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
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...........................................................................................11
4.1 Method description. ...................................................................................11
4.2 Parameter estimation. ...............................................................................11
4.3 Problems encountered during the study. ...................................................14
4.4 Calibration .................................................................................................14
4.5 Final results ...............................................................................................15
Section 5: Hydraulics ..........................................................................................16
5.1 Method description. ...................................................................................16
5.2 Work study maps.......................................................................................16
5.3 Parameter estimation. ...............................................................................16
5.4 Cross section description. .........................................................................17
5.5 Modeling considerations............................................................................17
5.6 Floodway modeling ...................................................................................17
5.7 Problems encountered during the study. ...................................................18
5.8 Calibration. ................................................................................................18
5.9 Final results. ..............................................................................................18
Section 6: Erosion and Sediment Transport .......................................................19
Section 7: Ratio of the top width of 100-yr and 25-yr floodplain..........................19

2

List of Figures:
Figure 1.1 Watershed Map ...................................................................................6
Figure 1.2 Study limitFigure 1.3 Soil Classification ..............................................7
Figure 1.3 Soil Classification................................................................................8
Figure 4.1 – Flow Chart of Mapping Process......................................................12
List of Tables:
Table 4.1 - Methods used for a Hec-HMS analysis.............................................11
Table 4.2 - Sub-basin Characteristics .................................................................13
Table 4.3 - Sub-basin discharges .......................................................................14
Table 4.4 – Summary of 100-yr Peak Discharge Values ....................................15
Table 4.5 – Summary of 25-yr Peak Discharge Values ......................................15
Table 4.6 – Summary of 500-yr Peak Discharge Values ....................................15
Table 4.7 – Comparison of 100-yr Peak Discharge Values ................................15
Exhibit
Exhibit 1 Floodplain Limit Map
Exhibit 2 Annotated Flood Insurance Rate Map
Attached CD
Wentworth Wash 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
Wentworth 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 Wentworth Wash.
Study reach of the Wentworth Wash is in FEMA Zone X, as shown on the current Flood
Insurance Rate Map (FIRM) number 04019C-2280K (February 8, 1999).
The watershed is 5.28 square mile. The study watershed was divided into five sub-basins
(Fig.1.1). The study limits for the Wentworth Wash extends from a junction with Tanque
Verde Wash to the upstream end of Subbasin A (Fig.1.2).
1.4 Methodologies Used for Hydrology and Hydraulics
Topographic, hydrologic and hydraulic analyses were performed to determine drainage
conditions in the Wentworth wash. ArcGIS, Version 9.3.1, HEC-HMS version 3.4 (HECHMS), HEC-RAS Version 4.0 (HEC-RAS), FLO-2D Version 2007.06, and HECGeoRAS, Version 4.2.93 (HEC-GeoRAS) 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 Wentworth Wash at the confluence with the Tanque
Verde Wash is 4719 cfs, where the area is 5.28 square miles.
Preliminary floodplain boundaries for areas less than one square mile are also shown in
this study. The Wentworth Wash watershed is mostly located within Federal land
(national forest, FEMA Zone D). The floodplain was mapped in the downstream area of
the Wentworth Wash. The study found some homes at risk for flooding during the 100-yr
flood.

5

WENTWORTH

TANQUE VERDE LOOP

CP A

(
!

Figure 1.1
Watershed Map
Wentworth Wash

SPEEDWAY

( Discharge Point
!

Contour 50ft
River
Subbasin
WEW A

FREEMAN

WEW B
WEW C
WEW D
WEW E
Areial Photo 2008 Pima Association of Governments
Topo: 2008 Pima Association of Governments
Vertical Datum: NAVD 1988

CP D CP B

(
!

CP E

(
!

Pima County Index Map

Index Map Scale 1:5,250,000

CP C
ESCALANTE

(
!
OLD SPANISH

22ND

MELPOMENE

BROADWAY

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.
Pima County Regional Flood Control District

1,000

500

0

10/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Wentworth\Wentworth_Fig1_1.mxd

1,000 Feet

WENTWORTH

TANQUE VERDE LOOP

Figure 1.2
Study Limit Map
WentworthWash

CP A

(
!
SPEEDWAY

Study Limit

(
!

BROADWAY

Existing Floodplain
ZONE A
ZONE AE
ZONE X - SHADED

FREEMAN

MELPOMENE

WEW A

22ND

Discharge Point
River
Subbasin

Areial Photo 2008 Pima Association of Governments

CP D CP B

(
!

