Final-Gardiner_Hydraulics_Report-combined-1r.pdf

MT NPS YELL 11(2)

MT PARK 2013(1)

Vancouver, WA 98661 360-619-7700 and Yellowstone National Park

September 22, 2014

U.S. Department of Transportation Federal Highway Administration

Western Federal Lands Highway Division 610 East Fifth Street

Gardiner Gateway and North Entrance Improvements

Final Hydraulics Report

Gardiner, MT

Park County, MT

G a r d i n e r G a t e w a y a n d N o r t h E n t r a n c e I m p r o v e m e n t s M T P A R K 2 0 1 3 ( 1 ) / M T N P S Y E L L 1 1 ( 2 ) F i n a l H y d r a u l i c s R e p o r t

Table of Contents Page

1.0 – Introduction 1

1.1 – Background 1

1.2 – Existing Drainage 1

1.3 – Proposed Drainage Approach 2

1.4 – Construction Sequence 3

2.0 – Proposed Design 4

2.1 – Peak Runoff 4

2.2 – Infiltration Basins and Swales 5

2.3 – Trench Drains 7

2.4 – Catch Basins 7

2.5 – Storm Sewers and Outfalls 7

2.6 – Culverts 9

2.7 – Ditches 10

3.0 – Operation and Maintenance 11

3.1 – Infiltration Basins and Swales 11

3.2 – Trench Drains 11

3.3 – Catch Basins 11

3.4 – Storm Sewers and Outfalls 12

3.5 – Culverts 12

3.6 – Ditches 12

Figure 1. Proposed Drainage.

Figure 2. Hillside Drainage Area.

Photos 1 to 38.

Appendix A – Proposed Drainage System Drawings

Appendix B – Drainage System Calculations

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1.0 Introduction

1.1 Background

Gardiner (City) and Park County (County), Montana, and Yellowstone National Park (Park) are constructing infrastructure improvements in Gardiner and the north entrance to the Park as part of a five year design and construction project. The improvements include paving streets, installing sidewalks, installing parking areas, and replacing water distribution systems within portions of the City and Park.

Governmental entities involved in the project include the National Park Service (NPS), Park County, Montana (County), Montana Department of Transportation (MDT), and the Federal Highway Administration (FHWA). FHWA, Western Federal Lands Highway Division (WFLHD) is designing the improvements. The proposed road, parking, and sidewalk improvements are shown in red on Figure 1. A summary of existing drainage, proposed drainage improvements, and recommended operation and maintenance is presented.

1.2 Existing Drainage

Stormwater and snow-melt runoff is generally conveyed northward through the City following the slope of the ground surface. A limited network of catch basins, storm sewer pipes, swales, ditches, and culverts collect and convey the runoff. Most of the existing network does not have adequate flow capacity for conveying the runoff expected from the proposed street and parking improvements.

The hillside drainage south of the City and within the Park contributes runoff to existing shallow roadside ditches along the North Entrance Road (Photo 1). The ditches convey the water through a series of inlets, storm sewers, and ditches to a storm sewer crossing Stone Street (Stone Street storm sewer), northwest of the school football field (Photos 2, 3, 4, and 5). The water mixes with other stormwater runoff, eventually being discharged at an outfall at the north end of 4th Street.

The existing 48-inch diameter corrugated-metal-pipe (CMP) Stone Street storm sewer line goes north from the football field ditch through private property, under at least two large structures, and outfalls into a discharge ditch (Photo 6). The storm sewer line has a 5-foot wide easement between Stone Street and Spring Street. At least two building structures are directly over the existing storm sewer. The existing storm sewer line’s condition is not known. A junction and slight bend in the pipe alignment is located where the line crosses Spring Street. A deteriorating pipe or separated joint could cause the soil backfill around the pipe to erode, creating a large sink-hole, possibly causing damage to nearby building structures. Repairing the failed pipe and sink-hole could be impossible without first removing the overlying or adjacent building structures. Using the existing storm sewer alignment increases the County’s liability for repairing any foundation damage caused by a sink-hole.

All storm sewer lines have a finite service life. Eventually, the existing storm sewer line would need to be repaired or replaced. A 5-foot wide easement is not adequate for constructing or maintaining the storm sewer line. A dedicated easement at least 20 feet wide would be needed where the storm sewer line runs through private property. This is not possible with the existing alignment. Federal funds cannot be used for constructing a storm sewer line without first having adequate easements. A new storm sewer line would need to follow a new alignment. Because of the lack of appropriate easement, using the existing storm sewer and storm sewer alignment is not recommended.

The roadside ditch south of the North Entrance Road and grated inlets immediately east of the Arch are undersized and particularly subject to clogging and overtopping (Photo 7). When this occurs, water flows

F i n a l H y d r a u l i c s R e p o r t | 2 down the road between the Arch columns and ponds at the low spot in the road profile west of the Arch, making passage by traffic unsafe (Photos 8 and 9). Water flowing over the road embankment increases flooding in Arch Park and adjacent streets (Photo 10).

Runoff from east of 2nd Street and north of East Park Street and a portion of the Robert Reamer Complex is collected by a privately installed and owned catch basin and storm sewer pipe at the north end of 1st Street (Photo 11). The storm sewer pipe and outfall are located on private property. The storm sewer pipe condition could not be verified. The County would need to obtain a permanent drainage easement or ROW to use the catch basin and storm sewer pipe. Installing a new storm sewer pipe would involve construction under stone masonry walls and near stone masonry building foundations.

Runoff from a portion of the Robert Reamer Complex flows towards the existing Boaters Access Road.

The access road is steep and highly erodible (Photo 12). The road surface is frequently eroded during major storm events (Photo 13).

Runoff from 2nd Street (State Highway 89) between the Highway Bridge and Park Street intersection is collected in a network of catch basins and storm sewers (Photos 14 and 15). The storm sewer outfall is on the Yellowstone River bank immediately downstream of the highway bridge. No deficiencies were noted with the system and it appears to have adequate capacity for existing and expected runoff.

