Stormwater and the Hydrologic Cycle
The Hydrologic CycleThe hydrologic cycle, illustrated in Figure 1, is the movement of water from the atmosphere to the earth’s surface. Water moves through one or more components of the cycle including evaporation, transpiration, runoff, precipitation, infiltration, percolation and its eventual return to the atmosphere.
In an undeveloped area, with natural ground cover such as forest or meadow, a significant portion of precipitation infiltrates into the soil. This water is filtered and cooled as it travels underground. Some infiltrated water is subsequently discharged into rivers and streams as baseflow. Baseflow provides a steady contribution of high quality water to lakes, streams and rivers. Other infiltrated water descends deeper underground to the water table and recharges aquifers. Groundwater recharge replenishes the supply of underground water that can be extracted for domestic use and irrigation. Another portion of precipitation is returned to the atmosphere through a combination of evaporation and plant transpiration called evapotranspiration. Where there is natural ground cover, all of these processes together serve to minimize the percentage of precipitation that becomes runoff, the water that flows over that land surface into streams and other surface water bodies.
Figure 1: The hydrologic cycle (Adapted from "The Physical Environment: An Introduction to Physical Geography")
Stormwater and the Hydrologic CycleUrbanization dramatically affects the hydrologic cycle by altering the relative percentage of precipitation that contributes to groundwater, evapotranspiration, and runoff relative to the natural ground cover. Specifically, urbanization increases runoff by decreasing the amount of water that infiltrates into the ground and is taken up and transpired by plants. This is because water cannot infiltrate into, and plants cannot grow on, impervious surfaces such as pavement and rooftops. Figure 3.1-2 illustrates how watershed imperviousness affects the magnitude of each of the hydrologic cycle components. Increased stormwater runoff not only decreases baseflow and groundwater recharge, but also increases the amount of water that runs off the surface, picking up and carrying pollutants to lakes, streams, rivers and wetlands. The increased surface runoff increases flooding frequency and severity while the increased input of pollutants degrades water quality and aquatic habitat.
Figure 2: Impact of impervious area on the hydrologic cycle fluxes (Adapted from Prince Georges County Department of Environmental Resources Programs and Planning Division. 1999. "Low-Impact Development Design Strategies: An Integrated Design Approach. Department of Environmental Resources", Prince Georges County, Maryland.
Figure 3: Baseflow is groundwater that constantly supplies rivers and streams with high quality water. When imperviousness decreases infiltration to groundwater, baseflow decreases. (Adapted from "Water Resources Management Practicum, 2000, Dam Repair or Removal: A Decision-making Guide.")
HydrographsStormwater hydrographs are plots of runoff discharge versus time. They illustrate a site’s response to a storm event. The highest point on a hydrograph represents the peak flow rate following a storm, and the area under the graph represents the total volume of runoff generated by the storm. Figure 4 shows the significant difference between a pre- and post- development hydrograph, specifically, that development increases the volume, peak flow rate and duration of stormwater runoff following a storm event.
Figure 4: Hydrograph showing site discharge for pre-development and post-development conditions
The increase in impervious surfaces increases the volume of runoff produced because it reduces infiltration, thus reducing baseflow. The impacts of these changes include increased flooding, erosion, channel widening, habitat loss, and streambed erosion.
Table 1: Effects of Imperviousness (Adapted from "Urbanization of Streams: Studies of Hydrologic Impacts", EPA 841-R-97-009, 1997)
The Dane County Erosion Control and Stormwater Management Ordinance sets management standards to attenuate the adverse impacts of stormwater. Specifically, stormwater management practices must be designed and installed at new developments to meet ordinance requirements. Management practices must be designed to maintain predevelopment peak flow for the 2- and 10-year, 24-hour storm events, so that the post-development hydrograph is similar to Figure 5. In order to attenuate the adverse impacts of sediment loading, the ordinance also requires that the stormwater management practices be designed to trap the 5 µm particle for the 1-year, 24-hour storm event.
Figure 5: Hydrograph showing site discharge for pre-development and post-development with detention practices installed
Note from Figure 5 that conventional, stormwater detention practices can affect the timing and magnitude of the peak flow rate, but do not equate the volume of pre- and post- development runoff. This is because these management practices retain water and release it at a peak rate equal to predevelopment conditions, but do not facilitate infiltration and evapotranspiration.
