Table of Contents:
Section 1: Introduction ..........................................................................................4
1.1 Purpose .......................................................................................................4
1.2 Project Authority ..........................................................................................4
1.3 Project Location...........................................................................................5
1.4 Methodologies Used for Hydrology and Hydraulics.....................................5
1.5 Acknowledgements .....................................................................................5
This study relied on assistance of RFCD GIS staff, who were integral to the
development of the models and maps...............................................................5
1.6 Study Results ..............................................................................................5
Section 2.0 Summary of Key Facts.......................................................................9
Section 2.1: General Information.......................................................................9
Section 2.2: Mapping Information......................................................................9
Section 2.3: Hydrology ......................................................................................9
Section 2.4: Hydraulics......................................................................................9
Section 2.5: Additional Study Information:.........................................................9
Section 3: Survey and Mapping Information .......................................................10
3.1 Field Survey Information ...........................................................................10
3.2 Mapping ....................................................................................................10
Section 4: Hydrology...........................................................................................10
4.1 Method description. ...................................................................................10
4.2 Parameter estimation. ...............................................................................10
4.3 Problems encountered during the study. ...................................................15
4.4 Calibration .................................................................................................16
4.5 Final results ...............................................................................................16
Section 5: Hydraulics .........................................................................................18
5.1 Method description. ...................................................................................18
5.2 Work study maps.......................................................................................18
5.3 Parameter estimation. ...............................................................................18
5.5 Modeling considerations............................................................................19
5.6 Floodway modeling ...................................................................................19
5.7 Problems encountered during the study. ...................................................19
5.8 Calibration. ................................................................................................20
5.9 Final results. ..............................................................................................20
Section 6: Erosion and Sediment Transport .......................................................20
Section 7: Ratio of the top width of 100-yr and 25-yr floodplain..........................21

2

List of Figures:
Figure 1.1 – Geronimo Watershed Map................................................................6
Figure 1.2 – Study limit for the Geronimo Wash ...................................................7
Figure 1.3 – Zoning Classification Map for the Geronimo Wash...........................8
Figure 4.1 – Flow Chart of Mapping Process......................................................12
Figure 4.2 – Hydrologic Soils Groups .................................................................14
5.4 Cross section description. .............................................................................19
List of Tables:
Table 4.1 - Methods used for a Hec-HMS analysis.............................................11
Table 4.2 - Sub-basin Characteristics .................................................................15
Table 4.3 - Sub-basin discharges .......................................................................15
Table 4.4 – Summary of 100-yr Peak Discharge Values ....................................17
Table 4.5 – Summary of 25-yr Peak Discharge Values ......................................17
Table 4.6 – Summary of 500-yr Peak Discharge Values ....................................17
Table 4.7 – Comparison of 100-yr Peak Discharge Values ................................17
Exhibit
Exhibit 1 100-yr and 500-yr Floodplain Limit Map for the Geronimo Wash
Exhibit 2 Annotated Flood Insurance Rate Map
Attached CD
Geronimo TDN with supporting models and GIS data.

3

Section 1: Introduction
1.1 Purpose
The purpose of this study is to provide flood and erosion hazard information for the
Geronimo Wash for use by the Pima County Regional Flood Control District (District) in
floodplain use permitting and floodplain management. More specifically, it provides:
• discharge values for sub-basins and important concentration points;
• hydrographs for use with floodplain mapping;
• floodplain mapping for channels with contributing areas greater than 1 square
mile, and channels with 100-yr discharges greater than 2000 cfs, which are treated
differently under the Pima County Ordinance.
1.2 Project Authority
The State of Arizona has delegated the responsibility to each county flood control district
to adopt floodplain regulations designed to promote the public health, safety and general
welfare of its citizenry as provided under the Arizona Revised Statutes, Title 48, Chapter
21, Article 1, Sections 48-3601 through 3627. More specifically, A.R.S. 3609 directs
county flood control districts to adopt floodplain regulations that:
A. Regulate all development of land, construction of residential, commercial or
industrial structures or uses of any kind which may divert, retard or obstruct flood
water and threaten public health or safety or the general welfare; and
B. Establish minimum flood protection elevations and flood damage prevention
requirements for uses, structures and facilities which are vulnerable to flood
damage; and
C. Comply with state and local land use plans and ordinances, if any.
In conformance with A.R.S. 3609, this ordinance provides for protection of the
public health safety and welfare by regulation of flood and erosion hazard areas to
control flood hazards and prevent repetitive loss from flood damage.
D. The flood hazard areas of Pima County are subject to periodic inundation
which may result in loss of life and property, create health and safety hazards,
disrupt commerce and governmental services, require extraordinary public
expenditures for flood protection and relief, and impair the tax base, all of which
adversely affect the public health, safety, and general welfare.
E. These flood losses are caused by the cumulative effect of obstructions in areas
of special flood hazards which increase flood heights, flow velocities, and cause
flood and erosion damage. Uses that are inadequately flood-proofed, elevated, or
otherwise protected from flood damage, also contribute to the flood loss. (Ord.
2005 FC-2 § 2 (part), 2005).
Section 16 of the Pima County Ordinance describes the provisions for floodplain
regulation in Pima County.

