Full Text
RESOLUTION NO. 99-17 A RESOLUTION ADOPTING CITY OF MOSCOW STORMWATER RUNOFF CONTROL STANDARDS AS GUIDANCE RELATED TO THE MOSCOW STORMWATER RUNOFF CONTROL ORDINANCE CONTAINED IN MOSCOW CITY CODE, CHAPTER 15, TITLE 5 AND MOSCOW CITY CODE SECTION 5-l-5(H) WHEREAS, City of Moscow, Idaho, has determined that it is in the hest interests of the City to adopt Moscow City Code Title 5, Chapter 1 5, which contains the Moscow Stormwater Rnnoff Control Ordinance and Moscow City Code Section 5-l-5(H) which requires stormwater runoff control in subdivisions; and WHEREAS, certain standards must apply to stormwater runoff control practices to promote consistency; and WHEREAS, the general purpose of this Resolution is to establish stormwater runoff control practices to limit the peak rate of stormwater discharge from new developments and subdivisions; and WHEREAS, the information, practices, and standards included in the Moscow Stormwater Runoff Control Standards are commonly accepted within the stormwater runoff control area and have been adapted to those areas which are under the purview of the City; and WHEREAS, the information, practices, and standards contained within the Moscow Stormwatcr Runoff Control Standards and in this Resolution are appropriate and desirable; NOW, THEREFORE, BE IT RESOLVED BY THE CITY COUNCIL OF THE CITY OF MOSCOW that the attached City of Moscow Stormwater Runoff Control Standards are hereby adopted as guidance for City of Moscow, Idaho, to be applied to Title 5, Chapter 15 of the Moscow City Code, as amended and to Moscow City Code Section 5-1-l as amended. ADOPTED THISL7 day RESOLUTION Page I ---PAGE BREAK--- MOSCOW STORMWATER RUNOFF CONTROL STANDARDS FOREWORD The objective of these standards is to set forth engineering design requirements and guidelines for Moscow's Stormwater Runoff Control Ordinance. The standards established herein are intended to represent the minimum; compliance with these standards does not relieve the designer of the responsibility to apply sound professional judgment of engineering practices relating to this subject. Special site conditions and environmental constraints may require runoff protection greater than would be required under these standards. Stormwater runoff control facilities shall be designed and constructed for 'zero net increase' in the rate of stormwater runoff in accordance with the ordinance and as set forth herein. GENERAL 1 . Engineering related requirements, verbatim, per City Code: a. All stormwater runoff control facilities shall be designed by a person duly licensed by the State of Idaho to design such a facility. Complete plans, stamped and sealed by a person duly licensed by the State of Idaho to design such facility, shall be submitted to the City Engineer, along with all reports and necessary calculations, for review and approval prior to installation. b. All methods and designs of stormwater runoff control facilities shall clearly demonstrate to the satisfaction of the City Engineer zero net increase runoff from the property. c. Any and all stormwater runoff from the property shall be diverted through a system of drainage ditches, drainage ways, pipes, culverts, and other such appropriate appurtenances in a manner which prevents and/or eliminates flooding and in a manner which minimizes erosion which may be caused by stormwater runoff. d. The design and installation of stormwater runoff control facilities shall be in accordance with the requirements contained within the 'Moscow Stormwater Runoff Control Standards' as adopted by Council from time to time. 2. Under no circumstances shall drainage be diverted in the proposed development and released to a property at points not receiving drainage prior to the proposed development. Flow may not be concentrated onto properties where sheet flow previously existed. 3. A Drainage Report (including computations), Construction Plans, and an Operation and Maintenance Plan shall be submitted to the City Engineer for review and approval. A print of the proposed final plat (if a subdivision) or final site plan (if non-residential) and the public utilities design drawings shall be submitted with the Drainage Report and Stormwater Runoff Control Facilities construction plans. 4. Detention basins are the most desirable stormwater runoff control facilities. 5. The plans and design of stormwater runoff control facilities for subdivisions must be approved by the City Engineer prior to his signing the final plat; the plans and design for all other developments must be approved by the City Engineer prior to the City issuing a building permit. 6. For the purpose of determining pre-development rate of stormwater runoff, as required herein and in accordance with the Ordinance, the pre-development condition of the property shall be considered that which existed at the time the Ordinance was adopted. 