12-11-2000 Council Packet - Laserfiche WebLink

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h t' K L requirements of MCWD Rule N - Stormwater Management. For a project of this nature, Rule N requires the developer to Incorporate BMP's for stormwater management. The project is less than 8 acres and is a low-density residential development. Therefore, the developer is not required to manage peak runoff rates or meet water quality standards in the rule. However, the developer has volu ntarily incorporated two wet-detention ponds into the project and has designed the grading of the site to attenuate and reduce peak rates of runoff to levels below that which exist now. In addition, the wet-detention ponds exceed the NURP volume required to reasonably expect 50% removal of nutrients and higher leveic of removal of sediment and other pollutants that partition themselves with the sediment. The wet-detention ponds also include temporary storage capacity for runoff events. The outlets are hydraulically restrictive with 4- Inch diameter outlets. Tiiese outlets are extremely small and are intended to mitigate and alleviate downstream drainage issues that exist independently of the project. Both ponds are designed with emergency controlled overflows in the event an extreme runoff event occurs. Lastly, the ponds receive the entire street, roof, driveway, and other runoff from the site. IV.Existing and Proposed Drainage Patterns This site drains to three general locations in the existing condition. The project will not change this general drainage pattern. Runoff drains to the east ravine, the north ravine or sheet drains down the slope to Wildhurst Trail from Lots 2 and 3.One effect the project will have is to divert some of the drainage area away from the east ravine and the slope to Wildhurst Trail. 2.4 acres will be diverted to a wet-detention pond for the north ravine area. V. 1. Runoff Rates East Ravine and the Slope to 'Wildhurst Trail (East Drainage) The drainage area and peak rates of runoff wh be reduced to these areas. The attached runoff summary entitled “East Drainage" is taken from the Landform Hydro-Cad model and illustrates the reductions. The reductions are the result of reducing the drainage area by 43% and because Pond 1 will detain the runoff it collects and release it at a slower rate through a 4 inch diameter pipe outlet to the ravine. 2.North Ravine The peak rates of runoff will basically remain unchanged for smaller runoff events and will be reduced for larger, more severe events. The attached runoff summary entitled “North Drainage" is taken from the Landform Hydro-Cad model and illustrates this effect. Although the drainage area to the North Ravine increases by 84% because of site grading, this is offset and mitigated by temporary storage of runoff in Pond 2, again releasing it at a slower rate through a 4 inch diameter pipe outlet. Runoff Volumes 1.East Ravine and the Slope to Wildhurst Trail (East Drainage)