The purpose of this section is to provide a basic overview of the hydrologic and hydraulic design requirements necessary for the design if highway culverts. This section is not intended for storm sewer design, although a lot of the procedures are applicable to both highway culverts and storm sewers. The materials contained in this section should provide the designer with the necessary tools to perform simple culvert designs, but the designer is highly encouraged to obtain copies of additional design materials, such as the Federal Highway Administration "Hydraulic Design of Highway Culverts", (HDS5) or the Minnesota Department of Transportation's Drainage Manual.

The methodology of culvert design presented in this section is consistent with the procedures described in the publications mentioned above. It is intended to be used by engineers with a good understanding of basic hydrology and hydraulic analysis. Inexperienced designers are encouraged to obtain additional instruction before embarking on real life projects.

Numerous computer programs are available to assist the designer with both the hydrologic and hydraulic analyzes. While the use of these programs can relieve the designer from considerable tedious hand calculations, it is recommended that the designer acquire a good understanding of the design concepts and analysis procedures, along with some experience using the manual techniques, before utilizing the computer programs.

Site Data Requirements

Field survey and site data acquisition plays a key role in hydrologic and hydraulic analysis for culvert design. Good field data is necessary to accurately determine tailwater elevations, roadway overflow capacity, flood damage potential, existing structure capacity, flood frequency relationships and a host of other items. Small culvert projects do not generally require as much information as larger projects, as the cost of the project doesn't warrant the expense of the data acquisition. A list of site data requirements for larger culverts projects is as follows:

1. Profile of channel bottom (thalweg) and water surface elevation for approximately 1000 feet upstream and downstream of site. If the slope is very flat or the channel bottom is irregular, a greater distance may be required to establish an average profile. With a steep slope, a shorter distance is acceptable.
2. A typical cross section of the channel perpendicular to the flow and extended on both sides of the flood plain to several feet above high water. If the channel andflood plain are relatively uniform, one cross section upstream and one downstream is sufficient. If the channel section varies considerably, several cross sections should be obtained.
3. Existing structure data, including clear span, low member elevation and invert elevations. A stream cross section at the face of the structure is desirable, and if the opening is not uniform, one at both the upstream and downstream face is desirable.
4. Pictures of the upstream and especially downstream channel and flood plain are very helpful for determining the Manning roughness coefficient. If no pictures are available, a good description of the channel, such as meandering, brush on banks, rocks in bottom, some trees in flood plain, pasture, etc., is very helpful.
5. Observed highwater, preferably obtained from a local resident or maintenance personnel. An indication of the period of observation is also desirable, i.e. "Highest water in 30 years". Stains or marks on the structure normally are not extreme highwater. The date of observed highwater, location of observation, cause (heavy rain, spring snow melt, ice or debris), and upstream and downstream measurements are also desirable.
6. Elevation of buildings or anything else, in or adjacent to the upstream flood plain, that could be damaged by temporary highwater should be noted.
7. If there are any structures downstream such as dams, bridges or culverts that could cause backwater at the study site, the size and elevation of these structures, along with observed highwater should be obtained. If overtopping is anticipated, a roadway profile should should be obtained, or as a minimum the roadway sag point should be obtained. Existing roadway profile, extended both ways from the site to at least one foot above the low point or to observed highwater, whichever is greater.
8. Other information of interest:
   
        a) Present and projected traffic counts
        b) Upstream and downstream structures
        c) Location maps
        d) Boating requirements
        e) Fish passage requirements (i.e. trout stream)
        f) Scour information
        g) Debris potential - ice, logs, etc.
        h) Description of channel bed material

A hydrologic analysis is required for each project to determine design flows. This analysis is one of the most important aspects of culvert design and is a prerequisite to the hydraulic analysis. Due to limited historical flow data and the ever changing conditions which affect rainfall and runoff, the prediction of flow frequency relationships are not as accurate as the hydraulic analysis.