SP
AN

ISH

WEW D
HOUGHTON

OL
D

Pima County Index Map

CP E

(
!

WEW B

WEW E
CP C
ESCALANTE

Index Map Scale 1:5,250,000

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.

1,000 500

0

1,000 Feet

Pima County Regional Flood Control District

WEW C

10/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Wentworth\Wentworth_wash_Watershed_Fig1_2.mxd

WENTWORTH

TANQUE VERDE LOOP

Figure 1.3
Soil Classification
Wentworth Wash

SPEEDWAY

Subbasins

Soil Classification
Soil Group: A (100%), ARIZO-RIVERWASH
COMPLEX, 0 TO 3 PERCENT SLOPES
Soil Group: B (100%), ANTHONY FINE SANDY
LOAM, 0 TO 3 PERCENT SLOPES
Soil Group: C (25%) D (75%),LAMPSHIRE-ROMEROROCK OUTCROP COMPLEX, 10 TO 65 % SLOPES
Soil Group: C (47%) D (53%),PANTANO-GRANOLITE
COMPLEX, 5 TO 25 PERCENT SLOPES

BROADWAY

Soil Group: C (53%) D (47%), PALOS VERDESJAYNES COMPLEX, 2 TO 8 PERCENT SLOPES

WEW A

Soil Group: D (100%), CELLAR-LEHMANS
COMPLEX, 5 TO 25 PERCENT SLOPES
Soil Group: D (100%), CELLAR-ROCK OUTCROP
COMPLEX, 30 TO 65 PERCENT SLOPES

Areial Photo 2008 Pima Association of Governments

FREEMAN

22ND

MELPOMENE

Soil Group: D (100%), CHIMENEA VERY GRAVELLY
FINE SANDY LOAM, 5 TO 15 PERCENT SLOPES

Pima County Index Map

WEW D
WEW B

WEW 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.

OLD SPANISH

ESCALANTE

HOUGHTON

Index Map Scale 1:5,250,000

This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.
Pima County Regional Flood Control District

1,000

500

0

WEW C

10/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Wentworth\Wentworth_wash_Fig1_3.mxd

1,000 Feet

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: Wentworth Wash
2.1.10 Reach Description: Wentworth 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-2280K
2.2.2 Mapping for Hydrologic Study: Lidar based on 2008 flight used to derive 2’
contour interval maps using ARC-GIS 9.3.1
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
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, FLO-2D version 2007.06.
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 2’ 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.

10

Section 4: Hydrology
4.1 Method description.
The 100-year peak discharges for the Wentworth Wash (WEW A, B, C, D and E; 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 by following the
Pima County Regional Flood Control District Technical Policy 018 (Tech-018). Tech018 is included in Appendix A. The HEC-HMS model is included in Appendix D.
4.2 Parameter estimation.
Methods are summarized in Table 4.1. The data processing methods are summarized in
Fig. 4
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 Wentworth Wash watershed is mostly
located within Federal land (national forest, FEMA Zone D). The floodplain was mapped
in the downstream area of the Wentworth Wash. The downstream of the Wentworth
Wash is mostly mapped as Zone D, as shown on the current Flood Insurance Rate Map
(FIRM) number 04019C-2280K. However, several properties in the downstream area are
privately owned. This study focuses on a drainage condition in Subbasin A, which
includes the private lands in the downstream.
The watershed is 5.28 square mile. The study watershed was divided into five sub-basins
(Fig.1.1). The upstream study limits is the upstream end of the Subbasin A, while the
downstream limit is the confluence with the Tanque Verde Wash (Fig.1.2).
4.2.2 Watershed work maps

11

A watershed work map is included in Exhibit 1. Eight subbasins were delineated for the
HEC-HMS hydrologic analysis. A 100-yr peak discharge at CP A was estimated in this
study. The peak discharge was used as input for the HEC-RAS and FLO-2D analysis.
Figure 4.1 – Flow Chart of Mapping Process

Topographic Data Preparation using ArcGIS with
DEM

Hydrologic Analysis using HECHMS

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 and FLO-2D

Floodplain Delineation using HecGeoRAS and FLO-2D

12

4.2.3 Gage Data.
None Available
4.2.4 Statistical parameters
None Available
4.2.5 Precipitation.
According to the Tech-018, the 3-hour storm shall be used as rainfall data in the HECHMS model in case that a time of concentration (Tc) is equal or less than three hours. A
3-hour storm was selected for a peak discharge calculation for the Wentworth Wash,
since Tc was less than 3 hours in all the subbasins.
A point 3-hour rainfall depth at the coordinates of the centroid of the watershed was
obtained from NOAA Atlas 14, upper 90% confidence interval precipitation frequency
estimate (NOAA 14 rainfall). Areal reduction factor was applied to watersheds larger
than 1 square mile, as described in Tech-018.

