Resilience Through Functional Design
Access – Wheeled Transport
Common Drainage Prescriptions For Low-Maintenance Vehicle Access
This post details the more common drainage and erosion repair/prevention treatments we at 7th Generation Design have utilized to date. The goal of this post is to introduce these lesser known methods and techniques, describe when and how they are appropriate in a given context, and provide helpful illustrations that illustrate their function so that you – the landowner, the designer, or land steward, can analyze your vehicular access trouble spots, and, paired with some appropriate re-designing, install long-lived, low-cost drainage solutions that repair and enhance the local hydrology.
When selecting a drainage treatment, choose the treatment that will get the job done efficiently while requiring the least future maintenance. NOTE: Efficiently does NOT necessarily mean the fastest or cheapest in the short term. Every treatment installed on a road surface is a potential future maintenance bill. Plan accordingly.
The following methods, techniques and installations are preferred methods of draining surface water off of low-standard roads. Ideally they are designed in from the start, but if not, these are the ‘low hanging fruit’ that we look to first to immediately solve drainage issues.
Crowning Or Sloping The Road Surface
What is crowning and what does it do?
Crowning a road surface means that the center of the road is higher than both of the road edges. This ensures that the road surface drains immediately to either side of the roadway. Generally one or both sides of the road will have ditches to collect and drain the water. The grade of the crown should generally be between 2-4%.
Wheel rutting can degrade and ultimately impair the function of a crown, as wheel ruts will trap water on the road surface. Any rutting that develops on the roadway should be repaired and the crown or slope restored prior to a rainy season or event.
What is outsloping and what does it do?
Similar to crowning, sloping the road surface drains water immediately from the road surface. Roads can be either insloped or outsloped.
Outsloped road surfaces drain water off the road surface and across the entire downslope edge of the road, away from the uphill cut bank, thus avoiding concentration of water in a ditch. Outsloping is generally desirable wherever feasible because it keep surface run-off dispersed over a broad area and removes it from the road way quickly.
Outsloping works well where the terrain is already sloping and coarse-textured materials make up the road surface and surrounding drainage areas. Any drainage areas should be well vegetated or rocked. On road grades that are too steep for water bars or rolling dips (>15%), outsloping is an effective way to drain the road surface. Outsloping is also an effective treatment when rounding the crest of a ridge to prevent water from being transferred from one sub-watershed to the next.
Outsloping should not be employed where the road surface is composed of fine textured materials (sands, clays) and can become slippery when wet (hazard for vehicles in slipping off the roadway). Outsloped road surfaces should have a cross slope profile of 2-5%. Outsloping can also be performed segmentally every 200-300 feet if it is not feasible to outslope the entire road section.
What is insloping and what does it do?
Insloped road surfaces drain water off the road surface and into the uphill cut bank. Because this water is effectively “held” by the road form, it needs to be kept moving through properly installed and well maintained road or borrow ditches to frequently placed cross drains (rolling dips or culverts).
Insloping is most effective when used to drain a climbing turn, to enhance safety on a slippery or steep roadway, or to protect sensitive areas (streams, wetlands, seeps, springs etc) from sediment-laden run-off water until a more appropriate discharge location is reached. With insloped roads, it is essential to drain early and often via the use of cross drains, and opportunities within the landform where the terrain “breaks” and provides an opportunity to drain water should be sought out and utilized.
Grading Guidelines For Crowning And Sloping.
- Grade roads when moist but not wet.
- Do not grade when heavy rains are predicted.
- Crown roads ¼” to ½” (2-4%) for each foot of road width to ensure good drainage.
What is a rolling dip and what does it do?
A rolling dip is a surface cross drain – meaning it transits water across the road surface in a non-erosive fashion while maintaining road surface integrity and normal-speed vehicle access. Rolling dips are the most reliable, cheapest, and most effective type of cross drain for low standard roads (as opposed to culverts and water bars). Rolling dips are able to collect water from both the road surface and from roadside ditches and direct the flow across and away from the roadway.
Rolling dips are excellent drainage elements for harvesting water from road surfaces. The dip-drain portion of the rolling dip can be let out into swales, ponds, infiltration basins, or lead out ditches patterned into keyline ripped soil where the water can be put to work in the landscape.
Under what conditions are rolling dips appropriate?
- Rolling dips can be used to drain roads that have gradients between 3 and 15%.
