Water Drainage Elements

Description & Function(s): A flat land drain is a type of “break” in an existing grade – a small increase in road elevation on a downhill slope, which leads water to flow off the road surface and into a lead-out ditch. As the name implies, flatland drains are employed in flat country where drainage opportunities are few. Flat land drains are effective at removing water from a road surface in flat land scenarios (road grades between 0-3% and adjacent slopes <5%). Flat land drains are especially useful in restoring dispersed overland flow or sheet flow to meadows and wetlands where V-shaped ditches might tend to concentrate runoff and initiate gully formation.

Context-Specific Design Criteria for Flatland Drains

Flatland Drains are generally a good fit for a specific context when:

• Flat land drains are appropriate where the road grade is <3% and the surrounding hill slope grade is <5%. This kind of terrain is often found when crossing broad valleys or bottom-adjacent land.
• Flat land drains are effective treatment for slightly to moderately incised roadways where it is not feasible to relocate the road.
• Roads with low-volume and low-speed traffic.

Flatland Drains may not be a good fit for a specific context when:

• The roadway is severely incised – in this circumstance the road will most likely need to be decommissioned relocated, ideally to higher ground with better drainage opportunities.
• Road has high-volume or high-speed traffic.

General Design & Installation Considerations for Flat Land Drains

• Material scraped from the lead out ditch can be used to create the elevated road surface (like a long, gentle speed bump) downgrade of the drain.
• The primary difference between a flat land drain and a rolling dip is the grade of the surrounding terrain. 
  • Rolling dips are employed on landforms with a cross-slope greater than 5%.
  • Flat land drains are employed on landforms with a cross slope less than <5% and road grades between 0-3%.
• A long, broad, deeply excavated lead out ditch is necessary in order to remove water from the roadway and avoid any pooling at the road edge. The lead out ditch should be as wide as the dozer blade, 1 to 2 feet deep at roadside, 30 to 50 feet long and taper gradually downslope to a feathered edge at the downhill end of the ditch.
• There must be an elevated road surface down grade of the dip drain in order to divert water off the roadway.
• The dip-drain should be slightly steeper than the road grade angling into the lead out ditch – this will help to ensure the dip-drain is self-cleaning and won’t accumulate sediment.
• Drain may soften during wet weather; prevent wet weather use or harden structures with coarse gravel hauled to site.

Resources for Continued Learning about Flatland Drains

Description & Function(s): A water bar is a mound or hump of earth with an accompanying ditch used to divert run-off from a road surface. Water bars and quick, easy, and cheap to build on low-standard roads using earth, and for this reason they are often deployed in situations as stop-gap measures. They can also be built on footpaths and low-standard roads using logs, sandbags, and rocks. Log water-bars are suitable to occasional vehicle traffic, earth, sandbag and rock water bars not so much. Water bars are good at deflecting small amounts of water off road surfaces when spaced at frequent intervals.

Context-Specific Design Criteria for Water Bars

Water Bars are generally a good fit for a specific context when:

• In emergency situations (such as lots of water flowing down a low-standard road way) water bars are one of the most effective band-aids simply because they can be constructed so quickly.
• They are very helpful when restoring native overland flow patterns when decommissioning roads.
• They are appropriate for seasonally closed roads as well (driving over a water bar made from soil during wet conditions is likely to destroy its function immediately).
• Where road grades are less than 15%

Water Bars may not be a good fit for a specific context when:

• Where road grades are steeper than 15%.
• Water bars are not appropriate for high-traffic or high-speed roads. They are very unpleasant to drive over (very jarring and bumpy) and roads with any significant vehicle traffic at all will quickly see water bars degrade into useless speed bumps.
• Where soils are fine or silty – water bars are very prone to silting in, at which point any inbound water will flow right over the top to continue its erosive journey down the access way. For this reason, water bars require frequent maintenance and reconstruction.
• Water bars are a poor choice where the road will have to be driven while wet – they will rapidly deform and fail to function as intended if driven over while wet.

General Design & Installation Considerations for Water Bars

• Water bars should be spaced roughly according to the grade of the landscape and road surface in which they are being applied.

