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YNEZ BRIDGE OVER THE
SAN GERTRUDIS CREEK
BRIDGE HYDRAULIC ANALYSIS
Prepared for:
The State of California
Department of Transportation
1657 Riverside Drive
P.O. Box 496073
Redding, CA 96049-6073
Prepared by:
ROBERT BEIN, WILLIAM FROST & ASSOCIATES.
14725 Alton Parkway
Irvine, CA 92718
(714) 472-3505
Contact Persons:
John McCarthy, RCE 38635
Michelle MObily, EIT
RBF Job Number: 10101661
December 5, 2003
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TABLE OF CONTENTS
Introduction................................................................................................................... ..................1
Design Hydrology...................................................................................................................... .....1
Existing Channel Conditions ......... ...... ....................00...... ........ ................ .................. ..... ..... ..... .......1
Proposed Channel Conditions. .... ............. ........ ..... ....... ....... ......... ........ ...... ............. ............ .... .......3
Hydraulic Analysis. ... ............ ... .... ... ..... .... ... ........ ...... ... ........ ........ ..... ......... .... ......... ... ..... ..... ...... .....3
Discussion of Hydraulic Results.. ............ ................. .......... ......... ..... ......... .... ......... ...................... ...5
Scour Analysis.......................................................................................................................... ......5
General Scour........................................................................................................................... ..5
Local Scour..;....................................................................................................................... .......6
Recommended Bridge Pier Scour Protection..................................................................................8
LIST OF TABLES
Table NO.1 - Summary of Channel Hydraulic Analysis
Table NO.2 - Summary of Scour Analysis
LIST OF FIGURES
Figure 1 - Existing Upstream of Ynez Road Bridge
Figure 2 - Existing Ynez Road Bridge (facing downstream)
Figure 3 - Existing Downstream of Ynez Road Bridge
Figure 4 - Santa Gertrudis Concrete Lined Side Slope
Figure 5 - Existing Downstream Ynez Bridge Pier
TECHNICAL APPENDIX
A. HEC-2 Creek Analysis System Calculations
EXHIBITS
A. Ynez Bridge Widening Improvements
B. HEC-2 Floodplain Map
H:\Pdata\ 10101661 \Strmwater\SCOUR2.doc
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~\nchesterlYnez Road HYdlcS/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
Introduction
The purpose of this report is to provide the hydraulic analysis for a reach of the Santa Gertrudis
Creek Including the proposed Ynez Road bridge, The proposed Ynez Road widening is located
in the City of Temecula, County of Riverside. This report will assess the hydraulic impacts
associated with the proposed bridge widening as well as an estimate of the potential scour at
the proposed bridge upstream pier.
The preliminary proposed improvements include widening of Ynez Road 4,63 meters (15,2 feet)
at the West and East side of the roadway. The bridge is approximately 44.8 meters (147 feet) in
length and currently has a width of 95.9 meters (314,7 feet). Upon widening the resulting bridge
width will be approximately 105,2 meters (345.1 feet). The existing bridge consists of two
abutments at each end and one 0.61 meter (2 feet) pier in the center. The pier height varies
between 4.30 meters (14,1 feet) on the upstream side to 4.42 meters (14,5 feet) on the
downstream side. The proposed Ynez Road bridge widening will extend the pier and
abutments 4.63 meters (15.2 feet) at the upstream and downstream sides of the bridge. The
proposed pier height will vary between 4.21 meters (13.8 feet) at the upstream and 4.40 meters
(14.3 feet) at the downstream.
Desian HvdroloQV
The hydrology utilized to evaluate the channel hydraulics for the Ynez bridge widening
Improvements within Santa Gertrudls Creek was obtained from a previous Conditional Letter of
Map Revision (CLOMR) prepared for Assessment District 161, Riverside County, Hydraulic
Study for FEMA Conditional Letter of Map Revision for Santa Gertrudis and Tucalota Creeks,
Riverside County, prepared by Howard H. Chang in March, 1991, The estimated 100-year
flowrate for Santa Gertrudis Creek downstream from Tucalota Creek is 281,2 cms (10,000 cfs).
Existina Channel Conditions
Santa Gertrudis Creek is a large tributary watershed to Murrieta Creek, and Tucalota Creek is a
tributary of Santa Gertrudls Creek which continues immediately upstream of Winchester Road.
The existing channel section under Ynez Road bridge has an engineered prismatic trapezoidal
geometry with concrete slope lining and earthen stre,am bottom. The bottom width of the lower
reach (downstream of the Ynez Road bridge) is generally 35.4 meters (116.2 feet) and it is 35
meters (114,8 feet) for the upstream reach, The upstream face of Ynez Road bridge is located
approximately 1148 meters (3764 feet) downstream of the downstream face of Margarita Road
bridge. Also, downstream of the Ynez Road bridge, at an approximate station ----, there exists a
drop structure. This drop structure Is used for general scour reduction and control. Figures 1,2
and 3 are pictures of the existing channel and bridge at the Ynez Road bridge crossing,
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~'lnchesterlYnez Road HYdlcS/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
Figure 1 . Existing Upstream of Ynez Road Bridge
Figure 2 - Existing Ynez Road Bridge (facing downstream)
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Issue Date: 5-December-2003
Figure 3 - Existing Downstream of Ynez Road Bridge
Proposed Channel Conditions
The proposed bridge improvements include excavation as well as abutment and pier extension.
