GOES-West Enhanced Visible Image, July 10, 2008 at 6:30 PM MST

Storm Report: July Storms of 2008 in
Central Arizona

Stephen D. Waters, Engineering Division, Flood Warning Branch
October 8, 2008

TABLE OF CONTENTS
Storm of July 10th
Meteorology ................................................................................. 2
Precipitation ................................................................................ 4
Runoff........................................................................................ 10
Thoughts.................................................................................... 15

TABLES
Table 1

15-min. rainfall values for the six highest raingages ............. 7

Table 2

Precipitation frequency statistics for the Spookhill area ......... 8

Table 3

Precipitation frequency statistics for Whitlow Ranch Dam ...... 8

Table 4

Precipitation frequency statistics for Durango Complex ......... 9

Table 5

Precipitation frequency statistics for Magma FRS.................. 9

Table 6

Runoff data for selected Impoundment Stations..................10

Table 7

Runoff data for Selected Stream Stations...........................12

FIGURES
Figure I

GOES East/West Visible Image 7/10/2008 6:00 PM MST ....... 2

Figure II

FCDMC raingage 24-hour Rainfall Totals Map....................... 3

Figure III County-wide gage-adjusted 24-hour rainfall totals map ........ 4
Figure IV

East Valley gage-adjusted 24-hour rainfall totals map .......... 5

Figure V

Pinal County gage-adjusted 24-hour rainfall totals map ........ 6

Figure VI

Magma FRS Inflow Hydrograph ........................................11

Figure VII Spookhill FRS Data Page .................................................11
Figure VIII Rittenhouse FRS Inflow Hydrograph ..................................12
Figure IX

Spookhill @ McDowell Data Page ......................................13

Figure X

Spookhill @ McDowell Photo ............................................13

Figure XI

EMF @ Broadway Rd. Hydrograph.....................................14

Figure XII EMF @ Queen Creek Rd. Hydrograph ................................14
Figure XIII Spookhill Watershed Rainfall Amounts and Return Periods....15

Storm Report, July Storms of 2008; Page 1

Storm of July 13th
Meteorology ............................................................................... 17
Precipitation .............................................................................. 18
Runoff........................................................................................ 22
Thoughts.................................................................................... 25

TABLES
Table 8

15-min. rainfall values for the six highest raingages..........20

Table 9

Precipitation frequency statistics for Guadalupe FRS..........21

Table 10

Precipitation frequency statistics for Salt R. @ Priest Dr. ....21

Table 11

Runoff Data for Guadalupe FRS ......................................22

FIGURES
Figure XIV

GOES West Enhanced Visible - 7/13/2008 4:30 PM MST ....17

Figure XV

FCDMC rain gage 24-hour Rainfall Totals Map ..................18

Figure XVI

East Valley gage-adjusted 6-hour Rainfall Totals Map ........19

Figure XVII Guadalupe FRS Data Page. ............................................22
Figure XVIII Guadalupe FRS Inflow Hydrograph .................................23
Figure XIX

Guadalupe FRS Impoundment Photo ...............................23

Figure XX

Guadalupe Detention Basin Photo ...................................24

Figure XXI

July 13th Storm Photo ...................................................24

Figure XXII Average Summer Thunderstorm Season Rainfall Chart ......25

A special thank you to members of the NWS, USGS, Pinal County Flood Control
and the Naval Research Laboratory for supplying data and several of the figures
in this report.

Storm Report, July Storms of 2008; Page 2

Storm of July 10th, 2008
METEOROLOGY
In the days preceding July 10th a low pressure system moved slowly north from the
mountains of northern Mexico. This system drew large amounts of moisture from
Mexico and the Gulf of California into Arizona, to the point where the Phoenix
balloon sounding on the morning of the 10th measured 1.92 inches of precipitable
water – a very wet value. The sounding indicated a large degree of atmospheric
instability as well. A 65 mph wind gust was reported in Scottsdale near Scottsdale
Road and Palm Lane. The highest wind gust measured by an FCD gage was 35
mph at Rackensack Canyon north of Carefree. Storm-steering winds on the 10th
were from the northeast at greater than 10 knots.

Figure I

GOES East/West Visible Image, July 10, 2008 at 6:00 PM MST

Storm Report, July Storms of 2008; Page 3

PRECIPITATION

Figure II

Figure II – 24 hour precipitation values in the northeast County quadrant
ending 07/11/2008 at 05:00 MST. All readings are in inches and are from
automated Flood Control District rain gages.

