The design of a stream crossing requires a comprehensive engineering approach that involves data collection, hydrologic analysis, formulation of alternatives, evaluation and selection of the "best" alternative according to established criteria, and documentation of the final design. The design process provided herein will not attempt to address all situations or all areas of knowledge and experience the engineer should possess to be proficient in crossing design. It is strongly recommend that the engineer reference and study the bridge crossing chapter of the "AASHTO-Highway Drainage Guidelines" (1), and the FHWA floodplain policy statement in FAP-Guide, 23 CFR 650A (9). The design procedure presented herein will insure a systematic process that will adequately address most crossing situations. It will also help to identify conditions and situations requiring special study and/or consideration.

Design and analysis of stream crossings in the coastal region that are subject to the effects of tidal flow and storm surge follow a similar procedure to that outlined for riverine crossings. However, there are major differences in the hydrologic and hydraulic analysis phases. The engineer is referenced to the basic tidal prism procedure contained inHEC-18(12), as well as more detailed one and two dimensional tidal crossing models presented in, Tidal Hydraulic Modeling for Bridges(13)

(1) DATA COLLECTION

Information gathered during the pre-design study and field survey is to be assembled for the study site. This process will include:

(a) Review of the preliminary design and assessment report (Appendix D)

(b) Plotting of a plan and profile view of the topographical features for the crossing on the Bridge Survey and Hydraulic Design Report (Appendix E) PAGE 1 of 3,PAGE 2 of 3,PAGE 3 of 3 (PDF format - see note below)

NOTE: These PDF files may give an initial error message when opened; however, they are viewable by zooming in. Printing may be problematic, but it is possible. Be sure the "shrink to fit" toggle is on, and be prepared to wait about 5 minutes for the printing to complete. The problem has something to do with the graphics copied from the Microstation File to the PDF file.

[Click here for Microstation *.DGN file.(contains both Bridge & Culvert Survey and Hydraulic Design Reports)]

The drawing scale shall be 1 in.= 50 ft. horizontal,1 in.= 10 ft. vertical with existing manmade features shown with dashed lines .A larger sheet may be used if required for wide floodplain. It must be trimmed and folded to fit within the Survey Report.

Information to be included on the profile view:

• Centerline profile of the floodplain
• Historical flood data (high water elevations, date of occurrence, and estimated frequency)
• Show existing features(utilities, drainage structures, and crown grade profile of existing highway)
• Control elevations such as existing buildings
• Water surface elevation at date of survey and "normal" water surface elevation

Information to be shown on plan view:

• Natural features (limits of floodplain, stream channel showing base and top of bank, type of vegetative cover in floodplain, stream classification)
• Existing man-made features in floodplain buildings, houses, highways, utilities, etc.)

(2) HYDROLOGIC ANALYSIS

This phase involves the development of a number of discharges on which the performance of alternate designs will be evaluated. This entails:

(a) Determination of a drainage area for the site

(b) Developing discharge quantities for a range of floods to be studied. This shall include as a minimum:

Q₂, Q₁₀, Q₂₅, Q₅₀, Q₁₀₀, Q-overtopping (existing roadway), Q-overtopping (proposed roadway)

(c) If a crossing is in a FEMA Regulated Flood Insurance Program site where a detail study has been performed, the study discharges will be used to evaluate conformity of the project to flood zone regulations. If an error is found in the FEMA hydrological data or if there is considerable disagreement in the data and results from standard hydrological procedures presented in this guideline, a specific course of action shall be developed and approved by the Reviewing Engineer.

(d) Document the hydrologic analysis portion of the Bridge Survey and Hydraulic Design Reports.

(3) FORMULATION OF ALTERNATIVES

This and the next phase, Alternative Evaluation and Selection, is generally an iterative process through which a hydraulic analysis is performed for one or more alternatives, the results are evaluated, adjustments are made and further alternatives developed until the "best" alternative is selected. This hydraulic analysis of alternatives will be accomplished as follows:

(a)The Corps of Engineers HEC-RAS Step-backwater Analysis Program is recommended for the stream reach study. An exception is to be made for utilization of the HEC-2 when an existing detailed flood study crossing is involved. FHWA-WSPRO is another acceptable model.

(b)A minimum of three cross-sections shall be used (one each up and downstream and one at the crossing). Additional sections should be used when site conditions warrant.

(c)A run of the model with the selected discharge shall be made under existing conditions and a comparison made to at least one historical occurrence.

(d)Adjustment shall be made to calibrate the model to a "best" or reasonable" fit to the historical data.

(e)FHWA "Guideline for Selecting Manning's Roughness Coefficients for Channels and Flood Plains" (9), should be referenced for roughness factor selection.

(f)A profile plot of the adjusted model including the historical data shall be provided.

(g)Alternate structures and grade configurations can now be entered for hydraulic output development.

(4) EVALUATION AND SELECTION

The selection of a "best" alternative is accomplished by comparison of the study results and considerations to acceptable limitations and controls. These limitations are prescribed by general and specific criteria.

