AOP_Design_Summary_DIXON1-27-2018.pdf

USDA Forest Service Dixon Creek AOP Culvert Design Summary

Site: Chattahoochee – Oconee National Forest, FSR 4, MP 3.66 Date: 5 January 2018

Reference Reach Summary

Location narrative:

The surveyed reach is approximately 350 ft long with 285 ft upstream of the crossing. Bed gradient throughout the survey averages 4.25%. The selected reference reach is located approximately 200 feet upstream of the existing crossing. The bed is gravel/cobble and about 50% embedded.

Site field sketch:

Existing site photos:

Existing crossing is 42” CMP w/rock masonry headwalls

Slope: 4.24%

Length: 15.56 ft

Type and spacing of bedforms: Riffles with traverse ribs, moderate stability, spacing 13 – 15 ft

Bankfull widths:

Range: 13 – 15 ft

Pebble count data summary:

n D95 D84 D50 D30 D16 D10

107 140 mm 95 mm 24 mm <2 mm ‐‐‐‐‐ ‐‐‐‐‐‐‐

Key piece size data summary:

n D95 D84 D50 D16

31 540mm 400 mm 245 mm 180mm

Traverse rib size data summary:

n D95 D84 D50 D16

14 >250 mm 240 mm 180 mm ‐‐‐‐‐

Notes: Large cobbles and boulders were well embedded in fines ≈50% or more, and often mossy capped indicating infrequent movement. The largest pieces (key pieces) were rather randomly oriented in the bed. This size category may be influenced by steep, bedrock side slopes. Traverse ribs material, which composed the riffle crests, included some of the largest size particles, but was altogether a smaller size range.

Hydrology & Discharge Estimates

Basin characteristics:

Area: 1.42 square miles Average Annual Precipitation: 69.9 inches

Design discharges:

Q2: 144 cfs Q100: 739 cfs

Peak flow determination and validation: 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): 42” galvanized CMP, 23 ft long

Proposed structure description (type, size, gauge, coating, etc.): Bottomless aluminum box culvert, ConTech #28 R3 (14’‐10” x 4’‐10”)

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. Based on information provided by the Forest, the subsurface materials consist of cobbles and boulders. There is visible bedrock at the outlet at the existing structure.

Since there has been no formal geotechnical investigation, the design will assume there is bedrock approximately 0.85’ below the existing outlet, based on visible bedrock downstream of the crossing. The proposed structure foundation has been designed at 2’ below the existing stream elevation at the inlet which should limit the potential for scour. The calculated vertical adjustment potential from the long profile is 1.68’.

Horizontal alignment/skew modified? No

Structure/footing embedment depth: .85 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 0.96 = 1.68 ft

Note: Visible bedrock at outlet & downstream.

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 30 ft temporary bridge at an estimated cost of $23,300.

NOTE: Bypass option only provides a turning radius suitable for passenger cars. A larger turning radius was considered but rejected due to excessive excavation needed to accommodate larger design vehciles.

Design Channel

Design channel slope: .048 ft/ft

Cross section dimensions:

Bankfull width: 13’ ft

Bank width each side: 1 foot Bank slope: 0.2:1. Banks slope up at 20 percent, 1 foot vertical, to a flat ledge of about 1 foot wide.

Bedform type and spacing: Riffles with transverse ribs @ 14’

Hydraulic variables in proposed structure:

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

Q2 144 2.03 5.38 2.6 3.97

Q100 739 5.91 4.84 38 submerged

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, and grading upstream channel to about river station 1+14 to eliminate multi‐threaded pattern. Design span to remain in same location and centerline as present pipe.

The proposed structure will pass up through the 25‐year peak flow, beyond which it is submerged.

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 remains through the new span. An adjustment in analysis worksheet of >25% particle size for D84 did not change results.

Containing flow through the design span increases shear stress by a factor of 2 or more throughout the estimated peak flow range.

Using pebble count data only from the riffle/rib section increases shear resistance such that incipient motion is between the 10 and 25‐year event; 3 times the volume and roughly 2 times the depth of the 2‐ year peak flow.

The design reach, from approximately river station 1+14 through 0+33, and through the culvert, will utilize ribs spaced at Qb distance, or about 14 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. No attempt will be made to establish grade control on steep series of bedrock ledges immediately below the outlet.

General Comments/Notes

1. Xx

2. yy