CO FLAP MES 18(1) 18 Road Final Hydraulics Report with Addendum.pdf

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Addendum (Page 1) CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Subject: CO FLAP MES18(1) 18 Road – Final Hydraulics Report Addendum Date: May 2024

To: Neil Ogden, Project Manager, CFLHD

From: Aaron Estep, PE, Hydraulic Engineer, CFLHD Signature: ________________________

Through: Luis Calderón, PE, CFM, Hydraulics Team Lead, CFLHD Signature: ________________________

This Addendum is intended to supplement the Final Hydraulics Report (dated April 2022) for the subject project.

During project un-shelving in early 2024, six culverts were identified to have cover issues and were not providing positive drainage within the typical section. Several pipe alternatives were evaluated including smaller diameter circular pipes, horizontal elliptical, and arch pipes. The table below provides updated recommendations for each of the six locations. Given the issues with culvert fit, the standard design criteria (Section 2 of the Final Hydraulics Report) may not be met for all culverts below. For the culverts listed below, each was designed with a HW/D ratio of 1.5 and not to overtop the road for the 25yr event. The revised recommendations listed were selected as it provides the minimum acceptable rise and the most fall while still meeting the criteria.

Station Existing Culvert Final Report

Recommendation Revised

Recommendation

107+01 18” CMP 24” CMP 23”x14” HE RCP 134+66 18” CMP 24” CMP 30”x19” HE RCP 143+27 18” CMP 24” CMP 30”x19” HE RCP 210+35 18” CMP 24” CMP 30”x19” HE RCP 254+16 18” CMP 24” CMP 23”x14” HE RCP 285+88 None 24” CMP None

Station 285+88 After further analysis of the site hydrology, it was determined that this proposed culvert only drains approximately 2 acres. There is no culvert in this location and no evidence of overtopping of the road. The existing and proposed road is more or less flush with the surrounding landscape and thus significant grade raises would be necessary to accommodate a pipe of any size. For these reasons, this culvert is recommended to be removed from the design.

Cover Requirements Minimum cover for the above listed culverts was based on Section 12.6.6.3 of the AASHTO LRFD Bridge Design Specifications, 9th Edition. This requirement states a minimum 12” of cover (including the base and pavement layers) for a rigid pipe and flexible pavement combination.

2024-05-21T13:45:42-0600

AARON ESTEP, PE

2024-05-21T14:33:36-0600

LUIS GERARDO CALDERON ARCE

18 Road Mesa County, CO

CO FLAP MES18(1)

Final Hydraulics Report

Federal Highway Administration Central Federal Lands Highway Division

April 5, 2022

1 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Signature Sheet

Report prepared by: ____________________________________________________

Cassidy Cote, Hydraulic Engineer

Report reviewed by: ____________________________________________________

Aaron Estep, Hydraulic Engineer

Approved for distribution by: ____________________________________________________

Luis Calderón, Lead Hydraulic Engineer

Distribution

Electronic:

N:\Projects\CO\mes18(1)\Hydraulics\9_Final-Report Neil Ogden, Project Management Darin Lockhart, Project Development, Lead Designer Londen Mackey, Project Development Leslie Perry, Environmental Protection Rebecca Borst, Geotechnical Karl Eikermann, Bridge

2 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Executive Summary Hydrologic and hydraulic analyses were conducted for the CO FLAP MES18(1) 18 Road project in Mesa County, CO. The project consists of improvements to approximately 6.05 miles of roadway beginning at the Government Highline Canal and terminating at an existing BLM campground. Twelve existing culvert locations were identified by survey within the project limits. Of those twelve, two existing culverts are greater than 48-inches in diameter and are thus identified as major culverts for the purposes of this report.

Hydraulic capacity for all cross-route structures was evaluated on a 25-year frequency based on a low-standard roadway classification. Hydraulic performance was evaluated using HY-8. Proposed culvert replacement types, sizes and outlet protection recommendations are summarized Table 1. Additional details regarding outlet protection are provided in Section 5.2. The following report summarizes hydraulic design criteria, computational methods and final hydraulics recommendations.

Table 1: Recommendation Matrix

Station Existing Culvert

Proposed Culvert Notes

105+59 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

107+01 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

130+61 96-in. CMP 12-ft. x 10-

ft. CBC

Replace with 12-ft. x 10-ft. CBC equipped with headwalls, wingwalls and a class 6 outlet apron

134+84 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

143+20 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

156+74 30-in. CMP 42-in. RCP Replace with 42-in. RCP with FES and an outlet apron per C251-50

157+57 18-in. CMP 18-in. RCP Replace existing parallel culvert with 18-in. RCP and outlet apron per C251-50

175+54 30-in. CMP 60-in. RCP Replace with 60-in. RCP equipped with headwalls, wingwalls and a class 2 outlet apron

210+36 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

212+27 24-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

254+13 18-in. CMP 24-in. RCP Replace with 24-in. RCP equipped with FES and outlet apron per C251-50

271+12 72-in. CMP 9-ft. x 6-ft.

CBC

Replace with 9-ft. x 6-ft. CBC equipped with headwalls, wingwalls and a class 4 outlet apron

285+90 None 24-in. RCP Install new 24-in. RCP cross culvert with FES and outlet apron per C251-50

Note: FES = Flared End Sections, RCP = Reinforced Concrete Pipe, CBC = Concrete Box Culvert, CMP = Corrugated Metal Pipe, C251-50 = Standard C251-50: Placed Riprap at Culvert Outlets (CFLHD 2015).

3 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table of Contents

1 Project Background Information

2 Hydraulic Design Criteria

3 Hydrology

3.1 Hydrologic Setting

3.2 FEMA Flood Insurance Study

3.3 Available Hydrologic Data

3.4 Major Basin Hydrology

3.4.1 Regression Equation Estimates

3.4.2 Precipitation

3.4.3 Basin Parameters

3.4.4 Recommended Design Flows

3.5 Minor Basin Hydrology

4 Hydraulic Analysis

4.1 Hydraulic Model Development

4.2 Existing Culvert Crossings

4.3 Hydraulic Model Results

4.3.1 Existing Conditions

4.3.2 Proposed Conditions

5 Recommendations

5.1 Material Selection

5.2 Outlet Protection

5.3 Entrance Treatment

5.4 Minimum Replacement Culvert Size

5.5 Site Specific Comments

5.5.1 Minor Crossings

5.5.2 Major Crossings

6 References

4 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Tables Table 1: Recommendation Matrix

Table 2: Hydraulic Design Criteria

Table 3: Major Basin Properties

Table 4: Regression Equation Peak Flow Rates for Major Basins

Table 5: NOAA Atlas 14 24-Hour Point Precipitation

Table 6: Drainage Basin Parameters (NRCS Method)