The designer must decide whether to use peak flow rates or hydrographs. Peak flow rates are generally adequate for highway culverts and storm drains. However, where flood routing is necessary, such as when considerable ponding is anticipated or peak flow rates are being regulated, hydrographs are usually required. Because of the extremely tedious nature of hydrograph development, computer programs are usually incorporated.

Recommended methods for determining peak flow rates, listed in order of preference, are as follows:

- Gaging Station Records
- Flood Insurance Studies
- USGS Regression Equations
- Rational Method

Gaging station records for a site should almost always be used provided the period of record is sufficient for the desired design frequency. At least 10 years of record should be available for predicting the 10 year flood and at least 25 years for predicting the 100 year event. The Log Pearson Type iii method of analysis is the accepted method to analyze the stream flow measurements.

Flood Insurance Studies should be incorporated when available. The National Flood Insurance Program, NFIP, has compiled detailed flood data for numerous communities throughout the nation. If this data is available it should be utilized in your study. The Minnesota Department of Natural Resources, DNR, requires adherence to NFIP data for all studies involving DNR protected waters. In some situations where the study data is rather old, you may be able to justify not using the published data.

USGS Regression Techniques for determining peak flows have been developed for most of the states in the country. These equations should be used for most routine designs unless there are gaging station records or flood insurance study data available. The USGS first developed regression techniques for estimating peak runoff rates for Minnesota in 1977, and has released revised techniques again in 1987 and 1997. As aAs a general rule, the most recent technique should be utilized. Use of these techniques are rather straight forward and are detailed in the USGS Manual.

Rational Method of determination of peak discharges has been around for some time and is most applicable for drainage areas of less than 200 acres. Results from the Rational Method are often more reasonable than the USGS regression methods when applied to small drainage areas. The Rational Equation is:

Q = CIA

Where:
Q = discharge, cfs
C = runoff coefficient
I = rainfall intensity, in/hr
A = drainage area, acres

Charts for use in determining the time of concentration, rainfall intensity and runoff coefficients can be obtained from many of the sources listed in the references.

Hydrographic Methods recommended are as follows:

- SCS Unit Hydrograph, TR20 and TR55
- HEC-1

A detailed discussion of hydrographic methods is far too involved for discussion in this section. The designer is directed to SCS, now called NRCS, the Natural Resources Conservation Service, for more detailed information on the SCS method of hydrograph generation. The Army Corps of Engineers HEC-1 computer has detailed documentation which is also valuable.

Hydrologic Results

Whatever method or methods of hydrologic analysis are used, the designer should compare the predicted rates of runoff with historical flood events. All of the hydrologic methods utilize a combination of basin characteristics and historical data, be it rain fall events or stream flow measurements to determine rates of runoff. Due to the high degree of variability in these characteristics and the limited length of historical records, the accuracy of the resulting predictions are not as good as we would like. Therefore, back-calculating flow rates from observed highwater observations can be extremely valuable. For instance, if you back-calculate the flow rate for an observed highwater which occurred from a rainfall estimated to be approximately a 100 year event and the computed flow is closer to the predicted a 10 year flow, you know something is wrong. Numerous things could be wrong in either the back-calculations or the flow-frequency predictions. A close agreement between the two calculations is always reassuring, where as major disagreement indicates a need for further study.

Design Frequency

By definition, a design flood does not overtop the roadway. The selected design frequency should take into consideration traffic volume, risks due to flooding, geometric factors, project costs and agency regulations. Consideration should also be given to availability of a practical detour route. Higher design frequencies should be utilized at locations without available detours or with long detours. Most states have guidelines for selecting design frequencies, but these guidelines are all influenced by the considerations listed above. The Minnesota Department of Transportation, MNDOT, uses a risk assessment procedure to select design frequencies for highway culverts along with the following general guideline:

Due to the difficulty in applying the risk assessment procedure to small culverts, MNDOT has adopted a 50 year design frequency for minor centerline culverts.

