ETL 1110-2-367
31 Mar 95
Perform Lower Sub-basin Analysis (+ Upper as
1 day may be appropriate. If the problem is providing facilities
needed) (Option B). Execute Option A, if appropriate and if
to handle peak flow reaching the line-of-protection (such as for
an urban area with little or no ponding volume), then a short
not already executed. Then, compute a runoff hydrograph for
time interval is required. A good test is to analyze a plan
the lower interior subbasin using the precipitation record,
configuration using several time intervals until the results are
infiltration loss, base flow, and unit hydrograph parameters.
consistent, especially the stage-frequency relationship.
Add the auxiliary inflow for the lower subbasin. Combine the
routed hydrograph from the upper subbasin, if present as a result
(2) Effect of the Computational Time Interval on Other
of Option A above.
Computations. Selection of a computation time interval can
Perform Exterior Basin Analysis (Option C).
affect the validity and numerical stability of several
computations. Shorter time intervals generally provide more
Execute Options A and B, if appropriate and if not already
stable results. If the output results indicate a significant
executed. Then, compute the exterior stage hydrograph at the
difference between total inflow and outflow volume, a shorter
primary outlet location using the data specified in the exterior
time interval may be required.
stage module.
d. Interior analysis computation sequence. After the
Perform Pond Routing Analysis (+ Upper, Lower,
Exterior as needed) (Option D). Execute Options A, B, and
plan is specified, the screen illustrated in Figure 3-22 is dis-
played. This menu controls the interior analysis computations
C, if appropriate and if not already executed. Then, compute the
performed for a single operation. Five options are available:
pond stages and outflows for each time period throughout the
analysis using the data for the interior pond, gravity outlets,
Perform Upper Sub-Basin Analysis (Option A).
pumps, seepage, overflow, exterior stage, and combined inflow
Compute the runoff hydrograph for the upper interior subbasin
hydrograph.
using the precipitation record from the PRECIP module and the
Perform Frequency Analysis (+ Upper, Lower,
infiltration loss, unit hydrograph, and base flow parameters from
Exterior, Pond as needed) (Option E). Execute Options A,
the RUNOFF module. Add the auxiliary inflow for the upper
subbasin. Subtract the diversion from the upper subbasin. Route
B, C and D, if appropriate and if not already executed. Then,
the resulting hydrograph downstream to the lower subbasin.
compute a graphical annual or partial duration series interior
Figure 3-22. Interior analysis menu
3-17
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