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...........................................................................................10
4.1 Method description. ...................................................................................10
4.2 Parameter estimation. ...............................................................................10
4.3 Problems encountered during the study. ...................................................15
4.4 Calibration .................................................................................................15
4.5 Final results ...............................................................................................15
Section 5: Hydraulics ..........................................................................................17
Section 5: Hydraulics ..........................................................................................17
5.1 Method description. ...................................................................................17
5.2 Work study maps.......................................................................................17
5.3 Parameter estimation. ...............................................................................17
5.5 Modeling considerations............................................................................18
5.6 Floodway modeling ...................................................................................18
5.7 Problems encountered during the study. ...................................................18
5.8 Calibration. ................................................................................................19
5.9 Final results. ..............................................................................................19
Section 6: Erosion and Sediment Transport .......................................................19
Section 7: Ratio of the top width of 100-yr and 25-yr floodplain..........................20

2

List of Figures:
Figure 1.1 –Watershed Map .................................................................................6
Figure 1.2 – Study limit .........................................................................................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 .................................................................14
Table 4.3 - Sub-basin 100-yr discharges ............................................................15
Table 4.4 – Summary of 100-yr Peak Discharge Values ....................................16
Table 4.5 – Summary of 25-yr Peak Discharge Values ......................................16
Table 4.6 – Summary of 500-yr Peak Discharge Values ....................................16
Table 4.7 – Comparison of 100-yr Peak Discharge Values ................................17
Exhibit
Exhibit 1 100-yr Floodplain Limit Map for the Scotts Knob Wash
Attached CD
Scotts Knob 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
Scotts Knob 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 100-yr discharges greater than 100 cfs.
 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).

4

Section 16 of the Pima County Ordinance describes the provisions for floodplain
regulation in Pima County.
1.3 Project Location
The study was performed to provide drainage information for the Scotts Knob Wash. The
site includes Sections 01, 02, 11, 12, and 13 of Township 14 South, Range 16 East,
Sections 7, 8, 16, 17, 18, and 20 of Township 14 South, Range 17 East, Pima County,
Arizona. Entire watershed of the Scotts Knob Wash is in FEMA Zone X, as shown on the
current Flood Insurance Rate Map (FIRM) number 04019C-2285 K.
The watershed is 5.05 square mile. The study watershed was divided into seven subbasins (Fig.1.1). The study limits for the Scotts Knob Wash extends from a junction with
Tanque Verde Wash to the boundary of Saguaro National Forest (Fig.1.2).
1.4 Methodologies Used for Hydrology and Hydraulics
Topographic, hydrologic and hydraulic analyses were performed to determine drainage
conditions in the Scotts Knob wash. ArcGIS, Version 9.3, HEC-HMS Version 3.4 (HECHMS), Pima County Hydrology Procedure (PC-Hydro) Version 5.4.2, Hec-RAS Version
4.0 (HEC-RAS), and HEC-GeoRAS, Version 4.1.1 (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 Scotts Knob Wash at the confluence with the Tanque
Verde Wash is 6791 cfs, where the area is 5.05 square miles.
The Scotts Knob Wash watershed is mostly located within Federal land (national forest,
FEMA Zone D). The floodplain was mapped in the downstream area of the Scotts Knob
Wash. The study found some homes at risk for flooding during the 100-yr flood. A 500yr floodplain limit was also mapped. In general, the footprint of the 500-yr floodplain is
only slightly larger than the 100-yr floodplain.

5

E
ER

DIN

ON
GT

RD

Figure 1.1
Watershed Map
Scott's Knob Wash

(
!

! CP A
(

CP D

CP B
(CP C
!
(
!

Discharge Point
River

!
(
( CP E
!

100ft contours
Subbasins

( CP H
!

CP F

! CP G
(

SCK A
SCK B

! CP J
(
( CP K
!

E SPEEDWAY BL

SCK C
SCK D

( CP I
!

SCK E
SCK F
SCK G
SCK H
SCK I
SCK J

! CP L
(

SCK K
SCK L
Aerial Photo: 2008 Pima Association of Governments
Topo: 2008 Pima Association of Governments

Pima County Index Map

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.
Pima County Regional Flood Control District

1,000

500

Gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Scott's Knob Wash\Scott's_knob_wash_Figure1-1.mxd

0

1,000 Feet

D
RE

ING

N
TO

Figure 1.2
Study Limit
Scott's Knob Wash

Study Limit

CP A

(
!

