City of Concord
The City of Concord has enacted a Stormwater Ordinance in order to maintain public health and welfare by protecting existing and potential sources of groundwater supplies. The City of Concord is the permitting authority for all land disturbing activities and requires the land owner to maintain all on-site stormwater control facilities and all open space areas (e.g. parks or “green” areas) required by the approved stormwater control plan. The City of Concord will only provide construction permits to projects that establish a plan to manage stormwater runoff occurring during the construction process. The City of Concord, under the NPDES program, also has the authority to inspect properties for noncompliance and can issue a notice of violation (NOV) for any deficiency or infraction onsite. Property owners are responsible for the maintenance of any stormwater facilities or practices located on the property. The City of Concord has the authority to inspect stormwater facilities and practices in order to ascertain that they properly maintained and functioning.
Aquifer Protection (AP) District.
(a) Purpose of the AP District. The AP District is established for the purpose of protecting the quality and quantity of groundwater resources available to be used as current or future drinking water supplies in the City of Concord. This District is intended to:
(1) Maintain public health and welfare by protecting existing and potential sources of groundwater supplies;
(2) Prevent land use practices and development that could reduce the amount of recharge available to aquifers identified as current or potential sources of drinking water;
(3) Prevent land use practices and development that could contaminate or adversely impair the quality of groundwater within aquifers identified as current or potential sources of drinking water;
(4) Provide for future growth, in accordance with the City of Concord Master Plan, by protecting the long-term availability of clean, safe public water; and
(5) Identify land uses that can safely be sited in aquifer recharge areas and proximal to water supply wells.
(c) Limits of the AP District. The AP District, as displayed on the Zoning Map, The Zoning Overlay District Maps, of this ordinance, shall include the following areas within the City:
(1) Community Water Systems Protection Area. Within the AP District, an area comprised of the following groundwater development zones and wellhead protection areas:
a. The entirety of the land area which comprises the four (4) primary groundwater development zones identified on the Aquifer Protection District Map as APD-1, APD-2 (a and b), APD-3 and APD-4, together with any land within four hundred (400) feet of the perimeter of each zone.
b. That portion of the Wellhead Protection Area between Route 106 and the Soucook River and northerly of North Pembroke Road, identified on the Aquifer Protection District Map as APD-7, related to the City of Concord public water supply wells which are located easterly of the Soucook River in Pembroke.
c. That portion of the Pembroke Water Works Wellhead Protection Areas/Groundwater Reclassification Areas located westerly of the Soucook River adjacent to Manchester Street and to Route 106 in Concord, identified on the Aquifer Protection District Map as APD-5 and APD-6.
(2) Non-Transient, Non-Community (NTNC) Water Systems Protection Area. Within the AP District, a circular area centered on any single NTNC water supply well serving the system, with the radius of the circular area being equal to that defined for the Sanitary Protection Area established for a NTNC well by the New Hampshire Department of Environmental Services or by calculations of daily water use based upon the intended land use.
(3) Transient, Non-Community (TNC) Water Systems Protection Area. Within the AP District, a circular area centered on any single TNC water supply well serving a water system, with the radius of the circular area being equal to that defined for the Sanitary Protection Area established for a TNC well by the New Hampshire Department of Environmental Services.
(4) Domestic Water Systems Protection Area. Within the AP District, a circular area centered on any single domestic water supply well serving a water system, with the radius of the circular area being equal to seventy-five (75) feet.
(d) AP District—Community Water Systems Protection Area.
