Little_Sheep_Creek_Scour_Hydraulics_Memo.pdf

Memorandum

Western Federal Lands Highway Division 610 E. Fifth Street

Vancouver, WA 98661-3801

LITTLE SHEEP CREEK BRIDGE

SCOUR DESIGN HYDRAULICS MEMO

To: Susan Ellis, WFLHD Project Manager From: Sven Leon, P.E., WFLHD Hydraulics Engineer Date: February 17, 2017 Project: OR WALLOWA 4602(1) – Wallowa Mountain Loop Road

Background

Wallowa County (County), Oregon, is reconstructing five miles of the Wallowa Mountain Loop Road (NSFR 39) between State Highway 350 and the Wallowa National Forest Boundary. The proposed work will widen the roadway, add ditch relief culverts, and repair localized subgrade areas. Culverts in Little Sheep Creek will be replaced with a bridge (Fig. 1). The County will maintain the road and bridge structure. Western Federal Lands Highway Division (WFLHD) is designing the road improvements and new bridge. Recommendations for low chord and scour are presented.

Recommendations

Based on the hydraulic analysis and geotechnical borings, I recommend;

Minimum low chord elevation – 4193.1 feet (project datum) (50-year water surface plus 3 feet freeboard for woody debris passage).

100-year abutment scour elevation – 4182.7 feet.

500-year abutment scour elevation – 4180.9 feet.

Install Class 5 (FHWA FP-14) riprap revetment along each abutment wall for controlling approach road embankment erosion (4 feet thick, finished slope 2(h):1(v), crest elevation 4192.1 feet, toe elevation 4183.0 feet, 6 feet wide toe trench).

Install Geosynthetic Reinforced Soil Integrated (GRS-IBS) abutment footing bottoms below elevation 4180.0 feet.

Calculation Assumptions

Assumptions include;

GRS-IBS abutment is appropriate for the site.

90 degree flow impingement angle.

Gravel-cobble stream bed material gradation.

Attachments: Figure 1. Site Location Bridge Hydraulics & Scour Analysis Summary HEC-18 Scour Analysis

Project Site Location

Map printed from National Geographic TOPO

FIGURE 1

LOCATION MAP

Wallowa Mountain Loop Road

200 feet

N

Little Sheep Creek Bridge

To: By: Sven Leon, P.E.

Project: OR WALLOWA 4602(1) – Wallowa Mountain Loop Road WFLHD Hydraulics Engineer

Bridge: Date:

BRIDGE STRUCTURE SUMMARY

Low Chord Elevation 4193.3 feet (Project Datum)

Minimum Design Freeboard 3.0 feet above the 50-year flood * Needed for woody debris passage.

Stream Bottom Elevation 4185.0 feet

Clearance to Stream Bot. 8.3 feet

Waterway Width 25 feet

Skew 15.2 degrees relative to stream

Bridge/Stream CL Sta 11+41.6 Bridge Start 11+12.98 Bridge End 11+75.14

Abutment Type GRS-IBS, clear span, no piers.

HYDRAULIC & HYDROLOGIC SUMMARY Hydraulic Model: HECRAS 5.0

Flood Frequency Yr 2 10 25 50 100 500

Exceedance Probability % 50 10 4 2 1 0.2

Discharge cfs 452 834 1023 1207 1362 2180

Modeled Water Surface Elevation Elev 4188.3 4189.0 4189.5 4190.1 4190.6 4191.0

Elevation Datum Conversion Ft 0.0 0.0 0.0 0.0 0.0 0.0

Project Water Surface Elevation Elev 4188.3 4189.0 4189.5 4190.1 4190.6 4191.0

Clearance above WS Ft 5.0 4.3 3.8 3.2 2.7 2.3

Existing Water Surface Elevation Elev

Predicted Flood Rise Ft

Design Flow Velocity Ft/sec 7.0 8.4 9.2 10.1 10.9 11.5

Scour Base Elevation (Channel Thalweg) Elev 4184.0 4184.0

Contraction Scour Depth Ft 0.0 0.0

Stream Degradation Depth Ft 0.0 0.0

Left Abutment Scour Depth Ft 1.3 3.1

Right Abutment Scour Depth Ft 1.3 3.1

Total Left Abutment Scour Elevation ** Elev 4182.7 4180.9

Total Right Abutment Scour Elevation ** Elev 4182.7 4180.9

Pier Scour Depth Ft 0.0 0.0

Total Pier Scour Elevation ** Elev*

* or to bedrock when present.

