What is Storm Water Runoff?
Contaminants in storm water runoff that flow into municipal storm sewers may impact drinking water sources, recreational waters and aquatic life. Storm water problems are often most evident in urban areas. As communities grow, they often experience more storm water runoff problems due to their increasing impervious areas. Impervious areas reduce the amount of rainwater that can naturally infiltrate into the soil. This causes an increase in the volume and rate of storm water runoff. It can lead to more frequent and severe flooding, stream bank erosion, and potential damages to public and private property and water quality.
Managing Storm Water
Managing storm water runoff and its impacts is a serious issue facing Iowa and the nation. State and federal governments mandate local storm water programs to control storm water pollution. The intent of these regulations is to improve water quality by reducing storm water runoff and the contaminants carried by storm water.
Principles of Storm Water Management
The traditional approach to storm water management drains runoff water offsite through pipes and storm sewers as quickly as possible. Traditional detention based storm water systems only manage the large storm events for flood control and generally do not provide water quality projection. This approach does not minimize the amount of runoff generated or prevent or reduce the pollution to the receiving water body. The increased volume and duration of flows can cause water quality and quantity issues including stream bank erosion and property damage.
Today, more comprehensive storm water management is needed. Storm water must be managed for both flood control and water quality protection. Infiltration-based storm water management practices are the key to a more comprehensive approach. Infiltrating small rains and the first flush of larger storms minimizes the amount of runoff generated and the pollutant loads that are delivered to surface waters.
Runoff must be managed to protect quality in cost effective ways. Site planning that uses more natural ways to infiltrate or convey rainfall can save money over traditional pipe and detention-based systems. It can also reduce costs associated with long-term maintenance of infrastructure.
Storm water runoff degrades water quality
The urban landscape is partially covered with impervious surfaces such as roadways, parking lots, rooftops, and compacted soils. Pollutants accumulate on impervious and compacted urban surfaces between rainfall events. Storm water runoff is rainfall or snow melt that flows off the landscape.
Rains wash pollutants directly into streams, rivers, and lakes through storm drains without treatment. Pollutants moving in storm water runoff negatively impact drinking water sources, recreational waters, and aquatic life.
As communities grow, they often experience more storm water runoff problems. Adding impervious areas decreases the amount of rainwater that can naturally infiltrate into the soil. This causes an increase in the volume and the rate of storm water runoff.
The traditional approach to storm water management was to quickly drain storm water off urban landscapes. The increased runoff volume and rate compounds flooding problems. Therefore, many communities have adopted storm water ordinances that require storm water detention basins to control flooding.
Communities requiring storm water detention have only controlled the storm water from large storm events. The release rate is managed to control downstream flooding. The small storms were not managed because small rainfall events typically don’t cause flooding.
Although peak flows in stream corridors are decreased by storm water basins, the extended duration of elevated flows can cause stream bed and bank erosion. This degrades water quality and can cause property damage.
Small rainfall events account for the great majority of annual precipitation and most pollutants that accumulate on urban landscapes are transported to receiving waters during the first flush. These rain events flow through traditional detention basins as if they were not there. Therefore, basins generally do not provide water quality protection.
The new storm water paradigm that is being required of many communities calls for adding water quality protection to flood control priorities. Water quality can be protected by managing runoff from low intensity rainfall events. Managing runoff from small rains typically involves infiltration and filtration practices.
Storm water program regulations and the new paradigm
State and federal governments mandate local storm water programs control storm water pollution. The intent of these regulations is to improve water quality by reducing the volume of storm water runoff and the contaminants in the storm water. Regulations require a more comprehensive approach to storm water management that addresses storm water quality.
Infiltration-based storm water management practices are the key to a more comprehensive approach to storm water management. Infiltrating small rains and the first flush of larger storms minimizes the amount of runoff generated. Infiltrated runoff goes through a filtration process that traps pollutant loads in a soil matrix full of microbial activity. Microbes break down trapped pollutants that contaminate receiving waters when storm water runoff enters streams, lakes, or wetlands. Infiltrated storm water is slowly released as clean and cool water to maintain stable flows in stream corridors. Infiltration-based storm water management mimics the natural groundwater-driven hydrology that existed before impervious and compacted urban surfaces began preventing rainfall from soaking into the land. When rain is absorbed by healthy landscapes, stream flows are maintained by slowly released groundwater discharge rather than polluted surface runoff.
Managing the Water Quality Volume (WQv)
The Iowa Storm Water Management Manual provides guidance on managing storm water for water quality protection. The manual provides designers with a method for managing the water quality volume. The water quality volume (WQv) is defined as the runoff generated by a rainfall event that is equal to or larger than 90% of all rainfall events that occur in the course of a year.
For example, analysis of historical rainfall records from Sioux City indicate that nearly 95% of annual precipitation occurs as rainfall events that are less than 1.0 inch. These small rainfall events can be infiltrated to reduce runoff, protect water quality, and stabilize stream flows. In Iowa the water quality volume is defined as the 1.25 inch rain event.
