20250291.Final Drainage Report.pdf

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FINAL DRAINAGE REPORT

FOR

35 8/10 ROAD DRAINAGE IMPROVEMENT PROJECT

738 35 8/10 RD (MORRIS SUBDIVISION)

PALISADE, COLORADO

BHI PROJECT NUMBER: 20250291

MESA COUNTY PROJECT NUMBER: TBD

PREPARED FOR:

MESA COUNTY

200 S SPRUCE ST

GRAND JUNCTION, CO 81501

PREPARED BY:

BOHANNAN HUSTON, INC.

619 MAIN STREET

GRAND JUNCTION, CO 81502

PHONE: (970)-788-1140

DATE: OCTOBER 22ND, 2025

FINAL DRAINAGE REPORT 35 8/10 ROAD DRAINAGE IMPROVEMENT PROJECT

i

ENGINEER’S CERTIFICATION STATEMENT:

I hereby certify that this Final Drainage Report (Plan) for the design of 35 8/10 Road

Drainage Improvements was prepared by me (or under my supervision) in accordance with the provisions of the Stormwater Management Manual for the owners thereof. I understand that Mesa County will not and will not assume liability for drainage facilities designed by others.

SIGNATURE: _________________________________

State of Colorado No. ______________________

DEVELOPER’S CERTIFICATION STATEMENT:

Mesa County hereby certifies that the drainage facilities for 35 8/10 Road subdivision shall be constructed according to the design presented in this report. I understand that

Mesa County does not and will not assume liability for the drainage facilities designed and/or certified by my engineer and that Mesa County reviews drainage plans pursuant to the Municipal Code; but cannot, on behalf of Mesa County, guarantee that final drainage review will absolve Mesa County and/or their successors and/or assigns of future liability for improper design.

Name of Developer:

Authorized Signature:

MESA COUNTY APPROVALS

Engineering Division Date

Public Works Director Date

58679

TABLE OF CONTENTS

I. INTRODUCTION

A. Background B. Project Location C. Property Description D. Previous Investigations

II. DRAINAGE SYSTEM DESCRIPTION

A. Existing Drainage Conditions B. Master Drainage Plan C. Off-site Tributary Area D. Proposed Drainage System Description E. Drainage Facility Maintenance

III. DRAINAGE ANALYSIS AND DESIGN CRITERIA

A. Regulations B. Development Criteria C. Hydrologic Criteria D. Hydraulic Criteria E. Variance from Criteria

IV. POST-CONSTRUCTION STORMWATER MANAGEMENT

A. Stormwater Quality Control Measures

V. CONCLUSIONS

A. Compliance with Manual B. Design Effectiveness C. Areas in Flood Hazard Zone D. Variance From Manual

VI. REFERENCES

APPENDICES

APPENDIX A – LOCATION MAPS ........................................................................................ I

APPENDIX B – REFERENCE INFORMATION

APPENDIX C – HYDROLOGIC CALCULATION

APPENDIX D – HYDRAULIC CALCUATIONS

APPENDIX E – DRAINAGE PLANS

APPENDIX F – CHECKLIST

I. INTRODUCTION

A. BACKGROUND

This report is prepared by Bohannan Huston, Inc. and the purpose of the report is to provide a drainage design for the proposed drainage improvement project on 35 8/10 Road.

The project aims to address the existing inadequate drainage infrastructure and mitigate localized flooding issues, particularly on the east side (Cox Property) 738a and 738b 35 8/10

Road, in Mesa County, Colorado. The goal of this drainage improvement is to reduce stormwater flooding, especially on private properties impacted by changes in stormwater flow.

B. PROJECT LOCATION

The drainage improvement project is located along 35 8/10 Road, south of the

Government Highline and north of the Price Ditch. The site is located in Section 10, Township

13 South, Range 97 West of the 6th Principal Meridian within Mesa County, Colorado. The project area is primarily focused on evaluating and redesigning the stormwater infrastructure along the road, where localized flooding has caused issues for residents, particularly in the

Morris Subdivision east of 35 8/10 Road. The area is bordered by both agricultural land and private residencies in all directions. See Appendix A for the Vicinity Map.

C. PROPERTY DESCRIPTION

The project site covers approximately 6.7 acres, with the drainage improvement project affecting 0.12 acres of disturbance. The primary goal is to mitigate stormwater flooding, particularly on 738 35 8/10 RD (Cox property) on the east side of the road, by improving stormwater infrastructure.

Currently, the site consists of residential properties with gravel and asphalt surfaces, as well as natural landscapes, agricultural areas, and grasses. The ground cover includes asphalt, gravel, and vegetation and covers all analyzed areas. The site topography generally slopes from North to South, with slopes ranging from approximately 1% to 4%.

There is an 12” existing underground pipe that begins along the Cox property, directing stormwater west under 35 8/10 Road and then south along 35 8/10 Road, and discharging flow into the Price Ditch. Any remaining flow ponds in front of the Cox property before draining south to the adjacent properties yard and driveway. See the existing drainage plan in Appendix E.

The Price Ditch is an existing irrigation facility within the project area and is managed by the Palisade Irrigation District. The ditch serves as a conveyance system for irrigation water to agricultural users in the region. Any proposed drainage improvements will be designed to avoid adverse impacts on the Price Ditch and have been and will continue to be coordinated with the Palisade Irrigation District through the project design development phase.

According to the National Resources Conservation Service (NRCS) Web Soil Survey, the predominant soil types are Turley Clay Loam, 0 to 2 percent slopes, classified under

Hydraulic Soil Group C. The soil summary and analysis are sourced from the NRCS Web

Soil Survey website. The soil map, description, and hydrologic soil group classification can be found in Appendix B.

The project site is located on FEMA Flood Insurance Rate Map (FIRM) No.

