Floodplain Analysis for an unnamed wash at the
Intersection of
Hacienda Del Sol Road and River Road
For
Pima County Regional Flood Control

March 4, 2008

Floodplain Analysis for an unnamed wash at the Intersection
of Hacienda Del Sol Road and River Road
Introduction
This study was performed to provide drainage information for the unnamed wash
running along Hacienda Del Sol Road. The unnamed wash runs through Sections 16, 20
and 21 of Township 13 South, Range 14 East, Pima County, Arizona, and lies between
Camino Real Wash and Hacienda Del Sol Wash. The entire watershed of the unnamed
wash is in FEMA Zone X, as shown on the current Flood Insurance Rate Map (FIRM)
number 04019C-1645K. While the watershed is in the Zone X, which is defined as nonflood hazard areas, a potential flood and erosion hazard is known in foothills areas where
the unnamed wash is located. Limited drainage information is currently available in
foothills areas.
The purpose of this study is to provide flood and erosion hazard information for the
unnamed wash along Hacienda del Sol road. Topographic, hydrologic and hydraulic
analyses were performed to determine drainage conditions in the wash. ArcGIS, Version
9.2, Pima County Hydrology Procedures (PC-Hydro), Version 5.3.1, HEC-RAS 4.0 Beta
version (HEC-RAS), and HEC-GeoRAS, Version 4.1.1 (HEC-GeoRAS) were used for
the analyses.

Description of the watershed
Most of the watershed of the unnamed wash is located in Pima County, while the
southern part of the watershed is in the City of Tucson (south of River Rd. Fig. 1). The
watershed is 227.4 acres with a mean slope of 0.03. The watershed is located in a
suburban foothills area covered with desert brush. No riparian habitat is mapped along
the wash, based on the 2005 Pima County Ordinance (Fig. 1). Zoning classification of
the most of the watershed is CR-1 (single residence zone; Fig. 2).

Data processing procedures
Data processing method is summarized in Fig.3. Triangular Irregular Network (TIN)
derived from Light Detection and Ranging (LiDAR) data was used to create a
topographic map (Fig. 4). The contour interval is 5 feet. The boundary of the watershed
and slope break points were determined using the contour lines. The locations of the
stream centerline, cross-sections, river banks, culverts, and other physical attributes of the
wash were determined by using a topographic map and 2005 PAG aerial photo (Fig. 4).
Those physical attributes of the wash were digitized in ArcGIS with an extension of
HEC-GeoRAS and then exported as an ASCII text file. The data were imported into
HEC-RAS to create geospatially referenced geometric data. Other parameters for the
steady analysis, such as Manning’s n-values, culvert data, expansion and contraction
coefficients, normal depth boundary condition, ineffective flow areas, and peak discharge
rates obtained from PC-Hydro are manually input into HEC-RAS. The hydraulic data

obtained from HEC-RAS were imported into HEC-GeoRAS to delineate a floodplain in
the study area.

Hydrologic analysis
The 100-year return interval peak discharge rate for the watershed was computed by
using PC-Hydro (Arroyo Engineering, 2007). NOAA Atlas 14, upper 90% confidence
interval, rainfall data was used for the analysis. Hydrologic soils group map for the
watershed is presented in Fig. 5. Hydrologic Soil Group B is the dominant soil type in the
watershed, which occupies 97% of the watershed. The watercourse was divided into ten
segments (Reaches) using slope break points. Basin factors were based on the tables in
the PC Hydro User Guide. The basin factor for each segment was determined by using a
2005 PAG aerial photo (Figs. 6-7.5). The Basin Factor used for the hydrologic analysis
ranges from 0.040 to 0.045 which corresponds to Suburban-Valley (< 1 house/acre;
Arroyo Engineering, 2007). A vegetation cover density of 30% was used to select the
SCS Curve Number for the hydrologic calculation of the study watershed. Impervious
cover percentage was 15%, which was determined using a 2005 PAG aerial photo.
The results of hydrologic analysis are included in Appendix 1. The 100-year peak
discharge rate at Hacienda Del Sol Road is calculated to be 690 cubic feet per second
(cfs). The Time of concentration (Tc) for the peak discharge is 25.2 minutes, and the
rainfall intensity is 4.92 inches/hr at the time of concentration.

