Concepts in mitigating site impacts when using heavy equipment
Part 2 – Water Management
by Walker Korby
As any sandcastle builder knows, you can create the most elaborate shapes and contours with a substrate that has just enough moisture to stick together. But as soon as you add more water, all bets are off. Everyone in the hardscaping business has one or more stories about the sudden, unexpected rain that practically washed their site away before they were finished. Or even worse for the client, especially if the work was not guaranteed, the story involves a site getting washed away long after the contractor has taken away all the machinery.
Presuming that you’ve got proper silt fencing in place to keep from impacting any sensitive downhill ecology, erosion can still lead to other issues down the road. For example, the less compacted and lighter topsoil is what runs off in a heavy rain, leaving exposed subsoil which is much harder for plants to get a foothold in. Often plants that are more opportunistic and exotic are favored in such soil conditions, leaving you with another ecological problem later on as undesirable species cover your site.
The bottom line with water management is controlling how fast water travels through your site. The higher the velocity, the more likely it is to dislodge the particles of soil and put them somewhere not in your plan. Two main factors control the speed of water through our site as hardscapers – substrate and slope.
Control the Substrate
You can control the substrate using two different concepts I like to call “hardening” and “texturing”. Hardening refers to the strategy of making the substrate particles cling together in a way that makes them less likely to be pulled away from each other by a disturbance. An extreme example is the chemical bonding of particles that happens in the formation of concrete. At the other end of the spectrum is simple compaction.
Texturing is based around the concept that a substrate is less likely going to degrade if the force of the disturbance is minimized. As water travels downhill it gathers speed, and the greater the velocity, the greater force the water applies to the particles, loosening their grip on one another and pulling them away. But if the speed of the water is slowed down by barriers, the velocity never reaches the point where it will degrade the substrate. Laying woven mats, or straw on a surface will slow down the velocity of water traveling across a surface hopefully long enough to allow plants to start rooting which further slows the water.
A super simple technique to control the substrate employs both hardening and texturing: drive your tracked machine parallel to the slope along the entire surface of your site creating texture perpendicular to the slope at the end of the day. This is something I do even if the project isn’t complete, especially if there is a chance of precipitation. At the end of the project, this technique will help hold any seed you put down or that comes along naturally, thereby speeding up the recovery of your site.
Control the Slope
Working with the substrate is key, but alone it’s not enough to deal with water management. Taking slope into account is the more critical factor and should be worked into the design of any feature, whether an access trail, or the sides of a water retention berm. Obviously the steeper the grade, the faster water has a chance to go. A harder substrate, will hold a steeper grade. But, as wizened excavators know, water will always find that imperceptible flaw and turn it into a major malfunction. So it’s always better to rely more on proper grading rather than substrate control because it’s easier to check for potential problems.
As a general rule, if you are going to have exposed compacted soil, keeping your slopes less than 20% (the elevation gain is 20% of the distance) tends to make water more forgiving. For access roads and trails you want to avoid going any steeper than this for much of a distance. If you have a grade that needs to be steeper than 20%, you will have to have a really solid substrate – crushed stone, natural or synthetic erosion control mats, or even wood chips in a pinch – until vegetation has a chance to get roots into the soil to stabilize it.
Sometimes the only option is to channel the water into an area that either has less of a grade, or is particularly hardened. One way to do this is with water bars – hardened linear depressions that cross the trail at an angle, diverting the water off and away from the trail. As a rule I won’t design water bars into a permanent landscape. They are a last resort and to be used only for temporary water diversion, or when the design or site parameters are so restrictive that they are the only option.
Occasionally if I have a short, particularly steep slope that I want to keep water from washing out before vegetation grows in, I’ll walk along the side slope, angling slightly up hill creating a small “mountain goat trail,” as I like to call them, with my boots and a rake. I’ll do this several times in parallel lines at an angle along the slope. Any rain washing down the slope tends to follow these longer, slower, shallower “trails” down to the bottom, instead of running straight down the fall line (the direction parallel to the overall slope). When vegetation grows in these trails are small enough to disappear from sight but still maintain some underlying function of keeping the water from traveling downslope too fast.
Once you understand the concepts that determine how water moves across or through soil, it’s not hard to get proficient at creating your own techniques for controlling how water moves through your site and adapting your plans accordingly. If you have a customer who has even a little bit of ecological intelligence, they’ll appreciate the effort you’re putting into ecological solutions. And, I find customers are usually willing to pay for site-sensitive solutions, as long as you take the time to let them know what you are doing and why you are doing it. And once the dust clears and the incessant beeping of the backup alarm is gone, they will usually be most appreciative that their site doesn’t look like a construction zone – even if it still is!
The Dirt Truth Part 1 addresses preservation of basic soil horizons and factors to consider when choosing equipment.
With a graduate degree in Geography from UMass Amherst, over 10 years working in natural lands property management, and extensive experience as naturalist, teacher, and an artist, Walker Korby brings a unique perspective to connecting with, and shaping the land on which we live. With a passion towards building resilience into our landscapes, his skill sets run the gamut of prescribed fire management to pond construction. Walker lives in Montague, MA, and operates On Contour – Integrated stewardship services for the ecologically minded landowner. He can be reached at 413-367-7172 or at “mailto:firstname.lastname@example.org”>email@example.com