7. Design for detention basins which utilize parkland will be considered. P10SCOW STOR:-.1WATER Rt'KOFF CONTROl_, STANDARDS Pagel of 7 ---PAGE BREAK--- 8. Any viable design which substantially complies to the Moscow Stonmwater Runoff Control Standards will be considered for approval by the City Engineer. ENGINEERING SUBMITTAL 1. Drainage Report, including computations: a. The Drainage Report is to be inclusive, clear, legible, and reproducible, with a complete set of drainage computations. The computations are to be presented in a rational fonmat with infonmation included so as to allow a reviewer to be able to reproduce the same results. b. Copies of any design charts, hydrographs or other design aids used in the analysis shall be included. All assumptions and computer input and output data, and variables listed in the computer printouts shall be clearly identified. c. All relevant geotechnical infonmation related to the project, and all site specific soillogs and subsurface testing infonmation shall be included. d. Computations shall include as a minimum (as applicable) the following: 1. Hydrologic calculations; 2. Inflow and outflow hydrographs; 3. Level-pool routing calculations; 4. Listing of maximum water surface elevation; 5. Pond volume rating table; 6. Discharge rating table; 7. discharge analysis; 8. Design storm events; 9. Detention capacities; 10. Outflow structure capacities; 11 . Drainage conveyance system capacities; 12. Impervious surfaces summary. e. The Drainage Report shall include a basin map which shows as a minimum the following: 1. Site boundary; 2. Basin limits both on-site and off-site areas which contribute or receive stonmwater runoff onto or from the project; 3. Drainage sub-basins; 4. Topographic contours max.), which shall extend beyond the project or drainage basin boundaries to the extent necessary to confinm basin limrrs used in the calculations; 5. Significant drainage features, natural or manmade, such as creeks, drainage channels, ditches, depressions, manholes, catch basins, etc.; 6. Groundwater table elevations; 7. Time of concentration routes, clearly labeled and correlated with the calculations; 8. Footprint of proposed drainage features; 9. Floodplain limits; 10. North arrow and scale; 11. Wetlands; 12. Environmentally sensitive areas within and adjacent; 13. Easements; 14. Impervious surfaces summary. f. For any existing and/or potential off-site drainage problems of concern, the design engineer shall demonstrate that the proposed project engineering plan has been designed so that it neither aggravates an existing problem nor creates a new drainage problem. Aggravation of the problem can include increasing the magnitude, frequency, or duration of the problem. MOSCOW STORM\VATER Rl:"OFF CONTROL STA '\"DARDS Page 2 of 7 ---PAGE BREAK--- 2. Construction Plans: a. Construction plans shall include as a minimum, and as applicable, the following: 1. Existing contours (2 foot contours with reference datum); 2. Proposed contours (2 foot contours, 1 foot in the area of any detention basin); 3. A cross-section and/or elevation view of the proposed facilrty which shows complete elevation data and facility design detail; 4. Planimetric location of all proposed improvements with respect to the property; 5. All proposed and existing drainage conveyance appurtenances/accessories, such as ditches, basins, structures, pipes, inlets, outlets, swales, etc.; 6. Temporary erosion control and permanent slope stabilization methods for the disturbed areas in and around the facility; 7. Profile grades and elevations; 8. Ditch and pipe centertine locations, dimensioned, and ftow directions; 9. Facility access location and design; 10. All easements and rights-of-way required for the facility; 11. Complete detail of the outflow control structure, including all construction specifications; 12. Flood plain elevation data; 13. The location, planimetrically, of the route of the discharged runoff - as a minimum, to an existing drainage facility such as a ditch, storm drain system, creek, etc. that has prior to development received the site's runoff; 14. 'Complete' elevation data; 15. Earth material and compaction requirements; 16. Any significant feature(s) which may influence the facility's performance. b. Four sets of prints and one mylar set of the final approved plans shall be furnished to the City Engineer's office - immediately following their approval. DESIGN REQUIREMENTS 1 . Stormwater runoff control facilities shall be designed to permit a post-development rate of discharge rate which does not exceed the pre-developed discharge rate for all durations of a 25 year storm frequency. Under no condition shall the facility discharge at a rate greater than the pre-development rate where rainfall intensity equals 0.50 inches per hour. 2. The facilrry shall be provided with an emergency overftow structure designed to accommodate the runoff resulting from a 100 year, 24 hour storm event generated under developed conditions. This extreme storm condition is required to account for the seasonal variations that are not included in the hydrologic textbook computations such as high groundwater, frozen ground, and snow melt. 3. All stormwater runoff control facilities shall be designed to include an emergency overftow directing excess flow to the conveyance system. This assumes the facility will be full due to plugged orifices or high inftows and is intended to protect against breaching of the system. 4. Acceptable hydrologic analysis methods are the Rational Method, the SCS TR-55 Method, the Files Method, and the HSPF Model Method. Other methods will be considered. The factor used in the Rational Method shall be as shown in Table 1 herein. 