• Water bars are more effective when built at an angle of 30-45^o to the slope grade (similar to a rolling dip).
• Build to height of 6-15 inches (15-40 cm) above road surface.
• Protect drainage and prevent erosion at discharge point with stone, grass, sod or anything to reduce flow velocity, such as an armored drain or media luna flow spreader.

Resources for Continued Learning about Water Bars

Description: Armored drains are hardscape-lined ditches built to provide an erosion-proof flow path for concentrated, high-volume, high-velocity water flows. Armored drains can be constructed from a range of geologial materials – rock, slab stone, cobble, concrete, masonry, pre-fabricated materials like cement impregnated fabrics and plastic sheeting, wood and tiles. They can come in many shapes, sizes and forms based on the context in which they are implemented.

Function(s): Armored drains protect soil from erosion where concentrated, high-volume, high-velocity water flows must transit a grade that, if left unprotected, would otherwise erode.

Context-Specific Design Criteria for Armored Drains

Armored Drains are generally a good fit for a specific context when:

• Anticipated flow volume, velocity and concentration are greater than what the native soil can withstand as surface flow without eroding.
• Water flow must transit a steep, unstable, fragile or already eroding slope (the flow cannot be re-patterned to another area with a lower-energy drainage profile).

Armored Drains may not be a good fit for a specific context when:

• If the drainage can be vegetated and the vegetation has the appropriate characteristics to prevent erosion of the soil.
  • The climate and annual moisture distribution profile must support perennial vegetation capable of maintaining the integrity of the drain during high-flow events.
• Crossing a perennial water course.

General Design & Installation Considerations for Armored Drains

• Use armored drains to transfer water to a terrain feature where it can be pacified and spread out across the broad landscape.
• NOTE: Armored drains are NOT appropriate for stream crossings.
• Armored drains can be designed to be elastic – meaning they can shift and settle slightly while still retaining their function. These types of armored drains are called rough bed channels. Poles 3-4″ in diameter and 3′ in length are driven at right angle to water flow into a pre-excavated channel, generally 4-6 posts being sufficient to cross the channel. Rows of posts are set 6-15′ apart for the entire length of the drain. After this, a layer of coarse sand and fine gravel are put into the channel. Into this angular rocks are embedded and placed erect to maximize channel roughness. The joints between the rocks are filled with 1-2″ crushed rock (rip-rap). The sides of the channel are usually supported by live erosion control plantings. If the ground underneath is permeable it is advisable to place plastic sheeting or a layer of heavy clay to prevent seepage along the length of the drain.
  • EFFECT: Water flows in the gravel areas as well as above the rough rocks in the case of flood. The water cannot move any of the downslope material because it always hits an obstacle that prevents it from gathering energy. Slight shifting is possible, and the entire system is so elastic that any such minor changes correct themselves eventually. Rough bed channels naturally revegetate and blend right in with the landscape.

Resources for Continued Learning about Armored Drains

Description & Function(s): Grassed waterways are constructed earthen channels covered in sod or seeded to establish complete perennial vegetative cover of the drainage way. To convey surface water to a suitable outlet at non-erosive velocities. Grassed waterways should be used where gully erosion is a problem. The most common areas are in draws between hills, and other low- lying areas on slopes where water concentrates as it runs off a field. Grassed waterways may also be used to convey runoff from terraces, diversions, or other sources of water concentrations to a stable outlet.

Applications:

• Grassed waterways are especially useful as diversion drains placed above a steeper section of slope where they can divert water from moving across the slope face.
• Grassed waterways can significantly reduce gully erosion which commonly develops in row cropping fields in slight valleys and draws where water concentrates.
• Reed sods can be used in grass drains that will be consistently wet. Should water be removed from these drains for any significant period of time, the water loving vegetation can quickly perish or be replaced by woodier upland species which is likely to degrade the waterway’s drainage function.

Context-Specific Design Criteria for Grassed Waterways

Grassed Waterways are generally a good fit for a specific context when:

• Grades are relatively shallow.
• There is ample annual moisture with even distribution throughout the year to maintain a healthy, living sod.
• A regionally-adapted grass species with a known track record for performing well in grassed waterways is available for seeding or sodding.