(1) Performance Standards.
a. For any land use that will render impervious the ground surface for more than fifteen (15) percent of a total lot area or for more than two thousand five hundred (2,500) square feet of any lot, whichever is the greater, a stormwater management plan shall be prepared consistent with the requirements and standards contained in New Hampshire Stormwater Manual Volumes I—III, New Hampshire Department of Environmental Services, December 2008, as most recently revised.
b. For a use for which a conditional use permit is authorized, Conditional Use Permits Required for Certain Uses in the AP District—Community Water Systems Protection Area, of this ordinance, a stormwater management and pollution prevention plan shall be prepared which shall include information consistent with the requirements and standards contained in Developing Your Stormwater Pollution Prevention Plan: A Guide for Industrial Operators, US EPA, February 2009, as most recently revised, and New Hampshire Stormwater Manual Volumes I—III, New Hampshire Department of Environmental Services, December 2008, as most recently revised. The plan shall demonstrate that the use will:
1. Minimize, through a source control plan (that identifies pollution prevention measures), the release of regulated substances into stormwater.
2. Demonstrate that stormwater infiltrated or discharged to the ground has been treated with the best available technology appropriate to mitigate all likely contaminants associated with the proposed use or activity.
c. For a use for which a conditional use permit is authorized, Conditional Use Permits Required for Certain Uses in the AP District—Community Water Systems Protection Area, of this ordinance, that will render impervious the ground surface for more than one acre of any lot not including any building lot coverage, a deicing management plan shall be prepared in a manner consistent with the requirements and standards contained in Chapter 4, Section 4-2, subsection entitled Snow and Ice Management of the New Hampshire Stormwater Manual Volume II, New Hampshire Department of Environmental Services, December 2008, as most recently revised.
d. All uses for which a conditional use permit is authorized, Conditional Use Permits Required for Certain Uses in the AP District—Community Water Systems Protection Area of this ordinance, shall be controlled using best management practices pertinent to the specific use according to local, State, and Federal regulations.
e. Chloride salt use for deicing shall be restricted to the minimum amount needed for public safety.
f. Streets, roads and parking areas shall be constructed so that direct application of road salt is the minimum amount possible while maintaining safety.
g. Non-point source pollution shall be minimized through implementation of practices that use or mimic natural processes to infiltrate, evapotranspirate (the return of water to the atmosphere either through evaporation or by plants), or reuse runoff on the site where it is generated.
h. Animal manures, fertilizers, and compost shall be stored in accordance with Manual of Best Management Practices for Agriculture in New Hampshire, New Hampshire Department of Agriculture, Markets, and Food, July 2008, as most recently revised.
i. All regulated substances stored in containers with a capacity of five (5) gallons or more shall be stored in product-tight containers on an impervious surface designed and maintained to prevent flow to exposed soils, floor drains, and outside drains.
j. Facilities where regulated substances are stored shall be secured against unauthorized entry by means of a door or gate that is locked when authorized personnel are not present and shall be inspected periodically by the facility owner or designee.
k. Outdoor storage areas for regulated substances, associated material or waste shall be protected from exposure to precipitation and shall be located at least fifty (50) feet from surface water or storm drains, at least seventy-five (75) feet from private wells, and outside the Sanitary Protective Radius (SPR) of potable supply wells used by public water systems.
l. Secondary containment shall be provided for outdoor storage of regulated substances if any of the regulated substances is stored in a container with the capacity to hold five (5) or more gallons. The containment structure must include a cover to minimize accumulation of water in the containment area and contact between precipitation and storage containers.
m. Containers in which regulated substances are stored shall be clearly and visibly labeled and must be kept closed and sealed when material is not being used or transferred from one container to another.
n. Prior to any alteration of terrain associated with implementation of any building or site development, all inactive wells on the property not in use or properly maintained shall be considered abandoned and must be sealed in accordance with We 604 of the New Hampshire Water Well Board Rules.
o. All new subsurface disposal systems shall be designed and installed in compliance with New Hampshire Department of Environmental Services Rules and shall only receive discharges typical of residential domestic wastewater.
p. An applicant for a new groundwater withdrawal within the AP District shall provide copies to the City of all application materials submitted to the New Hampshire Department of Environmental Services for approval of new groundwater withdrawals.
q. All submittals to the New Hampshire Department of Environmental Services for Registered Water Uses within the AP District shall also be submitted to the City by the owner of the registered water use.
r. Any on-site blasting shall conform to best management practices contained in the New Hampshire Department of Environmental Services document "Rock Blasting and Water Quality Measures That Can Be Taken To Protect Water Quality and Mitigate Impacts", 2010, as most recently revised.