Table 7: Design Peak Flows for Major Basins

Table 8: Minor Basin Rational Method Peak Flow Rates - Cross Culverts

Table 9: Minor Basin Rational Method Peak Flow Rates - Parallel Culverts

Table 10: Existing Culvert Characteristics

Table 11: Culvert Layout and Invert Elevations

Table 12: Recommended Outlet Protection

Figures Figure 1: Project Location Map

Figure 2: HUC 12 - Subwatersheds

Figure 3: STA 130+61 Inlet and Tributary Channel Headcut

Figure 4: STA 175+74 Culvert Outlet and RTC Incised Ditch

Figure 5: STA 130+61 Culvert Outlet and RTC Incised Ditch

Figure 6: Incised Ditch Approaching STA 130+61 Outlet on RTC

Appendices Appendix A: Hydraulic Analysis

Appendix B: HY-8 Output Reports

Appendix C: Capacity Design

Appendix D: Stability Design

Appendix E: Site Photographs

5 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

1 PROJECT BACKGROUND INFORMATION

The Central Federal Lands Highway Division (CFLHD) in cooperation with Mesa County and the Bureau of Land Management plan to improve the existing 18-road corridor. The project is located in Mesa County, CO along 18 Road as depicted in Figure 2. The existing roadway is one lane in each direction and composed of gravel and chip seal at a width of 24 feet. The proposed project includes paving and widening the roadway to 30 feet (11-foot lanes and 4-foot shoulders). The scope of the project also includes earthwork, grading, paving, and additional drainage improvements.

Figure 1: Project Location Map

6 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

2 HYDRAULIC DESIGN CRITERIA

The Federal Lands Highway Project Development and Design Manual (PDDM) establishes the hydraulic design criteria for this project (Federal Lands Highway 2012). The proposed road has a posted speed of 35 mph, a design speed of 40 mph, a design average daily traffic of 188 vehicles (2016), and a projected design average daily traffic of 315 vehicles (2041). Based on these parameters the road is classified as low-standard roadway for hydraulic design purposes. Applicable design criteria are detailed in Table 2.

Hydraulic adequacy for roadway culverts is determined by conformance to applicable design criteria including allowable headwater (AHW) and the respective headwater to depth ratio (HW/D).

Table 2: Hydraulic Design Criteria

Low-Standard Roadway

Design Frequency

Check Frequency Design Criteria

Roadway Culverts 25-Year Overtopping

New: AHW ≤ bottom of aggregate base layer

Existing: AHW ≤ shoulder hinge point

Where D ≤ 48", HW/D ≤ 1.5 & Where D > 48", HW/D ≤ 1.2

Heavy debris or sediment load concerns: 0.8 ≤ HW/D ≤ 1.0

Crossing is stable at the design flood

Minimum pipe size: 24 in. or equivalent

Parallel Culverts 10-Year Overtopping

Same AHW, HW/D and stability criteria as roadway culverts

Minimum pipe size: 18 in. or equivalent

Longitudinal Embankments 25-Year

Greater of 100-Year and overtopping

Analyze when encroachment on base floodplain is unavoidable

Minimum 2 ft. of freeboard at the design flood

Embankments stable at the design flood

7 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

3 HYDROLOGY

3.1 HYDROLOGIC SETTING

The USGS National Map indicates that the 18 Road project crosses through three subwatersheds (U.S.

Geological Survey 2018). The southern 0.45 miles of the project are located within the Lower Big Salt Wash Subwatershed (HUC12: 140100051613) followed by 0.19 miles within the Lipan Wash Subwatershed (HUC12: 140100051611). The remaining 5.61 miles to the north are located within the Middle Big Salt Wash Subwatershed (HUC12: 140100051612), as detailed in Figure 2.

Figure 2: HUC 12 - Subwatersheds

The two major drainages along the project corridor are the Lipan Wash and an unnamed tributary to Dry Gulch, which crosses 18 Road at stations 130+61 and 271+12, respectively. Lipan Wash generally flows southwest to the confluence with Big Salt Wash, located approximately 1,600 feet west of the southern terminus of the project. Dry Gulch generally flows south along 18 Road until its confluence with Big Salt Wash approximately 2,000 linear feet upstream of the Lipan Wash confluence. The contributing drainage basins of the two major crossings were delineated using available elevation data obtained from the USGS National Map (U.S. Geological Survey 2018). General basin properties are provided in Table 3. A drainage basin map is included in Appendix A. Elevations reference the North American Vertical Datum of 1988

(NAVD88).

8 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table 3: Major Basin Properties

Project Station Stream Crossing Description Drainage Area (mi2) Mean Basin Elevation (ft)

130+61 Lipan Wash Existing 101-in. CMP 21.08 5,702 271+12 Tributary to Dry Gulch Existing 72-in. CMP 3.98 5,397

3.2 FEMA FLOOD INSURANCE STUDY

FEMA Flood Insurance Rate Map (FIRM) panel 08077C0100F indicates that the Lipan Wash culvert (STA 130+61) falls within FEMA Zone X (unshaded). Zone X (unshaded) is classified as minimal flood hazard, located outside of a special flood hazard area (SFHA). The SFHA comprises the land area that is shown in the FIRM as being inundated by the base flood and where the National Flood Insurance Program's (NFIP) floodplain management regulations apply. The Dry Gulch tributary crossing at station 271+12 is designated as FEMA Zone D. Zone D is classified as possible but undetermined flood hazards as no analysis of flood hazards has been conducted. An excerpt of the applicable FIRM is included in Appendix A. The project will not be subject to NFIP floodplain management regulations for backwater.

3.3 AVAILABLE HYDROLOGIC DATA

There are no USGS streamflow stations located within the project limits. The closest active streamflow station is located on the Colorado River located near Palisade, CO. The drainage area of the Colorado River at this station is reported as 8,813 square miles (USGS 2021). Due to the large size of the drainage basin compared to the project basins of interest, this stream gage data was not utilized on this project.

3.4 MAJOR BASIN HYDROLOGY

3.4.1 Regression Equation Estimates

The USGS regressions for Colorado relate peak flow to basin and precipitation characteristics. The equations and associated limitations are documented in USGS Report 2009-5136 (Capesius and Stephens 2009). The project is located in the Northwest Hydrologic Region. The Northwest Region peak flow regression equations were developed using the following ranges of predictor variables:

• Drainage area: 1.1 to 5,247 square miles

• Mean basin elevation: 6,500 to 10,166 feet

• Mean basin slope: 7.0 to 49.4 percent

• Mean annual precipitation: 7.9 to 48.6 inches

The 18 Road basins fall outside the applicable range of the predictor variables due to a lower mean basin elevation. Although the basin characteristics are not within the ranges specified above, the regression equations serve as a useful comparison for peak flow rates. Peak flow rates determined by the USGS regression equations for the two major culverts are outlined in Table 4. Due to the basin characteristics being outside of the range that the regression equations were developed, a HEC-HMS rainfall-runoff model was generated to estimate peak flows for the major basins as described in the following sections.