Runoff from areas north of Park Street, west of 2nd Street, and east of the school generally flow north and northwest following the ground topography towards the Yellowstone River. Most of the runoff is dispersed through two broad swales. Runoff from approximately 28 acres of City and Park area and the hillside drainage is conveyed through a system of roadside ditches and culverts to the outfall at the north end of 4th Street (Photo 16). The culverts and ditch are undersized and overtop during larger storm events and when clogged with ice and debris. Water flowing down 4th Street follows the slope of the ground surface and is directed towards private property along the east side of the road. The driveway and garage of the last property north has particularly low ground elevation relative to the road surface (Photo 17).

Flood water or storm runoff tends to be directed and pond in the driveway and garage entrance. An existing 18 feet by 18.5 feet shed is located in the middle of the County ROW (Photo 18). The outlet channel passes between the shed in the ROW and a shed on the property to the east (Photos 19 and 20).

Rounded stone lines the outlet channel bottom and edges. Some stone has been eroded from the channel bottom and edges. Approximately 10 feet of the right bank (looking downstream) has eroded approximately 2 feet into the private property on the east. The existing erosion does not appear to be undermining the shed on the private property to the east.

1.3 Proposed Drainage Approach

Stormwater and snow melt runoff is proposed collected by a series of ditches, swales, culverts, trench drains, catch basins, inlets, and storm drains and discharged at two primary outfalls; north end of 4th Street and east end of East Park Street. Three infiltration basins and one infiltration swale are proposed for reducing the amount of stormwater discharged and improving water quality. New storm sewer lines are proposed for Main Street, Yellowstone Trail Road, North Entrance Road, and East Park Street. The storm sewer lines are designed for conveying the expected 50-year runoff, assuming the infiltration basins and swale are not infiltrating water. Runoff from West Park Street is to be conveyed through sidewalk trench drains to a new infiltration swale located south of West Park Street.

To reduce the flooding near the Arch, runoff from the hillside drainage will be collected by the proposed system of ditches and culverts along the North Entrance Road and dispersed over the flat meadow area north of the road. Existing and proposed ditches and swales eventually convey the dispersed water to the

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North Entrance Road storm sewer. Dispersing the runoff is not expected to significantly reduce the volume conveyed to the storm sewer. Dispersing is expected to delay the flood peak enough so that the proposed storm sewer system can convey the runoff from both the improved City areas and the hillside drainage.

New streets, sidewalks, and parking areas are planned for East Main Street, West Main Street, Yellowstone Trail Road, West Park Street, East Park Street, North Entrance Road, and Robert Reamer Avenue. New streets, sidewalks, and parking areas are also planned for 1st Street, 2nd Street, 3rd Street, and 4th Street south of Main Street. Drainage improvements are planned only where new street, sidewalks, and parking areas are to be constructed (Figure 1). Existing drainage problems and deficiencies are expected to be improved within the existing street ROW where new street, sidewalks, and parking areas are to be constructed. Existing drainage problems and deficiencies on adjacent private property is not expected to be directly improved by the project. No work is proposed within private property for improving drainage deficiencies. Drainage problems and deficiencies beyond the project streets are expected to be reduced by redirecting, collecting, and conveying the stormwater runoff through the proposed drainage system.

1.4 Construction Sequence

Construction is expected to occur over a four year period. Water and sanitary sewer improvements throughout the project area are planned for construction in 2014 and 2015. The Main Street storm sewer is to be installed with the water and sanitary sewer improvements. Catch basins associated with the Main Street storm sewer are to be constructed in 2016 with the East and West Main Streets surface improvements. The infiltration swale, infiltration basins, 4th Street storm sewer, Yellowstone Trail Road storm sewer, North Entrance Road storm sewer and associated catch basins, inlets, ditches, culverts, and outfalls are to be constructed in 2015 and 2016 with the Parks Street, Yellowstone Trail Road, and North Entrance surface improvements. East Park Street storm sewer and associated catch basins, inlets, ditches, and outfall are to be constructed in 2017 with the East Parks Street surface improvements.

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2.0 Proposed Design

2.1 Peak Runoff

The rational method was used for estimating peak runoff for the urbanized areas within Gardiner and the Park. The rational method requires drainage area, runoff coefficient, time of concentration, and rainfall intensity. Drainage areas to the storm sewers, culverts, ditches, catch basins, infiltration basins, and infiltration swale were estimated from ground reconnaissance, aerial satellite imagery and LIDAR topographic mapping (Figure 1). Runoff coefficients were selected based on observed and measured impervious areas from Table 7-5, MDT Hydrology Design Manual. Drainage areas and runoff coefficients represent the expected future drainage conditions. Runoff coefficients were 0.3 for unpaved areas, 0.9 for paved areas, and 0.6 as an average. A 0.6 average runoff coefficient appropriately represents the expected rain on frozen ground or rapid snow-melt runoff conditions. Time of concentrations were estimated using the approach in FHWA, Urban Drainage Design Manual, HEC-22, third edition, August 2013 (FHWA_NHI-10-009).

Rainfall intensities are derived from rainfall-intensity-duration-frequency (RIDF) curves. It is best to use an RIDF curve generated from local rainfall data. The nearest and most applicable RIDF curve provided in the MDT Hydrology Design Manual is for Corwin Springs, MT, approximately 8 miles west of Gardiner, MT. Unfortunately, the period of record for Corwin Springs is 20 years. Rainfall intensities equal to and smaller than the 10-year storm can reliably be estimated with 20 years of data. The larger, more extreme rainfall events are not well represented in only 20 years of data. Small data sets tend to underestimate the extreme rainfall intensities. Caution and judgment should be applied when using the data for predicting any storm intensities larger than the 10-year.

Municipal storm sewer systems designed for conveying the 50-year runoff should be designed with data sets larger than 20 years, particularly when there is a real possibility of flooding private property. If enough data is not available, then an appropriate alternative method should be applied. The NOAA, Atlas 2 approach, is based on area rainfall data with typically longer record periods. The approach averages rainfall data from locations having both higher and lower annual precipitation. In the absence of appropriate local rainfall data, the NOAA, Atlas 2 approach, was used for developing the RIDF curves.

