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Design For Fish Passage at Roadway - Stream Crossings: Synthesis Report
9 Construction, Maintenance and Inspection
How to use this chapter
The following construction topics have unique applications in culverts designed for fish passage. Topics are not covered in-depth; however, links to pertinent references are included. No specific discussion of slip lining issues is included, because no mature guidelines existed at the time of this report.
Timing of in-stream work will need to correspond to specific periods allowable by resource agencies. An in-stream work permit will be required.
It is important to consider constructability of any culvert installation. The successful construction of culverts utilizing natural bed material is contingent on the ability of crews to place rock within the structure. In general, this leads to the requirement that culverts span a minimum of 1800 mm (6 ft) (i.e. Bates et al. 2003), although 1500 mm (5 ft) installations are reportedly placed routinely in Alaska (Gubernick, Personal Communication). Depending on size of pipe and bed materials, placement has been done by a number of methods including Dingo Loaders, rock chutes, wheel barrows and trail building equipment. Due to the difficulty involved with mixing bed materials on site, it is also recommended that material be mixed prior to placement, except when backfilling large key elements with fines. Rock bands and banks must be placed by hand (United States Forest Service 2006a).
Quality assurance and quality control (QA/QC) are very important for all vertical controls, especially for the placement of all interim rock bands, baffles and key sediment.
9.1.3 Bed Mix Specification
When specifying engineered bed material, the design engineer should ensure that materials and compositions are appropriate for the design. This should include a "pit run" where the design engineer examines the composition of rock piles to ensure adequacy.
When a pit cannot specifically guarantee the composition of a pile, it will be necessary to verify the adequacy of the material. WDFW recommends the following techniques:
The following example from Washington Department of Fish and Wildlife is intended to help clarify the process of material gradation for stream simulation (Bates et al. 2003).
For our example site, the required bed gradation has been determined to be:
D100 = 0.381 m (1.25 ft)
What this means is that 16% of the material is less than 0.019 m (0.75 in), including roughly equal proportions of small gravel, sand and silt. Sixteen percent is between o.15-0.381 m (0.5-1.25 ft), which, when viewed from above, will compose 1/6th of the channel surface. The remaining 68% is basically well-graded gravel and cobble. If a gravel pit is making up this mixture, then piles of material need to be assembled in proportions that approximate the desired gradation. One approach is to use parts or "scoops" of a given component. For the example mixture here, a very simple recipe could be: five scoops of 0.15 m-minus (6-in-minus) pit run with fines, plus one scoop of 0.2-0.38 m (8-15 in) rock. Care should be taken to ensure that the fines within the 0.15 m-minus (6-in-minus) pit run are appropriate for sealing voids and interstices. A third class designation for fines may be necessary.
9.1.4 Sealing Voids
In culverts with placed sediments, especially those involving the use of oversized sediment mixes, it is important to limit permeability. Without such considerations, a significant portion of flow may seep through interstitial voids, causing the stream to go subsurface. Methods to limit permeability include placement of filter fabric (Browning 1990), and including an adequate proportion of fine sediments in bed mixes (Bates et al. 2003; Bates et al. 2006). During construction, fines can be power-washed into voids to ensure, and expedite, bed sealing. This washing procedure will also decrease the sediment concentration entering the stream system after the first flow event.
For constructed bed culvert installations, bed material is placed in thin layers with thickness appropriate for the slope and for the size of the mix, compacted, and covered with filler material to be washed into voids (United States Forest Service 2006a). Smaller material should be well compacted around larger elements (Bates et al. 2006).
9.2 Maintenance and inspection
Culverts that qualify as bridges, total span exceeds 6.1 m (20 ft), must be inspected every two years using 23 CFR 650 Subpart C of the National Bridge Inspection Standards as a guide (FHWA 2004). This inspection includes checks of all underwater elements, and fill and scour at the crossing.
Unfortunately, there are few if any documented schedules for culvert inspection and maintenance. Standard culvert problems and treatments are listed in the Federal Highway Administration Culvert Repair Practices Manual Volume I(Ballinger and Drake 1995), and CALTRANS has supplemental guidelines for use in their transportation system (CALTRANS 2006).
Inspection is advisable at regular intervals and ideally during flood events. This may be especially important at installations in areas with significant amounts of LWD, or at crossings with a propensity to collect debris (baffled culverts, fishways). Properly designed and constructed fish passage culverts will still require regular maintenance and monitoring to ensure continued performance, especially in the first few years to evaluate the potential to collect debris or to scour/aggrade the streambed.