Appendix B: Port Elizabeth Storm: 1 September 1968

Memo PE rainfall 22 May 2006 Heavy rain and some flooding in Port Elizabeth on TV tonight. 100mm was reported at the airport. This is only a quarter of the 400 mm that fell on 1 September 1968. Imagine the consequences if 400 mm fell today. This appendix is from pp 298-301 of my handbook Flood Risk Reduction Measures (Alexander 2000). Again notice that the highest recorded rainfall was nearly 40 years ago with no sign of a climate change related increase since then. Regards Will [I do not have the digitised copies of the two figures. Sorry.]

Appendix B: Port Elizabeth storm: 1 September 1968

Details of this storm are provided in the Weather Bureau newsletter No 234 of January 1968 (Hayward and v. d. Berg 1968), and the statistical analyses of the recording rain gauge record at the airport in the Weather Bureau publication WB 36. Intense rain averaging a sustained 20 to 30 mm per 15 minutes fell over the city of Port Elizabeth during a four-hour period from about 08:00 on Sunday 1st September 1968. Severe damage was caused and nine lives were lost, eight of them by drowning. The intense rainfall was associated with the passage of upper air cut-off low and high vertical air velocities aggravated by the topography of the city as well as other factors. A total of 429 mm of rain was recorded on the recording rain gauge at the airport, of which 417 mm fell between 08:00 and 12:00, with a maximum of 552 mm further to the north.

The 15, 30, 45 and 60-minute rainfalls associated with this storm were the highest on record at this station for the period of record from 1938 to 1972. (See table 22.2). The expected maxima for a 100 year return period derived by various methods are shown in Table 3B.1:

Table 3B.1 Extreme values of rainfall in millimetres for Port Elizabeth: (a) from WB 36 (based on data for each month), and (b) from WB 36 (based on annual maxima).

Highest on record Method Expected maxima for 100-year return period 15 min 30 min 60 min 15 min 30 min 60 min 1 day 35 61 112 (a) 16 22 31 103 (b) 33 47 69 179

PE Rainfall.doc 22 May 2006 2

The table of extreme values for rainfall in WB 36 has the following interesting footnote: For Port Elizabeth the expected maximum rainfalls based on the distribution of the annual maximum values are given as well", (line (b) in Table 3B.1). "The monthly expected values are too small compared with the monthly recorded because an exceptional single high maximum rainfall occurred in almost every month with a return period of hundreds to thousands of years. The second maximum rainfall was approximately one third of the first maximum. However, Hayward and van den Berg (1968) in their discussion on the Port Elizabeth storm comment that heavy falls over the coastal and adjacent inland areas of southern and eastern Cape and Natal are not at all unusual and can be expected to occur once or twice a year somewhere along this coast, although not of the same severity as the Port Elizabeth storm. They quote rainfalls of 360 mm in the Outeniqua Mountains in 1932, 312 mm in Durban in 1953, 265 mm in East London in 1953, 459 mm near Port Shepstone in 1959. Other heavy one-day falls were those of 590 mm at Eshowe in 1940, 475 mm at Kaapsche Hoop and 460 mm at Mariepskop both in 1939, with the Eshowe figure being the largest up to that time. Further falls exceeding 250 mm were experienced in Port Elizabeth in 1981. This storm produced 372 mm of rain at Loerie Dam, which was overtopped but did not breach (duPlessis, 1984). The East London maximum was exceeded in 1970 when 447 mm was recorded. The South African maximum at Eshowe has also since been exceeded.

Comment There are several points of interest. The time distribution in Fig. 3B.1 shows a fairly even spread of rainfall over the 4- hour period. It is obvious that for any catchments with response times less than about three hours, the rain falling towards the end of the storm would have been most critical as the catchments would have been saturated, streams would have been in flood, and the drainage systems would have been full while the storm continued to produce rain at the sustained intensity. The precipitation pattern and possibly the intensity as well, were influenced by the local topography (Fig.22B.2).