In order to decrease runoff and partially mitigate the adverse impacts of increased imperviousness, the county ordinance requires that a percentage of the average annual predevelopment infiltration (stay-on) be infiltrated. Both residential developments and nonresidential developments must achieve 90 percent of the average annual predevelopment infiltration (stay-on). When more than 2 percent of a site is needed to meet the stay-on performance standard, a performance standard aimed at meeting a recharge goal may be utilized. The recharge standard requires that 100 percent of predevelopment recharge is maintained on an average annual basis. Predevelopment recharge is determined using the Wisconsin Geological and Natural History Survey’s 2009 report, Groundwater Recharge in Dane County, Estimated by a GIS-Based Water Balanced Model (WGNHS Report) or subsequent updates to this report. An in-depth explanation of the county infiltration standards, practices and modeling guidance can be found on the Infiltration page. The county also strongly recommends infiltration practices be used to meet thermal impact standards, where appropriate, since they have the added benefit of decreasing runoff. Finally, site planners should use techniques that minimize imperviousness and reduce runoff, as discussed on the Erosion Control and Stormwater Management Ordinance page.
If all of these techniques are utilized, the volume of post-development runoff will approach the volume of predevelopment runoff, reducing the effects of development on lakes and streams.
Dane County stormwater standards should be met through the most effective, economical, and practical combination of management practices. Selection must be site specific and depends on the site conditions (land use, topography, slope, water table elevation, and geology) and applicable standards (rate, volume, sediment, oil and grease and thermal control).
There are three types of management practices that can be used to attenuate stormwater impacts. Dane County recommends utilization of these three methods in the order listed below:
- Site planning to minimize the volume of runoff originating from the site.
- Nonstructural techniques, including “good housekeeping” practices, to minimize the amount of pollutants that come into contact with runoff.
- Construction and maintenance of structural management practices to capture and treat stormwater runoff.
Incorporating these management techniques into the site planning process requires that project proponents identify the site’s physical characteristics, use models and other analyses to determine if applicable standards are being met, and consider the cost and feasibility of maintaining the proposed management practices.
Figure 6: Dane County stormwater site planning and permitting process
Stormwater Management Standards and RequirementsThe Dane County Erosion Control and Stormwater Management Ordinance requires that all sites needing a stormwater plan and permit install practices that comply with the following standards.
Submitted plans must also satisfy all items on the stormwater management application checklist.
In order to assist in meeting the ordinance requirements, Tables 2 and 3 list practices that could be used to achieve the stormwater performance standards. The table briefly describes where management practices should be used along with maintenance requirements, environmental concerns and any special considerations for the practice. Other practices may be used to meet erosion control or stormwater management standards if first approved by the Dane County LWRD Director.
Table 2: Non-structural stormwater management practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)
|Non-Structural Practices||Applicable Standard||Site Applicability||Maintenance Requirement||Environmental Concerns||Special Consideration|
|Minimizing Impervious Areas||Thermal, Rate Control, Infiltration||Limited application to retrofit sites||Low||None||May reduce improvement costs|
|Native Plants||Infiltration, Rate Control||Widely applicable||Low||None||Careful selection of native species; Requires a cover crop during establishment|
|Parking Lot and Street Sweeping||20% TSS Goal||Widely applicable||Moderate||Sediment and debris collected may be contaminated with heavy metals||Hi-Vac trucks are more efficient|
|Tree Planting||Thermal||Widely applicable (excluding berms and streambanks)||Low||Canopy may shade out ground level vegetation||Careful selection of native species; Size; Proper spacing|
Table 3: Structural stormwater management practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)
