(a) Stormwater management systems for minor and major site plans shall detain, retain, or result in the infiltration of stormwater from the twenty-four-hour storms of the two-year, ten-year, and twenty-five-year frequencies such that the peak flows of stormwater from the project site do not exceed the peak flows of stormwater prior to undertaking the project. The Planning Board may waive the flooding standard in accordance with the following criteria:
 

[1] Insignificant increases in peak flow rates from a project site. When requesting a waiver for a project resulting in an insignificant increase in peak flow rates from a project site, the applicant shall demonstrate that insignificant increases in peak flow rates cannot be avoided by reasonable changes in project layout, density, and stormwater management design. The applicant shall also demonstrate that the proposed increases will not unreasonably increase the extent, frequency, or duration of flooding at downstream flow controls and conveyance structures. In making its determination to allow insignificant increases in peak flow rates, the Planning Board shall consider cumulative impacts.
 

(b) The applicant shall demonstrate that on- and off-site downstream channel or system capacity is sufficient to carry the flow of a minimum twenty-five-year storm without adverse effects, including but not limited to flooding and erosion of drainage channels and shoreland areas, or that he/she will be responsible for whatever improvements are needed to provide the required increase in capacity and/or mitigation. The Staff Review Committee or Planning Board may require capacity for a storm of greater than 25 years due to soil, topographic, or other factors that affect stormwater drainage.

(c) All natural drainageways shall be preserved at their natural gradients and shall not be filled or converted to a closed system unless approved as part of the site plan review.

(d) The design of the stormwater drainage system shall provide for the disposal of stormwater without damage to streets, adjacent properties, downstream properties, soils, and vegetation.

(e) The design of the storm drainage systems shall be fully cognizant of upstream runoff which shall pass over or through the site to be developed and provide for this movement.

(f) Major site plans, regardless of size, shall submit a stormwater management plan that complies with Section 4C(2) and Section 4C(3) of the General Standards of the DEP Chapter 500, Stormwater Management, as amended.

(g) Major and minor site plan projects located within the Highland Lake watershed shall also meet the following standards:

[1] Projects in the Highland Lake watershed must include a stormwater plan designed to meet the standards of DEP Chapter 500, Section 4D, Phosphorous standards. The option to pay the compensation fee for exceeding the parcel's phosphorus allocation is not allowed in this watershed. Notwithstanding 1 M.R.S.A. § 302, this amendment shall be applicable to and shall govern any application for site plan approval that did not receive lawful final approval by, and so was pending on or at any time after, September 5, 2017, regardless of whether that application was a pending proceeding under 1 M.R.S.A. § 302.
 

(a) A qualified third-party inspector hired by that person shall, at least annually, inspect the stormwater management facilities, including but not limited to any parking areas, catch basins, drainage swales, detention basins and ponds, pipes and related structures, in accordance with all municipal and state inspection, cleaning and maintenance requirements of the approved stormwater management plan.

(b) If the stormwater management facilities require maintenance to function as intended by the approved stormwater management plan, that person shall take corrective action(s) to address the deficiency or deficiencies.

(c) A qualified third-party inspector hired by that person shall, on or by May 1 of each year, provide a completed and signed certification to the enforcement authority, in a form provided by the Town of Windham, certifying that the person has inspected the stormwater management facilities and that they are adequately maintained and functioning as intended by approved stormwater management plan, or that they require maintenance or repair, describing any required maintenance and any deficiencies found during inspection of the stormwater management facilities, and, if the stormwater management facilities require maintenance or repair of deficiencies in order to function as intended by approved stormwater management plan, the person shall provide a record of the required maintenance or deficiency and corrective action(s) taken.