Code of Maine Rules
06 - DEPARTMENT OF ENVIRONMENTAL PROTECTION
096 - DEPARTMENT OF ENVIRONMENTAL PROTECTION - GENERAL
Chapter 500 - STORMWATER MANAGEMENT
Appendix 096-500-E - Stormwater basins, ponds and underdrained vegetated soil filter beds

Current through 2024-38, September 18, 2024

This appendix applies to all projects using stormwater basins, ponds or underdrained, vegetated soil filters, and outlines the criteria for siting, designing, and constructing detention basins and ponds used for meeting the Department's stormwater management rules. The Department may require additional measures to address geotechnical, hydrologic, structural, hydraulic, and construction concerns. For example, the Department may require that the designer assess the impoundment's hazard potential for determining the appropriate design storm for the impoundment because the design storm may have a greater rainfall depth and larger recurrence interval than the 25-year, 24-hour storm used for the Department's stormwater management program.

1. Basin and pond types. A variety of stormwater management basins and ponds are used to control runoff quantity and improve runoff quality from developments. All need professional siting and design to avoid unreasonable impacts to wetlands, surface waters, and groundwater and to ensure long-term stability, pollutant removal performance, and control of peak flow rates. General restrictions and requirements for all basins and ponds are provided below.

NOTE:Examples of siting and design criteria for each type of stormwater management structure can be found in the Department's manual "Stormwater Management for Maine: Best Management Practices."

2. Siting restrictions. The Department has placed the following general restrictions on the siting of stormwater basins and ponds. Additional restrictions may be required based on drainage, geotechnical, wildlife, and safety concerns.

(a) Rivers, streams and brooks. A basin or pond may not be located in or within 75 feet, measured horizontally, of the normal high water line of a river, stream, or brook (intermittent or perennial) unless approved by the Department pursuant to, or exempted from, the Natural Resources Protection Act (NRPA).

(b) Wetlands. A basin or pond may not be constructed in or adjacent to a wetland and no dam, wall, berm, or embankment may be placed within or adjacent to a wetland as part of a stormwater management system, unless approved by the Department pursuant to the NRPA. The use of natural wetlands for runoff detention or retention storage to meet the standards in this chapter is prohibited unless the flooding standards in Section 4(H) are met and a Natural Resources Protection Act permit is obtained if required.

(c) Discharge of flows. When detention is used on a site, the pre-construction flow condition to off-site areas, whether sheet or concentrated, must be maintained in the post-construction condition unless drainage easements are obtained from affected property owners.

(d) Underground detention. Where underground detention is required because of limited space or other restrictions, runoff must at a minimum receive treatment to remove sediment and debris prior to discharge to the underground storage facility. The Department may require treatment to remove other pollutants if it determines that underground storage poses a threat to groundwater quality in which case all requirements in Appendix D must be met. The outlet control structure and the storage chambers for the underground detention structure must be accessible from the surface for maintenance, debris removal, and, if necessary, future modification.

3. Pond and basin design requirements

(a) Principal spillways. Basins and ponds designed to control flows so as to meet the flooding standard must have principal spillways capable of controlling runoff from 24-hour storms of the 2-year, 10-year, and 25-year frequencies. Basins and ponds designed to provide channel protection detention must have principal spillways capable of providing extended detention of twelve hours for runoff from a 24-hour storm of a one-year frequency. In both cases, the principal spillway must control the maximum flows from the design storm(s) without activating the emergency spillway.

(i) Trash racks. Any pipe, orifice, or culvert serving as a basin or pond outlet must have a trash rack to control clogging by debris and to provide safety to the public. The surface area of each rack must be at least four times the outlet opening it is protecting. A significantly larger trash rack ratio may be required for openings less than twenty-four inches in diameter. The spacing between rack bars must be no more than six inches or one-half the dimension of the smallest outlet opening behind it, whichever is less. If possible, trash racks should be inclined so to be self-cleaning.

(ii) Seepage controls. All smooth outlet pipes greater than eight inches and all corrugated outlet pipes greater than 12 inches must have seepage controls to prevent the piping of soil along the outside of the pipe. This standard applies to both dry detention basins and ponds with permanent pools.

(iii) Anti-floatation design. All outlets employing a riser structure must be designed to prevent the riser from floating.

