the problem of flooding in ladysmith, natal, south africa ... · the problem of flooding in...

The problem of flooding in Ladysmith, Natal, South Africa

F. G. Bell & T. R. Mason

Department of Geology and Applied Geology, University of Natal, Durban 4041, South Africa

Abstract. Ladysmith was founded in the mid-19th century alongside one of the meanders of the Klip River. The location was chosen for protection against native tribesmen, but in the last 110 years, Ladysmith has experienced 29 notable floods. The most recent floods occurred in February 1994, when the highest water levels in the last 71 years were recorded. An attempt to control flooding was made with the construction of the Windsor Dam in 1949. Unfortunately, the reservoir has been largely silted up so that its live storage capacity has been reduced to around 5%. The present flood walls in Ladysmith can be overtopped by a flood with a recurrence period of once every five years. This happens when channel flow exceeds 700m 3 S - 1 .

A number of flood alleviation schemes have been discussed over the years including the construction of artificial levees and river canalization. The scheme in vogue at present would involve the construction of another dam and reservoir. Although the reservoir would offer protection to the town, to do so it would have to remain at 10% of its storage capacity in order to retain a 100 year flood. An alternative solution, which would be less expensive, would be to relocate those properties affected most by the recent floods.

Introduction Ladysmith is situated in the Natal Midlands. It is located on the flood-plain of the Klip River, approximately 7 km downstream of the confluence between the Klip and its tributary, the Sand River. The town was founded in 1851 by the Imperial British Government to establish greater control over that area of recently annexed Natal. The problems of flooding are directly related to its location, the original settlements being situated in a bend of the Klip River which afforded the residents protection from marauding natives. The meander that was chosen encloses that part of the town which experiences the worst flood conditions. Since then the town has grown along the meander belt of the Klip River and so is subject to periodic flooding. In addition, the meanders tend to migrate laterally and downstream, so undermining the river banks. The area worst affected by the river floods covers approximately 8 km 2.

The geology in the Ladysmith area consists of shales, mudstones and fine-grained sandstones assigned to the Ecca and Beaufort Groups of the Karoo Supergroup (late Carboniferous to early Permian). These were intruded by doleritic sills and dykes of Jurassic age. Most of the area drained by the Klip and Sand Rivers is an extensive plain of low relief, interrupted by dolerite- capped hills. Due to the generally low gradient of the river the velocity of the flood water is low relative to its volume. This means that there is usually enough time to

issue evacuation warnings. The response time of a flood in the area is about 20 h, i.e. the time taken for rain to travel from where it falls to the gauging station (V1HO38-AO1) at Ladysmith. In fact there is a flood warning system in use. This involves relaying reports of heavy rainfall in the catchment area of the Klip River to Ladysmith in order to give the population a chance to move to safer ground when necessary. The system allows for the prediction of the size of the flood wave and its time of arrival.

The Klip River has its source in the Drakensberg Mountains some 40 km to the west of Ladysmith. The high rugged source area and the relatively flat area around Ladysmith play a part in the problem of flooding in that river water which has been constrained in relatively narrow, steep-sided valleys in the upper course suddenly issues onto a wide flood-plain over which it meanders. The meanders are highly pronounced to the south west of the town and over the flood-plain where their sinuosity is 1.9, increasing to over 2 further downstream. The criteria for meandering is taken to be a sinuosity of 1.5. Furthermore, river water is again somewhat constrained to the southeast of Ladysmith, restricting flow from the area. The catchment areas of the Sand River and the Klip River fall within the larger catchment area of the Tugela River (Fig. 1). The Klip River joins the Tugela River 20 km to the southeast of the town. The catchment of the Klip River and the Sand River area upstream of Ladysmith is approximately

BELL, F. G. & MASON, T. R. 1998. The problem of flooding in Ladysmith, Natal, South Africa. In: MAUND, J. G. & EDDLESTON, M. (eds) Geohazards in Engineering Geology. Geological Society, London, Engineering Geology Special Publications, 15, 3-10.

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Fig. 1. Catchment of the Klip River, showing proposed Mount Pleasant Reservoir. Broken stream courses indicate non-perennial streams�9

1650 km 2. Rainfall varies between 800 and 1000 mm per year over this area and falls in the spring-summer months between October and March. The mean annual temperature is 17-18~

Flood hydrology

Figure 2 summarizes the flood hydrology of the Klip River at Ladysmith. The Klip River has a bankfull discharge of approximately 700m 3 s -1 flooding occurring once this is exceeded. This figure corresponds to a flood with a 5 to 6 year return period. As would be expected from the seasonal rainfall, the period of flood risk occurs from October to March. However, the duration of floods is relatively short (1-2 days) due to the relatively small catchment area and to the fact that rainfall occurs as thunderstorms of limited duration.