Precipitation frequency data was obtained for Gardiner, MT, from the Hydrometeorological Design Studies Center, NOAA/National Weather Service.

The hillside south of Gardiner contributes stormwater runoff to the drainage system (Figure 2). The drainage area is estimated using topographic mapping to be 0.83 square miles. Suggested maximum drainage area for the rational method is 50 to 200 acres. The time of concentration is estimated to be 63 minutes. Suggested maximum time of concentration for the rational method is 30 minutes. Peak discharges were estimated with regression equations from Peak-flow Characteristics of Wyoming Streams (WRIR 03-4107, 2003). Elevation and longitude were obtained from topographic mapping. The design assumes the hillside drainage flood peak occurs after the street runoff flood peak has passed through the drainage system. To accomplish this, the hillside drainage runoff must be dispersed by the proposed system of culverts, ditches, and swales across the flat meadow area.

The rational method and regression equation calculations are included in Appendix B.

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2.2 Infiltration Basins and Swale

To reduce volume and peak of stormwater discharged from the City, install three infiltration basins and one infiltration swale (Figure 1). Locate one infiltration basin at the outlet of the ditch on the east side of the school football field, south of Stone Street (Photo 21). Locate the other two infiltration basins near the outlet of the proposed infiltration swale. Stormwater runoff from the streets and parking areas contain sediment. Erosion of the road embankments, swale sides, and non-vegetated ground surfaces also contribute sediment to the stormwater runoff (Photo 22). Sediment carried by the stormwater is expected to clog the gravel packs and underlying soils, reducing infiltration rates. Infiltration basins with gravel packs are susceptible to clogging and are not easily maintained. The infiltration swale located south of Park Street and east of the proposed bypass road is designed to retain sediment and be easily accessed and cleaned.

During construction and until the disturbed soil surfaces in the contributing watershed are stabilized, keep the gravel pack surface free of sediment. This might be accomplished by completely covering the exposed gravel pack surface with a non-woven geotextile fabric until construction activities are completed.

The basins and swale were designed using level-pool routing. Universal hydrograph method was used for developing runoff hydrographs. Porosity of the gravel pack was assumed to be 40 percent. Do not install geotextile filter fabric between gravel pack and underlying soils. Geotechnical borings were completed near the infiltration basins and swale (Gardiner Gateways Improvements Geotechnical Report, WFLHD 2014). Infiltration rates were measured in the borings using a falling-head test method. Consistent with commonly accepted design guidance, the infiltration rate was reduced by one-half for offsetting expected clogging of the soils with sediment. The measured infiltration basins and swale were designed to infiltrate runoff from the 50-year storm event. Design calculations are included in Appendix B.

Stone Street Infiltration Basin. The infiltration basin accepts stormwater discharged from the Main Street Sewer and Yellowstone Trail Road/West Park Street storm sewer, approximately 31.8 acres. The area available for the infiltration basin allows only a basin with a bottom 16 feet wide and 71 feet long, 1:1 side-slopes, and 8 feet deep. Based on geotechnical boring completed for the project, the soils adjacent to and beneath the infiltration basin are sands, gravels, cobbles, and boulders with an infiltration rate of 20 inches per hour. Assuming 10 inches per hour infiltration, the basin cannot infiltrate all of the 2-year stormwater runoff from the contributing drainage area. Assuming the gravel pack surface is free of sediment, debris, and ice, the basin can infiltrate a runoff volume equivalent to the 50-year stormwater runoff from Main Street (approximately 10.3 acres).

Install three 48-inch diameter, perforated, open-bottom concrete drywells in the infiltration basin for increasing the rate stormwater is introduced into the gravel pack. Equip each drywell with a median drain frame and grate (Appendix A, Sheet I.21, I.22, and I.34). Set basin bottom at elevation 5266.0 feet.

The basin includes a cast-in-place concrete outlet headwall with a removable, heavy, steel safety-debris grate (Appendix A, Sheet I.23). The grate is designed for passing the expected 100-year stormwater discharge, assuming 50 percent clogging. Installing vertical bars sloped towards the road embankment facilitates cleaning and debris removal. A vertical bar spacing of 4.5 inches maximizes debris passage while minimizing the potential for a child to become wedged in the grate.

The County and Park would like to hide the basin as much as possible. They are also concerned that an open basin with ponded water would attract children and become a possible drowning hazard. Fencing the basin would look out of context with the area and not provide the needed security. The existing ditch

F i n a l H y d r a u l i c s R e p o r t | 6 is narrow and lined with riprap. Covering approximately one-half of the proposed basin width with 2 to 4 feet of conserved soil emulates the existing ditch. Stormwater is introduced into the gravel pack through a 10-foot wide ditch running the basin length. Coarse, rounded gravel and cobles line the ditch.

An existing sanitary sewer line limits the basin outlet pipe invert elevation depth to no deeper than 5271.8 feet. A deeper basin outlet invert would need to be deep enough for going under the sanitary sewer, resulting in 20 feet deep trenches and manholes within the school yard area. An existing 24-inch diameter storm sewer serving a portion of the school parking lot, outlets near the proposed basin outlet structure, invert elevation 5269.6 feet. The existing pipe invert is 2.2 feet below the proposed basin outlet pipe invert. The school parking lot catch basin rim elevation is 5273.4 feet. Rim elevation of the proposed basin outlet structure is 5274.0 feet.

Calculated design water surface elevations at the inlet rim;

100-year flow is 5275.9 feet.

50-year flow is 5275.8 feet.

25-year flow is 5275.6 feet.

10-year flow is 5275.4 feet.

Water starts flowing over the parking lot curb from the proposed basin at approximately 5274.8 feet.

Water ponding in the proposed basin will tend to flow back to and out of the school parking lot catch basin. Larger than 2-year flows are expected to flow out from the proposed basin and overtop the parking lot curb, flowing across the parking lot and into the school parking lot catch basins.

Minimize flood water from the proposed basin flowing onto the school parking lot by;

Realigning the parking lot storm sewer from the existing catch basin to a proposed manhole.