9 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table 4: Regression Equation Peak Flow Rates for Major Basins

Percent Chance Exceedance (%) Return Period (year) Lipan Wash, 130+61

Peak Flow (cfs) Dry Gulch Trib., 271+12

50 2 150 40 20 5 470 120 10 10 940 230 4 25 2,000 510 2 50 3,200 820 1 100 4,900 1,300

0.2 500 10,900 2,900

3.4.2 Precipitation

The point precipitation values for the project were obtained from NOAA Atlas 14, Volume 8, Version 2 (Perica, et al. n.d.) using the Precipitation Frequency Data Server (National Oceanic and Atmospheric Administration 2011). Point precipitation values were used to represent precipitation over the entire drainage basin. A SCS Type II temporal distribution was selected for the project. The point precipitation values for a duration of 24 hours associated with various return periods are shown in Table 5.

Table 5: NOAA Atlas 14 24-Hour Point Precipitation

Percent Chance Exceedance (%) Return Period (year) Point Precipitation (inches) 50 2 1.10 20 5 1.35 10 10 1.56 4 25 1.88 2 50 2.14 1 100 2.41

0.2 500 3.10

3.4.3 Basin Parameters

The NRCS curve number (CN) method (Natural Resources Conservation Service 1997) was used to estimate runoff. A composite CN for the basins was estimated using SSURGO soil information (Natural Resources Conservation Service 2015) and the 2016 National Land Cover Dataset (Multi-Resolution Land Characteristics Consortium 2018). Soils within the 18 Road project drainage basins are classified predominately as hydrologic soil groups B, C, and D. Land cover consists primarily of shrub/scrub and grassland/herbaceous classes with a smaller portion of forested areas and barren lands. Detailed CN computations are included in Appendix A. The NRCS lag time equation and unit hydrograph were used to estimate peak flows from the two major basins. Basin parameters are shown in Table 6.

10 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table 6: Drainage Basin Parameters (NRCS Method)

Station Basin Drainage Area (mi2)

Composite

CN

Mean Basin Slope (%)

Basin Length (mi)

Lag Time (hr)

130+61 Lipan Wash 21.08 76.1 27.3 8.63 1.32 271+12 Tributary to Dry Gulch 3.98 77.0 21.6 3.88 0.56

3.4.4 Recommended Design Flows

The hydrologic computations were carried out using HEC-HMS (U.S. Army Corps of Engineers 2017) using the input parameters described in preceding sections. The resulting design flows for the two major crossings are detailed in Table 7. Additional information pertaining to the hydrologic analyses are provided in Appendix A.

Table 7: Design Peak Flows for Major Basins

Percent Chance Exceedance (%) Return Period (year) Lipan Wash, 130+61

Dry Gulch Trib., 271+12

50 2 110 40 20 5 340 120 10 10 610 230 4 25 1,200 440 2 50 1,700 640 1 100 2,300 890

0.2 500 4,300 1,600

Contrasting the results from the HEC-HMS model to the USGS regression equations, the peak flow rates at smaller storm events are comparable but as the return period increases, the differences also increase.

Due to the average basin elevation being well below the minimum basin elevation used to develop the regression equations, the peak flow results derived from the HEC-HMS model were selected to evaluate the major culvert crossings along the project corridor.

3.5 MINOR BASIN HYDROLOGY

There are 11 existing minor culvert locations within the project limits, defined on this project as having a basin size of less than 1 square mile (640 acres). The existing culvert crossings are CMPs that range in diameter from 12 to 30 inches. The contributing basins to the minor culverts were delineated using the DEM information previously described. A drainage basin map is included in Appendix A. The Rational Method was used to estimate peak flow rates for the minor drainage crossings (Federal Highway Administration 2002). The time of concentration was estimated using the NRCS method for sheet, shallow concentrated, and channel flow. A minimum time of concentration of 10 minutes was used for all basins.

Rainfall intensity-duration-frequency (IDF) curves were obtained from the NOAA Precipitation Frequency Data Server previously referenced. A runoff coefficient (C) of 0.25 – 0.30 was used for the undeveloped basins. Table 8 summarizes the basin parameters and peak flow rates for the 25 and 100-year return periods.

11 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table 8: Minor Basin Rational Method Peak Flow Rates - Cross Culverts

Station Drainage

Area (acres)

Time of Concentrat ion (min)

Rational Runoff

Coefficient (C)

25-year Rainfall

Intensity (in/hr)

100-year

25-year Peak Flow

(cfs)

105+59 2.9 18 0.30 2.4 3.5 2.1 3.0 107+01 23.2 50 0.30 1.1 1.5 7.7 10 134+84 19.6 37 0.30 1.6 2.3 9.4 14 143+20 17.5 21 0.30 2.2 3.2 12 17 156+74 150.9 48 0.30 1.2 1.6 54 72 175+541 622.5 75 0.25 0.9 1.3 140 200 210+36 2.9 28 0.30 1.8 2.5 1.6 2.2 212+27 50.9 52 0.30 1.1 1.5 17 23 254+13 18.6 43 0.30 1.5 2.0 8.4 11 285+90 4.4 10 0.30 3.1 4.4 4.1 5.8

The Rational Method was also used to estimate peak flows for the parallel culvert at STA 157+57. The 10-year frequency is the design event for this parallel culvert, which conveys drainage captured along a roadside ditch. Table 9 summarizes the basin parameters and peak flow results for the 10 and 100-year return periods at this crossing.

Table 9: Minor Basin Rational Method Peak Flow Rates - Parallel Culverts

Drainage

Area (acres)

Time of Concentrat ion (min)

Rational Runoff

Coefficient (C)

10-year

10-year

157+57 7.9 38 0.30 1.3 2.3 3.1 5.5

4 HYDRAULIC ANALYSIS

4.1 HYDRAULIC MODEL DEVELOPMENT

Hydraulic performance was evaluated using HY-8 (Federal Highway Administration 2017). Site specific hydraulic models were generated for each of the 12 existing crossings including the proposed configurations. An additional hydraulic model was generated for the new proposed cross culvert at STA 285+90. HY-8 output reports are included in Appendix B. Hydraulic capacity design results are provided in Appendix C.

1 Although the PDDM recommends limiting the Rational Method to drainage areas of 200 acres or less, it was selected for STA 175+54 as it produced more conservative results than the NRCS method described in Section 3.4.