SCK A

CP B !
(

CP D CP E

SCK D

!
(
(
!

( SCK B
!

CP C

SCK C

CP H

SCK E

(
!

(CP G
!
!
(

(
!

CP F

SCK H
SCK F

Discharge Point

CP J

(CP K
!
!
(

River
Subbasins

CP I

(
!

SCK J

Existing FEMA Floodplain
ZONE A

SCK G

ZONE AE
SCK K
SCK I

ZONE X - SHADED
Aerial Photo: 2008 Pima Association of Governments

CP L

(
!

SCK L
Pima County Index Map

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.
Pima County Regional Flood Control District

1,000

500

0

1,000 Feet

09/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Scotts_Knob\Scotts_knob_wash_Watershed_Fig1_2.mxd

E
ER

DIN

ON
GT

RD

SCK A

Figure 1.3
Soil Map
Scott's Knob Wash

SCK D
River

SCK B
SCK C

Subbasins

SCK E

Soil Types
Soil Group: A (100%)

SCK H
E SPEEDWAY BL

SCK F

Soil Group: B (100%)
Soil Group: C (100%)

SCK J

Soil Group C (25%) D (75%)
Soil Group: C (50%), D (50%)
Soil Group: C (53%) D (47%)

SCK G

Soil Group: C (56%) D (44%)

SCK K

Soil Group: D (100%)
Soil Group: D (100%)

SCK I
Aerial Photo: 2008 Pima Association of Governments

SCK L

Pima County Index Map

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.
Pima County Regional Flood Control District

1,000

500

0

09/2010
Gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Scott's Knob Wash\Scott's_knob_wash_Figure1-3.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: Scotts Knob Wash
2.1.10 Reach Description: Scotts Knob 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-2285 K
2.2.2 Mapping for Hydrologic Study: Lidar based on 2008 flight used to derive 20’
contour interval maps using ARC-GIS 9.2
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. 3.2) 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 Scotts Knob Wash (SCK A, B, C, D, E, F and G; Figure 3) were
calculated using U.S. Army Corps of Engineers Computer Hydrologic Modeling System,
(HEC-HMS) version 3.1.0. 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.

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

4.2.1 Drainage area boundaries.
The limits of this study are shown in Fig.1.2. The Scotts Knob Wash watershed is mostly
located within Federal land (national forest, FEMA Zone D). The floodplain was mapped
in the downstream area of the Scotts Knob Wash.
The watershed is 5.05 square mile. The study watershed was divided into seven subbasins (Fig.1.1). The upstream study limits is the boundary of the national forest, while
the downstream limit is the confluence with the Tanque Verde 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 (Fig.1.1) 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 20-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 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
(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: 32.229, Longitude: 110.653).
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.3.1. The study
watershed is mostly covered with Desert brush. Hydrologic Soil Groups D are the
dominant soil types in the Scotts Knob 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 5-30%, 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. The
CN selections and impervious cover selections are summarized in Table D1.

13

Table 4.2 - Sub-basin Characteristics
Sub-Basin
SCK A
SCK B
SCK C
SCK D
SCK E
SCK F
SCK G
SCK H
SCK I
SCK J
SCK K
SCK L

Area
(sq mi)
0.04
0.04
0.02
0.09
0.19
0.16
0.31
0.58
1.08
0.68
0.62
1.24

CN
91.2
91.0
88.5
90.6
90.3
90.2
89.9
87.9
89.5
89.3
89.4
86.3

Impervious Area
(%)
30
20
10
7
7
5
5
5
5
5
5
5

Vegetation Cover
(%)
30
30
30
30
30
30
30
30
30
30
30
30

Lag Time
(min)
4.4
6.6
4.9
9.3
7.1
6.2
13.6
9.1
15.4
15.1
15.3
11.1

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’, 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.

