The objective of the study was to update and improve the reliability and accuracy of short
duration (s 24 h) design rainfall values for South Africa. These were to be based on
digitised rainfall data whereas previous studies conducted on a national scale in South Africa
were based on data that were manually extracted from autographic charts. With the longer
rainfall records currently available compared to the studies conducted in the early 1980s,
it was expected that by utilising the longer, digitised rainfall data in conjunction with
regional approaches, which have not previously been applied in South Africa, that more
reliable short duration design rainfall values could Ix: estimated.
A short duration rainfall database was established for South Africa with the majority of the
data contributed by the South African Weather Bureau (SAWB). Numerous errors such as
negative and zero time steps were identified in the SAWB digitised rainfall data. Automated
procedures were developed to identify the probable cause of the errors and appropriate
adjustments to the data were made. In cases where the cause of the error could be
established, the data were adjusted to introduce randomly either the minimum, average or
maximum intensity into the data as a result of the adjustment. The effect of the adjustments
was found to have no significant effect on the extracted Annual Maximum Series (AMS).
However, the effect of excluding erroneous points or events with erroneous points resulted
in significantly different AMS. The low reliability of much of the digitised SAW B rainfall
data was evident by numerous and large differences between daily rainfall totals recorded
by standard, non-recording raingauges, measured at 08:00 every day, and the total rainfall
depth for the equivalent period extracted from the digitised data. Hence alternative
techniques of estimating short duration rainfall values were developed, with the focus on
regional approaches and techniques that could be derived from daily rainfall totals measured
by standard raingauges.
Three approaches to estimating design storms from the unreliable short duration rainfall
database were developed and evaluated. The first approach used a regional frequency
analysis, the second investigated scaling relationships of the moments of the extreme events
and the third approach used a stochastic intra-daily model to generate synthetic rainfall
series.
In the regional frequency analyses, 15 relatively homogeneous rainfall clusters were
identified in South Africa and a regional index storm based approach using L-moments was
applied. Homogeneous clusters were identified using site characteristics and tested using
at-site data. The mean of the AMS was used as the index value and in 13 of the 15 relatively
homogeneous clusters the index value for 24 h durations were well estimated as a function
of site characteristics only, thus enabling the estimation of 24 h duration design rainfall
values at any location in South Africa.
In 13 of the 15 clusters the scaling properties of the moments of the AMS were used to
successfully estimate design rainfall values for duration < 24h, using the moments of the
AMS extracted from the data recorded by standard raingauges and regional relationships
based on site characteristics. It was found that L-moments scaled better and over a wider
range of durations than ordinary product moments.
A methodology was developed for the derivation of the parameters for two Bartlett-Lewis
rectangular pulse models using only standard raingauge data, thus enabling the estimation
of design values for durations as short as 1 h at sites where only daily rainfall data are
available.
In view of the low reliability of the majority of short duration rainfall data in South Africa,
it is recommended that the regional index value approach be adopted for South Africa, but
scaled using values derived from the daily rainfall data. The use of the intra-daily stochastic
rainfall models to estimate design rainfall values is recommended as further independent
confirmation of the reliability of the design values. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/7756 |
| Date | January 1998 |
| Creators | Smithers, Jeffrey Colin. |
| Contributors | Schulze, Roland E., Pegram, Geoffrey. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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