(2) Spill Prevention, Control and Countermeasure (SPCC) Plan. For all uses for which a conditional uses [sic], as authorized pursuant to Section 28-3-6(d)(4), Conditional Use Permits Required for Certain Uses in the AP District—Community Water Systems Protection Area, of this Ordinance, where regulated substances are to be stored or used, a Spill Control and Countermeasure (SPCC) Plan shall be submitted which the Planning Board finds contains adequate and sufficient measures to prevent, contain, and minimize releases from ordinary or catastrophic events such as spills, floods or fires that may cause large releases of regulated substances. A SPCC shall, as a minimum, include:
a. A description of the physical layout and a facility diagram, including all surrounding surface waters, water supply wells, and Wellhead Protection Areas.
b. Contact list and phone numbers for the facility response coordinator, cleanup contractors, and all appropriate Federal, State, and local agencies that must be contacted in the event of a release to the environment.
c. A list of all regulated substances in use and locations of use and storage on the premises.
d. A prediction of the direction, rate of flow, and total quantity of regulated substance that could be released where experience indicates a potential for equipment failure.
e. A description of containment and/or diversionary structures or equipment to prevent regulated substances from infiltrating into the ground.
f. If the conditional use permit is granted, then the information required to be included in the SPCC Plan shall be updated annually, and a copy of the updated SPCC Plan shall be provided to the City.
(3) Prohibited Uses. The following uses are prohibited in the AP District—Community Water Systems Protection Area:
a. The development or operation of a hazardous waste disposal facility as defined under RSA 147-A.
b. The development or operation of a solid waste landfill, defined under RSA 149-M;
c. The outdoor storage of road salt or other deicing chemicals in bulk;
d. The development or operation of a junkyard;
e. The development or operation of a snow dump;
f. The development or operation of a wastewater or septage lagoon;
g. The development or operation of a petroleum bulk plant or terminal;
h. The development or operation of gasoline stations;
i. The development or operation of dry cleaning or laundry facilities using or storing dry cleaning chemicals on site;
j. Subsurface storage of petroleum products, except for propane;
k. Disposal by burial of tree stumps or brush which are transported from off site;
l. The development or operation of a commercial animal feedlot;
m. The development or operation of automotive repair, servicing, or autobody repair facilities;
n. Excavation of sand and gravel and other mining within 4 feet of the high water table; and
o. The above-ground storage of petroleum products, except for propane, in excess of a total of five hundred (500) gallons on any lot.
(4) Conditional Use Permits Required for Certain Uses in the AP District—Community Water Systems Protection Area.
a. The Planning Board may grant a conditional use permit for the following uses:
1. Storage, handling, and use of regulated substances in quantities exceeding one hundred (100) gallons or eight hundred (800) pounds dry weight at any one time, subject to the provision of an adequate Spill Prevention, Control and Countermeasure (SPCC) Plan, in accordance with the provisions of Section 28-3-6(d)(2), Spill Prevention, Control and Countermeasure (SPCC) Plan, of this ordinance.
2. Any use that will render impervious more than fifteen (15) percent or two thousand five hundred (2,500) square feet of any lot, whichever is greater.
3. Any activities that involve blasting of bedrock.
b. Prior to granting a conditional use permit, the Planning Board must find said use shall be in compliance with performance standards listed in Section 28-6-3(d)(1), Performance Standards, of this ordinance, as well as all applicable local, State, and Federal requirements. The Planning Board may impose conditions to the extent the Board concludes such conditions are necessary to minimize any adverse effect of the proposed use or activity upon groundwater, consistent with the intent of this ordinance, and to ensure compliance with State and Federal Drinking Water Quality Standards. The Planning Board may, at its discretion, require a performance guarantee or bond, in an amount and with surety conditions satisfactory to the Board, to be posted to ensure completion of construction of any facilities required for compliance with the performance standards.
