CHAPTER 5 FIELD OBSERVATIONS
5. Field observations
5.4. Discussion
Of the various physical evidences of flood elevations, the highest level of flood deposits can be considered the most suitable for determining the highest historical flood elevations. While evidence such as debris lines and vegetation scouring may only reflect the height of the most recent flood, flood deposits can be preserved for a long time. Fieldwork has shown that the well preserved flood deposits can be used to determine the highest flood elevations. In urbanised areas the highest elevation flood deposits are generally not preserved but may still be extant in open space areas and parks. In the five representative areas only the coastal strip of T40G is urbanised. As expected the flood deposit elevations vary across the quaternary catchments reflecting different levels of water accumulation from the lowest stream order tributaries to the highest stream order main river. In general flood deposits are lowest along sub-tributaries and tributaries and reach progressively higher elevations towards the quaternary catchment discharge point.
Figure 5.31. Flood clays (A) overlying the associated gravels (B). This couplet overlies older alluvial deposits (C).
Although river terraces were not used in this study to determine highest flood elevations, it was generally observed that the highest flood level coincided with the top of the third terrace in cross sectional profile (e.g. Fig. 5.15). Terrace one appeared to be a function of increased seasonal discharge to bank-full conditions and terrace two was the probable result of small to medium scale flooding. Terraces from four upward are generally poorly preserved and most likely represent palaeo- floodplains abandoned during river incision to the modern level (e.g. Charlton 2007).
As quaternary catchment T40G is located on the coastline it does not have a single discharge point like the other four representative quaternary catchments. Instead the accumulated runoff is distributed amongst nine main drainage points. Drainage lines are relatively short, longest being 32.6 km (Vungu River). T40G (Fig 5.11) the highest flood level was measured at point 17 (6.4 m) and upstream at point 10 (4.7 m) on the Umhlanga River. A similar relationship can be seen on the Zotsha River where point 1, near the river mouth, has the highest flood deposit level of 5.8 m and inland at point 12, 4.9 m. The lower flood deposit levels are along tributaries, e.g. point 9 (3.3 m) compared to point 4 (4.3 m) on the Vungu River. There are no historical flood discharge records for this catchment but the highest recorded discharge for the Mzimkhulu River along the northern border of the T40G quaternary catchment was 7050 m3/s in May 1959 (van Bladeren 1992). The flood levels recorded are most likely related to the 1959 flood event.
Quaternary catchment U20H (Fig. 5.14) has no upstream inflowing river. It is the headwater for the Umsunduze River which has a 39.8 km draining length through the U20H quaternary catchment. The highest flood deposit levels along the tributaries of the Umsunduze River range from approximately 10.2 m to 12.6 m (Points 20, 21, 51, 55). There is a second set of distinctive flood levels ranging from 5.4 m to 7.1 m (Points 39, 40, 47, 52) that have not been completely eroded by the 10.2 to 12.6 m event. The highest recorded flood level at the discharge point is a gauge height of 3.85 m and a discharge of 492 m3/s during the 1987 floods (van Bladeren & Burger 1989). The 1987 flood level is too low to account for the flood levels recorded in the field. This quaternary catchment has been subjected to a number of flash floods in the past that may account for the recorded flood deposit levels.
The 68 km length of the Mzimkhulu River drains through the T52D quaternary catchment (Fig 5.18).
The highest flood deposit levels along the Mzimkkulu River are approximately 14.2 m (Point 68) and 15.7 m (Point 71). Along the tributaries of the Mzimkhulu River the highest flood deposit levels range
from 2.8 m (Point 62) to 4.1 m (Point 73). The highest recorded discharges were recorded in 1959 at a value of 3795 m3/s (estimated level of 12.16 m) at the gauge station corresponding to the entrance of the quaternary catchment and 7050 m3/s below the T52D quaternary catchment (van Bladeren 1992).
Quaternary catchment V12G is primary drained by the 48.7 km Klip River with the secondary 41.3 km drainage line of the Ndakane River. The highest flood deposits recorded along the Klip River are approximately 11.1 m at point 89 and 10.6 m at point 93. Along the Ndakane River the highest flood level found is 3.3 m (Point 83). The highest flood levels along the tributaries ranged from approximately 0.9 m (Point 76) to 2.3 m (point 84). This quaternary catchment has experienced repeated flooding particularly along the Klip River affecting the town of Ladysmith. Up to 1998 the Klip River has flooded 29 times in 110 years (Bell & Mason 1998) with the most recent in 2012.
According to data presented by Bell & Mason (1998) peak estimated discharges for 1886, 1918 and 1923 were between 1600 m3/s and 1750 m3/s. The highest flood levels identified along the Klip River are probably related to one of these events.
The Mfolozi River (39.5 km) forms the main drainage line through quaternary catchment W23A (Fig.
5.25). The highest flood deposits along the Mfolozi River was found at point 102 (21.5 m) near the quaternary catchment discharge point and further upstream at points 107 (19.9 m) and point 108 (18.2 m). Along the tributaries the highest flood deposit levels range from 2.1 m (Points 103 and 106) to 2.4 m (Point 104). According to Kovács et al. (1985) and van Bladeren (1992) peak estimated discharge at the quaternary catchment discharge point was 16 000 m3/s with a gauge level of 12.8 m during Tropical Storm Domoina in 1984. However according to van Heerden & Swart (1986) a storm event in 1925, thought to be a tropical cyclone, produced flooding in excess of that of Tropical Storm Domoina in 1984. The gauge level of 12.8 m is based on an estimated flood level as the river gauges had been damaged or destroyed during Tropical Storm Domoina in 1984 (Kovács et al. 1985). It is apparent that the gauge station was destroyed before the flood level reached its peak elevation of 21.5 m as evidenced by the edge of the flood deposits at point 102.