7.8 Selecting Tailwater Elevations without Downstream Profile Data

Designing a culvert without first performing a backwater analysis to establish a reasonable tailwater elevation is not recommended. The most defensible method of design is to start well downstream of the culvert site and use a water surface profile analysis to develop a tailwater rating curve at the culvert for a full range of discharges. Because one or more profiles will be computed, it is easy to obtain a tailwater value when using HEC-RAS. But what procedure should be used when no downstream profiles are being computed for the culvert analysis? If the funding is unavailable to obtain the necessary cross sections and operate HEC-RAS, the engineer must fall back on less desirable methods to determine a design tailwater elevation. These choices include the following:

Locate past reports that include the site, such as a flood insurance study, to provide flood elevation and discharge information that can be used for the culvert design.
Use a highwater mark from a past flood at the culvert site.
Use the highest value found for the tailwater from the following three options:
Highest channel bank elevation at the culvert site.
The elevation corresponding to the top of the culvert.
After computing a normal depth rating curve for the full downstream cross section at the culvert site, use the elevation corresponding to the design discharge for the culvert.

There may be situations for which the use of a sophisticated, data-intensive tool such as HEC-RAS in a potentially low-risk situation, such as a small culvert through a low embankment, may not be justified. However, the engineer should consider the risks inherent in a simplified tailwater decision, especially as a worst-case scenario.

If the risk of structure overtopping is significant and potential damages from this occurrence are high, a tailwater elevation should be developed with HEC-RAS or a similar tool. Obtaining necessary cross-section data and using HEC-RAS may be much cheaper than greatly increasing the culvert capacity to pass the design discharge for a chosen tailwater elevation through one of the simplified methods listed above. The engineer should feel certain that a defensible tool was used to develop the design tailwater elevation for the particular situation.