Storm Report, July Storms of 2008; Page 4

Figure III

Figure III – This map was generated using GIS and a gridded rainfall product
provided by our weather vendor. The colored cells are approximately 1 km
on a side and represent an average rainfall depth for that area. The depths
are accumulated for the 24-hour period prior to 07/11/2008 at 5:00 AM
MST. The cell values are derived from an algorithm that sums the weather
radar returns, then adjusts them using the raingage readings. Black dots
represent ALERT raingage locations. White pixels within the red area at
lower right are values greater than 4.25 inches – see Figure IV for more
detail in this area.

Storm Report, July Storms of 2008; Page 5

Apache Junction

Florence Junction

Figure IV

Florence

Figure IV – This map was generated using GIS and a gridded rainfall product
provided by our weather vendor. The colored cells are approximately 1 km
on a side and represent an average rainfall depth for that area. The depths
are accumulated for the 24-hour period prior to 07/11/2008 at 5:00 AM
MST. The cell values are derived from an algorithm that sums the weather
radar returns, then adjusts them using the raingage readings. The black
numbers are ALERT raingage ID numbers. ID 6715 is Magma Dam in Pinal
County.

Storm Report, July Storms of 2008; Page 6

Figure V

Apache Junction

US 60

Vineyard FRS

Rittenhouse FRS

Figure V – This map was generated using GIS and a gridded rainfall product
provided by our weather vendor. The colored cells are approximately 1 km
on a side and represent an average rainfall depth for that area. The depths
are accumulated for the 24-hour period prior to 07/11/2008 at 5:00 AM
MST. The cell values are derived from an algorithm that sums the weather
radar returns, then adjusts them using the raingage readings. In the white
boxes – the value on the left is the 6-hour peak rainfall (inches) recorded by
the gage, and the value on the right is the estimated 6-hour return period
(years) for that point in space from NOAA Atlas 14 version 4. The two-digit
numbers within the grid cells are the estimated average rainfall amounts in
millimeters.

Storm Report, July Storms of 2008; Page 7

TABLE 1
DeviceID
StatType
DataType
Units
07/11/08
0500
0445
0430
0415
0400
0345
0330
0315
0300
0245
0230
0215
0200
0145
0130
0115
0100
0045
0030
0015
07/10/08
2400
2345
2330
2315
2300
2245
2230
2215
2200
2145
2130
2115
2100
2045
2030
2015
2000
1945
1930
1915
TOTALS:

6690
rain
precip
in

6715
rain
precip
in

6635
rain
precip
in

4560
rain
precip
in

6740
rain
precip
in

4700
rain
precip
in

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.04
0.00
0.00
0.04
0.04
0.04
0.00
0.04
0.00

0.04
0.00
0.04
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.16
0.16
0.75
0.43

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.04
0.00
0.04
0.04
0.04
0.08
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.04
0.04
0.08
0.04
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.12
0.00
0.04
0.04
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.04
0.04
0.04
0.00
0.04
0.04

0.00
0.00
0.00
0.00
0.04
0.04
0.00
0.04
0.00
0.00
0.00
0.00
0.04
0.43
0.16
0.35
0.71
0.59
0.43
0.00

0.24
0.47
0.08
0.04
0.00
0.08
0.39
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.08
0.00
0.00
0.00
0.00
0.12
0.31
0.16
0.28
0.51
1.06
0.12
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.04
0.04
0.04
0.00
0.00
0.31
0.59
0.16
0.31
0.59
0.47
0.00
0.00

0.08
0.47
0.16
0.20
0.51
0.43
0.12
0.08
0.00
0.08
0.35
0.08
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.08
0.35
0.31
0.39
0.91
0.28
0.00
0.00
0.00
0.00

3.07

2.95

2.91

2.83

2.80

2.64

Device ID 6690
6715
6635
4560
6740
4700

is
is
is
is
is
is

Spookhill Brown
Magma FRS
Spookhill McKellips
Spookhill FRS
Whitlow Ranch Dam
Durango Complex

Table 1 above presents 15-minute rainfall values for the six highest storm-rainfall
totals at FCD ALERT raingages. The bulk of the rainfall in the Spookhill area fell
between 7:30 PM and 9:00 PM, while further east at Magma and Whitlow Ranch
Dams it rained hardest between 10:30 PM and 12:30 AM.

