The Field

by Liia Koiv-Haus, ASLA, AICP

This post is the third installment in a series on the role of landscape architects in roadway design projects. In case you missed them, see part 1, The Role of Landscape Architects in Roadway Design Projects, and part 2, Slope Rounding in Roadway Design Projects.

Slope warping is a landscape grading method that emulates how undisturbed slopes have natural variations in cross slope. The concept essentially involves incorporating berms and swales into the landscape to slow stormwater and improve the aesthetics and safety of our roadsides. Using slope warping in roadside grading can break up long expanses of smooth, steep slopes and improve revegetation success, especially in arid climates. While the concept is similar to slope rounding in that it involves undulating slopes to slow stormwater runoff (see Slope Rounding in Roadway Design Projects), slope warping is more complex because it varies more depending on a site’s unique climate and topography.

Before delving into the details of slope warping, it is important to understand why the concept is rarely implemented. We see a lot of smooth, steep slopes along roadways because they are simple to design. It is less expensive and less time consuming to excavate and grade within existing right-of-way than to acquire more right-of-way or construct a retaining wall to allow a more gradual slope. It is easier to grade a perfectly smooth slope than to customize grading to site context. The image below shows a typical roadside slope before seeding.

Traditional engineering justifies grading smooth slopes because smooth slopes spread the energy of flowing water over a broader area. Concentrated flows, on the other hand, concentrate the energy of water and are more likely to erode soil. While this is true, it oversimplifies erosion. In arid climates, slope warping, which creates concentrated flows in valleys, can actually mitigate erosion by facilitating revegetation. If roadsides in an arid climate are graded with smooth slopes, vegetation won’t establish if they are too long and steep, and concentrated flows will form rills and gullies as a result (see image below).

Minimizing slope length and steepness improves both safety and aesthetics because flowing water accelerates and increases potential for erosion as it moves downhill. CalTrans’ Erosion Control Toolbox states that conventional linear slopes concentrate flow and thus increase erosion at the toe of slope. Minimizing slope steepness prevents erosion and provides “better recovery for errant vehicles that may run off the road,” according to the CalTrans Highway Design Manual.

Slope steepness and length are not the only important factors to consider in preventing erosion and encouraging revegetation along slopes. The impact of raindrops alone on bare dirt can dislodge soil at the top of the slope, carving a path for concentrated flows and erosion even on shorter slopes. In fact, “the energy of [raindrop] impact would be found to be much higher than the energy of flowing water” (Indian Roads Congress) so slope length is arguably less important than the ability of a slope to revegetate before an intense storm event. Vegetation protects the soil surface from the impact of raindrops and breaks up the energy of stormwater flowing downhill. Root systems stabilize slopes by holding soil particles together and creating soil pore spaces that allow for water infiltration. The more water that infiltrates, the less will flow across the landscape as runoff. The photo below helps illustrate the susceptibility of bare soil to erosion, and a time-lapse video shows this dynamic process even better.

While stormwater can be destructive, it is also necessary for revegetation. Seeds need that initial inundation to germinate, especially in more arid climates. Slope warping slows stormwater and creates pockets of pooled water that are ideal microclimates for seed germination, helping vegetation establish sooner. Without slope warping, an intense storm event following seeding will cause seed and sediment particles along the slope to be washed away before germination can occur.

Even if a drainage design is sound, because runoff and slope stability calculations often assume successful revegetation, slope failure might result if precipitation is atypical. Slope failure creates a safety hazard as debris can fall into the roadway and injure travelers or damage the pavement (see image below). A 2011 article about recommended practices for reducing erosion risk along slopes noted that “Most…surficial landslides occur as a result of loss of vegetation cover on soil slopes due to a cut being made for road construction…as a result of erosion due to flowing water.”

Because we often see steep slopes in nature that flourish without human intervention, there is a common misconception that disturbed roadside slopes can also revegetate “naturally.” Over time, native seed banks will accumulate, and plants will recolonize through natural succession. Soil structure will improve as the movement of fine root hairs through the soil and their eventual decomposition leave voids for water and oxygen to enter the soil. Beneficial fungi and microorganisms, all of which aid in revegetation, will accumulate in the soil. But this all takes time and won’t always happen before noxious weeds take over or natural hazards impact our transportation infrastructure. Topsoil salvage and stockpiling can help preserve some characteristics of native soil, but the soil structure cannot be replicated quickly or easily. For this reason, it is of utmost importance to minimize disturbance in the first place.

If disturbance cannot be minimized, certain measures can supplement slope warping to facilitate revegetation and reduce erosion risk. Saving onsite material such as boulders, rocks, duff, stumps, and logs and repurposing them in the landscape can slow stormwater runoff and help replenish organic matter in the soil. Seeding or planting a variety of species, including a mix of forbs, grasses, shrubs, and trees, will not only slow stormwater but create the biodiversity needed for a healthy ecosystem. Both planting trees and minimizing tree clearing can supplement slope warping to reduce risk of erosion and slope failure. Tree canopy coverage can intercept raindrops, slow stormwater runoff, and stabilize soil.

While tree clearing can improve roadway safety by increasing visibility, it can also increase landslide risk. An October 2024 Transportation Research Interdisciplinary Perspectives article by Ellen Oettinger White called “Daylighting decision-making at state departments of transportation: A case study of roadside tree removal” discusses how the Georgia Department of Transportation (DOT) clear-cut almost 25% of forests along state highways beginning in 2018 with the goal to “enhance roadside safety.” Georgia did not conduct environmental review before clear-cutting and they did not consider the erosion control benefits of trees in enhancing roadside safety. A 1975 study on the impact of clear-cutting and road construction on soil erosion by landslides found that clear-cut areas had landslides three times worse than those in forested areas.

Georgia is not alone; countless DOTs have prioritized the more obvious solutions to hazard mitigation over more interdisciplinary and complicated ones. Other DOTs can learn from Georgia’s experience by ensuring that the appropriate environmental point persons at their agencies are involved in decision-making regarding slope stability. It is the responsibility of landscape architects to elevate their roles at DOTs to improve the resiliency of our roadsides in the face of increasingly unpredictable storm events due to climate change.

Liia Koiv-Haus, ASLA, AICP, is a Landscape Specialist for the Colorado Department of Transportation. She also serves as an officer for ASLA’s Landscape—Land Use Planning Professional Practice Network (PPN).