(A) Application for peak discharge and runoff volume estimation. The Rational Method may be utilized for computing peak runoff from small watersheds for the purpose of designing drainage conveyance facilities and storm water detention/retention basins. The Rational Method should be applied within the following limitations:
(1) The drainage area should not exceed 160 acres.
(2) The minimum time of concentration (Tc) is 5 minutes. The maximum Tc should not exceed 60 minutes.
(3) The watershed should be homogenous (i.e. the watershed should not consist of two or more land-uses of significantly different character). Lack of basin homogeneity can lead to errors in estimates of Tc and runoff coefficient “C”.
(4) The watershed should not contain storm water facilities that require routing and combination of hydrographs.
(B) Watersheds that do not meet the criteria in division (A) of this section should be modeled using the Clark Unit Hydrograph method.
(C) A number of variations of the Rational Method are currently in use by communities throughout the Southwestern United States. These methods vary mostly in the method used to compute the time of concentration, Tc. It is required that the Rational Method presented within Chapter 2 of the ADOT Hydrology Manual be utilized for hydrologic analysis within the town. This methodology was developed for use within Arizona, and the manual is readily available. Other Rational Methods may also be acceptable, but must be approved in writing by the Public Works Engineer prior to use. Refer to § 152.34 for the Payson Intensity-Duration-Frequency (I-D-F) for use with the Rational Method.
(D) Estimation of runoff coefficient, “C”. The runoff coefficient, “C”, for undeveloped watersheds shall be estimated using Figures 3.2 and 3.3. The hydrologic soil group (HSG) and an estimate of vegetation cover density for the area of concern can be obtained using Table 3.2 in combination with the town soils map. “C” values for developed land uses shall be obtained from Table 3.5.
(Res. 1637, passed 2-28-02)
Rational "C" Coefficient, Mountain, Juniper and Grass
As a function of Rainfall Depth, Hydrologic Scil Group (HSG), and % of Vegetation Cover (From ADOT Hydrology Manual Figure 2-7)
As a function of Rainfall Depth, Hydrologic Scil Group (HSG), and % of Vegetation Cover (From ADOT Hydrology Manual Figure 2-7)
Rational "C" Coefficient, Mountain, Ponderosa Pine
As a function of Rainfall Depth, Hydrologic Soil Group (HSG), and % of Vegetation Cover (From ADOT Hydrology Manual Figure 2-8)
As a function of Rainfall Depth, Hydrologic Soil Group (HSG), and % of Vegetation Cover (From ADOT Hydrology Manual Figure 2-8)