Construction stormwater practices - temporary stream crossing
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A temporary steam crossing is a structure designed for short-term (typically less than one year) use that is installed across a watercourse to allow construction vehicles to safely cross without damaging the stream bed or channel. They may also be referred to as “temporary access” or “temporary access crossings.” The most common types of temporary stream crossings include bridges, culverts (or “pipe crossings”), and fords (or “drive-through crossings”).
Contents
Purpose and function
The primary function of a temporary stream crossing is to prevent construction equipment from damaging the stream bed and banks, impeding stream flow, blocking fish migration, and tracking sediment and other pollutants into the stream.
Applicability
Because of the potential disturbance created by construction of a temporary stream crossing, they should be used only when they are completely necessary for construction purposes. Refer to Planning considerations for additional discussion. In general, temporary stream crossings are applicable when heavy equipment must be moved from one side of a watercourse to the other, or where lighter-duty construction vehicles require frequent, short-term access for crossing a stream channel.
Site applicability
Temporary stream crossings should be limited to locations with low erosion potential and should be constructed during dry periods to minimize stream disturbance and reduce costs. Select crossing sites away from trees, steep banks, channel bends, and deep pools. Stream channel conditions, including expected flow rates during the operation of the temporary crossing, should be thoroughly evaluated before choosing the appropriate type of stream crossing. For example, culverts and access fords may not be appropriate for large streams or during high flow conditions, as they may constrict flow and create potential safety hazards. Temporary bridges may be more suitable for these conditions and are also less likely to impede fish and aquatic life passage.
Although bridges are generally the most expensive to design and construct, they are often the preferred method for temporary stream crossings because they provide the least disturbance of the stream bed and constriction of stream flow. Culverts are less expensive than bridges, relatively easy to construct, and able to support heavy equipment loads. Fords are typically the least expensive to construct but are generally not appropriate for construction during rainy periods or on high-flow perennial streams. The table below provides additional information on the applicability of each temporary crossing and limitations of use.
Typical temporary stream crossing methods and their applicability.
Link to this table
| Stream crossing type | Applicability | Limitations |
|---|---|---|
| Temporary bridge |
|
Use in large channels may require pier(s) or other supports, which increase disturbance to stream (see Construction recommendations and specifications) |
| Temporary culvert (pipe) |
|
|
| Temporary ford (drive-through) |
|
|
Permit applicability
Temporary stream crossings require coordination with regulatory agencies, and may require local, state, or federal permits. A permit is required for placement of fill in a waterway under Section 404 of the Clean Water Act. The local U.S. Army Corps of Engineers (USACE) office should be contacted concerning the requirements for obtaining a 404 permit. In addition, a permit from the U.S. Fish and Wildlife Service (USFWS) may be needed if endangered species are known to be present in the work area.
Section 23.11 of the MPCA Construction Stormwater General Permit notes that when a water crossing is necessary in order to complete a construction project, disturbance of the required undisturbed buffer zone may be allowed if fully documented in the SWPPP. Specifically, Section 23.11 states: “Permittees must include an undisturbed buffer zone of not less than 100 linear feet from a special water (not including tributaries) and must maintain this buffer zone at all times, both during construction and as a permanent feature post construction, except where a water crossing or other encroachment is necessary to complete the project. Permittees must fully document the circumstance and reasons the buffer encroachment is necessary in the SWPPP and include restoration activities.”
A DNR Public Waters Work Permit may be required if the waterway is a public water, depending on the impact of the stream crossing. The following guidance is provided by the MN DNR:
To Construct A Bridge or Culvert, or to Fill or Excavate the Bed of a Public Watercourse Having a Total Drainage Area, at its Mouth, of Less Than 5 Square Miles (3,200 Acres) - A DNR Public Waters Work Permit is not required, provided:
- County zoning officials and local Soil and Water Conservation District are given at least 7 days prior notice to determine that the project will not result in downstream erosion or sedimentation;
- The project will not divert water to a different watershed;
- The project will not impound water by damming the watercourse; and
- The watercourse is not an officially designated trout stream.
Low Water Ford Crossings (on streams only) - A DNR Public Waters Work Permit is not required when:
- The crossing conforms to the shape of the natural stream channel;
- The original stream bank is no higher than 4 feet;
- The normal summer flow does not exceed 2 feet in depth.
- The normal summer flow is not restricted or reduced;
- No special site preparation necessary;
- The crossing is constructed only of gravel, natural rock, concrete, steel matting or other durable, inorganic material not more than 1 foot thick;
- The finished graded slope is no steeper than 5:1 (horizontal to vertical);
- The graded banks must be seeded or mulched to prevent erosion and sedimentation; and
- The site is not an officially designated trout stream, wild, scenic or recreational river or officially designated canoe and boating route.
Temporary Bridges (on streams only) - A DNR Public Waters Work Permit is not required when:
- The stream bank can support bridge without pilings, foundations, culverts, excavation, or other special site preparations;
- Nothing is placed in the bed of the stream;
- The bridge is capable of removal for maintenance and flood damage prevention;
- The bridge is firmly anchored at one end and can swing away during flooding;
- A minimum 3 feet of clearance between the lowest portion of the bridge and normal summer stream flow is maintained; and
- The bridge construction is consistent with floodplain, shoreland, and wild and scenic river ordinances.
Effectiveness
As a standalone practice, temporary stream crossings are not intended to provide significant water quantity or water quality benefits. In fact, when they are not properly planned, constructed, maintained, and removed, temporary stream crossings can be a direct source of water pollution.
As noted above, temporary stream crossings provide a means for construction vehicles to cross streams without moving sediment into them, damaging the stream bed or channel, or creating flooding or safety hazards. To protect water quality, stream crossings should be located in areas where erosion potential is low, and a natural buffer should be preserved along the stream on both sides of the crossing. Additional recommended measures to protect water quality are discussed in Planning considerations.
If the appropriate considerations and protective measures are implemented, temporary stream crossings can serve as a stream protection measure by effectively reducing soil erosion and transport of sediment into streams.
Expected performance benefits for temporary stream crossings