4 DESCRIPTION AND METHODOLOGY OF PROPOSED COASTAL WORKS

Transcription

1 4 DESCRIPTION AND METHODOLOGY OF PROPOSED COASTAL WORKS 4.1 INTRODUCTION Rivière des Galets site is remarkably different from the Mon Choisy site. The critical coastal hazard to be addressed at Rivière des Galets is the overtopping of the sea wall and inundation of the houses located immediately behind it during large wave events. There is no high tide beach and very little sand to consider at Rivière des Galets, rather it is a fortified beach where the erosion of the land is inhibited by the presence of revetment/seawall constructed from concrete, gabion baskets and placed rocks. 4.2 WAVE OVERTOPPING The primary environmental and social problem at Rivière des Galets is the overtopping of the beach berm by waves during large swell events caused either by deep low pressure systems in the southern Indian Ocean or by nearby tropical cyclones. Coastal investigations carried out by the project consultants concluded that the more severe overtopping seems to occur in the relatively rare combination of large swells (>4 m) and long wave period (>16 sec). The main factors and processes identified as being the root causes of over-topping and inundation of the village include: (iii) (iv) Wave set-up during large wave events is greatest at the head of the bay, which is also the location of the village. The large volume of water pushed up onto the reef flat is driven into the bay due to the process of refraction bending waves, resulting in the larger set-up. The fringing reef is narrowest in front of the village which results in compression of the surf-zone and higher set-up in this location. The topography of the village indicates that it is lowest where the greatest inundation occurs. Overtopping is a problem in that it causes flooding of the land or structures behind a coastal structure or in the case of dykes or levees, the overtopping flow can cause erosion on the backside of the structure. Most overtopping studies are done in the context of the latter example, that of the over-topping embankment or dyke. 4.3 ROCK REVETMENT STRUCTURE AS IMPROVED DEFENCE BASIC STRUCTURE A rock revetment is defined by the US Army Corps of Engineers Engineering Manual (CEM) as a facing of erosion resistant material such as stone or concrete to protect a scarp, embankment, or other shoreline feature against erosion. 4-1

2 The type of coastal structure which has been selected by the project consultant for Rivière des Galets is a rock revetment with a crown wall parapet. Basically it is vertical face parapet wall with an integrated wave return lip that extends 150 mm out from the face of the pre-cast block as shown in Figure 4.1 below. In case of Rivière des Galets, the following basic design components for a rock revetment have been proposed: Structure slope of 2 Horizontal to 1 V vertical, crest berm of 2-3 stone widths, i.e. 1.5 to 2 m. (iii) (iv) (v) (vi) Stone armour construction with outer armour stone size of tonne 1.5 tonne (or if possible, larger) units preferentially placed at the toe of the revetment 2 layers of principal armour 2 layers of under armour ( kg stone) Foundation of the structure toe on reef platform by the necessary excavation (vii) Vertical crown wall of height 0.7 m with crest of crown wall set to 4.1 m AMSL. (viii) Wave return lip to be integrated into crown wall cross section. (ix) (x) Crown wall to be constructed of pre-cast concrete units Precast units to be set on existing compacted soil. Figure 4.1 : Layout Of Proposed Rock Revetment 4-2

3 4.3.2 DESIGN LIFE The rock revetment structure is designed to withstand damaging overtopping and wave runup through the 50 year planning horizon. This is considered to be adequate because for all such major construction works a balance must be struck between design life and the costs of works. One of the longer-term advantages of a rock revetment is that even though it is a high cost coastal protection option, it is of low maintenance CROWN PARAPET WALL The design allows for a 600mm thick crown parapet wall of total height of 1000mm, including a 300mm thick and 150mm wide lip. The top surface of the crown parapet wall will thus be 750mm thick. The top surface of the crown parapet wall will be 700mm higher than the existing ground/walkway between the existing gabion wall and the existing masonry backstop wall and will be lower than the top of the backstop wall by 500mm. Thus the finished level of the crown parapet wall will not adversely impact on the inhabitants and they will not complain about loss of view to the ocean. The height of the pre-cast parapet was based on the overtopping modelling, which required the wall to be at 4.1 metres above mean sea level. Figure 4.2 below shows the typical section of the proposed parapet wall. Figure 4.2 : Section of the proposed parapet wall 4-3