4.2.6 Physical parameters.
A hydrologic soils group map for the study watershed is presented in Fig.3.1. The study
watershed is mostly covered with Desert brush. Hydrologic Soil Groups C and D are the
dominant soil types in the Wentworth 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.
Table 4.2 - Sub-basin Characteristics
Sub-Basin
WEW
WEW
WEW
WEW
WEW

A
B
C
D
E

Area
(sq mi)
1.24
0.71
1.36
0.87
1.12

CN
88.0
88.9
89.7
88.7
88.6

Impervious Area
(%)
10
5
5
5
5

Vegetation Cover
(%)
30
30
30
30
30

Lag Time
(min)
42.1
25.2
22.7
18.3
15.8

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

13

was identified. The length of sheetflow was estimated at 100’, 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 (Table D2).
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
WEW
WEW
WEW
WEW
WEW

A
B
C
D
E

Area
(sq mi)
1.24
0.71
1.36
0.87
1.12

Rainfall Depth
(in)
3.39
3.39
3.39
3.39
3.39

Runoff Volume
(in)
2.17
2.25
2.32
2.23
2.22

Peak Discharge
(cfs)
1499
1181
2511
1759
2487

4.3 Problems encountered during the study.
None
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

14

4.5 Final results
4.5.1 Hydrologic analysis results
As described above, this study mainly focuses on drainage information in the
downstream of the Wentworth Wash. The 100-year peak discharge at CP A was
determined using the HEC-HMS. Seven hours were simulated on a 1 minute time step
with rainfall occurring in the first three hours. The following discharge was obtained
from the hydrologic analysis:
Table 4.4 – Summary of 100-yr Peak Discharge Values
Concentration
Point

Location

CP A

South of Speed way

Area (sq Rainfall
Runoff
Q100
mile)
Depth (in) Volume HMS (cfs)
(in)
5.3
2.90
1.80
4719

Time to
Peak
2:28

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

Location

CP A

South of Speed way

Area (sq Rainfall
Runoff
mile)
Depth (in) Volume
(in)
5.3
2.6
1.2

Q25 HMS Q25 RRE
(cfs)
(cfs)
2878

1933

Time to
Peak
2:47

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

Location

CP A

South of Speed way

Area (sq Rainfall
Runoff
Q500
mile)
Depth (in) Volume HMS (cfs)
(in)
5.3
3.8
2.6
7347

Time to
Peak
2:25

4.5.2 Verification of results.
Peak discharge estimated using a HEC-HMS model was compared with an existing 100year regulatory discharge at CP A. The comparison showed that the HMS-derived peak
discharge was higher than the one derived from the Regression Equation. The higher
HMS-derived peak discharge compared to the RRE-derived peak discharge would be
expected, because these steep watersheds could be expected to produce higher than
average at the sub-basin scale. No regulatory discharge point data is available along the
Wentworth Wash.
Table 4.7 – Comparison of 100-yr Peak Discharge Values
Concentration
Point

