Culverts serve many purposes and fit into multiple applications. For instance, bridge culverts provide a supportive structure for roadway and railway traffic while directing water flow in a non-interruptive manner. In addition to hydraulic fundamentals, a basic understanding of terminology and governing principles help lay the foundation for applying culvert applications.

So, here is the ultimate culvert hydraulic principles guide, where we’ll cover the basics of channels, culvert hydraulic principles, and other details. From understanding flow conveyance to covering inlet and outlet control mechanisms, you’ll better grasp why culvert hydraulics matter.

Basic Overview of Culverts

So, what are culverts? Culverts primarily function as hydraulic conduits, helping direct water from one side of a railway or roadway embankment to the other. Culverts serve dual purposes, acting as hydraulic and load-bearing traffic structures.

Culverts solve the issue of getting facilities over, around, under, or in between each other, such as canals, ditches, and streams. Culverts contain different materials, including steel, plastic, aluminum, concrete, and density polyurethane.

What Are Culvert Hydraulics?

The concept of culvert hydraulics includes the following:

  • Headwater, or the depth of upstream flow of the culvert
  • Barrel cross-sectional area
  • Barrel cross-sectional shape
  • Inlet configuration
  • Barrel roughness
  • Length
  • Slope
  • And tailwater, or the downstream flow depth

Moreover, there are two primary flow regimes for culverts: outlet and inlet control.

Outlet control occurs when water flows into a culvert faster than it can flow out, limiting the flow to either friction or roughness along the culvert barrel or the tailwater outlet depth. Conditions downstream of the culvert can impact the flow rate. Meanwhile, inlet control occurs when the culvert inlet constricts the flow more than other factors. The inlet impacts the amount of water passing below the road and regulates its rate of discharge downstream.

Basic Hydraulic Principles

Here is an essential guide to understanding governing culvert hydraulic principles, further detailing its basic terminology. This section includes discussions of general flow characteristics, energy principles, openings and weirs, and other notable hydraulic factors.

General Flow Characteristics

General flow characteristics include flow conveyance, various radiuses, and velocity discussions. In addition, understanding basic flow characteristic terminology helps lay the foundation for more complex analyses when applied to multiple culvert projects.

Flow Conveyance

Flow conveyance can refer to gravity-fed water pathways that originate from points of higher energy to lower energy, culminating at a point of equilibrium. The equilibrium point can be a significant body of water, like the ocean. The presence of natural conveyance channels, including streams, brooks, and rivers, facilitates this water travel and often dictates the path of least resistance.

The water’s flow can achieve its journey with the help of man-made structures, including pipes, culverts, canals, and drainage swales. Both man-made structures and natural features must obey the basic hydraulic principles.

Area, Wetted Perimeter, and Hydraulic Radius

The area of a hydraulic culvert refers to the cross-sectional area of flow within a channel. A prismatic channel consists of cross-sectional slopes with varying roughness and shape.

A section’s wetted perimeter is the portion of the channel in contact with the flow line. Lastly, the hydraulic radius is not a directly measurable characteristic but is frequently used during hydraulic calculations, especially as it relates to the geometric properties of the channel.


The flow velocity variation within a cross-section adds a layer of complexity to the hydraulic analysis. As a rule of thumb, calculations become simplified by averaging the fluid velocity through the section.

Energy Principles

Energy refers to the total power of the hydraulic culvert systems, consisting of potential, kinetic, and internal forms of energy. In hydraulic applications, energy values often become converted into units of energy per weight, resulting in length units. Therefore, it’s crucial to understand hydraulic and energy grades and how this may add or subtract from the stormwater system.

Hydraulic Grade

The hydraulic grade is equivalent to the water surface elevation for an open channel flow. The hydraulic grade represents the height a water column could rise in a piezometer, a tube open to the atmosphere vertical to the pipes. The hydraulic grade line, or HGL, is a profile plot of the hydraulic grade depicting the sectional conveyance length.

Energy Grade

The energy grade refers to the sum of the hydraulic grade and velocity head. This consists of the rate at which a water column rises in a Pitot tube—like a piezometer while accounting for fluid velocity. You plot this parameter as an energy grade line (EGL).

Energy Losses

Many factors can cause energy loss, including internal friction between fluid particles traveling at different velocities. Energy loss can also occur from localized areas of increased turbulence and streamline disruptions, such as disrupted valves or disruptions from a changing river section shape. You can calculate this loss by determining the friction slope (or the rate at which energy becomes lost along a given channel length).

Orifices and Weirs

Orifices and weirs are crucial for their widespread and reliable utility because the equations that describe them serve as the foundation for mathematical descriptions of complicated devices like culverts and drainage inlets. An orifice is a regular-shaped, submerged opening in which water flows, propelled by different upstream and downstream energy. When the water stream—or jet—expels from orifices, adverse velocity components cause contractions to a point where the flow area remains constant, and flow lines become parallel.

Weirs are the notches or gaps in which fluid flows. Depending on the weir design, the flow may contract as it exits over the top. You can also distinguish them by their rectangular, v-notch, and broad shapes. Weirs serve as emergency spillways to regulate high-return event flows for dams and retention ponds, measure flow, and regulate channel flow.

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The Ultimate Guide to Culvert Hydraulic Principles