Chapter 13.12 STORM SEWER FACILITIES

This chapter is adopted for the purpose of providing a standardized, safe and orderly manner of collecting, transporting, and disposing of stormwater runoff. The primary goal of this chapter, will, therefore, be to protect and safeguard human health, life, and development and subdivision of all lands. The territorial jurisdiction of the regulations found in this chapter shall include all of the incorporated lands located within the town. (Ord. 130 §1, 1988)

13.12.020 Definitions.

As used in this chapter:

1. "Collector street" means those roads defined as urban collector roads on the approved state Urban Systems Transportation Map.

2. "Council" means the town council of West

Yellowstone, Montana.

3. "Dedication" means the setting apart of land or interests in land for use by the public. Land becomes dedicated only when accepted by the town as a public dedication, either by the passage of an ordinance, or by entry of a resolution of approval in the official minutes books of the official meetings of the town.

4. "Design criteria" means guidelines upon which planning and engineering decisions and judgments are based.

5. "Design storm" means a precipitation event that, statistically, has a specified probability of occurring in any given year (expressed either in years or as a percentage).

6. "Developer" means an individual, firm, or group who undertakes the subdividing of a lot, tract, or parcel of land for the purpose of transfer of ownership or building development, and including dedication of street or changes in street or lot lines.

7. "Drainage" means interception, collection, and removal of excess storm water from an area into another area or into a receiving water body.

8. "Easement" means a grant by a property owner to specific persons or to the public to use land for specific purposes.

9. "Flood control" means preventing the entry of storm water into an area from another area, or from a stream or other water body.

10. "Land development" means the division of a lot, tract, or parcel of land into more than two parts for the purpose of transfer to ownership or development, and also surveys for clarification of property ownership and/or street rights-of-way.

11. "Master planning" means a systems approach to the planning of facilities, programs, and management organizations for comprehensive control and use of storm water with in a defined geographical area.

12. "One hundred year flood" means a flood stage that statistically has a one percent probability of occurring in any given year.

13. "Plat, final" means a map of all or part of a subdivision providing substantial conformance to an approved preliminary plat, prepared in accordance with Montana Code as amended or subsequently codified.

14. "Plat, preliminary" means a preliminary plan and other documentation of a proposed subdivision, planned unit development, or dedication containing the elements and requirements set forth in the subdivision ordinance.

15. "Protective covenants" means a written agreement between seller and purchaser stating restrictions or conditions.

16. "Rainfall intensity" means the rate at which rainfall falls to the ground in inches per hour. The value of this rainfall will vary depending on the duration of the storm event.

17. "Receiving waters" means streams, lakes, bays, etc., into which storm waters are discharged.

18. "Shall" means mandatory.

19. "Special building permits" wherever used in this chapter, the special building permits shall refer to those building permits required by the town, in conjunction with construction of multifamily dwellings, commercial and industrial facilities, and all public facilities. Building permits required for single-family dwelling units or duplex units shall not be considered a special building permit in this chapter.

20. "Standards" means detailed engineering drawings and/or specifications promulgated by public or private organizations that leave little choice to design engineers and techniques (e.g., manhole, catch basin, and inlet standards).

21. "Storm sewers" means usually enclosed conduits that transport storm water runoff toward points of discharge (sometimes called "storm drains").

22. "Stormwater drainage" means temporary storage of runoff on, below, or above the surface of the earth for the purpose of attenuating runoff.

23. "Street" means a right-of-way which provides vehicular and pedestrians access to adjacent properties, the dedication and maintenance of which the town has accepted by official action. It shall include the following terms: street, drive, court, circle, private street, road, avenue, boulevard, lane, place, and other such terms.

24. "Time of concentration" means the time period necessary for surface runoff to reach the outlet of a subbasin from the hydraulically most remote point in the tributary drainage area.

25. "Town engineer" means the designated registered engineer who is responsible for the administration of urban runoff matters.

26. "Travel time" means the sum of the time intervals for overland flow, sewer or gutter flow, and pipe and channel flow from the hydraulically most remote point in the tributary to the discharge point of interest.

