Section R. 280.2 - Computation of basic benefitsRule 2.
(1) Basic benefits for county roads and state highways shall be computed in direct proportion to the pro rata share of storm water from county roads and state highways within the drainage district to the total design storm water runoff from the drainage district.(2) Storm water runoff quantities (Q) shall be determined for the drainage district by use of the accepted engineering formula, Q = ciA, this being the product of the runoff coefficient (c), referred to in table I set forth in R 280.9, the rainfall intensity (i), and the area of the drainage district (A). Rainfall intensity is assumed to be of uniform rate throughout the drainage district.(3) Storm water runoff quantities (Q) for a county road or a state highway shall be determined by using the same method as that used for the entire drainage district. Computation examples are set forth in R 280.7 and R 280.8.(4) If the rainfall intensity used in the design of a county road or a state highway drainage facility is not the same as the rainfall intensity used in the design of the enclosed receiving county drain or intercounty drain, the basic benefit computed for the county road or the state highway shall be multiplied by the number which expresses the ratio of the design runoff from the county road or state highway to the design runoff for the receiving county or intercounty drain according to the recurrence interval factors. (See table II set forth in R 280.9.)Mich. Admin. Code R. 280.2