(b) Emergency spillways. Each stormwater basin and pond must have an emergency spillway designed to independently convey the routed runoff from at least the 25-year, 24-hour storm (as described under Section 3(c)(i) (crest elevation) of this Appendix. All spillways must meet the following criteria.

(i) Location. Emergency spillways must be located on undisturbed, non-fill soil wherever possible. If the spillway must be located on fill soils, then the spillway must be horizontally offset at least 20 feet from the principal outlet and be designed with a riprap lining, reinforced-turf lining, or a non-flexible lining.

(ii) Exit channel grade. The maximum grade of the spillway's exit channel may not exceed 20% unless a non-flexible lining is used to control erosion within the channel. Vegetation, reinforced turf, riprap, and modular blocks are considered flexible linings. All linings must be evaluated for stability at the channel grade chosen.

(iii) Flow depth. The design flow depth in the exit channel may not exceed one-half the d50 stone size for channels lined with riprap. The design flow depth in the exit channel may not exceed three inches for channels lined with un-reinforced vegetation.

(c) Embankments. Basin and pond embankments must be designed by a professional engineer registered in the State of Maine. The design must include an investigation of the subsurface conditions at the proposed embankment location to evaluate settlement potential, groundwater impacts, and the need for seepage controls. The Department will require the submittal of a geotechnical report from a geotechnical engineer for any embankment over 10 feet in effective height or which the Department determines poses a significant threat to downstream property or life.

(i) Crest elevation. The minimum elevation of the top of the settled embankment must be at least one foot above the peak water surface in the basin with the emergency spillway flowing at design depth for the design storm routed through just the emergency spillway.

(ii) Crest width. The minimum crest width for any embankment must be as shown in Table 6:

Table 6

Effective Height of Embankment Based on Crest Width

(iii) Construction. The selection of fill materials must be subject to approval of the design engineer or inspecting engineer. Fill must be free of frozen soil, rocks over six inches, and sod, brush, stumps, tree roots, wood, or other perishable materials. Embankment fills less than 10 feet in fill height must be compacted using compaction methods that would reasonably guarantee that the fill density is at least 90% of the maximum density as determined by standard proctor (ASTM-698). All embankment fills more than 10 feet in fill height must be compacted to at least 90% of the maximum density as determined by standard proctor (ASTM-698) and must have their density verified by field density testing.

(iv) Slopes. The embankment's slopes may not be steeper than 3 horizontal to one vertical.

(d) Gravel outlet with an underdrain. The outlet of wetponds discharging directly to a stream must be designed to meet the general standards for channel protection pursuant to Section 4(c)(2) of this Chapter and must be fitted with a gravel outlet with an underdrain.

(i) Pond bench. The bed of the gravel outlet must be built on a pond bench having a width of at least eight feet and a length that equivalent to 3 feet per 1000 cubic feet of volume for channel protection. The bench elevation must be set at the permanent pool elevation such that the channel protection volume will be stored between the bench surface elevation and the elevation of the principal spillway's lowest control outlet. The bench must be located at or near the end of the pond furthest from the principal inflow.

(ii) Underdrain pipe. The underdrain pipe must be installed down the centerline of the gravel trench. The pipe may be either perforated PVC pipe or corrugated, polyethylene drainage tubing. The slope of the installed underdrain pipe must be 1% or greater.

(iii) Gravel trench. A gravel filled trench with a minimum width of 4 feet and a minimum depth of 3 feet must be installed in the pond bench at least 2 feet from the pond side edge of the bench. The underdrain pipe must be bedded in clean, well-graded gravel (MDOT specification 703.22 Type B) extending 24 inches over the top of the drainage pipe, with at least six inches to the sides of the pipe, and six inches below the pipe.

(iv) Underdrain outlet. The underdrain outlet must discharge to an area capable of withstanding concentrated flows and saturated conditions without eroding.

4. Vegetated Soil Filter Bed Design Requirements. Soil filters are designed to provide pollutant removal and channel protection as they provide the slow release of runoff. The filter also provides cooling of the discharge reducing thermal

NOTE:Examples of specific design criteria for each type of filter bed structure and design can be found in the Department's BMP manual "Stormwater Management for Maine."

(a) Soil filter beds designed to meet the general standards for soil filters must be designed to meet the following criteria.

(i) Volume stored and treated. The soil filter basin must store and filter at least 1.0 inch of stormwater runoff from the impervious area draining to it, and 0.4 inches of stormwater runoff from the landscaped area draining to it. A stable overflow outlet must be provided for stormwater in excess of the volume to be stored for treatment.