27. "Utilities" means facilities for service to and use by the public. (Ord. 130 §2, 1988)

13.12.030 Drainage plans--General requirements.

Before any special building permit is issued or approved by the town, a drainage plan shall be submitted and approved to the town in accordance with the requirements of Sections 13.12.040 through 13.12.100. Also, before any subdivisions of land or development occur within the town or its impact zone, a preliminary and final drainage plan shall be submitted and approved by the town and its engineer. The procedures for these submissions and approval of these drainage plans shall follow the procedure outlined in Sections 13.12.040 through 13.12.100. (Ord. 130 §3, 1988)

13.12.040 Drainage plans--Control authority--Appeal.

A drainage plan shall be submitted and approved to the town in accordance with the following requirements:

A. Review of all preliminary and final drainage plans;

B. Approval, disapproval, or conditional approval of the drainage preliminary and final plans. The reasons for any disapproval or conditional approval shall be stated in writing to the applicant.

C. The town shall have the right to modify or vary the enforcement of these rules hereunder where extraordinary hardship exists; provided, that substantial justice is done and the public interest is secured. In granting any such variance or modification, the town may require conditions at will in its judgment to secure substantial compliance with the general principles thereof.

D. Appeals may be had by any person, firm, or corporation from a decision of the town or the town engineer within fifteen days after its decision. (Ord. 130 §4, 1988)

13.12.050 Preliminary drainage plan--Submittal and approval.

A. A preliminary drainage plan shall be required and submitted to the town engineer in conjunction with all preliminary plats and shall be mandatory for all changes in land use from that existing as of the date of the ordinance codified in this section. This drainage plan shall be reviewed and approved by the town engineer before a final drainage plan and final plat will be accepted by the town for consideration. The town shall approve conditionally or disapprove the drainage plan in writing within thirty calendar says of the submittal. If the preliminary drainage plan is disapproved, resubmittal of the entire drainage plan will be required before reconsideration of the plan will be undertaken by the town engineer.

B. A drainage plan shall not be required for single-family or duplex unit building permits, but shall be required for all other building permits (multifamily—more than two—commercial, industrial, public facilities, etc.), and for all land development or subdivisions of lands. In addition, these special building permit drainage plans will conform with Sections 13.12.080(D) and (E). (Ord. 130 §5(a), 1988)

13.12.060 Preliminary drainage plan--Contents.

The preliminary drainage plan shall contain design calculations and other supporting data showing how the development pro poses to meet the general stormwater drainage standards stated in this chapter. This preliminary drainage plan and calculations must be prepared and signed by a professional engineer registered in the state. It shall, in general, also contain the following:

A. A preliminary engineering plan which shall identify how the runoff from major rainstorms will be conveyed by the site’s swales, storm sewers, natural drainage ways, and streets during and after construction.

B. A preliminary drawing that shows the development’s position in the drainage basin and how it would be developed so as not to adversely effect upstream and downstream stormwater flows.

C. Preliminary hydrological and hydraulic calculations and a general layout drawing showing the proposed location and approximate sizes and grades of storm sewer lines, catch basins, pumps, detention basins, and other stormwater drainage structures. (Ord. 130 §5(b), 1988)

13.12.070 Final drainage plan--Submittal and approval.

A final drainage plan shall be submitted and approved in conjunction with all land development. Each final drainage plan shall be approved and signed or disapproved by the town engineer. The reasons for disapproval or conditional approval shall be stated in writing within thirty calendar days of receipt by the town engineer. If a final drainage plan is disapproved by the town engineer, a complete new resubmittal will be required before the town engineer will consider the drainage plan for approval. The approval of the final plat will not be given until the final drainage plan has been reviewed and approved by the town engineer. (Ord. 130 §6(a), 1988)

13.12.080 Final drainage plan--Contents.

The final drainage plan shall contain final design calculations and other supporting data showing how the development proposes to meet the surface drainage standards stated in this chapter. The final drainage plan and calculations must be signed by a professional registered engineer in the state. In detail, the following items shall be submitted as part of the final design plan:

A. Final design calculations in accordance with this chapter criteria shall show the storm drainage flows anticipated in each street, closed conduit, and/or open channels of the proposed drainage facilities. A standard form provided by the town shall be used in preparing these storm drain calculations.

