§ 152.32 METHODOLOGY.

   The town will normally accept two methods for hydrology computations, the Rational Method and the Clark Unit Hydrograph Method. The Rational Method may be applied to watersheds less than 160 acres in area where detailed hydrograph operations are not necessary. The Rational Method may be applied by use of hand computations or appropriate available computerized methods. The Clark Unit Hydrograph Method or the FLO-2D Method shall be applied for all watersheds greater than 160 acres in area, unless prior approval of an alternative method is received in writing from the Public Works Department. The COE HEC-1 Flood Hydrograph Package (1991) (HEC-1) is the preferred computer program for application of the Clark Unit Hydrograph Method. Other methods may be used, with prior written approval from the Public Works Department.

(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)

§ 152.33 DRAINAGE MASTER PLAN.

   The town PDMP, prepared by Tetra Tech, Inc. in January 2001, has been adopted by the town and is incorporated as a working part of this chapter. The PDMP hydrology models shall be used as a basis for all development projects within the town wherever practical. The PDMP hydrology models are based on future condition land-uses. New studies must adjust the land-uses to reflect existing and proposed conditions for the specific site and contributing watershed being studied.

(Res. 1637, passed 2-28-02)

§ 152.34 RAINFALL.

   Rainfall parameters for hydrologic modeling shall be derived from the ADOT Hydrology Manual. Rainfall intensity estimates for use with the Rational Method shall be taken from the appropriate charts on the NOAA ATLAS 14 web site. Point precipitation values for hydrologic modeling shall be taken from the appropriate charts on the NOAA Atlas 14 web site.

(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)

§ 152.35 RAINFALL LOSSES FOR THE UNIT HYDROGRAPH METHOD.

   (A)   General. Rainfall losses are to be estimated in accordance with the procedures in the ADOT Hydrology Manual. The Green & Ampt infiltration method is to be used with the Clark Unit Hydrograph Method.

   (B)   Determination of soil texture. The rainfall loss characteristics of soils for undeveloped land within the town and the contributing watersheds have been prepared as a part of the PDMP. The results are summarized in Table 3.2. A soils map and report for the Town of Payson is available from the Engineering Department. This information is to be used for all 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 Department.

(Res. 1637, passed 2-28-02; Am. Res. 2871, passed 9-17-15; Am. Ord. 872, passed 9-17-15)

   Table 3.2: Rainfall Loss Characteristics for Undeveloped Land

Soil Class (1)	Hydrologic Soil Group (HSG) (2)	Bare Ground XKSAT In/hr (3)	Natural
			RTIMP % (4)	Veg. Cover % (5)	IA In (6)
1A	D	0.04	0	90	0.25
2A	C	0.25	0	90	0.25
3A	C	0.40	0	80	0.25
4A	C	0.40	0	75	0.25
4B	C	0.40	0	75	0.25
4C	C	0.40	0	75	0.25
5A	C	0.25	0	75	0.25
5B	D	0.06	5	75	0.25
6A	D	0.04	0	85	0.25
6B	D	0.04	0	75	0.25
6C	D	0.06	20	85	0.25
7A	C	0.40	50	25	0.25
7B	C	0.40	30	10	0.25
73	C	0.40	0	80	0.25
3730	C	0.40	5	62	0.25
3731	C	0.40	5	62	0.25
3753	C	0.40	20	62	0.25
3821	C	0.40	0	62	0.25
4140	D	0.06	0	45	0.25
4170	D	0.06	0	51	0.25
4175	C	0.40	15	76	0.25
4240	C	0.40	5	78	0.25
4241	C	0.40	10	80	0.25
4242	C	0.40	10	82	0.25
4457	C	0.40	15	75	0.25
4468	C	0.40	10	47	0.25
4469	C	0.40	10	47	0.25
5350	C	0.40	0	83	0.25
5351	C	0.40	0	83	0.25
5352	C	0.40	15	83	0.25

 

(Res. 1637, passed 2-28-02)

§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
	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)
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
	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)