Name: _______________________________________________ Section:_______________________
part 1: Topography and Drainage Basins
1. On the topographic map of Thorofare Buttes,
2. Mark, in red, and label, the drainage divide that separates the streams draining into Pass Creek from the streams draining into Silvertip Creek. The drainage divide runs between these sets of streams. It does not cut across any stream. It is defined by the highest points separating these two stream networks so look for closed contour lines.
3. Four profile lines, A, B, C, and D are marked on the Thorofare Buttes map (Figure 19.6). Figure 19.7 contains the topographic profiles for these four lines. These topographic profiles show the slope shape in the vertical dimension. Use Figures 19.6 and 19.7 to answer the following questions.
a. Indicate whether the contour lines along each profile closer together at the top of the slope, the bottom of the slope, or whether they’re evenly spaced along the slope. Then indicate the shape of the profile line as linear, concave or convex.
Spacing Profile line shape (Fig. 19.7)
A to A’ TOP CONCAVE
B to B’ TOP CONCAVE
C to C’ BOTTOM CONVEX
D to D’ TOP CONCAVE
b. Examine the spacing of the contour lines along lines E-E’ and F-F’. What shape would a profile drawn along each of these lines have?
E to E’ LINEAR (EVEN SPACING)
F to F’ CONVEX
c. What shape would a profile of the intermittent stream labeled Z have? CONCAVE
4. Determine the shape of each hillslope in the horizontal dimension (from one side to the other). Examine the shape of the contour lines where they cross the line identifying the location of the profile. First determine which end point has the highest elevation. Second, decide whether the contour lines bend uphill as the cross the profile line, downhill as they cross the profile line, or go straight across the profile line. Third, indicate the hillslope shape in the horizontal dimension as concave (a valley), convex (a ridge) or linear.
Highest Direction contour
end point lines curve towards Hillslope shape
A - A' A (10000') bend apex points uphill concave
B - B' B' (10530') bend apex points downhill convex
C - C' C (10400') straight linear
D - D' D' (11400') straight (1/8" each side) linear
E – E’ E' (9600') straight linear
F – F’ F (10530') bend apex points downhill convex
Stream Z bend apex points uphill concave
5. Based on your answers to question 4, which of the profile lines could be a drainage divide?
B, C, or F
6. Combine your answers to questions 3 and 4 to determine which hillslope shape from Figure 19.3 matches each of the profile lines. Then indicate whether the slope will “gather water”, “spread water” or whether water will flow straight downhill. Last, indicate whether the erosive energy of water will be concentrated, spread out, or evenly distributed.
Hillslope shape
from Fig. 19.3 Water flow Erosive Energy
A - A' concave- concave collect concentrate
B - B' concave- convex shed disperse
C - C' convex- linear sheet wash uniform
D - D' concave- linear sheet wash uniform
E – E’ linear - linear sheet wash uniform
F – F’ convex- convex shed disperse
FIGURE 19.6 USGS
Topographic map
Thorofare Buttes,
Original scale was 1:24,000
contour interval = 40 feet
7. a. If contour lines are bending uphill, do they define a valley or a ridge? VALLEY
b. If contour lines are bending downhill, do they define a valley or a ridge? RIDGE
c. If you want to locate a drainage divide, do you want to look for contour lines which are bending uphill or downhill? DOWNHILL
d. Given your answers to (a), (b) and (c) above, draw the drainage divide separating the tributary labeled "Y" from the tributary labeled "Z".
e. Why is this drainage divide more difficult to locate at lower elevations than at higher elevations? What happens to the shape of the slope, as indicated by the shape of the contour lines, that makes it more difficult to locate? BECOMES A Linear-Linear SLOPE BELOW 9200'
f. Is it really correct to draw this drainage divide all the way to
Pass Creek as a single line? Why or why not? NO; DRAINAGE IN
BOTTON FACET PROBABLY REACHES
Name: _______________________________________________ Section:_______________________
part 4: Flood Magnitude and Frequency
Table 19.4 below contains a
record of the highest flood that occurred each year for a 57 year period on the
1. a. Calculate the missing
values for the percent of time floods are equaled or exceeded (% Time Q³Q)
and add this information to Table 19.4.
b. Calculate the missing values for recurrence intervals (return periods) and add this information to the table.