5. Rainfall intensity for design shall be as per the Moscow area intensity-duration-frequency chart included in this Standard. 6. Off-site stormwater runoff passing through the site shall be conveyed by hydraulically adequate conveyance systems. Off-site surface water entering the site shall be received and discharged at the existing locations with adequate energy dissipaters to minimize damage. There shall be no diversion at either of these points unless specifically approved otherwise. MOSCOW STOR\.lWATER R\Jl'OFF CONTROL S'L'cNDARDS Page 3 of 7 ---PAGE BREAK--- 7. Design of stormwater runoff control facilities shall include provisions for efficient and accessible maintenance, public safety, and general aesthetics. 8. Detention basins are the generally accepted means of controlling stormwater in Moscow. Other systems will be reviewed and considered. 9. Infiltration systems, evaporative systems, and closed depression type of stormwater runoff control systems are strongly discouraged. 10. Outfalls shall include provisions for energy dissipation. 11. Stormwater runoff control facilities shall not be located in dedicated public road right-of-way. Access roads for facilities to be maintained by the City shall have 20 foot wide access easements dedicated to the City. The Design Engineer shall furnish any necessary easement legal descriptions to the City Engineering Department and the City will prepare the easement document(s) for owner signature. Easements must be secured prior to final system design approval. 12. Detention basin construction shall generally conform to the following minimum requirements: a. Pond bottoms shall be level and be located a minimum of 0.5 feet below the inlet and outlet; b. Exterior side slopes shall be no steeper than 3H:1V. interior side slopes shall be no steeper than 4H:1V; c. Basins with berms greater than 4 feet in height shall include a keyway and a clay or geotextile liner; d. The minimum top of berm width shall be 10 feet, level; e. A minimum of 6 inches of compacted 3/4 crushed basalt aggregate on the surface of the berm's top; f. the berms shall be constructed from locally available loess or clay. Minimum compaction shall be 95% of AASHTO T-99; g. 1.5 foot minimum freeboard is required, measured from the gravel surface of the top of the berm; h. Maximum stormwater storage depth = 8 feet; i. The base of the berm shall be firm, unyielding, compacted subgrade, free of organics; j. Short type grasses shall be seeded, mulched and established on all surfaces of the basin - interior, exterior, complete, within the confines of the site. The developer is responsible for assuring these grasses 'establish'; k. Access shall be available to all detention basins. Those to be maintained by the City shall meet the following minimum requirements: 1. Maximum grade not to exceed 15%; 2. 12 foot minimum traveled way with 1 foot minimum shoulders; 3. Traveled way surfacing shall consist of 6 inches minimum of compacted 3/4 crushed basalt with geotextile separating it from subgrade; 4. 40 foot minimum outside turning radius; 5. Shall originate from a City street- with curb drops, as applicable. I. Erosion control measures for all new earthwork must be included for protection until ground cover is established; m. Shall not take longer than 48 hours to completely drain following a storm event; n. A setback of 5 feet from the toe of the exterior slope to the easement or property line is required for City-maintained basins. 13. The effects of sediment shall be considered and addressed in the design, operation and maintenance of the system. 14. Fencing may be required where public safety is determined to be a concern. MOSCOW STORMW ATER Rl');OFF COl\TROL STANDARDS Page 4 of 7 ---PAGE BREAK--- OPERATION AND MAINTENANCE PLAN 1. For all stormwater runoff control facilities that are to be operated and maintained by the City a thorough, neatly prepared Operation and Maintenance Plan shall be submitted to the City for review and approval by the City Engineer. Said 0 & M Plan shall include as a minimum the following: a. A description of 0 & M work required; b. 0 & M work frequency I schedule; c. Expected design life of the various components; d. An exhibit of the drainage facilities applicable to the 0 & M Plan; e. Recommendations for necessary/anticipated replacements or renovations. 2. Once approved 4 copies of the 0 & M Plan shall be furnished to the City Engineer. 3. For all stormwater runoff control facilities that are to be operated and maintained by the property owner or developer an 0 & M Plan shall be submitted in accordance with the above requirements. Submit only one copy of the approved plan - to be kept on file at the office of the City Engineer. MOSCOW STORMWATER RU;-.;:OFF CO:-.:TROL STAc'iDARDS Page 5 of 7 ---PAGE BREAK--- RUNOFF COEFFICIENTS FOR THE RATIONAL FORMULA Type of Cover Cultivated Cropland Lawns, Pasture, Turf Wooded, Brushy Gravel Residential Single Family/Duplex Multi-family, detached Multi-family, attached Commercial, Industrial, Institutional Heavy Light Pavement & Roofs Bare, resculpured ground 110SCOW STORMW ATER RlQOFF CO)