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Stormwater


Stormwater and the Hydrologic Cycle

The Hydrologic Cycle

The 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 Cycle

Urbanization 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.")

Hydrographs

Stormwater 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:

    1. Site planning to minimize the volume of runoff originating from the site.
    2. Nonstructural techniques, including “good housekeeping” practices, to minimize the amount of pollutants that come into contact with runoff.
    3. 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 Requirements

The 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 PracticesApplicable StandardSite ApplicabilityMaintenance RequirementEnvironmental ConcernsSpecial Consideration
Minimizing Impervious AreasThermal, Rate Control, InfiltrationLimited application to retrofit sitesLowNoneMay reduce improvement costs
Native PlantsInfiltration, Rate ControlWidely applicableLowNoneCareful selection of native species; Requires a cover crop during establishment
Parking Lot and Street Sweeping20% TSS GoalWidely applicableModerateSediment and debris collected may be contaminated with heavy metalsHi-Vac trucks are more efficient
Tree PlantingThermalWidely applicable (excluding berms and streambanks)LowCanopy may shade out ground level vegetationCareful selection of native species; Size; Proper spacing

Table 3: Structural stormwater management practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)

Structural PracticesApplicable StandardSite ApplicabilityMaintenance RequirementEnvironmental ConcernsSpecial Consideration
Bioretention Basin80% TSS; 40% TSS; Infiltration; Oil and Grease; Thermal; Rate ControlWidely applicableModeratePotential for groundwater contamination if not designed, sited, constructed and maintained properlyCost; Use native plus or root stock; contamination from salt; construction timing
Dry Basin80% TSS; 40% TSS; Thermal; Rate ControlWidely applicable, Larger drainage areas neededLow to ModerateProvides less water quality improvement than Wet BasinsSufficient/suitable land area; Design considerations; Sediment forebay
Constructed Wetland80% TSSApplicable on sites with medium-fine textured soils; Requires a large drainage areaHighPossible downstream warming, releases nutrients in the fallSufficient/suitable land area, Cost; Careful design; Biomass harvesting
Gabion80% TSS; 40% TSS; Stable OutletWidely applicableLow to ModerateDoes not remove smaller suspended solidsCarefully size stone
Grassed SwaleStable OutletWidely applicableLow to ModerateRestricted use for areas with high pollution potentialPretreatment; Check dams; Careful design
Infiltration BasinInfiltration; Rate Control; Stable Outlet; ThermalModerately restricted to sites with suitable soils; Requires a substantial area to meet standardsLow to ModeratePotential for groundwater contamination; Restricted use for areas with high pollution potentialSufficient/suitable land area; Proper construction; Compaction avoidance 80% TSS pretreatment
Infiltration TrenchInfiltration; Rate Control; ThermalHighly restricted to sites with small drainage areas and proper soils; Depth to water table and bedrock; SlopesHighPotential for groundwater contamination; Restricted use for areas with high pollution potentialRecommended with careful soils evaluation & 80% TSS pretreatment
Lined Waterway or OutletStable OutletWidely applicableLow to ModerateAlters natural coverSufficient/suitable land area; Runoff velocities
Oil and Grease FilterOil and Grease ControlApplicable on small impervious areasModerate to HighLimited pollutant removalCost and Frequent Maintenance
Oil and Grease SeparatorNoneApplicable on small impervious areasModerate to HighLimited pollutant removal, does not remove soluble pollutantsProprietary device must be approved
Permanent DiversionStable OutletApplicable to vegetated ditches and swalesModeratePossible erosion of diversion structure if diverted runoff carries a large sediment loadMust be carefully designed to prevent property damage
Pervious PavementInfiltration; Thermal; Rate ControlApplicable on areas with very low traffic volumesModeratePotential for groundwater contaminationLimited use in cold climates, Durability, Potential to clog
Rain Garden80% TSS; 40% TSS; Rate Control; InfiltrationApplicable on sites with drainage areas less than 2 acresLowSusceptible to cloggingSufficient/suitable land area, proper soils
Stone Check Dam80% TSS; 40% TSS; Rate Control; Stable OutletApplicable to vegetated ditches and swalesLow to ModerateDoes not remove smaller suspended solidsUse clear or washed stone
Stone CribThermalWidely applicable, especially in urban areasLow to ModerateLimited effectiveness with large storm eventsNeeds to be properly sited
Stone Outlet ProtectionStable OutletWidely applicableLowLimited effectiveness with large storm eventsSufficient/suitable land area; Carefully size stone
Stone WeeperWidely applicable to outletsApplicable to vegetated ditches and swalesLow to ModerateDoes not remove smaller suspended solidsCarefully sized stone
Subsurface DrainThermal; Rate ControlWidely applicableLowProvides limited sediment and pollutant removalMust have stable outlet
Vegetated Buffer Strip80% TSS; Rate ReductionWidely applicableLowNoneSufficient/suitable land area; Careful selection of species; Must be used in conjunction with other BMPs
Wet Basin80% TSS; 40% TSS; Rate ControlWidely applicableLowPossible thermal impacts; low bacteria removal; May attract undesirable wildlifeSufficient/suitable land area; Design considerations; Sediment forebay