In fact, Ladysmith has experienced 29 major floods in the past 110 years (Fig. 2). In other words, on average a flood occurred once every 3.9 years.

It is estimated that a flood with a 10 year return period would affect 211 families and 296 properties, the respective figures for a 20 year flood being 573 and 564. Figure 3 shows the relationship between the cost of the damage and the flood return period. It indicates that a flood with only a 10 year return period could inflict R2000000 worth of damage on the town. The most severely affected areas occur in the southern part of the town. The last flood occurred on 17 February 1994 (Fig. 4) and was the largest for the previous 71 years. It caused approximately R67 million in damage. The clean-up costs borne by the municipality were approximately R500 000 00. If we consider that the real costs of a flood have not changed since Ladysmith was founded, the lowest total cost of floods to the Ladysmith economy



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Fig. 2. Major floods experienced at Ladysmith between 1884 and 1994 inclusive. Data supplied courtesy of the Department of Water Affairs and Forestry.

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8 F. G. BELL & T. R. MASON

has been R28 million; the highest total cost, conserva- tively estimated, could be as much as R700 million.

Flood control measures

The most recent flooding (February 1994) of low lying areas of Ladysmith has highlighted the need for a flood control plan for the town. Several proposed schemes have been advanced over the years to mitigate the flood problem. All of the proposals are expensive and money from the central government will be needed to carry any of them out. As a rough guide, the cost of relocation is about ten times the cost of the cheapest amelioration scheme, which does not include the water storage option. Relocation, however, is likely to prove a socially less acceptable solution.

An attempt to control flooding was made in 1949 with the construction of the Windsor Reservoir. It is retained by an earth embankment dam and is located about 7.8 km N N W of Ladysmith. The capacity of the reservoir after its construction was 4.68 x 106m 3 with a surface area at top water level of l l0ha . The initial estimated annual rate of sedimentation in the reservoir was about 83 000 m 3. In fact this rate increased with time so that by 1989 the reservoir capacity had been reduced to 600 000 m 3. In other words, in 40 years it had been reduced to 12.8% of its original capacity and in 1995 the figure was about 5% of the original capacity (i.e. approximately 235 000 m3).

The proposed Mount Pleasant Reservoir is the most popular of the flood control schemes and if constructed would be designed to accommodate the 100 year flood and so will reduce the risk of flooding in any year from 20% to 1%. The gross capacity of the reservoir would be 205 x 106 m 3 with a net flood retention capacity of 183 x 106 m 3. The minimum height of the dam above the river bed would be 30 m, which would mean that the area covered by overflow flood would be 2600 ha. The dam has been designed to withstand the force of the maximum possible flood for the area, i.e. one which has an inflow to the reservoir of 8000 m 3 s -l . The estimated volume of sedimentation over 50 years is 22 x 106 m 3.

The hydrographs in Fig. 5 indicate that for up to a 100 year flood the maximum outflow from the reservoir would be 330m 3 s -1 at its maximum storage capacity. This outflow is well below the bankfull discharge of the Klip River. Furthermore, Fig. 5 shows that for a 200 year flood, outflow would be 720m 3 s -1 at peak water levels in the reservoir basin, which is near the flood peak for a 10 year flood (Tables 1 and 2). Levees designed to retain 10 year flood are included in the cost of the scheme. If they functioned effectively this outflow would not cause damage in the town. In fact Ladysmith has not suffered a 200 year flood (flood peak of 2090m 3 s-l), and has experienced only one 100 year flood (flood peak of 1790m 3 s-l), and that was in 1886.

Table 1. Estimated flood peaks and volumes at Ladysmith

Return period Flood peak Flood volume (years) (m 3 s -1 ) ( x 106 m 3)

10 725 107 20 945 139 50 1500 229

100 1790 272 200 2090 314 RMF a 4000 655 PMF b 8000 790

a RMF = regional maximum floods. b PMF = probable maximum floods. Data supplied by the Department of Water Affairs and Forestry.

Table 2. Risk of flooding

Flood return period (years)

Risk of flooding (exceeded at least once)

In any one year Over a period of 20 years

5 20% 99% 10 10% 88% 20 5% 64% 50 2% 33%

100 1% 18%

Data supplied by the Department of Water Affairs and Forestry.

Figure 6 shows the proposed site for the reservoir and the height of the water at 20 year and 100 year flood levels. It can be seen that a road and a railway line would have to be relocated to accommodate the reservoir and some flooding of access routes would occur during a 100 year flood.