Location

CP A

South of Speed way

Area (sq
Q100
Q100
mile)
HMS (cfs) RRE (cfs)
5.28

4719

3452

15

Section 5: Hydraulics
5.1 Method description.
The hydraulic modeling for the Sweetwater was performed using HEC-RAS, Version 4.0
(HEC-RAS), HEC-GeoRAS, Version 4.1.1 (HEC-GeoRAS), ArcGIS, Version 9.3, and
FLO-2D (Version 2007-6). A floodplain within Subbasin A is mapped in this study.
Steady flow analysis was performed using HEC-RAS in order to determine a floodplain
limit for the upstream of the Wentworth Wash (from the upstream end of the subbasin A
to approximately 2190 feet upstream of the Speedway BL.). The locations of the stream
centerline, flowpath, and cross sections of the Sweetwater Wash were determined using a
2-ft contour map and 2008 PAG aerial photos. The physical attributes of the wash were
digitized in ArcGIS using the HEC-GeoRAS extension and exported to HEC-RAS to
create geospatially referenced geometric data (cross section, reach profile). Other
parameters for the steady-state analysis, such as Manning’s n-values, expansion and
contraction coefficients, boundary condition, and ineffective flow areas were manually
input into HEC-RAS. Normal-depth with a slope of 0.016 was assumed for the
downstream boundary condition. The hydraulic data obtained from HEC-RAS were
imported into HEC-GeoRAS to delineate a floodplain boundary for the Wentworth Wash.
FLO-2D was used for the downstream distributary area (from approximately 2190 feet
upstream of the Speedwar BL. to a confluence with the Tanque Verde Creek). Geometric
data for the FLO-2D model were derived from the 2008 Lidar data. Grid cell size of 50
feet was used to map a floodplain in the distributary area. The time interval used for the
computation was 1 minutes. The model does not include infiltration or rainfall. A
hydrograph from the HMS at CP A (at the confluence with the Tanque Verde Creek) was
used as inflow data at 2190 feet upstream of the Speedway BL.
5.2 Work study maps
The work study map for the Wentworth Wash is included in Exhibit 2.
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. In the HEC-RAS model, Manning’s n value of
0.055 was assigned for the overbank with desert brush, and the value of 0.045 was
assigned to a channel. In the FLO-2D model, selected Manning’s n values are 0.045-0.06
for a floodplain and overbank, 0.03-0.035 for roads (Speedway BL. and Wentworth RD.).

16

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.
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 HEC-GeoRAS.
5.5 Modeling considerations.
5.5.1 Hydraulic Jump and drop analysis.
No hydraulic, drop analyses or adjustment of the floodplain was conducted in this study.
5.5.2 Bridges and culverts.
There are no culverts.
5.5.3 Levees and dikes.
None.
5.5.4 Islands and flow splits.
None.
5.5.5 Ineffective flow areas.
In the HEC-RAS model, ineffective flow option was modeled in the following situations.
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.

17

5.7 Problems encountered during the study.
5.7.1 Special problems and solutions.
Flow in the area from approximately 2190 feet upstream of Speedway BL. to the
confluence with the Tanque Verde Creek was modeled with FLO-2D. For a floodplain
mapping with FLO-2D, shallow flow depth less than 0.2 feet is considered to be
negligible and cells with flood depth less than 0.2 feet were removed from a 100-year
flood hazard area. In other words, cells with flow depth deeper than 0.2 feet were
considered as a floodplain in this study.
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 the HEC-RAS errors is available in Appendix E.
5.8 Calibration.
None.
5.9 Final results.
5.9.1 Hydraulic analysis results.
The HEC-RAS and FLO-2D modeling results are shown in Exhibit 2.
5.9.2 Verification of results.
Existing floodplain maps are not available along the Wentworth Wash. The new map
tends to follow the floodplain topography. The results suggest that the mapping is
reasonable.

18

Section 6: Erosion and Sediment Transport
6.1 Method description.
None – not applicable
6.2 Parameter estimation.
None – not applicable
6.4 Modeling considerations.
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 1.25 is included
in Addendum 1. The average ratio of 100-yr to 25-yr floodplain topwidth for the HECRAS study reach is 1.09. Cross sections with the ratio less than 1.25 are defined as
“Canyon Wash”.

19

2640

2640

2650

30
26

40
26

2640

2660

Study Limit

0
265

2650

2650

265
0

2650

26
50

2640

2660

266
0

2660

60
26

2660

2650

266
0

26
60
0
266

266
0

60
26

2660

0
266

CP A

60
26

2660

2660

26
50

0
266

2640

!
(

26
60

26
60

60
26

2640

26
50

0
266

N WENTWORTH RD

2640

26
50

2640

40
26

2650

2640

60
26

2640
0
264

0
264

0
264

2640

0
266

266
0

0
266

60
26

2660

2660

0 2660
266

60
26

2660

26
60

70
26

!
(
2670

2670

2660

2650

2640

26
70

2660

2670

2670

2670

2670

2670

26
70

267
0

2680

E SPEEDWAY BL

2680

2670

2670

26
70

26
70

26
70

2680

2670

80
26

V
U

268
0

2680

2680

80
26

4586

V
U

80
26

4868

2680

2690

2690

4586

2690

2690

2700

00
27

26
90

90
26

00
27

Areial Photo 2008 Pima Association of Governments
Topo: 2008 Pima Association of Governments
Vertical Datum: NAVD 1988

