This is an analysis of a 2006 planned Arrowhead/Quail Ridge development (85 homes on 90 acres). This planned development was in the interior tract on Woodstock Road of Arrowhead. The 2006 project was scuttled due to terrain analysis (topography) and stormwater runoff analysis that made the land unsuitable for development.
Summary: James Gregory found land issues (steep slope areas and elevations, intermittent stream areas, 100 year flood stormwater flow calculations, potential flooding) and used scientific analysis as evidence that grading over the stream beds and steep slopes would prohibit development per County Ordinance (too much impervious surface streets, driveways, homes would flood downstream road and houses). Lochnora and DOT maintained Erwin Road would have been flooded, as well as other ‘downstream properties.'
From: James Gregory <wlfpkr@nc.rr.com>
Date: Fri, 15 Sep 2006 01:40:37 -0400
To: beckyheron@nc.rr.com ...Frank.Duke@durhamnc.gov, Gail.Sherron@durhamnc.gov,
Steve.Medlin@durhamnc.gov, Teri.Danner@durhamnc.gov ..
Subject: Stormwater Analysis for proposed Quail Ridge Subdivision
Hello Folks,
I had the opportunity Thursday afternoon to review the revised preliminary plat maps and documents for the proposed Quail Ridge subdivision. Many of the items required in a preliminary plat that previously were not in compliance with the Code of Ordinances, County of Durham, Appendix B, City-County Subdivision Ordinance have been addressed in the revised documents submitted to Durham County planning staff on September 13, 2006. However, I am very concerned that the major problems with the stormwater analysis have not been corrected.
I discussed all the pertinent issues in the meeting with the Durham County staff on August 31, 2006 and in a meeting with Chris Brown, Office Manager, Dewberry & Davis on September 6, 2006. I will be happy to provide a detailed written report as time permits if any addressees so desire, but I wish to discuss the key issue in this message prior to today's meeting of the Durham County Development Review Board.
The Quail Ridge Subdivision Stormwater Impact Analysis (SIA) is not correct because the predictions of pre- and post-development streamstormflow volumes and hydrograph peak flows and the design of the network of stormwater detention basins are based on landscape units that are NOT WATERSHEDS. The landscape units described in the Dewberry and Davis SIA report dated September 12, 2006 and shown on the pre- and post-development maps were described in the SIA report on page 1, "This results in an inverse basin layout with the basin divide along the stream and ridge in the center of the basin". That method of delineating landscape units for a hydrologic analysis and for designing a network of stormwater BMPs is NOT standard practice in the field of watershed hydrology and is NOT standard engineering practice. That method of delineating landscape units, therefore, produces erroneous results in the prediction of pre- and post-development stream stormflow volumes and peak flows and in the design of the required stormwater BMPs.
A watershed is a natural landscape unit that is the basic unit of all hydrologic analyses, is the basic unit of engineering hydrologic analyses, and is the basic unit for all stormwater BMP designs. Please note the verbiage in the Durham County Code of Ordinances, Chapter 14, Article 5, Sec. 14-151 defining the one-year 24-hour storm, "One-year 24-hour storm means the surface runoff resulting from a rainfall of an intensity expected to be equaled or exceeded, once a year, and of a duration which will produce the maximum peak runoff flow FROM THE WATERSHED OF INTEREST under average antecedent wetness conditions."
Therefore, the Quail Ridge Subdivision Stormwater Impact Analysis is not in compliance with the requirements of the Durham County Code of Ordinances. The ordinance lists 3 acceptable methods for predicting watershed peak stormflow that are simple empirical approaches: the rational method, the USDA SCS peak discharge method (commonly referred to as TR-55) and the Putnam Method. Those methods are all designed to predict the stream stormflow volume or peak flow rate for design storms (i.e. specific return period storms) from WATERSHEDS. The ordinance also specifies in Sec. 14-153 that, "The same methodology must be used for calculating both the pre- and post-development flow rates." The "flow rates" referred to are the peak streamflow rates from the WATERSHEDS used in the peak stormflow analysis and BMP network design process.
The definition of watershed and the methods for delineating a watershed on a topographic map are so basic to the hydrologic and engineering sciences and so widely understood in the hydrology and engineering professions that most textbook writers do not waste the page space todiscuss them. I have 32 hardback monographs on various aspects of surface water hydrology, hydrologic modeling, engineering hydrology, and stormwater system design on the bookcase in my office. Only one of those books describes in detail the characteristics of a watershed and the methods for delineating watersheds on topographic maps: McCuen, Richard H. 2005. Hydrologic Analysis and Design. 3d ed. It is important to note that Dick McCuen's textbook, first published in 1989, is one of the most highly respected textbooks on quantitative and engineering hydrologic analyses and engineering hydrologic design in the country and is widely used in graduate courses in engineering hydrology. *Dick McCuen's book describes standard engineering practice in engineering hydrology. * The attached pdf file contains the scanned pages from the book that define watersheds and describe how to delineate watersheds on topographic maps for hydrologic analyses (including stormwater analyses) and hydrologic design (including stormwater BMP design).
If any of the addressees have questions about my comments above, I will be happy to discuss them. I plan to be in the office most of this morning preparing for an afternoon laboratory in my watershed hydrology class, the focus of which is, methods of watershed assessment on topographic maps.
Regards,
Jim Gregory
James D. Gregory, CPSS, PWS, PhD
Professor of Forestry, Watershed and Wetlands Hydrology
Dept. of Forestry and Environmental Resources
North Carolina State University
Box 8008, 2800 Faucette Drive, 3110 Jordan Hall
Raleigh, NC 27695-8008
Phone: (919) 515-7567; Fax: - 6193
Email: jim_gregory@ncsu.edu
COL AV USA RET