08077C0835F, revised on 7/6/2010. Based on this map, the entirety of the site is located in

FEMA-designated Zone X, outside the 0.2% annual chance (500-year) floodplain. FIRM No.

08077C0835F can be found in Appendix B.

D. PREVIOUS INVESTIGATIONS

The subject project is not located within any major drainage basins as defined by the

Mesa County GIS database. There were no drainage reports for adjacent developments available.

II. DRAINAGE SYSTEM DESCRIPTION

A. EXISTING DRAINAGE CONDITIONS

The Site topography generally slopes from northwest to southeast through the Site, with more mild slopes around (1% - 4%). Runoff is generally captured by the Price Irrigation

Ditch on the south end of the site and is discharged offsite from there. A copy of the Mesa

County GIS database map depicting the project location and 2-ft contour data that extends beyond the property limits can be found in Appendix A. See the existing drainage plan in

Appendix E.

The Site has 5 historic drainage basins, and three off-site basins. A drawing identifying the predevelopment (historic) drainage basins and conditions for the project site is provided in Appendix E.

Existing Basin 1, approximately 0.49 acres, primarily consists of the northwest portion of the project site. Runoff from this basin is channelized and flows south towards basin 2 where it is captured by the existing storm drain. Once captured, runoff continues to flow south through the existing storm drain to the Price Ditch where it is discharged offsite into the

Colorado River. Basin 1 provides 1.05 cfs of flow during the 100-yr storm event.

Existing Basin 2, approximately 0.25 acres, is located on the western side of the site and receives additional flow from basin 1. Runoff from this basin is directed into a swale along the road, flowing into the existing storm drain at Design Point 2. Once captured, the runoff continues south in the existing storm drain to Design Point 5, where it flows into the Price

Ditch and is discharged offsite to the Colorado River. Basin 2 provides 0.81 cfs of flow during the 100-year storm event.

Existing Basin 3, approximately 1.31 acres, is located north to northeast of the site.

Runoff from this basin sheet flows and then channelizes in the natural waterway before entering Design Point 3 on the east side of 35 8/10 Rd, where it flows south to Design Point

4. Eventually, the flow channelizes and drains to an offsite inlet into the Price Ditch east of the project site, where it is conveyed offsite to the Colorado River. Basin 3 provides 2.72 cfs of flow during the 100-year storm event.

Existing Basin 4, approximately 0.06 acres, is located in the southeast corner of the site and is intended to convey flow west under 35 8/10 Rd from Design Point 4 to Design Point 2.

However improper grading causes the flow to pond at Design Point 4 flow southeast through the Cox Property. Runoff from this basin sheet-flows southeast into OS-3 through the Cox property, then channelizes before entering an offsite inlet into the Price Ditch in Basin OS-2.

Basin 4 provides 0.21 cfs of flow during the 100-year storm event.

Existing Basin 5, approximately 0.19 acres, is located on the southwest side of the site and receives flow from Basins 1,2,3, and 4 via the existing storm drain. Runoff from this basin sheet-flows south toward the Price Ditch, where it is captured by the existing 8-inch inlet pipe at Design Point 5 and conveyed offsite to the Price Ditch. Basin 5 provides 0.64 cfs of flow during the 100-yr storm event.

There are three offsite basins surrounding the site. OS-1, approximately 0.61 acres, is located along the southwest perimeter of the site and does not contribute flow to the site. OS-

2, approximately 3.00 acres, is located along the east perimeter of the site and does not contribute flow to the site. The offsite inlet into the Price Ditch is located on the southwest end of Basin OS-2 and currently receives flow from Basins 3 and OS-3. OS-3 is approximately

0.75 acres and is located southeast of the perimeter of sub-basin 4, and flows into Price Ditch.

Table 1 provides a summary of drainage basin characteristics and runoff values corresponding to the drainage basin map provided in Appendix C. Table 2 provides a summary table of the flows received by each Design Point on the existing surface.

Table 1: Summary of Drainage Basin Characteristics and Runoff Values

Sub-Basin Area (ac) % Impervious Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 0.49 26.16% 0.10 0.32 1.05 2 0.25 44.15% 0.13 0.31 0.81 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

OS-3 0.75 53.20% 0.31 0.71 1.70

Total Site 6.66 24.80% 1.24 3.67 11.62

Table 2 – Design Point Summary Table

EX Design Pts Tributary Design Point Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 1 0.10 0.32 1.05 2 1,2,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72 4 3,4 0.47 1.14 2.93 5 1,2,3,4,5 0.80 2.02 5.43

B. MASTER DRAINAGE PLAN

There are no master drainage reports or adjacent property drainage reports that were available within the vicinity of this project.

C. OFF-SITE TRIBUTARY AREA

There are no off-site basins tributary to the Site. All site runoff flows into the Price

Ditch where it is conveyed into the Colorado River.

There will be no change in land use or drainage patterns and offsite peak flows do not have a significant effect on this project.

D. PROPOSED DRAINAGE SYSTEM DESCRIPTION

The proposed drainage system will involve regrading the existing storm drain alignment from Design Point 3 to Design Point 5 to provide proper drainage from the east side of 35 8/10

Road to the Price Ditch. An 18-inch RCP storm drain will be installed in place of the existing system from Design Point 3 to Design Point 2b. From Design Point 2a a 12-inch HDPE storm drain will convey flow on the west side of 35 8/10 Road to a storm drain junction structure (at

Design Point 2b), where the system will transition to a 30-inch RCP storm drain, that will discharge into the Price Ditch at Design Point 5 and be conveyed offsite to the Colorado River.

The proposed basins will follow the same hydrologic characteristics as the existing basins. No changes to basin areas, or outflow conditions are proposed.

