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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)
(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)
(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)
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