** includes contraction and stream degradation scour.

Streambed material grain size (inches) D100/D85/D50/D15 9 5 3 1

Scour Design Basis

Project Elevation Datum

RECOMMENDED SCOUR COUNTERMEASURES

Type

Top Elevation 4192.1 Slope (H:V) 2:1

Bottom Elevation 4179.0 Toe Width (Ft) 5

Thick (Ft) 4 Toe Thickness (Ft) 4

Riprap Design Basis

Riprap, Class 5 (FHWA FP-14)

HEC-23

BRIDGE HYDRAULICS & SCOUR ANALYSIS SUMMARY

Susan Ellis, WFLHD Project Manager

2/14/2017Little Sheep Creek Bridge

Project

HEC-18

Little_Sheep_Creek_Bridge-Hydraulics-Scour_Summary.xls

Little Sheep Creek Bridge

FOUNDATION SUMMARY

GEOTECHNICAL SUMMARY

STREAM PLAN & PROFILE STABILITY

Bankful width 18 to 25 feet. OHW is 2 feet. Flood prone width is 50 to 60 feet. Flood-prone depth 4 feet. Stream gradient is 1 to 2 percent. Pool-riffle dominant bedform. Large woody debris recruitment and transports potential is high. Debris flow potential is low. Banks are coarsely armored and moderately stable where vegetated. Channel confined against right bank (road embankment). Channel migration potential is low.

Bed grain size from four digital photos taken at two sites upstream of bridge. Two geotechnical borings indicate abutments founded on approximately 20 feet of poorly graded gravel with sand, cobbles, and boulders.

Assume GRS abutments.

Little_Sheep_Creek_Bridge-Hydraulics-Scour_Summary.xls

SCOUR ESTIMATE

Project: OR WALLOWA 4602(1) – Wallowa Mountain Loop Road File:

Desc: Little Sheep Creek Bridge Date:

Units: ENG By:

Location Description

0-ye ar

0-ye ar

0-ye ar w

/b ac kw at er

0-ye ar w

/b ac kw at er

CONSTANTS

UNITS SI or ENG ENG ENG ENG ENG g ACCELERATION OF GRAVITY, 9.81 m/s2, 32.2 ft/s2 32.20 32.20 32.20 32.20

Du D UNIT CONVERSION, 0.001 SI, 0.00328 English 0.00328 0.00328 0.00328 0.00328

LIVE-BED OR CLEAR-WATER DETERMINATION

y AVERAGE FLOW DEPTH, m, ft 4.5 4.7

D50 DIAMETER 50% FINER BED PARTICLES, mm 60 60

V AVERAGE VELOCITY, m/s, ft/s 8.0 8.0

Ku UNIT COEFFICIENT, 6.19 SI, 11.17 English 11.170 11.170 11.170 11.170

Vc (6.1) CRITICAL VELOCITY, m/s, ft/s 8.35 8.41 0.00 0.00

LB / CW LIVE BED or CLEAR WATER CW CW

LIVE-BED CONTRACTION SCOUR

y1 AVERAGE U/S DEPTH, MAIN CHANNEL, m, ft 4.5 4.7 y0 AVERAGE CONTRACTED DEPTH BEFORE SCOUR, m, ft 5.0 5.3

Q1 FLOW IN UPSTREAM CHANNEL, m3/s, ft3/S 1362.0 2180.0

Q2 FLOW IN CONTRACTED CHANNEL, m3/s, ft3/S 1362.0 2180.0

W1 WIDTH OF THE UPSTREAM CHANNEL, m, ft 120.0 130.0

W2 WIDTH OF THE CONTRACTED SECTION, m, ft 45.0 45.0

S1 ENERGY SLOPE OF MAIN CHANNEL, m/m, ft/ft 0.0100 0.0100 w (Fig 6.8) D50 FALL VELOCITY, m/s 0.500 0.500

