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§ 11.05.040.010 GENERAL.
   Storm drain systems shall be designed and constructed, in accordance with these engineering/design and construction standards and the Montana Department of Environmental Quality’s design circular DEQ-8, Montana Standards for Subdivision Stormwater Drainage, if the stormwater system is associated with a subdivision development.
(Prior Code, § 11.05.040.010) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.020 STORM DRAINAGE PLANS AND REPORT.
   (A)   A storm drainage plan shall be submitted for development or redevelopment that includes the creation of more than 5,000 square feet of new impervious surface area.
   (B)   (1)   A detailed stormwater report shall be prepared by a registered professional engineer.
      (2)   The stormwater report shall contain the stormwater calculations and a discussion of the design rationale. The plan shall include the following:
         (a)   A map showing building site(s), open areas, drainage ways, ditches, culverts, storm sewers, inlets, storage ponds, roads, streets and any other infrastructure improvements which may affect drainage characteristics. The map shall also include identification of the various ground surfaces (i.e., vegetation, gravel and pavement) and their respective coverage (square feet);
         (b)   Topographic contours (one-foot intervals) and sufficient spot elevation data;
         (c)   Description of the ultimate destination of stormwater run-off from the project and an evaluation of its impact on down slope drainage facilities;
         (d)   Design calculations determining run-off quantities, storage requirements and peak rates;
         (e)   Description of water quality design method and calculations;
         (f)   A storm drainage facilities operation and maintenance plan. The plan shall:
            1.   Identify ownership of all facilities;
            2.   Establish a schedule for maintenance activities necessary to keep the system operationally effective; and
            3.   Identify the responsible party in charge of the specific maintenance duties.
         (g)   Details and specifications (including invert and other pertinent elevation information) for all storm drainage improvements, such as storm sewer, manholes, inlets, discharge structures, and retention/detention pond dimensions and volume, side slope, and top, bottom and maximum water surface elevations.
   (C)   Pre-development and post-development run-off calculations for sizing drainage facilities shall include:
      (1)   Depiction of the drainage area on a topographical map, with acreage indicated (the off-site contributing area must be included).
         (a)   Provide a pre-development topography and drainage basin map and a post-development topography and drainage basin map.
         (b)   The change in peak discharge and volume of surface water entering and leaving the subject property.
      (2)   The capabilities of all downstream drainage systems within one-fourth mile of the project site downstream boundary to handle the change in peak discharge and volume of surface water caused by the development.
      (3)   In areas where ground water may be a factor, the analysis shall include the effects of the ground water on the total storm drainage system proposed for the development.
(Prior Code, § 11.05.040.020) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.030 METHODS OF DESIGN.
   (A)   Storm event peak rates and volume. Two methods are to be used for determining peak stormwater run-offs for the design of storm sewers, open channels and culverts.
      (1)   (a)   The rational formula method is to be used to design storm sewer systems.
         (b)   Information on the rational method is available in Chapter 7 of the MDT Hydrology Manual, including rainfall-intensity return periods.
      (2)   Use the one-hour precipitation return periods identified for station Butte - eight miles south.
         (a)   When it is necessary to know the volume of water and discharge in addition to the peak rate of flow, the SCS TR-20 Hydrologic Analysis Model or pre-approved hydrologic simulation model will be used together with a detailed routing analysis.
         (b)   For flow routing through detention facilities models such as; SWMM, StormNet and HydroCAD are also acceptable methods.
   (B)   Culverts and open channel design.
      (1)   Culverts shall be designed in accordance with the U.S. Department of Transportation, Hydraulic Engineering Circular No. 5.
      (2)   Open channels shall be designed using the energy concept and the Manning formula.
      (3)   Channels in earth shall have sides with a maximum slope of one-foot vertical rise for every two feet of horizontal distance.
      (4)   Steeper slopes will be permitted only if some type of bank protection is used to prevent erosion and sloughing of the sides into the bottom of the channel.
