Morphology and Flow in Streams
61
hydrograph (Fig. 5.5). Hydrographs provide useful information on response of stream
discharge to storm events and for comparisons among streams. The hydrograph
depends on various features ofthe storm and ofthe watershed. The rising limb (Fig. 5.5)
of the curve is usually concave, reflecting the infiltration and storage capacity of the
watershed. The peak of the curve represents maximum runoff. The falling limb
indicates flow from storage within the soil after precipitation has stopped.
Discharges that exceed the bankfull capacity ofthe channel are called floods. Floods
essentially are random events (Leopold et aI., 1964). The mean annual flood is used
widely as a hydrological measurement. It corresponds to the average value for the
largest annual discharge over a series of years.
The mean annual flood is an important event in producing the flood plain. The
return period, or recurrence interval, of flooding is calculated by
N + 1
Tr=-M
where Tr is the return period in yr, N is the number of years of record, and M is the rank
of an individual event in the set. The mean annual flood has a return period of 2.33 yr
(Leopold, et aI., 1964). Thus, once every 2.33 yr, on the average, the highest discharge of
the year will equal or exceed the value of the mean annual flood (Fig. 5.6).
EXERCISES
OPTION 1
In the Laboratory
1. Locate and draw the drainage basin boundaries (B) on a U.S. Geological Survey map for the
stream you plan to study.
a. Draw drainage basin boundaries (divide lines) perpendicular to contour lines (elevations);
overland flow and shallow subsurface water flows perpendicularly to contour lines.
b. Draw divide lines through the center of "saddles" and closed contour lines.
Saddle
Closed Contour
__ ~B
B"""-'r--B
0
B
c. Water flows to the opening of the V's of contour lines. Draw divide lines accordingly.
2. Determine basin and channel characteristics as described in Table 5.2 (see also Exercise 1).
3. Plot the average stream length, number of streams, and drainage area versus stream order on
semilogarithmic paper for the drainage network.
4. Plot the percentage of total channel length against order.
5. Using the data in Table 5.3, make a regression of length and average discharge versus
drainage area each on a log-log plot.
6. Calculate the regression constants for the above relationships.
61
hydrograph (Fig. 5.5). Hydrographs provide useful information on response of stream
discharge to storm events and for comparisons among streams. The hydrograph
depends on various features ofthe storm and ofthe watershed. The rising limb (Fig. 5.5)
of the curve is usually concave, reflecting the infiltration and storage capacity of the
watershed. The peak of the curve represents maximum runoff. The falling limb
indicates flow from storage within the soil after precipitation has stopped.
Discharges that exceed the bankfull capacity ofthe channel are called floods. Floods
essentially are random events (Leopold et aI., 1964). The mean annual flood is used
widely as a hydrological measurement. It corresponds to the average value for the
largest annual discharge over a series of years.
The mean annual flood is an important event in producing the flood plain. The
return period, or recurrence interval, of flooding is calculated by
N + 1
Tr=-M
where Tr is the return period in yr, N is the number of years of record, and M is the rank
of an individual event in the set. The mean annual flood has a return period of 2.33 yr
(Leopold, et aI., 1964). Thus, once every 2.33 yr, on the average, the highest discharge of
the year will equal or exceed the value of the mean annual flood (Fig. 5.6).
EXERCISES
OPTION 1
In the Laboratory
1. Locate and draw the drainage basin boundaries (B) on a U.S. Geological Survey map for the
stream you plan to study.
a. Draw drainage basin boundaries (divide lines) perpendicular to contour lines (elevations);
overland flow and shallow subsurface water flows perpendicularly to contour lines.
b. Draw divide lines through the center of "saddles" and closed contour lines.
Saddle
Closed Contour
__ ~B
B"""-'r--B
0
B
c. Water flows to the opening of the V's of contour lines. Draw divide lines accordingly.
2. Determine basin and channel characteristics as described in Table 5.2 (see also Exercise 1).
3. Plot the average stream length, number of streams, and drainage area versus stream order on
semilogarithmic paper for the drainage network.
4. Plot the percentage of total channel length against order.
5. Using the data in Table 5.3, make a regression of length and average discharge versus
drainage area each on a log-log plot.
6. Calculate the regression constants for the above relationships.