Plugging the existing parking lot storm sewer.

Installing a berm with spillway at proposed basin, setting the crest elevation to 5277.0 feet for retaining 100-year flow.

Fill in the low depression in the existing ground surface west of the proposed spillway with cobbles similar to the landscape cobbles south of the school access road and west of the proposed spillway (Photo 21). Grade the placed cobbles to blend the finished surface into the existing ground surface. The placed and existing cobbles help disperse any water discharged over the spillway.

Park Street Infiltration Swale Basins. The infiltration swale basins accept stormwater runoff from portions of West Park Street, East Park Street, and Robert Reamer Avenue, approximately 13.7 acres.

Based on geotechnical boring completed for the project, the soils adjacent to and beneath the infiltration basins are sands, gravels, cobbles, and boulders with an infiltration rate of 20 inches per hour. Assuming 10 inches per hour infiltration and a gravel pack surface free of sediment, debris, and ice, two basins, each 10 feet wide, 40 feet long, and 8 feet deep can infiltrate the expected 50-year stormwater runoff from the contributing drainage area (Appendix A, Sheet I.19 and I.24).

Install one 48-inch diameter, perforated open-bottom concrete drywell in each infiltration basin for increasing the rate stormwater is introduced into the gravel pack. Equip each drywell with a median drain frame and grate.

Park Street Infiltration Swale. The infiltration swale accepts stormwater runoff from portions of West Park Street, East Park Street, and Robert Reamer Avenue, approximately 13.7 acres. Based on

F i n a l H y d r a u l i c s R e p o r t | 7 geotechnical boring completed for the project, the soils adjacent to and beneath the infiltration basins are sands, gravels, cobbles, and boulders with an infiltration rate of 20 inches per hour. Assuming 10 inches per hour infiltration and a swale soil surface free of sediment, debris, and ice, a swale 10 feet wide, 500 feet long can infiltrate the expected 50-year stormwater runoff from the contributing drainage area. Swale side-slopes vary between 3(h):1(v) and 5(h):1(v). Swale depth varies between 4 and 6 feet (Appendix A, Sheet I.19 and I.24).

To pond water in the swale and promote infiltration and sediment deposition, construct the swale bottom flat and 12 inches lower than the proposed bypass road culvert invert. Avoid compaction of the underlying soils during swale construction. Disk the swale sides and bottom after final excavating and grading for loosing up the surface and encouraging vegetation establishment and stormwater infiltration.

2.3 Trench Drains

Install trench drains with a 9-inch wide concrete channel and 12-inch wide cast-iron grating for portions of East Park Street, West Park Street, and West Main Street for reducing the number of catch basins and length of storm sewer pipe (Figure 1 and Appendix A, Sheet I.8, I.9, I.11, I.14, and I.35). Photo 23 shows an example of a trench drain installed in Portland, Oregon. Locate the trench drains in sidewalks for draining parking and street areas. Each trench drain has capacity for conveying the 25-year stormwater runoff (0.4 cfs) from approximately 6,500 square feet drainage area, assuming it is free of debris and ice.

To control erosion, install cobble discharge aprons at trench drains discharging onto non-paved areas.

Design calculations are included in Appendix B.

2.4 Catch Basins

Install catch basins along Yellowstone Trail Road, North Entrance Road, East Park Street, West Park Street, East Main Street, and West Main Street (Figure 1 and Appendix A, Sheet I.31 and I.32). Use Type 1 for road segments with longitudinal slope (Photo 24). Use Type 2 at low spots in the road profile (Photo 25). Type 1 is designed for capturing the 25-year stormwater runoff with minimal flow bypass.

Type 2 are designed for capturing the 50-year stormwater runoff with less than 8 feet of street width submerged by water and no curb overtopping. Type 1 is consistent with MDT drop inlet type I. Type 2 is consistent with MDT curb inlet type II. Design flow capacities assume 50 percent clogging.

Install Type 1 catch basins for draining the non-paved areas immediately east of the arch. Place one catch basin on either side of the road. These catch basins collect runoff from the ground surface immediately east of the Arch. They are not intended to convey all of the discharge from the North Entrance Road ditches. Install a Type 1 catch basin for the west end of the Boaters Access Road for collecting stormwater runoff from the Robert Reamer complex before the stormwater flows down the access road.

Set rim elevations at least 6 inches below adjacent ground surface and grade the area towards the catch basins. To reduce the visual impact, install no masonry collars around the catch basins (Photos 26 and 27).

Remove the existing privately owned catch basin at First Street and East Main. Storm water runoff currently collected the catch basin will be collected by the proposed catch basins and conveyed to the Main Street storm sewer.

2.5 Storm Sewers and Outfalls

Storm sewer lines are proposed for East Park Street, Yellowstone Trail Road, Main Street, and 4th Street (Figure 1 and Appendix A). Designs assume all of the infiltration basins and swale are not infiltrating and that the storm water and snow melt runoff is conveyed by the storm sewers. Consistent with HEC-22

F i n a l H y d r a u l i c s R e p o r t | 8 guidelines, initial pipe diameter for each storm sewer line was calculated with Manning’s equation assuming full gravity flow and Manning’s roughness coefficient of 0.0130. Initial pipe slope was selected to minimize pipe size and trench depth. Pipe slopes were adjusted for avoiding conflicts with existing and proposed utilities. Energy losses in pipe runs and junctions were estimated and the energy grade lines (EGL) and hydraulic grade lines (HGL) calculated using the energy loss method. Pipe sizes and slopes were adjusted until the storm sewer lines conveyed the estimated 50-year peak discharge without the pipes becoming pressurized and the 100-year peak discharge without stormwater flowing out of manholes or catch basins. To reduce the potential for the storm sewer line becoming clogged with sediment, each pipe segment full-flow velocity was verified to exceed 3 feet per second. Design calculations are included in Appendix B.

Install rodent guards at all storm sewer pipe outfalls. The guards pivot on a hinge, allowing most debris to pass without clogging the pipe. Install the guards inside the pipe as needed for preventing snow, ice, and debris from covering them and keeping them from lifting.

