A culvert is a conduit that conveys flow through the embankment. The most commonly used shapes are circular, rectangular, elliptical, pipe arch and arches. They range in size from large multiple barrel box culverts and metal arch structures to single 18 inch pipes. The design process for culverts as well as all drainage structures is much like the bridge crossing in that it involves: data collection, hydrologic analysis, formulation, evaluation and selection of an alternate, and documentation of the design. Some of the larger structures must be analyzed by the same procedures and methods as a bridge crossing. The procedure presented here is summary in nature and is intended for the common box or pipe culvert crossing. The extent of design effort for a particular culvert must be commensurate to its cost and potential risk to the public. The engineer should reference FHWA, Hydraulic Design Series No. 5 (15), for more detailed guidance. He must also reference this document for nomograph charts and tables required for a manual design process.

The forms used for documentation and the information required differ for box and pipe size culverts. Any culvert structure providing conveyance greater than a single 72 inch pipe will follow the design procedure and documentation on the "Culvert Survey and Hydraulic Design report"(Appendix F). Smaller culvert design will be documented on a pipe data sheet (Appendix G).

(1) Data Collection Information gathered during the pre-design study and field survey relative to each particular crossing or all crossings in general is to be assembled. This process will include:

(a) For all box culverts or any other structure that preliminary estimates indicate requiring a total crossing conveyance greater than a single 72 inch pipe, plot a plan and profile view of the stream crossing on the "Culvert Survey and Hydraulic Design Report" (Appendix F) 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 is to be 1 inch = 50 feet horizontal and 1 inch = 10 feet vertical. Existing features are to be in ink with manmade features shown with dashed lines. This information is to be limited to that which is pertinent to the structure sizing and location. Information to be provided on the profile view:

(1) There are to be two profiles - one along the centerline of the roadway showing the flood plane section and roadway profile both existing and proposed. The second profile is to be along the centerline of the structure showing the stream bed grade, top of bank and normal water surface profile.

(2) The centerline of the roadway profile should show: ground line, channel base and banks, grade line, water surface elevations (date of survey, normal if different), flood plain limits, historical flood elevations (including date of occurrence, and estimated frequency), utility elevations, controlling backwater feature elevations (building floor levels, yards, cultivated fields, roadways, drives, other drainage structures, overtopping controls), general classification of stream bed and bank materials (clay, silt, sand, gravel, cobble, rock), plot rock line if identified

(3) The centerline of structure should show: stream bed, top of bank, existing and proposed roadway cross-section, normal water surface profile, historical flood levels, controlling feature elevations properly positioned along the profile, rock line if identified.

(4) Any additional stream cross-sections utilized for design or needed for structural excavation estimates are to be plotted on the survey report. The drawing scale for these sections can be adjusted as needed to fit the report.

Information to be provided on the plan view:

(1) Natural features - stream channel showing base and banks, limits of the floodplain

(2) Type of cover

(3) Manmade features -buildings, houses, highways, existing drainage structures, utilities

(4) The proposed roadway section and fill slope limits

(b) For 72 inch pipe size and smaller, the site data will be summarized on the pipe data sheet. The engineer will also need to reference the drainage plans for topographical and proposed layout information.

(2) Hydrologic Analysis - There are four discharge levels that must be evaluated for each culvert design. These are:

(a) A "design discharge" as listed and defined in the hydrology section (Table 4-3,Chapter VI)

(b) Q₁₀₀ base flood

(c) Q-overtopping. This discharge is computed after a trial size is selected.

(d) Q₁₀ for outlet protection and erosion control measures.

Other discharges may be required on a site specific basis. Examples are:

(a) Q-average - for permit determination

(b) Q-bank full - for fish passage, channel stability or floodplain analysis.

(3) Hydraulic Design

(a) The first step in hydraulically analyzing a culvert is to address criteria and information that must be quantified prior to commencing actual structural sizing and location. This would include:

Material Selection

A material selection recommendation must be provided for each pipe culvert. The general selection policy is as follows.

Culvert pipe shall be concrete with the following exceptions :

• The expected fill height over the structure exceeds the maximum values for concrete as provided in the N.C. Division of Highways charts,(Appendix H)
• The required invert slope is greater than 10%.
• If a majority of the installations for a project require metal, then all culvert pipe for the project can be metal.

Other site or project specific factors such as, corrosive conditions, accessibility, environmental requirements, handling and initial cost may dictate the use of a particular material.