2690

U
V
V
U

Discharge Points
River
Cross Sections
Contour 2ft
Contour 10ft
Proposed 100yr Flooding
Existing Zone AE

Flow Depth at Cell
Var
= > 0.200
= > 0.500
= > 1.500
= > 2.500
= > 3.500

80
26

2680

2680

Exhibit 1
100-year Floodplain
with cross sections
Wentworth Wash
Sheet 1 of 2

2690

2690

5983

26
90

V
U
V
U
3492

2710

U
V
V
U
7033

V
U

2700

2710

2710

2701

3614

2703.218

U
V
V
U
V
U

7287

2705.12

3733

2706.65

3838

271
0

271
0

270
0

2698.98

2700

27
00

2710

2700

2700

3359

93
2707.5

3902

V
U
8027

.0
11
27

V
U

271
0

271
0

9.82
270

5

V
U
8354

0
404

27

V
U

V
U
7

2720

868

2720

0
273

273
0

4
416

02
6.6
271

V
U
30
27

2720

2710
2
9.25
271

45

5

3
0.53
272

96
4756 32

4868

2730

V
U
106

Index Map Scale 1:1,500,000

24
2724.8

88
2726.8

V
U
5140

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.

85

09
2728.8

V
U
5280

U
V
V
U
5408

64
2730.4

5543

2730

2730

5661

2734.107

5744

2740

50
27

5863

V
U

400
Feet

27
30
2737.192

40
27

6212

10

U
V
V
U

2740.66
1
2742.0
75

6418

2743.4
76

2760

0
276

SEE SHEET 2 OF 2

V
U
6629

2747.24
7

2740

2750

2738.523

6309

2750

200

6079

80

V
U
123

0

2735.479

5983

120

This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.
Pima County Regional Flood Control District

2732.517

2730

U
V
V
U
V
U
V
U
V
U
V
U

2750

2720

V
U

2720

.134
2723

5029

27
40

27
20

20
27

UU
V
V
V
U
V
U
V
U
86

30
27

2730

78
42

945

20
27

Pima County Index Map

3
.76
12

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

11/2010

gislib\rfcd\projects\imd\xavi\mdx\AKITSU\Wentworth\Wentworth_48x36_exh_1_s1of2.mxd

2750

2738.523

V
U

Exhibit 1
100-year Floodplain
with cross sections
Wentworth Wash
Sheet 2 of 2

6212

2740

2740.66
1
2742.0
75

6418

2740

U
V
V
U
6309

0
273

SEE SHEET 1 OF 2

2743.4
76

V
U
6629

2747.24
7

U
V
V
U

2760

6951

2752.56

V
U

2760

2760

2750.454

V
U

2760

2760

2760

2748.884

6833

0
275

2750

0
2760

0
275

6
27

6724

7033

2753.716

2760

2755.567

U
V
V
U
V
U
V
U
V
U
7287

2757.703

7367

2758.935

7474

2760.593

7586

!
(

2762.299

7731

2770

2764.621

U
V
V
U
7828

2766.122

0
277

2767.593

7921

U
V
V
U

27
8

8027

2769.158

0

8133

2771.164

V
U
27
7

0

2773.21
9

2770

8265

U
V
V
U
V
VU
U
8354

2774.9
09

8460

86
8

2790

V
U
87
9

8579

2776
.8

7

5

V
U

7

4586

V
U

V
U

7

4868

2.1
5

92
0

93
1

27
8

27
9

27
9

27
9

U
V
V
U

7.0
7

4586

5

2

6

2810

2810

97
3

4

5.6
88

27
9
27
9

7.0
84

9.1
87

5983

1.2
71

4.0
06

8.9
46

2790

96
3

2

4.2
68

2790

95
3

27
8

7

27
8

V
U U
V
V
U
V
U

0.3
27

4

U
V
V
U
94
5

27
8

0

27
8

V
U
90
8

Areial Photo 2008 Pima Association of Governments
Topo: 2008 Pima Association of Governments
Vertical Datum: NAVD 1988

