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

precipitation intensity (TTF). These primary
drainage-basin characteristic measurements
and the scale of maps to use for each manual
measurement are described in Appendix A and
Appendix B.

(B). The topographic maps used to delineate
the drainage-divide boundary for this ungaged
site are the DMA 1:250,000-scale Fairmont
topographic map and the USGS 1:100,000-scale
Osceola County map. Contributing drainage
area (CDA) is calculated from the primary
drainage-basin characteristics total drainage
area (TDA) and noncontributing drainage area
(NCDA). The drainage-divide boundary for this
basin is delineated on the 1:250,000-scale map,
and the total drainage area (TDA) for the
ungaged site is listed in Larimer (1957, p. 313)
as 120 mi2. The total drainage area published
for the gaged site, Otter Creek near Ashton
(station number 06483460; map number 139,
fig. 1), is 88.0 mi2 (table 9). Because the total
drainage area of the ungaged site is 136.4
percent of the total drainage area of the gaged
site and within the 50- and 150-percent limits
for application, the adjustment procedure is
determined to be applicable to the ungaged site.
Inspection of the 1:IOO,OOO-scale map does not
show any noncontributing drainage areas
within the drainage-divide boundary of this
basin. The contributing drainage area (CDA) for
the ungaged site is calculated using equation 10
as

CDA = TDA - NCDA,
= 120-0,
:::: 120mi2,

(C). Relative relief(RR) for the ungaged site
is calculated from the primary drainage-basin
characteristics basin relief (BR) and basin
perimeter (BP). The difference between the
highest elevation contour and the lowest
interpolated elevation in the basin measured
from the 1:250,000-scale topographic map gives
a basin relief of 286 ft. The drainage-divide
boundary delineated on the 1:250,000-scale
topographic map is used to measure the basin
perimeter, which is 57.8 mi. Relative relief(RR)
is calculated using equation 11 as

RR = BR
BP'
286
- 57.8'

:::: 4.95 ftjrni.

(D). Drainage frequency (DF) for the
ungaged site is calculated from the primary
drainage-basin characteristics number of
first-order streams (FOS) and contributing
drainage area (CDA). A total of 57 first-order
streams are counted within the drainage-divide
delineation for the ungaged site on the
1:100,OOO-scale topographic map. Drainage
frequency (DF) is calculated using equation 12
as

DF = FOS
CDA'

57
- 120'

:::: 0.475 first-order streams/mi2.

(F). The 2-year, 24-hour precipitation
intensity (TTF) for the ungaged drainage basin
is determined from figure 5. Because the
drainage-divide boundary of this ungaged site
overlies two of the 2-year, 24-hour precipitation
intensity polygons shown in figure 5, a weighted
average for the basin is computed using
equation 19 as outlined in Appendix B.
According to figure 5, approximately 60 percent
of the total drainage area (TDA) for the ungaged
site is located within the polygon labeled as
2.85 in., and approximately 40 percent of the
total drainage area is located within the polygon
labeled as 2.95 in. The weighted average for the
2-year, 24-hour precipitation intensity (TTF) is
calculated using equation 19 (Appendix B) as

TFF = IA,)(TFF,) + (A,)(TFF,),
= (0.60) (2.851 + (0.401 (2.95),
:::: 2.89 in.

(G). The 50-year flood estimate for the
ungaged site using the drainage-basin equation
(table 2) is calculated as

Q50 = 231 (CDA)o.694 <RR)O.656 (DF)OAOl (TTF _ 2.5)0.491,

= 231 (120)0.694 (4.95)0.656 <0.475)0.401
12.89 _ 2.5)049',

= 8,550 ft'/,.

Because Q50 = Q50(db), and Q50(rul (equation
17) = SiiOldb) in this example, then Q50(ru) =
8,550 ft f s.

APPUCATION AND RELIABILITY OF FLooD.ESTIMATION METHODS 39