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Kokopelli Riverfront Trail FIR Hydraulic Design Report August 2016

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TableofContents

1.0  INTRODUCTION 4 

1.1  Purpose 4 

1.2  Project Location 4 

1.3  Area Description 4 

2.0  Hydrology 5 

2.1  Basin Description 5 

2.2  Precipitation Data 5 

2.3  Flood History 5 

2.4  Design Flood Frequency and Discharge 6 

3.0  Existing Structure 6 

3.1  Description 6 

4.0  Design Discussion 6 

4.1  Circumstances Influencing Design 6 

4.2  Improvements 6 

4.3  Reed Wash Pedestrian Bridge 6 

5.0  Conclusions 6 

APPENDIX Mesa County GIS Floodplain Map

Kokopell

1.0 INTR

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Kokopelli Riverfront Trail FIR Hydraulic Design Report August 2016

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NRCS Web Soil Survey indicates the project area primarily contains Sagers silty loam, 0 to 2 percent slopes (66 percent), Killpack-Badlands-Persayo complex, 3 to 25 percent slopes(31 percent) and Killpack silty clay, 2 to 5 percent slopes(2 percent). A portion of the trail is adjacent to the Colorado River. The Colorado River in the project area is a FEMA regulated floodplain and is on the Corp’s list of known traditional navigable waters and jurisdictional Waters of the United States (WUS). This segment has identification COLCLC03 on Colorado Department of Public Health and Environment’s Section 303(d) List of Impaired Waters. It is classified as a “sensitive” water due to its designation as critical habitat for the threatened Colorado pikeminnow and razorback sucker under the endanger species act. The proposed trail alignment crosses I-70 at mile point 15.703 along Reed Wash under bridge structures H-01-I and H-01-J. Small areas of wetlands were delineated on both the east and west bank of Reed Wash. Reed Wash is also on CDPHE’s Section 303(d) List (id COLCLC13A) but is not listed as impaired.

2.0 Hydrology

2.1 Basin Description

The proposed trail project is in two very large drainage basins and will have drainage improvements within smaller watershed basins along the proposed alignment. The Colorado River drainage basin is over 17,000 square miles upstream of the project area. The FEMA FIS for Mesa County has published hydrology, floodplain and floodway delineations. Reed Wash drainage basin is approximately 18,913 acres of semi-arid desert reaching up to elevation 4970 feet. It contains regulated floodplain as well but ends at the 100 – year floodplain elevation of the Colorado River floodplain upstream of the proposed pedestrian bridge crossing Reed Wash. The proposed alignment crosses minor drainage basins which will be delineated and hydrologically analyzed to appropriately size culvert crossings.

2.2 Precipitation Data

Drainage analysis will follow the recommendations of the the Mesa County/City of Grand Junction Storm Water Management Manual (SWMM). Intensity, duration and frequency values for rainfall data will utilize NOAA Atlas 14, Volume 8, Version 2 point precipitation frequency estimates.

2.3 Flood History

Flood history and corresponding flood information is well documented in the FIS for the Colorado River. Research will also be conducted to determine other significant events within the project area.

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2.4 Design Flood Frequency and Discharge

Flood frequency and discharge will be determined by the methods recommended in the SWMM and CDOT’s Drainage design manual.

3.0 Existing Structure

3.1 Description

I-70 bridge structures H-01-I and H-01-J at mile point 15.703 were built in 1970 and have a maximum span of 97 feet.

4.0 Design Discussion

4.1 Circumstances Influencing Design

The segments of proposed trail within the Colorado River floodplain have the greatest influence to the design of the trail and drainage features associated with the alignment. Trail improvements will impact the floodplain and the floodway. The design of the trail profile under the I-70 bridges is directly influenced by the Colorado River spring runoff. Consequently the bank of the Colorado River and Reed Wash will need to be lowered by 4 to 5 feet to accommodate the vertical clearance required to safely pass cyclists and pedestrians. The lowering impacts a small area of the Colorado River floodway and will have no adverse impact to conveying the 100-year flow and by adding conveyance area may improve the river’s capacity.

4.2 Improvements

The work along the banks of the Colorado River and Reed Wash give the project a unique opportunity to repair and improve the rip rap scour protection within the project limits in this area. Site investigations have shown the rip rap on the left bank of the Colorado downstream of Reed Wash has been eroding and in some places very unstable and washed away. Since the trail structure needs a stable platform to rest upon the proposed approach to the construction is to first excavate down to subgrade and replace and re-nestle the rip rap in areas deemed to be unstable. Essentially the project will first perform maintenance of the bank scour protection and then construct the trail.

4.3 Reed Wash Pedestrian Bridge

The Reed Wash Bridge will be placed north of the I-70 bridges and is completely within the backwater of the Colorado River 100-year floodplain. The structure is not expected to experience high velocity flows during a flooding event. Design considerations will take into account submergence and debris flow. The vertical placement of the bridge is designed to be now lower than the lowest elevation of the I-70 Bridges.

5.0 Conclusions The project is within environmentally and hydraulically sensitive waters. While permanent impacts will not adversely affect those conditions care must be taken to improve stability conditions of the scour protection and provide permanent and construction BMP’s to mitigate runoff pollutants. Final design of the trail and drainage facilities will address mitigation efforts to preserve the natural and beneficial values of the project area.

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APPENDIX

Print Date: July 7, 2016

I

The Geographic Informat ion System (GIS ) and i ts components are designed as a source of reference for answering inquiries, for planning and for model ing. GIS i s not intended or does not replace legal description inform ation in the chain of tit le and other informati on contained in of fic ial governm ent records such as the County Clerk and Recorders offi ce or the courts. In addit ion,the representati ons of locat ion in this GIS cannot be substi tute for actual l egal surveys.The informati on contained herei n is bel ieved accurate and suitabl e for the lim ited uses, and subject to the limi tat ions, set forth above. Mesa County makes no warranty as to the accuracy or sui tabili ty of any informati on contained herei n. Users assume al l risk and responsibilit y for any and all damages, including consequential damages, which may flow from the user's use of this inform ati on.

Mesa County Map 0 0.1 0.20.05 mi

0 0.1 0.20.05 km