City of Sheridan regulations are compliant with the Wyoming State Stormwater Management Program requirements for the city's small municipal separate storm sewer system (MS4) permit, and with those of Sheridan County.
"The street maintenance division maintains the City of Sheridan’s public roadway network and storm water drainage system. Their work is essential to public safety."
This manual shall apply to all development, redevelopment and construction activities on public and private property within the City of Sheridan. Additional guidance on the City of Sheridan development process is provided in the Developers Handbook provided on the City of Sheridan website. The purpose of this Manual is to provide the minimum standards to be used for the analysis and design of storm drainage systems for private development projects and City contracted projects within the City of Sheridan. This manual provides direction and guidance to allow responsible development in and around the City of Sheridan while improving water quality within the Goose Creek drainage. This manual provides guidance for the policy, design and permitting process to address stormwater runoff and treatment from proposed development and land disturbance. Design criteria and methods other than those described in these design standards shall be applied only after requesting and receiving approval from the City Public Works Department.
The following sections provide the requirements for stormwater management related to various types of construction and development activities within the City of Sheridan. Detailed design criteria for stormwater management facilities are provided in subsequent chapters. The rate and volume of stormwater runoff from proposed land developments shall be estimated in accordance with this Manual and shall be the foundation of the Storm Drainage Plan. Storm Drainage Plans shall be prepared by a Professional Engineer licensed in the State of Wyoming and shall be submitted to the City Engineer’s Office for review and approval.
The requirements of the Storm Drainage Plan vary based on the classification of the project. The City of Sheridan has classified construction and development projects into five categories: 1) City Contracted Projects; 2) One to Two-Family Residential Lot Developments; 3) Commercial and Multi-Family Residential Developments; 4) Subdivisions and Planned Unit Developments; and 5) Fully-developed Areas/Redevelopment. The following sections describe and summarize each classification and the associated stormwater requirements.
Stormwater runoff from the project development shall produce no adverse impact to downstream properties. Where practicable, the site shall be designed such that runoff rates and drainage patterns following development shall be the same as those which existed prior to development. A change from unconcentrated sheet flow to concentrated flow constitutes a change in the drainage pattern.
Natural drainages, such as depressions, swales, ditches, drains, channels, etc. shall be preserved to the maximum extent possible. If preservation of existing drainages is not possible, Developer shall provide adequate accommodations ensuring flows through natural drainages are properly mitigated.
When discharge from the site will be concentrated in comparison to pre-development conditions, energy dissipation or erosion control measures shall be employed to mitigate the increased potential for erosion.
When the runoff rate or location of discharge from the site will be changed by a proposed development, in comparison to pre-development conditions, a downstream offsite capacity analysis shall be required.
A physical inspection of the existing on-site and off-site drainage system shall be performed to identify any existing or anticipated future problems. The analysis must extend from the proposed project discharge location to the point downstream where the site runoff would join the main drainage course. The makeup and general condition of the drainage system shall be investigated including collecting such information as pipe sizes, channel characteristics, drainage structures, and evidence of existing or anticipated problems.
At each location with an existing or anticipated drainage problem, develop runoff hydrographs or Rational Method peak flow rates for the major (100-yr) storm event for the total composite drainage area tributary to that location under existing conditions and the conditions that will exist following the proposed development. Determine the capacity of the existing drainage system and evaluate impacts of adding the peak runoff from the proposed project site to the peak runoff from the total composite drainage area tributary to these locations.
For any potential off-site problem resulting from the development or redevelopment, the Developer must demonstrate that the proposed project has been designed to mitigate the anticipated problem.
As an alternative, the Developer, with approval by the City, may arrange with the owners of the off-site properties to install measures which will mitigate the anticipated problem.
In some cases, anticipated public drainage system problems may already be scheduled for correction by the City. The Developer should contact the City Engineer’s Office to determine current capital improvement project schedules. Provide information with the Drainage Report to document the capacity of the downstream drainage system and to illustrate that potential impacts have been adequately mitigated.
Where the development will result in a change in the rate of runoff or location of discharge, in comparison to pre-development conditions and no downstream drainage system exists adjacent to the property, the downstream drainage system shall be extended up to the property line and all runoff from the property shall be conveyed across the downstream properties to an approved discharge location. The Developer shall secure drainage easements from the downstream property owners and record such easements prior to drainage plan approval. If the Developer demonstrates that easements are not reasonably obtainable as determined by the City, then all runoff shall be conveyed to an on-site retention system.