4

1.3 Project Location
The study was performed to provide drainage information for the Geronimo Wash. The
site includes Sections 32 of Township 1 South, Range 14 East, Sections 5, 6 and 7 of
Township 13 South, Range 14 East, and Section 13 of Township 13 South, Range 12
East, Pima County, Arizona. The entire watershed of the Geronimo Wash is in FEMA
Zone X, as shown on the current Flood Insurance Rate Map (FIRM) number 04019C1630 and 1635K.
The watershed is 3.37 square mile. The study watershed was divided into six sub-basins
(Fig.1.1). The study limits for the Geronimo Wash extends from a junction with the Pima
Wash to the north of Calle sin Desengano (Fig.1.2). Major zoning classification of the
watershed is CR-1 (Fig.1.3).
1.4 Methodologies Used for Hydrology and Hydraulics
Topographic, hydrologic and hydraulic analyses were performed to determine drainage
conditions in the Geronimo Wash. ArcGIS, Version 9.3, HEC-HMS Version 3.4 (HECHMS), Hec-RAS Version 4.0 (HEC-RAS), and HEC-GeoRAS, Version 4.1.1 (HECGeoRAS) were used for the analyses.
1.5 Acknowledgements
This study relied on assistance of RFCD GIS staff, who were integral to the development
of the models and maps.
1.6 Study Results
The modeled discharge for the Geronimo Wash at the confluence with the Pima Wash is
4762 cfs, where the area is 3.37 square miles. The floodplain was mapped in the
downstream of Calle sin Desengano. The study found that some homes at risk for
flooding during the 100-yr flood. A 500-yr floodplain limit was also mapped. In general,
the footprint of the 500-yr floodplain is only slightly larger than the 100-yr floodplain.

5

Figure 1.1
Watershed Map
Geronimo Wash

OR
AC

LE

OR
AC

LE

HARDY

GER_CP
Geronimo_contour20ft
Geronimo Subbasins
GER B
GER C
GER D
GER E
GER F
GER G
2008PAGclr01ft.ecw
(
!

(
CP F !

INA

INA

(
CP E !

Pima County Index Map

(
CP D !

1ST

LE
OR AC

Index Map Scale 1:5,250,000

LL
PBE

ORANGE GROVE

SU

CAM

(
CP C !

SKYLINE

(
!
!
CP A (
CP B

2,000

1,000

0

NR

IS
E

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.
This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.

SU

Pima County Regional Flood Control District

NR

Scale 1:3000

IS
E

2,000 Feet

02/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Geronimo_wash_Watershed_Fig1_1.mxd

Figure 1.2
Study Limit
Geronimo Wash

OR
AC

LE

OR
AC

LE

HARDY

(
!

GER_CP
River
Geronimo Subbasins

Existing FEMA Floodplain
ZONE A
ZONE AE

GER F
GER E

ZONE X - SHALLOW 100-YEAR
2008PAGclr01ft.ecw
Study Limit

(
CP F !
GER D
INA

INA

(
CP E !
GER C

GER G

Pima County Index Map

(
CP D !

1ST

LE
OR AC

Index Map Scale 1:5,250,000

Study Limit
LL
PBE

ORANGE GROVE

SU

CAM

(
CP C !
GER B

SKYLINE
NR

IS
E

(
!
!
CP A (
CP B
2,000

1,000

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.
This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.

