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Dewatering

General

Dewatering is a practice where sediment-laden water is pumped into a compartmented container, settling basin, filter, or other appropriate best management practice to trap and retain sediment. This practice detains sediment generated during the removal of water from a site prior to discharging it off-site and/or to waters of the state.

Dewatering applies where sediment-laden water must be removed for construction. The selection of a dewatering practice is dependent upon the predominant soil texture encountered at the dewatering site with consideration given to pumping rates, volumes and device effectiveness. Users of the practice of dewatering should be conscious of applicable federal state, and local laws, rules, regulations, or permit requirements governing the use and placement of dewatering.

Advantages

  • Reduces the amount of sediment leaving the site
  • Allows for a more in-depth site assessment – additional necessary erosion control measures may be identified

Disadvantages

  • Must abide by multiple government laws and standards and obtain appropriate permits
  • Requires frequent maintenance
  • May be costly

Site Assessment

The proposed site must be assessed and documented to determine the site characteristics that will affect the placement, design, construction and maintenance of dewatering activities. Characteristics such as ground slopes, drainage patterns, runoff constituents, soil types, soil conditions, sinkholes, bedrock, proximity to regulated structures, natural resources, and specific land uses must be included in the site assessment. The documented site assessment should include the following:
  • Soil textural class for dewatering areas with investigation extending below grading and trenching depths
  • Storm sewer and sanitary locations
  • Potential contaminates already in the soil, such as odor or discoloration other than sediment, or an oily sheen on the surface of the sediment-laden water – notify DNR Spills Reporting if present
  • Seasonally highest water table depth
  • Transport method and distance to receiving waters
  • Discharge outfall locations

The Wisconsin Department of Natural Resources (WDNR) must be contacted when dewatering discharge will enter a WDNR listed Exceptional Resource Water, Outstanding Resource Water, or a wetland in an area of special natural interest. Additionally, if the discharge of the dewatering activity were to directly or indirectly enter a stream, the discharge flow rate must not exceed 50 percent of the peak flow rate of the 2-year, 24-hour storm event. General criteria applicable to dewatering activities are outlined in WDNR Conservation Practice Standard 1061 - Dewatering.

Soils

The selection of the dewatering practice depends upon the predominant soil texture encountered at the dewatering site. Refer to Figure 1, the USDA Soil textural triangle, to assist with classifying the soil of the site. Figure 2, Dewatering Practice Selection Matrix, illustrates acceptable dewatering options and their effective ranges. Pumping rates, volumes, and device effectiveness must also be considered when selecting a practice.

Figure 1: USDA Soil Textural Classification

Practices

Dewatering can be preformed in a variety of ways and should be selected based upon the individual site characteristics. The accepted dewatering practices are geotextile bags, gravity based settling systems, passive filtration systems, and pressurized filtration systems.

Geotextile Bags

Geotextile bags are gravity-based filter bags not contained within any vessel or enclosure. Geotextile bags are widely used on sites where there is no available space for a sediment basin. The footprint of the bag, however, should be no smaller than 100 square feet. They lie on the ground and are designed to collect silt and sediment from pumped water. Sediment-laden water is pumped from the site and discharged into the bag that is securely attached to the discharge pipe. The almost sediment-free water discharges through the walls of the bag, while the sediment is retained inside the bag. Disturbing the bag may break up the cake of collected sediment and reduce its efficiency.

Geotextile bags should be sized according to the particle size being trapped, expected flow or pumping rate per square foot of fabric and a 50% clogging factor. Geotextile bags should meet the criteria listed in Table 1.

PropertyTest MethodType I ValueType II Value
Maximum Apparent
Opening Size
ASTM D-47510.212 mm0.212 mm
Grab Tensile StrengthASTM D-4632200 lbs300lbs
Mullen BurstASTM D-3786350 psi580 psi
PermeabilityASTM D-44910.28 cm/sec0.2 cm/sec
FabricNominal
Representative
Weight
8oz12 oz
Polymers may be used to enhance the efficiency of the geotextile bags, but must meet the performance requirements of WDNR.

Gravity Based Settling Systems

The settling of particles is the primary means of treatment for gravity based settling systems. Therefore, sufficient detention time is required when using this practice. Practices include portable sediment tanks, sediment traps, sediment basins and wet detention basins. WDNR approved polymers can also be used to enhance the settling.

Portable sediment tanks are intended to settle only sands, loamy sands, and sandy loams. If polymer is added, these tanks will additionally be appropriate for settling loams, silt loams and silts. Portable sediment tanks should be at least three feet deep and have a minimum of two baffled compartments. The inlet and outlet pipe should be a minimum diameter of three inches. To account for the settling of suspended sediments, one must determine the appropriate size of a tank. Multiply the pumping rate (gallon per minute) by 1.83 (a factor that includes the conversion from gpm to cfs and the particle settling velocity for Soil Class 1) to calculate the surface area of a tank in square feet.

Sediment traps and basins are temporary sediment control devices, while wet detention basins are generally permanent structures designed to address post-construction pollutant reduction requirements. The design, installation, and operation of sediment traps and basins should meet WDNR requirements.

Passive Filtration Systems

Passive filtration systems also rely on filtration as the main means of removing sediment. The distribution of particle size in the stormwater influences sediment removal efficiency. Manufactured filters should be sequenced from the largest to the smallest pore opening. Available are sand media filters with automatic backwashing features that can filter to 50 μm particle size, screen or bag filters that can filter down to 5 μm, and fiber wound filters that can remove particles down 0.5 μm.

Other practices include filter tanks, filter basins, vegetative filters, grassed swales, and filtration fabric and should be installed, operated, and maintained according to manufacturer recommendations and WDNR.

Pressurized Filtration Systems

Designed to handle higher flow rates, pressurized filtration systems have the water flowing through the media pressurized, rather than depending solely on filtration. Pressurized filters are composed of individual filters that are most effective when larger particles have been removed by prior treatment with a weir tank, sand filter, etc. Practices include portable sand filters, wound cartridge units, membranes and micro-filtration units.

Pressurized filters have automatic backwash systems that are activated by a fixed pressure drop across the filter. Returning backwash water to the tank may be necessary if the volume of the backwash water is minor or substantially more turbid than the stormwater stored in the tank. Further means of treatment, such as land application, and disposal may be necessary to complete treatment.

Wound cartridge units are used when secondary filtration of sediments is necessary to remove fine particles such as clays. It is capable of removing sediment larger than 0.002 mm, but is most effective when used after larger particles have been removed by other treatment methods. Other practices include portable sand filters, membranes, micro-filtration, and polymers and should be installed and maintained according to manufacturer recommendations and WDNR.

Maintenance

  • Sediment must be frequently removed from devices and properly disposed of to maintain effectiveness
  • Dewatering must be monitored and recorded on a daily log
  • Install, operate, and maintain pressurized filtration systems by following manufacturer recommendations

Method to Determine Practice Efficiency

Dewatering practices reduce the amount of suspended sediment in water that must be removed from a site through filtering methods. The efficiencies for these practices vary by the type of device used and soil texture, pumping rates, volumes and device effectiveness. Devices that are constructed on site will have an efficiency that is determined by calculating the settling efficiency for the device.

References