AOP_Design_Summary_MULKY1-27-2018.pdf

USDA Forest Service Mulky Creek AOP Culvert Design Summary

Site: Chattahoochee – Oconee National Forest, FSR 4, MP Date: 5 January 2018

Reference Reach Summary

Location narrative:

The surveyed reach is approximately 506 ft long with 380 ft upstream of the crossing. Bed gradient throughout the survey averages 2.5%. The selected reference reach is located approximately 165 feet upstream of the existing crossing. The bed is gravel/cobble and about 50% embedded.

Site field sketch:

Existing site photos:

Existing crossing is 36” CMP w/rock masonry headwalls

Slope: 2.59%

Length: 14.68 ft

Type and spacing of bedforms: Riffles with traverse ribs, moderate stability, spacing 12 – 19ft

Bankfull widths:

Range: 13 – 15 ft

Pebble count data summary:

n D95 D84 D50 D30 D16 D10

100 205 mm 95 mm 25 mm <2 mm ‐‐‐‐‐ ‐‐‐‐‐

Traverse ribs size data summary:

n D95 D84 D50 D16

28 ≈300 mm ≈205 mm 75 mm 10 mm

Notes: Large cobbles and boulders were well embedded in fines ≈50% or more, and often mossy capped indicating infrequent movement. Channel margins usually a thin band of fines (sands and silts). The largest pieces were mostly integrated into the riffle crest/traverse rib features. The two categories are presented above as one dataset.

Hydrology & Discharge Estimates

Basin characteristics:

Area: 1.08 square miles Average Annual Precipitation: 70.1 inches

Design discharges:

Q2: 116 cfs Q100: 608 cfs

Peak flow determination: The USGS SteamStats version 4 (https://streamstats.usgs.gov/ss/ ) online program was used to generate flood flow peaks. Estimates are based on Gotvold et al 2009 (http://pubs.gov.sir/2009/5043/ )

Structural

Existing structure (size, length, material): 36” galvanized CMP, 22 ft long

Proposed structure description (type, size, gauge, coating, etc.): Bottomless aluminum box culvert, ConTech # 43 R3 (19’‐0” x 6’‐1”)

Length: 27 ft

Loading: HL‐93

Foundation type: Aluminum footing pads or Contech EXPRESS (if no bedrock)

Minimum cover: 1.4 ft

Headwall/wingwalls:

Option 1: Extend ALB approx. 2 ft each end & construct stacked rock façade

Option 2: Construct CIP concrete headwalls w/rock veneer

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

Geotechnical assumptions:

No formal geotechnical investigation has been done for this site. During the field survey in October

2017, a mini‐excavator provided by the Chattahoochee NF dug three test holes in three locations at the site. The following is what was found:

Test hole #1 located upstream 10’ from the inlet (in the stream) o At 1 ft down, bedrock/solid ground was encountered. Took survey shot #51 at bottom of hole.

Test hole #2 was dug on the upstream side, approximately 15’ right of the inlet (looking upstream). The soil was a sandy loam, no clay content observed at this location. Shot survey shot #50 at the bottom of the test pit.

o 0‐3’ ‐ Sandy loam soil and small cobbles o 3.2’ ‐ Began taking on water o 3.4’‐ Hit bed rock/solid ground.

Test hole #3 was dug on the downstream side, approximately 15’ left of the outlet (looking downstream).

o 0‐1’ ‐ Silty loam soil o 1‐1.5’‐ Silty loam with clay content (rolled 1/4” snake) o 2’ – Fractured granitic rock o 2.5’ – bedrock/solid ground

It is unknown if the hard excavation was bedrock or boulders. The subsurface materials consisted of cobbles and silty loam.

Since there has been no formal geotechnical investigation, the design will assume there is bedrock approximately 1.2’ below the existing outlet, based on visible bedrock downstream of the crossing. The proposed structure has been designed at 2’ below the stream elevation which should limit the potential for scour. The calculated vertical adjustment potential from the long profile is approximately 1.3’.

Horizontal alignment/skew modified? No

Structure/footing embedment depth: 2.0 ft at outlet

Narrative on determination of VAP lines and application to vertical alignment:

LVAP:

LVAP = 1.75 x PMD (Maximum Pool depth) = 1.75 x 0.75 = 1.31 ft

Roadway

Width: 12 feet

Existing surface: Aggregate

Proposed surface: Aggregate

Proposed fill slope angle: xV:yH

Horizontal alignment modified? No

Vertical alignment modified? No

Traffic control plan during construction:

The Forest has elected to install a 40 ft temporary bridge at an estimated cost of $21,600.

Design Channel

Design channel slope: 0.025ft/ft

Cross section dimensions:

Bankfull width: 16 ft Bank width each side: 2 ft Bank slope: 5.5:2.0

Bedform type and spacing: Riffles with transverse rib @ 13 feet spacing

Hydraulic variables in proposed structure:

RI Discharge (cfs) Depth (ft) Velocity (fps) HW/D Freeboard (ft)

Q2 116 1.91 5.96 9 2.22

Q100 608 3.99 9.95 5 0.25

Modeling technique: HECRAS model Q2 through Q100 flows through design box culvert. Assumptions were a 0.025 bed slope (consistent with upstream survey reach) for material in embedded pipe. Design span to remain in same location and centerline as present pipe. The proposed structure will pass up through the 100‐year peak flow.

Mobility/stability analysis and results: Under all flows modeled, Q2 through Q100, there is general bed mobility in the natural channel, with the exception of the distinct traverse rib features, which generally contain, and are largely made of, the largest size particles. This condition is the same for the new design. However, shear stress is increased only slightly in the new span. An adjustment in analysis worksheet of >25% particle size for D84 did not change results.

Using pebble count data only from the riffle/rib section increases shear resistance such that incipient motion is between the 5 and 10‐year event within the span and approximately the same for the reference reach

The design channel will utilize ribs spaced at Qb distance, or about 13 feet. The same sized material will form the overbank bank/ledge through the structure into which the ribs will be keyed, and substrate layer to the bed material.

General Comments/Notes

1. Xx

2. yy