WaterSubject

Owing to development pressure for land bordering the Vaal river, in South Africa. Rand Water are revising their development policy. Restrictions were in place to avoid flooding and obstructing the flood flow of the river. Relaxation of the regulations will permit controlled development along the river. A flood hazard-risk index was developed to indicate where development could be permitted. An economic comparison of costs and benefits supports the relaxation.The Vaal Barrage is the original water source for Johannesburg and surrounding areas. The banks of the Vaal river upstream of the Barrage are becoming sought after for urban development. The managers of the Barrage, Rand Water, have evolved a policy for controlling urban development along the banks.

The prescriptive method of limiting development on the banks of the Vaal River Barrage has been applied for many years and has successfully protected the Vaal River against pollution and flooding. Rand Water abstracts potable water from the Barrage for supplying some 10 million consumers within its supply area.

The importance of water quality and at the same time responsibility to the property owners along the banks of the Vaal River and Barrage with respect to flooding are paramount.

The Vaal River and the Wilge River which flow into Vaal dam upstream of the Vaal River Barrage are the main contributors of water and the main sources of floods. There are however other tributaries downstream of the Vaal Dam, namely the Taaibosspruit, the Suikerbosrand, the Klip and the Riet rivers. Some of these rivers take return flow from Gauteng and the Vaal Triangle area so that the flow through the Barrage and downstream is considerably different to the flow entering the Vaal Dam.

Rand Water owns the Vaal Barrage and is responsible for its operation. The Barrage backs up water along the Vaal River past Vanderbijlpark, Vereeniging and Sasolburg. Water levels along this reach of river are controlled by the Barrage gates as well as inflow from the tributaries and Vaal Dam. Flows, particularly in times of flood, are liable to spill onto the banks of the river and there is development along a considerable length of river which could be affected by floods. There have been occasions when property has been flooded owing to the high water level in the river.

Buildings along the banks of the Vaal and trees tend to obstruct flow during floods and cause the water to back up behind those obstructions with consequent hazardous effects on other property upstream. A backwater is created, i.e. the blocking of the flow path increases water depth upstream and this effect can be felt for a few kilometres upstream. It is for this reason that Rand Water in 1982 prohibited development below the maximum recorded flood line which occurred in 1975. This policy had some advantages, inter alia that it preserved vegetation along the banks in the form of a green strip. If further protected the river against pollution. However, pollution from stormwater reaches the river and on site treatment may become a necessity.

The designated waterway should be the zone within which obstructive or dangerous development should be prohibited.

The suggested prohibition line below which development should be prohibited could be based on a hazard-risk index of above 2. The boundary corresponds to the 10-year flood line (the 10-year line nearer the Barrage but nearer the 5-year line further upstream). To use a more frequent (e.g. 1-year) flood line would expose developers and Rand Water to a greater risk. The value of the additional land thus available is evaluated below. It is demonstrated that the value of the additional land thus made available is greater than the cost of additional flooding if building is on stilts. The policy suggested is to restrict development for a hazard-risk index of 2 or less, and use economic principles in allowing development above that zone.

Developers may propose altering the flood lines on the flood plain, for example by replacing exotic gum trees by indigenous shrubs or reshaping the ground, but the onus should be on them to prove this is beneficial from an economic, social, environmental, water quality and hydrological point of view and it is sustainable. This may enable them to make even more area available for development.

For the upper limit of the flood plain, the 25 year flood line or 1975 year flood line is suggested. The 25-year flood line corresponds with the 1975 flood, measured and marked along the riverbanks. This is based on the 1997 revised flood estimates. Hence it is a practical line to apply.

It would not be wise for floor levels of buildings to be below the 25 year flood line as the cost of flooding could be excessive and the backup effect would be greater. Hence, apart from surface development, e.g. roads, recreation areas on the ground, buildings will have elevated floors, by up to 3 m, or more if the consequences of flooding are great.

The flood fringe (between the 25-year and 50-year or 100-year now flood lines) may also be developed to an even greater density and development need not be banned in the flood fringe as long as the developers and owners are aware of and responsible for the risk of flooding and indicate this in writing. The flood lines should be indicated on all development plans, as well as floor levels. The hazard risk index is low and backup effect also negligible due to development on this fringe. In all cases, the responsibility needs legal opinion, and insurance against hazards should be considered.

The RI is adequate for indicating risk of flooding and the HI for degree of hazard. The combined HRI is of most use in demarcating zones adjacent to waterways.

Development in the waterway along the Vaal river barrage should be prohibited as it is dangerous to developers and will obstruct floods, causing backup of water upstream. The waterway corresponds to the 10-year frequency flood lines and a HRI of 2.

Restricted development may be permitted on the flood plains, i.e. between the waterway and the 25 year flood lines. Restricted development includes construction on stilts to raise the floor level above the 50-year flood level. The support structure should not occupy more than 1% of the land (erf) area. Structures and associated development, earthworks and planting of trees should only be commenced if the approval is obtained.

At-risk development may be permitted on the flood fringes, i.e. beyond the 25 year flood line and the developer shall indicate on all relevant plans the position of the flood lines and indemnify the authorities from any.