(5) Nonconforming Uses. Existing nonconforming uses may continue provided that there is no expansion of the nonconforming use or any change of use to anything other than a conforming use.
a. The following uses shall be exempt from the provisions of Section 28-3-6, Aquifer Protection (AP) District, of this ordinance as long as they remain in compliance with applicable local, State and Federal requirements:
1. The sale, transportation, and use of pesticides as defined under RSA 430:29 XXVI.
2. Agricultural and horticultural operations.
b. The following uses shall be exempt from the provisions Section 28-3-6(d)(1), Performance Standards, subsections (j) through (m), of this ordinance as long as they remain in compliance with applicable local, State and Federal requirements:
1. Any business or facility where regulated substances are not stored in containers with a capacity of five (5) gallons or more.
2. Storage of motor fuel in tanks attached to vehicles and fitted with permanent fuel lines to enable the fuel to be used by that vehicle.
3. Storage and use of office supplies.
4. Temporary storage of construction materials on a site where they are to be used within six (6) months of their being brought on site.
5. Household hazardous waste collection projects regulated under New Hampshire Code of Administrative Rules Env-Hw 401.03(b)(1) and 501.01(b).
c. Storage of heating fuels for on-site use or storage of fuels for emergency electric generation is exempt from the provisions of Section 28-3-6(d)(1), Performance Standards, subsection (j), of this ordinance, provided that storage tanks are located indoors on a concrete floor or have corrosion control, leak detection, and secondary containment in place.
d. Underground storage tank systems and above ground storage tank systems that are in compliance with applicable State rules are exempt from the provisions of Section 28-3-6(d)(7), Maintenance and Inspection, of this ordinance.
e. Any single-family or two-family dwelling is exempt from the provisions of Section 28-3-6(d)(1), Performance Standards, of this ordinance.
(7) Maintenance and Inspection.
a. For uses requiring Planning Board approval for any reason, a narrative description of future maintenance requirements for structures to continue to comply with Section 28-3-6(d)(1), Performance Standards, of this ordinance, shall be recorded at the Merrimack County Registry of Deeds. The description of maintenance requirements so recorded shall comply with the requirements of RSA 478:4-a.
b. Except for those facilities where the storage of all regulated substances is exempt from this ordinance, all properties within the AP District known to the City as using or storing regulated substances in containers with a capacity of five (5) gallons or more shall be subject to inspection.
c. Where inspections are required in order to verify compliance with Section 28-3-6(d)(1), Performance Standards, of this ordinance, the City shall perform such inspections at reasonable times with prior notice to the landowner.
(e) AP District—Non-Transient, Non-Community (NTNC) Water Systems Protection Area.
(1) Performance Standards.
a. The AP District shall be left in an undisturbed and unaltered state to the greatest extent possible.
b. All proposed land use activities within the AP District shall conform to Env-Ws 373 Design Standards for Non-Community Water Systems, as well as other applicable State and Federal regulations.
c. Drainage from impervious surfaces including parking areas or storage areas shall be sloped to drain away from a NTNC water supply well and outside the AP District.
d. A distance of at least fifty (50) feet shall be maintained from a NTNC water supply well to the nearest edge of a road right-of-way, driveway, or parking area in order to minimize contamination from deicing salts.
e. Wastewater piping must be composed of ductile iron or an approved equivalent pressure-type pipe that is tested for water-tight construction after installation.
