§ IV. Culverts.

At the intersection of open channels and roads, streets, or highways, it is necessary to provide an adequately sized culvert; sizing these structures is very important to avoid bottlenecks in the drainage system. Design of culverts involves determination of the hydraulic performance of the structure under certain conditions in addition to its structural adequacy. The more commonly used culvert types include both concrete and metal; circular pipes and arch pipes, and concrete box culverts.

A culvert will operate with either inlet-controlled or outlet-controlled flow. Under inlet control, the inlet geometry and the amount of headwater or ponding at the entrance are of primary importance. Outlet control involves the additional consideration of the elevation of the tailwater in the outlet channel and the slope, roughness and length of the culvert barrel.

INLET CONTROL

Most culverts in the city will operate under inlet control. Culverts with inlet control are usually on steep slopes and flow partly full. The headwater or depth of ponding at the entrance is an important factor in the culvert capacity. The headwater depth, HW, is the vertical distance from the culvert invert at the entrance to the energy line of the headwater pool. Because of the low velocities in most entrance pools, the water surface and the energy line at the entrance are assumed to be the same.

Figures 11 through 14 were developed by the Federal Highway Administration (Bureau of Public Roads) to aid in sizing culverts. Enter the charts or nomographs with a trial culvert size and the design flow, and read the value for HW/D on the line for the appropriate entrance type. HW can then be computed by multiplying the trial structure diameter or depth, D, by the value from the nomograph.

OUTLET CONTROL

Culverts with outlet control can flow with the culvert barrel full or part full, for part or all of the barrel length. The head, H, or energy required to pass a given quantity of water through an outlet-controlled culvert with the barrel flowing full throughout its length is made up of three major parts, which include a velocity head, H _v ; an entrance loss, H _e ; and a friction loss, H _f . This energy is obtained from ponding of water at the entrance and is expressed as:

H = H _v + H _e + H _f

Figures 15, 16, and 17 which were also developed by the Federal Highway Administration (Bureau of Public Roads), are included to aid in sizing structures with outlet control and, in particular, to determine the value for H.

The headwater depth, HW, can be expressed by a common equation for all outlet control conditions, including all depths of tailwater. This is accomplished by designating the vertical dimension for the culvert invert at the outlet to the elevation from which H is measured as h _o . Then,

H = H + h _o - LS _o

in which:

L = the length of the culvert in feet.

S _o = the barrel slope in feet per foot.

When the water surface elevation in the outlet channel is equal to or above the elevation of the top of the culvert opening at the outlet, h _o is equal to the tailwater depth, TW. TW is the distance in feet from the outlet culvert invert to the water surface in the outlet channel.

If the tailwater elevation is below the top of the culvert opening at the outlet, h _o is more difficult to determine. The discharge, size, and shape of culvert, and the TW must be considered. In these cases, h _o is the greater value of TW as defined above, or

d _c + D

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The critical depth is d _c , and D is the culvert height. Tailwater depth is not a factor for culverts flowing below critical depth at the outlet.

Figures 18 and 19 give critical depth, d _c , at various discharges for different sizes of circular and pipe-arch culverts.

To determine the value of H from the charts, select the proper value of k _e from Table 6 and enter the chart with the quantity of flow, Q, and the size and length of the structure. Values of k _e different than the scales shown can be located by a straight-line interpolation between the scales. Connect the length on the k _e scale to the culvert size to establish a turning point. Then connect the discharge and the turning point to determine the head, H.

The value of HW should be found for both the inlet and outlet control condition. The flow condition that produces the highest value for HW governs and indicates the flow control existing under the given conditions for the trial size selected.

(Ord. No. 4119, § 1(Exh. A), 6-16-08)