Final. Contact person: Colin Whittemore Aurecon Centre 1 Century City Drive Waterford Precinct, Century City Cape Town, South Africa

Transcription

1 Review Report and Recommendations for the Remediation of Flood Damage at the Berg River Causeway and the Dam Bypass Channel on Portion of Farms 1646 and 1014, Franschhoek Contact person: Colin Whittemore Aurecon Centre 1 Century City Drive Waterford Precinct, Century City Cape Town, South Africa T: F: E: colin.whittemore@aurecongroup.com Submitted to: Two Rivers Development Company (Pty) Ltd P O Box 2262 Somerset West Submission date: 17 February 2014 Final

2 Report Control Sheet CLIENT : Two Rivers Development Company (Pty) Ltd CLIENT REPRESENTATIVE : Mr Iain Banner ; Mr Stephan Ekbergh REPORT NUMBER : 8783/ REPORT TITLE : Review Report and Recommendations for the Remediation of Flood Damage at the Berg River Causeway and the Dam Bypass Channel on Portion of Farms 1646 and 1014, Franschhoek AUTHOR(S) : Colin Whittemore REVISION : Final PREPARED BY : Aurecon (South Africa) (Pty) Ltd ( PROJECT NUMBER : AB and AB3 DATE : 17 February 2014 Author(s): C R Whittemore Pr Eng (Associate) Full name and title Signature Approved by: M A Killick Pr Eng (Technical Director) Full name and title Signature P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx i

3 Two Rivers Development Review Report and Recommendations for the Remediation of Flood Damage at the Berg River Causeway and the Dam Bypass Channel on Portion of Farms 1646 and 1014, Franschhoek 1. Introduction This report is submitted to the Two Rivers Development Company (Pty) Ltd and provides a review of the performance of the Berg River causeway and the dam bypass channel during periods of heavy rainfall and high river flow in August This report also presents recommendations for work to be undertaken to remediate the damage caused during the August 2013 events and which will minimise the risk of similar damage occurring in future. This report should be read in conjunction with Aurecon s previous report on the causeway and bypass channel ( 1 ) as well as Aurecon drawings C-001 and These drawings give details of the previously recommended construction works for the dam and bypass channel (see Figures 1 and 2 Annexure A). 2. Berg River Flow Rates August 2013 Rainfall and river flow records indicate that August 2013 was one of the wettest months experienced in many years. In order to put this, and the flow rates experienced at the causeway and dam bypass channel into perspective, river flow data for gauge G1H020 on the Berg River in Paarl (at Daljosafat) was downloaded from the DWA website (see Annexure B). As shown in Figures B1 and B2 (Annexure B), the following peak flow rates were recorded at G1H020 during August 2013: 13 August 2013: 450 m 3 /s 15 August 2013: 600 m 3 /s 17 August 2013: 580 m 3 /s 28/29 August 2013: > 700 m 3 /s (gauge exceeded or malfunctioned at 700m 3 /s) The annual maximum flow rates recorded at G1H020 (also downloaded from the DWA website - see table of annual maximum flow rates in Annexure B), indicates that the flow rate in excess of 700 m 3 /s on 28/29 August was the highest on record dating back to July Flow rates in excess of 600m 3 /s have, however, been recorded on seven previous occasions in the 48 year record with the last occurrence being in June Therefore, while the August 2013 flow rates were high, based on the G1H020 flow record, it is apparent that similar flow rates can be expected every 5 to 10 years. It also should be noted that the flow rates experienced at the Berg River causeway would have been considerably less than those recorded at gauge G1H020 as the gauge is located much further downstream in the Berg River catchment. Based purely on upstream catchment area, the flow rates at the causeway could have been in the order of 50% of those experienced at the gauge. Confirmation of the causeway flow rates would, however, require a detailed analysis of rainfall over the catchment which fell outside the scope of work for this study. Attempts made to acquire flow data recorded at the Berg River Abstraction Works (approximately 3km downstream of the causeway) were unfortunately, unsuccessful. 1 Report on the Hydrologic, Hydraulic and Engineering Assessments informing the Final Rehabilitation Plan for the Freshwater Ecosystems associated with the Berg River Causeway and Three Local Tributaries of the Berg River on Portion of Farms 1646 and 1014, Franschhoek (Aurecon Report No. 6669/108669, dated 23 November 2012). P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 1