Grassed Waterways may not be a good fit for a specific context when:

• Water must be drained across a slope – the chances of water penetrating the slope and causing erosion or slippage is elevated. Utilize an armored drain or culvert instead to keep water from direct soil contact.
• If annual rainfall and moisture distribution throughout the year are insufficient to maintain a living sod that will perform as required when rain does arrive.

General Design & Installation Considerations

• Remove all trees, brush and stumps prior to construction.
• Construct the waterway in a parabolic shape using the relative dimensions outlined in the image below.

• Place earth fill in layers of 9″ or less, each layer being properly compacted by tracked or wheeled heavy equipment to obtain proper compaction.
• Preserve topsoil during excavation for adding back after the final form is complete to help establish grass.
• DO NOT use the grassed waterway as vehicle access. Ruts will develop, leading to incising of the channel and re-initiating gully formation.

Resources for Continued Learning about Grassed Waterways

Description: A culvert is a buried section of pipe crossing underneath the roadway from the uphill (cutslope) side to the downhill (fill slope).

Function(s): To drain water from the uphill side of an access route underneath the road surface to the downhill side of the access route. This keeps the water off the road surface and maintains 4-season access even during wet conditions. Culverts can also be used where roads cross perennial streams or seasonal waterways.

Context-Specific Design Criteria for Culverts

Culverts are generally a good fit for a specific context when:

• When there is a ditch on the uphill side of the road formed by insloped or crowned roadways that must be drained.
  • NOTE: Whenever possible, choose a lower maintenance surface drain, such as a rolling dip, instead of a culvert.
• When the volume or energy of water to be drained across the road is too great for a surface drain or will pose a hazard to traffic (i.e. creek crossings, “first chance” drainage opportunities following an extended “no chance” section of road where significant amounts of water are accumulated).

Culverts may not be a good fit for a specific context when:

• Routine inspection and maintenance will not be not feasible, affordable or possible – instead utilize a lower maintenance cross drain like a rolling dip.

General Design & Installation Considerations for Culverts

• NOTE: Think of culverts as a last resort. They are among the most maintenance intensive and failure-prone drainage options available. However, in certain contexts, they are sometimes the best, or more often, the only option available. Do everything possible with the preferred interventions to help reduce the need for and stress on culverts – your watershed, road and wallet will thank you!
• Culverts are only appropriate if planned maintenance will occur. If planned maintenance is not possible for the lifetime of the road, utilize rolling dips and flat land drains instead.
• Culverts, by their nature, concentrate and accelerate water flows, and care must be taken at the culvert discharge on the downslope to dissipate the water’s energy and disperse its flow over as wide a drainage area as possible. Failure to do this will lead to gouging and headcut formation, which will migrate up and begin to threaten the roadway from below. Poorly managed culvert outlets have degraded large swathes of land around the world – don’t let yours be one of them!
• Culverts are very prone to plugging, and when they plug, the water is forced up and onto the roadway while creating a pooling effect immediately adjacent to the road medium, all of which can lead to rapid degradation and damage of the road surface and the surrounding ecology.
• Culverts MUST be sized according to the size and soil run-off potential of the catchments they drain. The Rational Method for estimating peak flood flows for different recurrence interval events uses the formula Q=CIA where:
  • Q = predicted peak run-off from a 100-year run-off event (in cubic feet per second)
  • C = Run-off coefficient (the percent of rainfall that becomes run-off)
  • I = uniform rate of rainfall intensity (in inches per hour)
  • A = drainage area (in acres)
    • Use the Rational Method Calculator from LMNO Engineering to quickly calculate peak flood flow discharges for watersheds smaller than 200 acres (you can go larger, but the model doesn’t hold up as well for larger watersheds).
    • Plot a straight line connecting the calculated discharge rate and the Headwater Depth Ratio on the Federal Highway Association Culvert Capacity Inlet Control Nomograph chart below to determine the appropriate diameter of the culvert. Additional nomographs for different types of culvert materials and different types of culvert inlets are available in the FHWA’s Hydraulic Design of Highway Culverts manual. Most rural culverts will have a headwater depth ratio of less than 1.
      • Headwater Depth Ratio – this is the ratio between the Headwater Depth and the culvert Diameter. A ratio of less than 1.0 is recommended for most rural applications, and if you want to size your culvert approximately 12″ larger than the diameter required to safely pass a 100-year flood flow, use an HW/D ratio of .67.
        • Headwater Depth is the vertical head distance between the bottom of the culvert and the “high water mark” on the uphill side of the road, above which water would spill over the road surface if the culvert were plugged.
        • Culvert Diameter is the width of the culvert.

