Section 5
Links from this page are included to assist the reader, and are not part of the rule. Note: the section below references 12 MRSA 7755-A, which has been repealed. The requirement contained in that section is now located at 12 MRSA 12806. Text amended effective December 27, 2006.
5. Other applicable standards. The following standards apply to a project as described in this section in addition to the basic, general, urban impaired stream, and flooding standards.
A. Management of stormwater discharges. A project discharging concentrated stormwater runoff through an open-channel or pipe to any point that is not an open channel, an inlet to a storm drain system, or a natural or man-made impoundment must convert the concentrated flow to sheet flow to prevent erosion of the downstream receiving area. The conversion of concentrated flow to sheet flow must be done using properly designed level spreaders meeting the criteria below.
(1) Discharge to a level spreader. The peak stormwater flow rate to a level spreader due to runoff from a 10-year, 24-hour storm must be less than 0.25 cubic feet per second (0.25 cfs) per foot length of level spreader lip.
(2) Drainage area. The maximum drainage area to the spreader may not exceed 0.10 acre per foot length of level spreader lip.
(3) Length of level spreader. The level spreader length may not be more than 25 feet unless approved by the department.
(4) Siting of level spreader. The level spreader must be sited so that flow from the level spreader will remain in sheet flow until entering a natural or man-made receiving channel.
This standard is not applicable for level spreaders discharging runoff to vegetated buffers used to meet the general standards. Requirements for these level spreaders can be found in Appendix F.
B. Discharge to freshwater or coastal wetlands. Stormwater standards for the waterbody must be met before the stormwater enters a wetland, unless otherwise approved by the department or unless the affected area of wetland qualifies for an exemption pursuant to the Natural Resources Protection Act, 38 M.R.S.A. § 480-Q(17). Wetlands must receive stormwater in the same manner as before the project unless otherwise approved or required by the department. In general, new or increased stormwater discharges into wetlands must be put into sheet flow using level spreaders designed to meet the requirements in Section 5(A). The department may allow alternate stormwater treatment measures if those measures will not unreasonably adversely affect the wetland.
The discharge of runoff to a wetland due to a 2-year storm may not increase the mean storage depth within a wetland more than two inches above pre-development levels for more than 24 hours from the end of the storm event, unless otherwise approved by the department. The department may consider cumulative impacts due to runoff from other projects when applying this standard to any wetland.
C. Threatened or endangered species. Additional stormwater standards may apply on a case-by-case basis if the department determines that such standards are necessary to avoid significantly altering the habitat of a threatened or endangered plant or animal species or violating protection guidelines.
NOTE: Title 12 M.R.S.A. § 7755-A prohibits state agencies from issuing a permit that will significantly alter the habitat of any species designated as threatened or endangered species or violate protection guidelines.
D. Additional controls. If the department determines that additional controls are necessary to avoid an unreasonable impact on any wetland or waterbody due to pollutants that are not adequately addressed by the standards described in Sections 4 and 5, a stormwater project that results in three acres or more of impervious area or 20 acres or more of developed area, requires review pursuant to the Site Law, or is a modification of any size as described in Section 16 of this chapter may be required to use additional controls. This is a case-by-case determination based upon factors such as the size, nature and intensity of the development, characteristics of the affected natural resource, topography and soils.
For example, stormwater from a metallic mineral mining or advanced exploration activity regulated under the department's regulations, Metallic Mineral Exploration, Advanced Exploration and Mining regulations (06-096 CMR 200), may contain contaminants, such as high concentrations of dissolved metals, or be very acidic or alkaline, for which stormwater best management practices (BMPs) for other commercial or industrial developments do not provide adequate treatment.
E. Authorization for discharges to public storm sewer systems. If runoff from a project site will flow to a publicly-owned storm sewer system, then the applicant must obtain authorization from the system's owner to discharge into the system. At its discretion, the department may require the applicant to demonstrate that the system has adequate capacity for any increases in peak flow rates and volumes to the system.