SU

Pima County Regional Flood Control District

NR

Scale 1:2000

IS
E

0

2,000 Feet

02/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Geronimo_wash_Watershed_Fig1_2.mxd

Figure 1.3
Zoning Classification
Geronimo Wash

OR
AC

LE

OR
AC

LE

HARDY

Geronimo Subbasins
Pima County Zoning
CR-1
CR-4
CR-5
IR
SR

GER F
GER E

TR
2008PAGclr01ft.ecw

GER D
INA

INA

GER C
GER G

Pima County Index Map

1ST

SU

CAM

OR ACLE

PBE

ORANGE GROVE

GER B

SKYLINE

LL

LE
OR AC

Index Map Scale 1:5,250,000

2,000

NR

IS
E

1,000

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.
This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.

SU

Pima County Regional Flood Control District

NR

Scale 1:2000

IS
E

0

2,000 Feet

02/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Geronimo_wash_Watershed_Fig1_3.mxd

Section 2.0 Summary of Key Facts
Section 2.1: General Information
2.1.1 Community: Pima County Regional Flood Control
2.1.2 Community Number: NFIP Community Number 04019C
2.1.3 County: Pima
2.1.4 State: Arizona
2.1.5 Date Study Accepted: Not Accepted
2.1.6 Study Contractor: Pima County Regional Flood Control District – Akitsu Kimoto
2.1.7 State Technical Reviewer: Not Applicable
2.1.8 Local Technical Reviewer: Suzanne Shields
2.1.9 River or Stream Name: Geronimo Wash
2.1.10 Reach Description: Geronimo Wash
2.1.11 Study Type: Hydrology and Hydraulics study of a Riverene System
Section 2.2: Mapping Information
2.2.1 FIRM Panels: 04019C-1630 and 1635K
2.2.2 Mapping for Hydrologic Study: Lidar based on 2008 flight used to derive 10’
contour interval maps using ARC-GIS 9.3
2.2.3 Mapping for Hydraulic Study: Lidar based on 2008 flight used to derive a DEM
(5-ft cell size) for use with GeoRAS
Section 2.3: Hydrology
2.3.1 Model or Method Used: HEC-HMS (v. 3.4) model parameterized using methods
of RFCD Draft Tech Policy 018 (October 10, 2008)
2.3.2 Storm Duration: 3-hr
2.3.3 Hydrograph Type: SCS Unit Hydrograph
2.3.4 Frequencies Determined: 100 yr
2.3.5 List of Gages used in Frequency Analysis or Calibration: None
2.3.6 Rainfall Amounts and Reference: SCS Type II, NOAA 14 Upper 90%
Confidence Interval
2.3.7 Unique Conditions and Problems: None
2.3.8 Coordination of Q’s: Comparison with previous studies on file with RFCD and
discharge estimates
Section 2.4: Hydraulics
2.4.1 Model or Method Used: HEC-RAS 4.0, GeoRAS to parameterize
2.4.2 Regime: Modeled as subcritical
2.4.3 Frequencies for which Profiles were computed: 100 yr
2.4.4 Method of Floodway Calculation: No Floodway
2.4.5 Unique Conditions and Problems: Boundary set at normal depth.
Section 2.5: Additional Study Information:
None

9

Section 3: Survey and Mapping Information
3.1 Field Survey Information
No field survey was used.
3.2 Mapping
The 2008 Light Detection and Ranging (LiDAR) data was used for the analysis.
Coordinates were in Pima County projection:
Projection = State Plane, Arizona Central Zone
Datum = NAD83 HARN
Units = International Feet
North American Vertical Datum of 1988 (NAVD, 1988)
The LiDAR was used to derive a Digital Elevation Model (DEM) and a contour map.
DEM derived on 5’ centers provided the basis for delineating the watershed and subbasins. DEM was also used to characterize the topography along channels used for the
floodplain mapping process. Contour map derived from the DEM allowed modelers to
visualize topographic differences in making decisions on how to model different areas.

Section 4: Hydrology
4.1 Method description.
For the floodplain mapping, a 100-yr discharge is required. The 100-year peak discharges
for the sub-basins of the Geronimo Wash (GER B, C, D, E, and F; Fig.1.1) were
calculated using U.S. Army Corps of Engineers Computer Hydrologic Modeling System,
(HEC-HMS) version 3.4. The HEC-HMS model requires the parameters regarding
rainfall, topography, soil, vegetation, and channel characteristics to determine runoff
volume and peak discharge. Those parameters were determined according to the Pima
County Regional Flood Control District Technical Policy 018 (Tech-018). Tech-018 is
included in Appendix A.
4.2 Parameter estimation.
Methods are summarized in Table 4.1. The data processing methods are summarized in
Fig. 4.1.