Table 3 – Summary of Proposed Drainage Basin Characteristics and Runoff Values

Sub-Basin Area (ac) % Impervious Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 0.49 26.16% 0.10 0.32 1.05 2a 0.22 44.15% 0.11 0.27 0.71 2b 0.03 44.15% 0.02 0.04 0.10 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

OS-3 0.75 53.20% 0.31 0.71 1.70

Total Site 6.66 24.80% 1.25 3.67 11.62

Table 4 – Design Point Summary Table

The project is not proposing detention for the Site due to its location being directly adjacent to the 100-yr floodplain and adequate conveyance facilities downstream of the site exist to discharge flows to the Colorado River.

The Site will replace the existing 12” with an 18” RCP storm system to comply with

Mesa County requirements. All Inlets and Manholes will be designed and constructed in accordance with Mesa County Standards.

There are no proposed easements or tracts needed for drainage purposes.

E. DRAINAGE FACILITY MAINTENANCE

Mesa County will be responsible for maintenance of drainage facilities.

The design will require routine maintenance to remove trash and debris that accumulates in the storm drain inlets.

Construction BMPs at the storm inlet grates will be critical during the construction and final ground cover stages of the project to minimize sediment discharge into the storm inlets.

All storm infrastructure is accessible as the site consists of gravel surfacing. No storage items shall be placed on top of storm infrastructure.

EX Design Pts

Tributary Design Point Q2 (cfs) Q10 (cfs)

Q100 (cfs)

1 1 0.10 0.32 1.05 2a 1,2a 0.21 0.59 1.76 2b 1,2a,2b,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72

5 1,2a,2b,3,4,5 0.80 2.02 5.43

III. DRAINAGE ANALYSIS AND DESIGN CRITERIA

A. REGULATIONS

Analysis and design were prepared in accordance with the Mesa County Stormwater

Management Manual.

The latest edition of Mile High Flood District’s Urban Storm Drainage Criteria Manuals, Volumes 1, 2, and 3, was also utilized in the design.

B. DEVELOPMENT CRITERIA

There are no drainage design constraints on this project from major drainage studies, floodplain studies or adjacent property drainage reports.

Per discussions with the Palisade Irrigation District, the proposed storm improvements will need to drain to the same location as the existing system which is into the Price Ditch on the southwest end of the site.

C. HYDROLOGIC CRITERIA

Drainage analysis was performed for the 2-year storm for the water quality event, the

10-year storm for the minor event and the 100-year storm for the major event. All infrastructure was sized for the major event. The rainfall intensity was calculated per the

MHFD Criteria Manual, and the rainfall data was obtained from NOAA Atlas 14 Point

Precipitation data for the 1-hour storm. See Appendix B for the precipitation data and

Appendix C for runoff calculations.

D. HYDRAULIC CRITERIA

35 8/10 Rd passes through the project site and will be affected by the proposed system. No other storm drain systems will be affected.

The proposed on-site storm drain system consists of 12-inch HDPE pipe, 18-inch to

30-inch RCP pipe, type 13 area inlets and Mesa County Standard Manholes. The final segment of the 30” RCP will connect to an existing irrigation structure located downstream.

To prevent backflow from entering the system a flap gate will be installed in the pipe upstream of the irrigation structure connection. This will prevent backflow while still allowing the system to perform how it is intended. The system was analyzed using Hydraflow

Express, an extension of AutoCAD. The Manning’s N values used for RCP pipe were 0.013 and 0.009 for HDPE.

The proposed design was further evaluated using Stormwater Studio, which provides detailed hydraulic modeling of storm drain systems. Within the software, the tailwater condition for the 60-inch pipe inside the irrigation structure was configured to assess the hydraulic grade line (HGL) of the proposed system. Field observations and survey data documented a water surface elevation of 4741.23 feet, corresponding to a flow rate of approximately 189.4 cfs. To test the system’s performance under increased tailwater conditions, the elevation was raised by 0.5 feet in the model, producing a flow rate of about

198.02 cfs. This analysis confirmed that the proposed system can accommodate the additional headwater without surcharging upstream. As the project does not include any detention facilities, no detention volume or release rate was calculated. The proposed storm system is discharging into an existing drainage ditch. Limits of disturbance will not affect the

100-year floodplain.

There are no deviations being requested from the Mesa County Drainage Standards.

E. VARIANCE FROM CRITERIA

There are no variances being requested. There are no pre-existing conditions that would cause a variance.

IV. POST-CONSTRUCTION STORMWATER MANAGEMENT

A. STORMWATER QUALITY CONTROL MEASURES

The final stabilization of the site includes asphalt, and areas of landscaping.

No detention is proposed for the Site as it is not required due to adequate conveyance facilities downstream of the site to discharge flows to the Colorado River.

V. CONCLUSIONS

A. COMPLIANCE WITH MANUAL

This report has been prepared in accordance with the Mesa County Drainage

Standards. There are no master drainage reports or drainage reports for adjacent developments available when designing drainage for this site.

B. DESIGN EFFECTIVENESS

The storm drain along 35 8/10 Rd has been redesigned to improve flow during storm events by rerouting and replacing the existing storm system. The site will be regraded to enhance drainage, and the system will ensure proper flow into the Price Ditch conveying flow into the Colorado River.

C. AREAS IN FLOOD HAZARD ZONE

The project Site is located on FEMA Flood Insurance Rate Map (FIRM) No.

08077C0835F, Revised October 16, 2012. According to this map, a majority of the Site is located in FEMA designated Zone X, determined to be outside the 0.2% annual chance floodplain. FIRM No. 08077C0835F can be found in Appendix B.

D. VARIANCE FROM MANUAL

There are no variances requested from Mesa County Standards.

