Atch 2 Design Summary.pdf

USDA Forest Service Willet Creek AOP Culvert Design Summary

Site: Bighorn National Forests and Grasslands, Road 226, MP x.xx Date: 05-12-2021

Reference Reach Summary

Location narrative: Because the two road-stream crossings are new crossings, the reference reach channel is where the road crosses the stream.

Site field sketch: project file

Existing site photos: project file

Slope: For the new Willet Creek road-stream crossing, the reference reach slope is 0.388%. For the new tributary road-stream crossing, the reference reach slope is 1.12%.

Length: For the new Willet Creek road-stream crossing, the reference reach length is 50 ft. For the new tributary road-stream crossing, the reference reach length is 50 ft.

Type and spacing of bedforms: For the new Willet Creek road-stream crossing, the reference reach bedform is a shallow pool. For the new tributary road-stream crossing the reference reach, the reference reach bedform is a wet meadow system.

Bankfull widths:

New Willet Creek Crossing: Range: 21 – 26 ft; Median: 22 ft

New Triburarty Crossing: Range: 4 – 8 ft; Median: 6 ft.

Pebble count data summary, new Willet Creek crossing:

n D95 D84 D50 D30 D16 D10 202 78.8 mm 55.0 mm 27.5 mm 14.0 mm < 2 mm < 2 mm

Key piece size data summary, new Willet Creek crossing:

n D95 D84 D50 D16

14 651 mm 609 mm 395 mm 321 mm Notes:

Pebble count data summary, new tributary crossing:

n D95 D84 D50 D30 D16 D10 NA NA mm NA mm NA mm NA mm NA mm NA mm

Key piece size data summary, new tributary crossing:

n D95 D84 D50 D16

NA NA mm NA mm NA mm NA mm Notes: The substrate consists primarily of sand, silt, and fine gravels.

Hydrology & Discharge Estimates

Basin characteristics (new Willet Creek crossing):

Area: 11.9 square miles Mean basin elevation: 9500 ft Longitude_of_Basin_Outlet = 107 degrees C

Basin characteristics (new tributary crossing):

Area: 0.33 square miles Mean basin elevation: 9500 ft Longitude_of_Basin_Outlet = 107 degrees C

Design discharges (new Willet Creek crossing):

Q2: 197 cfs (low predicted value of 81.6 cfs) Q100: 539 cfs (low predicted value of 262 cfs)

Design discharges (new tributary crossing):

Q2: 9.05 cfs Q100: 34.7 cfs

Narrative of peak flow determination and validation:

For the new Willet Creek crossing, the low Q2 discharge water-surface elevation was similar to the elevation of the bankfull flow indicators.

Structural - ALBC

Existing structure (size, length, material): Unimproved ford.

Proposed structure description (type, size, gauge, coating, etc.): 25’-2” x 6’-2” aluminum box culvert (ALBC). Manufacturer to design plate and ribs.

Length: 22.5’ long

Loading: HL-93

Foundation type: Precast concrete inverted T over 6” of geocell

Minimum cover: 1.4’. 2’ will be designed on the roadway to accommodate future grading and maintenance.

Narrative for basis of selection (including relevant geotechnical issues):

• Concrete stemwalls allow placement of rip rap banks before placing the structure on top

• Structure has adequate waterway and clearance to meet hydraulic criteria

• Flexible structure will be easier to transport to site and is accommodating of any settlement.

Horizontal alignment/skew modified? Yes, new road location, see below.

Structure/footing embedment depth: 13’

Narrative on determination of VAP lines and application to vertical alignment: Bottom of footing is above possible scour. This was to keep footing weights down and transporting possible in this very remote location. Scour will be remediated through large rip rap protection.

Geotech - ALBC

Geotech at this site limited to surface observations, soil maps and Williamson Drive Probe (WDP).

Geology at the valley floor is glacial deposits. The Forest indicated that glaciers did not extend to this site, indicating soils are limited to the looser outwash of sand, gravel and boulders. The rock on the surrounding hills is listed as plutonic rock of granite gneiss with diorite and quartz. Bedrock outcroppings seen on site are consistent with this.

NRCS lists the near surface soils in the valley as loam with layers of very cobbly sandy clay loam and extremely cobbly loamy coarse sand to a depth of 60 inches. Soils on the hills are loam, cobbly sandy loam, very stony sandy clay loam and extremely stony sandy loam to a depth of 60 inches.

Results of the 5 WDP tests are below. In general, the tests confirm the information from the geology and NRCS with loose loam on the first foot, followed by medium dense to dense soils. Isolated higher values correspond to temporary driving cobbles down or aside.

Foundation level is approximately at 8’ deep from where the abutment WDP tests were done. See attached bearing capacity calculations. A conservative factor of safety of 4 was used, as judged appropriate for the level of geotechnical exploration performed. An allowable bearing capacity at service loading of 4,000 psf will be used.