(2) Prohibited Uses. The following uses are prohibited in the AP District—Non-Transient, Non-Community Water Systems Protection Area:
a. The installation or operation of wastewater disposal systems, including septic tanks, grease traps, and effluent disposal areas;
b. The use of fertilizers;
c. The location, installation, or operation of dumpsters for the temporary disposal of solid waste or grease;
d. The installation or operation of retention ponds or infiltration basins;
e. The installation or operation of storage tanks for oil, gasoline, or other toxic chemicals or regulated substances; and
f. The location or operation of any use which involves or is in any way associated with regulated substances.
(3) Maintenance, Testing, and Inspection Requirements.
a. Inspections of NTNC water systems may be conducted by the City in order to verify compliance with all local, State and Federal groundwater quality requirements. The City shall perform such inspections at reasonable times with prior notice to the landowner.
b. The owner of the NTNC water supply well shall submit to the City copies of all information submitted to the NHDES in compliance with the wellhead protection requirements and groundwater withdrawal permit for the NTNC well.
c. On-site wastewater disposal systems situated within the Sanitary Protective Area of a NTNC water system well shall be inspected by a qualified licensed professional engineer or NHDES licensed subsurface disposal systems designer prior to any sale or conveyance of such property. A report describing the engineer/designer's findings relating to the current condition of the on-site wastewater disposal system shall be filed with the City prior to said sale or conveyance.
d. Water quality testing of groundwater samples from NTNC water supply wells shall be performed by a qualified individual prior to any sale or conveyance of said property. The findings resulting from said testing shall be filed with the City prior to said sale or conveyance. At a minimum, said water quality testing shall evaluate each sample for the presence and concentration of radon, volatile organic compounds (VOCs using EPA method 524.2), nitrate, nitrite, lead, arsenic, sodium, chloride, iron, manganese, hardness, fluoride, pH and bacteria.
e. In the event of the installation of a new NTNC water system to serve a new land use for which building permits have been issued, prior to the issuance of a certificate of occupancy for that land use, the NTNC water system shall be subject to the same testing requirements as specified in Section 28-3-6(e)(3)d. for the sale or conveyance of a property served by a NTNC water systems, with the test results of the constituent analysis provided to the City.
f. Where inspections are required in order to verify compliance with sections 28-3-6(e)(1) and (2), Performance Standards, and Prohibited Uses, respectively, of this ordinance, the City shall perform such inspections at reasonable times with prior notice to the landowner.
(4) Nonconforming Uses. Existing nonconforming uses may continue provided that there is no expansion of the nonconforming use or any change of use to anything other than a conforming use. Best management practices shall be utilized to prevent groundwater contamination resulting from existing nonconforming uses.
(f) AP District—Protection of Transient Non-Community (TNC) Water Systems Protection Area.
(1) All land uses upon property served by a transient, non-community (TNC) water system well shall comply with all pertinent local, State and Federal groundwater quality requirements in order to prevent the contamination of groundwater proximal to the TNC.
(2) All onsite wastewater disposal systems upon the property on which a TNC water system well is located shall be properly maintained and kept in good working order.
(3) In the event of a discontinuance for more than one hundred eighty (180) days of a land use served by a transient, non-community (TNC) water system, or the expansion or change of such a land use, and prior to the sale or conveyance of said property on which such a land use is located, all TNC water systems shall have water samples collected from them by a qualified individual and the sample subjected to analysis by an accredited lab for the following constituents: arsenic; bacteria; chloride; copper; fluoride; hardness; iron; lead; manganese; nitrate/nitrite; pH; and sodium. A copy of the results of the constituent analysis shall be provided to the City.
(4) In the event of the installation of a new transient, non-community (TNC) water system to serve a new land use for which building permits have been issued, prior to the issuance of a certificate of occupancy for that land use, the TNC water system shall be subject to the same testing requirements as specified in Section 28-3-6(f)(3) for the sale or conveyance of a property served by a TNC water systems, with the test results of the constituent analysis provided to the City.
(g) AP District—Protection of Domestic Water Systems.
(1) All land uses upon property served by a domestic water system shall comply with all pertinent local, State and Federal groundwater quality requirements in order to prevent the contamination of groundwater proximal to the domestic wells.