10

Table 4.1 - Methods used for a HEC-HMS analysis

Rainfall Depth
Rainfall Distribution
Rainfall Loss
Time of Concentration
Transform
Routing

Selected Method
NOAA 14, upper 90% Confidence Interval
3-hr SCS Type II Storm
SCS Curve number
SCS Segmental Method
SCS Unit Hydrograph
Modified-Puls and Kinematic Wave

4.2.1 Drainage area boundaries.
The limits of this study are shown in Fig.1.2. The site includes Sections 32 of Township
1 South, Range 14 East, Sections 5, 6 and 7 of Township 13 South, Range 14 East, and
Section 13 of Township 13 South, Range 12 East, Pima County, Arizona. The entire
watershed of the Geronimo Wash is in FEMA Zone X, as shown on the current Flood
Insurance Rate Map (FIRM) number 04019C-1630 and 1635K.
The watershed is 3.37 square mile. The study watershed was divided intofive sub-basins
(Fig.1.1). The upstream study limits is the north of Calle sin Desengano, while the
downstream limit is the confluence with the Pima Wash (Fig.1.2).
4.2.2 Watershed work maps
The boundary of the watershed and internal sub-basins were determined using Hydrology
function in ArcGIS with DEM derived from the 2008 Lidar. The sub-basins reflected
predominant topographic, soils, cover and development conditions, so that the sub-basins
would represent hydrologic response from the sub-basin. The locations of the stream
centerline, cross-sections, culverts, and other physical attributes of the wash were
determined by using the 2-ft interval contour map and 2008 aerial photo.

11

Figure 4.1 – Flow Chart of Mapping Process

Topographic Data Preparation using ArcGIS with
TIN or DEM

Hydrologic Analysis using PCHydro

Geometric Data Preparation using
ArcMap and Hec-GeoRAS
(stream network, stream centerlines,
cross sections, river banks, culverts,
and/or block obstruction)

Hydraulic Analysis using HEC-RAS
(Manually input the following data; Manning’s nvalues, culvert data, expansion and contraction
coefficients, normal depth boundary condition,
ineffective flow areas, adjustment of reach length if
necessary)

Floodplain Delineation using HecGeoRAS

12

4.2.3 Gage Data.
None Available
4.2.4 Statistical parameters
None Available
4.2.5 Precipitation.
Rainfall depth was selected from the NOAA 14 Upper 90% rainfall data used in PC
Hydro. The point rainfall depth for the 3-hour storm was obtained, based on the
coordinates of the centroid of the watershed (Latitude: 32.342, Longitude: 110.938).
Areal reduction factor was applied to watersheds larger than 1 square mile as noted in
Tech-018. The 3-hr, SCS Type II rainfall distribution described in Haan et al (1994) was
used.
4.2.6 Physical parameters.
A hydrologic soils group map for the study watershed is presented in Fig.4.2. The study
watershed is mostly covered with Desert brush. Hydrologic Soil Groups B and D are the
dominant soil types in the Geronimo Wash watershed. The SCS Curve Number was
determined using maps obtained from NRCS (http://soildatamart.nrcs.usda.gov/) as a
basis for preparing a Hydrologic Soil Group Map for Pima County. The CN charts in the
PC Hydro Manual (Arroyo Engineering, 2007) were the basis for CN selection. A
vegetation cover density of 30% was used to select the SCS Curve Number for the
hydrologic calculation of the mountainous watersheds. Impervious cover percentage
from 5-15%, were selected based on lot size, the fraction of the sub-basin that is
developed and the tables in the PC Hydro manual. Sub-basin characteristics are
summarized in Table 4.2 The detail of the CN calculation is included in Appendix D.