Culvert Installation Do’s and Don’ts

• DO:
  • Install culverts at an angle across the road – a culvert installed this way will carry more water and cause less erosion and be less prone to plugging. 
  • Discharge first into an energy dissipator and then through a flow-spreading element like a media luna into a broad, well-vegetated area.
  • Construct headwalls and endwalls at the inlet and outlet to handle turbulent flow and prevent erosion around the pipe inlet and outlet.
  • ALWAYS lay the culvert pipe at the same or slightly steeper grade than the inbound flow grade in order to speed the water up as it passes through the culvert. This makes the culvert “self cleaning” and decreases the likelihood of sediment deposition and clogging.
  • The discharge zone should be appropriately armored to dissipate water energy prior to patterning it onto the landscape over as broad an area as possible. Geologically armored energy dissipation pools paired with tips-up media lunas are excellent for this.
  • Install culverts during a period of low or no water flow.
  • Design culverts to handle at least a 100 year or greater storm.
• DON’T:
  • Install at right angles to direction of water flow when crossing the road – this will create bank erosion due to turbulence which will increase the likelihood of plugging and over-topping the roadway.
  • Discharge directly into a stream. The high-energy water and likely high-sediment load can create scouring and deposition patterns that will destroy sensitive habitats and alter creek flow, potentially inducing bank erosion.
    • If there is significant sediment being carried through the culvert, install a sediment trap that can be regularly cleaned immediately after the point of discharge but before reaching the stream or creek.

Resources for Continued Learning about Culverts

Description: A level-sill spillway is a broad, perfectly level area that serves as a predetermined discharge location for water flows greater than what the associated impoundment (swale, pond, lake, infiltration basin etc.) is capable of infiltrating or holding.

Function(s): To discharge water from a swale, pond, infiltration basin or other water-impounding structure over a broad area in a non-erosive fashion while maintaining the integrity of the impounding structure.

Context-Specific Design Criteria for Level-Sill Spillways

Level-Sill Spillways are generally a good fit for a specific context when:

• ALWAYS necessary for any water impounding structure with a catchment area.
  • Even an earth tank water storage (“turkey’s nest”) with no contributing catchment area but its own surface area should have a designated overflow point in the event of rainfall intensity beyond the design capacity of the earth tank.

Level-Sill Spillways may not be a good fit for a specific context when:

• If the “only” place for the spillway would be on the dam wall then 1) an impounding structure probably should be built there in the first place and 2) more advanced discharge systems, such as pipes embedded in the dam wall, may be necessary.

