7.1 Terminology

Culvert analysis uses a number of terms to describe the different parts of the system, as illustrated in Figure 7.1. A few of these terms were introduced earlier in this book and are reviewed here.

Headwater elevation - The elevation of the energy grade line at the culvert entrance (section 3). This can also be considered equal to the water surface elevation at the culvert entrance if the velocity head is assumed negligible. Figure 7.1 shows separate water surface and energy grade line elevations upstream of the culvert, as would be computed by HEC-RAS, since the model does not ignore velocity head. The water surface elevation at section 3 is designated WS_U on the figure.

Headwater depth (HW) - The difference in elevation between the energy grade line just upstream of the culvert entrance (section 3) and the invert of the culvert entrance. Since the velocity head immediately upstream of the culvert entrance is normally quite small, the energy grade line elevation is often assumed to be equal to the water surface elevation. Therefore, the headwater depth is also assumed equal to the water surface elevation. This assumption gives a conservative (higher) solution for the headwater depth.

Tailwater elevation - The elevation of the water surface at the exit of the culvert. Figure 7.1 shows the tailwater elevation as WS_D.

Tailwater depth (TW) - The difference in elevation between the water surface elevation at the culvert exit and the invert of the culvert at the exit.

Entrance - The opening of the culvert at the upstream end.

Exit - The opening of the culvert at the downstream end.

Barrel - The body of the culvert that connects the entrance and exit.

Culvert invert - The lowest interior elevation of the culvert at any selected point. On Figure 7.1, the entrance and downstream exit inverts are designated ZB_U and ZB_D, respectively.

Outlet control, tailwater control, or exit control - Outlet control exists when the culvert entrance is capable of passing more discharge than the barrel can convey. The headwater elevation resulting from a certain discharge is a function of downstream conditions. The flow in the culvert is subcritical or pressure flow. Under outlet control, the tailwater elevation at section 2 (or the water surface elevation at the culvert exit, section BD) is the governing factor in water surface profile computations through the culvert. In Figure 7.1, the entrance and exit of the culvert are submerged, indicative of outlet control with pressure flow in the culvert resulting from the high tailwater elevation. Other examples of outlet control are presented in Section 7.3.

Head, culvert head, or culvert head loss - The difference between the headwater energy grade line elevation and tailwater (water surface) elevation, or the tailwater energy grade line if the velocity head at section 2 is not negligible. The more restrictive the culvert, the greater the head, and the higher the upstream water level caused by the culvert. Because the downstream velocity head for Figure 7.1 may not be negligible, the head represents the difference in the upstream and downstream energy grade lines (a value often very close to the difference in upstream and downstream water surface elevations).

Inlet control, headwater control, or entrance control - Inlet control exists when the culvert barrel is capable of passing more discharge than the culvert entrance can supply. A control exists near the culvert entrance and flow passes through critical depth at this point. The flow in the culvert is supercritical. The headwater elevation resulting from any discharge is a function of the entrance shape only. Examples of inlet control conditions are presented in Section 7.3.

Hydraulic grade line (HGL) - The sum of the datum (base elevation) and pressure head at a section. In open channels, the hydraulic grade is equal to the water surface elevation. The HGL is the line showing the hydraulic grade at any point on the conveyance element. Figure 7.1 shows the hydraulic grade line for a culvert under pressure, with the HGL above the top of the culvert.

Culvert velocity head - The average culvert velocity is used to obtain the culvert velocity head as V²/2g. The velocity head is a constant value for a culvert flowing full; therefore, the energy grade line and hydraulic grade line are parallel through the culvert, as shown in Figure 7.1.

Entrance loss - The entrance loss is the difference in the energy grade line elevation between section 3, just upstream of the culvert mouth, and section BU, just inside the culvert mouth. This loss of energy at the entrance is designated h_en. The loss is computed by multiplying a coefficient representing the degree of streamlining of the culvert entrance by the culvert velocity head.

Friction loss (also called barrel loss) - The loss of energy through the culvert, between the sections just inside the upstream end (BU) and downstream end (BD) of the culvert. The friction loss through the culvert is computed in the same fashion as friction loss through a bridge. The friction loss is indicated by the symbol h_f.

Exit loss - The difference in the energy grade line elevations between section BD, just inside the culvert exit, and section 2, just outside the culvert exit. This loss of energy is designated h_ex and is computed by multiplying the difference in velocity head at these two locations by a coefficient. For a conservative result, this coefficient is taken as 1 and is further addressed in Section 7.3.

Total loss - The total loss is the sum of the entrance, exit, and friction losses. Under outlet control, total loss and culvert head are used interchangeably. Total loss is indicated in Figure 7.1 by the symbol H_L.

Culvert shape or culvert cross section - The configuration or cross-sectional shape of the culvert structure. The most common culvert shape is circular; however, many other culvert cross-section shapes may be used, depending on the required flow capacity and the site and cover conditions. All shapes are defined in HEC-RAS by the rise and span, except for a circular pipe. Rise represents the vertical distance between the top of the culvert and the base. Span represents the horizontal distance between the widest points of the culvert. Figure 7.2 shows the nine shapes available for modeling within HEC-RAS.