re: request for bridge scour analysis for complex pier ...findings are based on the updated hec-ras...

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Technical Memorandum

To: Theresa Maahs-Henderson, Stantec

Dale Grove, Stantec

Keith Farquhar, HNTB Corporation

From: Hugh Zeng, P.E. & Mark Abrahams, HZ United, LLC

Date: 03/22/2016

RE: Request for bridge scour analysis for complex pier foundations

HZU has completed the requested bridge scour analysis for complex pier foundation. Our

findings are based on the updated HEC-RAS model (as discussed below). The proposed

bridge geometry was provided for two design alternatives, 4-span and 5-spans bridge.

Scour analysis was conducted using the procedure outlined in FHWA HEC-18 for scour at

complex pier foundations.

The revised HEC-RAS model included the confluence of the Baudette River and the Rainy

River. A newly created junction in the HEC-RAS model represents the river confluence.

Discharges of the Baudette River was obtained from the USGS Streamstat website, which

incorporates USGS Regression Equation using GIS drainage boundaries and the stream

gauge data. Because the drainage tributary area of the Baudette River is much smaller than

that of the Rainy River, coincidental factor was considered to correlate the peak discharges

from each river.

The results of the HEC-RAS model with the confluence showed minor water surface

increases. The Baudette River connects to the Rainy River near a right angle. The velocity

is slow compared to the Rainy River. Sand delta bars formed at the mouth of the Baudette

River indicate a low stream energy from the tributary. As a result, the effects of the

Baudette River on the mainstream is minimum. The stream attack-angle of 18.7° was

unchanged in the scour calculation.

The scour calculation took into account of both contraction scour and pier scour. The

contraction scour was calculated from the HEC-RAS model. HEC-18 equations were used to

predict the pier scour instead of using HEC-RAS model due to the complex pier footing

configurations, which consist of pile caps over pile groups. Results for the two proposed

pier configuration bridges are provided below:

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Table 1: Complex Pier Foundation Scour Analysis (100-yr Event)

Contraction

Scour (ft) Pier Stem Scour [ft]

Pile Cap (Footing) Scour

[ft] Pile Group Scour [ft]

Total Scour [ft]

Alternative 1: Continuous Steel I-Girder, 4-Span

3.90 4.80 19.07 3.08 30.85


4.17 4.84 19.23 3.10 31.34

Table 2: Complex Pier Foundation Scour Analysis (500-yr Event)

Contraction

Scour (ft) Pier Stem Scour [ft]

Pile Cap (Footing) Scour

[ft] Pile Group Scour [ft]

Total Scour [ft]


4.36 5.03 20.06 3.24 32.69


4.69 5.08 20.25 3.26 33.28

The results shown above are preliminary and subject to revision. The predicted scour

depths are comparable to the 30ft scour depth, which was documented in a June 19, 2009

MnDOT memorandum. Our analysis shows only a slight difference between the two

proposed pier configurations. Preliminary bridge design provides identical pier geometry

for the two proposed alternatives, so the only source of variance is due to any impact in

water surface and velocity in the channel at the Bridge. It should also be noted that this

analysis was conducted using a pier skew at 18.7° relative to the channel thaweg. Aligning

piers normal to the channel flow will effectively reduce the overall scour estimates.

Detailed calculations for the complex scour analysis are attached.

Attachment A: Complex Scour Calculations for 4-span Alternative

3025 Harbor Lane, Suite 121, Plymouth, MN 55447

763-551-3699 (Office) 763-390-9270 (Fax)

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S.P. 3905-09

Scour for Complex Pier Foundations

Baudette Bridge PE – 3 Pier Alternative

Rev Date By Ck

0 03/22/2016 MBA HZ

Live-bed or clear water scour

Ref: FHWA HEC-18, sect. 6.3 – Live-Bed Contraction Scour

V* = shear velocity in the upstream section

g = Acceleration of gravity (32.2 ft/s

2)

y1 = average depth in the upstream main channel = 25.69 ft (7.83 m)

S1 = slope of energy grade line of main channel (ft/ft) = 0.000092

ft/s (0.084 m/s)

T = fall velocity of bed material based on the D50, (use D50 = 0.50 mm)

for fall velocity in English units multiply T in m/s by 3.28

Use T = 10°C,

ω ≈ 0.06 m/s

Use live-bed scour.


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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1. Contraction Scour Depth

Contration Scour computed from HEC-RAS river analysis. Refer to HEC-RAS model

for contraction scour computation and inputs < R:\1502-Baudette Br

PE\Design\Calculations\HEC-RAS>.

Channel Contraction Scour:

Ref: HEC-18, sect. 7.5 – Scour for Complex Pier

ys = Total Scour from superposition of components

100-yr event

2. Pier Stem Scour Depth

(7.23)

Where:

f = Distance between front edge of pile cap or footing and pier ≈ 9 ft. (2.74 m)

apier = pier width = 7 ft (2.13 m)

h1 = h0 + T = height of the pier stem above the bed before scour = 0

Kh pier = Coefficient to account for the height of pier stem above bed and shielding

effect by pile cap overhang distance “f” in front of pier stem.