Non-stormwater discharge (illicit discharge) is prohibited from entering the storm drain system. This includes groundwater, wash water, interior building drainage water, irrigation water, etc. There exists the potential for irrigation/drain ditches to overtop during rain events, resulting in flooding to adjacent properties. Developers shall mitigate for this potential by requiring flood proofing of buildings, establishing appropriate building elevations, constructing overflow channels, developing appropriate site grading, or employing other measures as appropriate. In addition, there exists the potential for adjacent irrigation/drain ditches to leak, contributing to seasonally high groundwater conditions within the development. Developers shall mitigate for this potential through the construction of appropriate groundwater drainage systems. The degree of improvements required will depend on the character of the adjacent ditch and the proximity to the development. Irrigation ditches shall not be used as an outfall for stormwater discharge. Exceptions, if granted, shall require approval from the ditch company and the City of Sheridan. A Development Agreement between the City and the developer/property owner shall be required for all developments involving public infrastructure. This includes installation of new public infrastructure as well as extension and/or replacement of exiting public infrastructure.
The activities listed below are considered to be “maintenance” and are therefore not governed by the requirements of this manual. Exclusion from these stormwater management requirements does not relieve the development of other required permits and submittals. Contact the City Engineer’s Office to determine what (if any) permits or submittals will be required.
Private drainage system connections to the public storm drain system shall be approved by the City of Sheridan and shall comply with the following criteria. Such connections shall be entirely owned and maintained to the storm drain main by the Development that the connection services. Stormwater runoff from private drainage systems shall be controlled in accordance with the requirements of Chapter 2 of this manual. Connections shall only be made with approval from the City Engineer’s Office and if it is deemed to be in the best interest of the City. Private connections to the City storm drain shall be made by the following (in order of preference). Connecting the discharge pipeline to an existing manhole or catch basin; or Constructing a new manhole or catch basin on the existing storm drain main and connecting the discharge pipeline to this new structure. Minimum pipe diameter discharging to the City’s storm drain system shall be 6 inches with a minimum slope of 1 percent to provide adequate scour velocity. The maximum pipe diameter allowed will depend on an evaluation of the capacity of the City’s storm drain system and approval from the City Engineer’s Office. Private stormwater connections shall include backflow prevention to prevent stormwater from the City’s storm drain system from surcharging onto private property. Backflow preventer must be installed outside the public right-of-way.
Storm drain outfall facilities shall be accessible for operation and maintenance. When vehicle access is necessary, for facilities constructed outside of the street section, access roads shall be provided in dedicated access easements. The minimum clear driving lane width of access roads is 12 feet. Access roads shall have a maximum grade of nine percent and shall be constructed with gravel, pavement, concrete or an appropriate all-season surface. Gates and/or bollards are required when necessary to restrict access to stormwater facilities. Cables and/or chains stretched across access roads are not acceptable.
Detention basins can be designed as a standalone facility, also known as a dry basin, where runoff is routed over time until the basin completely drains out. They can also be designed to “stack” on top of water quality facilities such as retention/infiltration basins or wet basins where temporarily stored runoff will drain down to the original wet basin water surface elevation or to the top of the retention/infiltration basin designed to store the water quality volume (WQV). Further details of these water quality facilities are provided in Chapter 8. Detention basins shall be designed to limit the runoff from the site to pre-development rates for the full range of potential storms including the 2-year, 10-year, and 100-year events. Detention storage is not required for sites less than 10,000 square feet in area or where site improvements result in less than 5,000 square feet of impervious surface. LID techniques are however, recommended for these sites. Design of detention basins shall follow the procedures presented in Chapter 8 of the HEC-22 Manual, as modified herein.
Detention basins shall be landscaped to provide for slope stability, erosion control, and low maintenance. Landscape materials shall be compatible with use in a stormwater detention facility and associated water quality treatment facility. Utilize plant species native to the Sheridan area to the maximum extent practicable. In general, basins shall be irrigated and either seeded or installed with sod to provide an amenity to the community. Floatable or erodible material (i.e., wood chips, straw mulch, etc.) shall not be allowed within the basin. The interiors of the basin shall be stabilized with gravel, rock, and vegetation. Vegetation on basin embankments shall be limited to shallow rooted varieties. Points of inflow to the basin shall be armored to prevent erosion.
