Agenda - 04-23-2013 - 7b - Laserfiche WebLink

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28 report has reviewed both peak flow attenuation and nutrient loading/treatment requirements. Proposed Drainage: The proposed roadways on-site will mirror existing grades as closely as possible and limited clearing and grading are anticipated. Roadways will be public roads with curb and gutter and drainage from roadways will be discharged to swales located along natural drainage ways wherever possible. Where necessary, treatment and detention measures will be provided and the design and sizing of said devices will be coordinated heavily with Orange County Erosion Control in an effort to prevent unnecessary removal of mature trees in the area. REQUIREMENTS: Stormwater Orange County requires that new development comply with the following rules Management regarding stormwater management and conveyance: 1. Provide Nutrient Treatment and removal of 85% TSS from the runoff produced from the 1-in storm event. Due to its location in the Upper New Hope watershed, this would require removal of both Nitrogen and Phosphorous. 2. Provide peak rate reduction for the 1-year, 24 hour storm events to pre-development rates. Although this report is preliminary, we have assumed these conditions would be the most restrictive cases and have also reviewed the 2-yr and 10-yr storms for informational purposes. ANALYSIS: Stormwater The new Lake Jordan Accounting Tool was used to determine nutrient loading Management requirements based on the preliminary design and impervious limits discussed (Quality) with the Owner. Breakdowns of the area are presented with attached calculations. Stormwater Based on the area breakdown prepared above and incorporating the upstream Management(Peak areas contributing to the entire basin, Pre and Post Development discharges Flow Attenuation) were calculated using Hydraflow Hydrographs. RESULTS: Detailed calculations are provided in the Attachments at the end of this report. The results of the required calculations are summarized below. Stormwater Management Based on values calculated using the Lake Jordan Accounting Tool, the site will require minimal amounts of treatment. It appears that treatment for this site could be provided by level spreaders and filter strips located below a portion of the houses and the roadways. Per the values noted in the accounting tool, use of level spreaders and filter strips below the roadway and 7 of the 11 lots would provide adequate treatment for the site. Note that final design will be heavily coordinated design with Orange County Erosion Control and may incorporate alternative treatment strategies to those presented.