Vehicle Access For Regenerative Landscapes – Guiding Questions For Watershed Assessment
Welcome To Vehicle Access Fundamentals
This series covers how to best lay out, construct, maintain and retrofit vehicle access routes for maximum function and minimal maintenance while repairing, preserving and enhancing ecosystem integrity.
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, low-maintenance access (for vehicles, humans and animals) that maintains or improves watershed hydrology.
“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, A Good Road Lies Easy On The Land
Essential Landscape Terminology
The following terms are important to know and understand when planning vehicle access through a landscape. Each of these terms represents essential data that should inform the design of any whole site access and water harvesting plan.
Topography
Relief
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, while grade is measured in percentages based on rise over run (i.e. a 45 degree slope = 100% grade because for every 1 unit of vertical change there is unit of horizontal change).
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 will 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
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 southern 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.
Hydrology
Drainage Patterns
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 (i.e. don’t let the road move water from one watershed to another, drain it frequently and disperse discharge flows appropriately so that the receiving landscape can accept them and won’t be degraded).
Run-Off
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 and velocity increases with steepness of slope.
- Soil: permeability and infiltration capacity.
Soils
Texture
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).
Particle Size
From smallest to largest: clay > silt > sand > gravel > cobble > larger > boulders etc.
Coarser texture gives you more drainage 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 when building a road).
Guiding Questions For Watershed Assessment
Use the following questions to guide the assessment of any watershed as a precursor to planning new vehicle access routes or retrofitting existing ones.
Where Is The Water Coming From And How Much Is There?
- What land area is draining into this location and how much run-off does it generate during a given rain event?
- Figure out the actual area of the watershed (natural or man-altered) that drains into the particular point of interest.
- Then determine the total amount of water that will run off of that area during a given size and intensity of rain event.
- What type of soils or substrates constitute that area?
- Is it a high or low run-off area?
- Is the area vegetated, and if so, with what (ag fields will behave very differently from forests or chaparral).
Where Is The Water Going?
- What watercourse does the water follow at present?
- If dealing with an already existing road, has the road substantially altered the flow paths of water within its contiguous watersheds?
Where Should The Water Be Going?
- Is the water staying in its parent drainage?
- Ideally run-off stays within its parent watershed. When water from one watershed is moved into another, it dehydrates the parent watershed and overloads the receiving watershed, both of which lead to soil erosion and degradation.
- Is the water passing by areas where it could be discharged and infiltrated?
- Are you taking advantage of all available best chance drainage locations?
- Is there anything below the potential or actual discharge points that might be harmed by extra water flowing by, on or through it?
What Treatment Is Needed To Make It Go There?
- Which drainage treatments can move the water along the best course with the least maintenance?
- Find the best drainage option for the specific context – see Water Drainage Elements.
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.
