578 Stream Crossing

578 - 1 NATURAL RESOURCES CONSERVATION SERVICE CONSERVATION PRACTICE STANDARD STREAM CROSSING (No.) CODE 578 DEFINITION areas of listed species. Avoid wetland areas. A stabilized area or structure constructed Discourage livestock loafing in the stream by across a stream to provide a travel way for locating crossings, where possible, out of people, livestock, equipment, or vehicles. shady riparian areas or by including gates in the design. PURPOSE Install stream crossings perpendicular to the • Provide access to another land unit direction of stream flow where possible. Fully consider the natural lateral migration pattern of • Improve water quality by reducing the stream in the design. Avoid skews on all sediment, nutrient, organic, and but the smallest streams. inorganic loading of the stream Access Roads. Where the stream crossing is • Reduce streambank and streambed installed as part of a roadway, size the erosion crossing according to NRCS Conservation Practice Standard, Access Road, Code 560. CONDITIONS WHERE PRACTICE APPLIES Width. Provide an adequate travel-way width This practice applies to all land uses where an for the intended use. Make “livestock- only” intermittent or perennial watercourse exists crossings no less than 6 feet wide and no and a ford, bridge, or culvert type crossing is more than 30 feet wide, as measured from the needed. upstream end to the downstream end of the stream crossing, not including the side slopes. CRITERIA Side Slopes. Make all side slope cuts and fills Apply this standard in accordance with all stable for the channel materials involved. Make local, State, Tribal, and Federal regulations, the side slopes of cuts or fills in soil materials including flood plain regulations and flowage no steeper than 2 horizontal to 1 vertical (2:1). easements. Make rock cuts or fills no steeper than 1.5 Identify significant cultural resources or horizontal to 1 vertical (1.5:1). threatened or endangered species that could Stream Approaches. Blend approaches to the be affected by the implementation of the stream crossing with existing site conditions, practice. where possible. Use streambank soil Location. Locate stream crossings in areas bioengineering practices as appropriate and where the streambed is stable or where it can feasible. Make the approaches stable, with be stabilized (see NRCS Conservation gradual ascent and descent grades which are Practice Standard, Channel Bed Stabilization, not steeper than 4 horizontal to 1 vertical (4:1), Code 584). Do not place crossings where and of suitable material to withstand repeated channel grade or alignment changes abruptly, and long term use. Make the minimum width of excessive seepage or instability is evident, the approaches equal to the width of the overfalls exist (evidence of incision and bed crossing surface. instability), where large tributaries enter the Divert surface runoff around the approaches to stream, or within 300 feet of known spawning prevent erosion. Direct roadside ditches into a Conservation practice standards are reviewed periodically and updated if needed. To obtain NRCS, NHCP the current version of this standard, contact your Natural Resources Conservation Service State Office or visit the Field Office Technical Guide. September 2011 578 - 2 diversion or away from the crossing surface. Bankfull flow is the discharge that fills a stream channel up to the elevation at which flow Configure the crossing approaches (gradient begins to spill onto the floodplain. and curves) to properly accommodate the length and turning radii of vehicles using the Adequately protect bridges protected so that crossing. out-of-bank flows safely bypass without damaging the culvert or eroding the banks. Rock. All rock must be able to withstand exposure to air, water, freezing, and thawing. Vehicle and pedestrian bridges must be Use rock of sufficient size and density to resist designed in accordance with the current mobilization by design flood flows. American Association of State Highway and Transportation Officials Load and Resistance Use appropriate rock sizes to accommodate Factor Design (LRFD) bridge design the intended traffic without damage to the specifications (AASHTO, 2010). livestock, people, or vehicles using the crossing. Evaluate the need for safety measures such as guardrails and reflectors at bridge crossings. Fencing. Exclude livestock access to the crossing through the use of fence and gates, Acceptable bridge materials include concrete, as needed. steel, and wood. Install cross-stream fencing at fords, with Culvert Crossings breakaway wire, swinging floodgates, hanging Design culverts in a manner that is consistent electrified chain, or other devices to allow the with sound engineering principles and passage of floodwater and large woody adequate for the use, type of road, or class of material during high flows. vehicle. For culverts associated with a road, Design and construct all fencing in accordance culvert design flow shall meet the criteria in with NRCS Conservation Practice Standard, NRCS Conservation Practice Standard, Fence, Code 382. Access Road, Code 