4 4.3.4 ARMOUR STABILITY The rocks for the revetment wall will weigh between 1 and 1.5 tonnes, making the RDG sea defence wall one of the larger coastal protection measures in Mauritius. Armour stone size was computed for the proposed revetment at present sea level, 20 year SLR and 50 year SLR projections. Wave conditions were assumed to be based on 100 year return ( sec). Since, the revetment site is fronted by a shallow nearshore reef shelf, the wave heights that affect the revetment would be limited by the depth of the water over the reef shelf and will therefore be governed by the water level. The reef flat in front of the site sits at a depth of approximately 1.0 m AMSL and extends offshore to a minimum of 30 to 40 m. Depth limited wave height is assumed to be 0.7d where d is the depth of the water over the reef flat i.e. 1.0 m plus the super elevation of that water level due to high tide, sea level rise and wave set up. Based on the above, the Van de Meer shallow water formulation was used by the project consultant to provide the most conservative results for the recommended stone size as follows:- (iii) (iv) Stone armour construction with outer armour stone size of tonnes 1.5 tonne (or if possible, larger) units preferentially placed at revetment toe 2 layers of principal armour 2 layers of under armour ( kg stone) WALKWAY AND BACKYARD ACCESS The existing coastal protection layout at Rivière des Galets has been considered in the design. This includes an appreciation for the present location of the sea walls, gabion baskets and walkway as shown in Figure 4.3 below. Figure 4.3: Existing Walkway Each house has also a back yard access as shown in Figure 4.4 below. 4-4

5 Figure 4.4: Backyard access to individual house. Figure 4.5: Existing Wall at the backyard of the houses The proposed parapet wall will be only 700mm higher than the existing ground between the gabion wall and the backstop wall. It is recommended that the walkway should be preserved as recommended through the social consultation exercise and upgraded into a promenade/waterfront offering leisure potential to the villagers. It is also recommended to construct weep holes in the existing wall structure which shall serve for evacuating water back into the sea in case there is an overtopping of waves and subsequent accumulation of water in the yard of the inhabitants STABILITY OF CROWN PARAPET WALL According to Figures 4.1 and 4.2 above, the crown parapet wall will be designed to sit on top of the existing gabion wall-which is pictured in Figure 4.3. To ensure the appropriateness of this design element, it is recommended that during the detailed design stage of the rock revetment structure (which is yet to come), the design consultant should imperatively assess the stability of the existing gabion wall should they intend to retain it in the final analysis. 4-5

6 The preliminary design makes provision for an underlying geotextile sheeting, over which a 100mm thick crusher run layer will be placed to provide a uniform and level surface to accommodate the precast sections of the parapet wall. The rock armour, as being indicated in the project consultant s preliminary design, will surround and encompass the gabion nets which will thereby be consolidated, so that it will be structurally safe and sound to accommodate the crown parapet wall. The final and detailed design of this crown parapet wall (and its structural stability as well as that of the existing gabion wall) is beyond the scope of this EIA Study. Notwithstanding this fact, it is recommended that the precast parapet wall sections should be provided with 32mm thick dowels on its underside which would be made to penetrate into the gabion walls to improve its stability under the action of the huge heavy crashing waves. Finally, where required and where possible, any gabion basket which is found to be defective should be removed and replaced by new ones before the precast sections of the crown parapet wall are placed upon them. The above proposals should be considered as a study of alternatives forming part of the essence of an environmental impact assessment TYPES OF ROCK ARMOUR For the construction of such a rock revetment wall as proposed, from experience it is known that two methods are generally used: namely the rip-rap approach or alternatively a dressed-type armour. The rip-rap design normally leaves an irregular surface after completion of the works, while the dressed-type offers a regular uniform surface which is more appealing to the eye and more aesthetic. It is recommended that within the overall philosophy of the design of the revetment structure, and its integration within the livelihood of the RDG village and further because of its exposure on the national level, the dressed-type armour should be adopted in the design. Notwithstanding this, nothing with regards to the design aspects can be imposed on the project s design consultants; the approach adopted in this EIA study is only to consider possible alternatives to the proposed components and design aspects of the rock revetment structure which are being submitted to the project consultant s attention for his final decision. 4.4 MANAGED RETREAT As mentioned earlier, the rock revetment structure has a design life of 50 years, therefore an alternative option has been proposed by the project consultant which is managed retreat. Managed Retreat can be considered to be a good option for Rivière des Galets village given the relatively small number of affected families (approximately 97) and the exposed location of the site. However, moving an entire community can be a complicated issue giving rise to varying social perception amongst the community. This issue has been thoroughly discussed in Chapter 5 entitled Social Consultation and Chapter 7 Socio Economic Consideration. 4-6