The proposed bridge widening will add approximately 4.63 meters (15.2 feet) in width to each
side of the existing Ynez bridge and will result in a total bridge span of approximately 105.2
meters (345.1 feet) in width, as shown in Exhibit X. The existing bridge widening will be
concrete with a low chord elevation of 1054,3 feet and a minimum top of roadway at elevation
332,8 meters (1058.8 feet). The proposed channel section under Ynez Road bridge will remain
an engineered prismatic trapezoidal geometry with concrete slope lining and earthen stream
bottom, The bottom width of the lower reach as well as the bottom width of the upstream reach
will be consistent with the existing channel conditions, Similarly, the total length of the bridge
will remain approximately 44.8 meters (147 feet) in length.
Hydraulic Analysis
The hydraulic analysis of the channel utilized the U.S. Army Corps of Engineers (ACOE) HEC-2
water surface profile computer model. The HEC-2 program is intended for calculating water
surface profiles for steady and gradually varied flow in natural and manmade channels. The
effects of various obstructions such as bridges, culverts, weirs, and structures in the floodplain
may be considered in the computations. The computational procedure is based on solving the
one-dimensional energy equation with energy loss due to friction evaluated with Manning's .
equation. The computational procedure is generally known as the standard step method. HEC-
2 is a rigid boundary model which assumes that the channel bed does not fluctuate,
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Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
The existing water surface elevations for the location of the channel prior to Ynez bridge
widening were obtained for the aforementioned report titled, Hydraulic Study for FEMA
Conditional Letter of Map Revision for Santa Gertrudis and Tuca/ota Creeks, Riverside County,
prepared by Howard H. Chang dated March, 1991, The existing hydraulic analysis modeled the
Ynez Road bridge utilizing the Special Bridge routine in HEC-2, The depth and velocity
upstream of Ynez Road bridge for a 1 OO-year event are estimated to be 2,88 meters (9.46 feet)
and 2,60 mps (8.50 fps), respectively. The existing HEC-2 model extended approximately 360
meters (1170 feet) downstream of the Ynez Road bridge. The manning's roughness coefficient
corresponding to the invert and concrete slope lining are 0.04 and 0.015, respectively,
The proposed Ynez Road widening improvement was analyzed by modifying the HEC-2 run
from the previous report entitled Basis of Design Report, Santa Gretrudis Creek Hydraulics-
Bikeway Project, prepared by RBF Associates dated February 1999. The proposed condition
will include a 4.63 meter (15,2 foot) road widening both upstream and downstream of Ynez
Road bridge as well as pier and abutment extension. The proposed improvements were
designed with the channel containing the 1 OO-yr flowrate.
Channel characteristics near Ynez bridge crossing include concrete lined side slopes, as seen
In Figure 4, using a manning's "n" value of 0.015. The channel has an earthen bottom with an
alluvial bed, The channel within the project reach is assigned an "n" value of 0.03 for scour
analysis. Bridge analysis for Ynez Road bridge utilized the "special bridge" option in the HEC-2
program, The "special bridge" option analyzed the pressure plus momentum effects of the
bridge piers, and weir flow,
Figure 4 - Santa Gertrudis Concrete Lined Side Slope
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WinchesterlYnez Road HYd!ics/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
Results for the widened bridge condition are presented in Table No, 1 for the 1 OO-year flow rate.
Station
Cross
Section
Table No.1 - Summary of Channel Hydraulics Analysis
100-Year HEC-2 Model Results
Proposed Ynez Road Bridge Widening
Q100 = 10,000 cfs
Existln Condition Station
Water Cross
Surface Section
321 m
1053 ft
320.9 m
1052.4 ft)
19 + 66.3
19 + 51.1
osed Condition
20 + 30
20 + 45.2
Discussion of Hydraulic Results
The bridge widening will result in minor increases to water surface elevations both upstream
and downstream of the bridge. The maximum increase in the upstream water surface
elevations due to the Ynez bridge widening is 0.03 m (0,1 feet) over the existing water surface
elevations modeled for this study. Similarly, downstream of the Ynez bridge widening, the
proposed water surface elevations are 0,1 m (0.328 ft) over the existing water surface
elevations modeled for this study. This slight change in water surface elevation is due to the
change in available flow area under Ynez Road bridge, The channel cross sections which are
of significance are those at the face of the proposed Ynez bridge, Station 1951.1 downstream
and Station 2045,2 upstream. These stations correspond to existing upstream and downstream
stations 1966.3 and 2030, respectively,
The HEC-2 computer output files for the existing and proposed condition for the combined
model are included in Appendix A. The locations of the cross sections are indicated on the
enclosed HEC-2 Floodplain Map, Exhibit X.
