Regarding the use of Monthly Maximum Rainfall Intensity
data, you need to think about the application for your IDF curves and then
decide whether to due Partial Duration Series (looking at all values, or Monthly
Maximums. When you take only the largest rainfall in a month, you
eliminate other high values that may be also be detrimental to your objective
function. For example, if you are looking at scour in streambeds, and you
have two large storms in a month, both will produce significant scour, but
if you use only Monthly Maximum data, you neglect the second storm; hence you
show lower frequencies for each rainfall intensity than if you do a Partial
Duration Series analysis, and you underestimate the frequency with which your
objective function is subjected to a given rainfall
intensity.
I think that I am not saying this well (this is my
first day back after the holidays), but if your objective function is
susceptible to SEQUENTIAL DAMAGE or degradation from multiple storms of a given
intensity, then you should use partial duration series. Examples are
streambed scour (noted above), frequency of combined sewer overflows,
frequency of overflow of detention basins, etc. However, if one
storm of the above a certain intensity creates FAILURE of your objective
function, (loss of the dam for a storage facility, washout of a road, flooding
that destroys infrastructure, etc.) then it is OK to use Monthly
Maximums.
Hope
this is helpful, Happy New Year...LAR
-----Original Message-----
From:
A list for discussion and information exchange concerning the research, pla
[mailto:[log in to unmask]]On Behalf Of Ir Chong Sun
Fatt
Sent: Friday, December 28, 2001 2:59 AM
To:
[log in to unmask]
Subject: Derivation of IDF for Frequent
Storms using Monthly Maxima Data
Dear Members,
I am currently trying to develop
Intensity-Duration-Frequency (IDF) curves for frequent storms ( return periods
of 2, 3, 6, 12, 18 & 24 months ) for various urban centers in Malaysia, a
country which experiences humid tropical climate. I intend to use conventional
Frequency Analyses ( Gumbel, Pearson, GEV, GLO, etc ) on " Monthly
Maximum Rainfall Intensity Data " of various durations , to derive
the said IDF values . This method is analogous to the derivation of IDF values
using Annual Maxima Series for return periods >= 2 years.
I wish to solicit advisories and
comments from fellow members as to the appropriateness and suitability on
the proposed methodology. Suggestions on more suitable methods are also
welcome.
Thank you.
Mr Chong Sun-Fatt,
Hydrology Engineer,
Department Of Irrigation And
Drainage,
Malaysia