FLOOD09697

• County: Statewide
• Community: Nationwide
• Basin: Statewide
• Title: Mathematical Modeling of a Sociological and Hydrologic Decision System
• Date: 6/1/1978
• Prepared By: Institute for Social Science Research on Natural Resources, Utah State Univ.
• Floodplain - Doc Type: Educational/Technical/Reference Information

company the treatment of ordinal variables as
interval, this is offset by the use of more power-
ful, more sensitive, more highly developed, and
more clearly interpretable statistics with known
sampling error. For example, well defined mea-
sures of dispersion (variance) require interval or
ratio based measures. Furthermore, many more
manipulations (which may be necessary to the
problem in question) are possible with interval
measurement, e.g., partial correlation, multi~
variate correlation and regression, analysis of
variance and coevariance, and most pictorial
presentations (labovitz, 1970: 515).

For the purpose of this study, the specification
of imporlanl variables and of the general nature of the
relationships among them, the data have been formu-
lated and Irealed as interval information. Dummy
variables are generally not used in Ihe regression equa-
lions since the dala are trealed as if they measured
the underlying variable continuously. It is expected
thai measurement methods can later be improved to
approximale continuous scales more closely in the
real sense.2

Evaluation and Analysis of the Physical
and Hydrologic Data from
the Study Area

The physical data provide a description of the
real world hydrologic system thai establishes a basis
for formulation and testing of the combined socio-
logical-hydrologic model. The accuracy of predictions
from a modei are governed by the reliabilily of the
information used 10 develop the model and the accur-
acy of the input dala used in predictions.

As shown on Figure 1.1, the Ihree streams with-
in Ihe study area are tribularies to the Jordan River.
The urban portions of Ihe drainages of Mill Creek,
Big Cottonwood and little Cottonwood Creeks (Fig.
ure 2.2) contain approximalely 14,23, and 10 square
miles, respectively, and exlend from the foot ofthe
Wasalch Mountains to the Jordan River. Urbanization
is predominately residential in nature with a few areas
of light industrial and commercial development. The
rural portions of the three watersheds (Figure 1.1)
extend to the lops of Ihe Wasatch Mountains. Most
of the water flow is generated within Ihe mOlll1tain-
ous rural areas.

Topography

The generallopography of the urbanizing por-
tion of the study area isshown by Figure 2.1. Ele-

2Recently methods for attaining actual ratio scale
levels has been developed that should be applicable to some
of the variables in this study. (See Stevens, 1966; Hamblin,
1971,1974.)

vations range from 4200 feet at the Jordan River 1.0
4800 feet along the Wasatch Boulevard on the easl.
The slopes also steepen 10 the easl. The fast runoff
down the steep slopes tends 10 accumulale in ditches,
curbs, and gutters on Ihe flatter areas near the Jordan
River.

Geology

Where the steep mountain slopes merge into the
upper planes of the valley, rocks and gravel are over-
lain with sand and soil. Vegetalion is of the scrub oak
variely mixed with some grasses. Because of its high
gravel and sand contenl, the infiltration capacity of
the soil is generally high. The sand soil is easily eroded
by high velocity flows 10 form gullies, and erosion as
increased by grading, trenching, or other movement
of the soil during oonstruclion of buildings and roads.
Near the Jordan River the soils are heavy. Water tends
to pond in surface depressions rather than inftltrate,
and lower flow velocities reduce erosion hazards.

Degree of Urbanization Within
the Study Area

In order to model urban runoff, it is necessary
to select readily determined parameters which corre-
late with changes in the runoff hydrograph due to
urbanization. Two parameters proposed by Narayana
et al. (1969), Ihe percentage impervious cover, Cr, and
the characteristic impervious length faclor, 4, are
used in this study. These two parameters represent
physical conditions existing on the watershed and Can
be estimated from aerial photos.

Computalion of Urban
Parameters

Some size of spatial unil musl be adopted for
the model. Narayana et al. (1969) chose the entire
walershed as the primary catchmenllll1it. Evelyn et
al. (1970) found thai accurate synthesis of hydro-
graphs at selecled localions within a basin required
that small sub watersheds be chosen as the primary
calchment units. The oulflows from the subzones
then can he routed and combined 10 delermine out-
flow hydrographs al downstream points. An even
smaller unit, the urban block, would permit synthesis
of inlel hydro graphs for storm drain and gutter de.
sign under various assumed degrees of urbanization.

Evelyn el al. (1970) proposed the following pro.
cedure for evaluating Ihe urban parameters, and this
procedure was adopled for this study.

I. Divide the watershed into sub-zones as
illustrated by Figure 2.2.
A. Factors which influence Ihe num.
ber of subzones and their bound.
aries are:

14