(2) All on-site wastewater disposal systems upon the property on which a domestic water system well is located shall be properly maintained and kept in good working order.
(3) In the event of a sale or conveyance of a property on which a domestic well is located, the seller of the property shall have a water sample collected from the domestic well by a qualified individual and the sample subjected to analysis by an accredited lab for the following constituents: arsenic; bacteria; chloride; copper; fluoride; hardness; iron; lead; manganese; nitrate/nitrite; pH; and sodium. A copy of the results of the constituent analysis shall be provided to the City.
(4) In the event of the installation of a new well to serve a new land use for which building permits have been issued, prior to the issuance of a certificate of occupancy for that land use, the well shall be subject to the same testing requirements as specified in Section 28-3-6(g)(3) for the sale or conveyance of a property served by a domestic well, with the test results of the constituent analysis provided to the City.
A detention basin is an impoundment designed to temporarily store runoff and release it at a controlled rate, reducing the intensity of peak flows during storm events. Detention basins may consist of surface basins (pond-type structures) or subsurface basins (enclosed structures located below ground). Detention basins may be combined with treatment BMPs discussed in this guidance document, to provide for other stormwater management objectives. For example, a stormwater pond may be designed to provide treatment as well as detention. However, a detention basin is not by itself considered a “Treatment Practice.”
General Requirements Applicable to Detention Basins
• Detention basins are prohibited in areas of RSA 482-A jurisdiction unless a wetlands permit has been issued;
• Detention basins receiving stormwater from high-load areas must be lined;
• Provide small trash racks for outlets ≤ 6 inches in diameter or weirs ≤ 6 inches wide;
• Provide energy dissipation at inlets and outlets to prevent scour;
• Provide vegetation suitable to the soil type, moisture content, sun exposure, and the level of inundation anticipated for all areas of the detention basin, including the basin floors, side slopes, berms, impoundment structures, or other earthen structures;
• Underground detention basins must have access manholes located upstream, downstream, and at intermediate locations for providing maintenance.
• The bottoms, interior and exterior side slopes, and crest of earthen detention basins should be mowed, and the vegetation maintained in healthy condition, as appropriate to the function of the facility and type of vegetation.
• Vegetated embankments that serve as “berms” or “dams” that impound water should be mowed at least once annually to prevent the establishment of woody vegetation.
• Embankments should be inspected at least annually by a qualified professional for settlement, erosion, seepage, animal burrows, woody vegetation, and other conditions that could degrade the embankment and reduce its stability for impounding water. Immediate corrective action should be implemented if any such conditions are found.
• Inlet and outlet pipes, inlet and outlet structures, energy dissipation structures or practices, and other structural appurtenances should be inspected at least annually by a qualified professional, and corrective action implemented (e.g., maintenance, repairs, or replacement) as indicated by such inspection;
• Trash and debris should be removed from the basin and any inlet or outlet structures whenever observed by inspection;
• Accumulated sediment should be removed when it significantly affects basin capacity.
Stone Berm Level Spreaders
A stone berm level spreader is an outlet structure constructed at zero percent grade across a slope used to convert concentrated flow to “sheet flow.” It disperses or “spreads” flow thinly over a receiving area, usually consisting of undisturbed, vegetated ground. The conversion of concentrated flow to shallow, sheet flow allows runoff to be discharged at non-erosive velocities onto natural ground. To stabilize the spreader outlet, a stone berm is provided to dissipate flow energy, and help disperse flows along the length of the spreader.
Level spreaders are not designed to remove pollutants from stormwater; however, some suspended sediment and associated phosphorous, nitrogen, metals and hydrocarbons will settle out of the runoff through settlement, filtration, infiltration, absorption, decomposition and volatilization.
General Requirements Applicable to Stone Berm Level Spreaders
• The spreader must discharge to a vegetated receiving area with capacity to convey the discharge without erosion;
• The receiving area must be stable prior to construction of level spreader.