Maintenance shall be performed by the HOA or commercial site owner, unless this responsibility is accepted by the City. Further details are provided in Appendices E and F. Maintenance will be required to remove invasive plants and debris accumulated at inlet and outlet structures. Stormwater detention basin outlet control structures shall be accessible for maintenance and operation. When vehicle access is necessary, access roads shall be provided in dedicated access easements or right-of-way of at least 20 feet wide. The minimum clear driving width of access roads shall be 12 feet and the minimum turn-around radius shall be 25 feet or hammerhead. Access roads shall have a maximum grade of nine percent and shall be constructed with gravel, pavement, concrete or an appropriate all-season surface. Gates and/or bollards are required when necessary to restrict access to stormwater facilities. Cables and/or chains stretched across access roads are not acceptable. Access roads shall be maintained clear at all times to allow for maintenance access.
Retention/Infiltration Systems are generally not acceptable for disposing of stormwater runoff and the Developer shall demonstrate that this is the only feasible alternative available to provide drainage. Exceptions shall be pre-approved by the City Public Works Department. If a deviation for an infiltration system is approved, the system shall meet the requirements set forth in this section. Retention/Infiltration facilities are used to fully retain the site runoff volume where no viable outfall exists and shall be designed to fully store the post-development site runoff from the Major Storm event (100-year, 24-hour storm). The open water surface of the retention facility shall return to the pre-storm level within 72-hours after cessation of the Major Storm. Retention basins and infiltration basins are very similar in function and consist of a basin with the only means of emptying being through evaporation, evapotranspiration and infiltration. Retention and Infiltration basins are used to control runoff, but are also used to provide water quality treatment by filtration into the soil. This section discusses the design criteria for retention/infiltration basins for both runoff control and water quality treatment of the water quality volume (WQV). Further details of the water quality design are discussed in Chapter 8. Retention/infiltration basins can either be constructed as open systems or subsurface systems. Open systems typically include excavation of a basin designed to slowly infiltrate the collected runoff into the underlying soil. Subsurface retention/infiltration basins take the form of boulder pits, vault systems, and injection wells which are typically used on sites with limited available space. Retention storage is not required for sites less than 10,000 square feet in area or where site improvements result in less than 5,000 square feet of impervious surface. LID techniques are however, recommended for these sites.
Maintenance shall be performed by the HOA or commercial site owner, unless this responsibility is accepted by the City. Further details are provided in Appendices E and F. Infiltration facilities shall be accessible for operation and maintenance. When vehicle access is necessary, for facilities constructed outside of the street section, access roads shall be provided in dedicated access easements of 20-foot minimum width. The minimum clear driving width of access roads shall be 12 feet and the minimum turn-around radius shall be 25 feet or hammerhead. Access roads shall have a maximum grade of nine percent and shall be constructed with gravel, pavement, concrete or an appropriate all-season surface. Approval of retention/infiltration systems shall obligate the owner to repair, replace, or reconstruct the system if it fails to operate as designed. The maintenance and operation schedule for retention/infiltration systems shall include such a provision. Failure to maintain retention/infiltration systems will be subject to the terms set forth in the Development Agreement for the subdivision or commercial site.
Boulevard swales shall be protected from high sediment loads during construction and untilvegetation is established in the swale and on adjacent areas contributing runoff. This may require periodic cleaning of the boulevard swale until vegetation is fully established. Maintenance for swales and associated culverts shall be addressed in the HOA or by the adjacent property owner and shall follow the requirements of Appendix E and F and the Development Agreement. Maintenance requirements shall address vegetation heights, mowing, watering, fertilizing and frequency for sediment removal and erosion repairs. If access from the adjacent public right-of-way is not available, an access easement to the swale shall be provided to facilitate inspection, monitoring, and maintenance. Appropriate access shall be considered when accounting for maintenance of culverts within the swale. Failure to maintain boulevard swales will be subject to the terms set forth in the Development Agreement.