FLOOD06431

• County: Statewide
• Basin: Statewide
• Title: Estimating Design-Flood Discharches for Streams in Iowa Using Drainage-Basin and Channel-Geometry Characteristics
• Date: 1/1/1993
• Prepared For: Iowa City
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

RR = BR
BP'

(11)

181
- 33.5'

= S.40 Nmi .

Step 4. Drainage frequency (DF) is
calculated from the primary drainage-basin
characteristics number of first-order streams
(FOS) and contributing drainage area (CDA). A
total of 28 first-order streams are counted
within the drainage-divide delineation for this
gaging station on the 1: 100,000-scale
topographic map (table 1). These first-order
streams are shown in figure 4B. Drainage
frequency (DF) is calculated as

DF = FOS
CDA'

(12)

28
- 56.9'

= 0.492 first-order streams/mi 2 .

Step 5. The 2-year, 24-hour precipitation
intensity (TTF) for the drainage basin is
determined from figure 5. Because the
drainage-divide boundary for this gaging station
is completely within the polygon labeled as
3.15 in., the 2-year, 24-hour precipitation
intensity is given a value of 3.15 in. (table 1).

Step 6. The 100-year flood estimate using
the drainage-basin equation (table 2) is
calculated as

Q'00 = 277 (CDA10.681 (HmO.6'6 (DF)0.409 (TTF _ 2.S)0389,

277 (S6.9)0681 (S.40)'.656 (0.492)0.409 (3.1S _ 2.S)0.389,

8,310 ft'is.

,
,

Discharge estimates listed in this report are
rounded to three significant figures. The
difference between the above estimate of
8,310 ft3/s and the estimate of 7,740 ft3/s listed
In table 8 (method GISDB) is due to
measurement differences between manual
measurement and GIS procedure techniques
(table 1).

ESTIMATING DESIGN-FLOOD
DISCHARGES USING
CHANNEL-GEOMETRY
C~CTERISTICS

The channel-geometry flood-estimation
method uses selected channel-geometry
characteristics to estimate the magnitude and
frequency of floods for stream sites in Iowa. The
channel-geometry method is based on measure-
ments of channel morphology, which are
assumed to be a function of streamflow
discharges and sediment-load transport.
Multiple-regression equations were developed
by relating significant channel -geometry
characteristics to Pearson Type-III, design-flood
discharges for 157 streamflow-gaging stations
in Iowa (fig. 2).

Channel-Geometry Data Collection

The channel-geometry parameters that
were measured for each of the gaging stations
are as follows:

ACW - average width of the active channel,
in feet;

ACD - average depth of the active channel,
in feet;

BFW - average width of the bankfull
channel, in feet;

BFD - average depth of the bankfull
channel, in feet;

SCbd - silt-clay content of channel-bed
material, in percent;

SC1bk - silt-clay content ofleft channel-bank
material, in percent;

SCrbk silt-clay content of right
channel-bank material, in percent;

DSO - diameter size of channel-bed particles
for which the total weight of all particles
with diameters greater than Dso is equal
to the total weight of all particles with
diameters less than or equal to Dso, in
millimeters; and

ORA - local gradient of channel, in feet per
mile.

ESTIMATING DESIGN-FLOOD DISCHARGES USING CHANNEL-GEOMETRY CHARACTERISTICS 19