• Inspect at least once annually for accumulation of sediment and debris and for signs of erosion within approach channel, spreader channel or down-slope of the spreader.
• Remove debris whenever observed during inspection.
• Remove sediment when accumulation exceeds 25% of spreader channel depth.
• Mow as required by landscaping design. At a minimum, mow annually to control woody vegetation within the spreader.
• Snow should not be stored within or down-slope of the level spreader or its approach channel.
• Repair any erosion and re-grade or replace stone berm material, as warranted by inspection.
• Reconstruct the spreader if down-slope channelization indicates that the spreader is not level or that discharge has become concentrated, and corrections cannot be made through minor re-grading.
Conveyance swales are stabilized channels designed to convey runoff at non-erosive velocities. They may be stabilized using vegetation, riprap, or a combination, or with an alternative lining designed to accommodate design flows while protecting the integrity of the sides and bottom of the channel. Conveyance channels may provide incidental water quality benefits, but are not specifically designed to provide treatment.
General Requirements Applicable to Conveyance Swales
• Swales receiving stormwater from high-load areas must be lined (impermeable liner) to isolate the runoff from contact with underlying soil and groundwater.
• Grassed channels should be inspected periodically (at least annually) for sediment accumulation, erosion, and condition of surface lining (vegetation or riprap). Repairs, including stone or vegetation replacement, should be made based on this inspection.
• Remove sediment and debris annually, or more frequently as warranted by inspection.
• Mow vegetated channels based on frequency specified by design. Mowing at least once per year is required to control establishment of woody vegetation. It is recommended to cut grass no shorter than 4 inches.
Terraced Slopes or Benching
The land grading practice of providing terraced slopes or benching consists of shaping disturbed land surfaces to control the length of flow down steep slopes. Intermediate terraces (or benches) are incorporated into slopes that exceed 4:1 gradient. These terraces are then used to convey runoff laterally to a safe discharge (or to a constructed drainage system). The purpose of this practice is to provide for erosion control and vegetative establishment on those areas where the existing land surface is to be reshaped by grading.
Provisions should be made to safely conduct surface runoff collected by the terraced slope to storm drains, stabilized channels, or other stable conveyance practices or water courses. Runoff should also be intercepted at the top of the slopes and directed to a stable outlet.
General Requirements Applicable to Conveyance Swales
• Benches are required wherever the vertical height of a slope meets the conditions listed in the Design Criteria table;
• Benches, when required, must divide the slope face into equal parts;
• Benches must convey stormwater to a stable outlet.
• Grassed slopes should be mowed to grass height and frequency specified by design.
• Vegetated slopes should be inspected periodically for signs of vegetation loss or damage, with restoration as needed.
• Terraces and slopes should be inspected periodically for any sign of rill or gully erosion, and if such conditions are noted, the area should be immediately investigated and repaired as needed.
A flow splitter is an engineered structure used to divide flow into two or more directions. The structure typically consists of a manhole, precast concrete vault, or other structure divided into chambers, with the chambers separated by hydraulic control elements. Various hydraulic devices (such as pipes, weirs, or orifices) can be used to control the direction and quantity of flow entering the structure. Generally, a flow splitter consists of a structure with one inlet and two outlets set at different elevations. One outlet conveys low flows, such as those during small storms or at the beginning of a large storm. The other outlet conveys high flows occurring later in the storm. The flows are conveyed in different directions for water quantity or quality control.
The flow splitter is typically used to direct base flows and smaller storm flows to an “off-line” water quality treatment or pretreatment practice, with larger storms directed to an alternative outlet to bypass, and thus prevent overloading of, the treatment system. This simple type of device works on hydraulic principles and does not require mechanical components or instrumentation.
• Flow splitters should be inspected concurrently with the conveyance and treatment practices served by the devices. It is recommended that the device be inspected and maintained at least once annually.
• Sediments and debris should be removed and disposed as for other components of the drainage system.
