Parallel areas - Two or more areas which are discharging runoff into the same dependent downstream
TOCx - TOC redefined to be the maximum travel time from the most hydraulically remote representative
point in the series of areas to a particular outlet, X.
Inlet timex - Formerly called TOC. Inlet timex is the maximum representative flow time for runoff
originating in area X arriving at outlet X, whether dependent or independent
Inlet timew-x - The sum of TOCx (the TOC at the outlet upstream in area W) plus DTw-x (the "ditch time
(DT)," transit time through area X, from outlet W to outlet X).
Subarea - An individual drainage area in the series of drainage areas.
Runoff Estimation Summary
Preparatory work should be done before doing an estimation summary by using the following steps.
STEP 1: Delineating every subarea in the series of areas.
STEP 2: Determining the intensity of the two-year, 60-minute rainfall.
STEP 3: Determining for each subarea--
C, and C weighted, if necessary.
STEP 4: Determine for each dependent subarea the ditch time (or transit time) ditch time from the upper outlet to
the lower outlet.
Culvert flow -- Assume 300 fpm, unless you can be more precise.
Ditch flow -- Use figure 2-2 on page 2-7.
Specific Q Determination
Working systematically, start at the uppermost subarea and proceed downstream (subarea X is upstream of
Determine the subarea TOC (TOC, using the new definition) using these simple rules.
Rule for an independent subarea: TOCx = Inlet timex
Rule for a dependent subarea: First compare Inlet timex and Inlet timex-y = TOCx + DTx-y. Then select
TOCy equal to the larger value based on the comparison.
Adjust I60 minute, 2 year to ITOC, 2 year.
Calculate Q for subarea X: Qx = Cx ITOCx Ax