B. Detailed plans and specifications which will be needed for grading the site and constructing streets, storm sewers, detention basins, ponds, and other storm sewer control facilities. These construction drawings shall be prepared in accordance with the town standards on twenty-four inch by thirty-six inch mylar sheets.

C. Where the proposed drainage facilities are not located in dedicated street easements, there shall be provided an easement or drainage right-of-way conforming with the proposed watercourse and of such width to contain all construction activities, all future facilities, and of adequate width to also provide for future maintenance of the proposed facilities, or as determined by the town engineer.

D. Approval from the Corps of Engineers, Federal Emergency Management Agency (FEMA), and other governing agencies for modification to major waterways. (Ord. 130 §6(b), 1988)

13.12.090 Record drawings--Submittal and approval.

Final record drawings on twenty-four inch by thirty-six inch mylar sheets shall be submitted, approved, and signed, or disapproved by the town engineer for all storm sewer improvements to be constructed within the town public street or right-of-way. The reason for disapproval or conditional approval shall be stated in writing within fifteen days of receipt by the town engineer. Final acceptance of the improvements to be maintained by the town will not be given until the required record drawings have been submitted and approved by the town engineer. (Ord. 130 §7(a), 1988)

13.12.100 Record drawings--Contents.

A. The record drawings shall contain all as-constructed items pertaining to storm drainage facilities. These record drawings shall. be prepared in accordance with the town drawing standards and shall be inked on mylar so as to provide a permanent, reproducible copy of all existing stormwater facilities.

B. The record drawing submittals shall also include documentation of the closed conduit and open channel storm sewer facilities by listing pipe diameter, open channel storm sewer facilities by listing pipe diameter, open channel size and dimensions, flow line elevations, and contributing areas. A standard sheet to be used for summarizing this submittal shall also be used. (Ord. 130 §7(b), 1988)

13.12.110 Hydrologic design--Generally.

Hydrologic design will be predicated upon full urban development of the tributary watershed. Stormwater runoff of these urban development areas shall be in accordance with the type of development stipulated by the zoning ordinance and by the adopted land use plan in the town’s impact area. (Ord. 130 §8, 1988)

13.12.120 Hydrologic design--Waterways.

A. Waterways are those drainage basins with contributing drainage areas less than four square miles. These storm drainage facilities shall be designed for a design storm of ten years for all transport facilities and for a one-hundred-year design storm for all detention facilities.

B. For all watersheds and waterways, the design discharge shall be determined by the use of the rational method as per subsection C of this section or the simplified method as per subsection G of this section.

C. Rational Method. The rational formula shall be applied and used as follows:

Q = C I A

Where:

Q = Design discharge in cubic feet per second C = Runoff coefficient (see Table I below)

I = Intensity of rainfall, inches per hour (see Table II or III below)

A = Tributary watershed area in acres

Table IRUNOFF COEFFICIENTS
Type of Area	Runoff Coefficients
Rural, park, forested, agricultural		.10 - .30
Low residential (single-family dwellings)		.30 - .50
High residential (multiple family dwellings)		.60
Business and commercial		.80
Industrial		.80
Impervious		.90
Schools		.50

Table II10 YEAR DESIGN RAINFALL
INTENSITY – DURATION DATA
Duration in Minutes	Intensity(in/hour)
5.0	2.46
6.0	2.31
7.5	2.04
10.0	1.73
12.5	1.53
15.0	1.46
20.0	1.16
25.0	1.02
30.0	0.94
45.0	0.69
60.0	0.59
90.0	0.45
120.0	0.35
180.0	0.27
240.0	0.20
300.0	0.14
360.0	0.12

Table III100 YEAR DESIGN RAINFALL
INTENSITY – DURATION DATA
Duration in Minutes	Intensity (in/hour)
5.0	4.28
6.0	4.03
7.0	3.69
8.0	3.41
9.0	3.19
10.0	3.00
12.5	2.64
15.0	2.55
20.0	2.02
25.0	1.77
30.0	1.63
45.0	1.26
60.0	1.02
90.0	0.94
120.0	0.72
180.0	0.52
240.0	0.36
300.0	0.18
360.0	0.15
420.0	0.14
480.0	0.12

D. The time of concentration shall be based on an initial lot to street time of ten minutes for lots smaller than one-half acre, and fifteen minutes for lots equal to or greater than one-half acre.