UNIT COEFFICIENT, 1.0 SI, 3.28 English 3.28 3.28 3.28 3.28

D50 FALL VELOCITY, m/s, ft/s 1.640 1.640 0.000 0.000 k1 (p6.10) TRANSPORT COEFFICIENT 0.64 0.64 0.00 0.00

BED MATERIAL TRANSPORT MODE BL/SL BL/SL

V* SHEAR VELOCITY, m/s, ft/s 1.20 1.23 0.00 0.00 y2 (6.2) AVERAGE DEPTH, CONTRACTED SECTION, m, ft 8.43 9.27 0.00 0.00 yS (6.3) AVERAGE SCOUR DEPTH, m, ft 3.43 3.97 0.00 0.00

As AVERAGE SCOUR AREA, m2, ft2 154.35 178.54 0.00 0.00 scour18-5.xls

2/14/2017

S. Leon

CLEAR-WATER CONTRACTION SCOUR

y0 AVERAGE CONTRACTED DEPTH BEFORE SCOUR, m, ft 5.0 5.3 0.0 0.0

D50 MEDIAN DIAMETER BED MATERIAL, mm 60.0 60.0 0.0 0.0

Q DISCHARGE THROUGH THE BRIDGE, m3/s, ft3/s 1362.0 2180.0 0.0 0.0

W BOTTOM WIDTH OF THE CONTRACTED SECTION, m, ft 45.0 45.0 0.0 0.0

Ku UNIT COEFFICIENT, 0.025 SI, 0.0077 English 0.0077 0.0077 0.0077 0.0077

Dm DIA. SMALLEST NONTRANSPORT PARTICLE, m, ft 0.2460 0.2460 0.0000 0.0000 y2 (6.4) AVERAGE DEPTH, CONTRACTED SECTION, m, ft 3.45 5.16 0.00 0.00 yS (6.5) AVERAGE SCOUR DEPTH, m, ft 0.00 0.00 0.00 0.00

As AVERAGE SCOUR AREA, m2, ft2 0.00 0.00 0.00 0.00

PRESSURE FLOW CONTRACTION SCOUR

Hb DISTANCE LOW CHORD TO STREAM BED, m, ft y1 FLOW DEPTH UPSTREAM FACE OF BRIDGE, m, ft

D50 D50 OF BED MATERIAL IN OPENING, mm

Va AVERAGE VELOCITY THROUGH OPENING, m/s, ft/s

Ku UNIT COEFFICIENT, 6.19 SI, 11.17 English 11.170 11.170 11.170 11.170

Vc (5.1) CRITICAL VELOCITY, m/s, ft/s 0.00 0.00 0.00 0.00 ys VERTICAL SCOUR DEPTH, m, ft #DIV/0! #DIV/0! #DIV/0! #DIV/0!

PIER SCOUR

y1 FLOW DEPTH U/S OF THE PIER, m, ft

V1 APPROACH VELOCITY, m/s, ft/s a PIER WIDTH, m, ft

L PIER LENGTH, m, ft

FA FLOW ANGLE, degrees

K1 (Tbl 7.1) CORRECTION FOR PIER SHAPE (0.9 to 1.1, >5deg, 1)

K3 (Tbl 7.3) CORRECTION FOR BED FORM (1.1 to 1.3)

Sg SEDIMENT SPECIFIC GRAVITY 2.65 2.65 2.65 2.65

D50 DIAMETER 50% FINER BED PARTICLES, mm

D84 DIAMETER 95% FINER BED PARTICLES, mm

Ku UNIT COEFFICIENT, 6.19 SI, 11.17 English 11.17 11.17 11.17 11.17

Vc50 (6.1) CRITICAL VELOCITY D50, m/s, ft/s 0.00 0.00 0.00 0.00

L/a PIER LENGTH/WIDTH RATIO (MAX. L/a = 12)) 0.00 0.00 0.00 0.00

Fr FROUDE RATIO 0.00 0.00 0.00 0.00

GC SEDIMENT GRADATION COEFFICIENT, D84/D50 #DIV/0! #DIV/0! #DIV/0! #DIV/0!

H DENSIMETRIC PARTICLE FROUDE NUIMBER #DIV/0! #DIV/0! #DIV/0! #DIV/0!

