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TC Res. No. 1980-13i • RESOLUTION NO. 80-13 SERIES OF 1980 BE IT RESOLVED that all plans, including plans for drainage, for structures of five units or more or any commercial structure submitted to the Avon Design Review Board also be submitted to the Town's engineer at least 9 days in advance of submittal of the same to the Design Review Board. BE IT FURTHER RESOLVED that the criteria contained in Claycomb Engineering Associates, Inc.'s letter of July 3, 1980, on file herein, are adopted as temporary regulations of the Town with respect to drainage and urban runoff. ,,D INTRODUCED, READ, AND ADOPTED THIS g_`' DAY OF JULY, 1980. ATTEST: 9=2~ TOWN OF AVON CLAYCON* ENGINEERING ASSOCIATES, INC. July 3, 1980 Town of Avon P.O. Box D Avon, CO 81620 ATTN: Mayor Alpi and Council Members RE: Drainage Criteria Job No. 1776.015 Dear Mayor Alpi and Council: 0 • ~~%G ENe A a w c, cooNC~` We have developed some base criteria for handling drainage and urban runoff in the Town of Avon that should be beneficial to the long term interests of the Town. These criteria, if approved by yourselves, will be presented in final form in the completed drainage study. We should set the guidelines for drainage policy and pollution control even before completion of the drainage study. As you know, Avon Center, Wingspread, Peregrine and numerous other large projects are currently in the Town review process. As soon as the Town sets the basic outline of policy for drainage, we can work with the developers during the review process. In this way, the developers can be made aware of the Town's criteria early in the review process, and plan submittals accordingly. Our initial recommendations cover four_ areas: I Design Criteria II Ownership and Maintenance III Enforcement IV Construction Phasing I. Design Criteria A) Urban Pollution Control. All of the larger multi- family projects should incorporate detention facilities to allow oils, sands, greases, animal wastes and the like from paved areas to be caught before discharging the runoff. In addition, the outflow rate must be less than inflow to compen- sate for the large percentage of impervious area in the development and the high runoff rates. SUITE 207 VILLAGE PLAZA GLENWOOD SPRINGS. COLORADO 81601 303-945-8676 Town of Avon Page 2 July 3, 1980 Specific recommendations for detention facilities are as follows: Number of Units. All parcels larger than four- plex should submit a specific detention plan. Sizin . Detention ponds should be sized to take into account both snowmelt and rainfall runoff. Snowmelt occurs over a period of several months, with a near constant flow rate during that time. Rainfall runoff occurs at intervals, at greater intensities. The detention pond should provide a base volume of four hours detention for snowmelt, plus an additional surcharge volume for rainfall. The 10 year event should be the basis for sizing both rainfall and snowmelt capacities. Release Rate. Since snowmelt is a constant flow over a relatively long time, the snowmelt release rate from a detention pond will be the same as the inflow. The rainfall release rate should be such that a four hour average deten- tion time is achieved for a 10 year recurrance interval runoff. Grease Traps'. All ponds should incorporate some type of skimming baffle or grease trap to prevent oils from reaching the receiving waters. At its simplest, the device can be a baffle around the outlet pipe: At.the developer's option, the grease trap can be placed at the pond inlet. This location would avoid the aesthetic problem of oils floating in the pond surface, but may be more difficult to maintain. Maintainability. All ponds should be maintainable by machine. Small ones should be designed narrow enough so that conventional excavating equipment can reach in and scoop out accumulated silt and sand. Larger ones should be provided with an access ramp and a surfaced bottom so that machinery can enter the pond to clean it without becoming mired. Where space will not allow a detention pond, a filter curtain should be provided. This device would be a trench backfilled with crushed rock, Town of Avon Page 3 July 3, 1980 with sand or other pervious surfacing on top. Urban runoff flows along the trench and enough ponding occurs to allow water to infiltrate down through the trench. Oils and other pollutants are retained on the surface. Periodically the surface layer (with pollutants) needs to be scraped off and replaced. The filter curtains or drains can be accommodated to the site in many ways: i.e. parking lot medians, borders and landscaped areas. B) General Drainage. The main requirements under this area are: Culvert Sizing. All roadside culverts should be capable of passing the 10 year storm event (rainfall plus snowmelt). No culvert should be smaller than 18 inches. 100 Year Flood. In general, it is not necessary to size roadside and cross-road culverts for the 100 year flood. The site grading should be de- signed so that no major structural damage occurs as a result of the 100 year flood. Flow along the roadway itself may be utilized to take care of conveying the 100 year flood. Major structures (i.e. bridges, large box culverts) should be de- signed to accommodate the 100 year flood. Similarly, any diversion swales around Nottingham Lake (for urban runoff) should be able to carry the 100 year runoff from their tributary areas. II. Ownership and Maintenance Detention facilities should remain in the ownership of the developer or the homeowner's association. Regular maintenance is imperative, otherwise these facilities become useless eyesores. To accomplish that, standardized language should be inserted into each subdivision agree- ment, or made a condition of Design Review Board approval. The gist of the statement would require the homeowner's association to clean and maintain the facilities at regular intervals to the satisfaction of the Town (i.e. the Build- ing Department, the Town Engineer). If not done, the Town would have the power to clean out the facilities with its own forces or on contract, at the homeowner's expense. Town of Avon Page 4 July 3, 1980 III. Enforcement. Once the initial requlations have been reviewed, modified as necessary and approved by the Town, a specific ordinance amending the subdivision regulations should be enacted. This ordinance would set forth the drainage policy of the Town, and outline specific design criteria to be followed in the submittal of subdivisions for approval. IV. Construction Phasing. The construction stage is itself critical for pollution control. Any regulations adopted by the Town should include the following: A) Construction of detention ponds to be done before anything else on site, and all construction runoff diverted through them. The ponds need not be in final form. B) Use of hay bales and other measures to prevent silt-laden runoff form reaching streams, irrigation ditches or Nottingham Lake. We would like to review these items with Council at the July 8, 1980 work session. Very truly yours, CLAYCO ENG EERING ASSOCIATES, INC. n Gannett, P. E. Proj ct Manager WG/njh cc: All Council Members