CULVERT HYDRAULICS
FHWA

DEFINITION
A culvert is a hydraulically short conduit which conveys streamflow through a roadway embankment or past some other type of flow obstruction.
Culverts are constructed from a variety of materials and are available in many different shapes and configurations.
The selection of a culvert material may depend upon structural strength, hydraulic roughness, durability, and corrosion and abrasion resistance.
The three most common culvert materials are concrete (reinforced and nonreinforced), corrugated aluminum, and corrugated steel.

CULVERT HYDRAULICS
A complete theoretical analysis of the hydraulics of a particular culvert installation is time-consuming and difficult.
Flow conditions vary from culvert to culvert and they also vary over time for any given culvert.
The barrel of the culvert may flow full or partly full depending upon upstream and downstream conditions, barrel characteristics, and inlet geometry.
A culvert barrel may flow full over all of its length or partly full.
Full flow in a culvert barrel is rare.
Generally, at least part of the barrel flows partly full.
A water surface profile calculation is the only way to accurately determine how much of the barrel flows full.
The hydraulic condition in a culvert flowing full is called pressure flow.
The capacity of a culvert operating under pressure flow is affected by upstream and downstream conditions and by the hydraulic characteristics of the culvert.

FLOW CONTROL
Inlet and outlet control are the two basic types of flow control.
The basis for the classification system is the location of the control section.
The characterization of pressure, subcritical, and supercritical flow regimes plays an important role in determining the location of the control section and thus the type of control.
The hydraulic capacity of a culvert depends upon a different combination of factors for each type of control.
Inlet control occurs when the culvert barrel is capable of conveying more flow than the inlet will accept.
The control section of a culvert operating under inlet control is located just inside the entrance.
Critical depth occurs at or near this location, and the flow regime immediately downstream is supercritical.
Hydraulic characteristics downstream of the inlet control section do not affect the culvert capacity.
The upstream water surface elevation and the inlet geometry represent the major flow controls.
The inlet geometry includes the barrel shape, cross-sectional area, and the inlet edge.
Outlet control flow occurs when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept.
The control section for outlet control flow in a culvert is located at the barrel exit or further downstream.
Either subcritical or pressure flow exists in the culvert barrel under these conditions.
All of the geometric and hydraulic characteristics of the culvert play a role in determining its capacity.
These characteristics include all of the factors governing inlet control, the water surface elevation at the outlet, and the slope, length, and hydraulic roughness of the culvert barrel.
Energy is required to force flow through a culvert.
This energy takes the form of an increased water surface elevation on the upstream side of the culvert.
The depth of the upstream water surface measured from the invert at the culvert entrance is generally referred to as headwater depth.
Tailwater is defined as the depth of water downstream of the culvert measured from the outlet invert.
It is an important factor in determining culvert capacity under outlet control conditions.
Tailwater may be caused by an obstruction in the downstream channel or by the hydraulic resistance of the channel.
Since a culvert usually constricts the available channel area, flow velocities in the culvert are likely to be higher than in the channel.
These increased velocities can cause streambed scour and bank erosion in the vicinity of the culvert outlet.
Energy dissipators and outlet protection devices are sometimes required to avoid excessive scour at the culvert outlet.

PERFORMANCE CURVE
A performance curve is a plot of headwater depth or elevation versus flow rate.
The resulting graphical depiction of culvert operation is useful in evaluating the hydraulic capacity of a culvert for various headwaters.
Among its uses, the performance curve displays the consequences of higher flow rates at the site and the benefits of inlet improvements.
In developing a culvert performance curve, both inlet and outlet control curves must be plotted.
This is necessary because the dominant control at a given headwater is hard to predict.
Also, control may shift from the inlet to the outlet, or vice-versa over a range of flow rates.

TYPES OF CONTROL
A culvert flowing in inlet control has shallow, high velocity flow categorized as "supercritical."
For supercritical flow, the control section is at the upstream end of the barrel (the inlet).
Conversely, a culvert flowing in outlet control will have relatively deep, lower velocity flow termed "subcritical" flow.
For subcritical flow the control is at the downstream end of the culvert (the outlet).
The tailwater depth is either critical depth at the culvert outlet or the downstream channel depth, whichever is higher.
Since the control is at the upstream end in inlet control, only the headwater and the inlet configuration affect the culvert performance.
The headwater depth is measured from the invert of the inlet control section to the surface of the upstream pool.
The inlet area is the cross-sectional area of the face of the culvert.
Generally, the inlet face area is the same as the barrel area, but for tapered inlets the face area is enlarged, and the control section is at the throat.
The inlet edge configuration describes the entrance type.
Some typical inlet edge configurations are thin edge projecting, mitered, square edges in a headwall, and beveled edge.
The inlet shape is usually the same as the shape of the culvert barrel; however, it may be enlarged as in the case of a tapered inlet.
Typical shapes are rectangular, circular, and elliptical.
Whenever the inlet face is a different size or shape than the culvert barrel, the possibility of an additional control section within the barrel exists.

HYDRAULICS OF INLET CONTROL
Inlet control performance is defined by the three regions of flow: unsubmerged, transition and submerged.
A weir is an unsubmerged flow control section where the upstream water surface elevation can be predicted for a given flow rate.
The relationship between flow and water surface elevation must be determined by model tests of the weir geometry or by measuring prototype discharges.
These tests or measurements are then used to develop equations for unsubmerged inlet control flow.
For headwaters submerging the culvert entrance, the entrance of the culvert operates as an orifice.
An orifice is an opening, submerged on the upstream side and flowing freely on the downstream side, which functions as a control section.
The relationship between flow and headwater can be defined based on results from model tests.
The flow transition zone between the low headwater (weir control) and the high headwater flow conditions (orifice control) is poorly defined.
This zone is approximated by plotting the unsubmerged and submerged flow equations and connecting them with a line tangent to both curves.

FACTORS INFLUENCING OUTLET CONTROL
All of the factors influencing the performance of a culvert in inlet control also influence culverts in outlet control.
In addition, the barrel characteristics (roughness, area, shape, length, and slope) and the tailwater elevation affect culvert performance in outlet control.
The barrel roughness is a function of the material used to fabricate the barrel.
Typical materials include concrete and corrugated metal.
The roughness is represented by a hydraulic resistance coefficient such as the Manning's n value.
The barrel area and barrel shape are self explanatory.
The barrel length is the total culvert length from the entrance to the exit of the culvert. Because the design height of the barrel and the slope influence the actual length, an approximation of barrel length is usually necessary to begin the design process.
The barrel slope is the actual slope of the culvert barrel.
The barrel slope is often the same as the natural stream slope.
However, when the culvert inlet is raised or lowered, the barrel slope is different from the stream slope.
The tailwater elevation is based on the downstream water surface elevation.
Backwater calculations from a downstream control, a normal depth approximation, or field observations are used to define the tailwater elevation.

HYDRAULICS OF OUTLET CONTROL
Full flow in the culvert barrel is the best type of flow for describing outlet control hydraulics.
Outlet control flow conditions can be calculated based on energy balance.
The total energy (H_L) required to pass the flow through the culvert barrel is made up of the entrance loss (H_e), the friction losses through the barrel (H_f), and the exit loss (H_o). Other losses, including bend losses (H_b), losses at junctions (H_j), and losses at grates (H_g) should be included as appropriate.
H_L = H_e + H_f + H_o + H_b + H_j + H_g

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