12 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

4.2 EXISTING CULVERT CROSSINGS

The existing culvert characteristics shown in Table 10 were obtained from the project field survey completed by CFLHD. The structure at STA 130+61, previously reported as a 101-in. CMP has been field verified and adjusted to a 96-in. CMP. Photographs of the existing culvert crossings are provided in Appendix D.

Table 10: Existing Culvert Characteristics

Station Type Diameter (in) Length (ft)

Inlet Invert

Outlet Invert

Relief

105+59 CMP 18 41.1 4751.7 4750.9 4753.7

107+01 CMP 18 45.2 4753.2 4751.4 4755.0

130+61 CMP 96 72.8 4764.9 4763.7 4777.6

134+84 CMP 18 39.6 4774.4 4773.5 4776.2

143+20 CMP 18 50.0 4780.9 4780.0 4783.0

156+74 CMP 30 41.3 4784.8 4783.4 4790.2

157+57 CMP 18 40.2 4788.4 4788.4 4790.2

175+54 CMP 30 44.7 4803.4 4802.4 4808.1

210+36 CMP 18 43.1 4849.4 4849.4 4852.1

212+27 CMP 24 40.0 4850.4 4848.1 4854.4

254+13 CMP 18 50.2 4919.0 4918.1 4921.7

271+12 CMP 72 61.1 4934.1 4932.2 4944.6

4.3 HYDRAULIC MODEL RESULTS

A summary of the hydraulic capacity design results is included in Appendix C. This attachment details capacity design results pertaining to both existing and proposed configurations.

4.3.1 Existing Conditions

Hydraulic modeling indicates that 7 of the 12 existing culvert crossings are hydraulically adequate based on the standards established in this report. Several of the hydraulically adequate culverts were considered for a leave in place or culvert extension alternative based on satisfactory existing condition (e.g. STA 105+59). However, the project partners indicated a desirable to replace all the culverts along the project corridor so that the structures are at a similar life span and replacement cycle.

4.3.2 Proposed Conditions

Proposed culvert types and sizes are detailed in Appendix C, along with the associated capacity design results for each structure. Geometric configurations have been derived based on existing conditions and modified to accommodate proposed roadway geometry based on the pipe plots provided by the roadway design discipline. The majority of the proposed structures have increased in length to accommodate the widened roadway cross section through the project corridor.

13 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

5 RECOMMENDATIONS

5.1 MATERIAL SELECTION

Existing culvert condition was evaluated and during the April 2021 field visit. Five of the existing CMPs had inlets or outlets that had observed some level of crushing (STA 107+01, 134+84, 143+20, 156+74, 254+13).

This may be the result of traffic or maintenance driving on the shoulder where there is limited cover over the structures. Several of the minor drainage structures also had sediment accumulations at the inlets, which may be associated with low culvert slopes and/or crushed inlet conditions. Material preference was discussed at the on-site field visit with project partners. The partners indicated a preference for concrete (RCP, CBC) or plastic (CPP) over that of steel (CMP) due to acidic soils. Due to the crushed nature of several of the existing structures inlets, CPP is not recommended for replacement. Concrete has been recommended for all replacement culverts.

5.2 OUTLET PROTECTION

Culverts with diameters less than 48-inches and pipe slopes less than 10% may be sized in accordance to Standard Detail C251-50, Placed Riprap at Culverts (CFLHD 2015). The layout for structures exceeding the 48-in. threshold have been iterated between the hydraulics and roadway design disciplines to select a configuration that accommodates local site constraints and the proposed roadway geometry. The layout for the three sites exceeding the 48-in. threshold is summarized in Table 11.

Table 11: Culvert Layout and Invert Elevations

Crossing

Proposed Structure Size

Structure

Type

Proposed Structure Layout

Span (ft.) Depth (ft.)

Inlet Invert Elevation

(ft.)

Outlet Invert

Projected

Length (ft.)

Slope

130+61 12 10 CBC 4765.4 4764.0 60.3 2.2%

175+54 5 5 RCP 4803.4 4802.4 54.0 1.9%

271+12 9 6 CBC 4934.1 4932.0 63.0 3.3%

Outlet protection recommendations were developed using the Hydraulic Toolbox (FHWA 2018), the results from the HY-8 hydraulic models, and the layout detailed in Table 11. The 25-year hydraulic design event was used to evaluate stability. Recommend riprap class and outlet apron dimensions for the three crossings exceeding the 48-inch diameter threshold applicable to Standard C251-50 are summarized in Table 12. Some of these crossings outlet into confined channel settings. In these locations, recommended riprap limits (particularly the end width) can result in aprons that extend well beyond the vertical extents of the channel. In an effort to limit the elevation that riprap is placed for any particular apron, it is proposed that the outlet apron extend the recommended depth, length and end width, up to an elevation that is at least one foot above the top of the opening, adjusted along the channel profile. For the purpose of stability, riprap should tie into the existing ground at a slope not exceeding 1:1.5 (V:H).

14 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Table 12: Recommended Outlet Protection

Crossing

Proposed Structure Size

Outlet Protection

Span (ft.) Depth (ft.) Riprap Class

Apron Length (ft.)

Apron Thickness

Apron End Width (ft.)

130+61 12 10 CBC 6 99.0 4.0 103.0

175+54 5 5 RCP 2 20.0 3.0 28.5

271+12 9 6 CBC 4 50.0 3.0 58.0

The riprap apron sizes reported in Table 12 are based on the output produced from the HY-8 model in conjunction with a Hydraulics Toolbox analysis. Actual riprap apron dimensions shown in the plan set have been modified to accommodate local site constraints and stay within right-of-way to the extent practical.

5.3 ENTRANCE TREATMENT

A headwall entrance treatment is recommended to offset buoyant forces for structures exceeding a 48-inch diameter (Federal Lands Highway 2012). Cutoff walls are recommended to mitigate undermining (piping), which is known to lead to embankment failure. It is desirable for structures in excess of 48-inches to be equipped with flared wingwalls, matched into the channel banks at the inlet and outlet, to smoothly transition flow and protect the structure and roadway approaches from erosion. Beveled edges are recommended to maximize hydraulic efficiency on all headwalls (Federal Lands Highway 2012). Flared end sections have been proposed for all culverts below a 48-inch threshold.

The three structures reported in Table 12 are the only proposed structures meeting or exceeding a 48-inch diameter threshold. Headwalls, cutoff walls and wingwalls are proposed for the 60-inch RCP recommended for STA 175+54, as well as the concrete box culverts recommended at STA 271+12 and STA 130+61. Standard Detail C601-5, Concrete Headwall/Wingwall for Single Skewed 42-inch to 84-inch Pipe Culvert (FLH 2005) is recommended to be included the plan set for the STA 175+54 crossing.