Although the reservoir would protect the town from floods, a number of other factors need to be considered. First, the reservoir would have to remain at 10% capacity to maintain the net flood retention capacity. This means that the reservoir would not be of much value in terms of water supply for the surrounding area. This is a serious consideration since Ladysmith only has adequate water provision until 2010. Moreover, the reservoir would generate very little revenue from recreational activities. The cost of the reservoir in 1994 was estimated at between 200 and 250 million rands. If the reservoir is designed and constructed to enhance the water storage capability of the scheme, to allow for 50% water storage flood attenuation, then the cost escalates by R100 million.

An alternative scheme involves strengthening and upgrading of the existing artificial levees. The estimated cost of construction of artificial levees to withstand a 10 year flood is R10 million, that for the 20 ,.gear flood being R40 million However, the construction of levees can pose problems; for example, if levees are overtopped water cannot readily escape back into the river channel




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Fig. 6. Proposed location of the proposed Mount Pleasant showing 5, 20 and 50 year flood boundaries.

after the flood. Built-in fuse-plugs are required. Rapid failure of a levee during a flood means that rapidly flowing water discharging from the river can cause damage greater than if no levees were present. Disposal of storm water on the town side of the levees during heavy rainfall would be very difficult.

Canalization increases the bankfull discharge by increasing the velocity of flow and straightening the path of a river. However, there could be the possibility of added erosion and flooding occurring downstream of the canal due to water flowing from it at a higher velocity than the flow in a natural channel. In addition, a canal would have to be continuously maintained and cleaned of vegetation that would grow during periods of low water. In 1994 the cost of canalisation (plus levees) was estimated as around R70 million for protection against a 20 year flood. To accommodate a 50 year flood the cost would rise to over R85 million.

It has been suggested that suitable areas for the relocation of those properties which would be affected by a 20 year flood are available. This would also involve curtailing any further development in designated high- risk areas. The present cost of such relocation is approximately R197 million, which includes purchase of the land and the establishment of the necessary infrastructure as

well as the reciting of properties. Figure 7 indicates that the area between the 20 and 50 year flood lines is approximately 10% of the area below the 20 year flood line. Extending the relocation project to the 50 year flood line would cost an estimated extra R20 million, but could be carried out at a later stage. Relocation does not interfere with the natural river system. It could provide flood protection indefinitely for floods with a magnitude of the 50 year flood. The scheme would not require the maintenance cost that the reservoir would in order to keep if functioning. The land vacated could be used as low maintenance cost community facilities such as public parks, sports fields, or an environmental reserve. Such land use is cheaply restored after flooding.

The land vacated would have to be cleared and the rubble waste disposed of in landfills. Because of the rapid rates of urbanization it would be highly likely that informal settlements would spring up on the vacated land. Their removal would present an acute political problem even though the squatters would be in serious danger if the Klip River flooded. Similar squatter settlements in Durban were badly damaged by relatively minor flash floods caused by localized thunderstorms in April 1995. Small dry valleys were instantly transformed into raging torrents. Moreover it would be very difficult



10 F. G. BELL & T. R. MASON

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to undertake an orderly and prompt evacuation of the area where little formal organisation existed. A potential solution requires a major act of political will allied to enlightened legislation to prevent uncontrolled squatting on recreational land.

Public opinion has not been reliably tested in the past and people affected by flooding presumably would be reluctant to move, preferring to have a flood protection reservoir constructed. In addition, the flat land of a flood-plain is easier to develop and a ready source of water is available to industry from the river. On the other hand, if the public do support the relocation scheme they will need to be involved in the development of the new areas. New development can take place only on the outskirts of Ladysmith.

Conclusion

Ladysmith in the Natal Midlands was located within a meander loop of the Klip River for defensive purposes.

Unfortunately, however, this has meant that the town has been subjected to repeated flooding since its establishment in the mid-19th century. After emerging from the Drakensburg Mountains the Klip River flows over a flood-plain which experiences an average of 800-1000mm of rainfall annually. This falls primarily between October and March, and characteristically as thunderstorms.

The southern part of the town is most seriously affected by flooding, which has occurred more or less once every 4 years. The costs associated with flooding dictate that an adequate system of flood control must be implemented. Several proposed schemes have been advanced, the most popular but most expensive being a flood control reservoir. This scheme would protect the town from a 100 year flood, indeed with the associated levees from a 200 year flood. The other schemes which include the enhancement of the levee system, canalization and relocation of properties are less expensive but would accommodate a 50 year flood. Relocation, however, does not interfere with the natural river system and should involve little future maintenance cost.



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