Assessing the performance of techniques for disaggregating daily rainfall for design flood estimation in South Africa.
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Date
2020
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Abstract
Design Flood Estimation (DFE) and other hydrological modelling methods are used to limit
the risk of failure and ensure the safe design of infrastructure and for the planning and
management of water resources. The temporal distribution of rainfall has a significant impact
on the magnitude and timing of flood peak discharges. Rainfall temporal distributions are
therefore an important component of DFE approaches. In order to improve DFE methods
which are based on event or continuous simulation rainfall-runoff models, it is generally
necessary to use sub-daily time step rainfall hyetographs as input. However, the number of
recording raingauges which provide sub-daily timesteps in South Africa is relatively scarce
compared to those which provide daily data. Rainfall Temporal Disaggregation (RTD)
techniques can be used to produce finer resolution data from coarser resolution data. Several
RTD approaches have been applied in South Africa. However, application of RTD
approaches locally is relatively limited, both in terms of diversity of approaches and cases
of application, compared to those developed and applied internationally. Therefore, a need
exists to further assess the performance of locally applied approaches as well update the list
of available approaches through inclusion of internationally developed and applied RTD
techniques. A pilot study was performed in which selected locally applied and internationally
applied approaches were applied to disaggregated daily rainfall data. Some approaches were
applied in their original form while others were modified. Temporal distributions of rainfall
were represented by dimensionless Huff curves, which served as the basis for comparison of
observed and disaggregated rainfall. It was found that for daily rainfall, the SCS3, SCS4
and Knoesen model approaches performed considerably better than the other approaches in
the pilot study. The RTD approaches were further assessed using data from 14 additional
rainfall stations. For the additional stations, the Knoesen model disaggregated depths
provided the most realistic temporal distributions overall, followed by the SCS-SA approach.
In additional, an adapted form of the Triangular distribution was found to show potential for
disaggregation when a generalised value for the timing of the peak was utilised.
Description
Masters Degree. University of KwaZulu-Natal, Pietermaritzburg.