(A) 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.
(B) The quantity of runoff which is generated as the result of a given rainfall intensity may be calculated as follows:
(1) Development sites less than or equal to 5 acres in size, with a contributing drainage area less than or equal to 50 acres and no depressional storage. The rational method may be used.
(a) In the rational method, the peak rate of runoff, Q, in cubic feet per second (cfs) is computed as:
Q=CIA | ||
Where: | C= | Runoff coefficient, replacing 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. | |
(b) Values for the runoff coefficient "C" are provided in Tables 1 and 2, which show values for different types of surfaces 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.
(c) In the instance of undeveloped land situated in an upstream area, a coefficient or coefficients shall be used which are related to the use or uses which can be reasonably anticipated after development occurs. Interpolation, extrapolation and adjustment for local conditions shall be based on engineering experience and judgment.
(d) Rainfall intensity shall be determined from the rainfall frequency data shown in § 51.10 below. In general, the time of concentration (tc) methodology to be used for all stormwater management projects within the City of Brazil shall be as outlined in the U.S. Department of Agriculture (USDA)-NRCS TR-55 manual.
(e) In urban or developed areas, the methodology to be used shall be the sum of the inlet time and flow time in the stormwater 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 distance in feet divided by velocity of flow in feet per second. The velocity shall be determined by the Manning's equation (see § 51.11). 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 areas such as lawns, fields and other graded surfaces.
(2) Development sites greater than 5 acres in size or contributing drainage area greater than 50 acres or with significant depressional storage.
(a) The runoff rate for these development sites and contributing drainage areas shall be determined by a computer model that can generate hydrographs based on the NRCS TR-55 time of concentration and curve number calculation methodologies and the huff third quartile (50%) rainfall distribution.
(b) A sample time of concentration sheet can be found in figure 1 and TR-55 curve numbers are given in Tables 5a-5d. Hydrologic soil groups for each soil present in the City of Brazil are given in Table 6. Rainfall depths for various frequencies and durations shall be taken from Table 7.
(c) The huff third quartile (50%) is found in Table 8. Examples of computer models that can generate hydrographs include TR-20 (NRCS) and HEC-1 (COE).
(d) Other models may be acceptable and should be approved by the city engineer prior to their utilization.
(3) Development sites with drainage areas greater than or equal to 1 square mile. For the design of any major drainage system as defined in § 51.04, the discharge must be obtained from the IDNR. Other portions of the site must use the discharge methodology in the applicable section of this subchapter.
(Ord. 01-2000, passed 7-18-2000)