4 The size of cobbles / boulders to be used on the river bank is dependent on the level of erosion protection required. For the annual release from the Berg River Dam (160m 3 /s with a flow depth of approximately 1,7m downstream of the causeway), the average cobble / boulder size should be 180mm with a minimum layer thickness of 360mm (i.e. minimum layer thickness = twice the average cobble / boulder size). For protection at a flow rate of 350m 3 /s (approximating a 1:10 year flood event with a flow depth of 2,7m downstream of the weir), the average cobble / boulder size should be 300mm with a minimum layer thickness of 600mm. The eroded secondary channel at the toe of the current embankment should be filled using river cobbles and boulders and a more central, secondary channel created (see Figures 35 and 36). These tasks must be carried out with hands-on, on-site supervision of the process by a suitably qualified and experienced river ecologist who must ensure that the resulting embankment and secondary channel achieve a level of heterogeneity / variation in slope and profile that mirrors the natural bank and bed profile along this reach of the river. 4. Dam and Dam Bypass Channel 4.1 Background Modifications to the dam and construction of the dam bypass channel, as recommended in Aurecon s previous report ( 1 ) and detailed in Figures 1 and 2, were partially implemented by the end of June Following heavy rainfall and high river flows during August 2013, which caused damage to the bypass channel, a site inspection was carried out and several aspects of the construction were noted as being incomplete and / or contrary to the Aurecon design. These were: The cross-sectional profile of the bypass channel. The longitudinal profile of the bypass channel. The location of the first step in the bypass channel invert and combining of this with the grade control gabion. Omission of reno-mattress erosion protection to the dam embankment at the start of bypass channel. The termination point of the bypass channel erosion protection. Omission of the concrete sill at the dam inlet / outlet. Omission of the emergency spillway and associate concrete sill. Modification of the road side-drain. Provision of gabion / reno-mattress erosion protection at the road culvert downstream of the bypass channel. 4.2 Recommendations for Remediation Cross-section profile of bypass channel As shown in Figures 37A and 37B, the as-built trapezoidal cross-section of the bypass channel differs from designed rectangular cross-section profile. The as-built channel is larger than the designed channel and is, in fact, oversized with larger capacity than necessary. This is not considered to be problematic and no remediation work is considered necessary. The side slopes of the trapezoidal channel should provide for easier establishment of riverine vegetation Longitudinal profile of bypass channel As shown in Figures 38A and 38B, the uniformly sloping longitudinal gradient of the as-built channel differs from the designed stepped profile. This results in super-critical flow and higher flow velocities in the channel. During the site visit, it was noted that some movement of stone in the invert reno-mattresses had occurred. This was partly due to small stone size and high flow velocities and partly due to inadequate filling of the mattresses. It is recommended that the invert mattresses be re-packed as per the manufacturer s instructions and that sufficient stone of appropriate size is placed into them to enable the required tensioning of the lids prior to fastening them down. P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 4

5 The mattresses should be inspected periodically (at the end of winter) for further signs of deformation (stone movement) which should be addressed as necessary. The establishment of riverine vegetation should assist to stabilise the channel invert over time. Such vegetation should, however, not cause a significant reduction in channel capacity Step and grade control gabions As shown in Figures 39A and 39B, the location of the step in the bypass channel invert and the combining of this with the grade control gabions, is not in accordance with the design. The as-built configuration could result in the gradual lowering of the channel bed upstream of the step. This is more likely to occur if the geotextile filter material was omitted from the upstream side of the step / grade control gabions. Photographs taken during construction show no evidence of the geotextile filter being in place (see inset in Figure 39B). This situation will have to be monitored as lowering of the bed of the bypass channel will result in more water bypassing the dam and less being diverted into the dam. If not installed initially, the geotextile filter will have to be installed to rectify this situation. The surveyed levels of the top of the step / grade control gabions vary between 158,13m and 157,85m. The design level for the top of this structure is 157,7m. It is important for the top of the step / grade control gabions to be at the correct level (157,7m) as this is related to the level of the dam inlet sill (157.9m) and controls the amount of water bypassing and entering the dam under various flow conditions. The step / grade control gabions should be repacked to achieve the correct crest level Erosion protection to dam embankment As shown in Figures 40A and 40B, omission of the reno-mattress erosion protection to the dam embankment adjoining the bypass channel has resulted in the development of a flow path behind the gabion wall and erosion of the dam embankment. This erosion should be made good and the required reno-mattress protection installed Termination point of bypass channel erosion protection As shown in Figures 41A and 41B, the erosion protection provided at the end of the bypass channel is not in accordance with the design and stops short of the required extent. This has resulted in localised erosion of the unprotected stream bed and banks. The higher flow velocities in the bypass channel, resulting from the steeper bed slope due to omission of the steps in the longitudinal profile, now requires that the erosion protection be extended further downstream as shown in Figure 41B. The slope of the extended erosion protection on the banks should not exceed 1: Concrete sill and reno-mattresses at dam inlet / outlet Omission of the concrete sill and reno-mattress protection at the dam inlet / outlet (as shown in Figure 42) jeopardises the intended and safe operation of the dam and bypass channel. These are important features of the design and must be constructed as detailed Emergency spillway and concrete sill Omission of the emergency spillway and associated concrete sill (as shown in Figure 43) jeopardises the intended and safe operation of the dam. These are important features of the design and must be constructed as detailed Modification of road side-drain The modification of the existing road side-drain, recommended in Aurecon s previous report, has not been implemented as yet (see Figure 44). This modification will allow for the diversion of excess flow from the upstream catchment away from the undersized road crossings (culverts) and around the dam. Diversion of flow away from the culverts will reduce the extent of road overtopping that currently occurs. P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 5