14

Table 4.3 - Sub-basin 100-yr discharges

Sub-Basin
SCK A
SCK B
SCK C
SCK D
SCK E
SCK F
SCK G
SCK H
SCK I
SCK J
SCK K
SCK L

Area
(sq mi)
0.04
0.04
0.02
0.09
0.19
0.16
0.31
0.58
1.08
0.68
0.62
1.24

Rainfall Depth
(in)
3.57
3.57
3.57
3.57
3.57
3.57
3.57
3.57
3.57
3.57
3.57
3.57

Runoff Volume
(in)
2.63
2.61
2.38
2.57
2.54
2.53
2.51
2.32
2.47
2.45
2.46
2.18

Peak Discharge
(cfs)
164
149
75
297
678
590
845.7
1768.2
2708.6
1715
1556.6
3289.7

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
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 Scotts Knob Wash. The 100-year peak discharge at CP A was
determined using the HEC-HMS. Twenty four hours were simulated on a 1 minute time
step with rainfall occurring in the first three hours. The following discharges were
obtained from the hydrologic analysis:

15

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
CP G

Confluence with Tanque Verde Wash
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E

Area (sq Rainfall
Runoff
mile)
Depth (in) Volume
(in)
5.05
3.27
1.93
3.44
3.23
2.03
1.57
3.41
2.34
0.09
NA
NA
3.31
3.24
2.03
1.24
3.45
2.36
0.31
NA
NA

Q100 HMS or
PC-Hydro(cfs)

Time to
Peak

6790
5289
3326
397
5264
2669
1169

1:55
1:55
1:42
NA
1:54
1:43
NA

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
CP G

Confluence with Tanque Verde Wash
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E

Area (sq Rainfall
Runoff
mile)
Depth (in) Volume
(in)
5.05
2.38
1.3
3.44
2.50
1.4
1.57
2.65
1.6
0.09
NA
NA
3.31
2.51
1.4
1.24
2.67
1.65
0.31
NA
NA

Q25 HMS or Q25 RRE
PC(cfs)
Hydro(cfs)
4325
1886
3408
1524
2315
961
259
127
3394
1491
1868
830
741
327

Time to
Peak
1:57
2:11
1:43
NA
1:58
1:44
NA

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
CP G

Confluence with Tanque Verde Wash
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E

Area (sq Rainfall
Runoff
Q500
Time to
mile)
Depth (in) Volume HMS (cfs)
Peak
(in)
5.05
3.97
2.76
10286
1:51
3.44
4.18
2.9
7726
1:55
1.57
4.42
3.28
4672
1:42
0.09
NA
NA
577
NA
1:54
3.31
4.19
2.91
7677
1.24
4.46
3.32
3731
1:42
0.31
NA
NA
1723
NA

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 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 Scotts Knob Wash.

16

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
CP G

Confluence with Tanque Verde Wash
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E
Section 02, Township 14S, Range 16E

Area (sq
Q100
Q100
mile)
HMS (cfs) RRE (cfs)
5.05
3.44
1.57
0.09
3.31
1.24
0.31

6790
5289
3326
397
5264
2669
1169

3368
2713
1689
195
2653
1451
543

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. Floodplain
boundary outside of the national forest boundary was mapped in this study.
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 the 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). Most reaches within the study area is wide. Rather than
assign a channel and overbank Manning’s n, an average n for the whole cross-section of
0.04 or 0.045 was assigned except the downstream end of the reach in Subbasin A.
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.

17

5.4 Cross section description.
A 20-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 no culverts along the study reaches of the Scotts Knob Wash.
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 Scotts Knob 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.
5.7.1 Special problems and solutions.
None.

18

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 Scotts Knob 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.
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.

19

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
The ratio of 100-yr to 25-yr floodplain topwidth for the reach with peak discharge over
2000 cfs was calculated. The average ratios of 100-yr to 25-yr floodplain topwidth for the
Reach A, B, C, E and F are 3.19, 1.17, 1.13, 1.15 and 1.14. Cross sections with the ratio
less than 1.25, which are defined as “Canyon Wash”. A map showing the cross sections
with the ratio of the topwidth less than 1.25 is included in Addendum 1.

20

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33

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.

0
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This product is subject to the Department of Transportation
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Exhibit 1
100-year Floodplain
with cross sections
Scott's Knob Wash

34
8

0

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35

0

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

12/2010

gislib\rfcd\projects\imd\xavi\mdx\AKITSU\Scotts_Knob\Scott's_knob_wash_100\exh1.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.

Appendix B: General Documentation & Correspondence
None.

Appendix C: Survey Field Notes
C.1 Survey field notes for aerial mapping control.
C.2 Survey field notes for hydrologic modeling.
C.3 Survey field notes for hydraulic modeling.