Geology Reference
In-Depth Information
7. There are two major rivers in the Big Thompson River sys-
tem: The Big Thompson (BT) in the south and the North Fork
Big Thompson (NFBT) in the north. The flood peak at site 22,
east of Drake, occurred at the same time as the minor peak at
site 21 (2110 MDT) on the NFBT (see Figure 10 .9).
a. How did the flood crest moving down the BT impact
the water level on the NFBT at Site 21?
have had on the size of the flood at site 23 at the mouth of Big
Thompson Canyon (Figure 10.9, Inset map)?
9. The flood crest reached Site 23 at the mouth of Big Thompson
Canyon, 7.7 miles east of Site 22,30 minutes after the crest at Site
22. What was the average speed of the flood crest in miles per
hour and feet per second? Could you outrun this flood crest?
b. Why was there a second (main) peak at Site 21 on the
NFBT?
10. Refer to Table 10.6
a. At what three sites were the unit discharges or dis-
charge/area ratios of drainage basins (cfs/mi 2 ) the
greatest?
8. If the main peak at site 21 had occurred 30 to 35 minutes
earlier than shown in Figure 10.10, what impact would this
TABLE 10.6 Hydrologic Data for Selected Flood Sites in Figure 10.9
Drainage Area
(mi 2 )
Unit Discharge
(cfs/mi 2 )
Site No.
Stream and
Location
Peak Discharge
(cfs)
Average
Velocity
(ft/s)
Average Depth
(ft)
4
Dry Gulch
2.00
3,210
1,600
12
3.3
6
Big Thompson
R., Lake Estes
4,330
481
8
4.6
9.00 a
7
tributary near
Loveland Heights
1.37
8,700
6,350
26
5.5
8
Dark Gulch
1.00
7,210
7,210
28
5.1
9
Noels Draw
3.37
6,910
2,050
21
5.7
10
Rabbit Gulch
3.41
3,540
1,040
13
4.7
11
Long Gulch
1.99
5,500
2,760
19
5.8
12
Big Thompson
R. above Drake
28,200
829
22
8.3
34.0 a
14
Fox Creek
1,300
9
2.8
7.18 b
16
Devils Gulch
0.91
2,810
3,090
12
2.1
17
Tributary near
Glen Haven
1.38
9,670
7,010
29
5.6
18
Black Creek
3.17
1,990
628
11
4.5
20
Tributary west of
Drake
1.26
3,240
2,570
18
3.0
21
North Fork Big
Thompson R. at
Drake
8,710
12
5.2
85.1 b
22
Big Thompson
R. below Drake
30,100
16
10.3
276. b
23
Big Thompson
R. at mouth of
canyon
31,200
26
10.6
305. b
a. Approximate contributing drainage area during flood of July 31-August 1, 1976.
b. Contributing drainage area for flood of July 31-August 1, 1976, unknown,
(from McCain et al., 1979)
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