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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Kh pier ≈ 0.32 (from figure 7.6)

For 100-year event (from HEC-RAS output)

WSE = 1066.40 ft (325.04 m)

Ground = 1040.70 ft (317.20 m)

y1 = 25.7 ft (7.83 m)

V1 = 3.74 fps (1.14 m/s)

K1 = correction factor for pier nose shape; round nose = 1.0

K2 = correction factor for angle of attack of flow; 18.7° ≈ 1.50

K3 = correction factor for bed condition; (From Table 7.3, use plane bed) = 1.1

g = acceleration of gravity = 32.2 ft/s2


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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3. Pile Cap (Footing) Scour Depth

Use Case 2; bottom of pile cap is on or below bed

yf = distance from the bed (after degredation, contraction scour, and pier stem

scour) to the top of footing.

ks = Grain roughness, use estimate D84 for sand, 2.5 mm = 0.0082 ft (0.0025m)

Average velocity of flow at the exposed footing (Vf) is determined using the

following:

(7.25)

The scour component equation for the footing can be written as:

(7.24)

KW = wide pier factor for shallow flow, not applicable.

apc = pile cap width = 20.0 ft (6.10 m)


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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4. Determination of Pile Group Scour Depth Component

(7.28)

Where:

aproj = sum of non-overlapping projected widths of piles = 6.67 ft

Ksp = coefficient for pile spacing

a = 1.33 ft (16 in)

s = 4 ft

Ksp ≈ 0.49 (from figure 7.11)


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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Km = coefficient for number of aligned rows ≈ 1.4 (from figure 7.12)

(7.29)

(7.30)

Kh pg 0.45 (from figure 7.13)

(7.31)

Total Pier Scour (100yr):

Total Scour (100yr):


500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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Scour Design Check Flood Frequency (500-yr)







Where:

f = Distance between front edge of pile cap or footing and pier ≈ 9 ft.

apier = pier width = 7 ft

h1 = h0 + T = height of the pier stem above the bed before scour = 0.




500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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WSE = 1068.11 ft (325.56 m)

Ground = 1040.70 ft (317.20 m)

y1 = 27.41 ft (8.36 m)

V1 = 4.10 fps (1.25 m/s)









ks = Grain roughness, use estimate D84 for sand, 2.5 mm = 0.0082 ft


following:


500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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ft/s


KW = wide pier factor, not applicable.

apc = pile cap width = 20.0 ft

4. Determination of the Pile Group Scour Depth Component

(7.28)

Where:



a = 1.33 ft (16 in)

s = 4 ft

,



500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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(7.29)

(7.30)

Kh pg ≈ 0.45 (from figure 7.13)


500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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(7.31)



Attachment B: Complex Scour Calculations for 5-span Alternative


763-551-3699 (Office) 763-390-9270 (Fax)

1

S.P. 3905-09

Scour for Complex Pier Foundations

Baudette Bridge PE – 4 Pier Alternative

Rev Date By Ck

0 03/22/2016 MBA HZ

Live-bed or clear water scour

Ref: FHWA HEC-18, sect. 6.3 – Live-Bed Contraction Scour

V* = shear velocity in the upstream section

g = Acceleration of gravity (32.2 ft/s

2)

y1 = average depth in the upstream main channel = 25.69 ft (7.83 m)

S1 = slope of energy grade line of main channel (ft/ft) = 0.0001

ft/s

T = fall velocity of bed material based on the D50, (use D50 = 0.50 mm)

for fall velocity in English units multiply T in m/s by 3.28

Use T = 10°C,

ω ≈ 0.06 m/s

Use live-bed scour.


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

2






Ref: HEC-18, sect. 7.5 – Scour for Complex Pier

ys = Total Scour from superposition of components

100-yr event


(7.23)

Where:

f = Distance between front edge of pile cap or footing and pier ≈ 9 ft. (2.74 m)

apier = pier width = 7 ft (2.13 m)

h1 = h0 + T = height of the pier stem above the bed before scour = 0




100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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WSE = 1066.40 ft (325.04 m)

Ground = 1040.70 ft (317.20 m)

y1 = 25.7 ft (7.83 m)

V1 = 3.81 fps (1.16 m/s)






100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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ks = Grain roughness, use estimate D84 for sand, 2.5 mm = 0.0082 ft (0.0025m)


following:

(7.25)


(7.24)

KW = wide pier factor for shallow flow, not applicable.

apc = pile cap width = 20.0 ft (6.10 m)


100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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4. Determination of Pile Group Scour Depth Component

(7.28)

Where:



a = 1.33 ft (16 in)

s = 4 ft



100-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

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(7.29)

(7.30)

Kh pg 0.45 (from figure 7.13)

(7.31)




500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

7

Scour Design Check Flood Frequency (500-yr)







Where:

f = Distance between front edge of pile cap or footing and pier ≈ 9 ft.

apier = pier width = 7 ft

h1 = h0 + T = height of the pier stem above the bed before scour = 0.




500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

8



WSE = 1068.11 ft (325.56 m)

Ground = 1040.70 ft (317.20 m)

y1 = 27.41 ft

V1 = 4.17 fps (1.27 m/s)









ks = Grain roughness, use estimate D84 for sand, 2.5 mm = 0.0082 ft


following:


500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

9

ft/s


KW = wide pier factor, not applicable.

apc = pile cap width = 20.0 ft

4. Determination of the Pile Group Scour Depth Component

(7.28)

Where:



a = 1.33 ft (16 in)

s = 4 ft

,



500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

10


(7.29)

(7.30)

Kh pg ≈ 0.45 (from figure 7.13)


500-yr Event

763-551-3699 (Office) 763-390-9270 (Fax)

11

(7.31)



re: request for bridge scour analysis for complex pier ...findings are based on the updated hec-ras...

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