Storm Report, July Storms of 2008; Page 8

Table 2 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of the Spookhill McKellips (6635) raingage. The red lines list the
recorded values from Spookhill McKellips, Spookhill Brown (6690) and Spookhill FRS
(4560) from 7/10/2008. These three stations are close enough in space and elevation that
one frequency table can be applied to all. The blue lines are the interpolated return periods
for each rainfall amount in years. AEP is “Annual Exceedance Probability”.
Precipitation Frequency Estimates (inches) 33.4511N 111.6678W
AEP*
5
10
15
30
60 120
12
(1-in3 hr 6 hr
min min min min min min
hr
Y)

24
hr

48
hr

4
7
10
day day day

20
day

30
day

45
day

60
day

2

0.22 0.34 0.42 0.57 0.71 0.80 0.85 1.01 1.15 1.43 1.53 1.92 2.14 2.34 2.92

3.42

4.03

4.51

5

0.33 0.50 0.62 0.83 1.03 1.15 1.20 1.39 1.57 1.98 2.15 2.74 3.07 3.34 4.16

4.87

5.73

6.40

10

0.40 0.61 0.76 1.02 1.26 1.39 1.45 1.66 1.86 2.36 2.58 3.33 3.74 4.06 5.01

5.86

6.87

7.64

25

0.50 0.75 0.93 1.26 1.56 1.70 1.78 2.01 2.23 2.87 3.14 4.14 4.66 5.04 6.12

7.15

8.32

9.19

50

10.34

0.57 0.86 1.07 1.44 1.78 1.94 2.04 2.28 2.50 3.26 3.59 4.79 5.40 5.82 6.96

8.14

9.41

100 0.64 0.97 1.21 1.63 2.01 2.19 2.31 2.56 2.78 3.67 4.04 5.47 6.19 6.65 7.82

9.14

10.51 11.48

200 0.71 1.09 1.35 1.81 2.24 2.43 2.59 2.84 3.07 4.09 4.52 6.21 7.03 7.52 8.69

10.16 11.63 12.61

500 0.81 1.24 1.53 2.07 2.56 2.77 2.98 3.24 3.45 4.66 5.17 7.24 8.23 8.76 9.89

11.56 13.12 14.12

1000 0.89 1.35 1.68 2.26 2.79 3.02 3.29 3.55 3.75 5.12 5.68 8.09 9.21 9.76 10.82 12.65 14.29 15.27
6635 0.43 0.83 1.10 1.61 2.13 2.56 2.64 2.83 2.91 2.91
≈AEP

13

41

58

93

144

284

225 195 136

27

6690 0.31 0.59 0.75 1.38 2.20 2.72 2.76 2.95 3.07 3.07
≈AEP

4

9

10

40

177

437

298 257 200

36

4560 0.35 0.67 0.83 1.18 1.73 2.44 2.60 2.76 2.83 2.83
≈AEP

6

15

15

18

43

206

205 164 113

23

Table 3 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of the Whitlow Ranch Dam raingage (6740). The red line lists the
recorded rainfall values from 7/10/2008. The blue line is the interpolated return period for
each rainfall amount in years. AEP is “Annual Exceedance Probability”.
Precipitation Frequency Estimates (inches) 33.2991N 111.2759W
AEP*
12
5
10 15 30 60 120
3 hr 6 hr
(1hr
min min min min min min
in-Y)

24
hr

48
hr

4
day

7
day

10
day

20
day

30
day

45
day

60
day

2

0.27 0.41 0.51 0.68 0.84 0.96 1.00 1.19 1.42 1.76 1.94 2.34 2.63

2.89

3.63

4.32

5.13

5.86

5

0.39 0.59 0.74 0.99 1.23 1.37 1.41 1.63 1.92 2.41 2.70 3.27 3.70

4.03

5.08

6.01

7.12

8.07

10

0.47 0.72 0.89 1.20 1.49 1.65 1.70 1.94 2.27 2.87 3.23 3.94 4.48

4.86

6.08

7.18

8.45

9.53

25

0.58 0.88 1.10 1.47 1.82 2.01 2.08 2.34 2.71 3.48 3.93 4.85 5.55

6.00

7.39

8.71

10.17 11.37

50

0.66 1.01 1.25 1.68 2.08 2.29 2.37 2.65 3.04 3.95 4.48 5.58 6.43

6.91

8.40

9.89

11.46 12.73

100 0.74 1.13 1.40 1.89 2.33 2.57 2.67 2.96 3.38 4.43 5.04 6.36 7.37

7.88

9.43

11.11 12.76 14.08

200 0.82 1.25 1.55 2.09 2.59 2.85 2.99 3.29 3.72 4.93 5.63 7.18 8.39

8.91

10.50 12.35 14.07 15.42

500 0.93 1.42 1.76 2.37 2.93 3.22 3.42 3.72 4.17 5.62 6.43 8.34 9.85

10.38 11.96 14.06 15.83 17.20

1000 1.01 1.54 1.91 2.58 3.19 3.52 3.78 4.07 4.52 6.17 7.08 9.29 11.06 11.57 13.11 15.39 17.19 18.55
6740 0.16 0.31 0.43 0.83 1.30 2.01 2.52 2.76 2.80 2.80
≈AEP