General Criteria on which the design alternatives must be judged are:

• Backwater will not significantly increase flood damage to property upstream of the crossing.
• Velocities through the structure(s) will not damage the highway facility or unduly increase damages to adjacent property.
• Existing flow distribution is maintained to the extent practicable.
• Level of traffic service is compatible with that commonly expected of the class of highway and projected traffic volumes.
• Minimal disruption of ecosystems and values unique to the floodplain and stream.
• Cost for construction, maintenance and operation, including probable repair and reconstruction, and potential liabilities are affordable.
• Pier and abutment location, spacing, and orientation are such to minimize flow disruption, debris collection and scour.
• Proposal is consistent with the intent of the standards and criteria of the National Flood Insurance Program.

Specific criteria on which the design alternate must be judged:

(a)Design Discharge

This is the specific return period flood that has been established as being an acceptable level for roadway overtopping. When roadway overtopping is not involved, it will be the level of flood used for establishment of freeboard and/or backwater limitations. See Table 4-3, chapter VI, for desirable design discharge standards based on accepted inundation levels relative to roadway classification. Variation from these or other specific standard values must be justified by an assessment process which reflects consideration for risk of damages to the roadway facility and other properties, traffic interruption, environmental impacts and hazard to the public.

(b)Backwater

This is the increase in water surface elevation for a particular flood event measured relative to the normal water surface for this same event at the approach section. For National Flood Insurance Program designated floodplains, the backwater for the 100-year flood shall not exceed 1.0 foot. The normal water surface as it relates to a flood insurance site would include any restriction existing at the time of adoption of the regulation, such as an existing bridge. When a detail study area is involved, no increase in backwater is allowed when the crossing data is entered into the floodway model unless a modification proposal is developed and presented to the community and FEMA for approval. A modification proposal is to be a revision in the floodway boundaries to accommodate the crossing without increasing the 100-year flood elevation above the established floodway elevation.

(c)Minimum Length

The bridge ends will be located such that in the profile bridge section a line projection of the spill through slope face provides a minimum of 10 foot setback from any point on the channel bank or bed. Greater setback can be dictated by hydraulic conveyance needs and channel scour predictions.

(d)Freeboard

Provide 2 feet minimum clearance for bridge super- structures above the design flood for primary route structures and secondary crossings of major rivers.1.0 foot minimum for all other structures. There is no established freeboard for the roadway or other controlling features. However, this can be established as a project specific requirement if a specific need or condition warrants. This or a justified variance from the standard freeboard requirement must be approved by the Reviewing Engineer prior to completion of the design.

(e)Slope Protection

As a minimum class II stone rip rap shall be placed on the spill-through bridge slopes through the waterway opening extending to a point even with the bridge ends. The need for additional slope protection along the roadway fill approaches shall be evaluated on a site by site basis. Concentration, depth and velocity of flow in the overbank are factors to be considered in setting the rip rap limits. As a guide, the following equation can be used. If V₂ is considered to be less than a scourable velocity for the proposed fill slope, no further evaluation is necessary.

Z = (1-V₁/V₂)L

Where:

Z = Required distance of slope protection

V₁ = Average velocity in overbank approach

V₂ = Average velocity in bridge opening area adjacent to fill

L = Distance up stream to maximum backwater(bridge length)

The top of the rip rap elevation shall be 1.0 foot above the "design flood" which, for establishing slope protection limits, will not exceed the 50-year event.

(f)Deck Drainage

Standard practices for structural design at this time is to include 6 inch scupper drains at 12 foot centers in all waterway crossing structures. They can be eliminated in areas directly over channel when crossing identified sensitive streams. If review for variance from this standard is requested, the spacing requirement will be based on:

(1) Scupper capacity provided in HEC-21(11)

(2) 4 inches per hour rainfall intensity (maximum drivable)

(3) A minimum consideration of 30% blockage

(4) Maximum gutter spread of 2 feet.

Provision must be made to handle the flow from the bridge deck at all down grade ends. The capacity and adequacy of these drains can also be checked using the procedures of HEC-21(10).

Separation structures will have a very limited number of scuppers (adjacent to the piers). The potential gutter spread along the structure must be determined for acceptability. The acceptable spread is dependent on shoulder or special width provided on a structure, but should not extend into the travel lane of a shoulder approach structure. The few scupper drains can be ignored in this spread evaluation for separation structures. With the potential quantities of flow from the deck, it is very important to check the adequacy of the end drains and provide recommendations for additional measures when warranted.

(g)Channel Changes

As a general rule, the bridge crossing will be designed to accommodate the natural channel. Channel modification will be considered only when there is no practicable alternative from a cost or functional standpoint. Modification proposals with sufficient supportive data must be presented to the Reviewing Engineer for approval prior to completion of the design.

(h)Scour

An estimate of potential Scour depth is required for all bridge sites. The procedure for this analysis is presented in HEC-18, reference,(12). And HEC-20, reference(14)

(i)Economics

When more than one alternate will satisfy all control factors for a site, the evaluation and selection of a "best" alternate must include an economic analysis to insure that the selected alternate provides the least total cost from a construction, maintenance, and operation standpoint.

(j)Detour Bridges

The design process for these structures is also site specific. As general guidance a Q₅ design flood provides an acceptable level of risk for potential traffic interruption or damage to the detour. However, potential for damage to other developed properties if overtopping at this level of flood is not provided will warrant further consideration and a possible increase in the structure requirements. Spanning of the normal flow channel is recommended and scour consideration is limited to local scour at any in channel bents.