5.4 MINIMUM REPLACEMENT CULVERT SIZE

To limit maintenance problems associated with debris and sedimentation, a minimum 24-inch diameter replacement size is recommended for cross-culverts and an 18-inch diameter replacement size is recommended for parallel culverts (Federal Lands Highway 2012). If desired, the one parallel culvert replacement (STA 157+57) could be upsized to a 24-inch diameter on the basis of economy of scale. In discussion with the roadway design group, an 18-inch size was specified for this culvert replacement.

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5.5 SITE SPECIFIC COMMENTS

5.5.1 Minor Crossings

The project partners confirmed that this project will terminate prior to the BLM parking lot at the end of the project. As a result, the culvert at 419+57 that runs beneath this parking lot will not receive services as part of this project and has not been evaluated within this report.

Two options were considered for the crossing at STA 156+74. It may be desirable to equip the structure at this location with a headwall considering that the roadway design along this section includes a grade raise. Modeling indicates that alternative 1, a 36-inch RCP with a headwall results in headwater depths that are approximately 0.5 ft. higher than alternative 2, a 42-inch RCP equipped with flared end sections (FES). It is anticipated that the 42-inch RCP with FES will be less costly than a 36-inch RCP with a headwall.

The roadway design group was consulted in this regard. Due to the fiscally conservative option being more hydraulically efficient, the 42-inch RCP with FES alternative was selected for replacement of this crossing.

At the on-site field visit, a new culvert was proposed for installation at approximate STA 285+90 to convey flow captured between 18 Road and the access road on east side of the primary route. The new culvert will cross beneath 18 Road and outlet to a runoff swale that disperses water away from the predominant roadway prism. Waters from this swale will drain overland to the channel downstream of the drainage that crosses under 18 Road through the culvert at STA 271+12.

5.5.2 Major Crossings

The STA 130+61 and STA 271+12 drainages are particularly flashy in nature. The channels are significantly incised, compounded by erosive soils and the undersized nature of the existing structures. Facilitating hydraulically adequate structures equipped with riprap outlet aprons has been proposed to mitigate continued incision in these channels. The roadside ditches approaching these major drainage crossings require grading, fill/embankment repair, and armoring to stabilize the channels and address the existing incision. Riprap class 2 (minimum) at a depth of 18-inches and maximum longitudinal slope of 1:1.5 (V:H) has been recommended to stabilize the ditch in these locations.

5.5.2.1 STA 130+61 – Lipan Wash

The STA 130+61 location was noted by maintenance personnel as having overtopped in recent years. The hydraulic model for the existing crossing indicates that overtopping occurs prior to the 25-year design event. It is recommended that the existing 96-inch structure be replaced with a 12-ft. x 10-ft. concrete box culvert equipped with headwalls, cutoff walls, wingwalls and a class 6 outlet apron. This upsize will increase the waterway area from 50 square feet to 120 square feet, providing additional capacity for floodwater conveyance.

A minor drainage converges with the primary channel at the structure inlet as shown in Figure 3. The primary channel comes from the LTC (left side of the channel looking downstream). The minor drainage channel converges at the structure inlet from the RTC (right side of the channel looking downstream). The cross-functional design team selected flared wingwalls for the structure inlet to better route floodwaters into the structure. To facilitate flared wingwalls at the structure entrance without flanking the RTC inlet wingwall, the RTC minor drainage will be rerouted to the primary channel upstream of the proposed structure inlet. The project partners and cross-functional team discussed adding a small quantity of riprap to the drainage channel in an effort to stabilize the regraded drainage and address the approximate 3.5-

16 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

ft. deep headcut that has formed upstream, which is likely the result of circulating floodwaters from the primary channel, and is shown in the image on the right side of Figure 3.

Figure 3: STA 130+61 Inlet and Tributary Channel Headcut

5.5.2.2 STA 175+54 – Riprap Revetment

The proposed roadway geometry at this location includes an approximate 3.5-ft. grade raise and a widened cross section. Approximately 140 feet downstream, the channel resides adjacent to the existing roadway prism for approximately 100 longitudinal feet, as shown in Figure 4. To accommodate the proposed roadway side slopes and cross section, the channel will need to be realigned further from the road and riprap will need to be placed to stabilize the slopes.

Figure 4: STA 175+74 Culvert Outlet and RTC Incised Ditch

STA 175+74 Culvert

Flow

Area of Interest

17 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

It is proposed that the revetment use a class 2 riprap apron, matching the size of the outlet protection of the 175+54 crossing. For purposes of stability, the longitudinal slope of the riprap embankment should not exceed 1:1.5 (V:H). The class 2 revetment is recommended to be placed at a thickness of 1.5-ft. and be equipped with a 3-ft. thick, 1.5 ft. high mound at the toe of the slope. Riprap should be keyed in at the base of the embankment slope at the local thalweg elevation.

The PDDM establishes the hydraulic design criteria for longitudinal embankments (Federal Lands Highway 2012). For hydraulic capacity, riprap embankments along low-standard roadways are designed to provide a minimum of 2-ft. of freeboard at the 25-year design event. Using the tailwater surface elevation produced by the STA 175+54 HY-8 model (4805.3-ft.), the riprap revetment should extend up to a minimum elevation of 4807.3-ft. to meet hydraulic capacity criteria.

5.5.2.3 STA 271+12 – Dry Gulch Tributary

The hydraulic model for the existing crossing indicates that overtopping occurs prior to the 25-year design event. It is recommended that the existing 72-inch structure be replaced with a 9-ft. x 6-ft. concrete box culvert equipped with headwalls, cutoff walls, wingwalls and a class 4 outlet apron. This upsize will increase the waterway area from 28 square feet to 54 square feet, providing additional capacity for floodwater conveyance. A 2.5-ft. vertical drop and scour hole is present at the outlet of the existing structure. Extensive erosion is present in the ditch that approaches the culvert outlet on the RTC as shown Figure 5 and Figure 6.

Figure 5: STA 130+61 Culvert Outlet and RTC Incised Ditch

STA 175+74 Culvert

Ditch Erosion

Primary Channel Flow

18 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Maintenance personnel indicated that there are complex upstream hydrologic conditions at this site that have been subject to change in recent years. The BLM reportedly constructed a berm several hundred feet upstream of the crossing, intended to provide open range cattle a watering hole. The watering hole is no longer in use, however. During a recent flooding event, the berm allegedly routed floodwaters away from the primary channel, pushing flow down and across 18 Road and into the ditch just upstation of the STA 271+12 culvert outlet. Maintenance personnel indicated that this event resulted in the substantial existing incision that is present within this ditch, extending as much as 12 vertical feet as measured from the surface of the roadway.

