Stormwater Impacts

When stormwater is allowed to run off, it typically finds its way to a stream or other water body via the most direct, downhill route, and can cause a number of damaging impacts. Among these are erosion in the form of rilling and gullying; elevated streamflows caused by the rapid routing of runoff to stream channels, which causes additional erosion of the stream bed and banks; degradation of both water quality and aquatic habitat as sediment is mobilized; lowered groundwater levels, as less water infiltrates the soil and as the stream incises; and decreasing dry season streamflows. In addition to these direct impacts, excessive runoff of stormwater also results in a multitude of indirect impacts, further degrading water resources and aquatic habitat. These are discussed in greater detail below.

Water Savings

There are three important numbers to calculate when considering a stormwater retention project, and each of these numbers plays a role in determining how much water you will save by implementing a project.

Every gallon of water captured and stored during the rainy season is a gallon that will not have to be extracted, purchased or otherwise acquired during the dry season. There is usually a compromise to be made between desired volume, space requirements and costs for water storage, and collection capacity.

Applications

There are a number of proven strategies for effectively dealing with stormwater and runoff. These fall into three broad categories: stormwater dispersion, stormwater retention and infiltration, and stormwater retention and storage. The first two will work to locally recharge groundwater aquifers and alleviate the impacts of uncontrolled stormwater runoff, but will not result in any savings for water users. The retention and storage of stormwater has water supply impacts that can be directly quantified and put through a cost-benefits analysis.

Stormwater dispersion and infiltration

Keeping stormwater dispersed is perhaps the easiest and most cost-effective way of preventing the impacts that come from runoff. This approach essentially mimics natural conditions by dispersing stormwater over a large area, promoting infiltration, and is easier and less expensive to achieve if the stormwater is not allowed to concentrate in the first place.

For dispersion and infiltration to work, the infiltration surface should not be compacted. Dispersing runoff over compacted soil may still be successful in infiltrating the water, but during heavier rains the infiltration capacity of the soil may be exceeded. For compacted soil surfaces, decompaction may be the only treatment needed to eliminate stormwater and runoff, since it improves the infiltration capacity of the soil. Decompaction of soils can be done quickly using mechanical means such as hydraulic ripping or tilling, or can be achieved through the establishment of vegetation. As plants grow, their roots are very effective at decompacting soil, but it can take years for the soil to recover its permeability. Intentional incorporation of appropriate organic materials such as chipped woody debris or stabilized compost in some soil types can accelerate improved infiltration, permeability, and retention, and also assist in reducing erosion. All methods of dispersion and infiltration must be designed to accommodate the maximum intensity of rainfall that can be expected for the location.

• Dispersed drainage:
  The most simple and straightforward way to reduce or eliminate the impacts of stormwater runoff is to disperse it at the point where it is generated. In practical terms this means to drain impervious or compacted surfaces in a dispersed manner. For structures, this might involve removing rain gutters and allowing stormwater to flow evenly off all sides of a building. This allows the water to be dispersed to the maximum extent possible directly from the roof. In cases where water needs to be kept away from a building’s foundation, dispersion of stormwater involves the installation of multiple downspouts along rain gutters. The greater the number of downspouts, the more dispersed the runoff will be. The water can be conveyed away from the downspouts via pipes, but it is important that water from multiple downspouts not be combined or channeled into a single pipe or ditch, since this concentrates the runoff and increases its potential to cause erosion and negative impacts. Stormwater can also be conveyed away from a structure in perforated pipe, allowing the water to meter out of the pipe in small quantities at many locations. For structures associated with livestock, dispersing drainage must be carefully planned and executed to avoid unintended impacts to water quality during high intensity rainfall events. For paved areas, stormwater dispersion involves crowning or outsloping the paved surface, so that stormwater drains from as many sides as possible. If possible, paved areas should not have curbs, which concentrate stormwater. If curbs are necessary for safety, they should have multiple breaks or gaps to allow for the maximum possible dispersion of water. Stormwater dispersion from paved areas is most effective if the necessary grading and sloping are included in the original design. With any of these methods, the goal is to spread out the runoff, minimizing the amount of water conveyed to the ground at any single location.
  
  
• Keyline design describes a suite of practices designed to use water more efficiently, and includes methods to improve soil permeability and soil water storage while causing minimal disturbance to the soil and vegetation. Keyline plowing discourages the generation of runoff by allowing sheet flow to more effectively infiltrate the soil.

Stormwater retention and infiltration:

Stormwater can be retained for infiltration into the ground by conveying it to a location that has been specifically designed to allow water to sink into the soil over time. In this case, the primary goals are to minimize the ecological and property damage that result from the uncontrolled runoff, and to locally recharge the groundwater table. This approach is highly effective in recharging groundwater but is less practical for agricultural operations, as it can require substantial space and infrastructure development, and the potential benefits to water supply are indirect and difficult to measure.

• Retention basins
• Swales or bioswales: Stormwater can be conveyed away from a building or other impervious or compacted surface via pipes or low gradient ditches to a vegetated, low-gradient swale, or bioswales.