Hydraulic analysis
Steady flow analysis was performed to determine 100-year water surface elevations
in the study area by using HEC-RAS. The study area for the analysis is from the
confluence of an upstream tributary with a main channel to the confluence with Rillito
River. As described above, geometric data for HEC-RAS including stream centerline,
cross-sections, river banks, culverts, and block obstructions were obtained by using HECGeoRAS. The locations of cross sections used for the analysis are show in Fig. 8. There
are four culverts in the study area (Fig. 8). The culverts were named Culvert #1, 2, 3, and
4 from upstream to downstream (Pictures of the culverts are included in Appendix 2).
The dimensions of the culverts were summarized in Table 1. The data for Culvert #1, 2,
and 4 were obtained from Pima county Department of Transportation and Flood Control
District improvement plans (4BHDSR and 4TRRCA, Appendix 3). The data for Culvert
#3 was obtained by a field measurement. Culvert #1 and 2 were designed to convey a
100-year discharge of 379 cfs. Culvert #4 was designed to convey 575 cfs.
Manning’s roughness coefficients for the main channel and the over-bank areas were
determined by using a 2005 PAG aerial photo. An area-weighted average of the
Manning’s roughness coefficients for the main channel and the over-bank areas was used
to determine a water surface elevation at each cross section. The weighted average of the
roughness coefficients was determined by estimating area for a main channel and overbank areas and assigning weighing factors that are proportional to the total area (Jacb

Dividian, USGS, Technical of Water-Resources Investigations, Book 3, Chapter A15, see
Appendix 4). The method to assign composite n value to an entire cross section is used in
Maricopa County (Phillips and Tadayon, 2006)
Entrance loss coefficient and Manning’s roughness coefficient of the culverts are
obtained from HEC-RAS Hydraulic Reference Manual version 3.1. Culvert #4 has a
horizontal bend between the entrance and exit. Bend can result in additional losses for the
culvert operating under outlet control (Floodplain Online Book, provided by Haestad
Methods, Inc.). The detail of the method is provided in Appendix 5. The bend loss was
estimated following the method described in Floodplain Online Book. Angle of bend is
approximately 25 degree (4TRRCA). The culvert opening is assumed to be equivalent to
3.56 feet diameter, based on the opening of Culvert #4. The estimated entrance
coefficient for Culvert #3 is 0.64.
Contraction and expansion coefficients are 0.3 and 0.5 for just upstream and
downstream of culverts, and 0.1 and 0.3 for other cross sections, which were obtained
from HEC-RAS Hydraulic Reference Manual. Normal depth with a slope of 0.02 was
used as a boundary condition for the steady flow analysis.
The results of hydraulic analysis are shown in Figs. 9, 10 and 11. A 100-year water
surface elevation at each cross section is shown in Appendix 6. Fig. 9 shows the profile
of a 100-year water elevation. Fig. 10 shows a spatial variation of flood depths in the
study area. Fig.11 shows a floodplain limit in the study area. The flood prone area is
relatively wide between Culvert #3 and Culvert #4 (Figs. 10 and 11). There is a divided
flow at approximately 900 feet upstream of River Rd, based on the steady flow analysis
using HEC-RAS. The flood elevation exceeds 4 feet at the upstream of Culvert #1 2, and
#3 (Fig. 10). The water surface elevations at all the road crossings (Culverts #1, 2, 3, and
4) are higher than the roads (Fig. 9). This suggests flood water may run over the roads
during a 100-year flood event.
References
Floodplain Online Book, provided by Haestad Methods, Inc
http://www.haestad.com/library/books/FMRAS/FloodplainOnlineBook/javascript/wwhel
p/wwhimpl/common/html/wwhelp.htm?context=Floodplain_with_HEC_RAS&file=Floo
dplain%20with%20HEC-RAS-16-10.html
HEC-RAS Hydraulic Reference Manual version 3.1, US Army Corps of Engineering
Center, November 2002.
Jacb Dividian, USGS, Technical of Water-Resources Investigations, Book 3, Chapter
A15
PC-Hydro User Guide, PC-Hydro V5 Pima County Hydrology Procedures, A couputer
program for predicting peak discharge of surface runoff from small semi-arid watersheds
in Pima County, Arizona, Arroyo Engineering, LLC, 2007

Phillips J., and Tadayon, S., 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, Flood Control District of
Maricopa County, Scientific Investigations Report 2006-5108.