K2 (7.4) CORRECTION FOR FLOW ANGLE (1 to 5) 0.00 0.00 0.00 0.00 y1/a #DIV/0! #DIV/0! #DIV/0! #DIV/0!

a/D50 #DIV/0! #DIV/0! #DIV/0! #DIV/0!

V1/Vc #DIV/0! #DIV/0! #DIV/0! #DIV/0!

Kw (7.20-21) WIDE PIER CORRECTION FAC (y1/a<0.8, a/D50>50, Fr<1) #DIV/0! #DIV/0! #DIV/0! #DIV/0!

yS (7.3) SCOUR DEPTH, m, ft 0.00 0.00 0.00 0.00 yS (7.34) SCOUR DEPTH-COARSE BED, m, ft (D50>20mm & GC>1.5) #DIV/0! #DIV/0! #DIV/0! #DIV/0!

ABUTMENT SCOUR - NCHRP 24-20

y1 AVERAGE U/S DEPTH, MAIN CHANNEL, m, ft 4.5 4.7 y0 AVERAGE CONTRACTED DEPTH BEFORE SCOUR, m, ft 5.0 5.3

Qdis DISCHARGE THROUGH THE BRIDGE, m3/s, ft3/s 1362.0 2180.0

Bf FLOODPLAIN WIDTH, m, ft 120.0 130.0

WW WATERWAY WIDTH, m, ft 45.0 45.0

L PROJECTED LENGTH EMBANKMENT, m, ft 75.0 85.0

D50 DIAMETER 50% FINER BED PARTICLES, mm 60 60 alpha SCOUR AMPLIFICATION FACTOR (FIG. 8.9 - 8.12) 1.70 1.50

Ku UNIT COEFFICIENT, 6.19 SI, 11.17 English 11.17 11.17 11.17 11.17

L/Bf L/Bf > 0.75 USE LB, < 0.75 USE CW 0.63 0.65 #DIV/0! #DIV/0!

q1 UPSTREAM UNIT DISCHARGE, m3/s, ft3/s 11.35 16.77 #DIV/0! #DIV/0!

q2 UNIT DISCHARGE IN CONSTRICTED OPENING, m3/s, ft3/s 30.27 48.44 #DIV/0! #DIV/0!

q2/q1 2.67 2.89 #DIV/0! #DIV/0!

ymax (8.3) MAXIMUM FLOW DEPTH RESULTING FROM SCOUR, ft, m 6.33 8.35 #DIV/0! #DIV/0!

yS (8.4) ABUTMENT SCOUR DEPTH, m, ft 1.33 3.05 #DIV/0! #DIV/0!

TOTAL SCOUR

Base Elevation 4,184.0 4,184.0

Contraction Scour

Live Bed 3.4 4.0 0.0 0.0

Clear Water 0.0 0.0 0.0 0.0

Pressure Flow (assumes clear water) 0.0 0.0 0.0 0.0

Contraction + Pier (without Pressure Flow Scour)

Pier (max 2.4 x pier width) #DIV/0! #DIV/0! 0.0 0.0

Total Live Bed #DIV/0! #DIV/0! 0.0 0.0

Live Bed Scour Elevation #DIV/0! #DIV/0! 0.0 0.0

Total Clear Water #DIV/0! #DIV/0! 0.0 0.0

Clear Water Scour Elevation #DIV/0! #DIV/0! 0.0 0.0

Contraction + Pier (with Pressure Flow Scour)

Pier (max 2.4 x pier width) #DIV/0! #DIV/0! 0.0 0.0

Total Live Bed #DIV/0! #DIV/0! 0.0 0.0

Live Bed Scour Elevation #DIV/0! #DIV/0! 0.0 0.0

Total Clear Water #DIV/0! #DIV/0! 0.0 0.0

Clear Water Scour Elevation #DIV/0! #DIV/0! 0.0 0.0

Abutment

Abutment 1.3 3.1 #DIV/0! #DIV/0!

Total Bed Scour Elevation 4,182.7 4,180.9 #DIV/0! #DIV/0!

Note: S. Leon 7/12/12

HEC 18, 5th ED. 4/2012 (EQUATIONS SHOWN IN PARENTHESIS)