General Design & Installation Considerations for Level-Sill Spillways

• NEVER locate the spillway on or near a pond wall (the dam). Water discharged over an earthen embankment will degrade the structural integrity of the embankment, threatening the entire storage and everything downstream of it.
• The height of the level-sill spillway relative to the bottom of the impounding body determines the maximum water-holding capacity of that structure. The height difference between the spillway and the top of the impounding structure (dam wall, swale berm etc.) is known as the “freeboard”, and should generally be a minimum of 36” for constructed ponds and 12” for swales.
• Depending on the climate, soil type and anticipated demands upon the structure, a spillway should either be vegetated with perennial grasses (couch, kikuyu, para and others) or mat-forming vegetation with hairnet roots capable of binding soil that will “lay down” when heavy flows arrive, or geologically armored with knitted stone infilled with small gravel (see rough bed channels in Armored Drains) , or both.
• Level-sill spillways are sized to have the water discharge over a broad surface as gently as possible. In an ideal world, spillway size is determined by the anticipated peak discharge flow for a particular intensity of rain event (Q=CIA) such that the discharge will not erode the native soil base. In reality, spillways are sized according to a wide range of site-specific criteria, including landform and slope restrictions, property lines, legal restrictions, soil type, materials and labor costs etc.
  • Q = peak discharge in cubic feet per second.
  • C = run-off coefficient for the catchment area that drains through the spillway.
  • I = intensity of rainfall, measured in in/hr.
  • A = area of the catchment generating run-off that must drain through the spillway.
  • Time To Concentration: A factor influencing the magnitude of flood flow events that takes into account the length of the water shed and height differential between the top of the watershed and the discharge point.
    • see section on Culverts for more details on estimating peak flood discharge flows.
• Wherever small inflows persist for long periods of time (more than a couple days) that would lead to continuous low-volume discharge over a spillway, a trickle pipe should be embedded through the wall of the structure. Spillways that are already damp and saturated can be easily destroyed when heavy flows arrive. Trickle pipes allow the impoundment to discharge small volumes of water and keep the spillway dry, ready for use during a high-flow event.
• Rule Of Thumb (from Darren Doherty): Grassed level-sill spillways should never be designed to discharge more than 88.3 cubic feet/second (2.5 cubic meters/second), otherwise the risk of erosion is too high. Below are more ballpark estimates (converted to Imperial units) for spillway sizing from the Regrarians Earth Dam Design article linked below.

Resources for Continued Learning about Level-Sill Spillways

Description & Function(s): A trickle pipe is a drainage pipe set into the wall of a water impounding structure (i.e. dam wall, swale berm) and just below the top of the planned overflow spillway. A trickle pipe intercepts small and/or constant low-volume overflow at the top of the spillway and delivers it to the drainage bottom without transiting the spillway. This keeps the spillway dry and in good condition for emergency or seasonal use during high-flow events. Spillways that are constantly transiting water can become saturated and weak, or have small erosion incisions form, increasing the likelihood of failure during a heavy overflow event. By keeping smaller discharges off the spillway (and thus keeping the spillway dry) the lifespan of the spillway will be prolonged and the risk of spillway failure will be lessened.

Context-Specific Design Criteria for Trickle Pipes

Trickle pipes are generally a good fit for a specific context when:

• When the impounding structure receives prolonged or perennial inflow in small but relatively constant amounts.
  • i.e. valley-bottom ponds, ponds in climates with relatively even annual rainfall distribution, swales with relatively constant inflows (wastewater from processing, washing, laundry-to-landscape etc).
• When anticipated rainfall intensity can be safely and affordably discharged through a trickle pipe, and the spillway reserved for emergency use only.

Trickle Pipes may not be a good fit for a specific context when:

• There is no persistent inflow to the impoundment that will cause regular use of the spillway.
  • i.e. Winter-wet, summer-dry climates, impoundments that are isolated from significant catchment areas.

General Design & Installation Considerations for Trickle Pipes

• NOTE: A trickle pipe is NOT intended to carry all flows, but only those that are likely to cause regular saturation of the spillway. Once the runoff flows exceed the capacity of the trickle pipe, water moves down the spillway.
• Design of a specific trickle pipe system will depend on:
  • Design of the impounding structure (dam wall, swale berm).
  • Site topography.
  • Soil type(s).
  • Run-off characteristics (as determined by catchment size, anticipated rainfall intensity, presence of seasonal/perennial inflows and their time to concentration etc).
  • Valley floor stability.
• Parts of a trickle pipe system include:
  • The inlet pit.
    • An inlet pit helps to increase the amount of effective head.
    • It should be set either into the top of the spillway or at the other end of the impounding embankment. It should be set 12 – 18″ below the full supply level of the dam (which should be a minimum or 36″ below the top of the impounding structure).
    • If the catchment has trees or other debris in the inflow, a trash rack is necessary to screen debris and keep the trickle pipe from clogging. The inlet should be checked regularly to maintain unobstructed flow.
  • The pipe.
    • A trickle flow pipe should NOT be laid along the spillway surface otherwise it will create erosion problems.
    • Plastic or corrugated pipe can be used (flexible or rigid). Most importantly, the friction losses of the particular type of pipe need to be taken into account.
    • It is recommended to avoid using elbows throughout the run of the pipe. Elbows create points of restriction and can become blocked with debris or aquatic creatures, should they enter the pipe (this is why having a trash rack and appropriate screening is so important).
    • The size of pipe needed to carry the flows depends mainly on the effective head of water and on internal pipe friction. For ponds and other non-permeable impounding bodies, pipes of a diameter less than 18″ should not be used. Below this diameter they become quite susceptible to blockage. For swales and infiltration basins with much smaller impoundment volumes, smaller diameter pipes can be used.
    • Pipes need to be buried to protect them from sunlight and mechanical damage.
    • There must be continuous fall from the pipe inlet to the outlet.
      • NEVER have a steeper grade of pipe transition into a shallower grade of pipe unless right at the pipe outlet. If the water is carrying any sediment this creates an opportunity for that sediment to fall out of suspension and create a plug.
  • The outlet.
    • Ensure proper energy dissipation structures (energy dissipation pools, weirs, ramps etc.) are located at the pipe outlet to prevent scouring and erosion from high-energy water exiting the trickle pipe.

