Opinion ID: 2032266
Heading Depth: 1
Heading Rank: 2

Heading: culvert design

Text: LeRoy Gerrard, Boone County highway superintendent, testified that he used what is called the Dickens formula to size the culverts. The Dickens formula is used to determine how much water will flow in a particular drainage area during a 4-inch, 24-hour rain. The formula takes into account the size of the drainage area in acres and the slope and contour of the land. Gerrard initially recommended one 84-inch culvert, but a Boone County road foreman suggested two culverts, one 72 inches and the other 48 inches in diameter. Gerrard agreed, on that basis that that would increase the amount of water which could travel through the culverts. The Scholls' expert witness, John Payton, testified that Gerrard's use of the Dickens formula in sizing the culverts resulted in a negligent design and was not appropriate. Boone County offered the testimony of Patrick Diederich, a registered civil engineer who specializes in hydrology. Diederich testified that the Dickens formula used by Gerrard would yield a design for a 2- to 5-year storm, which by itself might be inadequate. However, Diederich noted that such formulas, including the modified rational formula used by Payton, fail to take into account any storage values for water that is expected to be routed through the culverts. Storage was defined as a backup of water due to the terrain in the area in front of the culvert which alters the result of both the Dickens formula and the modified rational formula. Diederich testified that routing formulas for watershed analysis have existed for many years and take such storage into account. Diederich testified that the HEC-1 formula, often utilized by the Army Corps of Engineers, is such a routing formula for watershed analysis. Applying the HEC-1 formula, Diederich opined that the culverts designed by Gerrard were designed for a 10- to 25-year storm. It was the further opinion of Diederich that Boone County's design of the culverts in question was valid and appropriate according to accepted engineering standards.