Box culverts are generally cast in place or precast concrete. There are large metal structures, arches and box shapes, with and without bottom plates, that can be considered for sites requiring large openings and/or spans. The primary source of information on available sizes and structural details is the manufactures literature.

Appendix H provides gage requirements and fill limitations for metal and concrete structures.

[LINK TO APPENDIX H,SHEETS 1,2,3,4,5,6,7,8,9,10,11]

End Treatment

Headwalls are generally used on the inlet end of pipe culverts 36 inch or larger. The outlet end does not require a headwall unless site specific conditions such as right-of-way limitation warrant placement of an outlet headwall. For guidance on end treatment of parallel pipes, reference section 5-20, of the Roadway Design Manual (16).

Allowable Headwater

The allowable headwater elevation is established based on an evaluation of natural flooding depths, upstream structures and land use, as well as the proposed roadway elevations.

Multiple Openings (width)

When the width of the structure opening is significantly wider than the natural channel, an evaluation must be made of the affect on flow capacity which will occur when the low flow area is restricted to its natural width by artificial or natural means.

Alignment

As near as is practicable, a culvert should intercept an outlet flow within the natural channel. When channel realignment is required, a natural channel design should be utilized (see section X).

Length and Slope

The slope of a culvert should approximate that of the natural channel. The invert elevation should be slightly below the natural bed ranging from 0.1 +/- feet for small pipes to 1.0 +/- feet for large box culvert. Where fish passage is a consideration, the invert should be a minimum of 1.0 feet below the natural bed. Baffles may be placed in the invert to promote retention of bed material and formation of a low flow channel. When a shallow (3-5 foot max. depth) non-erosive rock foundation is found throughout the proposed site, the structure can be built on footings without a bottom allowing retention of the natural channel bed. The Geotechnical Unit must confirm the foundation acceptability prior to final selection of the "bottomless" culvert.

Potential channel cleanout and improvements should also be considered particularly in the coastal plain. The length is established by the geometry of the roadway embankment, the bed elevation and skew.

Tailwater

The computed normal channel depth for each discharge level being evaluated generally establishes the tailwater. This can be determined by a simple single section analysis. Effects of downstream controls and constrictions must also be considered.

Debris

The structure opening should be reasonably sized to provide for debris. The limitation of structural height to headwater depths in the HW/D = 1.2+/- range has proven to limit problems of this nature to acceptable levels. Where experience or physical evidence indicates the water course will transport a greater than normal size or volume of debris, special debris controls should be developed and/or the estimated capacity of the structure reduced to reflect the potential for blockage.

(b)A trial size culvert can be determined using the design discharge, inlet control nomographs (HDS-5 ref.- 12)and an assumed HW/D = 1.2. Multiple openings may be selected by dividing the discharge.

(c)When a trial size selection is reasonable in regard to available sizes (see Appendix H) and allowable headwater limitations, the full inlet/outlet control analysis is performed. The higher of the computed headwaters governs.

(d)If the analyzed size is acceptable in regard to controls and criteria relative to the design discharge, verify it being the minimum acceptable by checking the performance of a smaller structure.

(e)If inlet control governs, improved inlet design must be investigated. This will be performed for all inlet control box culverts and for pipe culverts 36 inch and larger with lengths > 150 ft. If as much as one nominal size reduction can be achieved for box culverts, the improved inlet option can be selected. For pipe culverts, an economic analysis is required to justify the selected option.

(f) Determine the design values and acceptability of the selected culvert for the Q₁₀₀ and overtopping flood.

(g)Outlet velocities shall be determined for the Q₁₀ discharge. If this velocity exceeds the scour velocity for the receiving stream, rip rap outlet protection is required.

(1) See channel chapter for permissible velocity guidelines

(2) Use whichever is greater, tailwater depth or normal flow depth for culvert to determine outlet velocity.

(4) Design Documentation

All information pertinent to the culvert design shall be documented on either the "Culvert Survey and Hydraulic Design Report"; or the "Pipe Data Sheet". This will include:

(a) For box culverts, plot the proposed structure in plan and profile views. Note centerline station and skew. Show invert elevations and skew, or top of footing elevations.

(b) Show design water surface elevation on all views.

(c) Complete fill-in of data for selected structure on report or data sheet.

(d) If design is accomplished by computer program, private engineering firms must submit data file summaries on an IBM compatible disk.

(e) For large culverts (>72 inch), a plot of the performance curve for the selected structure with a plot of the natural stage-discharge relations is desirable.

(f) Provide stream classification.