750.3 Bridges

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Asset Management
Report 2009
See also: Innovation Library

Contents

750.3.1 Hydraulic Considerations for Bridge Layout

750.3.1.1 Survey Locations

Location of surveyed sections and profiles can greatly affect the quality of hydraulic models.

Districts should request guidance for survey activities related to hydraulics for Bridge designed structures using the Bridge Survey Location Request Form. Guidance for determining the surveying locations required is presented in the following articles.

Details of the Bridge Location Request submittal process can be found at EPG 747.1.1 Bridge Survey Location Request Submittal/Completion Process. EPG 238.3.36.1 General Bridge Survey Information provides guidance for surveying activities.

750.3.1.1.1 Existing Data

Flood Insurance Studies, Corps of Engineers data, Level II USGS scour studies and recent nearby projects should be reviewed to determine if existing survey data, hydraulic data or hydraulic models may be available. The Flood Insurance Study contains information on the hydrologic and hydraulic models used and also may provide information regarding 3rd party models that were used. Adequate existing data may reduce the need for additional survey data or provide a base model for design of a new structure.

FEMA Models:

HEC-RAS - Most FEMA HEC-RAS hydraulic models are available thru SEMA.

  • Zone A (Approximate) models do not contain details for manmade features, may not have surveyed sections in the right locations for adding details for structures and the manning “n” values are typically averaged for the entire stream. For these reasons Zone “A” models are not usually suitable for use to determine the bridge layout.
  • Detailed Steady-State Flow models in most cases can be used as the base model, with additional survey data incorporated as needed.
  • Unsteady-State Flow and 2D models will need to be acquired from SEMA and used as the base model, with additional survey data incorporated as needed.

HEC-2 - Some FEMA HEC-2 hydraulic models are available thru SEMA.

  • HEC-2 hydraulic models can be converted to Steady-State Flow HEC-RAS models
  • Detailed HEC-RAS models created from HEC-2 models can be used as the base model. Due to the age of these models it is recommended that new survey data be acquired in the vicinity of proposed and existing structures near the project site.

Other Models – Consult the Structural Hydraulics Engineer for Guidance.

Corps of Engineers Models:

The Army Corps of Engineers (Corps) has jurisdiction over several lakes in the state which provide flood control and regulate stream flow. In addition to these lakes the Corps has maintenance responsibilities for the Missouri River, Mississippi River, as well as several other smaller rivers and streams. To provide these functions the impacted stream would need to be modeled. If FEMA models are not available for Corps managed streams a model may be available from the Corps. The quality of these models varies and they should be reviewed in the same manner as FEMA models to determine if the model can be used. While there is not a comprehensive list of these streams the following table lists the known Corps lakes, their discharge streams and the Corps District with jurisdiction.

Corps Lakes in Missouri
Lake NameStream NameCorps District
Blue Springs LakeE. Fork Little Blue River Kansas City
Bull Shoals LakeWhite River Little Rock
Clearwater LakeBlack River Little Rock
Long Branch LakeE. Fork Little Chariton River Kansas City
Longview LakeLittle Blue River Kansas City
Mark Twain LakeSalt River St. Louis
Norfork LakeNorth Fork River Little Rock
Pomme De Terre LakePomme De Terre River Kansas City
Smithville LakeLittle Platte River Kansas City
Stockton LakeSac River Kansas City
Table Rock LakeWhite River Little Rock
Harry S. Truman LakeOsage River Kansas City
Wappapello LakeSt. Francis River
(upstream of dam)
St. Louis
St. Francis River
(downstream of dam)
Memphis


750.3.1.1.2 Bridge Survey Location Requests

The Bridge Survey Location Request Form received from the district should be filled out using the best data available. Data that is not required or that deviates from EPG guidance should be noted and explained on the Bridge Survey Location Request Form. In addition to the completed Bridge Survey Location Request Form, an image showing the location of the valley sections and a kmz file showing same sections should be included in the return submittal to the district.

750.3.1.1.2.1 Centerline and Offset Profiles

Estimate offset distances and terminal elevations for offset profiles. Provide additional offset profiles if required. See EPG 238.3.36.1.3 Centerline and Offset Profiles for location and elevation details.

750.3.1.1.2.2 Streambed Profiles

Multiple Defined Channels

A streambed profile is provided for all structures, including overflow structures that have a defined channel, even if that structure is not being replaced.

Overflow Structures

A streambed profile is not required for overflow structures that do not have a defined channel. When only the overflow structure is being replaced, the bridge survey is still developed based on all structures that are in the floodplain. Streambed profiles are provided for all other structures in the floodplain that have a defined channel.

Provide additional guidance for tributary streams as required. See EPG 238.3.36.3.6 Streambed Profiles for additional details.

