The Slope Conveyance Method, or Slope Area Method, has the advantages of being a relatively simple, usually inexpensive and expedient procedure. However, due to the assumptions necessary for its use, its reliability is often low. The results are highly sensitive to both the longitudinal slope and roughness coefficients that are subjectively assigned. This method is often sufficient for determining tailwater (TW) depth at non-bridge class culvert outlets and storm drain outlets.

The procedure involves an iterative development of calculated discharges associated with assumed water surface elevations in a typical section. The series of assumed water surface elevations and associated discharges comprise the stage-discharge relationship. When stream gauge information exists, a measured relationship (usually termed a “rating curve”) may be available.

You normally apply the Slope Conveyance Method to relatively small stream crossings or those in which no unusual flow characteristics are anticipated. The reliability of the results depends on accuracy of the supporting data, appropriateness of the parameter assignments (n-values and longitudinal slopes), and your selection of the typical cross section.

If the crossing is a more important one, or if there are unusual flow characteristics, use some other procedure such as the .

A channel cross section and associated roughness and slope data considered typical of the stream reach are required for this analysis. A typical section is one that represents the average characteristics of the stream near the point of interest. While not absolutely necessary, this cross section should be located downstream from the proposed drainage facility site. The closer to the proposed site a typical cross section is taken, the less error in the final water surface elevation.

You should locate a typical cross section for the analysis. If you cannot find such a cross section, then you should use a “control” cross section (also downstream). (Known hydraulic conditions, such as sluice gates or weirs exist in a control cross section.) The depth of flow in a control cross section is controlled by a constriction of the channel, a damming effect across the channel, or possibly an area with extreme roughness coefficients.

The cross section should be normal to the direction of stream flow under flood conditions.

After identifying the cross section, apply Manning’s (n-values). (See and Chapter 6 for more information.) Divide the cross section with vertical boundaries at significant changes in cross-section shape or at changes in vegetation cover and roughness components. (See Chapter 6 for suggestions on .)

Manning’s Equation for Uniform Flow (see Chapter 6 and Equation 6-3) is based on the slope of the energy grade line, which often corresponds to the average slope of the channel bed. However, some reaches of stream may have an energy gradient quite different from the bed slope during flood flow.

Determine the average bed slope near the site. Usually, the least expensive and most expedient method of slope-determination is to survey and analyze the bed profile for some distance in a stream reach. Alternately, you may use topographic maps, although they are usually less accurate.