compass can be used to draw the B 12 and E 12 curves. The value of the spillway
discharge Q is, respectively, added to and subtracted from the storage curve in ac-ft.
The B 6 and E 6 curves then are the midpoints between the respective B 12 curve and
the storage curve and the E 12 curve and the storage curve. These lines are indicated in
Fig. 6.14.
The next step is to choose the inflow hydrograph and determine the values of
inflow during a 12- or 6-h period. An example of computation for this is shown in
Table 6.4. The first column indicates the corresponding lines that have been drawn in
Fig. 6.14. The first few influent calculations are not indicated as lines because the
lines would be too short and would not show in the figure. The values for the first
five columns are the values to be used to enter the Fig. 6.14. It may be seen that 12-h
intervals may be chosen for the noncritical portion of the curve; however, a 6-h
interval should be chosen over the peak of the curve. This time interval may vary for
different drainage basin sizes. The values for column 3 are the instantaneous rates of
inflow in cfs determined from the storm hydrograph; and the values for column 4 are
merely the mean rate over the time period. Column 5 simply converts these values of
cfs to ac-ft during the time interval. As may be seen during the 12-h time interval,
these are exactly the same values as for cfs and for the 6-h period are half these
values. Then, choosing the predetermined elevation of the reservoir at the start of the
flood (for this example, the reservoir was considered to be empty), the amount of
runoff from column 5 is added to the appropriate B (B 12 ) line in Fig. 6.14. For
example, line AB occurring 48 h after the beginning of the flood represents an inflow
of 112,700 ac-ft and it is added to the B 12 line at an elevation of 433.6 ft, which is the
elevation at the end of the previous time period. The length of line AB represents the
112,700 ac-ft inflow. Since this line extends beyond the E 12 or ending curve for a
12-h period, line BC must be drawn in an upward direction in order to meet the E 12
curve. This establishes a new reservoir elevation at the end of the time period and a
new discharge rate that is equivalent to the Q value from the Q curve at the new
elevation. From this point, a 6-h time interval was used and line DE therefore is
drawn as the volume of flow of 98,600 ac-ft beginning with the B 6 curve and
extending to the right. Again, this line extends beyond the E 6 curve and therefore
must be extended as line EF upward to the E 6 curve. In all instances, when working
with a 12-h influent, one must begin and end with a 12-h curve, and correspondingly,
when working with a 6-h influent, one must begin and end with a 6-h curve. This
system is continued until the point where a line designating the influent does not
extend beyond the ending curve, and then a downward line would have to be drawn
to the corresponding ending curve, indicating a drop in elevation of the reservoir and
a decrease in the discharge rate. It may be seen from the values of Table 6.4, column
6, that the maximum reservoir elevation reached was 451.8 ft. at which time the
spillway discharge equaled the volume of inflow.
The final results of the calculations in Table 6.4 are shown in Fig. 6.15, which
plots the inflow vs. the outflow and also shows the stage or elevation at the
corresponding time periods. Note that the units for stage are in feet of elevation,
whereas the inflow and outflow units are given in terms of flow. It may be seen that
the peak of the stage occurs at the point where the inflow and outflow curves cross.
262
D. B. Aulenbach et al.
discharge Q is, respectively, added to and subtracted from the storage curve in ac-ft.
The B 6 and E 6 curves then are the midpoints between the respective B 12 curve and
the storage curve and the E 12 curve and the storage curve. These lines are indicated in
Fig. 6.14.
The next step is to choose the inflow hydrograph and determine the values of
inflow during a 12- or 6-h period. An example of computation for this is shown in
Table 6.4. The first column indicates the corresponding lines that have been drawn in
Fig. 6.14. The first few influent calculations are not indicated as lines because the
lines would be too short and would not show in the figure. The values for the first
five columns are the values to be used to enter the Fig. 6.14. It may be seen that 12-h
intervals may be chosen for the noncritical portion of the curve; however, a 6-h
interval should be chosen over the peak of the curve. This time interval may vary for
different drainage basin sizes. The values for column 3 are the instantaneous rates of
inflow in cfs determined from the storm hydrograph; and the values for column 4 are
merely the mean rate over the time period. Column 5 simply converts these values of
cfs to ac-ft during the time interval. As may be seen during the 12-h time interval,
these are exactly the same values as for cfs and for the 6-h period are half these
values. Then, choosing the predetermined elevation of the reservoir at the start of the
flood (for this example, the reservoir was considered to be empty), the amount of
runoff from column 5 is added to the appropriate B (B 12 ) line in Fig. 6.14. For
example, line AB occurring 48 h after the beginning of the flood represents an inflow
of 112,700 ac-ft and it is added to the B 12 line at an elevation of 433.6 ft, which is the
elevation at the end of the previous time period. The length of line AB represents the
112,700 ac-ft inflow. Since this line extends beyond the E 12 or ending curve for a
12-h period, line BC must be drawn in an upward direction in order to meet the E 12
curve. This establishes a new reservoir elevation at the end of the time period and a
new discharge rate that is equivalent to the Q value from the Q curve at the new
elevation. From this point, a 6-h time interval was used and line DE therefore is
drawn as the volume of flow of 98,600 ac-ft beginning with the B 6 curve and
extending to the right. Again, this line extends beyond the E 6 curve and therefore
must be extended as line EF upward to the E 6 curve. In all instances, when working
with a 12-h influent, one must begin and end with a 12-h curve, and correspondingly,
when working with a 6-h influent, one must begin and end with a 6-h curve. This
system is continued until the point where a line designating the influent does not
extend beyond the ending curve, and then a downward line would have to be drawn
to the corresponding ending curve, indicating a drop in elevation of the reservoir and
a decrease in the discharge rate. It may be seen from the values of Table 6.4, column
6, that the maximum reservoir elevation reached was 451.8 ft. at which time the
spillway discharge equaled the volume of inflow.
The final results of the calculations in Table 6.4 are shown in Fig. 6.15, which
plots the inflow vs. the outflow and also shows the stage or elevation at the
corresponding time periods. Note that the units for stage are in feet of elevation,
whereas the inflow and outflow units are given in terms of flow. It may be seen that
the peak of the stage occurs at the point where the inflow and outflow curves cross.
262
D. B. Aulenbach et al.