E. Where the size of the watershed is too large for the application of the rational method in one step, the waterways shall be subdivided into reaches of reasonable length, and the rational formula applied to each reach step-by-step, properly accumulating parameters and the time of concentration.

F. For all transport facilities, the engineer shall determine, using a design storm of one hundred years, the impact and/or surcharging that would occur in all proposed storm drainage facilities.

G. Simplified Method. For small drainage basins where the developer does not want to apply the rational formula and the step-by-step approach, 1.46 inches of water shall be used for a ten-year design storm, and 2.55 inches of water shall be used for as one-hundred-year storm. These values shall then be used for the intensity of rainfall or "I" in the rational formula in order to calculate the peak runoff flows for the appropriate contributing area, "A". The total volume of water discharged shall be calculated assuming the entire volume of water will be discharged from the site. (Ord. 130 §9, 1988)

13.12.130 Hydraulic design--Generally.

A. The design of all stormwater improvements within the development shall be such that all surface waters occurring within the development, as well as all surface waters flowing onto and/or through the development from the proposed future additional developments upstream, shall be conveyed through the development without damage to any improvement, building site, or dwelling which may be constructed within the development. All storm drainage facilities within the development shall be designed to adequately convey the stormwater runoff from the ultimate development of the drainage basin or watershed.

B. Also, all storm drainage facilities to be constructed outside of the subdivision shall be designed to adequately convey the stormwater runoff from the ultimate development of the drainage basin or watershed lying within and above the development to a natural watercourse having capacity to handle the generated present and future runoff. These waters shall be conveyed without damage to any improvement, building, or dwelling to the anticipated natural watercourse having capacity to carry the water or to a point designated by the town. (Ord. 130 §10(part), 1988)

13.12.140 Hydraulic design--Capacity.

A. The general capacity of the drainage facilities shall be determined by use of the Manning equation as stated below:


	0.667		0.5
Q =	A(1.486)	R	S
	n

Where:

Q = Flow rate in cubic feet per second

n = Manning roughness coefficient (see Table IV)

A = Cross-sectional area in square feet

R = Hydraulic radius in feet

S = Slope in ft./ft.

B. The minimum velocity at design flow shall be two feet per second using the Manning equation above.

C. Predicted surcharging and flooding due to a one hundred year design storm imposed upon the proposed facilities will also be calculated.

D. The design of drainage facilities shall allow for a minimum freeboard of 0.75 feet between the top of the inlet grade or manhole cover and the design surface water elevation.

E. Proper allowance for head losses through junction structures, inlets, and manholes shall be made in determining the hydraulic grade line or water surface elevations.

Table IV MANNING "n" VALUES
Type of Conduit or Channel	Roughness Coefficient "n"
Concrete gutters		.015
Corrugated metal pipe		.024
Reinforced concrete pipe--12" to 21"		.015
Reinforced concrete pipe--24" or larger		.013
Asbestos cement pipe		.012
Lined channels:
Concrete		.014
Air blown mortar		.016
Bituminous		.018
Sacked concrete		.025
Loose riprap		.035
Grassed		.035
Concrete--steel troweled or smooth form finish		.013
Concrete--wood float or broomed finish		.015
Asphaltic concrete		.017
PVC		.012

(Ord. 130 §10 (part) , 1988)

13.12.150 Closed conduit systems.

A. Closed Conduit Systems.

1. Minimum inside pipe diameter shall be twelve inches.

2. The maximum space of manholes or junction boxes shall not be more than four hundred feet.

3. Manholes or junction boxes shall be provided at all horizontal angle points and at all changes in pipe grade.

4. Curves in pipe systems will only be allowed when the pipe diameter is thirty-six inches or larger.

5. The maximum spacing of catch basins shall not be more than five hundred feet.

B. Minimum Cover. The minimum cover over pipes installed within the road right-of-way shall be as required for pipe strength, loading conditions, and hydraulic requirements, but in no case shall the minimum cover over storm sewer lines be less than one-and-one-half feet, and in no case shall the minimum depth to the flow line be less than four-and-one-half feet.

