This chapter establishes the minimum required standards for the planning and design of drainage systems and storm water management facilities within the county. All plans concerning drainage shall be prepared under the supervision of and certified by a registered land surveyor or professional engineer licensed by the State of Indiana. It is assumed that practicing engineers/surveyors involved with preparing drainage plans have adequate knowledge of the recommended procedures; therefore, there is no attempt in the document to provide step-by-step calculation methodologies.
(A) Storm sewer design parameters. All storm sewers, whether private or public, and whether constructed on private or public property shall be designed to handle the flow for a minimum storm return period of ten years (ten-year storm). In addition, storm systems shall be designed to store the return period of 100 years (100-year storm). These design parameters will allow surface water to drain into the storm system and not allow water to stand outside the public right-of-way and easements.
(B) Frequency/return period. The elevation of the 100-year pre- and post-development discharge shall be checked for all drainage system designs to assure conformance with the guidelines of the FEMA Program hi the areas of the county not covered by a flood insurance study, the design engineer or land surveyor must determine the pre-development 100-year flood elevation. The elevation for the 100-year post-development discharge shall be conveyed within the limits of the proposed easement.
(C) Runoff calculation methods (design flow). The following is the determination of runoff quantities:
(1) Runoff quantities shall be computed for the area of the parcel under development plus the area of the watershed flowing into the parcel under development. The quantity of runoff which is generated as the result of a given rainfall intensity shall be calculated as follows:
In the rational method, the peak rate of runoff, Q, in cubic feet per second is computed as:
Q = CIA
Where: C = Runoff coefficient, representing the characteristics of the drainage area and defined as the ratio of runoff to rainfall.
I = Average intensity of rainfall in inches per hour for a duration equal to the time of concentration (Tc) for a selected rainfall frequency.
A = Tributary drainage area in acres.
The rainfall intensity factor, I, should be obtained from the Louisville, Kentucky Rainfall Intensity-Duration Curves.
(2) The time of concentration (Tc) to be used shall be the sum of the inlet time and flow time in the drainage facility from the most remote part of the drainage area to the point under consideration. The flow time in the storm sewers may be estimated by the distance in feet divided by velocity of flow in feet per second. The velocity shall be determined by the Manning's Formula. Inlet time is the combined time required for the runoff to reach the inlet of the storm sewer. It includes overland flow time and flow time through established surface drainage channels such as swales, ditches and sheet flow across such areas as lawns, fields and other graded surfaces.
(3) The time of concentration, Tc, shall be determined by calculating the time for a particle of water to travel from the most hydrological remote point of the project area to the point of interest.
(4) Time of concentration to the first inlet or structure may be estimated by the Kirpick Equation (Tc = 0.0078*L^0.77*s^-0.385) where L equals length of travel in feet and S equals slope in foot per foot. Other methods to derive time of concentration such as TR-55, Kerby's Equation and the Kinematic Wave method will be acceptable. The minimum Tc shall not be less than ten minutes. Manning's Equation should be used to estimate any in-pipe or channel travel.
(5) Guidance to selection of the runoff coefficient “C” is provided by Table 1, in Appendix A, attached to the end of this chapter, which shows values for different types of surface and local soil characteristics. The composite “C” value used for a given drainage area with various surface types shall be the weighted average value for the total area calculated from a breakdown of individual areas having different surface types. A listing of soils groups with their corresponding hydraulic soil group can be found in the Appendix. They are classified into four categories: A, B, C, and D.
(D) Design flows. The professional engineer or professional land surveyor shall refer to the Ten State Standards for design methodology for storm sewers. The exception being that all storm sewer systems will be designed for the ten-year event. At a minimum, the facility must have the capacity to transport the ten-year post-development discharge except in unusual cases, such as retrofit projects. The CCDB shall determine design criteria for retrofit projects or other unusual cases. The water surface profile and through system capacity shall be checked for the 100-year post-development discharge. All drainage systems shall be capable of passing the 100-year design flow within die drainage easement. Additional facility specific requirements are found in the following portions of this chapter.
(Ord. 2010-11, passed 7-22-2010)