Drainmod Methods Approved by North Carolina
Health Services Commission

February 2003


At the November 15, 2002 meeting of the North Carolina Health Services Commission rules were adopted allowing alternative methods to determine depth to seasonally high water tables for subsurface wastewater systems (effective date of February 1, 2003). The rules established specific procedures to determine "soil wetness condition" of proposed sites. The three optional alternative methods include (1) monitoring of water table during January through April of a given year and determination of seasonally high water table using normalized rainfall chart, (2) monitoring of water table during January through April of a given year to calibrate DRAINMOD computer simulation of site using 30 year rainfall data, and (3) modeling of the site using detailed soil hydraulic measurements and DRAINMOD computer simulation of site using 30 year rainfall data.

This is the first time DENR regulations have specifically allowed use of the DRAINMOD computer simulation analysis. The computer program is considered to be a state-of-the-art simulation model. The program was developed by Dr. Wayne Skaggs of NC State University Department of Biological and Agricultural Engineering. The program has been previous used for precision wetland hydrologic evaluations, as well as, drainage and irrigation investigations. In recent years research has expanded use of the model for nitrogen impact evaluation of drainage waters.

The adopted rule establishes a standard: maximum 14-days of continuous saturation that occurs equal to or greater than a 30% frequency interval over 30 years of climatic records. This is the first time a precise quantitative standard has been established to define seasonally high water table under the rules.

Mr. David Stephenson petitioned the Commission and State Health Director pursuant to15A NCAC 24B .0101. A committee of the Commission reviewed the petition and allowed the petitioner, Department, and third parties to present verbal arguments regarding the petition's recommendations, alternate recommendations, technical justifications, and practical implications in two days of informal hearings. The committee then developed and recommended revised rules that were unanimously adopted by the Commission.

The link to the new rules is http://www.deh.enr.state.nc.us/oww/Rule1942CHS111502.pdf.

The link to the new definitions is located at http://www.deh.enr.state.nc.us/oww/Rule1935CHS111502.pdf.