Figure 6: Incised Ditch Approaching STA 130+61 Outlet on RTC

In discussion with the project partners, it was identified that maintenance has since installed a relief opening in the berm in an effort to allow floodwaters access to the primary channel. As part of this project, proposed roadway side slopes fill in the incised ditch. It is proposed that the top surface of the new ditch be armored with minimum class 2 riprap at an 18-inch depth in an effort to stabilize the ditch elevation and protect the toe of the roadway prism. The ditch is proposed to be regraded and directed downstream of the culvert to allow flow to enter the primary channel without flanking the proposed downstream RTC flared wingwall.

19 | P a g e CO FLAP MES18(1) 18 Road – Final Hydraulics Report

6 REFERENCES

Capesius, Joseph P., and Verlin C. Stephens. 2009. "Regional regression equations for estimation of natural streamflow statistics in Colorado." Scientific Investigations Report 2009-5136, U.S. Geological Survey. https://pubs.usgs.gov/sir/2007/5158/.

CFLHD, Central Federal Lands Highway Division. 2015. "Placed Riprap at Culverts (C-251-50)." Detail Drawings: FP-14.

Federal Highway Administration. 2017. HY-8 Culvert Analysis Program. Version 7.50.

Federal Highway Administration. 2002. Hydraulic Design Series No. 2, Second Edition, FHWA -NHI-02-001.

Washington, DC: Federal Highway Administration.

Federal Lands Highway. 2012. "Project Development and Design Manual, Chapter 7 Hydrology and Hydraulics." https://flh.fhwa.dot.gov/resources/design/pddm/.

FHWA. 2018. Hydraulic Toolbox. Federal Highway Administation, Aquaveo.

FLH, Federal Lands Highway Division. 2005. "Concrete Headwall/Wingwall For 42" to 84" Pipe Culvert."

FLH Standard Drawings.

Multi-Resolution Land Characteristics Consortium . 2018. National Land Cover Database 2011 (NLCD 2011). Multi-Resolution Land Characteristics Consortium (MRLC).

https://data.nal.usda.gov/dataset/national-land-cover-database-2011-nlcd-2011.

National Oceanic and Atmospheric Administration. 2011. Precipitation Frequency Data Server (PFDS).

Silver Springs, MD. Accessed April 2019. https://hdsc.nws.noaa.gov/hdsc/pfds/index.html.

Natural Resources Conservation Service. 1997. "National Engineering Handbook, Part 630 Hydrology."

Washington, DC. https://directives.sc.egov.usda.gov/viewerFS.aspx?hid=21422.

Natural Resources Conservation Service. 2015. Soil Survey Geographic (SSURGO) database. Fort Worth, TX. Accessed April 2019. http://websoilsurvey.nrcs.usda.gov.

Perica, Sanja, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, and Geoffery Bonnin. n.d. "Point Precipitation Frequency Estimates." NOAA Atlas 14, Volume 8, Version 2, National Oceanic and Atmospheric Administration , Silver Springs, MD.

U.S. Army Corps of Engineers. 2017. HEC-HMS, Hydrologic Engineering Center Hydrologic Modeling System. Version 4.2.1. Davis, CA.

U.S. Geological Survey. 2018. The National Map. https://viewer.nationalmap.gov/advanced-viewer/.

U.S. Geological Survey. 2018. The National Map. Accessed March 2019.

https://viewer.nationalmap.gov/advanced-viewer/.

USGS. 2021. National Water Information System: Web Interface. May 17.

https://nwis.waterdata.usgs.gov/nwis/inventory/?site_no=09106150.

A | A p p e n d i x CO FLAP MES18(1) 18 Road – Final Hydraulics Report

Appendix A

Hydrologic Analysis

0 1 2 3 4 Miles

²

Project Start

R oa d

Project End

Service Layer Credits: Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea, Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and the GIS User Community

Legend Basins

Main Flow Path

18 Road Alignment

CO FLAP MES18(1) - 18 Road Basin Delineation

August 2020

18 Road Project Corridor

Project Start

Project End

Zone D

Zone X

6/15/2020 Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=39.3127&lon=-108.6684&data=depth&units=english&series=pds 1/4

NOAA Atlas 14, Volume 8, Version 2 Location name: Fruita, Colorado, USA*

Latitude: 39.3127°, Longitude: -108.6684° Elevation: 5211.54 ft**

* source: ESRI Maps ** source: USGS

POINT PRECIPITATION FREQUENCY ESTIMATES

Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffery Bonnin

NOAA, National Weather Service, Silver Spring, Maryland

PF_tabular | PF_graphical | Maps_&_aerials

PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches)1

Duration Average recurrence interval (years)

1 2 5 10 25 50 100 200 500 1000

5-min 0.124 (0.099‑0.160)

0.157 (0.126‑0.203)

0.218 (0.174‑0.282)

0.274 (0.217‑0.357)

0.360 (0.277‑0.498)

0.433 (0.323‑0.604)

0.511 (0.367‑0.733)

0.597 (0.409‑0.881)

0.720 (0.472‑1.09)

0.820 (0.519‑1.25)

10-min 0.182 (0.145‑0.234)

0.231 (0.184‑0.298)

0.319 (0.254‑0.413)

0.401 (0.317‑0.522)

0.527 (0.406‑0.729)

0.634 (0.473‑0.885)

0.749 (0.537‑1.07)

0.875 (0.599‑1.29)

1.05 (0.691‑1.60)

1.20 (0.760‑1.84)

15-min 0.222 (0.177‑0.286)

0.281 (0.225‑0.363)

0.389 (0.310‑0.504)

0.490 (0.387‑0.637)

0.643 (0.495‑0.888)

0.773 (0.577‑1.08)

0.913 (0.655‑1.31)

1.07 (0.730‑1.57)

1.29 (0.842‑1.95)

1.46 (0.927‑2.24)

30-min 0.313 (0.250‑0.403)

0.398 (0.318‑0.513)

0.549 (0.437‑0.711)

0.685 (0.542‑0.892)

0.889 (0.682‑1.22)

1.06 (0.789‑1.47)

1.24 (0.888‑1.77)

1.44 (0.981‑2.11)

1.71 (1.12‑2.59)

1.93 (1.22‑2.96)

60-min 0.403 (0.322‑0.520)

0.501 (0.400‑0.646)

0.672 (0.534‑0.869)

0.825 (0.652‑1.07)

(0.807‑1.44)

(0.924‑1.72)

(1.03‑2.06)

1.66 (1.13‑2.43)

1.96 (1.28‑2.97)

2.20 (1.39‑3.37)

2-hr 0.494 (0.400‑0.628)

0.603 (0.487‑0.768)

0.794 (0.639‑1.01)

0.964 (0.771‑1.24)

1.22 (0.943‑1.64)

1.42 (1.07‑1.94)

1.64 (1.19‑2.31)

1.88 (1.30‑2.72)

