Abstract
Design rainfall estimates are primarily used in single-event deterministic design flood estimation methods where estimates of the peak discharge are based on the critical storm duration or time of concentration (TC) of a catchment. Therefore, daily design rainfall depths used in flood estimations must either be decreased or increased from durations less than or longer than 24 hours to the design rainfall depths for a rainfall duration of TC. This paper presents the comparison of two South African methods used to convert or scale 1-day fixed time interval observed rainfall (08:00 to 08:00) to continuous measures of n-hour rainfall for selected TC durations at a quaternary catchment level, in the C5 secondary drainage region in South Africa as pilot case study. In each quaternary catchment, the annual maximum series (AMS) of the 1-day fixed time interval point rainfall were extracted, infilled, converted and scaled to appropriate continuous measures of TC-hour point rainfall using conversion factors (Adamson, 1981) and scaling factors (Smithers and Schulze, 2003), respectively. Thereafter, all the TC-hour observed point rainfall values were averaged to observed catchment rainfall at a quaternary catchment level using the Thiessen polygon method. In using the two methods to estimate continuous short-duration n-hour (TC ≤ 24 hours) and long-duration n-hour (TC > 24 hours) catchment rainfall from 1-day fixed time interval point rainfall, an acceptable (0.71 < r2 ≤ 0.86) and high (r2 ≥ 0.93) degree of association were achieved, respectively, despite the different approaches used in each method. Overall, the results confirmed that fixed time interval rainfall should be scaled to continuous measures of rainfall using the Smithers-Schulze scale invariance approach for various TC durations in the case study area. In comparison to the Adamson conversion methodology, the Smithers-Schulze scaling methodology is also based on a more extensive and recent rainfall database as incorporated in software for design rainfall estimation in modern flood hydrology practice in South Africa.
Highlights
Observed rainfall data in South Africa can be obtained from continuously recording rainfall stations or from daily rainfall stations where rainfall depths are recorded at a fixed daily interval (08:00 to 08:00)
Design rainfall comprises of a depth of rain for a selected duration which is associated with a given annual exceedance probability (AEP) or return period (T) (Gericke and Du Plessis, 2011), and is primarily used in singleevent deterministic design flood estimation methods where estimates of the peak discharge are based on the critical storm duration or time of concentration (TC) of a catchment, while the catchment is assumed to be at an ‘average condition’ and the hazard or risk associated with a specific event is reflected by the probability of the 1:T-year rainfall event resulting in a 1:T-year flood event (SANRAL, 2013)
The studies focusing on long durations and daily point rainfall data include studies conducted by the South African Weather Bureau (SAWB; 1956), Schulze (1980), Adamson (1981), Pegram and Adamson (1988), and Smithers and Schulze (2000b)
Summary
Observed rainfall data in South Africa can be obtained from continuously recording rainfall stations or from daily rainfall stations where rainfall depths are recorded at a fixed daily interval (08:00 to 08:00). In South Africa, daily rainfall data are more abundant, reliable and generally have longer record lengths than the digitised sub-daily rainfall data, e.g., nearly 4 000 daily rainfall stations have record lengths of 20 years and longer, while more than 1 800 daily rainfall stations have more than 40 years of record (Smithers and Schulze, 2000b; 2004). Due to the availability and quality of daily rainfall data, these datasets could be used to estimate design rainfall, especially in those cases where practitioners do not use the methodology and design rainfall estimation software as developed by Smithers and Schulze (2003; 2004). Available, design rainfall depths for a duration equal to TC could be estimated using the approaches developed by Adamson (1981) and Smithers and Schulze (2003), respectively
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