Environmental Engineering Reference
In-Depth Information
The application of the rational method requires information with respect to rainfall data,
as well as a clear understanding of the concepts and principles involved. The main
parameters and procedures included in the determination of the i av value are explained
below:
• Time of Concentration - this is the time, required for the surface runoff flow to travel
from the most remote part of the drainage area to the point of consideration along a
conveying conduit. For urban drainage systems, the time of concentration consists of
the inlet time plus the time of flow in the conduit from the most remote inlet point to
the point under consideration. The time of flow may be estimated closely from the
hydraulic properties of the conduit. The inlet time is the time for runoff to reach
established surface drainage conduits. It would vary with surface slope, the nature of
surface cover, and the length of the path of surface flow, as well as with variables such
as the soil infiltration capacity and depression storage. Reported inlet times used for
design purposes vary from 5 to 30 minutes, with 5 to 15 minutes most commonly
used. In densely developed areas an inlet time of 5 minutes is often reported. In well-
developed districts with flat slopes 10 to 15 minutes is common, and in flat residential
districts 20 to 30 minutes are customary.
• Rainfall intensity - duration relationship - they are important characteristics of any
rainfall event. Usually storm events have varying intensity along its duration. The
curve representing the variation of the rainfall intensity during the storm duration is
called “hyetograph”. The i av of the rainfall event would be equal to the cumulative
depth of rainfall (in mm) divided by the storm duration. An “intensity-duration” curve
for a given storm event would be constructed by rearranging and averaging the
maximum consecutive rainfall intensities over an increasing storm duration, starting
with the peak intensity. A graphical representation of the formation of intensity-
duration curve for a specific storm event is shown on Figure 1.2. Different storms
usually have different intensity duration curves. Rainfall events are statistical
characteristics. The analysis of large data sets of storm records has found that the
Figure 1.2. Hyetograph and
corresponding intensity - duration
curve for a storm event.
Search WWH ::




Custom Search