6 4.2.9 Erosion protection at downstream culvert The required erosion protection at the road culvert downstream of the bypass channel has not been implemented (see Figure 45). This was a requirement of the final FCG rehabilitation plan and was also recommended in Aurecon s previous report. 5. Conclusions This report has provided an overview of the performance of the Berg River causeway and the dam bypass channel during periods of heavy rainfall in August A number of recommendations have been made regarding remedial works that are necessary as a consequence of damage caused during the August events. In the case of the Berg River causeway, the environmental damage which has been caused by inappropriate methods of bridging the river, is reversible and the recommended removal of the culverts, reinstatement of a low-level causeway and rehabilitation of the eroded left bank should go a long way towards achieving this. In the case of the dam bypass channel, damage caused was a result of deviation from and / or omission of certain aspects of the Aurecon design. This damage is readily reversible through implementation of the recommended remedial works with a low risk of reoccurrence. It may be the case that further detailed technical inputs / drawings are required to implement the recommendations but provision of these falls outside of Aurecon s current scope of work. It should be noted that the above recommendations have been made from an engineering / technical perspective and need to be approved by the project Environmentalist, Freshwater Ecologist, DEADP and DWA before being implemented. P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 6

7 ANNEXURE A: FIGURES N P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 7

8 Figure 1: Drawing No C-001 Rev A - Two Rivers Dam Upgrading : Plan and Sections

9 Figure 2: Drawing No C-002 Rev C- Two Rivers Dam Bypass Channel : Plan and Sections

10 Figure 37A: Bypass channel as designed - Stepped rectangular cross-section profile (red line). (Extract from drawing C-002) Figure 37B: Bypass channel as constructed Trapezoidal cross-section profile (red line). (Extract from drawing C-002)

11 Figure 38A: Bypass channel as designed - Stepped longitudinal invert profile (red line). (Extract from drawing C-002). Figure 38B: Bypass channel as constructed Uniformly sloping longitudinal invert profile (red line).

12 B B A A Figure 39A: Bypass channel as designed Location of grade control gabion (A) and step gabion (B). (Extracts from drawing C-002). Figure 39B: Bypass channel as constructed Location of combined grade control gabion (A) and step gabion (B). (Inset photograph shows no evidence of geotextile filter being installed at step / grade control gabion during construction).

13 Figure 40A: Bypass channel as designed reno-mattress protection to top of dam embankment (shaded red). (Extract from drawing C-002). Dam embankment area at risk (circled) due to stream flow behind gabion wall. Figure 40B: Bypass channel as constructed omission of reno-mattress protection to dam embankment and above gabion wall resulting in development of flow path behind wall putting dam embankment at risk of failure.

14 Figure 41A: Bypass channel as designed Extent of reno-mattress protection at end of bypass-channel (shaded red). (Extract from drawing C-002). Local erosion at end of current renomattress protection to be made good. Figure 41B: Bypass channel as constructed reno-mattress protection at end of bypass-channel to be extended further downstream as shown (to include area shaded red).

15 Figure 42: Concrete sill and reno-mattress protection at dam inlet (circled) - not constructed. See drawing C-002 for detail of the concrete sill and reno-mattresses. (Extract from drawing C-002). Figure 43: Location of emergency spillway (circled) - not constructed. See drawing C-001 for detail of the spillway and concrete sill. (Extract from drawing C-001).

16 Figure 44: Modification of side drain (circled) - not addressed. (Extract from drawing C-001).

17 Figure 45A: Gabion / reno-mattress / rip-rap protection upstream of road culvert to be repaired and extended. 10m 20m Figure 45B: Extent of gabion / reno-mattress / rip-rap protection required upstream of road culvert.

18 ANNEXURE B: RIVER FLOW DATA G1H020 P:\Projects\ Various Small Works Paarl\AB Two Rivers\3 Delivery\6 Reports\Review Report - Jan 2014\Final - Feb 2014.docx 37

19 Figure B1: Peak flow rates at G1H020 (Daljosafat) on 13, 15 and 17 August 2013 (Approximately 460m 3 /s, 600m 3 /s and 580m 3 /s) Figure B1: Peak flow rates at G1H020 (Daljosafat) on 28 / 29 August 2013 (Gauge exceedance or malfunction at 700m 3 /s)

20 Department of Water Affairs Yearly Peaks for Hydrological Years G1H020.A Berg Daljosafat Time Level at Peak Code Date of Peak Peak (m) Flow (cumec) : M : : : : : : M : M : : : : : : : : M : : : : : : : M : : : > : > : : M : : : M : : : M : : : : : : : : : Q : : Q : Q : M Explanation of codes: M... Missing Data Q... Data Not Audited >... Greater than