# HY8:Concrete Open Bottom Arch

HY-8 Version 7.3 and later has coefficients for computing inlet control depths for concrete open-bottom arch (commonly called Con/Span) culverts.

## Geometric Characteristics

Con/Span culverts have unique geometric configurations, and several sizes and shapes are available. The exact coordinates used in HY-8 to compute areas and other geometric cross section parameters are available in this document. Since the culverts can be made to accommodate any required rise for a given span, HY-8 contains culvert geometry in 3-inch increments of rise.

## Inlet Control Polynomial Coefficients

The polynomial coefficients used by HY-8 were derived from a study and document prepared by Don Chase at the University of Dayton, Ohio (1999). Dr. Chase determined a different set of coefficients for culverts with different span-to-rise ratios. Con/Span culverts with a 4:1 span-to-rise ratio performed better (resulted in a lower headwater) than culverts with a 2:1 span-to-rise ratio. Because of this, separate polynomial coefficients were determined for culverts with each of these span-to-rise ratios.

Dr. Chase's study determined the K, c, M, and Y NBS coefficients described in HDS-5, and these coefficients were fitted to a 5th-degree polynomial equation so they can be used in HY-8.

In HY-8, the 2:1 coefficients are used if the span:rise ratio is less than or equal to 3:1, and the 4:1 coefficients are used if the span:rise ratio is greater than 3:1. If the culvert being modeled has less than a 2:1 or greater than a 4:1 span-to-rise ratio, the user will see a note in HY-8 saying that the culvert is outside of the tested span-to-rise ratios. Further testing may be required to account for these large or smaller span-to-rise ratios, but it is likely that the computed headwater will be higher than the observed headwater if the span:rise ratio is greater than 4:1 and the computed headwater will be less than that observed if the span:rise ratio is less than 2:1.

For information on the exact coefficients used and to view diagrams showing the different culvert wingwall configurations, see the help describing the HY-8 polynomial coefficients.