in under fiber rolls to mitigate erosion and sediment. Fiber rolls are also used as check dams to control the velocity of run-on. Fiber rolls work as check dams in this situation because they can be properly anchored with stakes. This will allow for clean run-on.
In this example of run-on diversion BMPs, we see that source control was first addressed in the form of Bonded Fiber Matrix covering on the left side, mulch and silt fence on the right, and erosion control blanket in the middle of the channel to transport clean run-on. Diversion of Run-on
The envelope please. Number 4 Any of the above. This run-on BMP is good, but which BMP might complete this scenario? • Erosion Control Blankets • Mulch • Hydro-seeding • Any of the above
Temporarily Divert Run-on It pays to do it right the first time!
Environmental Barricades Environmental barricades come in various shapes, sizes and materials, from sand or gravel bags to manufactured systems such as the one in this photo. These manufactured barricades have a variety of advantages over sand or gravel bags in that they may be used for: Other advantages are: Rental Options Recyclable Replaces Sandbags Labor Saving Spill Containment Flood Control Erosion Control Water Filtration
Environmental Barriers such as this one are modular and may be configured to fit almost any situation. Each module is a hollow plastic mold that weighs 45 to 50 pounds. When filled with water each module will weigh more than 400 pounds. Environmental Barricade
Environmental Barricade used for a sediment holding tank Notice the plugs that allow for gradual release of water.
For more information onWater ManagementRefer to the Fishnet Roads Manual • Chapter 6: Working in and Around Stream Channels • Dewatering • Appendix A: BMP Toolbox – Water Management BMPs