Infiltration-based Best Management Practices
In most urban landscapes impervious surfaces discharge runoff into the storm drain system. One non-structural approach to water quality protection is to eliminate directly connected impervious areas and route runoff onto pervious surfaces. Many best management practices (BMPs) can infiltrate runoff from small rainfall events on-site.
Bioretention is the workhorse of infiltration-based storm water management. Bioretention involves the design of sites with landscaped areas to create micro-storage for runoff. Runoff from parking lots, streets, driveways, or rooftops is directed to these shallow depressions where it is temporarily impounded and infiltrated. Where soils are altered and compacted, an engineered sub grade is needed. The subgrade typically includes a gravel bed, an internal tile drain, and a manufactured soil matrix that includes a specified mixture of sand, compost, and topsoil. The size of the bioretention area is designed to receive the water quality volume from a rainfall event (up to a 1.5 inch rain).
Rain gardens provide bioretention but typically do not have an internal drainage system. Rain gardens rely on a healthy soil profile with good percolation rates. Impervious surface runoff is directed to the rain garden. Soil in the top foot of a rain garden is typically amended with sand and compost to enhance infiltration rates. The rain garden is vegetated to aid percolation and evapo-transpiration of water.
Bioswales are vegetated drainage ways that provide both conveyance and treatment of storm water runoff. Bioswales infiltrate runoff from small storms and provide vegetative filtering of runoff from large storms. Developments that maintain and utilize the green infrastructure of natural drainage ways can often reduce the cost of installing storm sewers and also provide water quality benefits.
Soil quality restoration is one of the easiest and best infiltration-based storm water management practices. A healthy soil profile can have up to 45% pore space, which creates significant storage capacity in lawns and green space. Protecting the soil profile from land disturbance and compaction should always be a priority. When land disturbing activities can’t be avoided, soil quality can be restored. Restoration involves deep tillage to reduce compaction and re-create pore space. Amend soils with compost to achieve organic matter content of 5% or more. Organic matter content and pore space is the key for landscapes with the ability to absorb, infiltrate, and store water.
Native landscaping can be used strategically to help restore and maintain soil quality. The deep root system of native prairie plants increase organic matter content and help increase pore space in the soil profile.
Pervious paving is important because parking lots, roads, driveways, and other transportation surfaces constitute 60% of total imperviousness. Porous pavements may include modular paver blocks, porous asphalt, pervious concrete, or grass systems. A rock chamber is installed beneath the infiltrating pavement to temporarily store water until it discharges into a drainage tile or is moved into the adjacent subsoil.
Green roofs provide a vegetated surface that has a 4 inch or deeper soil matrix below it. A green roof will eliminate 50% to 80% of roof runoff. Green roofs are especially applicable to large commercial or institutional buildings. Green roofs can significantly reduce energy consumption and the cost of heating and cooling buildings.
Wet ponds and wetlands can be installed instead of dry storm water basins. Permanent water bodies are more efficient at trapping sediment and can reduce nutrient loading if emergent vegetation is a component of the system.
Filtration practices such as sand filters can be used in high intensity urban areas to remove pollutants.
Mechanical separators can be installed as part of a storm drain system. Separators trap sediment and floatables such as trash. These BMPs typically do not remove nutrients, pathogens, hydrocarbons, or fine sediment particles.
There are also a number of good housekeeping or pollution prevention practices designed to limit the amount of pollutants contained in the runoff. Good housekeeping practices that prevent the accumulation of pollutants on impervious surface and their ultimate transportation to receiving waters might include:
- regular street sweeping,
- limiting fertilizer applications,
- directing runoff to green spaces,
- requiring pet waste clean up,
- requiring that lawn mowers blow clippings onto a yard rather than into a street gutter,
- not washing vehicles on impervious driveways that drain into the street,
- maintaining vehicles to prevent leaks of oil, antifreeze, or other hydrocarbons onto impervious transportation surfaces
Managing runoff from the entire rainfall spectrum is critical. BMPs should be selected to accomplish the three main goals of the storm water management paradigm:
- protecting water quality
- reducing the quantity of storm water runoff
- reducing peak flows
Consideration of a number of factors is necessary in order to select appropriate practices for a location or a given set of conditions. These factors include the size of the drainage area, space required for practices, soil characteristics, maintenance, and cost.
- Sediment from construction sites
- Pesticides and nutrients from lawns, parks, and roadsides
- Grass clippings
- Bacteria from pet waste
- Oil and grease from car leaks, gas stations, and industrial areas
- Trash such as cigarette butts, paper wrappers, and plastic bottles
- Illegally dumped pollutants
- Thermal impacts from sun-heated impervious surfaces
- Salt and sand from snow and ice control applications
- Illicit connections to storm sewers