Controlling water runoff should be a major objective of any shoreland landscape design. As water collects and flows through channels, it gathers energy for its erosive force. The faster water flows, the greater the particle size and quantity of pollutants it can carry along to the receiving water body (pond, lake, stream, river, wetland or coastal water).
Modifying the landscape with any type of development has the potential to cause changes to water flow, nutrient- and pollutant-loading, and groundwater recharge. Integrating runoff control strategies along with appropriate choices of plants and horticultural products into your landscape design are keys to ensuring a healthy shoreland environment.
Landscaping at the Water’s Edge is a manual for NH landowners and landscapers that covers the concepts and practices of ecological design for water quality protection in lakes, rivers, streams and coastal areas. This book, written in 2007 by several specialists from UNH Extension and others, remains a valuable tool that will help you understand the basics of how watersheds and shoreland ecosystems function so you can use the strategies and techniques below, as well as others, to help prevent soil erosion, nutrient and pesticide runoff, exotic plant invasions, and other detrimental processes associated with developed landscapes. Order a hard copy or download for free at extension.unh.edu/resource/landscaping-waters-edge-book
Runoff Control Strategies:
Detention – holding back or “ponding” a volume of water to slow the speed of its outflow. In some cases, water detention may also allow for infiltration and/or evaporation to reduce the final outflow volume.
Diversion – preventing water from traveling over the area of concern, thereby reducing surface runoff and minimizing the potential for erosion and the transport of nonpoint source pollutants.
Flow spreading – allowing a concentrated flow to spread out over a wide, gently sloped area to reduce the water velocity and encourage infiltration.
Infiltration – allowing water to percolate into the ground rather than running across the surface.
Plant absorption and transpiration – the movement of water from the soil into plant roots, up through the stems and leaves and the release of water vapor through the stomates to the atmosphere.
Techniques Used to Control Runoff:
Berm – A stabilized mound of dirt or stone to create a diversion and/or redirect water flow.
Check dam – A small mound of stabilized dirt or stone that breaks up the flow of water in a drainage ditch or trench to slow down velocity and allow for the settling of heavier materials.
Cut-in (or Cut-out) – A small trench that diverts water out of an existing channel, to be treated/infiltrated to prevent a significant volume of water from accumulating as it runs down a driveway, walkway or path. Multiple cut-ins may be required for long distances or high slopes.
Infiltration trench – A dug-in trench commonly used for roof runoff that allows for storage of runoff and encourages infiltration into the ground.
Plunge pool – A dug-in hole stabilized by stone, typically placed in a drainage ditch or trench. This allows water to fall below the level of the surface to slow the runoff velocity and capture heavy particles. These are often constructed in a series along a sloped route.
Rain garden – A shallow infiltration basin planted with water tolerant plant species, designed to capture concentrated runoff. Rain gardens are designed to pond water for just a few hours at a time, allowing it to be taken up and transpired by plants or infiltrate into the ground.
Swale – A stabilized trench that can act to store water (detention), sometimes also engineered to enhance infiltration.
Vegetative buffer – A relatively flat area stabilized with vegetation that allows water flow to spread out,
slow down, infiltrate and be filtered by the soil, and/or be intercepted and transpired by plants.
Waterbar – A diversion device that diagonally crosses a sloped trail, path or road to capture and divert runoff to the side. Commonly made of a log, a stone, a small, reinforced drainage channel, or a partially buried flexible material, a waterbar is most useful for small contributing areas (watersheds less than one acre) that receive foot and vehicle traffic.
Contact UNH Extension for more information about integrating landscape practices that protect the state’s waters. Resources for professionals and homeowners are available on the UNH Extension website: extension.unh.edu; or by contacting firstname.lastname@example.org.