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§152.36 LAND USE.
   The rainfall loss characteristics of future condition developed land within the town and the contributing watersheds have been prepared as a part of the PDMP. The results are summarized in Tables 3.3 and 3.4. This information is to be used for all existing and proposed condition unit hydrograph models prepared for studies within the town. The engineer may use more detailed information specific to a site with written approval of the Public Works Engineer. Land-use rainfall loss characteristics for developed land for use with the Rational Method ("C" coefficients) are to be derived from Table 3.5.
(Res. 1637, passed 2-28-02)
   Table 3.3: Rainfall Loss Characteristics for Developed Land
Land-Use Class (1)
Developed
Land-Use Class (1)
Developed
RTIMP % (2)
Veg. Cover % (3)
IA In (4)
VLDR
20
50
0.20
LDR
25
45
0.20
MDR
45
25
0.20
MFR
65
20
0.20
C
90
5
0.05
GC
0
98
0.20
GC/N
2
85
0.20
P
100
0
0.05
RW
75
12
0.08
N
---
---
---
NC
---
---
---
L
100
0
0.00
LDR1
51
29
0.16
LDR3
58
25
0.13
LDR5
45
33
0.15
S
45
52
0.13
RES
45
33
0.15
REDI
55
21
0.18
REDH
57
26
0.16
REDF
60
28
0.14
 
(Res. 1637, passed 2-28-02)
   Table 3.4: Descriptions of Land-Use Classifications
Land-Use Class (1)
Description (2)
Land-Use Class (1)
Description (2)
VLDR
Very Low Density Residential
LDR
Light Density Residential
MDR
Medium Density Residential
MFR
Multi-Family Residential
C
Commercial
GC
Golf Course/Park
GC/N
Golf Course - Not Designed
P
Paving
RW
Street Right-of-Way
N
Natural (see soils parameters)
NC
Non-Contributing
L
Lake; contributing drainage area
LDR1
Low Density PAD #1
LDR3
Low Density PAD #3
LDR5
Low Density PAD #5, #6, #7
S
School/Resort
RES
Reservation
REDI
Redevelopment Area in Sub-basin 120I
REDH
Redevelopment Area in Sub-basin 121H
REDF
Redevelopment Area in Sub-basin 121F
 
(Res. 1637, passed 2-28-02)
   Table 3.5: Developed Condition "C" Coefficients for Use with the Rational Method
Land Uses
Return Period
Land Uses
Return Period
2-10 Year
25 Year
50 Year
100 Year
Streets and Roads
Paved Roads
0.75 - 0.85
0.83 - 0.94
0.90 - 0.95
0.94 - 0.95
Gravel Road- ways and Shoulders
0.60 - 0.70
0.66 - 0.77
0.72 - 0.84
0.75 - 0.88
Industrial Areas
Heavy
0.70 - 0.80
0.77 - 0.88
0.84 - 0.95
0.88 - 0.95
Light
0.60 - 0.70
0.66 - 0.77
0.72 - 0.84
0.75 - 0.88
Business Areas
Commercial - Downtown
0.75 - 0.85
0.83 - 0.94
0.90 - 0.95
0.95
Commercial - Neighborhood
0.55 - 0.65
0.61 - 0.72
0.66 - 0.78
0.69 - 0.81
Residential Areas
Lawns - Flat
0.10 - 0.25
0.11 - 0.28
0.12 - 0.30
0.13 - 0.31
Lawns - Steep
0.25 - 0.40
0.28 - 0.44
0.30 - 0.48
0.31 - 0.50
Very Low Density Residential
0.30 - 0.40
0.33 - 0.44
0.36 - 0.48
0.38 - 0.50
Low Density Residential
0.45 - 0.55
0.50 - 0.61
0.54 - 0.66
0.56 - 0.69
Medium Density Residential
0.50 - 0.60
0.55 - 0.66
0.60 - 0.72
0.63 - 0.75
Multi-Family Residential
0.60 - 0.70
0.66 - 0.77
0.72 - 0.84
0.75 - 0.88
Parks/Cemeteries
0.10 - 0.25
0.11 - 0.28
0.12 - 0.30
0.13 - 0.31
Playgrounds
0.40 - 0.50
0.44 - 0.55
0.48 - 0.60
0.50 - 0.63
 
Note: Values of C for the 25-, 50- and 100-year frequencies were derived using frequency adjustment factors of 1.10, 1.20, and 1.25, respectively, with an upper limit of 0.95 for C for the 2- through 10-year values.
(Res. 1637, passed 2-28-02)
§ 152.37 RATIONAL METHOD.
   (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)
 
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)
§ 152.38 CLARK UNIT HYDROGRAPH METHOD.
   The Clark Unit Hydrograph Method shall be used in conjunction with HEC-1 for all watersheds greater than or equal to 160 acres, for design of detention basins; and where hydrograph channel or storage routing and combination is necessary. The Clark Unit Hydrograph method shall be applied in conformance with the procedures set forth in the ADOT Hydrology Manual.
(Res. 1637, passed 2-28-02)
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