Heritage Cases

The South African National Roads Agency (SANRAL) proposes to upgrade the National Route (N) 10 Section (/) 3 between Riet River Bridge (km 45.2) and Tarka Bridge (km 68.5) situated in the Inxuba Yethemba Municipal jurisdiction area (Figure 1). The project involves the complete upgrade of the section described above together with reconstruction and in some cases rehabilitation of appurtenant works. Climbing lanes will be constructed where required in the existing road reserve of 31,25m with possible expropriation along certain areas. The existing road will be reconstructed in half widths and deviations will be constructed where the entire existing road will be reconstructed. The alignment over this section will be corrected for an operating speed of 100km/h. WorleyParsons has been appointed by SANRAL as the project managers who subcontracted Coastal & Environmental Services (CES) as the Environmental Assessment Practitioner (EAP). The activity will make use of local borrow pits to source required fill material. A mining permit application has also been submitted for possible borrow pit sites to the Department of Mineral Resources (DMR). This is in accordance with the regulations pertaining to the Minerals and Petroleum Resources Development Act (Act No.28 of 2002) regulated by the Department of Mineral Resources. ACTIVITIES ASSOCIATED WITH THE PROJECT The proposed activity includes widening the existing section of road from a minimum width of 13.4 m to a maximum of 14.7 m. Climbing lanes may be required where necessary. The widening will also include the widening of all stormwater structures along the length of the project. There will also be bridge and major culvert construction. Design Flood Frequency It was confirmed by the SANRAL Regional Project Leader that along this section of road the bridges be analysed for a Class 2 road category, while the culverts be analysed for a Class 3 road category. Class 2 road category (Regional distributor) Riet River Bridge A road-over-river bridge exists across the Riet River at 45.18 km on the N10-3. This bridge has 2 spans, each 12.8 m wide and approximately 4.3 m high. The bridge was hydraulically analysed using the computer program HEC-RAS. The hydrological results obtained from the Standard Design Flood (SDF) method were used to analyze the bridge structure. The existing road bridge across the Riet River Bridge was hydraulically analysed with the available survey at hand. Table 1.2 shows the hydraulic performance, utilising the program HEC-RAS. The possible back-up flood effect of the Fish River (approximately 1,6km away) was not taken into account during the bridge hydraulic analysis. Mr Stegman ( Cell 082 4815 485), a farmer that has been farming for 56 years adjacent the Riet River, does not recall any overtopping of this bridge structure in the past. It was found that the Riet River bridge capacity is inadequate to accommodate the required design flood frequencies in accordance with the Drainage Manual (SANRAL, 2006) for Classes 2, 3 and 4 road categories. After thorough investigation to the cause of the inadequate bridge capacity, it was found to be a result of natural sedimentation in the river bed in the vicinity of and downstream of the Riet River Bridge. From the original design plans, later drawings prepared for the widening of the bridge and the current field survey, it is clear that the river bed had been elevated by approximately 1m due to sedimentation. The sharp curvature of the river stream downstream of the bridge, the flat gradient of the river in the vicinity of the bridge and downstream thereof, as well as dense vegetation downstream of the bridge are regarded as the main contributing factors for low flow velocities and consequential sedimentation. The river bed appears to have settled at this level and a larger bridge opening is required to accommodate the design flood. It thus proposed that the existing bridge be demolished and replaced with a new bridge with four 12,8m spans and a higher soffit level. A minimum bridge soffit level of 782.36 m.a.m.s.l. is required to comply to a Class 2 road category. This implies that the road vertical alignment must be raised by approximately 1.34m. The current bridge is located in a vertical sag. Blaaukrantz River Bridge A road-over-river bridge exists across the Blaauwkrantz River at 53.14 km on the N10-3. This bridge consists of 1 span, 12.0 m wide and approximately 4,0 m high. The bridge hydraulics was analyzed with the computer program HEC-RAS. The hydrological results, obtained by using the Standard Design Flood (SDF) method, were used to analyze the bridge structure. Peak floods obtained from the SDF method reflect slightly higher than the mean peak flow obtained from all the methods used. The existing road bridge across the Blaauwkrantz River was hydraulically analysed with the available survey at hand together with the computer program HEC-RAS. The results of the hydraulic analysis indicate that the bridge capacity is adequate to accommodate the required design flood frequencies in accordance with the Drainage Manual (SANRAL, 2006) for Classes 2, 3 and 4 road categories. Tarka River Bridge A road-over-river bridge exists across the Tarka River Bridge at 58.70 km on the N10-3. This bridge consists of 3 spans, each 19.1 m wide and approximately 7.2 m high. The hydrological results obtained from the Standard Design Flood (SDF) method were discharged as the values calculated were unrealistically high comparing with the other methods. The report (Report No 344512/1) by Van Bladeren (2005) recommends that the SDF method under estimate the run-off during extreme events and that the run-off should be increase by approximately 40%. Existing flow data were obtained from the two dam structures within the catchment area, namely the Lake Athur and the Kommandodrift dams. The data set was incomplete and could not be used statistically to determine the peak flows. An existing Department Of Water Affairs weir (Q4H013) is situated directly upstream of the Tarka River Bridge structure. This weir has a maximum rating curve of 106.91 m³/s and could also not be used. Data was obtained from 1980 onwards and in this period the recorded flows exceeded the weir’s rating curve on nine occasions. It is recommended that the alternative Rational peak flows be utilized for the hydraulic analysis. Mnr L Du Preez (cell 083 415 1374) a local farmer for 22 years can’t recall any overtopping of the bridge structures in the past. The existing road bridge across the Tarka River Bridge was hydraulically analysed with the available survey at hand together with the computer program HEC-RAS. It was found that the bridge capacity is inadequate to accommodate the latest required design flood frequencies in accordance with the Drainage Manual (SANRAL, 2006) for Class 2 road categories based on the freeboard requirement criterion. The other criterion, namely overtopping, was met for a Class 2 road. It is recommended that the existing bridge be retained based on the hydraulic performance for one category lower, namely a Class 3 road category. It is recommended that the existing road over river bridge at the Riet River be upgraded by replacing it with a new bridge to provide adequate capacity in order for the road to be classified as a Class 2 road category.