Loading...
HomeMy WebLinkAboutSW4240901_Stormwater Narrative_20241106 Lash Engineering, Inc. Civil/Consulting/Wastewater/Planning 1104 Cindy Carr Drive 7 Matthews,NC 28105 Phone: 704-847-3031 mikc1 ,LashEnginccring.com Lash Engineering, Inc. Engineers Report for: Mitchell's Gorge — Stormwater Narrative Alleghany County, NC. The project consists of 17 lots on a common cul-de-sac (Mitchell's Gorge). The lots will have a common wastewater drain field and be supplied with potable water by Aqua. The site boundaries encompass approximately 21.8 acres (951,383 sf.). The denuded area was along the ridge for about 170 feet on either side -just enough to install the RV pads and a small cottage. At the bottom of the slope on the north side of the development is a 50-foot buffer along the creek. The other side of Mitchell Gorge drains to an unnamed tributary that drains down to the Mitchell River Basin "Waters of the Mitchell River" with a 50' buffer requirement on the west side (off the property). These creek outfalls are shown in the plans. Mitchell River is a"High Quality" watershed that requires extra measures for stormwater protection. This development falls within that watershed and is impacted by those protection regulations. There is approximately 6,640 sf(0.152 ac.) of surface waters from the on-site creek. This project is to be developed as a "Low Density" development. Under the Low-Density guidelines for developments a 12% threshold has been established for a non-coastal county, high quality water, and no stormwater controls. This development will incorporate those guidelines. The existing creeks and drainage ways are to remain undisturbed so there are no street or utility crossings proposed. The entire cul-de-sac will be accessed through one intersection. As stated before, the development occurs on top of a ridge that drains down to creeks. Mitchell's Gorge is built along the ridge. The pavement is not crowned but sloped in such a way as to divert the stormwater off the pavement and onto a grassed area that discharges the stormwater onto the slope in a fashion that disperses it such that the grasses can remain stabilized. The houses have a roof gutter system with downspouts that are collected and piped down the slope underground with 4" black corrugated HDP pipe and allowed to discharge at the bottom of the ridge where the grade is flatter, and the discharge can disperse to sheet flow. On the north side of Mitchell Gorge there is a minimum of about a 100' of grassed and wooded land prior to the stormwater intersecting the swale. On the south side there are several hundred feet of grassed & wooded area. Several dimensions were added to the plan to show areas left natural, undisturbed, grassed or wooded. The soils are Soils: Group TII Clay Loam. The soil varies in slope from flat to steep. Summary: The road has been constructed, the RV pads have been installed and all but 8 of the cottages have been built. The plans show these improvements, and a table of impervious areas shows the square footage for each lot. • Eight cottages remain to be built. • All of the RV Pads have been accounted in the table. • There is no off-site drainage. The project is located on a ridge. • House gutters and downspouts are piped to the bottom of the ridge. • A total of 29,036 sf. is available to maximize the 12% threshold. • On a per lot basis this equates to 29,036/8= 3,629 sf. per cottage. • The maximum size cottage allowed by the approved&recorded HOA documents is 1,700 sf. • Assuming that there might be an additional 300 sf of porches & walks, the maximum impervious area per lot would be 2,000 sf. (This amount is shown on the plan). • 2,000 sf is less than 3,629 sf. so the project will be less than the 12% allowable impervious area. • Therefore, the project is in compliance with the Low Density 12%requirement. The project is in compliance with the Low-Density guidelines for a 12% impervious allowance utilizing dispersed flow and no stormwater controls as a result of the Mitchell River watershed being designated as a"High Quality" stream. This is a non-coastal county. Please contact Lash Engineering, Inc. if you have any questions or concerns at (704)847- 3031. Michael Lash, PE.