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 2024-29, July 17, 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
Effective height of embankment (feet)
Crest Width (feet)
less than 10
6
10 - 15
8
15 - 20
10
20 25
12
25 - 35
14
more than 35
15
(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.