Environmental Engineering Reference
In-Depth Information
relationship between the i av of a given storm and the storm duration are similar for
similar geographic conditions. This has led to the development of unified
intensity-duration curves for the specific geographic region and their linkage to
the frequency of occurrence of the rainfall event.
The frequency of a storm event with given i av , is a statistical parameter, which
shows how often this specific storm would be probable to occur, e.g. if a given
storm event with i av = 50 mm/h occurs once per two years, its frequency is 0.5. The
frequency of a storm occurrence is reciprocally connected to the term “return
period”, also known as “recurrence interval”, which shows the time period in which
this specific storm would be most probable to occur. Considering the storm with a
frequency of 0.5, its return period would be 2 years.
• Intensity-Duration-Frequency Relationships - these represent a set of intensity -
duration curves, for different return periods, which are determined based on the
statistical analysis of rainfall data for a given geographic region. The length of record,
gage accuracy, placement, and density, significantly affect the reliability of rainfall
data. Only the average intensity for a given duration may be determined from the
rainfall intensity duration curve. This set of curves are used to determine the design
value of i av , which will be used in the rational formula, based on the determined time
of concentration for the point under consideration. The method assumes that the peak
flow would be formed when the storm duration is equal to the time of concentration.
In practice, there are some cases when this assumption is not valid. Because of this,
the hydrograph method, described in the following section is considered to be more
accurate, but more data would be necessary for its application. One point, which
would require specific attention during the process of determination of the value of i av ,
is the choice of the correct “design storm”, which would be used for the procedure.
This choice would consider the possible effect and risks associated with the return
period of this specific storm. In the case of flood protection structures, the “design
storm” would have return periods of about 5, 10, 20 or 100 years. If the return period
is 10 years, it means that there is a probability that a peak flow, higher than the design
one, would occur once in 10 years. If the designer chooses a higher return period, then
the inevitable risks associated with this choice are lower. However, the cost of the
protection structure would rise considerably. Thus, the choice of the design storm with
a corresponding return period is a compromise between risks and cost of the structure,
which should be made after careful consideration of the possible undesirable
consequences. The procedure is known as “risk assessment”. In the case of diffuse
pollution abatement measures (Chapter 2), and specifically, the provision of retention
structures, two design storms must be considered-one with respect to the flood
protection, and another with respect to pollution abatement. The last would have a
much lower return period.
The Runoff Coefficient (C) is the variable least susceptible to precise determination. The
range of coefficients, classified with respect to the general character of the tributary area
reported in use, is very wide and could be found in numerous literature sources (White
1978, Novotny 2003). Actually, this coefficient gives a less accurate evaluation of the
Search WWH ::




Custom Search