<2

<2

<2

3

8

25

71

64

30

9

Storm Report, July Storms of 2008; Page 9

Table 4 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of the Durango Complex raingage (4700). The red line lists the
recorded rainfall values from 7/10/2008. The blue line is the interpolated return period for
each rainfall amount in years. AEP is “Annual Exceedance Probability”.
Precipitation Frequency Estimates (inches) 33.4266N 112.1186W
AEP*
10
5
(1-inmin min
Y)

15
min

30
min

60
min

120
12
3 hr 6 hr
min
hr

24
hr

48
hr

4
7
day day

10
day

20
day

30
day

45
day

60
day

2

0.23 0.35 0.43 0.58 0.72 0.81 0.86 1.02 1.14 1.31 1.40 1.53 1.69 1.83 2.24 2.62 3.04

3.38

5

0.34 0.51 0.63 0.85 1.06 1.17 1.23 1.42 1.58 1.83 1.98 2.19 2.41 2.61 3.19 3.73 4.33

4.80

10

0.41 0.62 0.77 1.04 1.29 1.42 1.48 1.70 1.87 2.20 2.39 2.65 2.92 3.16 3.83 4.47 5.17

5.71

25

0.51 0.77 0.95 1.29 1.59 1.75 1.83 2.06 2.25 2.68 2.95 3.28 3.62 3.90 4.65 5.43 6.22

6.85

50

0.58 0.88 1.09 1.47 1.82 1.99 2.10 2.34 2.53 3.05 3.38 3.78 4.17 4.49 5.27 6.16 7.01

7.68

100 0.65 0.99 1.23 1.65 2.04 2.24 2.38 2.63 2.82 3.44 3.83 4.31 4.75 5.10 5.90 6.89 7.78

8.49

200 0.72 1.10 1.37 1.84 2.28 2.49 2.67 2.92 3.11 3.84 4.30 4.87 5.36 5.74 6.53 7.62 8.54

9.28

500 0.82 1.25 1.55 2.09 2.58 2.83 3.06 3.31 3.50 4.40 4.95 5.66 6.22 6.64 7.37 8.61 9.54

10.30

1000 0.90 1.36 1.69 2.28 2.82 3.09 3.39 3.62 3.81 4.83 5.48 6.30 6.91 7.35 8.02 9.37 10.29 11.05
4700 0.39 0.75 1.02 1.42 2.17 2.32 2.32 2.60 2.64 2.64
≈AEP

8

22

35

41

146

125

86

93

65

23

Table 5 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of a point between three raingages: Magma FRS (6715, owned by
FCDMC), Magma Cattle Company (711, approx. 7 miles SE of Florence Jct.) and Magma
79 (721, approx. 4 miles S of Florence Jct.), both of which are owned by Pinal County Flood
Control. These three stations are close enough in space and elevation that one frequency
table can be applied to all, and are all in the watershed contributing to Magma FRS. The
blue lines are the interpolated return periods for each rainfall amount in years.
Precipitation Frequency Estimates (inches) 33.1692N 111.3267W
AEP*
(1-inY)