|Structural Practices||Applicable Standard||Site Applicability||Maintenance Requirement||Environmental Concerns||Special Consideration|
|Bioretention Basin||80% TSS; 40% TSS; Infiltration; Oil and Grease; Thermal; Rate Control||Widely applicable||Moderate||Potential for groundwater contamination if not designed, sited, constructed and maintained properly||Cost; Use native plus or root stock; contamination from salt; construction timing|
|Dry Basin||80% TSS; 40% TSS; Rate Control||Widely applicable, Larger drainage areas needed||Low to Moderate||Provides less water quality improvement than Wet Basins||Sufficient/suitable land area; Design considerations; Sediment forebay|
|Constructed Wetland||80% TSS||Applicable on sites with medium-fine textured soils; Requires a large drainage area||High||Possible downstream warming, releases nutrients in the fall||Sufficient/suitable land area, Cost; Careful design; Biomass harvesting|
|Gabion||80% TSS; 40% TSS; Stable Outlet||Widely applicable||Low to Moderate||Does not remove smaller suspended solids||Carefully size stone|
|Grassed Swale||Stable Outlet||Widely applicable||Low to Moderate||Restricted use for areas with high pollution potential||Pretreatment; Check dams; Careful design|
|Infiltration Basin||Infiltration; Rate Control; Stable Outlet; Thermal||Moderately restricted to sites with suitable soils; Requires a substantial area to meet standards||Low to Moderate||Potential for groundwater contamination; Restricted use for areas with high pollution potential||Sufficient/suitable land area; Proper construction; Compaction avoidance 80% TSS pretreatment|
|Infiltration Trench||Infiltration; Rate Control; Thermal||Highly restricted to sites with small drainage areas and proper soils; Depth to water table and bedrock; Slopes||High||Potential for groundwater contamination; Restricted use for areas with high pollution potential||Recommended with careful soils evaluation & 80% TSS pretreatment|
|Lined Waterway or Outlet||Stable Outlet||Widely applicable||Low to Moderate||Alters natural cover||Sufficient/suitable land area; Runoff velocities|
|Oil and Grease Filter||Oil and Grease Control||Applicable on small impervious areas||Moderate to High||Limited pollutant removal||Cost and Frequent Maintenance|
|Oil and Grease Separator||None||Applicable on small impervious areas||Moderate to High||Limited pollutant removal, does not remove soluble pollutants||Proprietary device must be approved|
|Permanent Diversion||Stable Outlet||Applicable to vegetated ditches and swales||Moderate||Possible erosion of diversion structure if diverted runoff carries a large sediment load||Must be carefully designed to prevent property damage|
|Pervious Pavement||Infiltration; Thermal; Rate Control||Applicable on areas with very low traffic volumes||Moderate||Potential for groundwater contamination||Limited use in cold climates, Durability, Potential to clog|
|Rain Garden||80% TSS; 40% TSS; Rate Control; Infiltration||Applicable on sites with drainage areas less than 2 acres||Low||Susceptible to clogging||Sufficient/suitable land area, proper soils|
|Stone Check Dam||80% TSS; 40% TSS; Rate Control; Stable Outlet||Applicable to vegetated ditches and swales||Low to Moderate||Does not remove smaller suspended solids||Use clear or washed stone|
|Stone Crib||Thermal||Widely applicable, especially in urban areas||Low to Moderate||Limited effectiveness with large storm events||Needs to be properly sited|
|Stone Outlet Protection||Stable Outlet||Widely applicable||Low||Limited effectiveness with large storm events||Sufficient/suitable land area; Carefully size stone|
|Stone Weeper||Widely applicable to outlets||Applicable to vegetated ditches and swales||Low to Moderate||Does not remove smaller suspended solids||Carefully sized stone|
|Subsurface Drain||Thermal; Rate Control||Widely applicable||Low||Provides limited sediment and pollutant removal||Must have stable outlet|
|Vegetated Buffer Strip||80% TSS; Rate Reduction||Widely applicable||Low||None||Sufficient/suitable land area; Careful selection of species; Must be used in conjunction with other BMPs|
|Wet Basin||80% TSS; 40% TSS; Rate Control||Widely applicable||Low||Possible thermal impacts; low bacteria removal; May attract undesirable wildlife||Sufficient/suitable land area; Design considerations; Sediment forebay|
Sediment Control RequirementsFor new development, the ordinance requires stormwater practices be designed to retain all soil particles greater than 5 microns (80% reduction) for the 1-year, 24-hour storm event.