750.3.1.1.2.3 Water Surface Profiles

See EPG 238.3.36.3.7 Water Surface Profiles for details.

750.3.1.1.2.4 Valley Sections

The layout of valley sections varies with stream size, slope, meander and other factors. As such, the guidance presented here considers a typical crossing of a natural stream.

Location for Structures

A minimum of three valley sections are required, one upstream and two downstream of the proposed structure. The ideal location for valley section placement for creation of a hydraulic model is upstream and downstream of the disturbance to flow caused by the structure and roadway fill.

Stream Type

For purposes of hydraulic modeling, streams (natural, manmade, altered, etc.) are considered to be either natural streams or drainage ditchs. To be considered as a drainage ditch, stream gradiant should be nearly flat with considerable overbank storage available compared to the volume of the stream flow. Streams that do not meet these criteria should be treated as natural streams.

750.3.1.1.2.4.1 Natural Streams

Initial Placement of Valley Sections

Initial placement of the upstream and the first downstream valley section should be based on a 1:1 contraction ratio (upstream) and a 2:1 expansion ratio (downstream) from the streamside end of the roadway fill to the limit of the 100-yr. floodplain. The slope of the contraction and expansion lines should be based on 100-yr. flood flow path. For locations without an existing structure or when the replacement structure may be shorter, the origin of the expansion line may be moved closer to the bank of the channel. Initial placements should be adjusted as specified below.

Initial Placement of Upstream andFirst Downstream Valley Sections
Initial Placement of Upstream and
First Downstream Valley Sections


Final Placement of Valley Sections

Upstream Valley Section – The upstream valley section is used to help determine the upstream water surface elevation and the flow velocity entering the bridge. Placement should be in a location representative of the average floodplain width upstream in the vicinity of the bridge and should not be placed at an excessively wide location in the floodplain or at junctions with tributaries. The location of the section should remain either at or upstream of the intersections of the Expansion lines and floodplain limits after any location or orientation adjustments are made. Orientation adjustments may be needed when the channel flow is not parallel to the 100-yr. flood flow. (See Valley Section Orientation for details).

First Downstream Valley Section – The first downstream valley section is used to establish the water surface elevation and flow velocity downstream of the bridge which is used to calculate the energy loss through the bridge caused by the bridge and roadway fill. Placement should be in a location near the initial section that provides a natural constriction. If a natural constriction does not exist, the section should be placed at a location representative of the average floodplain width downstream in the vicinity of the bridge and should not be placed at an excessively wide location in the floodplain or at junctions with tributaries. The location of the section should remain either at or downstream of the intersections of the expansion lines and floodplain limits after any location or orientation adjustments are made. Orientation adjustments may be needed when the channel flow is not parallel to the 100-yr. flood flow. (See Valley Section Orientation for details).

Second Downstream Valley Section –' The second downstream valley section is used to establish a starting water surface elevation and flow velocity for the hydraulic model. Placement should be in a location downstream of the final location of the first downstream valley section within a range of 0. 5 to 1.0 times the distance between the roadway centerline and the final location of the first downstream valley section (measured along the 100-yr flood flow path) that provides a natural constriction. If a natural constriction does not exist the section should be placed at a location representative of the average floodplain width downstream in the vicinity of the bridge and should not be placed at an excessively wide location in the floodplain or at junctions with tributaries. Orientation adjustments may be needed when the channel flow is not parallel to the 100-yr. flood flow (see Valley Section Orientation, below, for details).

Valley Section Orientation

Valley sections are taken through the entire valley including the stream channel and floodplain at right angles to both the channel and 100-year flood flows. To conform to right angles, the valley section may be "doglegged" so the first leg is at right angles to one side of the valley, the second leg is at right angles to the channel, and the third leg is at right angles to the opposite side of the valley. For hydraulic modeling purposes if the angle of the stream flow is within 15° of the 100-year flood flow it may be considered to be at a right angle and doglegging the valley section is unnecessary.

Placement of Valley Sections
Placement of Valley Sections


Valley Section Locations at Junctions

Stream junctions near bridges are relatively common. Below are some generalized cases for determining if valley sections are needed:

Small Tributaries – For bridges over the main channel with a small tributary entering either upstream or downstream of the bridge, additional valley sections will not be required when the drainage area of the tributary is less than 20% of the drainage area of the main channel.
For bridges over small tributaries near larger channels, valley sections will be required.
Larger Tributaries – Need for valley sections is determined on a case by case basis.

See EPG 238.3.36.3.8 Valley Sections for additional requirements for valley sections.

