About the author: Barney Blackburn, P.E., CPESC, CPSWQ, is environmental consultant for Wake County Environmental Services, Watershed Management. Blackburn can be reached at [email protected].
Improper construction techniques during pipe and culverts installation in streams can have a significant impact on water quality, aquatic habitat and stream ecology. The overall health of these aquatic systems affects many aspects of life, including drinking water supplies, recreation, travel and tourism, and aesthetics. Managing the stream during construction and installation of permanent conveyances is necessary to support aquatic wildlife, drinking water and recreational activities.
Components
Typical components of construction phasing for pipe and culverts include, but are not limited to, stilling basins, geotextile filter bags, impervious dikes, pipe, temporary channels and stream crossings.
Stilling basins are constructed above ground to settle large particles from sediment-laden water pumped from the work area. They are designed to dewater at a slow rate through a permeable stone layer, typically constructed with filter stone and riprap. To prevent internal and external erosion of the basin side slopes, stabilize with seed and mulch. Stilling basins are designed for longer projects such as culvert installation.
A geotextile filter bag is utilized to capture large sediment particles when effluent is pumped from the construction site. The maximum recommended pumping rate for the bags is 80 gal per minute per sq ft so the bag will not burst or fail. Additional bags always should be available on site, as they can quickly fill with sediment. Place the bags on a stone pad or wooden pallets to maximize the surface area flow through all sides. Geotextile bags typically are used in short-term projects or when site conditions prohibit construction of a stilling basin.
When using stilling basins and geotextile filter bags, design and construct inside perimeter sediment control measures to provide secondary treatment of the discharge. Construct and install the basins and bags on level ground to avoid extreme elevation ranges and prevent the bags from rolling downgrade and potentially into the stream. Lastly, basins and bags should be located so that construction storm water will not have to be pumped across the stream channel.
Impervious dikes are utilized as dams for streams on the upper and lower sides of the culvert or pipe construction area. They may consist of sandbags, metal sheet piles or stone wrapped with a low-permeability geotextile. Selection is based on project conditions, such as type of stream bottom or width of the stream. Impervious dikes also are used to anchor temporary pipe and may be used to create the side of a temporary channel in wet conditions.
Temporary pipe and channels are utilized to convey stream flow through or around the pipe or culvert work zone. They should be designed to carry the two-year storm at a minimum. Pipe should be anchored at the upstream and downstream ends with impervious dikes and is primarily used for culvert extensions. Temporary channels should be lined with geotextile and designed as a base ditch with 2:1 side slopes, and typically are designed for new culvert installation and extended-duration projects.
Stream crossings convey stream flow and allow construction vehicles to cross a temporary channel. The crossings should be temporary unless permitted by an environmental regulatory agency. More than one pipe can be considered depending on the width of the stream channel, and the stream crossing should be constructed with riprap and aggregate stone as the top surface.
Enhancements
Three porous baffles should be installed in a stilling basin and arranged so the inflow passes through each baffle prior to discharge. The baffles reduce turbulence within the basin by distributing the velocity of the inflow and enhance sediment capture as particles adhere to the baffle fibers. In addition, excavating 1 ft below ground within the basin is an option to obtain additional sediment storage and form a permanent pool to dissipate the energy of the inflowing effluent. Flocculants also may be utilized in conjunction with stilling basins and the pumping operation, but will clog the pores of geotextile bags.