      (5)   Constructed and natural channels and culverts shall be designed with sufficient capacity to convey, at a minimum, the depth associated with the 100-year 24-hour design storm peak flow rate, assuming developed conditions for on-site tributary areas and existing conditions for any off-site tributary areas.
   (C)   Storm drain systems and inlets. The ten-year 24-hour design storm shall be used to size the enclosed storm sewer system and inlets.
(Prior Code, § 11.05.040.030) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.040 GENERAL DESIGN REQUIREMENTS.
   (A)   All required stormwater retention/detention facilities must be constructed and in operation prior to paving and building construction, unless otherwise approved by the Department of Public Works.
   (B)   The capacity of the downstream drainage course is required to be evaluated for a minimum distance of one-fourth mile from the point of discharge of the development.
      (1)   The downstream conveyance system must be adequately sized to handle off-site peak flows conveyed through the development plus the on-site developed peak flow discharged to the drainage course from the controlling ten-year storm.
      (2)   This system should be shown on the drainage layout.
      (3)   The size of the controlled outlet shall be calculated for the total drainage basin when the total run-offs from both off-site and on-site areas are combined.
   (C)   The limits of the FEMA mapped 100-year floodplain shall be required to be delineated on the final plat, as well as the engineering drawings for plat construction.
   (D)   Arrows indicating drainage direction in all public and private property shall be shown on the construction drawings for all hydraulic conveyance systems.
   (E)   (1)   A topographical map sufficient in area to show all areas draining onto the proposed land development must be submitted and show enough of abutting downstream properties to indicate natural or human-made drainage course into which the proposed land development is to drain.
      (2)   The direction of flow, acreage of areas contributing drainage to the development, the outline of the development and location of intermittent streams or drainage courses which are within the site boundaries shall be indicated on this map.
   (F)   Surface water entering the subject property shall be received at the naturally occurring location, and surface water exiting the subject property shall be discharged at the natural location, with adequate energy dissipaters within the subject property to eliminate the potential for detriment to the existing downstream infrastructure.
(Prior Code, § 11.05.040.040) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.050 STORM SEWER SYSTEM; MINIMUM REQUIREMENTS.
   (A)   Manhole spacing. The maximum spacing between storm manholes shall be 500 feet.
   (B)   Minimum pipe size.
      (1)   The minimum diameter of any storm drain pipe shall be 12 inches, except that a maximum length of 60 lineal feet of eight-inch diameter pipe may be used between inlets and manholes.
      (2)   The minimum size of any storm drain culvert is 12 inches.
   (C)   Storm drain manhole ring and cover.
      (1)   The storm drain manhole ring and cover shall be East Jordan Iron Works Model 3 772Z 1 or an approved equal.
      (2)   The cover shall be marked “storm.”
   (D)   Curbs and gutters or swales.
      (1)   Streets and roads shall be designed to ensure proper drainage and curbs and gutters or swales shall be required in all subdivisions.
      (2)   Curbs and gutters of adjoining properties shall be extended to match any new curb and gutter. Gutter, ditch and swale flow line grades shall be greater than 0.5%.
   (E)   Culverts and drainage facilities. Culverts of adequate size shall be provided and installed by the developer where drainage channels intersect any street right-of-way or easement. All culverts shall extend at least the width of the base of the fill.
      (1)   The amount of backfill to be placed over the culvert and a culvert’s capacity shall be determined by a licensed professional engineer.
      (2)   This shall include arrangements for driveway culverts. Culverts larger than 24 inches shall have flared ends.
   (F)   Drain inlet frame and grate. The curb inlet frame and grate shall be Olympic SM 49, East Jordan Iron Works combination of product numbers 7222Ml or 7222M3 (Herringbone grate) or an approved equal.
   (G)   “Beehive” area drain frame and grate. The beehive area drain frame and grate shall be Neenah Foundry Model R-2560-D6 “beehive grate with frame,” East Jordan Iron Works 1205-2 or an approved equal.
   (H)   “Drive over” area drain frame and grate. The drive over area drain frame and grate shall be Neenah Foundry Model R-2556-A Type C frame with Type G grate, East Jordan Iron Works Model 1848-G or an approved equal.