- Too flat (<3% road grade or less than 5% cross slope for the dip-drain) is too flat to drain effectively, and the rolling dip will likely plug with sediment.
- Too steep (>15%) and the roll out will be too steep on the downhill side and vehicle traffic will damage the structure.
Rolling Dip Dimensions
- Generally the rolling dip roll out is twice as long as the roadway is wide, with the dip-drain running along the hypotenuse of the triangle (see below).
- The dip-drain should be broadly angled with a cross slope of 5-8%, steep enough to flush away accumulating sediments and be “self-cleaning”.
- NOTE: The dip-drain should always be as steep or slightly steeper than the inbound grade delivering the surface water. This will ensure that any sediments being carried with the water will not deposit on the road surface and plug the drain.
- Material excavated to create the dip-drain can be utilized in creating the roll out mound.
- Rollouts should be as long as required to suit the type of vehicle traffic. For example, a rolling out can be much shorter and steeper if it only needs to accommodate light-duty trucks, but will need to be very long and gentle if it needs to accommodate a dually with a horse-trailer (this is to make sure the trailer hitch doesn’t bottom out when crossing the dip-drain).
See more rolling dips and how they were installed in this video.
Detailed how-to instructions in Bill Zeedyk’s A Good Road Lies Easy On The Land, pages 21-25.
Flat Land Drains
What is a flat land drain and what does it do?
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.
Where are flat land drains appropriate?
Flat land drains are appropriate where the road grade is <3% and the surrounding hill slope grade is <5%. Flat land drains are effective treatment for slightly to moderately incised roadways where it is not feasible to relocate the road. Severely incised roadways in flat lands will most likely need to be decommissioned and the road relocated. If the drain is located on fine soils, it may be necessary to bring in some coarser aggregate to stabilize the mound.
Essential Flat Land Drain Features And General Guidelines
- Material scraped from the lead out ditch can be used to create the mound.
- 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 <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 dip-drain should be slightly steeper than the road grade angling into the lead out ditch.
Cut Off Drains
What is a cut off drain and what does it do?
Cut off drains allow water that is impounded in a road side ditch to move away from the road surface and into lower lying areas further from the road. By creating openings in the downhill ditch wall at strategic discharge points, water can be let out of the ditch at frequent intervals and discharged across acceptable terrain. This helps to prevent the depth of water in the trench from getting too deep (deeper water moves faster and has greater erosive potential) and limits the danger of pooling at the road edge should the ditch clog (pools of water at the road edge can infiltrate the road medium and create soft spots, which lead to rutting, potholing and other failure forms.
General Guidelines For Cut Off Drains
- Cut holes in ditch wall/berm above appropriate drainage areas (not sensitive areas like seeps, springs, creeks, or wetlands, or areas that are too flat to drain effectively)
- Properly armor (with geological and/or biological elements) transition points (especially if significant grade change is involved) to dissipate water energy or allow sediment to fall out. Discharge the water in as thin a sheet as possible over as broad an area as possible.
What is a borrow ditch and what does it do?
Borrow ditches run parallel with roadways and are used to drain crowned or insloped roads into some form of cross drain – a rolling dip, culvert, water bar etc. Borrow ditches gather and remove run-off quickly and reduce seepage into roadbed materials. They also provide an opportunity for sediments to be removed from runoff water before it enters surface water or groundwater.
When are borrow ditches appropriate?
On steep, climbing turns, steep road surfaces, with all crowned or insloped road surfaces, and wherever an uphill slope will shed water towards a road surface.
General Borrow Ditch Guidelines
- Locate ditches on the upslope side to prevent water from flowing onto the road from uphill.
- Design and grade ditches and bank slopes at a maximum 1:2 (rise:run) ratio.
- Plant ditches with <5% slope with grass to filter sediments.
- Line ditches with >5% slope with riprap stone to prevent erosion, and overseed with perennial grasses.
- Prevent water from standing in a ditch (this can weaken the road through subsurface infiltration)
- Check and clean ditches regularly, especially after major weather events
- DO NOT back blade or remove vegetation from cut banks or ditch slopes if not needed to maintain function – “if it ain’t broke, don’t fix it!”.
Last Resorts (Don’t Do These Unless You Have To)
The following drainage treatments are less preferable, though frequently they are the only options available, especially when dealing with pre-existing access that was poorly designed.
What is a water bar and what does it do?