750.3.1.1.2.4.2 Drainage Ditches

The upstream and first downstream valley sections should be placed at least 1 top width of the channel, but not less than 150 ft., from centerline of structure. The second downstream valley section should be placed at least ½ the top width of the channel, but not less than 100 ft., downstream of the first downstream valley section. Distances are measured along the stream centerline.

Junctions should be treated the same as for natural streams.

750.3.1.1.2.5 Typical Channel Sections

See EPG 238.3.36.3.9 Typical Channel Sections for details

750.3.1.1.2.6 Existing Bridges

See EPG 238.3.36.3.10 Existing Bridges for details.

750.3.1.2 Abutment Layout

Abutments shall be placed so that spill fill slopes do not infringe upon the channel; the toes of the spill fill slopes may be no closer to the center of the channel than the toe of the channel banks. The Soil Survey provided by the Geotechnical Section gives minimum spill fill slopes based on slope stability criteria. The minimum bridge length for stability criteria is thus determined by projecting the stability slopes outward from the toes of the channel slopes as shown below. For structures crossing an NFIP regulatory floodway, abutments shall be placed so that the toes of the spill fill slopes are outside the floodway limits.

750.3.1.3 Pier/Bent Layout

Piers should not be placed in the channel except where absolutely necessary. Where possible, piers are to be placed no closer to the center of channel than the toe of the channel banks. When the proposed bridge length is such that piers in the channel are necessary, the number of piers in the channel shall be kept to a minimum (See Abutment and Pier Location Limits above).

Bents shall be skewed where necessary to align piers to the flow direction, at the roadway design criteria frequency, to minimize the disruption of flow and to minimize scour at piers. For stream crossings, skew angles less than 10 degrees are not typically used, and skew angles should be evenly divisible by 5 degrees.

750.3.1.4 Roadway Fill Removal

When replacing an existing bridge, the bridge memorandum and design layout should note whether the existing roadway fill is to be removed. The designer should consult the district in regard to the limits of fill removal. Minimum removal should provide hydraulic conditions that minimize the bridge length. Normally, existing fill is removed to the natural ground line. The removal limits of existing roadway fill will be shown on the roadway plans.

750.3.1.5 Velocity

Average velocity through the structure and average velocity in the channel shall be evaluated to ensure they will not result in damage to the highway facility or an increase in damage to adjacent properties. Average velocity through the structure is determined by dividing the total discharge by the total area below the water surface. Average velocity in the channel is determined by dividing the discharge in the channel by the area in the channel below the water surface.

Acceptable velocities will depend on several factors, including the "natural" or "existing" velocity in the stream, existing site conditions, soil types, and past flooding history. Engineering judgment must be exercised to determine acceptable velocities through the structure.

Past practice has shown that bridges meeting backwater criteria will generally result in an average velocity through the structure of somewhere near 6 ft/s. An average velocity significantly different from 6 ft/s may indicate a need to further refine the hydraulic design of the structure.

750.3.1.6 Hydraulic Performance Curve

The hydraulic performance of the proposed structure shall be evaluated at various discharges, including the 10-, 50-, 100-, and 500-year discharges, which are the discharges typically found in a Flood Insurance Study. The risk of significant damage to adjacent properties by the resulting velocity and backwater for each of these discharges shall be evaluated.

750.3.1.7 Flow Distribution

Flow distribution refers to the relative proportions of flow on each overbank and in the channel. The existing flow distribution should be maintained whenever possible. Maintaining the existing flow distribution will eliminate problems associated with transferring flow from one side of the stream to the other, such as significant increases in velocity on one overbank. One-dimensional water surface profile models are not intended to be used in situations where the flow distribution is significantly altered through a structure. Maintaining the existing flow distribution generally results in the most hydraulically efficient structure.

750.3.1.8 Bank/Channel Stability

Bank and channel stability must be considered during the design process. HEC-20 provides additional information on factors affecting streambank and channel stability, and provides procedures for analysis of streambank and channel stability. At a minimum, a qualitative analysis (HEC-20 Level 1) of stream stability shall be performed. If this qualitative analysis indicates a high potential for instability at the site, a more detailed analysis may be warranted. See the AASHTO Highway Drainage Guidelines Chapter VI and HEC-20 for additional information.

750.3.1.9 Scour

Asset Management
Report 2009
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Hydraulic analysis of a bridge design requires evaluation of the proposed bridge's vulnerability to potential scour. Unanticipated scour at bridge piers or abutments can result in rapid bridge collapse and extreme hazard and economic hardship.

Bridge scour is composed of several separate yet interrelated components, including long term profile changes, contraction scour and local scour. Total scour depths are obtained by adding all of these components together. All bridges shall be evaluated for the scour design flood and scour check flood frequencies shown in the table below.