5
min

10
min

15
min

30
min

60
min

120
min

2

0.25 0.37

0.46

0.62

0.77

0.87

0.91

1.09

1.26

1.54 1.69 1.97 2.19 2.40

2.99

3.52

5

0.36 0.55

0.68

0.91

1.13

1.26

1.30

1.50

1.72

2.13 2.37 2.78 3.09 3.38

4.22

4.96

10

0.44 0.66

0.82

1.11

1.37

1.52

1.56

1.78

2.03

2.54 2.85 3.36 3.75 4.09

5.06

5.94

25

0.54 0.82

1.01

1.36

1.69

1.86

1.92

2.15

2.43

3.09 3.48 4.15 4.64 5.05

6.15

7.21

50

0.61 0.93

1.16

1.56

1.93

2.12

2.19

2.44

2.73

3.51 3.97 4.78 5.36 5.81

6.98

8.18

100

0.69 1.05

1.30

1.75

2.17

2.38

2.48

2.73

3.03

3.95 4.48 5.45 6.13 6.61

7.82

9.16

200

0.77 1.17

1.45

1.95

2.41

2.64

2.77

3.03

3.34

4.40 5.02 6.15 6.94 7.45

8.67

10.16

500

0.87 1.33

1.64

2.21

2.74

3.00

3.19

3.45

3.75

5.02 5.75 7.15 8.10 8.65

9.84

11.52

1000 0.95 1.45

1.79

2.41

2.99

3.27

3.52

3.78

4.07

5.52 6.34 7.96 9.04 9.61

10.74 12.57

3 hr

6 hr 12 hr

24
hr

6715

0.28

0.55

0.79

1.22

1.89

2.28

2.80

2.87

2.95

2.95

≈AEP

3

5

9

15

45

77

214

138

83

20

3.82

4.17

4.21

4.21

711

0.35

0.51

0.67

1.06

1.65

2.72

≈AEP

5

4

5

8

22

245

721

0.35

0.59

0.79

1.26

1.73

≈AEP

5

6

9

17

28

3.27

48
hr

4
day

7
day

10
day

20
day

30
day

>1000 >1000 >1000 149
3.74

4.13

4.49

4.69

1000 >1000 >1000 >1000 307

Storm Report, July Storms of 2008; Page 10

RUNOFF
There were two big runoff stories in this event:
The first was at Spookhill FRS and the new Spookhill levee along the section
of Loop 202 that was scheduled to open on July 15th. The runoff facilities
functioned well under the extreme rainfall, but numerous areas of bank
erosion were noted and repaired. FCD ALERT stations at McDowell,
McKellips and Brown Roads were moved after the event, from the sides of
the box culverts to their tops, because erosion was severe enough to nearly
topple the standpipes.
The second story occurred at Magma FRS in Pinal County, where “off the
scale” rainfall intensities (see bottom of page 9) filled the dam to 45% of its
spillway capacity. This was a good test of the Emergency Action Plan that
had recently been developed for the dam, as there were concerns regarding
the dam’s structural integrity from recent inspections. About 2 hours into
the fill event the District’s pressure transducer, which was mounted on the
upstream side of the dam’s principal outlet, was broken from its mounts by
the high-velocity inflow. However, the gage did its job as responders were
notified to visually monitor the dam.

Selected Impoundment Stations (Table 6)

Structure Name

ID

Peak
Stage
(ft)

Magma FRS
Rittenhouse FRS
Spookhill FRS
Vineyard FRS
Whitlow Ranch Dam

6718
6703
4563
6688
6739

18.74
10.06
8.90
0.73
11.14

Peak
Outflow
(cfs)

Peak
Storage
(ac-ft)

167
106
58
5
250

2,470
403
209
29
13

Peak
Date-Time
Capacity
(% full)
45
10
10
<1
<1

Unknown
7/11-06:56
7/10-23:10
7/11-09:26
7/11-01:12

Vineyard FRS is included above because of what didn’t happen there.
Looking at the rainfall values in Figure V one might assume that Vineyard (&
Powerline too) would experience a significant impoundment. This shows the
ability of these watersheds to absorb large amounts of water after being dry
for several months. On 7/13/2008 a rainfall event of much lesser
magnitude, falling on a wet watershed, caused a ponding elevation of 1.3
feet at Vineyard FRS.

Storm Report, July Storms of 2008; Page 11

Figure VI

Magma FRS - ID 6718
1624.00

Emergency Spillway Elevation: 1623.10

1622.00
1620.00

Maximum Fill Level: 1618.74

1618.00

Elevation (ft)

1616.00
1614.00
1612.00
1610.00
1608.00
1606.00
1604.00
Point at which transducer began to malfunction

1602.00
1600.00
7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008
0:00
1:00
2:00
3:00
4:00
5:00
6:00
7:00
8:00
9:00
10:00
11:00
12:00
Date / Time

Magma FRS – Inflow Hydrograph

Figure VII

Spookhill FRS near 10% of spillway capacity – 07/10/2008 @ 11:10 PM MST
Storm Report, July Storms of 2008; Page 12

Rittenhouse FRS
11.00
10% of Spillway Capacity: 10.02 feet

10.00
9.00
8.00

Stage (ft)

7.00
6.00
5.00
4.00
3.00

Figure VIII

2.00
1.00

0.00
7/10/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/12/2008 7/12/2008 7/12/2008 7/12/2008 7/13/2008 7/13/2008 7/13/2008
18:00
0:00
6:00
12:00
18:00
0:00
6:00
12:00
18:00
0:00
6:00
12:00
Date / Time

Rittenhouse FRS – Inflow Hydrograph

Selected Stream Stations (Table 7)
Water-level Sensor Name
ACDC @ 43rd Ave.
ACDC @ 67th Ave.
EMF @ Broadway
Granite Reef
Granite Reef Wash
Guadalupe Channel
Indian Bend Wash @ Shea
New River @ Glendale
Spookhill Brown
Spookhill McDowell
Spookhill McKellips

Peak Discharge
(cfs)