2.21 (1.46‑3.29)

2.47 (1.59‑3.73)

3-hr 0.557 (0.453‑0.702)

0.659 (0.536‑0.831)

0.840 (0.681‑1.06)

1.00 (0.809‑1.28)

1.25 (0.978‑1.67)

1.46 (1.11‑1.98)

1.68 (1.23‑2.34)

1.91 (1.34‑2.75)

2.25 (1.50‑3.33)

2.52 (1.63‑3.77)

6-hr 0.666 (0.549‑0.827)

0.774 (0.637‑0.963)

0.962 (0.789‑1.20)

1.13 (0.921‑1.42)

1.38 (1.09‑1.81)

1.58 (1.21‑2.10)

1.79 (1.33‑2.45)

2.02 (1.43‑2.85)

2.34 (1.59‑3.41)

2.60 (1.70‑3.82)

12-hr 0.804 (0.670‑0.984)

0.926 (0.772‑1.14)

1.14 (0.945‑1.40)

1.33 (1.09‑1.64)

1.60 (1.28‑2.06)

1.82 (1.42‑2.38)

2.06 (1.54‑2.77)

2.31 (1.65‑3.20)

2.65 (1.82‑3.79)

2.93 (1.94‑4.24)

24-hr 0.954 (0.805‑1.15)

1.10 (0.926‑1.33)

1.35 (1.13‑1.63)

1.56 (1.31‑1.90)

(1.52‑2.39)

2.14 (1.69‑2.76)

2.41 (1.83‑3.19)

2.70 (1.96‑3.68)

3.10 (2.15‑4.36)

3.42 (2.30‑4.87)

2-day 1.10 (0.944‑1.31)

1.27 (1.09‑1.51)

(1.33‑1.86)

(1.54‑2.18)

2.19 (1.79‑2.74)

2.49 (1.99‑3.16)

2.80 (2.15‑3.66)

3.14 (2.31‑4.22)

3.61 (2.54‑4.99)

3.97 (2.71‑5.57)

3-day 1.20 (1.04‑1.42)

1.39 (1.19‑1.64)

(1.46‑2.02)

1.99 (1.69‑2.36)

2.39 (1.98‑2.97)

2.73 (2.19‑3.43)

3.07 (2.38‑3.97)

3.44 (2.54‑4.57)

3.95 (2.80‑5.41)

4.35 (2.99‑6.04)

4-day 1.28 (1.11‑1.50)

1.48 (1.28‑1.74)

(1.57‑2.14)

2.12 (1.81‑2.50)

2.55 (2.11‑3.14)

2.90 (2.34‑3.62)

3.26 (2.54‑4.18)

3.65 (2.71‑4.81)

4.18 (2.98‑5.69)

4.60 (3.18‑6.34)

7-day 1.49 (1.30‑1.72)

(1.49‑1.98)

2.08 (1.81‑2.42)

2.40 (2.08‑2.80)

2.86 (2.40‑3.47)

3.23 (2.64‑3.98)

3.62 (2.84‑4.57)

4.02 (3.02‑5.23)

4.57 (3.29‑6.12)

5.00 (3.50‑6.80)

10-day 1.67 (1.47‑1.91)

1.90 (1.67‑2.18)

2.30 (2.01‑2.65)

2.64 (2.30‑3.05)

3.12 (2.63‑3.74)

3.50 (2.87‑4.26)

3.89 (3.08‑4.87)

4.30 (3.26‑5.55)

4.86 (3.53‑6.46)

5.30 (3.74‑7.15)

20-day 2.17 (1.93‑2.44)

2.45 (2.18‑2.77)

2.92 (2.59‑3.31)

3.32 (2.93‑3.78)

3.87 (3.30‑4.55)

4.30 (3.57‑5.14)

4.74 (3.80‑5.81)

5.18 (3.97‑6.55)

5.78 (4.26‑7.54)

6.25 (4.47‑8.28)

30-day 2.59 (2.32‑2.89)

2.92 (2.62‑3.27)

3.47 (3.11‑3.90)

3.92 (3.49‑4.43)

4.54 (3.90‑5.28)

5.02 (4.20‑5.93)

5.50 (4.44‑6.67)

5.98 (4.62‑7.47)

6.61 (4.90‑8.51)

7.08 (5.11‑9.29)

45-day 3.12 (2.83‑3.46)

3.54 (3.20‑3.93)

4.20 (3.79‑4.68)

4.74 (4.25‑5.30)

5.46 (4.71‑6.26)

5.99 (5.05‑6.99)

6.52 (5.30‑7.80)

7.03 (5.47‑8.67)

7.68 (5.74‑9.77)

8.16 (5.94‑10.6)

60-day 3.59 (3.27‑3.95)

4.08 (3.72‑4.50)

4.86 (4.41‑5.37)

5.47 (4.93‑6.08)

6.28 (5.43‑7.13)

6.86 (5.81‑7.94)

7.42 (6.06‑8.81)

7.96 (6.22‑9.73)

8.62 (6.47‑10.9)

9.09 (6.66‑11.7)

1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).

Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.

Please refer to NOAA Atlas 14 document for more information.

Back to Top

PF graphical https://www.commerce.gov/ http://www.noaa.gov/

6/15/2020 Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=39.3127&lon=-108.6684&data=depth&units=english&series=pds 2/4

Maps & aerials

Small scale terrain

7/17/2020 Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=39.3127&lon=-108.6684&data=intensity&units=english&series=pds 1/4

NOAA Atlas 14, Volume 8, Version 2 Location name: Fruita, Colorado, USA*

Latitude: 39.3127°, Longitude: -108.6684° Elevation: 5211.54 ft**

* source: ESRI Maps ** source: USGS

POINT PRECIPITATION FREQUENCY ESTIMATES

Sanja Perica, Deborah Martin, Sandra Pavlovic, Ishani Roy, Michael St. Laurent, Carl Trypaluk, Dale Unruh, Michael Yekta, Geoffery Bonnin

NOAA, National Weather Service, Silver Spring, Maryland

PF_tabular | PF_graphical | Maps_&_aerials

PF tabular PDS-based point precipitation frequency estimates with 90% confidence intervals (in inches/hour)1

Duration Average recurrence interval (years)

1 2 5 10 25 50 100 200 500 1000

5-min 1.49 (1.19‑1.92)

(1.51‑2.44)

2.62 (2.09‑3.38)

3.29 (2.60‑4.28)

4.32 (3.32‑5.98)

5.20 (3.88‑7.25)

6.13 (4.40‑8.80)

7.16 (4.91‑10.6)

8.64 (5.66‑13.1)

9.84 (6.23‑15.1)

10-min 1.09 (0.870‑1.40)

1.39 (1.10‑1.79)

1.91 (1.52‑2.48)