2. a. What is the discharge
for the largest recorded flood? 12,600 cfs
b. On the average, how often does a flood of this size occur? 58 yrs
c. If a flood of this magnitude occurred in 1991, can we predict when it will occur again? Why or why not? No. We can only predict a duration when another flood of this magnitude is likely.
3. a. What is the discharge for the smallest
recorded flood? 835 cfs
b. On the average, how often does a flood of this size occur? Every year
c. Would it be possible for a flood of this magnitude to occur two or three times in one year, even though the return period is one year? Yes; the one year is just an AVERAGE recurrence time.
4. Every time the flow on this river exceeds 5,500 cfs a small town on the river floodplain is flooded. Approximately how often does this town get flooded (years)? On average, once every 5.3 years
5. Plot the discharge and percent of time a flood is equaled or exceeded on the log-probability graph paper provided. The axes have been labeled. Once the points are plotted, use a straightedge to draw a trend line showing the general pattern of flood frequency. Remember, a trend line is one centered on the dots not one that connects the dots; some of the dots will fall above the line and some below it. The line should cover the entire width of the graph (from 0 to 99.99 percent) if possible.
TABLE 19.4 Ranked highest
annual floods for the
|
Rank |
Year |
Discharge (cfs) |
% Time Q³Q |
Return Period (yrs) |
|
Rank |
Year |
Discharge (cfs) |
% Time Q³Q |
Return Period (yrs) |
|
1 |
1978 |
12,600 |
1.7 |
58.0 |
|
30 |
1972 |
2,440 |
51.7 |
1.9 |
|
2 |
1966 |
10,000 |
3.4 |
29.0 |
|
31 |
1954 |
2,400 |
53.4 |
1.9 |
|
3 |
1961 |
9,920 |
5.2 |
19.3 |
|
32 |
1947 |
2,340 |
55.2 |
1.8 |
|
4 |
1951 |
7,530 |
6.9 |
14.5 |
|
33 |
1980 |
2,330 |
56.9 |
1.8 |
|
5 |
1935 |
6,710 |
8.6 |
11.6 |
|
34 |
1943 |
2,310 |
58.6 |
1.7 |
|
6 |
1959 |
6,620 |
10.3 |
9.7 |
|
35 |
1983 |
2,290 |
60.3 |
1.7 |
|
7 |
1946 |
6,470 |
12.1 |
8.3 |
|
36 |
1968 |
2,280 |
62.1 |
1.6 |
|
8 |
1956 |
6,030 |
13.8 |
7.3 |
|
37 |
1986 |
2,170 |
63.8 |
1.6 |
|
9 |
1950 |
5,840 |
15.5 |
6.4 |
|
38 |
1990 |
2,110 |
65.5 |
1.5 |
|
10 |
1934 |
5,830 |
17.2 |
5.8 |
|
39 |
1976 |
2,070 |
67.2 |
1.5 |
|
11 |
1965 |
5,500 |
19.0 |
5.3 |
|
40 |
1940 |
2,020 |
69.0 |
1.5 |
|
12 |
1948 |
4,920 |
20.7 |
4.8 |
|
41 |
1979 |
2,010 |
70.7 |
1.4 |
|
13 |
1973 |
4,400 |
22.4 |
4.5 |
|
42 |
1989 |
1,900 |
72.4 |
1.4 |
|
14 |
1937 |
4,190 |
24.1 |
4.1 |
|
43 |
1939 |
1,740 |
74.1 |
1.3 |
|
15 |
1952 |