C. Surcharging Within Closed Conduit Systems. Surcharging or back-up of stormwaters into catch basins and manholes will not be allowed under a ten-year design storm.

D. Culverts. Drainage culverts under roads shall have a minimum diameter of eighteen inches and under driveways a minimum diameter of twelve inches and shall be placed at a depth as required by subsection B of this section unless otherwise approved by the town engineer.

E. Catch Basins. Catch basin Types I, II, or III, shall be used as required by the hydraulic and slope conditions or as directed by the town engineer.

1. Catch Basin Type I. A Type I catch basin shall be used where a one to two block area is to be collected into the storm sewer facilities.

2. Catch Basin Type II. Catch basin Type II shall be used where a drainage area of four to five blocks or less is to be collected into the storm sewer facilities.

3. Catch Basin Type III. A Type III catch basin shall be used where a one to two block area is to be collected and where a standard manhole is to be used as the catch basin.

F. Cleanout Structures. A Type I or II cleanout structure shall be provided in the storm drainage facilities to facilitate the collection of debris and gravel from the streets and from surrounding erosion, or as directed by the town engineer.

1. Type I Cleanout Structure. A Type I cleanout structure shall be used in major transport lines when directed by the town engineer.

2. Type II Cleanout Structure. A Type II cleanout structure shall be provided at the entrance to all major retention facilities and at the bottom end of a complete waterway and prior to discharge into any receiving stream or basin. The placement of a Type II cleanout structure may be waived at the discretion of the town engineer. (Ord. 130 §11, 1988)

13.12.160 Open channels.

A. Curbs and Gutters. Curbs and gutters shall be designed to carry the ten-year design flow. Breaking of back of curbs for curb cuts shall not be allowed.

B. Concrete Valley Gutters. Concrete valley gutters will not be permitted across collector streets. Concrete valley gutters shall be designed to carry the ten-year de sign storm at a design depth not to exceed eighty percent of the maximum carrying capacity. All valley gutters shall be built as per the town standard storm water details. (Ord. 130 §12, 1988)

13.12.170 Channels and ditches.

A. Channels or open ditches may be used in place of pipe when the required pipe size would be larger than sixty inches in diameter and approval is received from the town engineer. Existing natural channels and creeks may be retailed unimproved in buildup areas if the flow of water is continuous throughout the year and if the channel or creek is adequate to carry a one hundred year design storm with one foot of freeboard or more. Existing inadequate creeks or channels shall be enlarged and grassed or otherwise improved as required to obtain the capacity to carry a one hundred year storm with one foot of freeboard within the boundaries of the area to be developed.

B. In general, open channels and ditches shall be lined or riprapped if the ten-year design storm velocity exceeds three feet per second. (Ord. 130 §13, 1988)

13.12.180 Maintenance.

The town will assume the maintenance responsibilities of the constructed storm drainage facilities which have been approved by the town engineer after the final acceptance of these facilities by the town. Any major repairs or adjustments required during the one year warranty period after the acceptance of the constructed facilities will be at the cost of the developer and not the town. After the termination of the one year warranty period and the final acceptance of the constructed facilities, the town will assume all maintenance responsibilities. (Ord. 130 §14, 1988)

13.12.190 Plan review fee--Special building permits.

The plan review fee for special building permits shall be included as part of the building permit fee. No additional fee shall be levied for the review of the final drainage plans or the record drawings in the case of special building permits. (Ord. 130 §15, 1988)

13.12.200 Plan review fee--Land development.

A. Review fees will constitute payment to the town for the time required by the town engineer and his staff to review the submittals to the town of storm drainage facilities. These reviews will be for the preliminary drainage plan review, the final drainage plan review, and the record drawings review.