An important component of both of these methods is an adequate overflow. When stormwater is retained it is necessarily concentrated in a limited space. Should the amount of stormwater exceed the space available for infiltration, the excess water must be conveyed away from the area without causing erosion. Generally this conveyance leads to a stream or other water body, reducing the benefit of stormwater retention. However, a properly designed retention basin, swale, or other retention structure should experience overflow only rarely, when a heavy rainfall occurs during saturated conditions.

Stormwater retention and storage

Stormwater retention and storage involves collecting stormwater and storing it for future use. This practice reduces damage from uncontrolled runoff and provides water supply flexibility, reducing long-term water costs. It is appropriate in most parts of California, where water is limited, where precipitation is seasonal or infrequent, or where chronic water shortages exist.

• Ponds:
  Agricultural ponds for water supply are relatively common. Ponds reduce the generation of stormwater by directly collecting rainfall, and in most cases are constructed to collect stormwater as sheetflow, before it has become channelized. Ponds can also be used to collect and store stormwater that is generated by agricultural drainage systems, such as vineyard drainage tiles, and from impervious surfaces such as roofs and paved areas (see “rainwater harvesting” below). Ponds do not require appropriative water rights in California as long as they do not collect water from a natural channel – sheetflow and runoff from agricultural drainage systems are not within the purview of the State Water Resources Control Board.
  
• Rainwater harvesting:
  Construction of rainwater harvesting systems on a variety of scales is becoming more popular in California. Most of the state has a Mediterranean climate with distinct wet and dry seasons, and the benefits of capturing and storing rainfall (before it becomes stormwater) are becoming more widely recognized. In the wet season, stormwater is generated when it rains, and in most cases this water runs off, causing erosion and depleting groundwater. During the dry season, water is in high demand but is scarce, so it is extracted from streams or groundwater, imported, etc., resulting in ecological damage, groundwater depletion and economic loss and inefficiency. Rainwater harvesting collects water that would otherwise run off from roofs or other impervious surfaces and stores it for future use, simultaneously addressing the problems of water scarcity and stormwater impacts. The only things necessary to harvest rainwater are a surface to collect water from and a place to store it.

Additional Benefits

As noted above, when stormwater is allowed to concentrate and run off in an uncontrolled manner, a number of detrimental impacts follow. The implementation of any of the strategies outlined above will work to limit or eliminate these impacts.

When stormwater is allowed to run off, it typically finds its way to a stream or other water body via the most direct, downhill route, and can cause a number of damaging impacts.

• Erosion:
  The most straightforward and observable impacts is erosion. The channelized flow of runoff over a surface that is not adjusted to it causes erosion in the form of rilling and gullying, which have a number of damaging effects. Gullying causes the degradation and loss of soil, which reduces the area of usable land and can impact soil fertility. Gullying that results from uncontrolled stormwater runoff causes millions of dollars in property damage throughout the world each year. The soil that is eroded by runoff is usually transported to a stream or other water body, causing a number of harmful effects, including decreased water quality.
  
• Impacts to surface hydrology:
  The addition of runoff to the existing network of stream channels has multiple impacts to surface hydrology. Unlike water that infiltrates the soil, runoff makes its way to a stream very rapidly, elevating peak streamflows, increasing the erosive power of the stream and causing in-channel erosion. This erosion typically occurs as stream channel incision, where the stream erodes and deepens its bed, and as channel widening. Incision of a stream can cause local drops in the water table, which means that less groundwater is stored in the soil (see below). Stream widening usually occurs through bank failure, where the stream undercuts and erodes its banks. Widening can lead to the loss of riparian vegetation, degrading wildlife habitat and impacting water quality.
  
• Water quality
  Stormwater that is allowed to run off into a stream or other water body degrades surface water quality in a number of ways. Runoff will transport contaminants that it encounters along the way, as well as sediment that is created through soil erosion. Runoff from paved surfaces can pick up oils and other substances that originate with motor vehicles, and runoff from agricultural facilities or pasture land with compacted soil can transport nitrogen compounds and other contaminants that originate in animal manure and urine, as well as pesticides, herbicides and other potentially harmful chemicals. Runoff may also degrade water quality by raising the temperature of the water body it enters.
  
• Aquatic habitat:
  All of these stormwater impacts degrade the quality of habitat for aquatic organisms, including fish, amphibians and invertebrates. A large variety of organisms are highly sensitive to contaminants, many of which can cause health and reproduction problems. Excess fine sediment delivered to lakes and streams by gullying and instream erosion can smother fish and amphibians and their eggs, as well as invertebrates that live in the gravels on the bottom.
  
• Groundwater:
  A less obvious stormwater impact is its effect on groundwater. Precipitation that infiltrates the soil is stored as groundwater. Like surface water, groundwater moves downhill through the soil toward streams and lakes. Unlike surface water, groundwater movement is extremely slow– where surface runoff can flow into a stream or lake in a matter of minutes or hours, groundwater may take weeks, months or even years to reach a surface water body. This slow movement allows groundwater to contribute to streamflow during times of dry weather or drought. This “baseflow” is reduced if more precipitation is converted into stormwater and runoff, and less water is allowed to infiltrate the soil. Where a stream has become incised due to elevated peak flows, the lowered “base level” of the stream causes water in the soil to drain out more quickly than it would in the absence of incision. All of these conditions result in less groundwater, reducing the amount of water available both to provide baseflow in streams, and for extraction through wells.