East Park Street. The East Park Street storm sewer conveys runoff collected from areas along and south of East Park Street (Appendix A). The storm sewer outfall is in a small depression and swale east of the City and north of the Boater Access Road (Photo 28). Use the rounded large cobbles and boulders available at the outfall location in place of riprap for energy dissipation and erosion control. Reshape the outfall to match proposed design. Remove the several large pieces of concrete rubble exposed at the surface. Install no headwall at the outfall.

Yellowstone Trail Road. The Yellowstone Trail Road storm sewer conveys runoff collected from areas along and south of West Park Street, Yellowstone Trail Road, and North Entrance Road. It also conveys runoff from Arch Park, the triangle, and the hillside drainage south of the City (Appendix A, Sheet I.7, I.8, I.13, I.14, I.19). The proposed storm sewer outfall is in the existing ditch south of the school football field (Photo 29). Remove, clean, and reinstall the existing riprap in the ditch for energy dissipation and erosion control (Appendix A, Sheet I.28). Reshape the outfall to match the proposed design. Install a masonry headwall at the outfall. Install masonry headwall and debris/safety racks on the 30-inch diameter inlet culvert.

Main Street. The Main Street storm sewer conveys runoff collected from areas along and south of East Main Street and West Main Street (Appendix A, Sheet I.10, I.14, and I.15). It also conveys runoff from approximately 0.6 acres on 2nd Street (State Highway 89). Abandon the private catch basin and storm sewer on the north side of the intersection of 1st Street and Main Street. The proposed Main Street storm sewer outfall is in the existing ditch east of the school football field (Photo 30). Install Class 3 riprap in the ditch for energy dissipation and erosion control. Reshape the outfall to match the proposed design.

Install no headwall at the outfall.

4th Street storm sewer. Initially, the proposed storm sewer will convey stormwater from Main Street storm sewer, Yellowstone Trail Road storm sewer, and existing school parking lot storm sewer (Appendix A, Sheet I.16 and I.17). To accommodate future City drainage improvements, the proposed storm sewer and outfall are sized for also conveying the estimated runoff from an additional 14.6 acres of city area north of Main Street, west of 2nd Street, and east of the school.

An alignment from the proposed infiltration basin going north between the school building and schoolyard fence, then east along Spring Street, and then north down 4th Street is recommended (Figure 1, Photo 31). The proposed storm sewer and manholes are 7 to 10 feet deep within the schoolyard and along Spring Street. The alignment takes advantage of existing public ROW, minimizes impacts to utilities, and minimizes construction cost.

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The storm sewer outfall is at the north end of 4th Street within County ROW. An existing 8-inch diameter sanitary sewer traverses the ROW approximately 12 feet from the proposed storm sewer outlet.

The outlet is immediately adjacent to a 24 inch by 35 inch corrugated metal pipe arch driveway culvert (Photo 32). The driveway culvert conveys stormwater from a roadside ditch. The proposed storm sewer system reduces the contributing drainage area for the driveway culvert and roadside ditch to an estimated 8 acres of City area. Assuming 8 acres drainage area and a ditch and culvert free of ice and debris, the existing ditch and driveway culvert are predicted to convey the 100-year runoff before overtopping.

Flood water or storm runoff flowing down the road will continue to be directed towards private property along the east side of the road. The low ground elevation at the driveway and garage of the last property north will continue to collect and pond surface water, potentially causing flood damage.

An existing 18 feet by 18.5 feet shed sets in the middle of the County ROW. There is approximately 8 feet between the northeast corner of the shed and the property line to the east. The narrow width is not large enough for a discharge channel with sloped sides. A discharge channel with vertical slopes is required (Appendix A, Sheet I.25 and I.26). Assuming 1-foot setback from shed and property line, 8-inch wall thickness, a 5-foot wide discharge channel can be constructed. The discharge channel will convey stormwater from both the proposed storm sewer and the existing driveway culvert. Because of the private property immediately adjacent to the discharge channel, the channel was designed for conveying the estimated 100-year discharge. Manning’s equations was used to size the discharge channel. To dissipate energy and encourage infiltration, riprap is proposed for lining the discharge channel. Assuming a 0.08 Manning’s roughness coefficient, 2 percent channel slope, 5,246.8 feet floor elevation, and a channel free of ice and debris, the predicted 100-year water surface elevation is 5,249.7 feet.

The steep, hillside area immediately downstream of the discharge channel has abundant rounded large cobbles and boulders (Photo 33). Under current discharge conditions the area experiences a minimal amount of erosion. Additional discharge is expected with the proposed storm sewer. To effectively control the erosion, augment the stone within 13 feet by 12 feet area with 3 feet of Class 5 riprap (Appendix A, Sheet I.25 and I.26). A constructed flow channel direct water away from the private property on the east side of the ROW. A moderate amount of erosion is expected immediately downstream of the riprap area. The erosion is not expected to undermine the shed or private property adjacent to the ROW.

The vertical concrete wall along the sides of the proposed discharge channel keeps the adjacent shed and private property from being undermined and eroded. A rockery wall would need to be thicker than the concrete wall and would make the ditch too narrow for providing adequate flow capacity. The proposed concrete floor near the storm sewer outfall allows construction to occur over the existing sanitary sewer without incasing it in concrete and allows the proposed storm sewer invert to be set at the lowest possible elevation (Appendix A, Sheet I.26). Relocate the existing sanitary sewer cleanout outside of the proposed discharge channel. The concrete wall top elevation is set low enough to allow re-grading the road and adjacent ground surface near the discharge channel, encouraging more surface drainage to flow towards the outfall channel and away from the private property.