560. The design flow for culverts not associated with a road will be the Vegetation. Plant all areas to be vegetated as 2-year, 24-hour storm discharge, or bankfull soon as practical after construction. If flow, whichever is less. Design culverts with completion does not coincide with appropriate sufficient capacity to convey the design flow planting dates for permanent cover, use a and transported material without appreciably cover of temporary vegetation to protect the altering the stream flow characteristics. site until permanent cover can be established. Native or functioning-as-native plant species Design culverts to minimize habitat are preferred. Use NRCS Conservation fragmentation and to minimize barriers to Practice Standard, Critical Area Planting, Code aquatic organism movement. 342, where vegetation is unlikely to become Do not use culverts where large flows of established by natural regeneration, or where sediment or large woody material are acceleration of the recovery of vegetation is expected, or where the channel gradient desired. exceeds 6 percent (100 horizontal to 6 In areas where the vegetation may not survive, vertical). use NRCS Conservation Practice Standard, Evaluate the need for safety measures such as Heavy Use Area Protection, Code 561. guardrails at culvert crossings. Bridge Crossings Crossings shall be adequately protected so Design bridges in a manner that is consistent that out-of-bank flows safely bypass without with sound engineering principles and damaging the structure or eroding the adequate for the use, type of road, or class of streambanks or the crossing fill. vehicle. Design bridges with sufficient capacity At least one culvert pipe shall be placed with to convey the design flow and transported its entire length set six inches below the material without appreciably altering the existing stream bottom. Additional culverts stream flow characteristics. Design bridges to may be used at various elevations to maintain fully span the stream, passing at least the terrace or floodplain hydraulics and water bankfull flow where the design flow is not surface elevations. dictated by regulation. NRCS, NHCP September 2011 578 - 3 Make the barrel length of the culvert adequate Use a minimum thickness of 5 inches of placed to extend the full width of the crossing, concrete. Pour the concrete slab on a including side slopes, and inlet or outlet minimum 4-inch thick gravel base, unless the extensions. foundation is otherwise acceptable. Acceptable culvert materials include concrete, Construct toe-walls at the upstream and corrugated metal, corrugated plastic, new or downstream ends of the crossing. Make the used high quality steel, and any other toe-walls a minimum of 6 inches thick and 18 materials that meet the requirements of NRCS inches deep. Extend the toe-walls in the Conservation Practice Standard, Structure for stream approaches to the bankfull flow Water Control, Code 587. elevation. Ford Crossings Precast concrete panels may be used in lieu of The following criteria apply to all ford cast-in-place concrete slabs. To the extent crossings: possible, the panels shall follow the contours of the stream bottom in order to avoid potential Make the cross-sectional area of the crossing problems with sediment accumulation. Use equal to or greater than the natural channel concrete units that have adequate cross-sectional area. Make a portion of the reinforcement for transportation and crossing depressed at or below the average placement. stream bottom elevation when needed to keep base flows or low flows concentrated. Dewatering of the site and toe-walls is required during placement of the concrete to maintain Match ford shape to the channel cross-section the proper water/cement ratio. Flowing water to the extent possible. will erode concrete that is not sufficiently Provide cutoff walls at the upstream and hardened. The stream must be diverted or downstream edges of ford-type stream retained from flowing over the concrete for at crossings when needed to protect against least 12 hours after placement of the concrete. undercutting. During construction, aquatic species must be Evaluate the need for water depth signage at removed from the construction area according ford crossings. to State protocols. To the extent possible, the top surface of the Rock Fords and the Use of Geosynthetics ford crossing shall follow the contours of the stream bottom but in no case shall the top Coarse aggregate or crushed rock ford surface of the ford crossing be higher than 0.5 crossings are often used in steep areas foot above the original stream bottom at the subject to flash flooding and where normal flow upstream edge of the ford crossing. is shallow or intermittent. When the site has a soft or unstable subgrade, use geotextiles in Make the downstream edge of the ford the design of rock ford crossings. crossing with a low-flow hydraulic drop less than 0.5 foot above the original stream bottom. Dewater and excavate the bed of the channel to the necessary depth and width and cover Concrete Fords with geotextile material.

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