Be structurally sound to safely support vehicular traffic.
Be cost effective.
Allow for aquatic organism passage.
Take into consideration land use goals of the property owners.
Not over-top or back up water during flood events.
The design must:
Support vehicular passage as heavy as emergency response vehicles like fire trucks and 20 ton dump trucks loaded with rock (HL-93 compliant).
Fit the available budget (to be determined from the landowner's budget, as well as possible funding from the NRCS).
Be able to convey the flows of a 10-year design storm.
Have base flow velocity (~1ft/s) and water depth (2 in) that meets aquatic organism passage requirements (considering species such as the Candy Darter and Roanoke Logperch, both of which are federally listed under the Endangered Species Act and inhabit this watershed).
Have a design life of at least 25 years.
Standards
Standards for Stream Crossings, Roads, Bridges, and Culverts
Virginia NRCS Conservation Practice Standard 578 - Stream Crossings (Virginia NRCS, 2012b).
Road slope must be less than 6:1, maintain width over stream crossing, and have a minimum width of 12 ft.
Cut and fill surfaces must have slopes less than 2:1 and 1.5:1 for soil and rock respectively and must be re-vegetated according to VA NRCS CPS 342 - Critical Area Planting (Virginia NRCS, 2017).
Culverts must have a minimum diameter of 24 inches, be countersunk at least 6 feet below stream bed, be covered by at least 1 foot of fill, and extend 1 foot beyond crossing on either side. Appropriate materials include concrete, corrugated metal/ plastic, and steel. Should not be used where stream grades exceed 6%.
Bridges must fully span the channel, pass bankfull discharge, and convey typical sediment and organic loads carried by stream.
Permit required by the Clean Water Act when crossing trout streams/ spawning areas.
Virginia NRCS Conservation Practice Standard 560 - Access Roads (Virginia NRCS, 2014).
Road must be crowned.
Other criteria covered by NRCS Stream Crossing Standard (Virginia NRCS, 2017).
VDOT: Hydraulic Design of Highway Culverts Third Edition (Schall et al., 2012).
VDOT: Hydraulic Design of Safe Bridges (Zevenbergen et al., 2012).
VDOT Structure and Bridge Division, Instructional and Informational Memorandum (VDOT, 2018)
Standards Related to Aquatic Organism Passage:
Virginia NRCS Conservation Practice Standard 396 - Aquatic Organism Passage (Virginia NRCS, 2012a).
Hydraulic performance of structure must be evaluated at bankfull and a 25-year peak flow event.
Structure shall be designed around the swimming abilities of a target organism, and a range of flows should be considered especially those occurring during migration periods.
Structure should be constructed out of non-toxic materials and in such a way that avoids impacts to aquatic organisms, riparian areas, and in-stream habitat.
Crossing bottoms should mimic natural channel geometry and morphology. Stream Simulation (Gubernick et al., 2008) is recommended for this element of the design, especially when designing culverts or bottomless arches.
VDOT - HEC 26 Culvert Design for Aquatic Organism Passage (Kilgore et al., 2010).
Standards Related Streams, Floodplains, and Riparian Buffers:
Virginia NRCS Conservation Practice Standard 582 - Channel Bed Stabilization (Virginia NRCS, 2011a).
Bed materials be compatible with present and future flow and sediment regimes, and resistant to natural forces (ice, debris, and different discharges).
Stream Simulation (Gubernick et al., 2008) can be used to select an appropriately sized material to maintain natural channel function.
Virginia NRCS Conservation Practice Standard 580 - Streambank and Shoreline Stabilization (Virginia NRCS, 2011b).
Virginia NRCS Conservation Practice Standard 391 - Riparian Forest Buffer (Virginia NRCS, 2015).
Riparian buffers must have a minimum width of 35 feet on each side of the stream.
Native species are preferable. Natural regeneration may be employed if seed-producing trees are within 100 ft of the entire planting area.
Hardwood seedlings for planting must be a minimum of 18 inches tall or a quarter inch in diameter. Tree whips must have a minimum root collar diameter of ⅜ inches and be approximately three feet tall.
Appropriate species should be selected for the environmental and hydrologic conditions of the site.
Virginia Department of Conservation Riparian Buffers Modification & Mitigation Guidance Manual (Baird and Wetlmore, 2006).
Virginia NRCS Conservation Practice Standard 342 - Critical Area Planting (Virginia NRCS, 2017).
Ripping can be performed to ameliorate compacted layers.
Species should be native, resistant to common pests and diseases, and not be noxious weeds.
The use of erosion netting, mulch, or a stabilizing agent is recommended prior to seeding.