03 Bridge Hydraulics 335b3b

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Bridge Definition From a hydraulic perspective, a bridge is defined as: • A structure built over a depression or obstacle for ageway. • Part of a stream crossing system that includes the approach roadway across the floodplain and any openings

• Involves the hydraulic considerations for bridge planning and design Hydraulic Considerations: • • • • • • • •

Discharge patterns Water levels Slopes Cross-sections Velocities Roughness Erosion and Sedimentation Scour

Local pier scour

Channel Shift and General Scour

Exposure of Foundation by Channel Bed Degradation

Floating Debris Action on Superstructure

Bridge Waterway and Length The determination of adequate waterway openings for river crossings is essential to the design of safe and economical bridges. Hydrologic and hydraulic studies of bridge sites are necessary in the determination of the bridge length (Refer Section 3). To avoid general scour in a bridge crossing, appropriate bridge length is necessary. From an economical point of view, a shorter bridge is preferred if there are no hydraulic constraints regarding bridge length. 1.

Approximate River Width, B B = (c) Q3/4

*

Q = discharge c = coefficient ranging from 0.5 – 0.8, determined considering flood plain obstruction (refer to Table 3-1 of DGCS Volume 3 Water Projects)

2. Desirable minimum bridge span length, L L = 20 + 0.005Q **

Hydrologic and Hydraulic Analysis for Bridges

Hydrologic and Hydraulic Analysis for Bridges

Methods of determination of Design Flood Level (DFL) Manning’s Formula

V = velocity, m/s n = Manning’s roughness coefficient R = hydraulic radius = A/P, m P = wetted perimeter, m S = slope of the energy grade line, m/m (For steady uniform flow, S = channel slope, m/m) The selection of Manning’s n is generally based on observation; however, considerable experience is essential in selecting appropriate n values. Typical ranges of n values for various types of channels and floodplains is given in Table 4-2, Table 4-3, Table 4-4 and Table 4-5.(DGCS Vol. 3)

Design Floods Design flood flows at the site using an approved method applicable to the river are essential in the hydrologic and hydraulic analysis. Where floods exceeding the design flood have occurred or where super floods will cause extensive damage to the ading property or the loss of costly structure, a larger waterway opening may be warranted.

As per AASHTO provisions for hydrologic studies, flood flows shall be investigated in the hydrologic studies: • For assessing flood hazards and meeting flood plain management requirements. • For assessing risks to highway s and damage to the bridge and its roadway approaches – overtopping flood and/or the design flood level for bridge scour. • For assessing catastrophic flood damage at high risk sites. • For investigating the adequacy of bridge foundations to resist scour. • To satisfy agency design policies and criteria – design floods for waterway opening and scour for the functional classes of highways. • To calibrate water surface profiles and to evaluate the performance of existing structures – historical floods. • To evaluate environmental conditions- low or base flow information and in estuarine crossing, the spring and tide range.

Some common terminologies for different types of flood levels include: • Ordinary Water Level or Normal Water Level (OWL/NWL)

Height of water level in the river under normal condition • Maximum Flood Level (MFL) Highest recorded flood level. Note that where gauging is not available, this might need to be based on anecdotal observations from the community. However, the values have to be verified or validated by the hydrologist. An equivalent return period may be computed which will guide the hydrologist whether the maximum flood level shall be used as the design flood level.

Some common terminologies for different types of flood levels include: • Design Flood

Discharge used to size the capacity of the bridge. The design flood frequencies for different road types is provided in Table 3.2.5-1. • Check Flood A less frequent flood which generate greater runoff than the design flood and may cause catastrophic effect on the bridge (refer to Volume 3 and Section 2.3 of AASHTO LRFD 2012)

Some common terminologies for different types of flood levels include: • Design Flood Level (DFL)

Design flood level is calculated from the design flood discharge identified above. The Design Flood Level (DFL) will be the reference from which the freeboard will be measured, refer Section 4.4. The hydrologist and the bridge engineer shall decide whether to use the maximum flood level or not if it is higher than the design flood level considering several factors such as the topography of the area and other aspects which will be greatly affected by raising the bridge excessively high. • Ultimate Limit State Flood (ULSF) Design flood against which the bridge is structurally designed to withstand the force of the water. Overtopping can occur in this event, but the bridge structure must be designed to withstand the loading.

