Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
Synthetic time-area relations can be used such as the default relation in the HEC-1 program:
A * = 1.414 (T*) 1.5 for 0 < T* < 0.5 (5.9)
1 - A * = 1.414 (1 - T*) 1.5 for 0.5 < T* < 1.0
where:
A* = contributing area in percent of total area and
T* = time in percent of T c .
Equation (5.9) is a symmetric relation and is not recommended for most watersheds in Maricopa
County.
Two other dimensionless time-area relations have been developed during the reconstitution of
recorded rainfall-runoff events as described in the Documentation Manual. These dimensionless
relations for urban and natural watersheds are shown in Figure 5.6 and Figure 5.7. Each of
those figures show a synthetic time-area relation and shaded zone where the time-area relation
is expected to lie. For an urban watershed, the synthetic time-area relation of Figure 5.6 is recommended,
and for a natural (undeveloped) watershed the synthetic time-area relation of Figure
5.7 is recommended. If a time-area relation is developed from the watershed map, which is generally
recommended for unusually shaped watersheds, then the resulting relation should lie
within the shaded zones in either Figure 5.6 or Figure 5.7. The HEC-1 default time-area relation
is shown for comparison in each figure. Tabulated values of the dimensionless time-area relations
are shown in Table 5.4.
August 15, 2013 5-19