2.41 (1.90‑3.13)

3.16 (2.44‑4.37)

3.80 (2.84‑5.31)

4.49 (3.22‑6.44)

5.25 (3.59‑7.75)

6.32 (4.15‑9.61)

7.21 (4.56‑11.0)

15-min 0.888 (0.708‑1.14)

1.12 (0.900‑1.45)

(1.24‑2.02)

(1.55‑2.55)

2.57 (1.98‑3.55)

3.09 (2.31‑4.32)

3.65 (2.62‑5.24)

4.27 (2.92‑6.30)

5.14 (3.37‑7.82)

5.86 (3.71‑8.96)

30-min 0.626 (0.500‑0.806)

0.796 (0.636‑1.03)

(0.874‑1.42)

1.37 (1.08‑1.78)

1.78 (1.36‑2.44)

2.12 (1.58‑2.94)

2.48 (1.78‑3.54)

2.87 (1.96‑4.22)

3.42 (2.24‑5.18)

3.86 (2.45‑5.91)

60-min 0.403 (0.322‑0.520)

0.501 (0.400‑0.646)

0.672 (0.534‑0.869)

0.825 (0.652‑1.07)

(0.807‑1.44)

(0.924‑1.72)

(1.03‑2.06)

1.66 (1.13‑2.43)

(1.28‑2.97)

2.20 (1.39‑3.37)

2-hr 0.247 (0.200‑0.314)

0.302 (0.244‑0.384)

0.397 (0.320‑0.507)

0.482 (0.386‑0.618)

0.608 (0.472‑0.820)

0.712 (0.536‑0.972)

0.822 (0.596‑1.15)

0.939 (0.650‑1.36)

(0.731‑1.65)

(0.792‑1.86)

3-hr 0.185 (0.151‑0.234)

0.219 (0.178‑0.277)

0.280 (0.227‑0.354)

0.334 (0.269‑0.425)

0.416 (0.326‑0.557)

0.485 (0.369‑0.658)

0.558 (0.408‑0.778)

0.637 (0.445‑0.915)

0.750 (0.500‑1.11)

0.840 (0.543‑1.25)

6-hr 0.111 (0.092‑0.138)

0.129 (0.106‑0.161)

0.161 (0.132‑0.201)

0.189 (0.154‑0.237)

0.230 (0.182‑0.302)

0.264 (0.203‑0.351)

0.300 (0.222‑0.410)

0.338 (0.238‑0.476)

0.391 (0.265‑0.569)

0.434 (0.284‑0.639)

12-hr 0.067 (0.056‑0.082)

0.077 (0.064‑0.094)

0.095 (0.078‑0.116)

0.110 (0.091‑0.136)

0.133 (0.106‑0.171)

0.151 (0.118‑0.198)

0.171 (0.128‑0.230)

0.191 (0.137‑0.265)

0.220 (0.151‑0.314)

0.243 (0.161‑0.352)

24-hr 0.040 (0.034‑0.048)

0.046 (0.039‑0.055)

0.056 (0.047‑0.068)

0.065 (0.054‑0.079)

0.078 (0.063‑0.100)

0.089 (0.070‑0.115)

0.100 (0.076‑0.133)

0.112 (0.081‑0.153)

0.129 (0.090‑0.182)

0.142 (0.096‑0.203)

2-day 0.023 (0.020‑0.027)

0.026 (0.023‑0.032)

0.033 (0.028‑0.039)

0.038 (0.032‑0.045)

0.046 (0.037‑0.057)

0.052 (0.041‑0.066)

0.058 (0.045‑0.076)

0.065 (0.048‑0.088)

0.075 (0.053‑0.104)

0.083 (0.056‑0.116)

3-day 0.017 (0.014‑0.020)

0.019 (0.017‑0.023)

0.024 (0.020‑0.028)

0.028 (0.024‑0.033)

0.033 (0.027‑0.041)

(0.030‑0.048)

0.043 (0.033‑0.055)

0.048 (0.035‑0.064)

0.055 (0.039‑0.075)

0.060 (0.042‑0.084)

4-day 0.013 (0.012‑0.016)

0.015 (0.013‑0.018)

(0.016‑0.022)

0.022 (0.019‑0.026)

0.027 (0.022‑0.033)

0.030 (0.024‑0.038)

0.034 (0.026‑0.044)

(0.028‑0.050)

0.044 (0.031‑0.059)

0.048 (0.033‑0.066)

7-day 0.009 (0.008‑0.010)

0.010 (0.009‑0.012)

0.012 (0.011‑0.014)

0.014 (0.012‑0.017)

0.017 (0.014‑0.021)

(0.016‑0.024)

(0.017‑0.027)

0.024 (0.018‑0.031)

0.027 (0.020‑0.036)

0.030 (0.021‑0.040)

10-day 0.007 (0.006‑0.008)

0.008 (0.007‑0.009)

(0.008‑0.011)

0.011 (0.010‑0.013)

0.013 (0.011‑0.016)

0.015 (0.012‑0.018)

0.016 (0.013‑0.020)

0.018 (0.014‑0.023)

0.020 (0.015‑0.027)

(0.016‑0.030)

20-day 0.005 (0.004‑0.005)

0.005 (0.005‑0.006)

0.006 (0.005‑0.007)

0.007

(0.007‑0.009)

0.009 (0.007‑0.011)

(0.008‑0.012)

0.011 (0.008‑0.014)

0.012 (0.009‑0.016)

0.013 (0.009‑0.017)

30-day 0.004 (0.003‑0.004)

0.004

(0.006‑0.009)

(0.006‑0.010)

0.009 (0.007‑0.012)

(0.007‑0.013)

45-day 0.003 (0.003‑0.003)

0.003

(0.004‑0.004)

(0.004‑0.006)

(0.005‑0.008)

(0.005‑0.009)

(0.006‑0.010)

60-day 0.002 (0.002‑0.003)

(0.003‑0.003)

(0.004‑0.007)

(0.004‑0.008)

(0.005‑0.008)

1 Precipitation frequency (PF) estimates in this table are based on frequency analysis of partial duration series (PDS).

Numbers in parenthesis are PF estimates at lower and upper bounds of the 90% confidence interval. The probability that precipitation frequency estimates (for a given duration and average recurrence interval) will be greater than the upper bound (or less than the lower bound) is 5%. Estimates at upper bounds are not checked against probable maximum precipitation (PMP) estimates and may be higher than currently valid PMP values.

Please refer to NOAA Atlas 14 document for more information.

PF graphical https://www.commerce.gov/ http://www.noaa.gov/

7/17/2020 Precipitation Frequency Data Server https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_printpage.html?lat=39.3127&lon=-108.6684&data=intensity&units=english&series=pds 2/4

Maps & aerials

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