Managing Runoff on Hilly Terrain: A Mountain West Specialty

Building in the Mountain West offers incredible views, but developing on the sloped terrains of Utah, Colorado, and Idaho introduces a massive engineering challenge: gravity.

When rain hits a flat job site, water moves slowly, giving your Best Management Practices (BMPs) time to pool and filter the runoff. When rain hits a steep, freshly graded slope, water accelerates. This high-velocity runoff acts like a buzzsaw, carving deep rills and gullies into your soil and carrying tons of sediment directly toward your property line.

If you treat a sloped site like a flat one, your SWPPP will fail. Here is how you manage stormwater when gravity is working against you.

The Physics of the Slope: Why Standard BMPs Fail

The biggest mistake contractors make on hilly terrain is relying entirely on a single perimeter control at the bottom of the hill.

If you place a standard silt fence at the base of a 30-degree slope, the sheer volume and velocity of the water rushing down will either flatten the fence completely or severely undercut it. Perimeter controls are designed for slow, shallow sheet flow—not high-velocity, concentrated water.

Breaking the Slope: The Strategy

To protect a hillside, you have to break the slope into smaller, manageable sections. The goal is to slow the water down before it ever reaches the bottom.

• Check Dams and Wattles: By trenching in rows of wattles (fiber rolls) horizontally across the face of the slope, you create a series of mini-dams. As water flows down, it hits the first wattle, slows down, drops its sediment, and then gently spills over to the next section.

• Erosion Control Blankets (ECBs): Exposed dirt on a steep incline cannot simply be left alone. ECBs are rolled mats made of straw, coconut fiber, or synthetic netting that are stapled directly into the hillside to hold the soil in place until vegetation can take root.

• Velocity Dissipation Devices: If you are channeling water off a hill through a pipe or a graded swale, the water exiting that channel will be moving incredibly fast. You must install riprap (large, rough rocks) at the outfall to break the water's energy before it hits natural soil or the municipal street.

We Know the Terrain

At Accena SWPPP Services, our experts are trained to read the topography. We map out the contour lines of your site and mathematically determine exactly where check dams, ECBs, and fortified perimeters need to be installed. We stop the water from gaining speed so your site stays perfectly intact.

Building on a slope? Let our Storm Troopers design a custom erosion control plan today.