VI. REFERENCES

1. FEMA Flood Insurance Rate Map (FIRM) No. 08077C0835F, revised October 16,

2. Soil Survey for Mesa County, Colorado, Web Soil Survey 3.2, Natural Resources

Conservation Service (NRCS), January 22, 2024

3. Mile High Flood District’s Urban Storm Drainage Criteria Manuals, Volumes 1, 2 and

3, latest edition

4. US Department of Commerce, NOAA Atlas 14, Volume 8, Version 2 PF Map:

Contiguous Us, US Department of Commerce, National Oceanic and Atmospheric

Administration, National Weather Service, 7 Nov. 2005

5. Mesa County Public Works Department, Stormwater Management Manual, Mesa

County, Colorado, Mesa County Public Works Department, December 31, 2007

Appendix A – LOCATION MAPS

TABLE OF CONTENTS

I. INTRODUCTION

A. Background B. Project Location C. Property Description D. Previous Investigations

II. DRAINAGE SYSTEM DESCRIPTION

A. Existing Drainage Conditions B. Master Drainage Plan C. Off-site Tributary Area D. Proposed Drainage System Description E. Drainage Facility Maintenance

III. DRAINAGE ANALYSIS AND DESIGN CRITERIA

A. Regulations B. Development Criteria C. Hydrologic Criteria D. Hydraulic Criteria E. Variance from Criteria

IV. POST-CONSTRUCTION STORMWATER MANAGEMENT

A. Stormwater Quality Control Measures

V. CONCLUSIONS

A. Compliance with Manual B. Design Effectiveness C. Areas in Flood Hazard Zone D. Variance From Manual

VI. REFERENCES

APPENDICES

APPENDIX A – LOCATION MAPS ........................................................................................ I

APPENDIX B – REFERENCE INFORMATION

APPENDIX C – HYDROLOGIC CALCULATION

APPENDIX D – HYDRAULIC CALCUATIONS

APPENDIX E – DRAINAGE PLANS

APPENDIX F – CHECKLIST

I. INTRODUCTION

A. BACKGROUND

This report is prepared by Bohannan Huston, Inc. and the purpose of the report is to provide a drainage design for the proposed drainage improvement project on 35 8/10 Road.

The project aims to address the existing inadequate drainage infrastructure and mitigate localized flooding issues, particularly on the east side (Cox Property) 738a and 738b 35 8/10

Road, in Mesa County, Colorado. The goal of this drainage improvement is to reduce stormwater flooding, especially on private properties impacted by changes in stormwater flow.

B. PROJECT LOCATION

The drainage improvement project is located along 35 8/10 Road, south of the

Government Highline and north of the Price Ditch. The site is located in Section 10, Township

13 South, Range 97 West of the 6th Principal Meridian within Mesa County, Colorado. The project area is primarily focused on evaluating and redesigning the stormwater infrastructure along the road, where localized flooding has caused issues for residents, particularly in the

Morris Subdivision east of 35 8/10 Road. The area is bordered by both agricultural land and private residencies in all directions. See Appendix A for the Vicinity Map.

C. PROPERTY DESCRIPTION

The project site covers approximately 6.7 acres, with the drainage improvement project affecting 0.12 acres of disturbance. The primary goal is to mitigate stormwater flooding, particularly on 738 35 8/10 RD (Cox property) on the east side of the road, by improving stormwater infrastructure.

Currently, the site consists of residential properties with gravel and asphalt surfaces, as well as natural landscapes, agricultural areas, and grasses. The ground cover includes asphalt, gravel, and vegetation and covers all analyzed areas. The site topography generally slopes from North to South, with slopes ranging from approximately 1% to 4%.

There is an 12” existing underground pipe that begins along the Cox property, directing stormwater west under 35 8/10 Road and then south along 35 8/10 Road, and discharging flow into the Price Ditch. Any remaining flow ponds in front of the Cox property before draining south to the adjacent properties yard and driveway. See the existing drainage plan in Appendix E.

The Price Ditch is an existing irrigation facility within the project area and is managed by the Palisade Irrigation District. The ditch serves as a conveyance system for irrigation water to agricultural users in the region. Any proposed drainage improvements will be designed to avoid adverse impacts on the Price Ditch and have been and will continue to be coordinated with the Palisade Irrigation District through the project design development phase.

According to the National Resources Conservation Service (NRCS) Web Soil Survey, the predominant soil types are Turley Clay Loam, 0 to 2 percent slopes, classified under

Hydraulic Soil Group C. The soil summary and analysis are sourced from the NRCS Web

Soil Survey website. The soil map, description, and hydrologic soil group classification can be found in Appendix B.

The project site is located on FEMA Flood Insurance Rate Map (FIRM) No.

08077C0835F, revised on 7/6/2010. Based on this map, the entirety of the site is located in

FEMA-designated Zone X, outside the 0.2% annual chance (500-year) floodplain. FIRM No.

08077C0835F can be found in Appendix B.

D. PREVIOUS INVESTIGATIONS

The subject project is not located within any major drainage basins as defined by the

Mesa County GIS database. There were no drainage reports for adjacent developments available.

II. DRAINAGE SYSTEM DESCRIPTION

A. EXISTING DRAINAGE CONDITIONS

The Site topography generally slopes from northwest to southeast through the Site, with more mild slopes around (1% - 4%). Runoff is generally captured by the Price Irrigation

Ditch on the south end of the site and is discharged offsite from there. A copy of the Mesa

County GIS database map depicting the project location and 2-ft contour data that extends beyond the property limits can be found in Appendix A. See the existing drainage plan in

Appendix E.

The Site has 5 historic drainage basins, and three off-site basins. A drawing identifying the predevelopment (historic) drainage basins and conditions for the project site is provided in Appendix E.

Existing Basin 1, approximately 0.49 acres, primarily consists of the northwest portion of the project site. Runoff from this basin is channelized and flows south towards basin 2 where it is captured by the existing storm drain. Once captured, runoff continues to flow south through the existing storm drain to the Price Ditch where it is discharged offsite into the

Colorado River. Basin 1 provides 1.05 cfs of flow during the 100-yr storm event.