   (I)   Sump requirements for catch basins and manholes. A 18-inch sump is required for catch basins and drain inlets. A 24-inch sump is required for storm drain manholes.
(Prior Code, § 11.05.040.050) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.060 FLOW CONTROL AND RETENTION/DETENTION POND DESIGN REQUIREMENTS.
   (A)   This section outlines the requirements for sizing flow control facilities. The flow control standards are based on the Stormwater Management Manual for Eastern Washington and the Spokane County Stormwater Manual.
      (1)   Standard flow control facilities are detention, retention (natural depressions) and infiltration facility.
      (2)   Any other facility is considered a non-standard system and shall be evaluated individually by the City PWD.
      (3)   Flow control facilities are necessary to mitigate potential adverse impacts on down-gradient properties due to the increase in stormwater run-off caused by land development.
   (B)   (1)   The peak rate of stormwater run-off from any proposed land development to any natural or constructed point of discharge downstream shall not exceed the pre-development peak rate of run-off.
      (2)   The post development volume of run-off can exceed the pre-development volume of run-off when the required down-gradient analysis demonstrates that there will be no adverse impacts on down gradient properties or existing natural and constructed conveyance systems.
(Prior Code, § 11.05.040.060) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
§ 11.05.040.070 DETENTION FACILITIES MINIMUM REQUIREMENTS.
   (A)   Facility volume.
      (1)   The CN method requires the selection of, or the input of, a rainfall distribution and the precipitation associated with a design storm.
      (2)   The NRCS Type I 24-hour ten- and 100-year storm events are the design storms to be used for all flow control facilities that use a surface discharge.
      (3)   Detention ponds shall be sized using one of the following methods:
         (a)   Sized to contain the difference between the post development and the pre-development 24-hour storm events; or
         (b)   Post development level pool routing method with a draw down time of 72 hours.
   (B)   Setbacks. When a detention facility is proposed upslope of developed property or at the top of a slope inclined 15 % or greater, the minimum setback from the slope must be greater than or equal to 50 feet or as determined by a licensed engineer with geotechnical experience.
   (C)   Release point.
      (1)   Generally.
         (a)   Stormwater run-off from a developed site shall leave the site in the same manner and location as it did in the pre-developed condition.
         (b)   The distance between the outlet structure and the inlet into the detention facility shall be maximized.
      (2)   Outflow control structures.
         (a)   Control structures are manhole or catch basins with a restrictor device used for controlling outflow from a facility to meet a desired standard.
            1.   Outflow structures are required for all stormwater detention facilities.
            2.   The restrictor device is usually multiple orifices, consisting of two or more orifices and/or a weir section sized to meet performance requirements.
         (b)   1.   Run-off shall enter the detention facility through a conveyance system separate from the control and outflow conveyance system.
            2.   The distance between the inlet and outlet shall be maximized to reduce sediment from accumulating in the outflow structure. The types of outflow control structures are listed below.
               a.   Multiple orifice restrictors.
                  i.   In most cases, control structure need only two orifices: one at the bottom and one near the tip of the riser;
                  ii.   Minimum orifice diameter is three inches; and
                  iii.   Orifice shall be constructed on a tee section or on a baffle.
               b.   Risers and weir restrictor.
                  i.   Properly designed weirs maybe used as flow restrictors. However, they must be designed to provide for primary overflow of the developed 100-year peak flow discharging to the detention facility; and
                  ii.   The combined orifice and riser (or weir) overflow may be used to meet flow requirements. However, the design must still provide for primary overflow of the developed 100-year peak assuming all orifices are plugged.
               c.   Outflow control structure minimum requirements. The following minimum access requirements shall be met. Additional requirements are specified in Appendix A.
                  i.   An access road to the control structure is required for inspection and maintenance; and
                  ii.   Manhole and catch basin lids for control structure shall be locking and rim elevation shall match proposed finish grade.
(Prior Code, § 11.05.040.070) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)
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