(ii) Soil filter. The soil filter basin must consist of depressional surface storage over a densely vegetated soil filter that is underlain with underdrain bedding and drained by perforated underdrain pipe. The soil filter material must be fine enough to filter fine sediments and provide effective adsorption of pollutants, but coarse enough to drain the stored volume over a period of no less than 24 hours and no greater than 48 hours. The soil filter material must be well blended and graded and must contain sufficient organic matter to facilitate the removal and treatment of hydrocarbons. Unless otherwise approved by the Department on a case-by-case basis, the soil filter must be at least 18 inches thick and the surface area of the filter must be no less than 5% of the contributing impervious drainage area.

(iii) Basin design. The soil filter must be constructed with an underdrain system and with a liner to prevent infiltration into the in-situ soil unless all the conditions for either (b) or (c) below are met, and must be designed to discharge its stored volume in one of the following two ways. Otherwise, the requirements from Appendix D must be met.

(b) If located over soil in hydrologic group C or D, unless the soil material consists of ablation till, stratified drift or aeolian sand soils, the soil filter may be constructed with an underdrain system and without a liner to prevent infiltration into the in-situ soil if the following conditions are met.

(i) There is a separation in in-situ soils of at least 18 inches between the bottom of the underdrain bedding material and bedrock, and

(ii) The bottom of the filter media is no lower than the elevation of the seasonal high water table; or

(c) If located over highly permeable soils, such as those derived from ablation till, stratified drift, aeolian sand, or other highly permeable overburden, the soil filter may be constructed without a liner to prevent infiltration into the in-situ soil if it has been constructed with an underdrain system and the following requirements are met:

(i) The filter drains no greater than one acre of asphalt or concrete paving or equivalent material, and no greater than 2 acres of developed land area;

(ii) There is a separation in in-situ soils of at least 18 inches between the bottom of the underdrain bedding material and both the high seasonal water table and bedrock;

(iii) The stored runoff volume is stored to a depth no greater than 18 inches;

(iv) The area draining to the filter does not include any:

a. public roads;

b. fuel service facilities;

c. high use parking lots and drive through lanes;

d. any storage or handling areas for petroleum products, road salt or other potential groundwater contaminants unless those areas are isolated within containment structures that are operated and maintained in accordance with a spill prevention, control and countermeasures plan; operation and maintenance plan; or equivalent document approved by the Department;

e. industrial facilities where facilities are stored and processed in areas exposed to precipitation;

f. vehicle maintenance facilities where maintenance or storage of equipment occurs in areas exposed to precipitation; and

g. lawns not on individual residential lots, unless all application of fertilizers, pesticides and similar turf-management chemicals are performed in accordance with a Department-approved management plan; and

(v) The filter is located no less than 300 feet from any private drinking water supply well, is not located within the delineated contributing area of or closer than 300 feet from any public drinking water supply well, and is located as far down gradient of any drinking water supply well as practicable. The filter must also be set back from any subsurface wastewater disposal system at least the distance required in the Maine Subsurface Water Disposal Rules, 144A CMR 241, for setbacks from watercourses;

(d) If the above requirements and the following additional requirements are met, the filter may be constructed without an underdrain system and may drain in less than 24 hours;

(i) The permeability of the in-situ soils at the elevation of the bottom of the soil filter is greater than the minimum permeability of the soil filter media; and

(ii) An analysis is performed demonstrating that the level of water table mounding under the system will be below the bottom of the soil filter 48 hours after the end of the storm event.

(e) Underdrain pipe bedding. The interface between the underdrain bedding material and the soil filter material must be designed to minimize the risk of clogging at the interface while preventing significant loss of fine soil material from the soil filter layer. The underdrain bedding material must be sufficiently coarse to allow flow of treated water to the underdrain pipe.

(f) Underdrain outlet. The underdrain system and the overflow must discharge to areas capable of withstanding concentrated flows and saturated conditions without eroding.

(g) Non-stormwater discharges, and stormwater from outside drainage systems. Non-stormwater discharges and stormwater from drainage systems outside the project area may not be discharged to the soil filter bed, unless the Department determines the additional discharge will not have a negative impact on groundwater quality.

5. Additional requirements. Additional requirements may be applied on a site-specific basis.