When site conditions prevent water from soaking into the ground—due to impermeable clay soils, a high water table, or contamination concerns—specialized solutions are necessary to manage surface water runoff. Standard drainage systems relying on infiltration are ineffective in these scenarios, requiring engineered approaches to manage water on the surface. These methods focus on directing water to a regulated discharge point, removing it through natural processes, or storing it for later use. The goal is controlling the water’s path and volume to prevent flooding or erosion without disturbing the groundwater.
Utilizing Natural Evapotranspiration
Natural evapotranspiration (ET) offers a passive method for water disposal by converting liquid water into vapor without requiring deep ground infiltration. This process combines evaporation (water movement from the soil surface to the atmosphere) and transpiration (water movement through plants and release as vapor through leaves). ET systems are especially feasible in semi-arid and warmer climates where the annual evaporation rate exceeds the annual rate of precipitation.
Engineered Evapotranspiration (ET) beds are designed to maximize this natural process, often using a lined system to prevent infiltration into the underlying soil. These beds are filled with specific material, such as fine sand, which allows water to rise through capillary action to the surface for evaporation. Specialized, shallow-rooted vegetation is planted to enhance transpiration, effectively pumping water from the root zone into the atmosphere.
For small-scale applications, surface spreading allows water to cover a wide, shallow area where ambient conditions encourage evaporation. Rain gardens can also function as an ET system, using densely planted, water-loving native species to maximize water uptake and subsequent transpiration. The performance of all ET systems is directly dependent on climate, and their feasibility is questionable where precipitation frequently exceeds the evapotranspiration rate.
Directing Water to Designated Surface Discharge Points
For managing larger volumes of water, the most common engineering approach involves conveying the water to a regulated surface discharge point, such as a municipal storm sewer or a natural watercourse. This process requires a controlled system to manage both the volume and quality of the water before release. Conveyance infrastructure includes lined swales (broad, shallow channels often vegetated to slow flow and filter sediment) and culverts or piping systems designed for rapid transport.
Discharging water off-site is a legally sensitive solution that requires specific permits, such as a National Pollutant Discharge Elimination System (NPDES) permit or local regulatory approvals. These permits ensure water quality is maintained and that the discharge does not cause erosion or flooding downstream. Regulated discharge differs significantly from uncontrolled runoff, which is often illegal if it damages a neighboring property by increasing water volume or velocity.
Discharge hierarchies generally prefer releasing water to a natural watercourse first, if infiltration is impossible, and then to a public surface water sewer as a last resort. When discharging to a natural water body, the flow rate must often be attenuated (slowed down) to match the natural flow rates of the area to prevent erosion and flooding. Sustainable Drainage Systems (SuDS) employ techniques like filter strips and vegetated swales to manage the speed of runoff and provide filtration before release.
Collection, Storage, and Reuse Systems
An alternative strategy is to capture and store surface water on-site, preventing disposal into the ground or discharge off-site. This approach requires infrastructure specifically designed to contain the water without allowing it to seep into the underlying soil. Storage infrastructure includes above-ground or underground tanks, cisterns, and lined retention ponds or basins. Capacity can range from a few hundred liters for residential use to tens of thousands of liters for industrial or commercial applications.
The collected water, which can include rainwater or stormwater runoff, is then used for non-potable applications, reducing demand on the public drinking water supply. Practical reuse applications include landscape irrigation, flushing toilets, washing vehicles, and supplying industrial processes like cooling towers. This water must undergo treatment appropriate for its intended use, ensuring it is safe and functional for the non-potable purpose.
Modern systems often combine storage for attenuation (flood control) with storage for reuse in a single, smart design, actively managing water levels based on predicted weather. This dual-purpose strategy reduces the overall volume of water needing disposal and provides a resilient, alternative water source. The integrity of the storage system is paramount, as its purpose is to manage the water on the surface and prevent unintended infiltration into the surrounding soil or groundwater.