Peak Stage
(ft)

Date-Time

ID
4823
5523
6573
4568
4728
6603
4693
5508
6693
6637
6638

2,344
1,627
1,402
751
39
739
589
3,216
58
57
58

3.85
5.50
2.78
1.75
3.00
2.45
1.88
1.33
5.42
7.43
6.60

7/10-21:48
7/10-22:34
7/10-22:44
7/10-22:02
7/11-02:09
7/10-20:35
7/10-22:24
7/11-01:14
7/10-21:13
7/10-22:29
7/10-22:44

Storm Report, July Storms of 2008; Page 13

Figure IX

Spookhill at McDowell Stream Gage near its peak, 07/10/2008 @ 11:00 PM MST

Figure X
These partially submerged culverts are on the upstream side of the Spookhill Levee, and pass water from south to
north beneath McDowell Road. The new section of the Loop 202 freeway is off the photo to the left. The water
surface appears brown because of all the floating straw washed down from recent landscaping.

Storm Report, July Storms of 2008; Page 14

EMF @ Broadway Rd.
1500

Figure XI

1350

1200

Discharge (cfs)

1050

900

750

600

450

300

150

0
7/10/2008 7/10/2008 7/10/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/12/2008 7/12/2008 7/12/2008 7/12/2008
12:00
16:00
20:00
0:00
4:00
8:00
12:00
16:00
20:00
0:00
4:00
8:00
12:00
Date - Time

EMF at Queen Creek Road
1500

Figure XII

1350

1200

Discharge (cfs)

1050

900

750

600

450

300

150

0
7/10/2008 7/10/2008 7/10/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/11/2008 7/12/2008 7/12/2008 7/12/2008 7/12/2008
12:00
16:00
20:00
0:00
4:00
8:00
12:00
16:00
20:00
0:00
4:00
8:00
12:00
Date - Time

Two hydrographs from gages on the East Maricopa Floodway. The X and Y scales are the
same to show the hydrograph translation as it moved downstream from Broadway to Queen
Creek Road.
Storm Report, July Storms of 2008; Page 15

THOUGHTS
The new ADOT levee along the Loop 202 freeway (which opened for traffic
on July 15th) was tested by this locally severe event. Rainfall on the
structure itself exceeded 150 year return periods, and the logarithmic
average of the seven raingages in the watershed directly feeding it was 47.5
years (see Figure XIII below). Luckily, the structure did not receive a large
contribution from the watershed feeding Signal Butte and Apache Junction
Flood Retarding Structures, and in general runoff was not as abundant as it
could have been because this was the first major storm of the season.
In areas near and including the watershed feeding Magma FRS in Pinal
County, approximately 117 square miles received three or more inches of
rain from this storm, with measured return periods exceeding 1,000 years
(see Table 5). This is a very rare storm for central Arizona, and would have
caused major damage to infrastructure and transportation had it occurred in
the urban area. The fact that the Magma FRS only filled to 45% of its
spillway capacity can again, I believe, be attributed to the dry condition of
the watershed prior to the storm.

Figure XIII

Storm Report, July Storms of 2008; Page 16

Storm of July 13th, 2008
METEOROLOGY
July 11th and 12th were relatively stable days in our area, but atmospheric moisture
remained at elevated levels. On the 11th a complex of storms developed in
northern Pima County that dropped more than 4 inches in some areas. Instability
returned to the region on the 13th. Storms formed over the mountains to our east
and by early evening were pushing outflow gusts into the Valley. The resultant
storm that formed over southeast Phoenix and north Tempe dropped more than
two inches of rain in a short period of time. The highest wind gust measured by an
FCD gage was 36 mph at McMicken Dam in Surprise.

Figure XIV
GOES-West Enhanced Visible Image, July 13, 2008 at 4:30 PM MST

Storm Report, July Storms of 2008; Page 17

PRECIPITATION

Figure XV

Figure XV – 24 hour precipitation values in the central metro area ending
07/14/2008 at 05:00 MST. All readings are in inches and are from
automated Flood Control District raingages.

Storm Report, July Storms of 2008; Page 18

Figure XVI

Figure XVI – This map was generated using GIS and a gridded rainfall
product provided by our weather vendor. The colored cells are
approximately 1 km on a side and represent an average rainfall depth for
that area. The depths are accumulated for the 6-hour period prior to
07/13/2008 at 9:00 PM MST. The cell values are derived from an algorithm
that sums the weather radar returns, then adjusts them using the raingage
readings. The black numbers are ALERT raingage ID numbers. ID 6500,
near the center of the red cells, is Guadalupe FRS.