13

Figure 4.2
Soil Classification Map
Geronimo Wash

OR
AC

LE

OR
AC

LE

HARDY

Geronimo Subbasins
Soil Group: A (100%), ARIZO-RIVERWASH COMPLEX, 0 TO 3 PERCENT SLOPES
Soil Group: B (100%),
Soil Group: B (100%), PINALENO VERY COBBLY SANDY LOAM, 1 TO 8 PERCENT SLOPES
Soil Group: B (100%), PINALENO-STAGECOACH COMPLEX, 5 TO 16 PERCENT SLOPES
Soil Group: B (82%) C (18%), PINALENO-STAGECOACH-PALOS VERDES COMPLEX, 10 TO 35 PERCENT SLOPES
Soil Group: C (47%) D (53%), PANTANO-GRANOLITE COMPLEX, 5 TO 25 PERCENT SLOPES

GER F

Soil Group: C (50%), D (50%)
Soil Group: D (100%), CELLAR-LAMPSHIRE-ROCK OUTCROP COMPLEX, 15 TO 60 PERCENT SLOPES
Soil Group: D (100%), CELLAR-LEHMANS COMPLEX, 5 TO 25 PERCENT SLOPES

GER E

GER D
INA

INA

GER C
GER G

Pima County Index Map

1ST

SU

CAM

OR ACLE

PBE

ORANGE GROVE

GER B

SKYLINE

LL

LE
OR AC

Index Map Scale 1:5,250,000

2,000

NR

IS
E

1,000

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.
This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.

SU

Pima County Regional Flood Control District

NR

Scale 1:2000

IS
E

0

2,000 Feet

02/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Geronimo_wash_Watershed_Fig4.2.mxd

Table 4.2 - Sub-basin Characteristics
Sub-Basin
GER B
GER C
GER D
GER E
GER F
GER G

Area
(sq mi)
0.21
0.43
0.22
0.69
0.99
0.83

CN
84.0
84.3
83.5
88.5
90.4
84.6

Impervious Area
(%)
12
12
7
10
5
15.0

Vegetation Cover
(%)
30
30
30
30
30
30

Lag Time
(min)
22.2
21.9
15.5
10.9
15.8
26.6

The SCS TR-55 segmental Time of Concentration (Tc) method with a combination of
kinematic wave method was used. The hydraulically most distant point on the sub-basin
was identified. The length of sheetflow was estimated at 100 feet, the distance from the
end of the sheetflow to a well-defined channel was selected as the shallow concentrated
portion of the flow path, and the channel portion was the path from the well-defined
channel to the sub-basin outlet was the ‘channel flow’ portion of the flow path.
Tc is the sum of the travel time for sheetflow, shallow concentrated flow and channel
flow. The travel time for sheetflow was calculated using kinematic wave method. The
travel time for shallow concentrated flow was calculated using the methods described in
the TR-55 manual (USDA-1986). The travel time for channels used estimates from a
HEC-RAS model. The lag time was calculated as 0.6 Tc. The detail of the Tc calculation
is included in Appendix D.
The SCS unit hydrograph method was used to produce hydrographs at the outlet of the
sub-basin in HEC-HMS. Runoff from sub-basins was routed using the Modified-Puls
method. A storage discharge table for the channel routing was developed using the cross
sections and slopes derived from HEC-HMS. Modified puls routing employed the
methods described in the HMS manual. The detail of the calculation of the number of
subreach is included in Appendix D. Sub-basin discharges are summarized on Table 4.3.
Table 4.3 - Sub-basin discharges
Sub-Basin
GER B
GER C
GER D
GER E
GER F
GER G

Area
(sq mi)
0.21
0.43
0.22
0.69
0.99
0.83

Rainfall Depth
(in)
3.44
3.44
3.44
3.44
3.44
3.44

Runoff Volume
(in)
1.89
1.92
1.85
2.26
2.43
1.93

Peak Discharge
(cfs)
318
669
412
1891
2411
1136

4.3 Problems encountered during the study.
None

15

4.3.1 Special problems and solutions
4.3.2 Modeling warning and error messages
The time interval of the rainfall data used in this study is 5 minutes, while the simulation
time interval is 1 minute. The HEC-HMS model interpolated the 5-minute time interval
of the rainfall data to 1-minute time interval.
4.4 Calibration
No Calibration
4.5 Final results
4.5.1 Hydrologic analysis results
The 100-year peak discharges at the concentration points along the Geronimo Wash were
determined using the HEC-HMS. Six hours were simulated on a 1-minute time step with
rainfall occurring the first three hours. The following discharges were obtained from the
hydrologic analysis:

16

Table 4.4 – Summary of 100-yr Peak Discharge Values
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E
CP F

Confluence with Pima Wash
Upstream of the confluence with Pima Wash
South of Orange Grove
At Skyline Dr.
At Ina Rd.
North of Calle sin Desengano