Resources for Continued Learning about Trickle Pipes

Description & Function(s): Armored fill crossings are a type of surface cross drain employed where low-standard roads cross seasonal valley bottom drainages or small streams. The stream flows over the top of the fill and down the protected outside fill face. Armored fill crossings utilize geotextile fabric, large rocks, rip-rap and road base to create a road surface that will hold up when completely saturated and discharging water and remain drive-able year-round for appropriately sized vehicles (even when flowing).

Context-Specific Design Criteria for Armored Fill Crossings

Armored Fill Crossings are generally a good fit for a specific context when:

• Armored fills are a good choice for seasonal drainages that will stay wet for significant periods of time or discharge larger volumes than could be safely handled by a simple road dip.
• Stream flow is ephemeral and intermittent and the majority of traffic will be crossing during low-flow or dry conditions (though armored fill crossings can be transited during high-flow or emergency conditions, assuming the vehicle is capable).
• Winter or wet-season maintenance is not possible. Armored fill crossings cannot plug with debris (unlike culverts) and are very low-maintenance when installed properly.
• The crossing is in low–volume traffic areas, such as ranches, seasonal logging roads,utility access routes, open space districts, and parklands (all low-standard roads, collectively).
• If rock armor is locally available they will be less expensive to install than culverts and bridges, and they require less frequent inspection and maintenance.

Armored Fill Crossings may not be a good fit for a specific context when:

• Crossing perennial or fish-bearing streams. An armored fill crossing will inhibit fish migration. In these cases a culvert with an appropriately designed fish passage should be utilized.

General Design & Installation Considerations for Armored Fill Crossings

• The dip crossing is generally constructed wide and deep enough to accommodate the anticipated 100 year peak flood flow.
• The width of the armored channel at the outboard edge of the dipped road should be at least 5 times the estimated design peak flow wetted perimeter in the upstream natural channel.
• The depth should be at least 1.5 times deeper than the average flood flow depth in the natural channel.
  • For example, a natural stream channel with an estimated peak flow width of 8 feet and depth of 2 feet should have an armored fill that is at least 40 feet wide (5×8’=40′) and 3 feet deep (1.5×2’=3′) at the outboard edge of the road crossing.
  • This will keep anticipated flood flows confined within the armored portion of the dipped crossing.
• If soils are fine grained it will be necessary to install geotextile fabric in the keyways prior to backfilling with armor. This fabric will prevent the rock armor from sinking into the finer grained soil.

Resources for Continued Learning about Armored Fill Crossings

Description & Function(s): Roadside ditches are drainage ditches that parallel a road. They collect and carry road-surface run-off, water from springs or seeps, and water from run-on sources to designated discharge locations. Roadside ditches require regular maintenance, and should be allowed to drain early and often to prevent saturation of the road bed.

Context-Specific Design Criteria for Roadside Ditches

Roadside Ditches are generally a good fit for a specific context when:

• Water expressed in the cutbank needs to be prevented from reaching the traveled roadway.
• Safety concerns dictate an insloped road with a berm on roads traversing steep cross-slope grades.
• High-standard roads require a road template shape that is super-elevated to accommodate higher speed traffic. All other roads should be outsloped or crowned to reduce the road-drainage density, to restore natural surface drainage patterns, and to reduce maintenance costs.

