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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
as functions of watershed characteristics. These forms of empirical equations indicate an interrelation
of T c and R, and such dependence was observed in the database, as discussed in the
Documentation Manual. The equation for estimating R for Maricopa County is:
1.11 – 0.57
R = 0.37T c A L 0.80
(5.8)
where:
R = storage coefficient, in hours,
T c = time of concentration, in hours,
A = drainage area, in square miles, and
L = length of flow path, in miles.
5.5.3 Time-Area Relation
Either a synthetic time-area relation must be adopted or the time-area relation for the watershed
must be developed. If a synthetic time-area relation is not used, the time-area relation is developed
by dividing the watershed into incremental runoff producing areas that have equal incremental
travel times to the outflow location. This is a difficult task and a well defined and reliable
procedure is currently not available. The following general procedure is often used:
1. Use a topographic map of the watershed to trace along the flow path, the distance
from the hydraulically most distant point in the watershed to the outflow location; this
defines L in both Equation (5.5) and Equation (5.8).
2. Draw isochrones on the map to represent equal travel times to the outflow location.
These isochrones can be established by considering the land surface slope and resistance
to flow, and also whether the runoff would be sheet flow or would be concentrated
in watercourses. A good deal of judgement and interpretation is required for
this.
3. Measure and tabulate the incremental areas (in an upstream sequence) as well as the
corresponding travel time for each area.
4. Prepare a graph of travel time versus contributing area (or a dimensionless graph of
time as a percent of T c versus contributing area as a percent of total area). The
dimensionless graph is preferred because this facilitates the rapid development of
new time-area relations should there be a need to revise the estimate of T c .
5-18 August 15, 2013