2.6 Culverts

Culverts are proposed for the Bypass Road and North Entrance Road (CUL 1, Figure 1). The twin 24-inch diameter Bypass Road culverts convey stormwater discharged from the infiltration swale (Appendix A, Sheet I.18). A 30-inch diameter culvert (IN 2, Figure 1) on the north side of the North Entrance Road serves as an inlet to the Yellowstone Trail Road storm sewer for stormwater collected on the north side of the North Entrance Road (Appendix A, Sheet I.7). A 30-inch diameter culvert (IN 3, Figure 1) on the south side of the North Entrance Road provides an inlet to the storm sewer for stormwater collected on

F i n a l H y d r a u l i c s R e p o r t | 10 the south side of road (Photo 34). Four 24-inch diameter culverts (CUL 2, 3, 4, and 5, Figure 1) are proposed for the North Entrance Road east of the Bypass Road for conveying stormwater runoff from the hillside drainage to the flat meadow area north of the road (Photo 35). Locate and space the culverts for dispersing the stormwater evenly over the flat meadow area. Dispersing the hillside stormwater runoff over the flat meadow is essential for delaying the peak of the hillside drainage and ultimately reducing the volume conveyed by the storm sewer systems.

Masonry headwalls are proposed for all culvert inlets and outlets. Debris-safety racks are proposed for culverts inlets connecting to storm sewers (Appendix A, Sheet I.29). The culverts are designed to convey the 50-year peak discharge with a headwater-to-pipe diameter ratio of 1.0. Design calculations are included in Appendix B.

2.7 Ditches

The existing ditch (D1 and D2, Figure 1) east of the football field has adequate capacity for the conveying the expected stormwater discharge. Except for at the proposed storm sewer outfalls, no modification to the existing ditch is needed. Existing ditches along 4th Street are not included in the project limits. No modifications to the ditches along 4th Street are proposed.

Install a ditch with 2 feet bottom width, 3 feet minimum depth, and 2(h):1(v) side-slopes along the north side of the North Entrance Road between the CUL 2 and 5 and along the south side of the road between IN 3 and CUL 5 (D3 and D4, Figure 1 and Photo 35). The ditches area needed for dispersing the stormwater from the hillside drainage over the flat meadow area north of the North Entrance Road and east of the Bypass Road. Install no ditches between the Arch and storm sewer culvert inlets (Photo 36).

Eliminating the ditches forcers more storm and snow melt runoff to enter the storm sewer away from the Arch.

Install a ditch with 1 foot bottom width, 1foot minimum depth, and 2(h):1(v) side-slopes along the south side of the Boaters Access Road (D5, Figure 1 and Photo 37). The ditch conveys stormwater runoff from the access road and from the Robert Reamer complex that bypasses the Type 1 catch basin near the west end of the access road. Line the ditch with 1-foot thick layer of Class 2 riprap for controlling erosion.

The ditch discharges at the top of the Gardiner River canyon rim over existing large boulders and cobbles (Photo 38). A minor amount of erosion at the discharge point is expected, but is expected to be limited by the boulders and cobbles. No additional riprap is needed at the discharge point for dispersing the flow or controlling erosion (Appendix A).

The ditches are designed to convey the 50-year peak discharge. Design calculations are included in Appendix B.

F i n a l H y d r a u l i c s R e p o r t | 11

3.0 Operation and Maintenance

Catch basins, trench drains, culverts, ditches, inlet structures, and infiltration basins cannot operate at full capacity when clogged with ice, snow, sediment, or debris. It is critical that the structures be free of ice, snow, sediment, or debris for reducing flooding potential.

3.1 Infiltration Basins and Swales

Stormwater runoff from the streets and parking areas contain sediment. Sediment carried by the stormwater tends to clog the gravel packs and underlying soils, greatly reducing infiltration rates.

Infiltration basins with gravel packs are susceptible to clogging and are not easily maintained.

Stormwater runoff also carries pollutants, including oils, greases, automotive fluids, and metals.

Sediment removed with cleaning activities may be classified as special waste and require special handling and disposal. Review applicable current Federal, State, and County regulations prior to initiating cleaning activities.

Infiltration Basins. Inspect the gravel pack surface and drywell bottoms annually and immediately following extreme storm events. Remove any accumulated sediment and debris. Jetting the surface with water tends to force sediment into the void spaces. Avoid jetting the surface of the gravel pack until as much of the sediment is removed as possible. Remove and replace the top 12 inches of gravel pack when becomes filled with sediment.

Infiltration Swale. Inspect the swale bottom annually and immediately following extreme storm events.

Remove any accumulated sediment and debris. Jetting the surface with water tends to force sediment into the underlying soil void spaces. Avoid jetting the surface until as much of the sediment is removed as possible. When the infiltration rate becomes less than 5 inches per hour, remove the top 6 inches of swale bottom soil. Disk the bottom completely to loosen the soil.

3.2 Trench Drains

Inspect all proposed trench drains annually and after periods of heavy stormwater or snow-melt runoff for excessive debris and sediment accumulation. Remove all debris and sediment as soon as possible.

Inspect all proposed trench drains after periods of heavy snow fall and prior to snow melt for excessive snow and ice blockage. Remove all excessive snow and ice blockage before expected rapid melt periods.

Most ice and debris may be removed from the concrete channel using a long-handled hoe. Heavy accumulations may require jetting the concrete channel with water. Severe accumulations may require removing the cast-iron grate. Avoid placing snow removed from the streets and sidewalks on the trench drains.

3.3 Catch Basins

All proposed catch basins should be inspected annually and after periods of heavy stormwater or snow-melt runoff for excessive debris and sediment accumulation. All debris and sediment should be removed as soon as possible. All proposed catch basins should be inspected after periods of heavy snow fall and prior to snow melt for excessive snow and ice blockage. All excessive snow and ice blockage should be removed before expected rapid melt periods. Avoid placing snow removed from the streets and sidewalks on the catch basins.

F i n a l H y d r a u l i c s R e p o r t | 12

3.4 Storm Sewers and Outfalls

Inspect all proposed storm sewer outfalls annually and after periods of heavy stormwater or snow-melt runoff for excessive debris/sediment accumulation and erosion. Remove all debris/sediment and repair erosion as soon as possible. Inspect all proposed storm sewer outfalls after periods of heavy snow fall and prior to snow melt for excessive snow and ice blockage. Remove all excessive snow and ice blockage before expected rapid melt periods.