Existing Basin 2, approximately 0.25 acres, is located on the western side of the site and receives additional flow from basin 1. Runoff from this basin is directed into a swale along the road, flowing into the existing storm drain at Design Point 2. Once captured, the runoff continues south in the existing storm drain to Design Point 5, where it flows into the Price

Ditch and is discharged offsite to the Colorado River. Basin 2 provides 0.81 cfs of flow during the 100-year storm event.

Existing Basin 3, approximately 1.31 acres, is located north to northeast of the site.

Runoff from this basin sheet flows and then channelizes in the natural waterway before entering Design Point 3 on the east side of 35 8/10 Rd, where it flows south to Design Point

4. Eventually, the flow channelizes and drains to an offsite inlet into the Price Ditch east of the project site, where it is conveyed offsite to the Colorado River. Basin 3 provides 2.72 cfs of flow during the 100-year storm event.

Existing Basin 4, approximately 0.06 acres, is located in the southeast corner of the site and is intended to convey flow west under 35 8/10 Rd from Design Point 4 to Design Point 2.

However improper grading causes the flow to pond at Design Point 4 flow southeast through the Cox Property. Runoff from this basin sheet-flows southeast into OS-3 through the Cox property, then channelizes before entering an offsite inlet into the Price Ditch in Basin OS-2.

Basin 4 provides 0.21 cfs of flow during the 100-year storm event.

Existing Basin 5, approximately 0.19 acres, is located on the southwest side of the site and receives flow from Basins 1,2,3, and 4 via the existing storm drain. Runoff from this basin sheet-flows south toward the Price Ditch, where it is captured by the existing 8-inch inlet pipe at Design Point 5 and conveyed offsite to the Price Ditch. Basin 5 provides 0.64 cfs of flow during the 100-yr storm event.

There are three offsite basins surrounding the site. OS-1, approximately 0.61 acres, is located along the southwest perimeter of the site and does not contribute flow to the site. OS-

2, approximately 3.00 acres, is located along the east perimeter of the site and does not contribute flow to the site. The offsite inlet into the Price Ditch is located on the southwest end of Basin OS-2 and currently receives flow from Basins 3 and OS-3. OS-3 is approximately

0.75 acres and is located southeast of the perimeter of sub-basin 4, and flows into Price Ditch.

Table 1 provides a summary of drainage basin characteristics and runoff values corresponding to the drainage basin map provided in Appendix C. Table 2 provides a summary table of the flows received by each Design Point on the existing surface.

Table 1: Summary of Drainage Basin Characteristics and Runoff Values

Sub-Basin Area (ac) % Impervious Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 0.49 26.16% 0.10 0.32 1.05 2 0.25 44.15% 0.13 0.31 0.81 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

OS-3 0.75 53.20% 0.31 0.71 1.70

Total Site 6.66 24.80% 1.24 3.67 11.62

Table 2 – Design Point Summary Table

EX Design Pts Tributary Design Point Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 1 0.10 0.32 1.05 2 1,2,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72

B. MASTER DRAINAGE PLAN

There are no master drainage reports or adjacent property drainage reports that were available within the vicinity of this project.

C. OFF-SITE TRIBUTARY AREA

There are no off-site basins tributary to the Site. All site runoff flows into the Price

Ditch where it is conveyed into the Colorado River.

There will be no change in land use or drainage patterns and offsite peak flows do not have a significant effect on this project.

D. PROPOSED DRAINAGE SYSTEM DESCRIPTION

The proposed drainage system will involve regrading the existing storm drain alignment from Design Point 3 to Design Point 5 to provide proper drainage from the east side of 35 8/10

Road to the Price Ditch. An 18-inch RCP storm drain will be installed in place of the existing system from Design Point 3 to Design Point 2b. From Design Point 2a a 12-inch HDPE storm drain will convey flow on the west side of 35 8/10 Road to a storm drain junction structure (at

Design Point 2b), where the system will transition to a 30-inch RCP storm drain, that will discharge into the Price Ditch at Design Point 5 and be conveyed offsite to the Colorado River.

The proposed basins will follow the same hydrologic characteristics as the existing basins. No changes to basin areas, or outflow conditions are proposed.

Table 3 – Summary of Proposed Drainage Basin Characteristics and Runoff Values

Sub-Basin Area (ac) % Impervious Q2

(cfs) Q10 (cfs)

Q100 (cfs)

1 0.49 26.16% 0.10 0.32 1.05 2a 0.22 44.15% 0.11 0.27 0.71 2b 0.03 44.15% 0.02 0.04 0.10 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

Table 4 – Design Point Summary Table

The project is not proposing detention for the Site due to its location being directly adjacent to the 100-yr floodplain and adequate conveyance facilities downstream of the site exist to discharge flows to the Colorado River.

The Site will replace the existing 12” with an 18” RCP storm system to comply with

Mesa County requirements. All Inlets and Manholes will be designed and constructed in accordance with Mesa County Standards.

There are no proposed easements or tracts needed for drainage purposes.

E. DRAINAGE FACILITY MAINTENANCE

Mesa County will be responsible for maintenance of drainage facilities.

The design will require routine maintenance to remove trash and debris that accumulates in the storm drain inlets.

Construction BMPs at the storm inlet grates will be critical during the construction and final ground cover stages of the project to minimize sediment discharge into the storm inlets.

All storm infrastructure is accessible as the site consists of gravel surfacing. No storage items shall be placed on top of storm infrastructure.

EX Design Pts

Tributary Design Point Q2 (cfs) Q10 (cfs)

Q100 (cfs)

1 1 0.10 0.32 1.05 2a 1,2a 0.21 0.59 1.76 2b 1,2a,2b,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72

III. DRAINAGE ANALYSIS AND DESIGN CRITERIA

A. REGULATIONS

Analysis and design were prepared in accordance with the Mesa County Stormwater

Management Manual.

