Land Development Handbook by Dewberry & Davis

Author:Dewberry & Davis
Language: eng
Format: epub
Tags: -
Publisher: McGraw-Hill Education
Published: 2012-04-10T16:00:00+00:00

Qw = CwL H3/2

(21.61)

where Cw is a weir coefficient that depends on the shape and depth of water above the weir relative to the depth of water below the weir, L is the length of the weir, and H is the depth of water above the weir. For a broad crested trapezoidal weir, Cw ranges between 2.5 and 3.1. Refer to hydraulic handbooks, such as Brater et al. (1996), for various other values for different types of weirs.

To determine whether the roadway overtops less than 1 foot, as was required in the example problem, we will assume that a 50-foot-long rectangular weir approximates the overtopping portion of the roadway. Using Equation 21.61, a Cw = 2.6, L = 50 feet, and H = 1 foot (allowable over-topping) yields a weir discharge of 130 cfs. Next, we determine whether the combination of weir flow and culvert flow, at the allowable elevation of 325.5 feet (1 foot above the road elevation), is sufficient to pass the 100-year discharge.

Using an elevation of 325.5 feet, the corresponding HW/D ratio is (325.5 – 314.2)/7 = 1.61. From the box culvert with inlet control nomograph (Figure 21.24), construct a straight line from HW/D scale (1) at HW/D = 1.61 to the left scale for D = 7 feet. Read Q/B = 90 and find Q = 7(90) = 630 cfs. The total flow of culvert and weir flow is 630 + 270 = 760 cfs. Our 100-year design discharge is equal to 690 cfs. Therefore, there is sufficient capacity within the culvert and the 1 foot of allowable overtopping of the road to pass the design discharge. A trial-and-error process using the same procedures could calculate the actual 100-year water surface elevation.

This example uses the culvert nomograph for inlet control to calculate the headwater depth. Equations 21.47, 21.48, and 21.49 could also have been used for the same purpose. The nomograph is convenient and quick, however, and the equations can be incorporated into computer programs or hand-held calculators. A variety of computer programs are available to assist and automate the culvert analysis process. There are also numerous inlet and outlet nomographs for various culvert shapes that can be found in Hydraulic Design of Highway Culverts by the FHWA, available from National Technical Information Service (NTIS), Springfield, Virginia.

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