3 - Structure Selection / Forest Road Drainage Installation Practices in the Pacific Northwest

Forest Road Drainage Installation Practices
in the Pacific Northwest

3 - Structure Selection	Open-Bottom Structures, Metal, Polyethylene, Steel

Considerations

Life of structure, i.e., short or long term, permanent or temporary;
need and ability of the structure to pass debris, sediment, ice flows;
expected debris and sediment movement upstream from crossing;
water quality requirements, i.e., domestic water users and proximity to fish bearing waters;
fish passage and protection of fish habitat;
risk of sediment generation during installation and deactivation;
level of access required following permanent deactivation, i.e., consider fords and modified fords to maintain access after deactivation.

Open-Bottom Structures

Log Culverts provide the most flexibility as length and width can be increased during construction if needed. As sill logs are placed outside the wetted perimeter of the channel, log culverts can be built outside the fisheries window as the creek bottom is not disturbed. Use geotextile on top and puncheon between logs to prevent fill material from falling into the stream. As a rule, most wood species begin to deteriorate quickly and may need to be replaced after 5 to 10 years, with the exception of Red Cedar which can last 40 years or longer. If suitable material for a log culvert is available, the reduced risk of sediment generation during construction and easy disposal of the log culvert during deactivation offset its more time-consuming installation compared to a pipe culvert.

Bridges and alternative drainage structures such as multiplates are dealt with in detail in the Bridge Infoflip.

Closed-Bottom Structures

Corrugated Metal Pipes are easy to install but vulnerable during transport (unloading) and installation. Intakes and outlets are also prone to damage by equipment during harvesting, grading and snowplowing, or by environmental conditions such as windthrow or ice break. In forest road applications, pipe culverts are ideally suited for cross drain culverts or small non fish-bearing streams. Using thicker gauge material will dramatically increase strength and recoverability of pipes, with a minimal increase in cost. Closed-bottom arches are a cost-effective alternative to round pipes as they are structurally lower risk, and create stream characteristics when embedded properly.

Polyethylene Pipes are much lighter than metal pipes, more forgiving, and also easy to handle. They may be difficult to install in a high flow, high energy stream as they tend to float. Uplifted "gun barrel" intakes may be the result of poorly installed pipes. Double wall products are available for increased strength and improved hydrology. Polyethylene pipes do not freeze up as readily and thaw sooner in the spring than metal pipes. Extend pipes down to the base of fill slopes as flumes to prevent fill slope erosion, or attach to a metal pipe as flume extension.

Thick-Wall Steel Pipes are considerably more expensive than corrugated metal pipes, and may be in limited supply. However, these extremely durable pipes are well suited as temporary structures during access development, or as short term, in-block drainage solutions during harvesting. Because of their strength, they require almost no fill. Pipe recovery during deactivation is simple and pipes can be used repeatedly, paying good dividends on the high initial investment.

3 - Structure Selection