Design Frequency*Scour Design Flood Frequency *Scour Check Flood Frequency
Q25 Q100 Q500
Q50 Q100 Q500
Q100 Q200 Q500
* The Overtopping Discharge and Frequency shall be evaluated as a flood scour event if it has a lesser recurrence interval than
the scour design flood or scour check flood (AASHTO LRFD Bridge Design Specifications 2.6.4.4.2, and HEC-18).

Lateral channel movement must also be considered in design of bridge foundations. Stream channels typically are not fixed in location and tend to move laterally.

For additional information on scour and stream stability, see HEC-18 and HEC-20.

750.3.1.9.1 Pile Footings

The top of pile footing elevations should be set at or below the calculated total scour design depth, provided the calculated depths appear reasonable. Consult the Structural Project Manager in regard to footing elevations if Total Scour design depth is less than 6.0 feet. Top of footing elevations on the overbanks should be designed at the same elevation as footings in the channel unless it can be determined with a reasonable degree of certainty that the channel will not migrate into the overbank during the life of the bridge. The bottom of footing elevation shall remain the same whether a seal course is used or not; do not adjust the bottom of footing if a seal course is used. Considerable exercise of engineering judgment may be required in setting these footing depths.

Pile Footing Placement
Pile Footing Placement

750.3.1.9.2 Spread Footings

Spread footings shall be keyed into the rock to prevent sliding and to protect the footing from scour. Keys shall be a minimum of 6 inches into harder rock, such as limestone, dolomite and hard sandstone and a minimum of 18 inches into softer rock such as soft sandstone, siltstone, mudstone, and shale. The sides of the footing shall be poured in contact with the sides of the intact rock excavation; all fractured or loose rock shall be removed. Since rock removal can damage the structure of the formation making it potentially less resistant to scour, the bottom of footing elevation should be placed at the lowest of the following elevations:

a) Top of footing at or below top of rock if rock will potentially be exposed by scour.
b) Keyed 6 in. into the loadbearing hard rock layer or 18 in. into the loadbearing soft rock layer.
c) 3 ft. below the total scour design flood depth (below frost line).
d) Below the total scour check flood depth.

Spread footings on rock highly resistant to scour (i.e. granite and rhyolite) shall be either keyed a minimum of 6 inches into the rock or have steel dowels drilled and grouted into the rock. Contact Geotechnical section for recommendation on whether to key into rock or use dowels.

750.3.1.10 List of References

1. Lagasse, J.D., et al., 2012, Stream Stability at Highway Structures – Fourth Edition - Hydraulic Engineering Circular No. 20 (HEC-20), Federal Highway Administration, Publication No. FHWA-HIF-12-004

2. AASHTO, 2007, Highway Drainage Guidelines, American Association of State Highway and Transportation Officials

3. United States Water Resources Council, 1981, Guidelines for Determining Flood Flow Frequency, Bulletin #17B of the Hydrology Committee

4. Southard, R.E. and Veilleux, A.G., 2014, Method of Estimating Annual Exceedance-Probability Discharges and Largest Record Floods for Unregulated Streams in Rural Missouri, USGS Scientific Investigations Report 2014-5165

5. Alexander, T.W. and Wilson, G.L., 1995, Technique for Estimating the 2 to 500 Year Flood Discharges on Unregulated Streams in Rural Missouri, USGS Water-Resources Investigations Report 95-4231

6. Southard, R.E., 2010, Estimation of the Magnitude and Frequency of Floods in Urban Basins in Missouri, USGS Scientific Investigations Report 2010-5073

7. Brunner, G.W., 2010, HEC-RAS River Analysis System User’s Manual, US Army Corps of Engineers

8. Brunner, G.W., 2010, HEC-RAS River Analysis System Hydraulic Reference Manual, US Army Corps of Engineers

9. Warner, J.C., et al., 2009, HEC-RAS, River Analysis System Applications Guide, US Army Corps of Engineers

10. Barnes, H.H., 1967, Roughness Characteristics of Natural Channels, USGS Water-Supply Paper 1849

11. Arcement, G.L. & Schneider, V.R., 1984, Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains, Federal Highway Administration, Report No. FHWA-TS-84-204

12. Chow, V.T., Open-Channel Hydraulics, McGraw Hill Book Company, 1988, pp. 108-123

13. Schall, J.D., et al., 2012 Hydraulic Design of Highway Culverts, Third Edition - Hydraulic Design Series No. 5 (HDS-5), Federal Highway Administration, Publication No. FHWA-HIF-12-026,

14. Kilgore, R.T., et al., 2016 Highways in the River Environment-Floodplains, Extreme Events, Risk and Resilience, Hydraulic Engineering Circular No. 17 (HEC-17), Federal Highway Administration, Publication No. FHWA-HIF-16-018

15. Arneson L.A., et al., 2012, Evaluating Scour at Bridges, Fifth Edition - Hydraulic Engineering Circular No. 18 (HEC-18), Federal Highway Administration, Publication No. FHWA-HIF-12-003,

16. Keaton J.R. et al., 2012, Scour at Bridge Foundations on Rock, NCHRP Report 717, Transportation Research Board

17. Ettema R. et al., 2010, Estimation of Scour Depth at Bridge Abutments (Draft Final Report), NCHRP Report 24-20, Transportation Research Board.

