Basic Hydrology & Hydraulics: DES 601

Module 20Culverts - II

Flow types

• Department practice is to consider flow through the conduit occurring in one of four combinations: • Free surface flow (Type A) through entire

conduit,• Full flow in conduit (Type B),• Full flow at outlet and free surface flow at inlet

(Type BA),• Free surface at outlet and full flow at inlet (Type

AB).

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Free surface flow (Type A)• If free surface flow is occurring in the culvert, the

hydraulic parameters are changing with flow depth along the length of the culvert.

• It is necessary to calculate the backwater profile based on the outlet depth, Ho.

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Full flow in conduit (Type B)• If full flow is occurring in the conduit, rate of energy

losses through the barrel is constant for steady flow• The hydraulic grade line is calculated based on

outlet depth, Ho, at the outlet.

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Full flow at outlet (Type BA)• If the friction slope is flatter than the conduit slope, it is

possible that full flow may not occur along the entire length of the culvert

• Determine: the length over which full flow occurs (Lf) using eq. 8-14, Type BA free surface losses, and Type BA HGL at inlet (if applicable).

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Free surface at outlet (Type AB)• When the outlet is not submerged, full flow will

begin within the conduit if the culvert is long enough and the flow high enough

• The following steps should be followed: check Type AB uniform depth, determine Type AB free surface losses, determine Type AB full flow losses, determine Type AB HGL at inlet (if applicable).

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Number of barrels• Culverts consisting of more than one box are useful

in wide channels where the constriction or concentration of flow must be kept to a minimum

• General recommendation – where a culvert consists of more than one barrel, shapes of uniform geometry and roughness characteristics should be used to maintain uniform flow distribution.

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End treatments• End treatments serve several different purposes but

typically act as a retaining wall to keep the roadway embankment material out of the culvert opening

• Secondary characteristics of end treatments include hydraulic improvements, traffic safety, debris interception, flood protection, and prevention of piping (flow through the embankment outside of the culvert).

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Parallel vs. cross–drainage• The inlet and outlet points of culverts handling

drainage parallel to the travel lanes, such as at driveways, side roads, and median crossovers, are concerns in providing a safe roadside environment.

• Flow quantities for these parallel drainage situations are generally low with drainage typically accommodated by a single pipeReference: Roadway Design Manual 2010; TXDOT; Chapter 2 Section 7

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Parallel vs. cross–drainage• Cross drainage culverts are those handling drainage

across and beneath the highway.

• Selection of an appropriate end treatment is primarily related to culvert size, culvert end location, side slope rate, terrain characteristics, drift conditions, right-of-way availability.

• Multiple barrels are often appropriate and used.Reference: Roadway Design Manual 2010; TXDOT; Chapter 2 Section 7

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