7 4.5 METHODOLOGY OF CONSTRUCTION WORKS PLAN OF WORKING AREA The EIA report has looked into the best options for the methodology of the construction works. In order to do this, a plan of the RDG village showing the access roads to the working site, the adjacent features such as the cemetery, the positioning of the rock bulk storage areas has been prepared and is enclosed at Annex 4A at the end of Chapter 4. The plan, compiled from a Google Plan, shows: The extent of the RDG village The 5No existing access roads leading to the village and to the seafront from the Coastal Road B9 The location of the adjacent cemetery to the north of the project site. The positions of the proposed 2No bulk storage areas (also called depots), one at the northern end of the site and the other at the southern end of the site) The proposed transitory rock stacking areas at the working fronts DESCRIPTION OF WORKING AREA As explained above and as shown on the drawing at Annex 4A, the southern point of the site coincides with the existing cemetery from which it is separated by a common access of 3 metres width over a length of 38 metres. This road leads from the coastal Road B9 to the northern tip of the existing gabion baskets. There is only one house entrance on this access road. So it is recommended that this access road should be used as access to the site. It is marked as Access Road No.1 and is called Cemetery Lane. To enable this, the following measures are required: (iii) ACCESS ROAD NO.2 The access road should be hoarded on the cemetery side so as not to impact upon the activities of the cemetery (over a length of 38 metres) The rock transport lorries (for local transportation) will have to reverse into the access road from the Coastal Road B9. To ensure safe operations, all local transportation lorries should imperatively be equipped with reversing alarms to warn about their reversing movements for safe operations. All the local transportation lorries should have a helper whose duty will be to get out of the lorry and direct the driver during his reversing operations in order to guarantee safety to B9 road users, the workers themselves, the inhabitants of the RDG village as a whole but more importantly the residents of the Access Road No.1. The second access road leading from the Coastal road B9 into the village is called Port Lane and is 36 metres south of Access No.1 called Cemetery Lane. This road is very narrow of up to 3 metres wide only and has several existing houses and house entrances on both sides. This road access is referred to as Access No.2 on the drawing at Annex 4A. It is not recommended to use this road for providing access to local transportation lorries to the working fronts. 4-7

8 4.5.4 ACCESS ROAD NO.3 Access Road No.3, on drawing at Annex 4A, is called Coconut Street and is 34 metres south of Access Road No.2 Port Lane. It is 35 metres long. This access road is very narrow and houses and house entrances exist on both sides. It will be difficult for a heavy truck or equipment transporting rocks and boulders to reverse into this road ACCESS ROAD NO.4 The Access Road No.4 is named Beach Street. It is 50 metres long and 5 metres wide, and its junction with the B9 road is 48 metres south of Access Road No.3 Coconut Street. It is recommended to use this Access Road No.4 as a second access to the site because it presents the following advantages: Being of width 5 metres, it allows for heavy truck manoeuvreing At its end, there exists an open space 60 metres long and 7 metres wide which can be used as a rock stacking area from which the hauling equipment will collect the rocks and place them within the rock armour comprising the rock revetment wall. This area is shown in the photographs enclosed at Figure 4.5a below: Figure 4.5a: Possible Rock Stacking Area 4-8

