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

Step 2. The weighted 50-year peak
discharge for the gaged site Q50(wgl (equation
16) is estimated next. Because table 8 lists both
the drainage-basin and channel-geometry
regression-equation estimates for this gaged
site, Otter Creek near Ashton (station number
06483460, map number 13!l, fig. 1), the
weighted estimate will be based on the Pearson
Type-III estimate and boJth of these
regression-equation estimates.

The 50-year Pearson TYPE--III estimate is
11,100 ft3/s, and the effective record length is 39
years (table 8). The 50-year drainage-basin
regression estimate is 6,710 ft3/s (listed as
method GISDB in table 8), and the average
equivalent years of record for this regression
equation is 9.5 (table 2). The 50-year Region I,
active-channel channel-geometry regression
estimate is 9,260 ft3/ s (listed as method ACRI in
table 8), and the average equivalent years of
record for this regression equation is 8.9 (listed
in the second set of equations in table 4). The
weighted 50-year flood estima1:e for the gaged
site is calculated using equation 16 as

9,260 ft3/s (listed as method ACRI in table 8),
and the standard error of estimate, in log units,
for this equation is 0.188 (listed in the second set
of equations in table 4). The weighted average,
50-year flood estimate for the gaged site is
calculated using equation 14 as

Q50(db) ISE(,g)12+Q5ol,g) ISE1db)12
Q50Idb,g) = (SE )2+ (SE )2
(db) (cg)

6.710 (0.188) 2 + 9. 260 (0.185) 2

(0.185) 2 + 10.188) 2
= 7.960 ft3/s.

Becaus=- Q50(dbcg~ = Q50(rgl in this example, then
Q50(rg) - 7,960 ft / s.

Step 4. The final step adjusts the 50-year
recurrence interval regression-equation est-
imate of 8,550 ft3/ s (Q50(ru)) calculated for the
ungaged site by the 50-year recurrence interval
information determined for the gaged site. The
adjusted 50-year flood estimate for the ungaged
site Q50(au) is calculated using equations 17 and
18 as

Q50(wgj

(Q50(g)) (ERL) + (<<'50(gdb)) iEidb)) + IQ50ig,g)) IEi,g))
ERL+Eidb) +Ei,g)

111.100) 1391 + (6.710119.5) + (9.260) (8.9)
39 + 9.5 + 8.9

=10,100 ft3/s.

Step 3. The regression-equation estimate for
the gaged site Q50(rg) (equation 18) is
determined next. Because table 8 lists both the
drainage-basin and channel-geometry
regression estimates for this gaged site, Otter
Creek near Ashton, the weighted average of
these regression estimates Q50ldbcg) (equation
14) is calculated to determine the regression
estimate Q50(rg)'

The 50-year flood estimate calculated for
this gaging station using the drainage-basin
equation is 6,710 ft3/ s (listed as method GISDB
in table 8), and the standard error of estimate,
in log units (base 10), for this equation is 0.185
(table 2). The 50-year flood estimate calculated
for this gaging station using the Region I,
active-channel channel-geometry equation is

~ [2I>TDAJ ]
Q50(au) = Q50iru) LAF- TDAg (AF-1) .

!1TDA is the absolute value of the difference
between the total drainage area of the gaged site
(88.0 mi2) and the total drainage area of the
ungaged site (120 mi2),

I>TDA = 32.0 mj';
TDA, = 88.0 mj';

Q50(wgj
AF = .
Q50irg)

AF = 10. 100
7.960 .

AF = 1.27;

Q = S. 550 [1.27 _ ( (2) 132.01111.27 -1)J '
50(au) 88.0

= 9,180 ft3/s.

This adjustment procedure has increased
the 50-year recurrence interval regression-
equation estimate for the ungaged site Q50lrul by

40 ESTIMATING DESIGN-FLOOD DISCHARGES FOR STREAMS IN IOWA