Abundant Water – Part 2: Designing Resilient Water Drainage Systems
Running water is the primary natural force that generates the need for maintenance of most man-made access routes. Most drainage needs throughout the landscape are in relation to access routes. Effective drainage is therefore the primary consideration when planning and designing for functional access (for vehicles, humans and animals) and healthy local hydrology. Standing water, snow, ice, frost, and subsurface seepage can also threaten access route integrity and need to be taken into account.
“A road lies easily on the land if it is located on a landform where it can be readily and effectively drained (neither too steep nor too flat); is functional when used as intended (class of vehicle, season and suitable weather conditions); has appropriate drainage features (closely spaced, properly situated and adequately maintained); preserves the natural drainage pattern of the landform; conserves water; does not cause or contribute to accelerated soil loss, lost productivity or water pollution; does not encroach on wetland or riparian areas; and is scenically pleasing.
A road is not easy on the land if it collects, concentrates or accelerates surface or subsurface runoff; causes or contributes to soil erosion; impairs or reduces the productivity of adjacent lands or waters; wastes water; unnecessarily intrudes upon key habitats, stream channels, floodplains, wetlands, wet meadows or other sensitive soils; and is aesthetically offensive.”—Bill Zeedyk
When patterning and designing drainage treatments within and through a landscape it is helpful to start with first principles.
Principles of Resilient Drainage Design
1. Last Chance, First Chance, Best Chance, No Chance
- FIRST CHANCE: Situations will arise where it is either infeasible or potentially damaging to sensitive ecosystems to discharge road surface run-off. In this situation, carry the water as far as is necessary and discharge accumulated run-off at the First Chance.
- LAST CHANCE: In situations where there is clearly no opportunity to discharge water from the access way surface for a significant distance down grade, such as when entering and incised road section or on a steep downhill slope, discharge all accumulated run-off before entering this “no go zone”. This is the Last Chance.
- BEST CHANCE: Also known as the “Least Worst Chance”. In some situations, the only available discharge point may be less than ideal, while also being the only option available – i.e. spilling onto less-permeable surfaces. In these situations, choose the best (least worst) of the options available. This is the Best Chance.
- NO CHANCE: Sometimes there is no opportunity to discharge water from the road surface – such as after a Last Chance and before a First Chance. In these situations we are forced to deal with the water downstream. These situations often necessitate expensive treatments, and are best identified during the planning phase and avoided if at all possible.
2. Drain More Frequently On Steeper/More Fragile Soil Types
The steeper the slope, the more erosive potential any water running over the surface will have. The more fragile the soil, the more susceptible it is to erosion. The table below gives maximum allowable distances between drainage treatments (try to space them way closer than this!).
“A more pragmatic approach to selecting drainage points is based on the principle of dispersing runoff at every opportunity along the way rather than at some predetermined spacing interval.”– Bill Zeedyk
3. Choose More & Smaller vs. Less & Larger
Smaller drainage treatments that occur more frequently will be more resilient, less prone to catastrophic failure, and less costly in the long-run than installing less frequent but much larger drainage treatments. Surface water run-off follows an exponential curve with regards to how much and what size of sediment it can move. Don’t let a small amount of water compound into a BIG problem.
4. Work From The Top Down vs. Bottom Up
This goes hand in hand with the preceding principle. Address drainage issues starting from the top of the watershed down towards the bottom – don’t start from the bottom up, or you’ll be designing massive (i.e. expensive) drainage treatments when you could have been installing much smaller elements distributed throughout the road/watershed.
5. Maintain Vegetative Cover In Drainage Areas
Vegetation is critical to help water infiltrate once it has been discharged from a road surface and spread over the landscape. It is also critical in preventing future erosion, whether from the direct impacts of rain droplets or during a high-discharge event (most often these two things happen nearly simultaneously).
6. Put The Water To Work
Drainage water can be a tremendously productive resource if it is planned for. It can be used to passively irrigate pastures, be directed into fish ponds, and to rehydrate broad acreages if properly dispersed. It can literally be as good as dollars in your pocket – dollars that will continue showing up year after year.
Information That (Should) Influence Design & Maintenance Decisions
Relief is the amount of elevation change between the current position and the top of a ridge or the bottom of a valley. More relief = more opportunities for drainage.
Slope / Steepness / Grade
Slope is similar to grade in that it is a measure of steepness, however slope is typically measured in degrees, where 0 degrees in flat and 90 degrees is a vertical cliff face. In general, the following is true:
- 2x slope = 2x run-off velocity = 8x particle size that can be transported by same volume of water
- 2x slope = 2x run-off velocity = 4x erosive power of the water (amount of bed load that can be carried)
The inverse of each of the above bullet points is also true:
- ½ the slope = ½ run-off velocity = ¼ the sediment moved (1/4 the erosive power)
- ½ the slope = ½ run-off velocity = 1/8 the particle size that can be transported
- This means changing from steeper grades to shallower grades will create deposition zones! This can clog drainage features without proper planning!
NOTE: Water velocity also increases with depth due to a relative decrease in surface tension – this is another reason why more smaller, well distributed drainage treatments are better than fewer larger treatments – they help keep the water spread out where it has less energy!
Aspect is the angle of the road surface relative to the sun’s position. In the northern hemisphere, northerly aspects are typically wetter, more prone to icing, and dry slower. Southerly aspects dry faster, exhibit freeze/thaw cycles, generally have thinner soils and are thus typically close to bedrock. The inverse of these is true in the souther hemisphere.
Length of Slope
- Longer slopes = more accumulated water.
- 2x Length of slope = 4x as much sediment moving power.
- 2x the length of slope = 8x sediment size that can be moved by same rain event on slope of half the length.
If working with an existing roadway, determine where the run-off is going. Then ask if that is where it went before the road was put in, and if not (as will likely be the case), how has the road altered the overland flow patterns? How might the road be retrofitted to harmonize its drainage with the native pattern to that place?
If considering installing a new road on virgin ground, identify the native drainage pattern and harmonize road drainage treatments to maintain the existing pattern.
Surface run-off can be an asset or a liability. All road surfaces produce surface run-off. How the run-off is handled will determine which balance sheet column it falls into.
- 1” rain = 27,000 gallons/acre ~ 25,000 gallons after evaporative loss
- On roads, 80-90% of this is run off.
- 1 mile of 12 foot wide road equates to ~1.5 acres surface area.
- The proportion of rainfall that becomes streamflow depends on:
- Size of drainage area: larger area means larger volume of run-off.
- Topography: run-off volume increases with steepness of slope.
- Soil: permeability and infiltration capacity.
Soil texture refers to the size, composition and proportion of different sized particles (a key factor in determining location, construction and drainage methods). Surface roughness can reduce shear force and erodibility of the exposed surface (think sand vs. small gravel vs. knitted rock rundown).
From smallest to largest: clay > silt > sand > gravel > cobble > larger > boulders etc. Coarser texture gives you more options. Valley bottoms tend to be composed of similar sized particles, whereas hill slopes have better mixes of sizes for creating more stable surfaces (yet another reason to avoid valley bottoms if at all possible).
Resources For Continued Learning
A Good Road Lies Easy On The Land…Water Harvesting From Low-Standard Rural Roads – by Bill Zeedyk, 2006. Download the PDF
Low Maintenance Roads for Ranch, Fire & Utilities Access, Guenther, 1999.