Table 3.2.5-1

Design Flood Frequencies (Minimum Requirements) for Bridges

Road Classification

River Structure

Bridge Drainage Hydraulic Scour

Design Flood

Check Flood

Design Flood

Check Flood

Design Flood

Check Flood

Expressway

100 yr

200 yr

*100 yr

*500 yr

25 yr

50 yr

National Road

50 yr

100 yr

*100 yr

*500 yr

10 yr

25 yr

Other Roads

25 yr

50 yr

50 yr

100 yr

5 yr

10 yr

•*or from an overtopping flood of lesser recurrence level, whichever is the more severe based on AASHTO LRFD 2012 Sec 2.6.4.4.2 Bridge Scour

Depth of water surface to scoured bed

Depth of materials removed below normal bed level

Cross-section at a bridge waterway opening

Hydraulic Design Data Contraction Scour Left

Channel

Right

10.59 6.80 9.68 1080 147.87 2.40 9981.22 138.51 0.640

6.79 4.53

Input Data Average Depth (m): Approach Velocity (m/s): Br Average Depth (m): BR Opening Flow (m3/s): BR Top WD (m): Grain Size D50 (mm): Approach Flow (m3/s): Approach Top WD (m): K1 Coefficient:

2.40

Results Scour Depth Ys (m): Critical Velocity (m/s): Equation:

1.19 1.23 Live

Pier Scour All piers have the same scour depth Input Data Pier Shape: Pier Width (m): Grain Size D50 (mm): Depth Upstream (m): Velocity Upstream (m/s): K1 Nose Shape: Pier Angle: Pier Length (m): K2 Angle Coef: K3 Bed Cond Coef: Grain Size D90 (mm): K4 Armouring Coef:

Round nose 2.00 2.40000 10.50 6.49 1.00 0.00 10.00 1.00 1.10

Scour Depth Ys (m): Froude #: Equation:

6.49 0.64 CSU equation

1.00

Results

Combined Scour Depths Pier Scour + Contraction Scour (m): Channel: 7.67

0.00 2.40 818.78 26.64 0.640

• Vertical clearance between the Design Flood Level, DFL, (or the Maximum Flood Level, MFL) and the (soffit) of the lowest member of the bridge superstructure shall not be less than 1.50 m for rivers carrying debris and 1.00 m for other bridges. For definition of DFL and MFL, refer to Section 3. • For navigable channels, the required vertical clearance, based on HPCG/CG-8, Memorandum Circular Number 01-14, Navigational Clearance for Road Bridges and Other Structures over Navigational Inland Waters, 16 April, 2012 shall be: Vertical Clearance = HWL + HV + K where: HWL = highest water level recorded within the AOR (Area of Responsibility) HV = height of vessel K = is a constant 1 meter allowance For bridges in the coastal environment, adequate freeboard shall be provided to prevent wave impact on the bridge superstructure under combined action of high tide, storm surge and design wave.

Thickness of riprap aprons should be at least twice the D50 size of the stones

Guide to Bridge Hydraulics (1973) by TAC

Velocity (m/s) 3 4 5 6

Stone Size (D50) mm 0.30 0.55 0.90 1.30

Loose boulder apron

Gabions and mattresses

Articulated blocks

Alternatives methods

Boulder riprap

Gabions

Grouted riprap

Reinforced concrete

Type of Revetment

Sodding Wooden Pile Fence Dry Boulder Riprap Gabion (spread type) Grouted Riprap (spread type) Gabion (pile up type) Grouted Riprap (wall type) Rubble Concrete Stone Masonry Crib Wall Reinforced Concrete with sheet pile

Allowable Design Velocity (m/s) <2 <4 <5 <5 >5 <6.5 >5 >5 >5 >6 >6

Slope (H:V)

Flatter than 2:1 Flatter than 0.6:1 Flatter than 1.5:1 Flatter than 1.5:1 Flatter than 1.5:1 1:1 to 1.5:1 Steeper than 1:1 Steeper than 1:1 Steeper than 1:1 Steeper than 1:1 Steeper than 1:1