9 Note: This area can also be used as equipment parking area. Sub-section refers Notwithstanding the above, the contract specifications should make it clear that the local transportation lorries should be fitted with reversing lights and warning whistles for warning and safety. Additionally, the lorry helper must control the reversing action of the lorry and direct the driver to ensure safe reversing operations ACCESS ROAD NO.5 Access Road No.5 (see Drawing at Annex 4A) is further down into the RDG village and its junction with Road B9 is 136 metres further south from Access Road No.4. This road cannot be used by the local transportation lorries to deliver rocks to the working fronts because of lack of direct access from B9 to the gabion wall JUNCTION B9/B10 The road junction B9/B10 is situated at 80 metres south from the Access Road no.5 and both the roads B9 and B10 can therefore be used to reach the project site. Hence the rock transportation lorries can use any of the two roads to reach the proposed bulk storage areas/depots, as described in sub-section below BULK ROCK STORAGE AREA/DEPOT NO.1 It is proposed, within the methodology for the works, to have 2No bulk storage areas, also referred to as Rock Depots. The first one, called Rock Depot No.1 (of dimensions 55m x 50m approximately), is proposed to be sited about 85 metres from the northern boundary of the site, as shown on the Drawing enclosed at Annex 4A at the end of Chapter 4. This north rock stacking area is proposed to be located inside an un-cultivated sugarcane area of big extent as shown on the photograph enclosed in Figure 4.5b below. The contractor can negotiate with the owner for such a stacking area, referred to as a Bulk Rock Storage Area/Depot from which boulders can be delivered to the northern working front of the revetment wall. Figure 4.5b: Bulk Rock Storage Depot No.1 4-9

10 The only disadvantage is that lorries or equipment used for the local transportation of the rocks will have to use the existing coastal Road B9 over a length of about 85 metres to reach the first site access (Access Road No.1) (see sub-section 4.5.2) and about 200 metres to reach the second access site Access Road No.4 (sub-section 4.5.5). But this inconvenience can be managed by planning proper road signs and road markings at the junction and along the stretch of Road B9 in question, and by providing police presence and guidance during times of local transportation of rocks BULK ROCK STORAGE AREA/DEPOT NO.2 Opposite the junction B9/B10 there exists a vast area of unoccupied land presently under thick foliage - which can easily be cleansed to provide a rock stacking area of 55m x 60 metres. The advantage of this stacking area is that it borders the bank of the Riviere des Galets along about 65 metres. The bank of the river along this stretch is 13 metres wide. This area is directly accessible to the southern end of the existing gabion wall hence the working site EQUIPMENT PARKING AREA Figure 4.5c: Bulk Rock Storage Depot No.2 The available space (7m x 60m) available at the end of Access Road No.4 Beach Street can be used by the contractor for parking of the construction equipment. Please see sub-section Thus, such equipment will not require to use any public road to reach the working fronts. Hence this spot is ideal for reasons of safety and ease of accessibility to the working sites for the construction equipment. 4.6 SOURCING OF ROCKS It has been estimated that an approximate volume of 13,000m 3 of rocks and boulders of size 1.0 to 1.5 tonnes will be required for the construction of the revetment wall, but may be a lesser volume of about 10,000m 3 if the existing gabion wall is incorporated in the final design of the rock revetment wall. At this point in the project procurement cycle, it is not yet precisely known where the rocks will come from or from which rock quarries they will be sourced, as this will be the responsibility of the contractor under the contract. 4-10

11 Notwithstanding the above, for rock transportation, it is suggested that the rock quarry /rock depot belonging to UBP at Ste Marie Crushing Plant Riviere des Galets could be used as it is the nearest source. If negotiations are successful, this site could also be used as a transit rock storage area. Note: At this point in time, this option should only be considered as a suggestion. From whatsoever source or quarry, the transport lorries of lorries capacity will have to transport the boulders to the bulk storage depots No.1 and No.2 as explained at Section 4.5 above. The assessment of the impact associated with the traffic generated by the inland rock transportation is exhaustively addressed at Section 6.17 of Chapter DESIGN REVIEW OF COASTAL ADAPTATION MEASURES AT RIVIERE DES GALETS BASELINE DATA The baseline data which was for the preparation of this review is the set of 5No reports prepared by the consortium of consultants appointed by the UNDP/MESDDBM, namely:- Detailed Technical Assessment Report Final Feasibility Study Report Coastal Adaptation Measures for Rivere des Galets Final Design Report Options for Adaptation Cost Benefit Analysis for Adapting to Sea Level Rise: Protection or Managed Retreat in Riviere des Galets REVIEW OF DESIGN PARAMETERS The Riviere des Galets is a small town located in the southern coast of Mauritius, which experiences flooding due to wave run-up during extreme events and storm surge resulting from cyclones. A view of the existing seawall is shown in Fig.4.6 below: Fig.4.6: A View of the Existing Seawall. 4-11