Outlet protection is typically provided at stormwater discharge conduits from structural best management practices to reduce the velocity of concentrated stormwater flows to prevent scour and minimize the potential for downstream erosion. Outlet protection is also provided where conduits discharge runoff into an in-ground stormwater management practice (e.g., pond or swale) to prevent scour where flow enters the BMP.
Standard engineering practices allow for many different types of outlet protection which provide energy dissipation. Common outlet protection measures include:
• Riprap aprons, the design of which is covered within this section;
• Riprap lined scour holes, stilling basins or plunge pools. Design references for stilling basins are provided under ‘Design References’.
Other outlet protection practices may be used, if documented by applicable technical literature.
• Inspect the outlet protection annually for damage and deterioration. Repair damages immediately.
Operation, Maintenance, Inspection & Source Control
The operation and maintenance of a stormwater management system and its individual components is as critical to system performance as the design. Also, implementation of source controls is an important aspect of the operation of a site to prevent contaminants from exposure to runoff, thus minimizing the pollutants that need to be treated by the stormwater management system. This Chapter addresses the operation and maintenance considerations of stormwater design, the preparation of an Inspection and Maintenance (I&M) Manual, and the preparation of a Source Control Plan.
Thus, the design process must give serious consideration to maintenance issues to develop stormwater management facilities with realistic maintenance expectations. Proper operation and maintenance will ensure that the stormwater system and individual BMPs will remain effective at removing pollutants as designed and meeting New Hampshire’s water quality objectives. Proper maintenance will:
• Maintain the volume of stormwater treated over the long term;
• Sustain the pollutant removal efficiency of the BMP;
• Reduce the risk of re-suspending sediment and other pollutants captured by the BMP;
• Prevent structural deterioration of the BMP and minimize the need for expensive repairs;
• Decrease the potential for failure of the BMP.
Without proper maintenance, BMPs are likely to become functionally impaired or to fail, providing reduced or no treatment of stormwater. Design must consider how facilities will be accessed for inspection and maintenance, what activities are needed to maintain each facility, the frequency these activities must be performed, and who will be responsible for inspection and maintenance. The location and sizing of BMPs must account for these considerations. Also, the site design may require development of easements or deed restrictions to provide for access to perform necessary maintenance and repairs.
In addition, the AoT regulations require the development of an Inspection and Maintenance (I&M) Manual for stormwater management systems, identifying responsible parties for implementing the required maintenance activities, detailing the activities that are necessary, and documenting the implementation of the activities.
Inspection and Maintenance (I&M) Manual
A formal operation and maintenance plan for a stormwater system will assist the party responsible for maintenance in understanding how the system functions and the maintenance activities needed to maintain that function. Such a plan clearly identifies inspection activities, schedules, record keeping requirements, and contingency measures for ensuring the long-term integrity of the stormwater management facilities. Typically, such a plan identifies each BMP used on the site and its specific maintenance activities and schedules.
The AoT regulations (Env-Wq 1500) require the long term maintenance of stormwater practices, and stipulate the establishment of a mechanism to provide for ongoing inspections and maintenance. Such a mechanism includes the preparation of an Inspection and Maintenance (I&M) Manual. This manual must include, at a minimum, the following:
1. The names of the responsible parties who will implement the required reporting, inspection, and maintenance activities identified in the I&M manual;
2. The frequency of inspections;
3. An inspection checklist to be used during each inspection;
4. An inspection and maintenance log to document each inspection and maintenance activity;
5. A deicing log to track the amount and type of deicing materials applied to the site;
6. A plan showing the locations of all the stormwater practices described in the I&M manual; and
7. Actions to be taken if any invasive species begin to grow in the stormwater management practices.
All record keeping required by the I&M manual shall be maintained by the responsible parties, and any transfer of responsibility for I&M activities or transfer in ownership shall be documented to the DES in writing.
Concord Code of Ordinance
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