750.3.2 Hydraulic Design Process

750.3.2.1 Overview

The hydraulic design process begins with the collection of data necessary to determine the hydrologic and hydraulic characteristics of the site. The hydraulic design process then proceeds through the hydrologic analysis stage, which provides estimates of peak flood discharges through the structure. The hydraulic analysis provides estimates of the water surface elevations required to pass those peak flood discharges. A scour analysis provides an estimate of the required depth of bridge foundations. A risk assessment is performed for all structures, and when risks to people, risks to property, or economic impacts are deemed significant, a least total economic cost analysis shall be performed to ensure the most appropriate and effective expenditure of public funds. Finally, proper documentation of the hydraulic design process is required.

The level of detail of the hydrologic and hydraulic analyses shall remain consistent with the site importance and with the risk posed to the highway facility and adjacent properties by flooding.

750.3.2.2 Data Collection

The first step in hydraulic design is collecting all available data pertinent to the structure under consideration. Valuable sources of data include the bridge survey; satellite imagery, aerial photography and various maps; site inspections; soil surveys; plans, surveys, and computations for existing structures; and flood insurance study data.

750.3.2.2.1 Bridge Survey Location Request

Location of the surveyed sections and profiles is an important factor in developing the best possible water surface profile model for the proposed structure. For this reason, inclusion of the Bridge Survey Location Request as an agenda item at an initial core team meeting is recommended.

The procedure for transmission of the Bridge Survey Location Request form between the district and Bridge Division is described in EPG 747.1.1 Bridge Survey Location Request Submittal/Completion Process.

750.3.2.2.1.1 Bridge Survey Locations.

Bridge Division will provide guidance for the bridge survey items as noted in the following:

Centerline and Offset Profiles – EPG 751.3.1.1.2.1
Streambed Profiles – EPG 751.3.1.1.2.2
Water Surface Profiles – EPG 751.3.1.1.2.3
Valley Section Locations – EPG 751.3.1.1.2.4
Typical Channel Sections – EPG 751.3.1.1.2.5
Existing Bridges – EPG 751.1.1.2.6

750.3.2.2.2 Bridge Survey

The bridge survey is prepared by district personnel and provides information regarding existing structures, nearby structures on the same stream, and streambed and valley characteristics including valley cross-sections along the centerline of the proposed structure, valley cross-sections upstream and downstream of the proposed structure, and a streambed profile through the proposed structure.

Bridge surveys are conducted in accordance with EPG 747 Bridge Reports and Layouts.

750.3.2.2.3 Photographs and Maps

Aerial photography, satellite imagery, USGS topographic maps, and county maps should be consulted to determine the geographic layout of the site. Aerial photographs, and satellite imagery in particular, can provide information on adjacent properties that may be subjected to increased risk of flood damage by the proposed structure, and may be available from the MoDOT Photogrammetry Section.

750.3.2.2.4 Site Inspection

A site inspection is a vital component of the hydrologic and hydraulic analyses, and is especially important for those sites subjected to risk of significant flood damage. A visit to the proposed site can provide the following information:

  • selection of roughness coefficients
  • evaluation of overall flow directions
  • observation of land use and related flood hazards
  • geomorphic observations (bank and channel stability)
  • high-water marks
  • evidence of drift and debris
  • interviews with local residents or construction and maintenance personnel on flood history

Photographs taken during the site visit provide documentation of existing conditions and will aid in later determination of hydraulic characteristics.

750.3.2.2.5 Flood Insurance Study Data

If a Flood Insurance Study (FIS) has been performed for the community in which the structure is proposed, the FIS may provide an additional data source. The FIS may contain information on peak flood discharges, base flood elevations (BFE) water surface profile elevations, and information on regulatory floodways.

750.3.2.2.6 Data Review

After all available data has been compiled, the data should be reviewed for accuracy and reliability. Special attention should be given to explaining or eliminating incomplete, inconsistent or anomalous data.

750.3.2.3 Bridge Hydrologic Analysis

Peak flood discharges are determined by one of the following methods. If the necessary data is available, discharges should be determined by all methods and engineering judgment used to determine the most appropriate. For accuracy of discharges, see EPG 751.5.2.1, Hydraulic Data.