For redevelopment resulting in exposed surface parking lots and associated traffic areas, the ordinance requires that stormwater practices be designed to retain soil particles greater than 20 microns (40% reduction) for the 1-year, 24-hour storm event.
Although not required by the ordinance, the following goals should be met whenever possible. The design, suggested location, and implementation of proposed practices should be included in the plans.
- For existing development, design practices to retain soil particles greater than 40 microns on the site, resulting from a 1-year, 24-hour storm event.
- For street reconstruction, design practices to retain soil practices greater than 20 microns on the site, resulting from a 1-year, 24-hour storm event.
Oil and Grease ControlThe ordinance requires that all stormwater plans for commercial and industrial developments and all other areas where the potential for oil or grease exists must include practices to treat oil and grease in the first 0.5 inches of runoff. The best available oil and grease removal technology must be used.
Oil and grease removal practices are generally combined with other stormwater runoff management practices and are obtained through commercial sources. Information regarding choice, installation and maintenance of these management practices is best obtained from the manufacturer.
Sites that must control the first half-inch of runoff for oil and grease include:
- vehicle fueling and service areas
- commercial buildings with drive-through areas
- parking lots with more than 40 stalls
- convenience stores
- other areas that are determined to have the potential for oil and grease pollution
Additional guidance and approved treatment practices can be found on the Oil and Grease Control page.
Runoff RateThe ordinance requires that all stormwater facilities be designed, installed and maintained to effectively accomplish the following:
- Maintain predevelopment peak runoff rates for the 1-year, 24-hour storm event (2.49 inches over 24 hours)
- Maintain predevelopment peak runoff rates for the 2-year, 24-hour storm event (2.84 inches over 24 hours)
- Maintain predevelopment peak runoff rates for the 10-year, 24-hour storm event (4.09 inches over 24 hours)
- Maintain predevelopment peak runoff rates for the 100-year, 24-hour storm event (6.66 inches over 24 hours)
The ordinance requirements for water quantity apply to individual sites and not the entire watershed. It is more difficult to control the larger storms with the practices installed on an individual site.
Municipalities may consider large regional facilities, sited as part of municipal and regional stormwater planning, in order to manage stormwater from larger storms.
Determining Runoff Rate Using TR-55Technical Release 55 (TR-55), or Urban Hydrology for Small Watersheds (NRCS 1986), is a model that calculates storm runoff volume, peak rate of discharge, hydrographs (refer to Section 3.1), and storage volumes for stormwater facilities. This model was developed for small watersheds (10 square miles or less), especially urbanizing watersheds, in the United States. A revision was made in June of 1986 that incorporated results of subsequent research and other changes based on experience with the original edition. TR-55 begins with a rainfall amount distributed uniformly over a watershed over a specified time period. Mass rainfall is converted to mass runoff and runoff travel time routed through segments of a watershed is used to create a runoff hydrograph.
The ordinance requires that TR-55 specified curve numbers for land uses must be used in hydrologic calculations. The amount of runoff generated by pervious surfaces depends heavily on the soil type and these surfaces are classified with a hydrologic soil group (HSG). The maximum allowable pre-development runoff curve numbers for hydrologic calculations are presented below. When dual HSG are specified, the drained condition shall be assumed.
|Land Cover||HSG A||HSG B||HSG C||HSG D|
Stable OutletsThe ordinance requires that discharges from new construction sites have a stable outlet capable of carrying designed flow at a non-erosive velocity. Outlet design must consider both flow capacity and duration. This requirement applies to both the site outlet and the ultimate outlet to stormwater conveyance or water body.
Stable outlets are an integral part of well-designed erosion control and stormwater management practices. Stable outlets allow stormwater and erosion control structures to function properly and provide a way for runoff to be discharged without causing damage to downstream properties or water bodies. A stable outlet can be a grassed waterway, vegetated or paved area, grade stabilization structure, underground outlet, rock chute, rock lined channel or stable watercourse.
Stable outlets must have the capacity to handle the designed outflow from the stormwater or erosion control structures they serve. If the outlet is to be vegetated, it should be constructed and established before installation of other stormwater or erosion control structures. Verify that the channel lining is adequate to carry the design to velocity and volume.