Storm Report, July Storms of 2008; Page 19

TABLE 8
DeviceID
StatType
DataType
Units
07/13/08
2000
1945
1930
1915
1900
1845
1830
1815
1800
1745
1730
1715
1700
1645
1630
1615
1600
1545
1530
1515
1500
1445
1430
1415
TOTALS:

6500
rain
precip
in

4520
rain
precip
in

4510
rain
precip
in

4600
rain
precip
in

6550
rain
precip
in

4525
rain
precip
in

0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.00
0.04
0.00
0.00
0.00
0.24
0.83
0.79
0.43
0.04
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.04
0.00
0.04
0.00
0.00
0.00
0.00
0.12
0.16
0.28
0.67
0.39
0.24
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.04
0.00
0.00
0.04
0.00
0.08
0.00
0.00
0.20
0.94
0.55
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.08
0.12
0.35
0.71
0.28
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.04
0.16
0.35
0.67
0.31
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.16
0.24
0.51
0.55
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

2.40

1.93

1.85

1.54

1.54

1.46

Device ID 6500
4520
4510
4600
6550
4525

is
is
is
is
is
is

Guadalupe FRS
Salt River @ Priest Dr.
Roeser @ 2nd St.
IBW @ McKellips Rd.
Ahwatukee
ASU South

Table 8 above presents 15-minute rainfall values for the six highest storm-rainfall
totals at FCD ALERT raingages. Note that the bulk of the rainfall fell in a one-hour
period between 4:00 and 5:00 PM in the afternoon.

Storm Report, July Storms of 2008; Page 20

Table 9 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of the Guadalupe FRS raingage (6500). The red line lists the recorded
rainfall values from 7/13/2008. The blue line is the interpolated return period for each
rainfall amount in years. AEP is “Annual Exceedance Probability”.

Precipitation Frequency Estimates (inches) 33.3719N 111.9705W
AEP*
5
10 15 30 60 120
12
3 hr 6 hr
(1min min min min min min
hr
in-Y)

24
hr

48
4
7
10 20 30
hr day day day day day

45
day

60
day

2

0.21 0.32 0.40 0.54 0.67 0.77 0.83 0.97 1.08 1.30 1.41 1.56 1.72 1.87 2.31 2.69 3.14 3.49

5

0.31 0.48 0.59 0.79 0.98 1.12 1.18 1.35 1.49 1.82 2.00 2.23 2.45 2.67 3.29 3.84 4.47 4.97

10

0.38 0.58 0.72 0.97 1.20 1.36 1.42 1.61 1.76 2.18 2.42 2.70 2.98 3.24 3.96 4.61 5.35 5.92

25

0.47 0.72 0.89 1.20 1.48 1.67 1.75 1.96 2.12 2.66 2.97 3.35 3.69 4.00 4.81 5.60 6.45 7.11

50

0.54 0.82 1.02 1.38 1.70 1.90 2.01 2.22 2.38 3.04 3.41 3.86 4.25 4.60 5.46 6.35 7.27 7.97

100 0.61 0.93 1.15 1.55 1.92 2.14 2.28 2.49 2.65 3.42 3.86 4.40 4.85 5.23 6.11 7.11 8.08 8.82
200 0.68 1.04 1.29 1.73 2.14 2.39 2.56 2.77 2.92 3.82 4.33 4.98 5.47 5.89 6.76 7.87 8.89 9.65
500 0.78 1.18 1.46 1.97 2.44 2.71 2.94 3.16 3.29 4.36 4.99 5.79 6.36 6.82 7.65 8.91 9.95 10.74
1000 0.85 1.29 1.60 2.16 2.67 2.97 3.25 3.46 3.58 4.80 5.52 6.45 7.07 7.57 8.33 9.70 10.75 11.54
6500 0.28 0.55 0.83 1.57 2.28 2.36 2.40 2.40
≈AEP

4

8

21

108 307 184 135

79

Table 10 below is the NOAA Atlas 14 Point Precipitation Frequency Estimate Table at the
latitude/longitude of the Salt River at Priest Dr. raingage (4520). The red line lists the
recorded rainfall values from 7/13/2008. The blue line is the interpolated return period for
each rainfall amount in years. AEP is “Annual Exceedance Probability”.

