Stormwater runoff is precipitation that flows over land or impervious surfaces, such as rooftops and paved areas, instead of soaking into the ground. Developed landscapes have fewer natural areas to absorb this water, causing runoff volume to increase rapidly. This water carries pollutants like sediment, chemicals, and debris into local waterways. Effective management is necessary to prevent soil erosion, reduce contaminant transport, and mitigate localized flooding.
Improving Landscape Absorption
Maximizing the “sponge effect” of the existing landscape is the most accessible way to manage runoff. This focuses on improving the soil’s ability to accept and retain water. Amending the soil with compost or other organic material significantly increases its porosity and water-holding capacity. Organic matter helps bind soil particles into stable aggregates, creating air pockets that allow water to infiltrate. Mechanical aeration can further assist in breaking up compacted soil layers, improving pathways for water to penetrate deeper into the ground.
Planting native vegetation with deep root systems is an effective biological tool. Certain native prairie plants, such as the Compass Plant, can develop roots reaching up to 15 feet deep, which stabilize soil and create channels for water to travel into the subsoil. This deep root matrix allows the landscape to hold water and withstand drought conditions.
In areas maintained as lawns, keeping the grass blades longer, typically around three inches, encourages deeper root growth. Taller grass shades the soil, reducing water loss from evaporation and increasing moisture available to the root system. This practice makes the lawn more resilient, contributing to the landscape’s capacity to absorb rainfall.
Capturing and Diverting Rainwater
Structural methods intercept water flow from hard surfaces, redirecting it to areas where it can slowly infiltrate or be stored for later use. Rainwater harvesting systems, such as barrels and cisterns, capture water directly from roof downspouts, which is the cleanest source of residential runoff. A single rain barrel typically holds 50 to 100 gallons, while larger cisterns can store thousands of gallons.
This collected water is non-potable but suitable for purposes like landscape irrigation, car washing, or refilling decorative ponds. Disconnecting downspouts from stormwater drains or impervious driveways immediately reduces runoff volume entering the municipal system. Redirecting downspout flow into a dry well allows water to infiltrate underground slowly.
A dry well is an excavated pit, lined with geotextile fabric and filled with crushed stone, which temporarily stores water until it leaches into the surrounding soil. For managing sheet flow, earthen structures like swales and berms can be constructed.
A swale is a shallow, vegetated channel designed to slow, spread, and sink water across a gentle slope. Swales are often designed with a longitudinal slope of 2% to 4% to prevent pooling and reduce water velocity. A berm, a low mound of soil, can be built along the downhill side of a swale to increase channel capacity and promote maximum infiltration.
Utilizing Permeable Materials
Replacing non-absorbent surfaces with permeable materials offers a long-term solution for managing runoff from high-traffic areas like driveways and patios. Permeable pavements, including permeable interlocking concrete pavers (PICP) and porous asphalt or concrete, are designed with internal void spaces that allow water to pass directly through the surface. These systems can exhibit very high infiltration rates, sometimes exceeding 4,000 centimeters per hour.
Permeable systems require a subsurface layer of coarse aggregate that acts as a temporary reservoir before water infiltrates into the subgrade soil. Proper maintenance, such as vacuum sweeping to remove fine sediment, is necessary to maintain the high infiltration rate. Function relies heavily on the permeability of the underlying soil, sometimes requiring a piped underdrain in areas with poor drainage clay.
Rain gardens are engineered systems designed as shallow depressions that utilize specialized soil mix layers to collect and filter runoff from adjacent impervious areas. These bioretention cells feature a specific blend of sand, topsoil, and organic matter to optimize infiltration and pollutant removal. The plants selected must tolerate both brief periods of standing water and extended dry spells, making deep-rooted native species the preferred choice.
Carefully grading the land is a foundational practice to ensure water flows predictably and slowly. A minimum 2% slope away from a building’s foundation is standard to direct water away from the structure. Managing the slope of the ground reduces the velocity of surface water flow, which increases the time for absorption and minimizes soil erosion.