· Michael Blair
Nashville sits on rolling ground, and a great many lots have a slope at the back or the side that the builder graded, seeded once and left. Our rain comes in short intense bursts that a thin layer of clay cannot absorb, so the water runs off the surface, picks up speed, and cuts channels. Once a channel starts it concentrates every subsequent storm into itself. Fixing it means dealing with the water first and the surface second.
Why seed alone fails
Grass seed on a bare slope has to germinate and root before the next storm, and on a slope of any steepness the next storm moves the seed and the top layer of soil to the bottom before that happens. Clay that has been exposed and compacted by rain has a crust that seed sits on rather than in. A slope that has been reseeded three times and is still bare is not a seed problem.
The water reaching the slope is usually more than the slope should be carrying. A downspout discharging at the top of it, a driveway or patio shedding onto it, or a neighboring lot draining across it are the common sources, and until they are redirected the surface has no chance.
Slow the water first
Extend downspouts past the slope or pipe them to the bottom. Catch runoff from hard surfaces at the top of the slope in a shallow swale or a trench drain and route it around rather than over. On a long slope, break it into shorter runs with a swale or a low berm across the grade every so often, so water is intercepted and spread before it gains speed.
For an existing gully, the channel itself can be lined with stone, a rock-lined swale that accepts that water will run there and stops it cutting deeper. A deep gully is filled and compacted in lifts, then protected, because simply filling it means it washes out again on the first storm.
- Redirect downspouts and hard-surface runoff away from the slope
- Swales or berms across the grade to break a long slope into short ones
- Stone-lined channels where water has to run
- Fill gullies in compacted lifts, then protect the surface
Hold the surface
A slope that is seeded should be covered. Erosion control blanket, a biodegradable straw or coconut fiber mat pinned to the soil, holds the seed and the top layer in place through the storms of the first season and rots away as the roots take over. Hydroseeding does something similar with a tackified mulch. Either is far more reliable than loose straw, which blows and washes.
Choose the cover for the slope. Grass is fine on a slope that can be mowed. Steeper than that, a groundcover or a mix of deep-rooted natives holds better and never needs cutting. Sedges, native grasses and shrubs that root deeply in clay do more to hold a bank than turf, and once established they need nothing.
When it needs structure
Some slopes are too steep to hold with plants, or too close to a structure to be allowed to move at all. Terracing with low walls turns one steep slope into several flat steps, each of which drains gently and can be planted. Segmental block walls are the usual choice and need drainage behind them, because a wall that holds water in our clay eventually leans.
A wall of any real height is engineered work and may need a permit through Metro Codes, and disturbing a large area of soil on a slope brings its own erosion control requirements during the work. A drainage contractor sorts the water; a landscaper plants the surface; on a slope that is actively failing, the water gets sorted first.