Precipitation Frequency Estimates (inches) 33.4320N 111.9613W
AEP*
5
10 15 30 60 120
12
3 hr 6 hr
(1min min min min min min
hr
in-Y)

24
hr

4
7
10 20 30 45
48
hr day day day day day day

60
day

2

0.21 0.32 0.39 0.53 0.65 0.75 0.80 0.95 1.07 1.29 1.42 1.57 1.73 1.89 2.33 2.72 3.17 3.52

5

0.31 0.47 0.58 0.78 0.96 1.09 1.14 1.33 1.47 1.81 2.01 2.24 2.47 2.69 3.32 3.87 4.51 5.01

10

0.38 0.57 0.71 0.95 1.18 1.32 1.38 1.58 1.74 2.17 2.44 2.72 3.00 3.26 3.99 4.65 5.40 5.97

25

0.46 0.71 0.88 1.18 1.46 1.62 1.70 1.92 2.09 2.65 3.00 3.37 3.72 4.03 4.86 5.66 6.51 7.17

50

0.53 0.81 1.01 1.35 1.68 1.86 1.96 2.19 2.36 3.02 3.44 3.89 4.29 4.64 5.51 6.41 7.34 8.05

100 0.60 0.91 1.13 1.53 1.89 2.09 2.22 2.46 2.63 3.40 3.90 4.44 4.89 5.28 6.16 7.18 8.16 8.90
200 0.67 1.02 1.27 1.71 2.11 2.33 2.49 2.73 2.90 3.80 4.38 5.02 5.52 5.95 6.83 7.95 8.97 9.74
500 0.77 1.17 1.45 1.95 2.41 2.65 2.87 3.11 3.26 4.35 5.05 5.84 6.41 6.88 7.72 9.00 10.05 10.84
1000 0.84 1.28 1.58 2.13 2.64 2.91 3.18 3.41 3.55 4.78 5.59 6.51 7.13 7.63 8.41 9.80 10.86 11.65
4520 0.24 0.43 0.63 1.02 1.61 1.85 1.89 1.93
≈AEP

3

4

7

13

40

49

42

26

Storm Report, July Storms of 2008; Page 21

RUNOFF
Runoff of consequence was in effect limited to the high-rainfall areas shown
in Figure XVI. Because most of this area is urbanized there are few
established drainage channels and therefore few stream gages. Two runoff
instances stand out – the inflow to Guadalupe FRS, which reached its highest
impoundment ever since records were established in 1989, and the
Superstition Freeway (US 60) from Priest Drive to Mill Avenue, which was
closed for several hours due to ponded water accumulating faster than it
could be pumped out.

Selected Impoundment Station (Table 11)

Structure Name
Guadalupe FRS

ID

Peak
Stage
(ft)

Peak
Outflow
(cfs)

Peak
Storage
(ac-ft)

6503

9.41

Gated

36

Peak
Date-Time
Capacity
(% full)
12.8

7/13-17:38

Figure XVII

Guadalupe FRS near its peak, 07/13/2008 @ 6:00 PM MST

Storm Report, July Storms of 2008; Page 22

Figure XVIII

Guadalupe FRS (6503)
10

9

8

7

Stage (feet)

6

5

4

3

2

1

0
7/13/2008
12:00

7/13/2008
16:00

7/13/2008
20:00

7/14/2008
0:00

7/14/2008
4:00

7/14/2008
8:00

7/14/2008
12:00

7/14/2008
16:00

7/14/2008
20:00

7/15/2008
0:00

Date - Time

Guadalupe FRS – Inflow Hydrograph

Figure XIX

Storm Report, July Storms of 2008; Page 23

Figure XX

Guadalupe Detention Basin in the Town of Guadalupe approximately 1 mile SE of Guadalupe
FRS. Although there is not a water-level monitoring station at the basin, this is likely the largest
impoundment experienced since its construction in 2003.
Figure XXI

This photo of the storm was taken by the author on 07/13/2008 at 4:15 PM. The view is to the
east from the Flood Control District office at 2801 W. Durango St., Phoenix.

Storm Report, July Storms of 2008; Page 24

THOUGHTS
This storm was much smaller in aerial extent than the storm on the 10th, but
was significant because of the area that was affected. For reasons of
geography, storm patterns, or just chance, the area east of the Salt River
Mountains (which includes South Mountain) is infrequently hit by severe
storms. As noted on page 22, this storm produced the largest volume of
water stored behind Guadalupe FRS since 1989, and is likely the largest
storm since 1969 – the storm that originally prompted the construction of
the dam. Had this area been wetted sufficiently by the storm on the 10th
(see Figure III), or if the core of the storm had been more centered over the
watershed, the impoundment would have been much larger.
As shown in the chart below, Monsoon 2008 was the second wettest in
central Arizona since 1999. These two storms (July 10th and 13th) were the
result of greater than normal moisture being advected into the State from
Mexico, and this condition continued into August where urban Phoenix
experienced three additional notable storms (7th, 25th and 28th).

Figure XXII

Storm Report, July Storms of 2008; Page 25