FLOOD01360

• County: Statewide
• Title: Hydraulic Design of Highway Culverts
• Date: 9/1/1998
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

for each selected flow rate. Use these
water surface depths and equation (8) to
calculate flow rates across the roadway.

4. Add the culvert flow and the roadway
overtopping flow at the corresponding
headwater elevations to obtain the overall
culvert performance curve.

Using the combined culvert perfor-
mance curve, it is an easy matter to
determine the headwater elevation for
any flow rate, or to visualize the per-
formance of the culvert installation
over a range of flow rates. When roadway
overtopping begins, the rate of headwater
increase will flatten severely, The
headwater will rise very slowly from
that point on. Figure III-16 depicts an
overall culvert performance curve with
roadway overtopping. Example problem
1II-4 illustrates the development of an
overall culvert performance curve.

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tROADWAY
OVERTOPPING
-......, ...- .....-........-...
ROAOWA'" CREST ..........--

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~-S;~FOR"'ACE
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FLOW RATE (ft~s)

Figure IIl-16--Culvert performance
curve with roadway overtoppiug.

C. Culvert Desi2u Method.

The culvert design method provides a
convenient and organized procedure for

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designing cui verts, considering inlet
and outlet control. While it is possible
to follow the design method without an
understanding of culvert hydraulics,
this is not recommended. The result
could be an inadequate and possibly unsafe
structure.

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1. Culvert Design Form. The Culvert
Design Form, shown in figure III-17, has
been formulated to guide the user through
the design process, Summary blocks are
provided at the top of the form for the
project description, and the designer's
identification. Summaries of hydrologic
data of the form are also included. At
the top right is a small sketch of the
culvert with blanks for inserting important
dimensions and elevations.

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The central portion of the design form
contains lines for inserting the trial
culvcrt description and calculating the
inlet control and outlet control hcadwatcr
elevations. Space is provided at the
lowcr center for comments and at the
lower right for a description of the
culvert barrel selected.

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The first step in the design process
is to summarize all known data for the
culvert at the top of the Culvert Design
Form. This information will have been
collected or calculated prior to per-
forming the actual cui vert design. The
next step is to select a preliminary
culvert material, shape, size, and entrance
type. The user then enters the design
flow rate and proceeds with the inlet
control calculations.

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2. Inlet Control. The inlet control
calculations determine the headwater
elevation required to pass the design
flow through the selected culvert configu-
ration in inlet control. The approach
velocity head may be included as part of
the headwater, if desired. The inlet con-
trol nomographs of appendix D are used
in the design process. For the following
discussion, refer to the schematic inlet
control nomograph shown in figure III-IS.

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