Geology Reference
In-Depth Information
FIGURE 10.13 Pattern of scour and deposition within canyons of Big Thompson and North Fork Big Thompson rivers. Floodway is
about 200 ft wide with gradients of 2-4 percent (Shroba et al., 1979).
19. The heaviest precipitation from this flood occurred
between 1930 and 2040 hours MDT at Glen Comfort and
1930 and 2200 hours at Glen Haven. There was little oppor-
tunity to warn anyone. Use this information with the other
information in this exercise to prepare two paragraphs, one
arguing for and the other against purchase of a waterfront
lot for a house in one of the many river valleys or canyons
similar to Big Thompson along the Front Range of the Rock-
ies. Use a separate sheet of paper for your answer.
enough time. The river determines how much time is
"enough"; it may be a few years, a few decades, or a
few centuries. But over time, rivers will occupy and
modify their floodplains.
On a shorter time period, floods will cover parts
of the river valley. How much gets covered is deter-
mined by how big the floods are.
QUESTIONS (10, PART D)
Refer to Figure 2.10 in Chapter 2, Bloomington, Indiana.
1. Sketch a topographic profile of the valley of Griffy Creek,
from the 750' contour in contact with the "L" in Bloomington
in the west to the zero in the 750' contour above the "16" in
Township 16 in the East. Mark the floodplain on the profile
with a different color.
PART D. FLOOD PLAINS AND FLOOD
INSURANCE RATE MAPS
Meandering Rivers and Identification of Flood
Plains on Topographic Maps
The first step in avoiding flood hazards is to avoid
building in rivers. Some geoscientists suggest that the
area known as the flood plain should instead just be
called part of the river. The implication of this is
important: Flood plains are natural parts of river sys-
tems that do not happen to be covered by water all of
the time. When flood plains are covered by water,
however, the damage to unprepared (and in some
cases even prepared) people and communities can be
disastrous.
In this part of the exercise we are going to use
several different topographic maps in the colored
plates section of the topic, and identify different flood
plains on the maps. First, refer to Figure 10.14, which is
a sketch of typical topographic features seen along a
flood plain.
Note on this figure that natural rivers meander
(bend) a lot. The floodplain is the topographically low
area adjacent to the river. Although when we look at a
river it may appear to have a permanent channel,
meandering rivers will change their channels and
erode and redeposit over their entire floodplain, given
2. Is the Drive-in Theater (Southwest of the dam) in a flood-
plain? Explain.
3. Is Payne Cemetery (east-central part of the map) in a
floodplain? Explain.
Refer to Figure 6.10 in Chapter 6, Draper, Utah.
4. Shade on this map the floodplain of Little Cottonwood Creek
in the area of Glacio Park, near the eastern edge of the map.
5. Do you think the floodplain west of Beaver Ponds Springs
is broad or narrow? What evidence do you use to support
your answer?
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