(1) The applicant shall submit a storm drainage plan to the reviewing authority. The plan must include a design report, calculations, and plan sheets sufficient to provide construction details of the storm drainage system and must conform with the requirements of either (2) or (3).

(2) Except as provided in (3), a storm drainage plan must be designed in accordance with Department Circular DEQ-8.

(a) for lots proposed for uses other than as single living units, a storm drainage plan submitted under (2) must be prepared by a professional engineer and the storm drainage system is subject to the requirements in ARM 17.36.314;

(b) a storm drainage plan submitted under (2) must include a maintenance plan for all drainage structures. The maintenance plan must describe the maintenance structures, provide a maintenance schedule, and designate the entity responsible for performing maintenance. The reviewing authority may require the applicant to create a homeowner's association or other legal entity that will be responsible for maintenance of storm drainage structures and that will have authority to charge appropriate fees. The maintenance plan must include easements and agreements as necessary for operation and maintenance of all proposed off-site storm drainage structures or facilities.

(3) Regardless of the type of use or the number of commercial or residential units proposed, a storm drainage plan is not subject to the requirements of (2) if all of the requirements in (3)(a) through (h) are met. To be exempt from the requirements of (2), a storm drainage plan must be submitted demonstrating that:

(a) the proposed subdivision has five or fewer lots;

(b) the area of disturbance within each proposed lot has a slope of three percent or less;

(c) unvegetated areas including, but not limited to, road surfaces, road cuts and fills, roofs, and driveways, comprise less than 15 percent of the total acreage of each proposed lot;

(d) drainage structures, such as road ditches, exist or, if necessary, will be constructed;

(e) completion of the proposed subdivision will not increase the amount of pre-development storm water runoff, during the 100-year 24-hour storm event, between proposed lots and from the proposed subdivision area to an adjoining property;

(f) the proposed subdivision will not alter pre-development pass-through water flow patterns;

(g) the applicant provides the reviewing authority with a 7 1/2 minute USGS topographic map showing the proposed subdivision and, if available, a map with contour intervals no greater than 20 feet that shows drainage patterns; and

(h) no buildings or drainfields in the subdivision will be flooded during the 100-year 24-hour storm event.

(4) If fill material will be placed within a delineated floodplain, the applicant shall provide evidence that the floodplain permit coordinator has been notified and that appropriate approvals have been obtained.

(5) If applicable, the applicant shall obtain an MPDES permit for storm water discharges, pursuant to ARM Title 17, chapter 30.

(6) Storm water that reaches state surface waters must be treated prior to discharge if the reviewing authority determines that untreated storm water is likely to degrade the receiving waters.

(a) minimum treatment of storm water consists of removal of settleable solids and floatable material. The reviewing authority may require more extensive treatment if deemed necessary to protect state waters from degradation;

(b) plans for the treatment facility must be approved by the reviewing authority.

(7) The department may grant a waiver from any of the requirements in this rule pursuant to the provisions of ARM 17.36.601.