Geology Reference
In-Depth Information
b.
How do the locations of these sites compare with the
distribution of maximum precipitation for the storm
(see Figure 10.9)?
14. Given that the previous peak discharge at Site 21 was
1290 cfs in the 1965 flood, how many times larger was the
1976 flood than the earlier flood at this site?
11. Plot the peak discharge and drainage area for each of the
stations with the three highest unit discharges for the 1976
storm on the peak discharge/drainage area diagram for east-
ern Colorado (Figure 10.11). Unit discharges provide a mea-
sure of the relative size of a flo^d event in cubic feet per
15. The ratio of the peak discharge of the 1976 storm to the
discharge for the 100-year flood was determined for selected
stations as follows:
second per square mile (
c
fe/
m
i )• Considering the three
data points plotted for the 1976 flood, do you think that even
more outstanding floods will occur on Big Thompson River,
or is this the largest that can be expected? Explain.
Site
Peak Discharge, 1976 (cfs)
Ratio
12 (BT)
3.8:1
28,200
21 (NFBT)
8,710
1.4:1
22 (BT)
30,100
2.9:1
12. Explain why unit discharge for Site 23 (see inset map,
Figure 10.9) could not be plotted on this graph (Figure 10.11;
see Table 10.6.)
23 (BT)
31,200
1.8:1
For the drainage basins upstream of Drake, was this flood
greater on the Big Thompson River (BT, Site 12) or on the
North Fork Big Thompson River (NFBT, Site 21)?
13. From Table 10.6, which site had the highest average
velocity of stream flow?
1
1
1 1—i—i i i i 1
1
i i i i i i i 1
1
i i i
I
i i i i i i i 1
1
1
1
'—'—
1
1
100Q
'
'
0.1
1
10
100
1000
10,000
DRAINAGE AREA, IN SQUARE MILES
FIGURE
10.11
Relation of peak discharge to drainage area for flood of July 31-August 1, 1976, and previous maximum floods in
eastern Colorado. The curve was drawn through the average of the eleven largest floods in the United States between 1890 and 1969.
The empirical equation for the curve below the 200 mi
2
area is Q = 4000 AO.61, where Q = discharge
anc
| A = drainage
are
a
(Modified from McCain et al., 1979; after Matthai, 1969)