Culvert Hydraulics

An exact analysis of the hydraulics of a particular culvert is extremely complex and time-consuming. Flow control may change with the flow rate and tailwater elevation changes, the culvert may change from full flow to partly full and hydraulic jumps may occur within or downstream from the culvert barrel. A detailed discussion of all of these conditions is beyond the scope of this section. This section will focus on location of control and determination of headwater elevations and outlet velocities.

The upstream headwater elevation for a culvert is either controlled by the inlet capacity of the culvert or the tailwater condition and associated energy losses. With inlet control, the capacity of the barrel is greater than the capacity of the inlet, hence, inlet control. With outlet control, the capacity of the inlet is greater than the capacity of the outlet, hence, outlet control. Since it is usually not obvious whether the culvert is operating in inlet or outlet control, both conditions are generally calculated, with the controlling headwater being the higher of the two.

Tailwater Determination

Tailwater elevations used in culvert hydraulic calculations are generally obtained using the slope area method of analysis (normal depth) with a single downstream channel section. For sensitive sites, a more accurate analysis using step backwater methods employed in computer programs such as HEC2 and WSPRO may be more applicable. The designer is also cautioned to check for backwater effects from downstream structures which may influence tailwater elevations. To assist in selection of appropriate "n" values for use with the Manning equation, a table with channel descriptions is included in the appendix.

Inlet Control

For inlet control, the control section is located just inside the entrance of the barrel and is governed by the inlet geometry, barrel shape, area, and inlet edge. Inlet control is classified as being either unsubmerged, transition or submerged. When the headwater is below the inlet crown, the culvert operates as a weir and is classified as unsubmerged. When the headwater is greater than about 1.2 times the culvert height, the culvert operates as an orifice and is classified as submerged. Transition flow occurs when the headwater depth is between 1.0 and 1.2 times the culvert height. Headwater depths for inlet control can be readily calculated using nomographs contained in the appendix. Additional nomomgraphs for a variety of other shapes and sizes are available from HDS-5 and the MnDOT Drainage Manual.

Outlet Control

For outlet control, the control section is located at or below the culvert outlet. All of the hydraulic characteristics of the culvert contribute to the culvert capacity. These include, culvert slope, roughness, barrel length, tailwater elevation and inlet configuration. Full flow conditions are assumed when the tailwater depth is greater than approximately 95% of the culvert height. The outlet control nomographs located in the appendix are applicable to full flow conditions. To accurately analyze partial flow conditions, it is necessary to use step backwater calculations to determine flow depths within the pipe. An alternative to the step backwater method is to use the approximate methods presented in the appendix. Used with care, the approximate methods will provide results adequate for most situations.

Roadway Overtopping

When headwater elevations exceed the low point on the roadway, a portion of the flow will pass over the roadway as well as through the culverts. Flow over the roadway is calculated using broad crested weir formulas and submergence factors as shown in the appendix. When roadway overtopping occurs, headwater determination is an iterative process. The basic concept behind the analysis is that the headwater for the culvert and the roadway must be the same and the combined flows must equal the total design flow. The solution usually involves assuming a trial headwater elevation and calculating the flow for the culvert and the roadway using that trial headwater and comparing the total flow to the design flow. Subsequent trial headwaters are assumed until the total combined flow matches the design flow. This is all determined within an instant by mother nature and almost as fast using modern computer programs. This is one area where use of the computer really pays off.

Computer Programs

There are numerous computer programs which can aid the designer considerably. The most common and most applicable for designing highway culverts are as follows:

- HY8 - FHWA- (culvert, roadway, tailwater, analysis etc.)
- WSPRO - FHWA/State DOT's - (step backwater, including culverts and roadways)
- HECRAS - Corps of Engineers - (step backwater, including culverts and roadways)
- CulvertMaster - Proprietary - (similar to HY8)
- HydroCad - Proprietary - (includes hydrology and culvert hydraulics)