Hydraulic Design of Highway Culverts - DOT On-Line Publications

1. Flow Control and Measurement. Flow control structures are used to measure and control
the rate of discharge in open channels. Culverts are often used as flow control structures due to
the in-depth understanding of culvert hydraulics, reliable and accessible design techniques, and
the availability of economical construction materials and methods. Discharge measurement and
control are required in irrigation canals, stormwater management ponds, and cooling water
channels for power plants, among others (Figure VI-1). In all three applications, a culvert could
be used to control water flow rates or flow distribution. The flow rates through the culvert are
easily calculated based on the geometry of the structure and coordinated records of headwater
and tailwater elevations. The routing procedures of Chapter V must be applied to determine the
corresponding inflow into the storage pond upstream of the culvert.
Culverts located on small watersheds can be utilized as flow measurement structures to provide
streamflow records. Shortly after a flood event, high water marks upstream and downstream of
a culvert installation can be measured and documented. Temporary staff gages placed at the
site would simplify these efforts. The peak discharge at the culvert site can then be
determined. These data help to improve runoff calculation methods and aid in verifying
computer models. If discharges for the entire flood event are required, a recording stage gage
is required. Harris details techniques and procedures for obtaining peak runoffs using culverts
as flow measurement structures (29).
2. Low Head Installations. Low head installations are culverts which convey water under a
roadway with a minimum headwater buildup and energy loss. These installations are typically
found in irrigation systems where the discharge is usually steady, and the available channel
freeboard and slope are small. Often the installations flow partly full over the length of the
culvert. Energy losses must be minimized to transport the water efficiently. The hydraulic
solution imposing the least energy loss would be to bridge the conveyance channel. However,
economic considerations may require the use of a low head culvert installation.
Reduction of energy loss and headwater at a culvert installation requires an understanding of
the background and theory utilized in the culvert design procedures discussed in Chapter III.
The minimal headwater rise, small barrel slope, and high tailwaters associated with these
installations usually result in outlet control. Therefore, minimizing entrance, exit, and friction
losses will reduce the required headwater (Equation (7)). Alignment of the culvert barrel with the
upstream channel helps to minimize entrance loss and takes advantage of the approach
velocity head. Inlet improvements, such as beveled edges, will further reduce entrance loss.
However, the hydraulic effects of further entrance improvements, such as side- and slopetapered
inlets are small in outlet control. Thus, the use of these inlets is usually not justified in
low head installations. The exit loss can be reduced by smoothly transitioning the flow back into
the downstream channel to take advantage of the exit velocity. Friction loss is reduced by the
utilization of a smooth culvert barrel.
In analyzing low head installations flowing partly full in outlet control, backwater calculations
may be necessary. Beginning at the downstream water surface (tailwater), the hydraulic and
energy grade lines are defined. Outlet losses are calculated using Equation (4c), considering
the downstream velocity. Thus, the calculations proceed upstream through the barrel, until the
upstream end of the culvert is reached. At that point, inlet losses are calculated using Equation
(4a) with the appropriate inlet loss coefficient, ke. The inlet loss is added to the calculated
energy grade line at the inlet to define the upstream energy grade line. Deducting the approach
velocity head from the upstream energy grade line results in the upstream water surface
elevation (hydraulic grade line).
144