12 The maximum possible wave characteristics, tide and water level variations as discussed in the above-mentioned reports are discussed below for reference: (iii) The Mean High Water Spring (MHWS) is CD and Mean Low Water Spring (MLWS) is CD. Hence the maximum possible tidal variation is of the order of about 0.5m. The depth limited wave height considering the sea level rise for 50 years of life span has been arrived as 2.1m A wave period of 16 to 18s has been assumed which could be the worst scenario. Based on the above considerations, the top level of the proposed sea wall crown has been fixed at 4.1m above MSL and boulders of 1.0 to 1.5 tonnes have been suggested by the project consultant. However, the length of the toe protection appears to be inadequate to retain the armour layer of the seawall. It is known from experience that the toe of any coastal structure is an important failure point and should imperatively be protected against any risk of scouring. Hence it is recommended that a minimum of 3m or width of 3 stones should be provided as toe width. Furthermore, it is highly recommended that a further flattening the armour slope should be considered. The adopted slope of 1:2 could be further flattened to 1:2.5 or 1:3 to ensure further stability of the seawall under extreme weather conditions. The reasons for this are given below: The flattening of the slope wall will increase the footprint of the sea defense structure increasing its stability against the expected huge wave swells. A flatter slope automatically produces a longer wave travel distance inducing better dissipation of wave energy as the waves crash against the sea defense structure FILTER LAYER The disposition (displacement) of the existing gabions is due to the seepage of flooded water behind and beneath the structure and their resulting scour as shown in Figure 4.7 below: Fig.4.7: Displacement Of The Existing Seawall At A Few Locations 4-12

13 This scouring can undermine the existing seawall in the future. Hence on the land side of the seawall, it is recommended that an excavated filter layer of about 0.3m deep and 0.5m width be provided INLAND UPGRADING Paving on the inland side will certainly enhance the aesthetic value of the area. It is understood, through discussion with the EIA Consultant that a promenade will be created in the space available between the sea defence wall and the existing masonry backstop wall. This is a good proposal. It is also understood that the EIA Consultant has proposed several enhancement measures for the available inland space in favour of the RDG village community for the social from the social perspective RE-USE OF GABION WALL The existing gabion seawall can be used as the core material, above which the amour layer can be placed by adopting a slope of 1:2.5 on the seaward side along with a proper toe as discussed above; this would constitute an economical and safe design. Removal of the entire rubble stones would be expensive and incorporating the existing gabions into the rock armours should be feasible if the recommendations made as above are considered for implementation CROWN PARAPET WALL The project consultant has proposed a crown parapet wall 700mm high with a 300m thick lip on top to optimise the diffraction of the crashing waves. This is considered a good design approach. However, it is believed that this expected diffraction would be improved if the 700mm high straight vertical body of the crown parapet wall is made into a concave shape. It will then serve the propose of streamlining the wave diffraction effect which will result in a marked reduction of the over-topping RECOMMENDATIONS AND CONCLUSION (iii) (iv) (v) (vi) While the appropriateness of the design and design parameters appears satisfactory, yet it is recommended that a toe protection of minimum 2 to 3m should be provided. A flatter slope of the sea defence wall of 1:2½ o 1:3 should be considered in the final design approach for the reasons enumerated above in this report. A filter layer should be provided as explained in this report. A concave/curved body of the crown parapet wall on the seaward side should be considered as it is believed that this shape will improve its wave diffraction potential. The existing gabion nets should be considered to be maintained in place in the design of the new sea defence wall because it will be cheaper this way and will continue to offer protection to the RDG village community even during the construction of the new sea defense wall. Rock filled sea defense walls are generally an expensive proposal, but they may be required in the interest of preserving lives and livelihoods of the RDG Village community. Moreover, it is known from experience that such rock revetment requires relatively low maintenance during its design life. 4-13