Geology Reference
In-Depth Information
Peak discharges exceeded the 100-year flood
discharge at several sites, although the rainfall, flood
discharges, unit discharges, and dollar losses were not
unprecedented for areas along the eastern foothills
and plains of Colorado (McCain et al., 1979). The
major component of the dollar losses was damage to
U.S. Highway 34; losses not counted included tax
receipts, tourist business, water wells, septic tanks,
and property devaluation because of location in a des-
ignated flood plain.
Significant geomorphic changes occurred in the
steep terrain with relief of more than 3,000 ft and
canyon walls as steep as 80 percent. These included
channel scouring for most of the river, where the stream
gradient was greater than 2 percent, on the outside of
meander bends and in narrow reaches. In reaches
where the gradient flattened to less than 2 percent and
where the stream channel widened, boulders (some
with an intermediate diameter of 7 ft) and finer sedi-
ment were deposited on point and channel bars and on
flood-plains. Overbank sedimentation was the primary
impact downstream of Big Thompson Canyon.
On the tributaries, sheetfloods transported boul-
ders onto lesser slopes of alluvial and debris fans and
eroded unprotected slopes. Debris avalanches, slides,
and flows were generated by the saturated soils and
provided debris that destroyed some buildings.
Both historic and geologic evidence suggests that
other basins along the Front Range are vulnerable to
flood events of magnitude similar to that of the one
that struck the Big Thompson Basin on the eve of Cen-
tennial Sunday (Shroba et al., 1979).
The primary objective of this exercise is to under-
stand the nature and causes of flash flooding in moun-
tain terrains. The secondary objective is to investigate
the impacts of the flooding and associated water erosion
and sediment deposition. For more information on the
Centennial Flood, see
www.coloradoan.com/news/
thompson/.
TABLE 10.5 Cumulative Precipitation, July 31-August 2,
1976, for Selected Sites in the Big Thompson River Basin
Mean value of precipitation for July here is 1.5-2.1
inches. The 2-and 4-inch rainfall contours (isohyets) are
shown in Figure 10.9.
Site
Cumulative Precipitation (in.)
2
5.2
3
6.0
4
8.1
6
9.9
7
10.5
8
10.4
9
10.6
10
8.0
11
10.0
13
10.0
14
9.9
15
6.3
16
8.0
17
8.8
18
8.1
19
8.2
20
8.0
27
7.9
A
6.3
B
5.8
C
10.0
D
10.1
E
9.8
QUESTIONS (10, PART C)
1.
Examine Table 10.5. Plot the cumulative rainfall for the
sites in this table on Figure 10.9 and draw in the lines of
equal precipitation (isohyets) for
6, 8,
and 10 inches.
5. Refer to discharge hydrographs (Figure 10.10) for
selected sites on the Big Thompson River and the North
Fork Big Thompson River. Compare sites 1, 21, and 23. Do
YOU think the flooding at Estes Park (site 1) was serious?
Explain.
2.
Where were the centers of precipitation?
6.
Do the hydrographs support the precipitation data in indi-
cating the area(s) of greatest runoff and potential damage?
Explain.
3.
What is the explanation for the intense, localized nature of
this precipitation event?
4.
From this map, where would you expect most flood and
mass-movement damage from the storm to occur?
