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”).

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	Results in least disturbance to stream bed and banks compared to other stream crossing methods Typically no time of year restrictions for use Generally most appropriate for streams with high flow velocities or steep gradients	Use in large channels may require pier(s) or other supports, which increase disturbance to stream (see Construction recommendations and specifications)
Temporary culvert (pipe)	Most common temporary stream crossing Most appropriate for short-term use Generally appropriate for perennial or intermittent streams Generally preferred over fords as disturbance to stream is limited to construction and removal	May not be appropriate for large streams or during high flow conditions Causes greatest obstruction to flow May damage channel cross-section; culvert crossings should be planned to limit the number of crossings needed Not appropriate for streams that are vertically degraded or incised
Temporary ford (drive-through)	Most appropriate for short-term use Generally appropriate for ephemeral or low flow perennial streams Used when bridge or culvert crossings are not feasible; typically very shallow streams where stream banks are less than 4 feet above the stream invert	Use should be limited to dry season Use should be limited to streams whose beds are armored May degrade water quality due to contact with construction vehicles and equipment

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.
Link to this table

Water quantity
Flow attenuation	Little or no design benefit
Runoff volume reduction	Little or no design benefit
Water quality
Pollution prevention
Soil erosion	Primary design benefit
Sediment control	Little or no design benefit
Nutrient loading	Little or no design benefit
Pollutant removal
Total suspended solids	Little or no design benefit
Total phosphorus	Little or no design benefit
Heavy metals	Little or no design benefit
Floatables	Little or no design benefit
Oil and grease	Little or no design benefit

Planning considerations

With careful planning, the need for temporary stream crossings may be minimized or avoided altogether. If stream crossings cannot be avoided, the following should be considered when planning for the use of temporary stream crossings:

• Plan for stream crossings to be in service for the shortest practical amount of time and removed as soon as their function is completed.
• Place crossings strategically to reduce the number of crossings needed during construction.
• To the extent possible, minimize the number of trips back and forth across stream by completing construction activities on one side of the stream and then moving to the other side.
• Maintain a natural buffer zone along the stream.
• When planning for temporary stream crossings, consider that bridges pose the least potential for creating barriers to aquatic migration.

Design

Temporary crossings require coordination with regulatory agencies including USACE for a permit. If in-channel disturbance is required to build the temporary crossing, temporary stream channel diversions should be used to minimize soil loss and erosion and removed upon completion of the crossing. The following general design criteria apply for all temporary stream crossings:

• Select stream crossing sites along relatively straight channel reaches and away from deep instream pools and trees if possible. For piped and ford crossings, look for places where bank slopes are low and long. For fords, broad and rocky riffle areas are preferred, if available.
• Design the temporary stream crossing to be as perpendicular to the stream flow as possible. Under certain site conditions, the crossing angle may by up to 15 degrees from perpendicular.
• Do not use earth/soil material for construction within the stream channel. Any material placed in flowing water must not cause siltation (e.g., clean rock and gravel).
• If the temporary crossing will be used for 2 weeks or less, design the crossing to convey the stream’s base flow.
• For temporary crossings to be used for longer durations, design the crossing with sufficient capacity to convey the 2-year, 24-hour flow without overtopping.
• Design the crossing such that the design flow velocity at the outlet is non-erosive for the receiving stream channel and install outlet protection.
• Design the temporary crossing to have a single traffic lane with a width between 12 feet and 20 feet.
• Temporary crossings should not significantly impede stream flow; therefore, do not design a crossing method that results in a significant water level difference between the upstream and downstream ends of the crossing.

Specific design considerations for each type of temporary stream crossing are provided below.

Bridges

Bridges The following design recommendations apply to bridges as temporary stream crossings:

• Temporary bridge crossings are typically constructed of wood, metal, or other approved materials; common materials include logs, sawn timber, prestressed concrete beams, or metal beams.
• Bridge deck materials should be capable of holding the maximum anticipated weight.
• Bridge crossings should be designed to minimize disturbance to the stream banks.

Culverts

The following design recommendations apply for culverts as temporary stream crossings:

• Design culverts with a minimum capacity to convey a 1-year flow or the stream’s base flow for projects shorter than two weeks and for the 2-year flows for all other projects. Multiple culverts or elliptical culverts may be used to achieve these flow capacities while providing minimum cover requirements.
• All culverts must be capable of supporting their cross-section area under the maximum expected loads of the project.
• Culvert diameters should be as wide as will fit into the existing stream channel without major excavation; the minimum culvert diameter used for a culvert crossing should be 12 inches; 18 or more inches is preferred.
• For channel widths greater than 3 feet, more than one culvert may be installed such that the cross-sectional area of the pipes exceeds 60 percent of the cross-sectional area of the existing stream.
• The culvert slope should mirror the slope of the existing stream.
• Culvert length should not exceed 40 feet
• Culverts should be designed to minimize the erosive power at their outlets so as to not damage the stream channel at the discharge point. Outlet protection can be installed to minimize this damage.

Fords

The following design recommendations apply for fords as temporary stream crossings:

• Approach roads should be no steeper than 5:1.
• Convey overland flow away from the temporary access ford approaches using diversions like shallow swale, ensuring that discharge from the swale is non-erosive. Outlet protection or level spreaders can be used as necessary.