Sediment Control Requirements

For 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 Control

The 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-though 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 Rate

The 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-55

Technical 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 CoverHSG AHSG BHSG CHSG D
Woodland30557077
Grassland39617178
Cropland51687883
Calculation of post-development runoff must account for changes in permeability class due to the soil characteristics and site compaction. Areas with high equipment traffic shall be considered heavily disturbed. Areas with limited equipment traffic will be considered lightly disturbed. Developers are required to lower one permeability class for all hydrologic calculations, unless practices such as deep tilling, chisel plowing, and incorporating organic matter into the upper soil surface have successfully restored soil structure.

Stable Outlets

The 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 Lining

To 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.

Infiltration

Infiltration 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 Control

Thermal Standards

The ordinance requires that the increase in runoff temperature originating from sites in cold-water community watersheds must be reduced, unless results of a thermal impact model approved by the Dane County LWRD Director show that the temperature increase of post-development runoff from the site will be zero.

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.

Thermal Consideration

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.

Thermal Model Description

One model that can be used to estimate thermal impacts is the Thermal Urban Runoff Model (TURM) (Norman, J.M. and A. Roa. 2000. Effects of the Natural Environment and Urban Runoff on Stream Temperatures). The University of Wisconsin and the Dane County Land Conservation Division developed the model to estimate runoff temperature from urban watersheds. The thermal impact from impervious areas was documented in a study at Token Creek subwatershed, where collected data was compared to the results calculated by TURM. This model accounts for the fact that storm water not only absorbs heat from impervious surfaces, but that it also cools these surfaces, reducing the ability of the impervious surfaces to heat runoff from additional rainfall. However, TURM does not account for the inherent variability of rainfall due to changes in intensity and the type of storm, as the model assumes that the rainfall is uniform over the entire duration of the event. Field data collected at Token Creek subwatershed indicates that storm water runoff from highly urbanized areas has the potential to increase the temperature of receiving waters by as much as 23° F (Roa, A., J.M. Norman, T.B. Wilson, and K. Johnson. 2002. Thermal Impact Analysis of Token Creek Subwatershed and Validation of Temperature Urban Model (TURM)).

Other model considerations include:

  1. the amount and temperature of impervious area;
  2. the ambient air temperature;
  3. the gain or loss of heat through the passage of water through management practices;
  4. the net change in heat due to tree canopy;
  5. the heat loss through evaporation;
  6. the time and duration of storm events, and;
  7. the difference in the time of concentration of vegetated areas and impervious surfaces.

Other thermal impact models may be used if they are approved by the Dane County LWRD Director.


Maintenance Requirements

All 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.