Roadside Ditches may not be a good fit for a specific context when:

• If the opportunity exists to drain the water off the road surface without first putting it into a ditch – do that! For example, if the road surface is suitable for outsloping (coarse texture, well-drain soils on mild slopes) then no ditch is necessary.
  • ALL ditches will require some maintenance. Don’t create one unless you have to.

General Design & Installation Considerations for Roadside Ditches

• Roads that require a ditch often have incised and degraded ditch lines. Too often, traditional practices designed to fix ditches address only the symptom (erosion) rather than the problem (water volume and steep road gradient), which causes increased water power.
• Borrow ditches typically have either a “V” or a “U” shape.
  • V-shaped borrow ditches concentrate flow and have higher energy, and thus pose a greater erosion risk. V-shaped ditches should generally be armored in some way to prevent ditch scouring and incision. Avoid this shape if at all possible.
  • U-shaped borrow ditches have a broader bottom, and tend to be vegetated and have less susceptibility to erosion, however because they encourage slower flows they are more prone to plugging from sediment deposition. Use this shape as a general rule of thumb.
• When possible, outlet ditches before curves or steeper sections of road to help reduce ditch erosion.
  • Try to install additional outlets before the ditch empties into a stream or intermittent channel to keep road drainage separate from natural flows.
• Look at the condition of the road ditch, and outlets as well. If the ditch outlet is eroded or plugged, consider installing a new outlet above the existing one to further disperse the drainage and reduce erosion and sediment.
  • Always drain the ditch at the first chance, last chance, best chance and before there is no chance. More drainage ditch outlets = less stress on the ditch and the road = less maintenance and improved road longevity.
• Never drain a ditch directly into a stream. Always pattern ditch drains into vegetated areas prior to connecting with waterways. This will improve water quality, decrease storm surges, and enhance habitat in the stream by having cleaner water with less sediment or surface contaminants entering it.

Resources for Continued Learning about Borrow Ditches

Description & Function(s): A belt diversion is a structure used on low-traffic roads to convey water off the road surface. The diversion consists of a conveyor belt bolted to rot-resistant timber that is buried in a trench that crosses the roadway at a diagonal. Belt diversions will not deform or crush and can be driven over in wet conditions without any adverse effect. They are very low-maintenance once installed.

Context-Specific Design Criteria for Belt Drains

Belt Drains are generally a good fit for a specific context when:

• Traffic volume is low (driveways, farm lanes,gated roads, and camp lanes).
• Roads will need to be driven while wet (belt diversions hold up well to vehicle passage in wet weather because they are not made of earth – unlike water bars, and to some extent rolling dips).

Belt Drains may not be a good fit for a specific context when:

• They are NOT suitable for roads that have heavy traffic, fast traffic, frequent grading, or plowing.

General Design & Installation Considerations

• Install on sloping sections of low volume roads with long, continuous grades and evidence of rills, gullies, and loss of road-surface fines.
• Materials Required
  • Use a conveyor belt approximately ½-inch wide by approximately 15 inches high by the necessary length.
  • Use treated 2×6″ lumber. Length and number depends on road width. Overlap any lumber joints with 4-foot length of board.
  • Use bolts and nuts 3/8-inch-diameter (length varies with belt) and wide-diameter washers.
  • Utility knife, drill, hammer, and adjustable wrenches.
• Be sure that the belt diversion is long enough to 1) be angled across roadway (a minimum of 30 degrees) and 2) ensure that water does not flow back to the roadway around the end of the diversion.
• Obtain free or low-cost used conveyor belts from any facility that uses conveyor belts (mines, mills, etc).
  • Belts typically come in 26-30″ widths and can be cut with a utility knife or a reciprocating saw.
• For longer diversions, consider constructing the belt offsite and then removing the 4-foot joint board. The diversion can then be folded in half for transport and reassembled onsite.
• Have a minimum of 1% grade to the belt drain, and ideally have it match or slightly exceed the grade of the inbound road surface to make it self-cleaning.
• Armor the outlet of the drain if significant flows are expected.

Resources for Continued Learning about Belt Drains