(1) The applicant shall submit a storm drainage plan to the reviewing authority. The plan 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-family dwellings, a storm drainage plan submitted under (2) must be prepared by a registered professional engineer;

(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) A storm drainage plan is not subject to the requirements of (2) if:

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

(b) the area of disturbance within the proposed subdivision has a slope of 3% or less;

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

(d) drainage structures, such as road ditches, will be constructed;

(e) completion of the proposed subdivision will not increase the amount of pre-development storm water runoff from the area;

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

(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.

(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.