A water bar is a mound or hump of earth running across a roadway used to deflect run-off from the road surface. Water bars and quick, easy, and cheap to build, and for this reason they are often deployed in situations as stop-gap measures – i.e. situations involving reaction to the consequences of poor or absent drainage design. Water bars are good at deflect small amounts of water off road surfaces when spaced at frequent intervals.
Under what conditions are water bars appropriate?
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 simple 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).
Water bars are poor long-term solutions however. 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 speedbumps. Water bars are also 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 should be spaced roughly according to the grade of the landscape and road surface in which they are being applied.
General Guidelines For Construction.
- Water bars are more effective when built at an angle of 30% to the slope grade (similar to a rolling dip).
- Build to height of 6-15 inches (15-40 cm) above road surface.
- Protect drainage at discharge point with stone, grass, sod or anything to reduce flow velocity. Install tips-up media luna flow spreaders when appropriate.
Culverts (Cross Drains / Piped Drains)
What is a culvert and what does it do?
A culvert is a buried section of pipe crossing underneath the roadway from the uphill (cutslope) side to the downhill (fill slope) side that is used to drain water from one side of the roadway to the other.
Where and when are culverts appropriate?
Culverts are helpful in draining insloped or crowned roadways that have a drainage ditch on the uphill side of the road. 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 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, 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!
Culvert Do’s And Don’ts
- 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 over a broad, vegetated and potentially geologically armored area.
- Construct sidewalls at the inlet and outlet to handle turbulent flow and prevent erosion around the pipe edges.
- Install at right angles to direction of water flow when crossing the road – will get bank erosion from this – more prone to plugging
- 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.
General Construction Guidelines
- Pipes should have headwalls and endwalls to prevent erosion at intake 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. 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.
- Extra length of culvert should be considered to accommodate for headwalls.
- Design culverts to handle at least a 100 year or greater storm.
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!
Impermeable hardscapes are exactly what they sound like – surfaces that completely prohibit the infiltration of water – asphalt, concrete, tightly packed masonry or pavers etc. They do have their place, especially around the immediate foundation of structures to keep water from infiltrating underneath foundations, however beyond that, it is almost always worth investing into permeable hardscapes, of which there are now a multitude of options. Impermeable hardscapes eliminate the infiltration capacity of whatever area they cover, and then impose a much greater demand on the areas immediately down-watershed of them to handle the excess run-off.
It is always best to get the rain into the ground where it falls rather than somewhere else.
The following are some of the most common Type 1 Errors that occur with regards to water drainage. If you do these things you may as well light your wallet on fire.
Downslope Berms That Trap Water On The Road Surface
Even a small berm on the downslope road edge can function to trap immense amounts of water on a road surface, which creates a compounding effect further down-watershed when the water ultimately exits the roadway, whether through a planned drainage treatment or by creating its own exit (always to the detriment of the surrounding ecology, and likely the integrity of the road).
By simply ensuring that no berms are present on the downslope road edge, water will be allowed to transition off the roadway along its entire length, instead of being concentrated at one drainage point.
Intentional berming can be useful when we might wish to bypass sensitive areas prior to discharging water from the road surface (springs, seeps, wetlands and creeks). Appropriate drainage measures must be taken in this instance to handle the accumulated flow at a later point.
Failure To Crown / Slope
Uncrowned or unsloped roadways will become rutted, entrenched, and ultimately unusable because water trapped on the roadway will stay on the roadway, where it will increase in speed, volume and erosive power.
Valley Bottom Roads
Placing roads along valley bottoms guarantees a high, perpetual maintenance bill and a steep ecological cost. While placing roads along valley bottoms is easy (at least when installing them), these roads are often impossible to drain (guess what else always goes to the valley bottom?) and are frequently unusable for significant parts of the year due to pooling and puddling. Valley bottom roads often force the straightening of water ways, which increases the erosive energy of the water flow and often initiates headcutting and accelerated dehydration of the entire valley watershed.
Don’t do it.
Moving Water From One Sub-watershed To Another
Whether by an active drainage measure (piped culvert, ditch drain etc.) or by a road or access route, water can be moved out of one sub-watershed and into another. This effectively starves and dehydrates the lower areas of the donor watershed, while creating a discharge and infiltration problem in the neighboring watershed. By draining water into the sub-watershed that it first fell into, ecological disruption can be minimized and costly drainage treatments avoided.