3.5 Culverts

Inspect all proposed culverts annually and after periods of heavy stormwater or snow-melt runoff for excessive debris and sediment accumulation inside the culvert and at the inlet and outlet. Remove all debris and sediment as soon as possible. Inspect all proposed culverts after periods of heavy snow fall and prior to snow melt for excessive snow and ice blockage. Remove all excessive snow and ice blockage before expected rapid melt periods. Avoid placing snow removed from the streets and sidewalks on the culvert inlets or outlets.

3.6 Ditches

Inspect all existing and proposed ditches annually and after periods of heavy stormwater or snow-melt runoff for excessive erosion and sediment deposition. Repair all erosion and remove deposited sediment.

Inspect all existing and proposed ditches after periods of heavy snow fall and prior to snow melt for excessive snow and ice blockage. Remove excessive snow and ice blockage before expected rapid melt periods. Avoid placing snow removed from the streets and sidewalks in the ditches.

The ditches proposed for the north side of the North Entrance Road between the CUL 2 and 5 and for south side of the road between IN 3 and CUL 5 (D3 and D4, Figure 1) are not lined with riprap and may experience frequent erosion and sediment deposition. They may also experience frequent ice and snow blockage.

MH MS3

MH MS2 MH MS4

OF2

MH PS6

MH PS8

CUL 2

CUL 3

CUL 4

CUL 5

CUL 1

IBDW 1

MH 4S4

EXISTING STORM SEWER

MH MS8

MH MS7

MH MS6

MH MS5

OF 1

EXISTING DITCH

MH MS1

D3

CATCH BASIN

REMOVE PRIVATE

EXISTING 2ND STREET OUTFALL

D1

D2

* Shared flow.

MH EPS9

MH EPS7

MH EPS4

IN5

IBDW 2

D4

D5

IN2

OF 3

IN4

IN1

OF 4

Swale Infiltration Proposed

MH PS2

t e e rt

S d r teertS kraP tsaE daoR liarT enotswoll eY s t S tr e e t

Yellow stone River

Stone Street

Spring Street

Water Street

Schools

Gardiner

Schools

Gardiner

N

Main Street

YN

P N orth Entrance R oad th S tr e e t

W Park Street

W Park Street n d S tr e e t

NORTH BOUNDARY

YELLOWSTONE NATIONAL PARK

14.6 ac

3.5 ac

4.1 ac

0.5 ac

0.6 ac

0.6 ac

2.1 ac

2.2 ac

2.4 ac

0.5 ac

1.7 ac

1.0 ac

13.7 ac

2.1 ac

0.8 ac

0.8 ac

Legend

Storm Sewer Pipe and Manhole

Storm Sewer Catch Basin

Culvert and Headwall

Red Line, Proposed Street Improvements

Storm Sewer Pipe Flow Direction

Infiltration Basin with Dry Well and Check Dam

Drainage Area for Calculating Peak Storm Runoff

General Storm Runoff Flow Direction

GARDINER GATEWAY IMPROVEMENTS

FIGURE 1

PROPOSED DRAINAGE

SCALE IN FEET

125 0 125 250

Hillside Drainage

Proposed Drainage Ditch

FLOW (cfs)ID DA (ac)

36,200 ac

0.83 sq. miles

2.6 ac

0.8 ac

5317 5317

IN3

53185318

5319 5320

53175

53185318

53195319

53225322

53235323

F.1

STATE PROJECT

NUMBER

SHEET

MT

PARK 2013(1)

P

M

S e p te m b e r

U

S S u r ft

D c m y fi le s p w p r o d u c ti o n d A ll

D A

H y d r a u li c s

R e p o r t_ p r o p

M a s te r

D r a in a g e

L O

P a r k

H Y .d g n

C h e c k e d b y

D e s ig n e d b y

D5

D4

D3

D2

D1

Ditches

IN5

IN4*

IN3

IN2

IN1

Major Inlets

CUL5*

CUL4*

CUL3*

CUL2*

CUL1

Culverts

IBDU2

IBDU1

Drywells

MH EPS4

MH EPS7

MH EPS9

MH PS8

MH PS6

MH PS2

MH MS9

MH MS7

MH MS6

MH MS5

MH MS4

MH MS2

MH MS1

MH 4S4

Manholes

OF4

OF3

OF2

OF1

Outfalls

4.1

36,200

36,200

22.5

31.8

4.1

36,200

15.8

2.4

31.8

36,200

36,200

36,200

36,200

13.7

13.7

10.3

7.5

3.4

2.6

15.8

16.6

19.8

3.5

4.0

5.2

7.3

7.6

9.8

10.3

32.8

7.5

19.8

10.3

47.4

G

G G

G

G

G

G G

G

G

G A

T E

G

G

N B

N B

N B

N B

N B

G

DI

JH 1001

JH 1002

JH 1028

JH 1029

JH 1048

JH 1049

JH 1050

JH 1058

JH 1059

JH 10001

JH 10188

JH 10189

JH 10577

POT 1

0+05.

50=

POT 1

01+

06.

POT 12+82.56=

POT 27+68.15

P O

T

.8

P O

T

.0

P O

T

.0

P O

T

.7

G a r a g e d o o r

' w id th

E n tr y d o o r w ith in ' fr o m w a ll?

G a r a g e d o o r

' w id th

S p a c e b e tw e e n g a r a g e d o o r s

Dennehotso Bridge

Project Site Location

Urban Drainage Areas

Hillside Drainage Area

Image date July 23, 2011

FIGURE 2

GARDINER GATEWAY IMPROVEMENTS

Hillside Drainage Area

N

DA = 0 .83 sq. mi.

0 1,000 feet

Photos

Photo 1. Ditches and road east of the Arch.

Photo 2. Undersized and easily clogged inlets east of the Arch.

Gardiner Gateway Improvements

Photo 3. Ditch along south edge of Arch Park.

Photo 4. Existing culvert inlet west side of Arch Park.

Photo 5. Ditch east of school football field.

Photo 6. Inlet to existing Stone Street storm sewer.

Photo 7. Ditch south side of the North Entrance Road flowing full.

Photo 8. Water flowing between Arch legs and down the North Entrance Road.