750.3.2.3.1 Historical USGS Stream Gage Data

See EPG 749.7 Historical USGS Stream Gage Data

750.3.2.3.2 NFIP Flood Insurance Study Discharges

See EPG 749.8 NFIP Flood Insurance Study Discharges

750.3.2.3.3 USGS Rural Regression Equations

See EPG 749.6.1 Rural Regression Equations.

750.3.2.3.4 USGS Urban Regression Equations

See EPG 749.6.2 Urban Regression Equations.

750.3.2.3.5 Other Methods

Other methods of determining peak flood discharges include the Corps of Engineers' HEC-1 and HEC-HMS hydrologic modeling software programs, the SCS TR-20 hydrologic modeling software program, and the SCS TR-55 Urban Hydrology for Small Watersheds method. See also EPG 749.9 Flood Hydrographs.

Use of these alternate methods should be limited to situations where the methods given above are deemed inappropriate or inadequate.

750.3.2.4 Hydraulic Analysis of Bridges

The Corps of Engineers Hydrologic Engineering Center's River Analysis System (HEC-RAS) shall be used to develop water surface profile models for the hydraulic analysis of bridges. Documentation on the use of HEC-RAS is available in references (6), (7), and (8).

Hydraulic design of bridges requires analysis of both the "natural conditions" and the "proposed conditions" at the site to show that the structure meets Backwater requirements and National Flood Insurance Program Requirements. It is also necessary to analyze the "existing conditions", when replacing an existing structure, see EPG 748.5 Matching Existing Structures.

For these reasons, water surface profile models for bridges shall be developed for three conditions:

  • Natural conditions - Includes natural channel and floodplain, including all modifications made by others, but without MoDOT structures
  • Existing conditions - Includes natural conditions and existing MoDOT structure(s)
  • Proposed conditions - Includes natural conditions, existing MoDOT structures if they are to remain in place, and proposed MoDOT structure(s)

Backwater from another stream is determined by comparing the water surface elevations upstream of the structure for either existing conditions or proposed conditions to the corresponding water surface elevation for the natural conditions.

For bridges near a confluence with a larger stream downstream of the site, additional models may be required. The water surface profile and resulting backwater should be evaluated both with and without backwater from the larger stream. The higher backwater resulting from the proposed structure shall be considered to control.

The hydraulic model in HEC-RAS is based on an assumption of one-dimensional flow. If site conditions impose highly two-dimensional flow characteristics (i.e. a major bend in the stream just upstream or downstream of the bridge, very wide floodplains constricted through a small bridge opening, etc.), the adequacy of these models should be considered. A two-dimensional model may be necessary in extreme situations.

750.3.2.4.1 Normal Water Surface Elevation

The normal water surface elevation is the elevation of the water surface across the flood plain without MoDOT bridges, culverts, or roadway fills in place. This elevation may be obtained using the slope-area method or from a "natural conditions" water surface profile.

750.3.2.4.2 Slope-Area Method

The slope-area method applies Manning's equation to a natural valley cross-section to determine stage for a given discharge. Manning's equation is given as:

Q=\frac{1.486}{n}AR^\frac{2}{3}S_o^\frac{1}{2}
where:
Q = Discharge (cfs)
n = Manning's roughness coefficient
A = Cross-sectional area (ft2)
R = Hydraulic radius = A/P
P = Wetted perimeter (ft)
So = Hydraulic gradient (ft/ft)

In order to apply Manning's equation to a natural cross-section, the cross-section must be divided into sub-sections. The cross-section should be divided at abrupt changes in geometry and at changes in roughness characteristics.

For a given water surface elevation, the discharge can be determined directly from Manning's equation. Determination of the water surface elevation for a given discharge requires an iterative procedure.

The slope-area method should not be used with the roadway centerline valley cross-section to determine the normal water surface elevation when the centerline cross-section is not representative of the stream reach, such as when the new alignment follows or is very near the existing alignment. The centerline cross-section should also not be used when the centerline cross-section is not taken perpendicular to the direction of flow, such as when the alignment is skewed to the direction of flow or is on a horizontal curve. In these cases, an upstream or downstream valley cross-section should be used to determine the normal water surface elevation. The water surface elevation for an upstream or downstream valley cross-section can be translated to the roadway centerline by subtracting or adding, respectively, the hydraulic gradient multiplied by the distance along the stream channel from the valley cross-section to the roadway centerline.

A computer program is available to assist in making the slope-area calculations.

750.3.2.4.3 Roughness Coefficients

Roughness coefficients (Manning's "n") are selected by careful observation of the stream and floodplain characteristics. Proper selection of roughness coefficients is very significant to the accuracy of computed water surface profiles. The roughness coefficient depends on a number of factors including surface roughness, vegetation, channel irregularity, and depth of flow. It should be noted that the discharge in Manning's equation is inversely proportional to the roughness coefficient (e.g., a 10% decrease in roughness coefficient will result in a 10% increase in the discharge for a given water surface elevation).

