Water runoff occurs when precipitation flows over the land surface instead of soaking into the ground. This happens when the rate of rainfall exceeds the soil’s capacity to absorb it, causing excess water to flow. Preventing this surface flow is important because it is the primary cause of soil erosion, which degrades landscapes and transports pollutants like fertilizers, pesticides, and debris into local waterways. Effective runoff prevention relies on a layered strategy that addresses soil capacity, utilizes vegetation, and incorporates structural systems.
Enhancing Soil Health for Maximum Absorption
Runoff control begins by maximizing the soil’s ability to absorb water. Soil health directly influences its porosity, which is the open space available for air and water within the structure. Compacted soil, often caused by heavy foot traffic or machinery, dramatically reduces this pore space, severely limiting the rate at which water can infiltrate the ground.
Reducing soil compaction is achieved through techniques like aeration, which physically creates open channels for water to travel downward. Integrating organic matter, such as compost or mulch, is highly effective because it acts like a binding agent, helping soil particles form stable aggregates. This improved structure creates both large and small pores, allowing for rapid infiltration and better retention.
Certain types of organic matter can hold up to 20 times their weight in water; studies show that a one percent increase in soil organic matter can increase the available water-holding capacity by up to 3.7 percent. Minimizing soil disturbance, especially during or immediately before heavy rainfall, also protects the fragile soil structure and prevents surface sealing, which otherwise hinders absorption.
Strategic Planting and Landscape Design
Building upon healthy soil, vegetation provides the next line of defense by slowing water flow and utilizing deep root systems to create pathways for infiltration. Deep-rooted native plants are particularly effective because their roots can extend several feet into the ground. These extensive root networks break up dense soil, creating macro-pores that facilitate the rapid movement of water deep into the subsoil, which minimizes surface sheet flow.
In contrast, common turf grass has shallow roots, often reaching only one to two inches deep, resulting in poor absorption qualities and higher rates of surface runoff. Replacing turf with native plants, which can absorb up to ten times more stormwater, is an effective landscape design choice. This principle is applied in the construction of rain gardens, which are shallow, intentional depressions filled with native plants that are strategically placed to capture and hold water from impervious surfaces.
Another effective application of vegetation is the use of vegetative buffer strips (VBS), which are densely planted areas situated between a runoff source and a sensitive area. The dense stems and leaves of the plants slow the velocity of the water, allowing suspended sediments and pollutants to settle out before the water can infiltrate the soil. Properly designed VBS can reduce the volume of runoff by 35 to 90 percent and trap over 75 percent of sediment, with effectiveness increasing significantly in strips wider than nine meters.
Physical Barriers and Water Redirection Systems
Structural and engineered elements offer the final layer of runoff control, managing water flow where soil and plants alone are insufficient. Permeable pavement, such as pervious concrete and porous asphalt, replaces traditional impervious surfaces. This allows stormwater to drain through the surface and into a crushed stone reservoir below, promoting infiltration back into the native soil instead of directing it to storm drains.
For managing roof runoff, rain barrels and cisterns provide effective source control by capturing water directly from downspouts. One inch of rain falling on a 1,000 square-foot roof yields approximately 600 gallons of runoff. Rain barrels typically hold 50 to 100 gallons and are useful for small storms, while cisterns can hold hundreds or thousands of gallons, often providing water for non-potable uses like irrigation.
Any overflow from these collection systems can be directed to dry wells, which are subsurface pits designed to collect clean water and allow it to percolate slowly into the subsoil. Landscape grading can also be used to create swales, which are broad, shallow channels, and berms, which are raised barriers. Swales slow the flow of water and increase the time available for infiltration, while berms are used to contain water on the downslope side of features like rain gardens.