Scour Analysis
The amount of scour associated with a hydraulic structure such as a bridge pier is the result of
the combination of local scour and general scour. General scour is due to the long term trends
of the channel degradation, whereas local scour is due to the flow contraction and the vortex
flow that occurs at the bridge piers, Contraction scour was not considered, as the channel base
will remain constant through the bridge section.
General Scour
Long-term general scour is defined as scour that occurs with a time scale of the order of several
years or longer, and includes progressive degradation or aggradation and lateral bank erosion
due to channel widening or meander migration. The long-term general scour is not significant
downstream of the Inez Bridge widening because the rate of scour development over time is
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WlnchesterlYnez Road HYdr!cs/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
relatively slow. The rate is slow due to the downstream drop structure located near station ------
Local Scour
Local scour is defined as the abrupt decrease in the channel bed elevation near a pier due to
the erosion of bed material by the local flow patterns introduced by the bridge pier. This erosion
process produces the increased bed shear stress at the nose of the piers, which results in eddy
and vortex systems. The parameters affecting the depth of scour at the bridge pier include: size
and shape of the pier, discharge, Froude number, velocity, and the mean size of the erodible
bed material. In addition, debris entrapment on a pier near the bed results in an increase to the
pier width, which will directly increase the amount of local scour, Figure 5 is a picture of the
existing pier downstream of Inez Bridge. A debris factor of 2.0 ft was used in addition to the pier
width to account for the excessive scour depth as a result of debris around the pier. The results
of the hydraulic analysis were used to calculate the local scour. Table NO.2 summarizes the
results of the hydraulic analysis at the upstream face of the proposed Ynez Road bridge
widening, HEC-2 station 2045.2,
Figure 5 - Existing Downstream Ynez Bridge Pier
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V\;inchesterlYnez Road HYd!cs/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
Table No.2 - SummarY of Scour Analvsis
Hydraulic Analysis 50-Year
Parameter
Discharae (a) 10,000 cfs
W.S. Elevation 1052,5 ft
Depth of Flow(d,' 9.44 ft
Velocity of Flow (V 8.55 fps
Froude Number (Fr) 0.49
Pier Width (a) + 2,0 ft Debris Factor 2.0 ft + 2.0 ft - 4,0 ft
The procedure used to determine local scour for this project is outlined by the U,S. Department
of Commerce in "Evaluating Scour at Bridges, 3rd Edition," dated November 1995, The report
investigates more than 5 equations associated with local scour studies, and compared the
equations using various types of graphical and statistical analysis, The results revealed that to
compute both live-bed and clear-water pier scour an equation based on Colorado State
University (CSU) equation is recommended. The pier scour equation based on the CSU
equation is given below.
Ys = 2.0y,K,K2K3K4C!!..f65 Fro 43
y,
where:
Ys = Scour depth, m
y, = Flow depth directly upstream of the pier, m
K, = Correction factor for pier nose shape
K2 = Correction factor for angle of attack of flow
K3 = Correction factor for bed condition
K. = Correction factor for armoring by bed material size
a = Pier width, m
L = Length of pier, m
Fr, = Froude Number
V, = Mean velocity of flow directly upstream of the pier, m/s
g = Acceleration of gravity (9.81 m/s2)
The coefficient K, is 1.0 for round nose piers.
The coefficient K2 is 1.0 for piers aligned with the fiow in the channel.
The coefficient K3 is 1.1 for clear water scour,
The coefficient K. is calculated using an equation developed by FHWA by Molinas at CSU and
is given below.
K4 = [1- 0.89(1- VR)2t5
where:
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~inchesterlYnez Road HYdliCS/Bridge Scour
Hydraulic Analysis and Scour Report
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Issue Date: 5-December-2003
v,=[~-v,J
V'90 -v,
v = 0 645[D50 JO,053V
I' c50
a
VR = Velocity ratio
V, = Approach velocity, m/s
Vi = Approach velocity when particles at a pier begin to move, m/s
VegO = Critical velocity for Dgo bed material size, m/s
V 050 = Critical velocity for Dso bed material size, m/s
a = Pier width, m
given:
v" = 6.19yIl6D;/3
and:
De = Critical particle size for the critical velocity Ve, m
According to the equation based on the CSU equation, the calculated scour depth around the
bridge pier is approximately 2.67 meters (8.75 feet).
Recommended Bridae Pier Scour Protection
Using this scour Information we can develop recommendations regarding the protection of the
proposed bridge piers. The estimated scour calculation indicates that the proposed column pier
should have a pile cap elevation protected to a depth of 2.67 meters (8.75 feet) below the
existinq flowline of the channel.
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