It is extremely important that roughness coefficients in overbank areas be carefully selected to represent the effective flow in those areas. There is a general tendency to overestimate the amount of flow occurring in overbank areas, particularly in broad, flat floodplains. Increasing the roughness coefficients on overbanks will increase the proportion of flow in the channel, with a corresponding decrease in the proportion of flow on the overbanks.

References (9), (10), and (11) provide guidance on the selection of roughness coefficients.

750.3.2.4.4 Hydraulic Gradient (Streambed Slope)

The hydraulic gradient (So) is the slope of the water surface in the vicinity of the structure. It is generally assumed equal to the slope of the streambed in the vicinity of the structure. Note that the hydraulic gradient is typically much smaller than the valley slope used in the USGS regression equations. Hydraulic gradient is a localized slope, while valley slope is the average slope of the entire drainage basin.

Hydraulic gradient is determined by one of two methods, depending on drainage area:

  • For drainage areas less than 10 mi2, the gradient is determined by fitting a slope to the streambed profile given on the bridge survey.
  • For drainage areas greater than 10 mi2, the gradient is determined from USGS 7.5 minute topographic maps by measuring the distance along the stream between the nearest upstream and downstream contour crossings of the stream. The hydraulic gradient is then given by the vertical distance between contours divided by the distance along the stream between contours.

750.3.2.4.5 Overtopping Discharge and Frequency

The overtopping flood frequency of the stream crossing system - roadway and bridge - shall be determined if the overtopping frequency is less than 500-years. An approximate method of determining the overtopping discharge uses the slope-area method given above and setting the stage to the elevation of the lowest point in the roadway. A more accurate method involves using a trial-and-error procedure, adjusting the discharge in the HEC-RAS proposed conditions model until flow just begins to overtop the roadway. The overtopping frequency can then be estimated by linear interpolation from previously developed discharge-frequency data.

750.3.2.4.6 Waterway Enlargement

There are situations where roadway and structural constraints dictate the vertical positioning of a bridge and result in small vertical clearances between the low chord and the ground. In these cases, significant increases in span length provide small increases in effective waterway opening. It is possible to improve the effective waterway area by excavating a flood channel through the reach affecting the hydraulic performance of the bridge. This is accomplished by excavating material from the overbanks as shown in the figure below; enlargement of the channel itself is avoided where possible as excavation below ordinary high water is subject to 404 permit requirements.

A similar action may be taken to compensate for increases in water surface elevations caused by bridge piers in a floodway.

There are, however, several factors that must be accommodated when this action is taken.

  • The flow line of the flood channel must be set above the ordinary high water elevation.
  • The flood channel must extend far enough upstream and downstream of the bridge to establish the desired flow regime through the affected reach.
  • Stabilization of the flood channel to prevent erosion and scour should be considered.

750.3.2.5 Scour Analysis

Current methods of analyzing scour depths are based mainly on laboratory experiment rather than on practical field data. The results should be carefully reviewed and engineering judgment used to determine their applicability to actual field conditions.

HEC-RAS includes the ability to calculate scour depths. The methods used to calculate those depths are based on the FHWA HEC-18 publication. Additional methods for abutment scour, scour in cohesive soils, scour in rock, and for scour in coarse bed streams not found in HEC-RAS are available in HEC-18. Rock scour information in HEC-18 is based on NCHRP Report 717.

750.3.2.5.1 Long Term Profile Changes - Aggradation and Degradation

Long term profile changes result from aggradation or degradation in the stream reach over time. Aggradation involves the deposition of sediment eroded from the channel and banks upstream of the site. Degradation involves the lowering or scouring of a streambed as material is removed from the streambed and is due to a deficit in sediment supply upstream. Aggradation and degradation are generally the result of changes in the energy gradient of the stream.

Aggradation and degradation over the life of a structure are difficult to predict. These long term profile changes are typically the result of human activities within the watershed including dams and reservoirs, changes in land use, gravel mining and other operations. HEC-18 and HEC-20 provide more information on predicting long term profile changes. Comparison of channel bottom elevations shown on plans for existing bridges to current survey data may be informative.

750.3.2.5.2 Contraction Scour

Contraction scour is generally caused by a reduction in flow area, such as encroachment on the floodplain by highway approaches at a bridge. Increased velocities and increased shear stress in the contracted reach result in transport of bed material. Contraction scour typically occurs during the rising stage of a flood event; as the flood recedes, bed material may be deposited back into the scour hole, leaving no evidence of the ultimate scour depth.