Photo 9. Water ponding at road profile sag west of the Arch.

Photo 10. Water escaping existing drainage system increases Arch Park flooding.

Photo 11. Existing private catch basin and storm sewer.

Photo 12. Boaters Access Road.

Photo 13. Erosion of Boaters Access Road surface occurs during storm and snow melt runoff.

Photo 14. North on 2nd Street drainage towards highway bridge.

Photo 15. Intersection of 2nd Street and Main Street, looking uphill to the south.

Photo 16. North on 4th Street with roadside ditches and culverts.

Photo 17. 4th Street outfall between existing sheds. Driveway surfaces near buildings are lower than road surface.

Photo 18. Existing shed in middle of County ROW. Outfall to the right.

Photo 19. Outlet channel between sheds.

Photo 20. Outlet channel between sheds. Red stake marks property line. Segment of channel edge eroded beyond property line.

Photo 21. Proposed Stone Street Infiltration Basin location.

Photo 22. Erosion and undermining of existing pavement east of the Arch.

Gardiner Gateway Improvements

Channel bottom limits Buried top of basin limits

Photo 23. Proposed trench drain concept.

Photo24. Proposed Type 1 catch basin.

Photo 25. Proposed combination catch basin.

Photo 26. Proposed catch basin location east of Arch.

Gardiner Gateway Improvements

Proposed Catch Basin Location

Existing Catch Basin

Photo 27. Proposed catch basin location east of Arch.

Photo 28. Proposed East Park Street storm sewer discharge apron location.

Gardiner Gateway Improvements

Proposed Catch Basin Location

Existing Catch Basin

Proposed Storm Sewer

Proposed Outfall Limits

Photo 29. Proposed Yellowstone Trail Road storm sewer discharge apron location.

Photo 30. Proposed Main Street storm sewer discharge apron location.

Gardiner Gateway Improvements

Proposed Outfall Limits

Proposed Storm Sewer

Proposed Outfall Limits

Proposed Storm Sewer

Photo 31. Proposed storm sewer alignment through school yard.

Photo 32. Driveway culvert immediately adjacent to proposed storm sewer outfall pipe.

Gardiner Gateway Improvements

Proposed Storm Sewer Alignment

Photo 33. 4th Street storm sewer outfall discharge apron location.

Photo 34. North Entrance Road storm sewer culvert inlets.

Gardiner Gateway Improvements

Proposed Outfall Limits

Property Limits

Proposed Culvert Inlets

Proposed Swale

Proposed Storm Sewer

Proposed Ditch No Ditch

Photo 35. Proposed ditches and culvert along North Entrance Road.

Photo 36. Construct no ditches between Arch and storm sewer inlet culverts.

Gardiner Gateway Improvements

Flat Meadow Area

Proposed Culvert

Proposed Ditches

No Ditches

Photo 37. Proposed ditch and storm sewer alignment down the Boaters Access Road.

Photo 38. Proposed ditch and outfall along the Boaters Access Road.

Gardiner Gateway Improvements

Proposed Ditch Limit

Proposed Storm Sewer Alignment

Proposed Ditch Limit

APPENDIX A

PROPOSED DRAINAGE SYSTEM DRAWINGS

DRAFT

FEDERAL HIGHWAY ADMINISTRATION

U. S. DEPARTMENT OF TRANSPORTATION

Project Location

APPROVED:

DATE

FEDERAL HIGHWAY ADMINISTRATION

U.S. DEPARTMENT OF TRANSPORTATION

TYPE OF CONSTRUCTION:

DESIGN DESIGNATION:

SPECIFICATION:

Western Federal Lands Highway Division

Director, Project Delivery, C. Kratovil

PROJECT MANAGER

VANCOUVER, WASHINGTON

WESTERN FEDERAL LANDS HIGHWAY DIVISION

PLANS PREPARED BY

PLANS FOR PROPOSED PROJECT

MT PARK 2013(1)

COMMITMENT TO EXCELLENCE

FEDERAL LANDS HIGHWAY

R

E N

O R

A I

N

N

ST F

R

I

D

D E P A

T

NTOFT A SP

T T

O

U

TE

ATESO

AM

E R

IC

A

M

MONTANA KEY MAP

COLUMBIA

BRITISH

ALBERTA

SASKATCHEWAN

N O

R T

H D

A K

O T

A

S O

U T

H D

A K

O T

A

WYOMING

ID

A

H O

UNITED STATES

CANADA

GLACIER

NATIONAL

PARK

YELLOWSTONE

NATIONAL

PARK

CHARLES M. RUSSELL

NATIONAL

WILDLIFE RANGE

Lake

Flathead

Elwell

Lake

M

Lake

Fort Peck

I SOS U IR

KALISPELL

MISSOULA

Anaconda

HELENA

BUTTE

BOZEMAN

Livingston

BILLINGS

City

Miles

Glendive

Point

Wolf Glasgow

HAVRE

Shelby

FALLS

GREAT

Lewistown

Dillon

E N

O T

S

WOLLEY

R E

VIR

90 15

2 2

MONTANA

REVIR

LENGTH 1.035 MILES

95% PLANS

See Sheet A.2 for Index to Sheets

SCALE IN MILES

20 0 20 40 60

W Y

O M I N

G

I D

A H

O

MONTANA

GARDINER

Helena

Bozeman

Billings

Butte Anaconda

B e a v e rh e a d

Mts.

Centennial Mts.

Falls

Idaho

MEM. PKWY.

JOHN D. ROCKEFELLER JR.

YELLOWSTONE

NATIONAL

PARK

PROJECT AREA

Vicinity Map

See Sheet A.4 for

Roadside Development.

Grading, Base, Paving, Drainage and

N

MONTANA

PARK COUNTY

YELLOWSTONE NATIONAL PARK

e n ot s w oll e Y reviR

Specifications, 2010

Montana Public Works Standard

Projects, FP-03 US Customary Units of Roads and Bridges on Federal Highway

Standard Specifications for Construction

Design Vehicle e (max)

V

ADT (2035)

ADT (2015)

45' Tour Bus

0.040

20 MPH

A.