Statistical Based Regional Flood Frequency Estimation Study for South Africa Using Systematic, Historical and Palaeoflood Data Pilot Study – Catchment Management Area 15
by
D van Bladeren, P K Zawada and D Mahlangu SRK Consulting & Council for Geoscience
Report to the Water Research Commission on the project “Statistical Based Regional Flood Frequency Estimation Study for South Africa using Systematic, Historical and Palaeoflood Data”
WRC Report No 1260/1/07 ISBN 078-1-77005-537-7
March 2007
DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use
EXECUTIVE SUMMARY
INTRODUCTION
During the past 10 years South Africa has experienced several devastating flood events that highlighted the need for more accurate and reasonable flood estimation. The most notable events were those of 1995/96 in KwaZulu-Natal and north eastern areas, the November 1996 floods in the Southern Cape Region, the floods of February to March 2000 in the Limpopo, Mpumalanga and Eastern Cape provinces and the recent floods in March 2003 in Montagu in the Western Cape. These events emphasized the need for a standard approach to estimate flood probabilities before developments are initiated or existing developments evaluated for flood hazards. The flood peak magnitudes and probabilities of occurrence or return period required for flood lines are often overlooked, ignored or dealt with in a casual way with devastating effects. The National Disaster and new Water Act and the rapid rate at which developments are being planned will require the near mass production of flood peak probabilities across the country that should be consistent, realistic and reliable.
At present the methodologies for flood frequency analysis in South Africa consists of three basic approaches, all of which have certain validity limits (Kovacs, 1993): Deterministic methods (Rational, Synthetic unit hydrograph, Direct runoff hydrograph, SCS, etc.) Statistical methods such as the LP3, GEV and Log-normal (annual maximum flood series data) Empirical methods (Midgley-Pitman, HRU 1/71, CAPA and RMF)
Experience in the Department of Water Affairs and Forestry (DWAF) has shown that these methods often give vastly different results and unless a certain amount of judgement and experience can be used, the selection of final values may be inconsistent and subjective. This pilot study provides a basis to develop simple and consistent methodologies for the rest of the country to estimate flood peaks and their associated probabilities for use by authorities, consultants and planners.
Since the last extensive review of the methodologies in 1970’s to early 1980’s the following should be noted that justify a review of the methodologies: The period of observation has been extended by a further 25 to 30 years i.e. more data, The number of sites are significantly more, South Africa has had several extreme flood events that added to the extreme flood peak data base, The technology regarding the statistical analysis of flood data has improved, The gathering of historical data by DWAF (van Bladeren, 1992) has in many areas increased the period of observation to between 100 to 150 years, and
i