B. The following fees will be levied against the development and shall be due and payable as follows: forty percent of the fee shall be paid prior to the approval of the preliminary drainage plan, and the remaining sixty percent of the review fee shall be paid prior to the approval of the final plat and final drainage plan. The review fees are: four hundred dollars per submittal, plus ten dollars per unit or acre, whichever is greater. (Ord. 130 §16, 1988)

13.12.210 Drainage water fee--Imposed.

This drainage water fee shall be imposed on all lands being developed within the town limits either as a special building permit or as a land development. The revenues derived therefrom shall be used only for the development and construction of improvements to the existing stormwater facilities and for the construction of additional needed stormwater facilities, such as new, main interceptor lines, detention facilities, and disposal facilities to serve the developing areas. It is not the intent of this development fee to pay for the construction of on-site storm drainage facilities for any developments, but to provide the major overall interceptor lines and detention facilities to serve the conglomeration of developments. (Ord. 130 §17, 1988)

13.12.220 Drainage water fee--Calculation.

A. This drainage water charge shall be imposed upon all new developments and lands to be improved under special building permits in the town. This charge shall not be imposed upon any already developed land within the town at the time of the passage of the ordinance codified in this chapter. The amount of the charge or fee shall be as follows:

1. Six hundred fifty dollars per acre if the amount or discharge is not limited by on-site detention;

2. Four hundred fifty dollars per acre if the discharge or storm drainage waters from the development is less than 0.20 of a cfs per acre at any time during a ten-year design storm event;

B. In computing the area of any tract of land for purposes of applying such fee, the area of any existing public street, alleyway, or any canal, irrigation lateral, or natural waterway shall be deducted. The amount of the charge or fee may be changed from time to time by council action. (Ord. 130 §17(a), 1988)

13.12.230 Drainage water fee--Payment.

A. All charges or fees shall be paid to the town treasurer.

B. Land being developed in the town shall be assessed this drainage water fee prior to the execution of the subdivision agreement or special building permit. Payment shall then be made for the tract of land being developed prior to final execution of the subdivision contract or issuance of the special building permit. (Ord. 130 §17(b), 1988)

13.12.240 Drainage water fund.

The moneys generated from the collection of this drainage water fee or charge shall hereby establish a drainage water fund. Moneys from this drainage water fund may be expended by approval of the town council for the following purposes, namely:

A. Design, land and right-of-way acquisition, and construction of drainage interceptor lines and channels and mayor transport facilities;

B. Design, land right-of-way acquisition, and construction of drainage detention and drainage disposal facilities. (Ord. 130 §18, 1988)

13.12.250 Inspection requirements.

A. All municipal improvements that shall require town maintenance after the one-year warranty period and final acceptance by the town shall have the construction supervised by a registered engineer in the state and shall have a qualified inspector present while construction is actually occurring. The developer may select a qualified, registered engineer firm of his choice to provide the inspection, or as part of his subdivision agreement, he may elect to have and negotiate a fee for the town to provide this inspection and construction supervision.

B. During the construction process, all materials shall be tested according to the standard engineering practices to assure that proper and adequate materials are utilized for the construction of these municipal improvements. Certified test results shall be submitted to the town engineer within five days after the test results have been taken to assure the quality of construction. (Ord. 130 §19, 1988)

13.12.260 Materials standards.

All materials utilized for municipal improvements and construction shall be new and of highest quality and shall conform to the materials specifications detailed in the appropriate sections of the town standard specifications for municipal construction. (Ord. 130 §20, 1988)

13.12.270 Liability insurance.

The town is authorized to require all persons constructing storm sewer facilities to post with the town liability insurance coverage in the amount of up to one million dollars as approved by the town council. (Ord. 130 §21, 1988)

13.12.280 Violation--Penalty.

No person shall make a change in land use or install urban stormwater facilities which are not in accordance with the urban stormwater run off control standards stated in this chapter. Violation of this chapter is a municipal infraction subject to the provisions of Sections 7-1-4150 through 7-1-4152, MCA. (Ord. 207 36, 2000; Ord. 130 §22, 1988)