The latest edition of Mile High Flood District’s Urban Storm Drainage Criteria Manuals, Volumes 1, 2, and 3, was also utilized in the design.

B. DEVELOPMENT CRITERIA

There are no drainage design constraints on this project from major drainage studies, floodplain studies or adjacent property drainage reports.

Per discussions with the Palisade Irrigation District, the proposed storm improvements will need to drain to the same location as the existing system which is into the Price Ditch on the southwest end of the site.

C. HYDROLOGIC CRITERIA

Drainage analysis was performed for the 2-year storm for the water quality event, the

10-year storm for the minor event and the 100-year storm for the major event. All infrastructure was sized for the major event. The rainfall intensity was calculated per the

MHFD Criteria Manual, and the rainfall data was obtained from NOAA Atlas 14 Point

Precipitation data for the 1-hour storm. See Appendix B for the precipitation data and

Appendix C for runoff calculations.

D. HYDRAULIC CRITERIA

35 8/10 Rd passes through the project site and will be affected by the proposed system. No other storm drain systems will be affected.

The proposed on-site storm drain system consists of 12-inch HDPE pipe, 18-inch to

30-inch RCP pipe, type 13 area inlets and Mesa County Standard Manholes. The final segment of the 30” RCP will connect to an existing irrigation structure located downstream.

To prevent backflow from entering the system a flap gate will be installed in the pipe upstream of the irrigation structure connection. This will prevent backflow while still allowing the system to perform how it is intended. The system was analyzed using Hydraflow

Express, an extension of AutoCAD. The Manning’s N values used for RCP pipe were 0.013 and 0.009 for HDPE.

The proposed design was further evaluated using Stormwater Studio, which provides detailed hydraulic modeling of storm drain systems. Within the software, the tailwater condition for the 60-inch pipe inside the irrigation structure was configured to assess the hydraulic grade line (HGL) of the proposed system. Field observations and survey data documented a water surface elevation of 4741.23 feet, corresponding to a flow rate of approximately 189.4 cfs. To test the system’s performance under increased tailwater conditions, the elevation was raised by 0.5 feet in the model, producing a flow rate of about

198.02 cfs. This analysis confirmed that the proposed system can accommodate the additional headwater without surcharging upstream. As the project does not include any detention facilities, no detention volume or release rate was calculated. The proposed storm system is discharging into an existing drainage ditch. Limits of disturbance will not affect the

100-year floodplain.

There are no deviations being requested from the Mesa County Drainage Standards.

E. VARIANCE FROM CRITERIA

There are no variances being requested. There are no pre-existing conditions that would cause a variance.

IV. POST-CONSTRUCTION STORMWATER MANAGEMENT

A. STORMWATER QUALITY CONTROL MEASURES

The final stabilization of the site includes asphalt, and areas of landscaping.

No detention is proposed for the Site as it is not required due to adequate conveyance facilities downstream of the site to discharge flows to the Colorado River.

V. CONCLUSIONS

A. COMPLIANCE WITH MANUAL

This report has been prepared in accordance with the Mesa County Drainage

Standards. There are no master drainage reports or drainage reports for adjacent developments available when designing drainage for this site.

B. DESIGN EFFECTIVENESS

The storm drain along 35 8/10 Rd has been redesigned to improve flow during storm events by rerouting and replacing the existing storm system. The site will be regraded to enhance drainage, and the system will ensure proper flow into the Price Ditch conveying flow into the Colorado River.

C. AREAS IN FLOOD HAZARD ZONE

The project Site is located on FEMA Flood Insurance Rate Map (FIRM) No.

08077C0835F, Revised October 16, 2012. According to this map, a majority of the Site is located in FEMA designated Zone X, determined to be outside the 0.2% annual chance floodplain. FIRM No. 08077C0835F can be found in Appendix B.

D. VARIANCE FROM MANUAL

There are no variances requested from Mesa County Standards.

VI. REFERENCES

1. FEMA Flood Insurance Rate Map (FIRM) No. 08077C0835F, revised October 16,

2. Soil Survey for Mesa County, Colorado, Web Soil Survey 3.2, Natural Resources

Conservation Service (NRCS), January 22, 2024

3. Mile High Flood District’s Urban Storm Drainage Criteria Manuals, Volumes 1, 2 and

3, latest edition

4. US Department of Commerce, NOAA Atlas 14, Volume 8, Version 2 PF Map:

Contiguous Us, US Department of Commerce, National Oceanic and Atmospheric

Administration, National Weather Service, 7 Nov. 2005

5. Mesa County Public Works Department, Stormwater Management Manual, Mesa

County, Colorado, Mesa County Public Works Department, December 31, 2007

Appendix A – LOCATION MAPS

Appendix B – REFERENCE INFORMATION

National Flood Hazard Layer FIRMette

0 500 1,000 1,500 2,000250 Feet

Ü

SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT

SPECIAL FLOOD

HAZARD AREAS

Without Base Flood Elevation (BFE) Zone A, V, A99

With BFE or Depth Zone AE, AO, AH, VE, AR

Regulatory Floodway

0.2% Annual Chance Flood Hazard, Areas of 1% annual chance flood with average depth less than one foot or with drainage areas of less than one square mile Zone X

Future Conditions 1% Annual Chance Flood Hazard Zone X

Area with Reduced Flood Risk due to Levee. See Notes. Zone X

Area with Flood Risk due to Levee Zone D

NO SCREEN Area of Minimal Flood Hazard Zone X

Area of Undetermined Flood Hazard Zone D

Channel, Culvert, or Storm Sewer

Levee, Dike, or Floodwall

Cross Sections with 1% Annual Chance

17.5 Water Surface Elevation

Coastal Transect

Coastal Transect Baseline Profile Baseline Hydrographic Feature

Base Flood Elevation Line (BFE)

Effective LOMRs

Limit of Study Jurisdiction Boundary

Digital Data Available

No Digital Data Available

Unmapped

This map complies with FEMA's standards for the use of digital flood maps if it is not void as described below.