Blow counts per foot

0 1 6 5 10 10 4 1 2 6 16 5 Hit cobble 11 10 2 3 15 41 23 25 3 4 30 24 18 Refusal at 4' 40 4 5 38 26 46 5 6 42 30 55 6 7 32 28 Groundwater 32 Refusal at 6.5' 7 8 30 33 8 9 37 35 9 10 36 38 10 11 36 30 11 12 49 33 12 13 49 34 13 14 47 37 14 15 35 Refusal at 14.5' 61 15 16 52 Hit cobble

River Right Abutment (west) #5 River Left Abutment (east) #1 Upper Terrace #2 Upper Terrace #3 Upper Terrace #4Depth

Structural - CMPA

Existing structure (size, length, material): Unimproved ford.

Proposed structure description (type, size, gauge, coating, etc.): 128” x 83” corrugated metal pipe arch (CMPA). Galvanized and increased 1 gauge for durability.

Length: 32’ long

Loading: HL-93

Foundation type: 6” of bedding over 18” of foundation fill wrapped in geotextile

Minimum cover: 24”

Narrative for basis of selection (including relevant geotechnical issues):

• Small structure needed. Very low water volume spread out over a wider flat bottom channel with little evidence of peak flows.

• Flexible structure will be easier to transport to site and is accommodating of any settlement.

Horizontal alignment/skew modified? Yes, new road location, see below.

Structure/footing embedment depth: 9’

Narrative on determination of VAP lines and application to vertical alignment: Limited scour expected.

Stream at this location is a low volume tributary in a flat marshy area of the valley. The structure will be embedded 2’ to accommodate the streambed and scour protection.

Geotech - CMPA

Geotech at this site limited to surface observations. See also general site discussion above for ALBC.

Existing channel bottom very loose loam with high organics. Assume 3,000 psf bearing capacity with embedding pipe 2’ into channel and overexavating an additional 2’ for 6” of bedding and 18” of foundation fill.

Roadway

Design vehicle: 30’ long truck for construction traffic. No logging traffic. 10 mph design speed.

Existing Width: 8-12 feet in vicinity of project

Proposed width:

On steep side slope entering valley: 12’ with 1’ shoulders = 14’ on side slope; 16’ with 1’ shoulders = 16’ on switchback In valley and on terrace, including pipe arch: 12’ with no shoulders Over aluminum box culvert: 14’ crowned width with adjacent 4’ of 1V:4H recovery zone

Existing surface: Native; Proposed surface: Native except over structures where use 4” crushed aggregate for traction, surface stability and water quality.

Proposed fill slope angle:

On steep side slope entering valley: 1V:1H Cut and 1V:1.5H Fill In valley and on terrace, including pipe arch: 1V:1.5H Over aluminum box culvert: 1V:4H. Use rip rap on embankment around wingwalls.

Horizontal alignment modified? Yes, new route. 0.80 new road miles and 0.46 decommissioned road miles. 50 feet minimum curve radius except on switchback with 40 foot radius.

Vertical alignment modified? Yes, new route. 14% max grade on steep side slope, less than 10% elsewhere. Curves per FSH 7709.56 with normal governing criteria being minimum length of 50 feet.

Traffic control plan during construction: Existing road can be used as bypass during construction. Will be decommissioned after new route constructed.

Design Channel New Willet Creek Crossing Design channel slope: 0.388%

Cross section dimensions:

Bankfull width: 22.0 ft Bank width each side: 1.75 ft (partial bank margins) Bank slope: 1.0V:1.5H

Bedform type and spacing: shallow pool

New Tributary Crossing Design channel slope: 1.12%

Cross section dimensions:

Bankfull width: 6.0 ft Bank width each side: No banks to be constructed.

Bank slope: NA

Bedform type and spacing: Wet meadow, no vegetation.

Hydraulic variables in proposed structure (new Willet Creek crossing):

RI Discharge (cfs) Depth (ft) Velocity (fps) HW/D Freeboard (ft) Q2 71.2 2.60 1.89 -- --

Q100 539 4.76 5.89 0.67 2.32

Hydraulic variables in proposed structure (new tributary crossing):

RI Discharge (cfs) Depth (ft) Velocity (fps) HW/D Freeboard (ft) Q2 8 0.48 2.96 -- --

Q100 35 1.31 4.80 0.30 3.11

Narrative of modeling technique, validation, etc.

HEC-RAS was used to model flow hydraulics at the new Willet Creek crossing.

HY-8 was used to model flow hydraulics at the new tributary crossing.

New Willet Creek Crossing Adjusted bed material gradation:

D95 D84 D50 D30 D16 D10

78.8 mm 55.0 mm 27.5 mm 14.0 mm < 2 mm < 2 mm

Note: No adjustments made for gradation. Channel-bed material excavated at the existing crossing will be used to construct the new channel bed.

Adjusted key piece gradation:

D95 D84 D50 D16 651 mm 609 mm 395 mm 321 mm

Note: No adjustments made for gradation.

Final specified key piece size range: 18 in – 26 in

New Tributary Crossing Adjusted bed material gradation:

D95 D84 D50 D30 D16 D10 NA mm NA mm NA mm NA mm NA mm NA mm

Note: Channel-bed material excavated at the existing crossing will be used to construct the new channel bed.

Adjusted key piece gradation:

D95 D84 D50 D16 NA mm NA mm NA mm NA mm

Final specified key piece size range: NA

Narrative on mobility/stability analysis and results:

EM-1601 was used determine the gradation of bank material at the new Willet Creek road-stream crossing.