Contraction scour may be one of two types: live-bed contraction scour or clear water contraction scour. Live-bed scour occurs when the stream is transporting bed material into the contracted section from the reach just upstream of the contraction. Clear-water scour occurs when the stream is not transporting bed material into the contracted section. The type of contraction scour is determined by comparing the average velocity of flow in the channel or overbank area upstream of the bridge opening to the critical velocity for beginning of motion of bed material.

Live-Bed Contraction Scour - Live-bed contraction scour depths can be determined using the equations in FHWA HEC-18.

Clear Water Contraction Scour - Clear-water contraction scour depths can be determined using the equations in FHWA HEC-18.

750.3.2.5.3 Local Scour

Local scour involves removal of material from around piers, abutments and embankments and is caused by increased velocities and vortices induced by the obstruction to flow. As with contraction scour, bed material may be deposited back into the scour holes as floodwaters recede.

Pier scour and abutment scour are considered two distinct types of local scour.

Pier Scour - Pier scour depths can be determined using the equation found in FHWA HEC-18.

The HEC-18 Pier Scour Equation is based on the Colorado State University (CSU) equation. The equation is best suited for non-cohesive soils, but has been used for cohesive soils. Additional equations are provided in HEC-18 for cohesive soils, coarse bed materials and erodible rock.

The pier scour depth is limited to a maximum of 2.4 times the pier width for Froude numbers less than or equal to 0.8, and a maximum of 3.0 times the pier width for Froude numbers greater than 0.8.

The pier width used in the equations is that projected normal to the direction of flow. Piers should be skewed to minimize this width. The effect of debris should be considered in evaluating pier scour by considering the width of accumulated debris in determining the pier width used in the above equations.

For multiple columns with a spacing of 5 diameters or more, the total pier scour is limited to a maximum of 1.2 times the scour depth calculated for a single column. For multiple columns spaced less than 5 diameters apart, a "composite" pier width that is the total projected width normal to the angle of attack of flow should be used. For example, for three 6 ft. diameter columns spaced at 25 ft. apart, the pier width is somewhere between 6 ft. and 18 ft. (three times six feet), depending on the angle of attack.

Top width of pier scour holes, measured from the pier to the outer edge of the scour hole, can be estimated as 2.0 x the scour hole depth, ys.

Abutment Scour - Abutment scour depths can be determined using the equations found in FHWA HEC-18.

The first equation, is the Froelich Abutment Scour Equation.

The HIRE Abutment Scour Equation, is recommended when the ratio of projected abutment length, L', to flow depth, ya, is greater than 25.

HEC-18 also provides another approach to calculating abutment scour which is based on NCHRP Report 24-20.

750.3.2.5.4 Rock Scour

Since scour in rock is mainly an issue for spread footings, and the number of spread footings used in or near streambeds is limited, use the following procedure to request rock scour parameters to prevent unnecessary geotechnical sampling and testing.

a) Spread footings should be specified on the request for soil properties form when soundings (borings) are requested. By default for spread footings and retaining walls, rock erodibility is automatically checked for investigation of rock parameters for scour by the Geotechnical Section.
b) The foundation investigation geotechnical report should provide erodibility index numbers and indicate if scour numbers will be provided for further recommended evaluation. Alternatively, if rock scour is determined to not be a concern it should be noted on the report.
c) If spread footing option requires further evaluation, Bridge Division should follow-up with the Geotechnical Section and request scour numbers when they are scheduled to be available. The testing used to determine scour numbers may require 2 weeks.

750.3.2.5.5 Total Scour

All the above types of scour are considered in determining proper depth of bridge foundations. The total scour is obtained by adding the individual scour components.

Provide justification if scour analysis is not performed (slope protection can eliminate the need for abutment scour calculations, etc.)

750.3.3 Documentation of Hydraulic Design

Documentation is viewed as the record of reasonable and prudent design analysis based on the best available technology. Documentation should be an on-going process throughout the design and life of the structure.

Proper documentation achieves the following:

  • Protects MoDOT and the designer by proving that reasonable and prudent practices were used (be careful to state uncertainties in less than specific terms)
  • Identifies site conditions at time of design
  • Documents that practices used were commensurate with the perceived site importance and flood hazard
  • Provides continuous site history to facilitate future construction

At a minimum, the following documentation of the hydraulic design is to be archived:

  • Bridge Survey Report form, associated plan and profile sheets
  • Bridge Hydraulics and Scour Report or Culvert Hydraulics Report
  • Any computation sheets used in the hydrologic and hydraulic analyses
  • Program input/output files from water surface profile model(s), HY-8, HEC-RAS or other computer programs.
  • Input and output data. Computer program input files must be reproducible from the data provided.
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