The basemap shown complies with FEMA's basemap accuracy standards

The flood hazard information is derived directly from the authoritative NFHL web services provided by FEMA. This map was exported on 2/4/2025 at 5:49 PM and does not reflect changes or amendments subsequent to this date and time. The NFHL and effective information may change or become superseded by new data over time.

This map image is void if the one or more of the following map elements do not appear: basemap imagery, flood zone labels, legend, scale bar, map creation date, community identifiers, FIRM panel number, and FIRM effective date. Map images for unmapped and unmodernized areas cannot be used for regulatory purposes.

Legend

OTHER AREAS OF

FLOOD HAZARD

OTHER AREAS

GENERAL

STRUCTURES

OTHER

FEATURES

MAP PANELS

B 20.2

The pin displayed on the map is an approximate point selected by the user and does not represent an authoritative property location.

1:6,000

108°23'36"W 39°6'57"N

108°22'59"W 39°6'29"N

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Soil Map—Mesa County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

7/18/2025

725770 725780 725790 725800 725810 725820 725830 725840 725850 725860 725870

725780 725790 725800 725810 725820 725830 725840 725850 725860 725870 725880

39° 6' 46'' N

8°

3'

9' ' W

39° 6' 46'' N

8°

3'

5'

' W

39° 6' 41'' N

8°

3'

9'

' W

39° 6' 41'' N

8°

3'

5'

' W

N

Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 12N WGS84 0 30 60 120 180

Feet 0 10 20 40 60

Meters Map Scale: 1:692 if printed on A portrait (8.5" x 11") sheet.

Soil Map may not be valid at this scale.

MAP LEGEND MAP INFORMATION

Area of Interest (AOI) Area of Interest (AOI)

Soils Soil Map Unit Polygons

Soil Map Unit Lines

Soil Map Unit Points

Special Point Features Blowout

Borrow Pit

Clay Spot

Closed Depression

Gravel Pit

Gravelly Spot

Landfill

Lava Flow

Marsh or swamp

Mine or Quarry

Miscellaneous Water

Perennial Water

Rock Outcrop

Saline Spot

Sandy Spot

Severely Eroded Spot

Sinkhole

Slide or Slip

Sodic Spot

Spoil Area

Stony Spot

Very Stony Spot

Wet Spot

Other

Special Line Features

Water Features Streams and Canals

Transportation Rails

Interstate Highways

US Routes

Major Roads

Local Roads

Background Aerial Photography

The soil surveys that comprise your AOI were mapped at 1:24,000.

Warning: Soil Map may not be valid at this scale.

Enlargement of maps beyond the scale of mapping can cause misunderstanding of the detail of mapping and accuracy of soil line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed scale.

Please rely on the bar scale on each map sheet for map measurements.

Source of Map: Natural Resources Conservation Service Web Soil Survey URL:

Coordinate System: Web Mercator (EPSG:3857)

Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts distance and area. A projection that preserves area, such as the Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required.

This product is generated from the USDA-NRCS certified data as of the version date(s) listed below.

Soil Survey Area: Mesa County Area, Colorado Survey Area Data: Version 15, Sep 6, 2024

Soil map units are labeled (as space allows) for map scales 1:50,000 or larger.

Date(s) aerial images were photographed: Jun 24, 2020—Jul 8, The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident.

Soil Map—Mesa County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

7/18/2025

Map Unit Legend

Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI

Tr Turley clay loam, 0 to 2 percent slopes

1.1 100.0%

Totals for Area of Interest 1.1 100.0%

Soil Map—Mesa County Area, Colorado

Natural Resources Conservation Service

Web Soil Survey National Cooperative Soil Survey

7/18/2025

Appendix C – HYDROLOGIC CALCULATION

Existing Basin Summary Table Sub-Basin Area (ac) % Impervious Q2 (cfs) Q10 (cfs) Q100 (cfs

1 0.49 26.16% 0.10 0.32 1.05 2 0.25 44.15% 0.13 0.31 0.81 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

Existing Design Points Summary Table EX Design Pts Tributary Design Point Q2 (cfs) Q10 (cfs) Q100 (cfs)

1 1 0.10 0.32 1.05 2 1,2,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72

Proposed Basin Summary Table Sub-Basin Area (ac) % Impervious Q2 (cfs) Q10 (cfs) Q100 (cfs

1 0.49 26.16% 0.10 0.32 1.05 2a 0.22 44.15% 0.11 0.27 0.71 2b 0.03 44.15% 0.02 0.04 0.10 3 1.31 44.89% 0.43 1.05 2.72 4 0.06 53.20% 0.04 0.09 0.21 5 0.19 46.70% 0.10 0.25 0.64

OS-1 0.61 7.00% 0.02 0.17 0.86

OS-2 3.00 8.80% 0.12 0.77 3.63

Proposed Design Points Summary Table EX Design Pts Tributary Design Point Q2 (cfs) Q10 (cfs) Q100 (cfs)

1 1 0.10 0.32 1.05 2a 1,2a 0.21 0.59 1.76 2b 1,2a,2b,3,4 0.70 1.77 4.79 3 3 0.43 1.05 2.72

Designer:

Company: 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Date: 1-hour rainfall depth, P1 (in) = 0.45 0.65 0.82 1.07 1.27 1.47 1.98 Project: a b c

Location: Rainfall Intensity Equation Coefficients = 28.50 10.00 0.786

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr Overland

Flow Length Li (ft)

U/S Elevation

(ft) (Optional)

D/S Elevation

(ft) (Optional)

Overland Flow Slope

Si (ft/ft)