Channel LiningTo prevent channels from eroding, an analysis of the channel velocity must be performed to determine the required control practice(s). Where velocities are higher than 5 feet per second or where the channel must carry prolonged flow, the channel should be lined with riprap or other armoring material. Channel linings shall be designed based on the expected channel velocity from the 10-year, 24-hour storm event.
InfiltrationInfiltration reduces runoff volumes and depends on rainfall intensity, slope of the infiltrating surface, the permeability of soils and subsoils, soil moisture, content, vegetation and temperature. During infiltration, water enters from surface storage into soils via the combined effects of gravity and capillary forces. The capillary forces are inversely proportional to the diameter of pores. As the process continues, the pore space becomes filled and the capillary tension decreases. Under saturated conditions, flow is mostly due to gravity.
The ordinance requires that a percentage of the average annual rainfall be infiltrated unless the applicant can demonstrate that the practice is likely to result in groundwater contamination. Infiltration is all precipitation that does not leave the site as surface runoff, and is referred to as “stay-on.” For both residential and non-residential developments, 90 percent of what infiltrated in the predevelopment condition (predevelopment infiltration) must be infiltrated. If more than two percent of a site is needed to meet the infiltration standard, infiltration practices may be alternatively designed to meet an average annual recharge goal determined by the WGNHS Report. If the ordinance requirement is met with the recharge methodology, a minimum of two percent of the site must be dedicated to the infiltration practices.
Thermal StandardsThe ordinance requires that the increase in runoff temperature originating from sites in cold-water community watersheds must be reduced. Affected sites are those located within the watershed of a Class I, Class II, and Class III Trout Streams, as identified in the WI DNR's Trout Stream Maps. These areas can also be identified by turning on the "Thermally Sensitive Areas" layer in the LWRD Viewer.
Figure 7: A watershed is the land area that drains to a common location (typically a water body and its tributaries). For this reason, watershed boundaries are also called drainage divides. Practices to reduce the temperature of runoff must be installed if a site is located anywhere within the watershed of a thermally sensitive water body.
The increase of impervious surfaces in urban areas is a major source of thermal pollution in cold climates and threatens the health of cold-water ecosystems (Galli, 1990). Research shows that the average stream temperature increases directly with the percentage of impervious cover in the watershed. Impervious areas absorb energy from the sun, which causes them to become warmer. As water runs over these areas, it absorbs some of that heat energy and is warmed, causing thermal pollution in lakes, rivers, and streams. Impervious areas also compound the problem by reducing infiltration, which in turn increases the volume of runoff that is created, leading to higher permanent stream temperatures in the summer months.
Stream water temperature is a major limiting factor for cold-water fisheries, as all biological activity is related to temperature. Temperature is a characteristic of water quality and is very important in chemical and biochemical processes, particularly those involving biochemical activity. Higher stream temperatures result in lower dissolved oxygen (DO) concentrations and may cause biological oxygen demand (BOD) to increase. Temperature increases in streams can also result in behavioral changes of fish and macro invertebrate communities (aquatic insects), as these species have specific water temperature preferences and tolerance limits.
Over time, the cumulative impact of individual development sites will increase water temperature, permanently affecting habitat in the stream. By mitigating runoff and water temperature impacts, the stream community will benefit not only from maintained stream temperature, but also from a decline in the amount of sediment, nutrients, and pollution that reaches receiving waters.
Guidance on designing sites in thermally sensitive areas can be found on the thermal mitigation page.
Maintenance RequirementsAll stormwater management practices must include a maintenance plan, which describes the entity responsible for long-term upkeep of the practice and the type of maintenance required. The maintenance plan must be deed recorded prior to permit issuance. The plan should also include accessibility to the site and the level of maintenance required. Long-term maintenance costs should be considered when selecting a practice. Some practices may be inexpensive to implement, but long-term maintenance activities of the practice may be costly. As part of an approved erosion control or stormwater permit, maintenance requirements are enforceable per Section 14.49(8) of the Dane County Erosion Control and Stormwater Management Ordinance.
The county will maintain a database of permitted stormwater practices and will periodically perform inspections to ensure the maintenance requirements set forth in the approved plan are being met.
Additional information can be found on the stormwater maintenance page.