Designing a stormwater conveyance system for steep slopes – balance performance and cost
When I look at solutions for stormwater conveyance systems at sites with steep slopes, I always want to come up with a solution that balances performance and cost and is not burdensome to the client in terms of risk and long term maintenance. I want to give them something that will hold up, will not bankrupt them, and does not have them pulling out their hair. There are easily 5 to 10 different drivers, and I look for something that blends them all into an overall optimal solution.
How steep is steep?
So, what is the threshold at which point a channel becomes “steep,” from an erosion potential perspective? (The definition I use here should not be confused with “hydraulically steep,” which refers to the occurrence of supercritical flows and is unrelated to the topic of erosion potential.) My view is that a steep channel is one that requires anything more than unreinforced vegetation to remain stable and not erode while conveying the design discharge. The tricky part about this definition is that it depends on the flowrate being conveyed and the channel’s cross-sectional geometry. In other words, virtually any slope could be considered steep if enough water is flowing through it. The higher the flowrate, the flatter the slope is that starts to become problematic.
For example, take a look at this photo. At first glance you might not consider it steep. So does it surprise you to know we had to install a permanent erosion mat here to avoid problems? Although I make it a point to always check every design to ensure the lining system will remain stable during the design flow, I really start to closely examine the performance once channel slopes get to 2-3 percent.
What are your options?
Based on my definition of a steep channel slope, unreinforced vegetation (grass) will not be a viable lining option. However, I almost always analyze channels with a lining of unreinforced vegetation as a first step because it determines whether we will need a more robust lining. In some instances (downchutes for instance), I know from experience that unreinforced vegetation will not hold and so I skip that first step. Once I know that unreinforced vegetation won’t work, I move on to other options, which may include any of the following:
So many options, how to decide?
- Natural vegetation with a turf reinforcement mat (TRM)
- Loose riprap
- Cellular confinement (infilled with soil, stone, or concrete)
- Cabled concrete/articulated revetment units
- Fabric-formed concrete – A good choice if you cannot get easy access to an area.
- Preformed HDPE/CMP channel sections
- Closed pipe sections
- Exposed geomembrane – Holds up well, requires minimal equipment to install and is relatively inexpensive
- Pavement (asphalt or concrete)
Several options may work on any given site. Begin by determining flow conditions to help rule options in or out. The accompanying graphic shows the suitability of a few of the lining types for certain flow conditions. The red and yellow lines represent applied shear stresses on a hypothetical channel bed. As the channel slope increases, so does the shear stress. The other lines represent allowable shear stresses for various lining types. Allowable shear stresses that plot above the applied shear stress lines are acceptable for consideration. Allowable shear stresses that plot below the applied shear stress lines are not. You can see that some lining types are only acceptable up to a certain channel slope, whereas others are acceptable for all channel slopes evaluated. It’s important to remember that the channel lining type can affect the flow conditions within the channel so lining selection is an iterative process wherein candidate lining types are identified, flow conditions for each are calculated, and suitability of the candidate lining types are confirmed.
I always consider past treatments used at a site. Is there familiarity with a particular system and will it work again? Often there is a comfort level in using something that has been successful at a site, has held up well, and is reasonably priced. But be very careful about analyzing the solution to check if it will work this time. (Remember what those financial disclaimers say – Past success is no guarantee of future performance…)
Then, evaluate material availability, construction budget, maintenance considerations, aesthetics, and any other factors that affect your particular site. The accompanying picture shows rock cross vanes I designed a few years ago for restoration of a site that suffered substantial erosion due to a culvert failure. For this site, community acceptance was crucial so I needed a more aesthetically pleasing solution. That aim, along with the relatively steep slope of the water course, led me to propose a series of “steps” along the channel. The steps used a combination of imported stone slabs and reuse of slabs that originally formed the box culvert that had previously failed. Thus, I achieved some cost savings by reusing material that would otherwise have had to be removed for disposal.
When performance limitations are reached
Most, if not all, lining systems have performance limits based on how much force the water applies. This often leads designers to move to a more robust lining system capable of withstanding higher forces. However, it may make more sense to look at options to reduce the forces on the channel lining so that lower performing (and, often, simpler and less expensive) systems can be used. For really steep slopes, or at sites where there is a lot of channel area to be treated, designers should look for ways to change conditions in the channel.
For example, you might be able to go from needing riprap to unreinforced vegetation by building a stormwater pond that provides a small amount of stormwater storage. The pond could fill up and take an aggressive, short-term flow that would destroy a grass-lined channel and release at a much less aggressive rate that could be tolerated by a less robust lining system.
Other approaches may include modifying the drainage plan to distribute the flows to more locations, increasing the channel width to lessen flow depth (and therefore, the applied shear stress), changing the alignment to reduce slope along flow line, or including energy dissipation/grade stabilization structures (for example, check dams) to reduce the water surface slope along flow line.
To sum it up
Don’t assume a relatively mild-sloped channel is stable, especially with higher flows. Be realistic when determining peak flows and know the performance limitations of the lining system. Also be sure to consider how the lining system will change over time and what repairs may become necessary. If you are using a manufactured system, solicit manufacturer for input on design and application. And last, but not least, remember that flow energy will likely be high at the base of the conveyance and needs to be dealt with to contain water and avoid erosion.
Do you have any other suggestions for designing stormwater conveyances for steep slopes?
Brian Stone, PE, is a Senior Project Manager with Cornerstone Environmental Group, and specializes in site design and water management.
Categories: Environmental Planning & Compliance, Landfill Engineering and Design, Solid Waste
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