277
8.6
41

27
8

89
0

Discharge Points
River
Cross Sections
Contour 2ft
Contour 10ft
Proposed 100yr Flooding
Existing Zone AE

V
U
998
5

280
0.8
13

102
23

280
2.96
5

0
28
0

U
V
V
U
V
U
V
U
V
U

7287

280

0

1034
8

7033

2810

280
4.9
58

280
7.24

U
V
V
U

0

V
U

28
1

V
U

1010
0

1046
5

2812
.8

106
85

75

28
11

2820

2820

280
9.3
06

1055
3

.60

V
U
8027

7

107
91

2815

V
U

V
U
8354

2816.3
2

109
39

34

28
3

0

2819
.1

Pima County Index Map

V
U

V
U

11065

868

7

2821.211

V
U
11207

V
U
113
6

2823.407

8

V
U

2825
.6

1148
5
2827
.

V
U
945

16

V
U
963

826

5

2

V
U
1164

7
2830
.

U
V
V
U

239

28
3

1175
7
2832
.2

1

Index Map Scale 1:1,500,000
0

1188
6

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.

V
U
106

V
U

2834
.0

11993

285
0

2860

124
86

123
83

283
5.71
4

283
9.0
05

2820

V
U

V
U

123
10

283
7.40
5

24

2840

U
V
V
U

121
98

85

284
1.3
01

284
3.92

This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.
Pima County Regional Flood Control District

0

200

400
Feet

284
2.34
2

V
U

V
U

284
5.71
2

120

V
U
123

0

10

0
285

7
28

80

2830

126
13

Study Limit

28
8

0

2830
2840

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

11/2010

gislib\rfcd\projects\imd\xavi\mdx\AKITSU\Wentworth\Wentworth_48x36_exh_1_s2of2.mxd

Appendix A: References
A.1 Data collection summary.
Include a list of previous studies, other applicable studies, published and unpublished
historical
flood information, and research contacts.
A.2 Referenced documents.
Arizona Department of Water Resources, Flood Mitigation Section
“Requirements for Flood Study Technical Documentation” SS1-97, November 1997
Arroyo Engineering. 2007. PC-Hydro User Guide. Pima County Regional Flood Control
District
Eychaner, J.H., 1984. Estimation of magnitude and frequency of floods in Pima County,
Arizona, with comparisons of alternative methods: U.S. Geological Survey WaterResources Investigations Report 84-4142, 69 p.
Haan, C.T., Barfield, B.J., Hayes, J.C. 1994. Design Hydrology and Sedimentology for
Small Catchments, Academic Press.
National Weather Service. 1984. Depth-Area Ratios in the Semi-Arid Southwest
United States, NOAA Technical Memorandum NWS Hydro-40
NOAA, 2006. NOAA Atlas 14, Precipitation Frequency Atlas for the United States:
Volume 1 - Version 4.0 The Semiarid Southwest. National Weather Service,
Hydrometeorological Design Studies Center. Available on the internet at:
http://hdsc.nws.noaa.gov/ hdsc/pfds/sa/az_pfds.html
Phillips, J., and S. Tadayon. 2006. Selection of Manning’s roughness coefficient for
natural and constructed vegetated and non-vegetated channels, and vegetation
maintenance plan guidelines for vegetated channels in central Arizona: U.S. Geological
Survey Scientific Investigations Report 2006–5108, 41 p.
Thomas, B.E., H.W. Hjalmarson, and S.D. Waltemeyer. 1997. Methods for Estimating
Magnitude and Frequency of Floods in the Southwestern United States. USGS Water
Supply Paper 2433. 195 p.
U.S. Army Corps of Engineers (COE). 1998. HEC-1 Flood Hydrograph Package, Users
Manual, CPD-1A, Hydraulic Engineering Center, Davis, CA.
U.S. Army Corps of Engineers (COE). 2001. HEC-RAS, River Analysis System,
Hydraulic Reference Manual, CPD-69, Hydraulic Engineering Center, Davis, CA.

U.S. Army Corps of Engineers (COE). 2006. HEC-HMS, Hydrologic Modeling System
User’s Manual, (v. 3.1.0) CPD-74A, Hydraulic Engineering Center, Davis, CA.
U.S. Department of Agriculture Natural Resources Conservation Service (NRCS), 1986.
Urban Hydrology for Small Watersheds, Technical Release 55. Washington, DC.