Area (sq Rainfall
Runoff
Q100
Time to
mile)
Depth (in) Volume HMS (cfs)
Peak
(in)
3.37
3.20
1.92
4894
2:00
2.54
3.20
1.98
4002
2:00
2.33
3.22
2.03
4132
1:52
1.9
3.26
2.13
4005
1:41
1.68
3.28
2.21
3713
1:38
0.99
3.44
2.43
2411
1:39

Table 4.5 – Summary of 25-yr Peak Discharge Values
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E
CP F

Confluence with Pima Wash
Upstream of the confluence with Pima Wash
South of Orange Grove
At Skyline Dr.
At Ina Rd.
North of Calle sin Desengano

Area (sq Rainfall
Runoff
mile)
Depth (in) Volume
(in)
3.37
2.48
1.3
2.54
2.48
1.36
2.33
2.50
1.39
1.9
2.53
1.48
1.68
2.54
1.5
0.99
2.67
1.72

Q25 HMS Q25 RRE
(cfs)
(cfs)
3052
2450
2342
2507
2563
1711

1507
1281
1218
1079
1001
720

Time to
Peak
1:46
2:00
1:53
1:48
1:39
1:39

Table 4.6 – Summary of 500-yr Peak Discharge Values
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E
CP F

Confluence with Pima Wash
Upstream of the confluence with Pima Wash
South of Orange Grove
At Skyline Dr.
At Ina Rd.
North of Calle sin Desengano

Area (sq Rainfall
Runoff
Q500
Time to
mile)
Depth (in) Volume HMS (cfs)
Peak
(in)
3.37
4.13
2.76
7996
1:51
2.54
4.13
2.84
6495
1:51
2.33
4.16
2.89
6592
1:44
1.9
4.21
3.02
5680
1:40
1.68
4.24
3.11
5206
1:38
0.99
4.45
3.39
3336
1:39

4.5.2 Verification of results.
Results are reasonable when compared with USGS Regression Equation 13 (Thomas et
al, 1997, Table 4.7). The equation 13 results were generally lower than the HMS results,
which would be expected, because these steep watersheds could be expected to produce
higher than average discharge on average. No regulatory discharge point data is available
along the Geronimo Wash.
Table 4.7 – Comparison of 100-yr Peak Discharge Values
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E
CP F

Confluence with Pima Wash
Upstream of the confluence with Pima Wash
South of Orange Grove
At Skyline Dr.
At Ina Rd.
North of Calle sin Desengano

Area (sq
Q100
Q100
mile)
HMS (cfs) RRE (cfs)
3.37
2.54
2.33
1.9
1.68
0.99

4894
4002
4132
4005
3713
2411

2681
2270
2156
1903
1762
1250

17

Section 5: Hydraulics

5.1 Method description.
Steady flow analysis was performed to determine 100-year water surface elevations in the
study area by using HEC-RAS with the discharge obtained from HEC-HMS.
5.2 Work study maps
As described above, geometric data for HEC-RAS including stream centerline, crosssections, and culverts, were obtained from HEC-GeoRAS. The locations of cross sections
and channels used for the 100-yr floodplain analysis are show in Exhibit 1. The 100-yr
and 500-yr floodplain limits are also shown in Exhibit 1.
5.3 Parameter estimation.
The watershed was modeled using methods consistent with District Tech Policy 019.
5.3.1 Roughness coefficients.
Manning’s roughness coefficients for the channel and the over-bank areas were
determined by using a 2008 aerial photo. Differentiation of channel and overbank ‘n’
values should be done only when channel flow is at least twice as deep as overbank flow
(Phillips and Tadayon, 2006). The roughness used in this study is 0.06 for overbank areas
and 0.035 for channel. An average n for the entire cross-section of 0.045-0.05 was
assigned when channel is shallow and not twice as deep as overbank flow.
5.3.2 Expansion and contraction coefficients.
Default HEC RAS expansion (0.3) and contraction (0.1) coefficients were used for the
most cross sections. The expansion coefficient of 0.5 and contraction coefficient of 0.3
were used for the cross sections immediately upstream or downstream of culverts.

