Important Parts of Culvert Design
(Figures, quotations from Hydraulic Design of Highway Culverts, US DOT, 2005
Cross-Sectional Shapes:
Factors of consideration:
“The shape selection is based on the cost of construction, the limitation on upstream water surface elevation, roadway embankment height, and hydraulic performance.”
Materials:
Three most coming are concrete, corrugated aluminum, and corrugated steel.
Factors of Consideration:
“The selection of a culvert material may depend upon structural strength, hydraulic roughness, durability, and corrosion and abrasion resistance. Culverts may also be lined with other materials to inhibit corrosion and abrasion, or to reduce hydraulic resistance. For example, corrugated metal culverts may be lined with asphaltic concrete.”
Inlets:
(Four standard inlet types)
Factors of Consideration: “Structural stability, aesthetics, erosion control, and fill retention are considerations in the selection of various inlet configurations.”
Flow Conditions:
a.Full flow- “The hydraulic condition in a culvert flowing full is called pressure flow. If the cross sectional area of the culvert in pressure flow were increased, the flow area would expand. One condition which can create pressure flow in a culvert is the back pressure caused by a high downstream water surface elevation. A high upstream water surface elevation may also produce full flow. Regardless of the cause, the capacity of a culvert operating under pressure flow is affected by upstream and downstream conditions and by the hydraulic characteristics of the culvert.”
b.Partly full (free surface) flow- “Free surface flow or open channel flow may be categorized as subcritical, critical, or supercritical. A determination of the appropriate flow regime is accomplished by evaluating the dimensionless number, Fr, called the Froude number: Fr = V/ (g yh)0.5.In this equation, V is the average velocity of flow, g is the gravitational acceleration, and yh is the hydraulic depth. The hydraulic depth is calculated by dividing the cross-sectional flow area by the width of the free water surface. When Fr > 1.0, the flow is supercritical and is characterized as swift. When Fr < 1.0, the flow is subcritical and characterized as smooth and tranquil. If Fr = 1.0, the flow is said to be critical.”
“The three flow regimes are illustrated in the depiction of a small dam below. Subcritical flow occurs upstream of the dam crest where the water is deep and the velocity is low.Supercritical flow occurs downstream of the dam crest where the water is shallow and the velocity is high. Critical flow occurs at the dam crest and represents the dividing point betweenthe subcritical and supercritical flow regimes.”
Types of Flow Control:
- Inlet Control
- Outlet Control
Headwater:
“Energy is required to force flow through a culvert. This energy takes the form ofan increased water surface elevation on the upstream side of the culvert. The depth of theupstream water surface measured from the invert at the culvert entrance is generally referred toas headwater depth. A considerable volume of water may be ponded upstream of a culvert installation under high fillsor in areas with flat ground slopes. The pond which is created may attenuate flood peaks undersuch conditions. This peak discharge attenuation may justify a reduction in the required culvertsize.”
Tailwater:
“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. In either case, backwater calculations from the downstream control point are required to precisely define tailwater. When appropriate, normal depth approximations may be used instead of backwater calculations.”
Outlet Velocity:
“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. Minor problems can occasionally be avoided by increasing the barrel roughness. Energy dissipaters and outlet protection devices are sometimes required to avoid excessive scour at the culvert outlet. When a culvert is operating under inlet control and the culvert barrel is not operating at capacity, it is often beneficial to flatten the barrel slope or add a roughened section to reduce outlet velocities.”
Performance Curves:
“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. At the design headwater, the culvert operates under inlet control. With inlet improvement the culvert performance can be increased to take better advantage of the culvert barrel capacity.”
Data Requirements for Culvert Design: