Well-designed bridges are the preferred option for permanent stream crossings and usually have the least impact on channel activities and fish passage.
Any bridge crossing design needs to consider:
- Ability to pass peak flows
- Ability of structure and road fill to accommodate flooding/backwater impacts
- Accommodate provisions for fish/aquatic organism passage as required by permitting agencies
- Weight allowance: will you get “unplanned” users of the bridge such as fire trucks, concrete trucks, septic tank pump trucks, etc?
- Consult guidelines for design of abutments, footings, scour depth, etc.
When utilizing a culvert to pass flows beneath a crossing, consider the following:
- Proper siting of culvert crossings in a stable, relatively straight reach is critical.
- Culverts must adequately pass peak flows, debris, ice and allow for fish passage.
- Culvert crossings should be avoided in aggrading streams, or on latterly unstable stream locations. – cleaned out
- Fish passage considerations may require oversized pipes, baffled culverts, open-bottom arches, or bridges.
- Corrosive soil or water conditions may damage metal pipe.
- Multiple side-by-side culverts should be discouraged because they catch debris and have greater tendency to wash out.
Fords are crossing where vehicles simply drive through the channel at low water. These are best located in shallow channels with gravelly bottoms and infrequent traffic. Even small Montana streams will likely become uncrossable during the spring/early-summer snowmelt.