Overland Flow Time ti (min)

Channelized Flow Length

Lt (ft)

U/S Elevation

(ft) (Optional)

D/S Elevation

(ft) (Optional)

Channelized Flow Slope

St (ft/ft)

NRCS

Conveyance

Factor K

Channelized Flow

Velocity Vt (ft/sec)

Channelized Flow Time tt (min)

Computed tc (min)

Regional tc (min)

Selected tc (min) 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

0.19 0.25 0.32 0.47 0.52 0.59 0.67 10.62 12.49 12.49 1.12 1.61 2.03 2.64 3.13 3.63 4.88 0.10 0.20 0.32 0.60 0.80 1.05 1.60

0.33 0.39 0.46 0.57 0.61 0.66 0.73 3.24 5.43 5.43 1.51 2.16 2.73 3.55 4.21 4.88 6.57 0.13 0.21 0.31 0.50 0.64 0.81 1.19

0.34 0.40 0.46 0.57 0.61 0.67 0.73 14.79 17.40 17.40 0.96 1.38 1.74 2.26 2.68 3.11 4.18 0.43 0.72 1.05 1.69 2.16 2.72 4.00

0.41 0.47 0.52 0.62 0.66 0.70 0.76 1.96 2.18 5.00 1.54 2.21 2.79 3.63 4.31 4.99 6.72 0.04 0.06 0.09 0.13 0.17 0.21 0.30

0.35 0.42 0.48 0.58 0.62 0.68 0.74 3.89 4.41 5.00 1.54 2.21 2.79 3.63 4.31 4.99 6.72 0.10 0.17 0.25 0.40 0.51 0.64 0.94

0.04 0.09 0.18 0.36 0.43 0.51 0.61 14.81 21.84 21.84 0.85 1.22 1.54 2.01 2.38 2.76 3.72 0.02 0.07 0.17 0.44 0.62 0.86 1.38

0.05 0.11 0.20 0.37 0.44 0.52 0.62 21.62 31.98 29.57 0.72 1.03 1.30 1.69 2.01 2.33 3.13 0.12 0.33 0.77 1.87 2.63 3.63 5.79

0.41 0.47 0.52 0.62 0.66 0.70 0.76 12.65 16.01 16.01 1.00 1.43 1.81 2.35 2.80 3.24 4.36 0.31 0.50 0.71 1.09 1.37 1.70 2.474742.50 4741.80 0.006 8 0.61 3.36

4747.40OS-1 0.61 C 7.0 125.40

18.59

29.57

OS-3 0.75 C 53.2 140.00 4744.20 4742.50 0.012 122.00

4744.60 4744.11 0.054 23.00 4744.11 4743.07 0.045

29.04

OS-2 3.00 C 8.8 300.00 4759.90 4750.90 0.030 448.00 4750.80 4741.50 0.021 5 0.72 10.36

4742.50 0.016 6 0.75 7.034750.70 4747.40 0.026 315.50

24.00 4751.90 4750.16 0.072 288.002 0.25

18.63

17.07

5 0.19 C 46.7 31.40 4745.30 4743.50 0.057 61.00 4743.50 4742.70 0.013 17 1.95 0.52

20.42

4 0.06 C 53.2 9.00

Cells of this color are for calculated results based on overrides

Joel Enciso Bohannan Huston 9/17/2025 35 8/10 Rd Drainage Improvements 738 35 8/10 Rd (Morris Subdivision)

Version 2.00 released May 2017

8 1.70 0.23

20.66

3 1.31 C 44.9 300.00 4760.20 4750.40 0.033 281.10 4750.40 4744.10 0.022 12 1.80

4750.16

2.61

4744.00 0.021 15 2.19 2.19C 44.2

0.042124.00 4763.57 4758.30

Rainfall Intensity, I (in/hr)

2.20 1.87 23.47130.0294751.204758.30247.00

Peak Flow, Q (cfs)

Calculation of Peak Runoff using Rational Method

Overland (Initial) Flow Time Channelized (Travel) Flow Time Time of ConcentrationRunoff Coefficient, C

Subcatchment Name

Area (ac)

NRCS

Hydrologic Soil Group

Percent Imperviousnes s

1 0.49 C 26.2

Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)

Cells of this color are for required user-input Cells of this color are for optional override values

I 𝑖𝑛/ℎ𝑟 = a ∗ Pଵ b + tୡ ୡ t୧ =

0.395 1.1 − Cହ L୧

S୧ .ଷଷ t୲ = L୲

60K S୲

L୲

60V୲

Computed tୡ = t୧ + t୲

Regional tୡ = 26 − 17i + L୲

60 14i + 9 S୲

Selected tୡ = max t୫୧୬୧୫୳୫ , min Computed tୡ , Regional tୡ t୫୧୬୧୫୳୫= 5 (urban) t୫୧୬୧୫୳୫= 10 (non-urban)

Q 𝑐𝑓𝑠 = CIA

Calculation of Existing Peak Runoff

Designer:

Company: 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr

Date: 1-hour rainfall depth, P1 (in) = 0.45 0.65 0.82 1.07 1.27 1.47 1.98 Project: a b c

Location: Rainfall Intensity Equation Coefficients = 28.50 10.00 0.786

2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr Overland

Flow Length Li (ft)

U/S Elevation

(ft) (Optional)

D/S Elevation

(ft) (Optional)

Overland Flow Slope

Si (ft/ft)

Overland Flow Time ti (min)

Channelized Flow Length

Lt (ft)

U/S Elevation

(ft) (Optional)

D/S Elevation

(ft) (Optional)

Channelized Flow Slope

St (ft/ft)

NRCS

Conveyance

Factor K

Channelized Flow

Velocity Vt (ft/sec)

Channelized Flow Time tt (min)

Computed tc (min)

Regional tc (min)

Selected tc…