The City of Dover has enacted Storm Water Ordinances in order to treat stormwater to maintain and protect the quality of receiving waters and to reduce the discharge of pollutants. The City of Dover 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 Dover will only provide construction permits to projects that establish a plan to manage stormwater runoff occurring during the construction process. The City of Dover, 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 Dover has the authority to inspect stormwater facilities and practices in order to ascertain that they properly maintained and functioning.
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.”
• 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.
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.
• 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.
• 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.
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.
• 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.
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.
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.