Two-stage Ditches (artificial floodplains)
Two-stage ditches are a relatively new but highly effective innovation in containing and directing flow of agricultural runoff. Typical ditches are excavated in a trapezoidal shape, which concentrates and speeds up flow when water levels rise. This leaves them vulnerable to erosion and flooding. Measures intended to disrupt the flow of water and minimize erosive forces usually involve the use of rip rap. Concrete berms may be employed to extend the walls of the ditch in hopes of reducing flooding. More often, these measures simply move the problem downstream.
Two-stage ditches are designed to reproduce the characteristics of the natural floodplains that form on the border of rivers. These floodplains serve many valuable functions, including reducing sediment and nutrient pollution. In cross-section, they have a traditional ditch structure, which intersects with the water table and collects and directs water in periods where the flow is light. Planted margins slow the water flow, provide filtration and pollution removal, and support important pollinator habitats. Since the ditches are less prone to flooding, they are more effective in removing excess moisture in the fields, where drainage outlets may be completely under water.
In rainy seasons, or after prolonged or heavy storm events, as the water level rises it is contained within a wider, gently sloping shelf that is planted with a variety of native plants. This floodplain effect allows the water to spread out and slow down, while deep rooted plants reinforce the soil and prevent erosion.
Two-stage ditches are especially helpful in regions where tile drainage is common, since buried tiles often bypass edge-of-field treatment efforts like vegetated buffer strips. Another advantage is that this ditch configuration can support and cleanse flows from the entire watershed, not just from tile systems. Two-stage ditches require little maintenance other than periodic mowing or removal of large debris. Since vegetation removes excess nutrients, typical problems leading to eutrophication in waterways, such as duckweed and algal blooms, rarely occur.
Sediment Control Basins
Sediment control basins are structures meant to detain and slow surface flow and allow suspended sediment to settle out. They’re frequently used in construction sites to control sediment washed out in areas where the soil is disturbed. In both construction and agricultural applications, berms are installed at the bottom of slopes to redirect water away from vulnerable waterways and into the basin. The sediment removal step prevents clogging of pipes, outlets and other conveyance structures. Once the runoff has slowed enough to drop most medium size sediment particles, it’s typically conveyed to another drainage field and feature like a vegetated swale where nutrients and other contaminants can be removed.
Some sediment basin designs include a permanent pool, especially where incoming flow is heavily laden or fast traveling. Longer detention times allow fine silt and clay particles to settle out, which are typically difficult to remove. When permanent pools are included, designs may include the installation of turbidity curtains. These are impermeable liners set up along the entire course of the water column in a winding pattern, interrupting the rapid, straight-line flow of incoming water. Furthermore, water in permanent pools can only discharge at the top, near the water surface. This ensures that water still carrying suspended sediment cannot escape and the outlet structure doesn’t become clogged.