18

5.4 Cross section description.
A 2-foot interval contour map derived from 2008 LiDAR data was used to select the
location of cross sections. Cross-section locations were determined primarily based on
the channel topography. The cross-section lines were drawn to be perpendicular to flow
paths in Geo-RAS and ArcGIS.
5.5 Modeling considerations.
5.5.1 Hydraulic Jump and drop analysis.
No hydraulic jumps were encountered.
5.5.2 Bridges and culverts.
There are four culverts along the study reaches of the Geronimo Wash. The photos of the
culverts are included in Appendix E.
5.5.3 Levees and dikes.
None.
5.5.4 Islands and flow splits.
None.
5.5.5 Ineffective flow areas.
Ineffective flow areas were noted on the study reach of the Geronimo Wash.
In general these ineffective flow areas were disconnected overbank areas that would not
convey flow to the next downstream cross-section.
5.5.6 Supercritical flow.
No supercritical reaches.
5.6 Floodway modeling
No encroachment calculations were performed.
5.7 Problems encountered during the study.

19

5.7.1 Special problems and solutions.
The modeling results showed that a 100-yrear flow would not be confined within the
“main channel” at cross sections 161427.79, 5118.424 and 5082.873. A lateral weir was
added at those cross sections in order to divert a portion of the flow from the “main
channel”. The discharges leaving the “main channel” were 4.2 cfs at the cross section
16147.79 and 183.2 cfs between cross sections 5118.424 and 5082.873.
5.7.2 Modeling warning and error messages.
No errors occurred. The following warning messages occurred:
Divided flow
Energy loss greater than 1.0
Energy equation could not be balanced and defaulted to critical.
Cross-section extended vertically.
Multiple critical depths calculated.
Conveyance ratio is less than 0.7 or greater than 1.4.
Inspection indicated that the modeling is accurate given the steep channel conditions.
Most of these errors force a critical solution which is reasonable for these steep
watercourses. A summary of errors is available in Appendix E.
5.8 Calibration.
None.
5.9 Final results.
5.9.1 Hydraulic analysis results.
The HEC-RAS modeling results were summarized in Appendix E.
5.9.2 Verification of results.
Existing floodplain maps are not available along the Geronimo Wash. The new map
tends to follow the floodplain topography. The results suggest that the mapping is
reasonable.
Section 6: Erosion and Sediment Transport
6.1 Method description.
None – not applicable
6.2 Parameter estimation.
None – not applicable
6.4 Modeling considerations.

20

None – not applicable
6.5 Problems encountered during the study.
6.5.1 Special problems and solutions.
None – not applicable
6.5.2 Modeling warning and error messages.
None – not applicable
6.6 Calibration.
None – not applicable.
6.7 Final results.
6.7.1 Erosion and sediment transport analysis results.
None – not applicable
6.7.2 Verification of results.
None – not applicable
Section 7: Ratio of the top width of 100-yr and 25-yr floodplain
A map showing the cross sections with the ratio of the topwidth less than 1.15 is included
in Addendum 1. An average ratio for the entire reach is 1.17.

21

Exhibit 2
Annotated Flood
Insurance Rate Map
Geronimo Wash

E121429

E121430

E121325

GER E

GER_CP
Geronimo Subbasins
Prpoosed 100-yr floodplain
LOMR Case Studies
FIRM - Flood Insurance Rate Map
Township, Range, Section
Existing FEMA Floodplain
ZONE A
ZONE X - SHALLOW 100-YEAR
!
(

GER F

E121432

E121431

E121336

(
CP F !

GER D

INA

INA

(
CP E !

1S T

2008PAGclr01ft.ecw
GER C

LOMR Case 08-09-1756X

GER G
(
CP D !

SK

E131406

E131405

YL

E131301

E
IN
E
IN
YL
SK

Pima County Index Map

GER B
Index Map Scale 1:1,500,000

LL
CA

MP

BE

(
CP C !

ORANGE GR OV E

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Department of
Transportation Technical Services Division makes no
claims regarding the accuracy of the information depicted
herein.
This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.

Scale 1":400'

Pima County Regional Flood Control District
FIRM Panel
1630 of 4700
Community
040073

E131312

E131408

E131407

CP B

0

400

800

1,600

2,400

3,200
Feet

CAMP BE LL

(
(!
CP A !

FIRM Panel
1635 of 4700
Community
040073

Pima County Regional Flood Control
97 East Congress Street - 3rd Floor
Tucson. Arizona 85701-1207
(520)243-1800 - FAX (520)243-1821
http://www.rfcd.pima.gov

02/2010

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