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EIA for Proposed Desalination Plant at Mile 6, Swakopmund Specialist Workshop, 20-21 April 2009 Abiotic Coastal/Shoreline Dynamics 20 April 2009 Andre.

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Presentation on theme: "EIA for Proposed Desalination Plant at Mile 6, Swakopmund Specialist Workshop, 20-21 April 2009 Abiotic Coastal/Shoreline Dynamics 20 April 2009 Andre."— Presentation transcript:

1 EIA for Proposed Desalination Plant at Mile 6, Swakopmund Specialist Workshop, 20-21 April 2009 Abiotic Coastal/Shoreline Dynamics 20 April 2009 Andre Theron Council for Scientific and Industrial Research

2 © CSIR 2009 1. Important issues to be addressed by the EIA 2. Methodology to address/investigate these issues 3. Coastal site characteristics 4. Main findings, potential mitigations 5. Overlaps with other specialists & final information required 6. Monitoring Main aspects of presentation:

3 © CSIR 2009 1.Important issues to be addressed by the EIA Mainly the pipelines crossing the coast, but also the intake basin Drivers of coastal processes: winds, waves, currents, (tides) Coastal processes: e.g. marine & aeolian sediment transport Coastal dynamics: beach profile, shoreline evolution, sed. composition Others: backshore area/dunes, sedimentation, dredging, construction damages, mitigations, etc.

4 © CSIR 2009 2.Methodology to address/investigate these issues Mainly desktop, qualitative Available information, literature, site visit, etc. Understanding of local processes & dynamics Limited modelling (previous presentation) Limited quantification, estimates

5 © CSIR 2009 3a. Some coastal site characteristics Historic shoreline variations: Significant variations despite rocky outcrops/reefs. (Major river floods 1934 & 1942) 5 beach sand samples: median grain sizes (D50) ranged from 0.21mm (fine) to 0.37mm (medium) “Dune” sand 0.58mm (coarse) 0.22 mm 0.27 mm Sediment ? ?

6 © CSIR 2009 Typical wave conditions (Hs, 50m): height = 1.1m wave period (Tp) = 12s originating from ~S-SSW 1-in-1 year height = 3.6m Total estimated vertical beach level variation at HW line = ~1.5m Vineta mean net longshore sand transport rate = 140 000 – 400 000 m 3 /a Energetic wave conditions & fine to medium sands lead to high sediment transport potential, but widespread occurrence of rocky areas & limited availability of sand reduce actual rate. 0.22 mm Southern location Tidal levels Water-level (m to CD) Lowest astronomical tide Mean low-water spring tide Mean low-water neap tide Mean level Mean high-water neap tide Mean high-water spring tide Highest astronomical tide 0,00 0,27 0,67 0,98 1,29 1,69 1,97 3b. More coastal site characteristics Wave modelling

7 © CSIR 2009 4a. Potential surfzone impacts related to jetty & pipes Shoreline crossing construction method assumed: 1) Both seawater intake & brine disposal - excavation & burial of pipelines across beach & dune, with subsequent rehabilitation; 2) Temporary sheetpile walls/coffer dam on beach and temporary jetty spanning surf zone for laying of pipes; 3) Blasting required to excavate trench through rocky areas. Effect of open jetty structure on coastal dynamics & processes is due to that part of under-structure which is in contact with sea. Due to the wide spacing & small diameter of columns, which present little resistance to water flows through structure, open jetty has small local effect on waves, currents & sediment transports at site, little on adjacent coastline. Thus no significant change from present dynamics & processes.

8 © CSIR 2009 4b. Potential impacts related to pipes crossing shoreline Local disruptions of intertidal, swash & supra-tidal zones and sed. transports. Construction activities temporarily increase local turbidity. Thus temporary direct local disturbance of beach (& sediments) and potentially small-scale permanent modification. Most significant environmental aspects i.t.o. effects on abiotic coastal (surf-zone, beach & dune) environments are short-term and mainly related to construction period. Mitigations: Excavation to adequate depth and burial across beach & surf zone so that pipeline does not interfere with coastal processes. Minimise operational area during construction.

9 © CSIR 2009 4c. Potential impacts related to intake basin option 500 m structure spanning much of surf zone. Thus longshore current & sand drift permanently changed. Shoreline accretion due to partial trapping of longshore drift. Sedimentation inside basin due to trapping of sand drift & deposition of suspended sediments in low energy area. Occasional dredging required to remove. Limited downdrift erosion possible due to sediment starvation, but rocky outcrops “pin” shoreline. Potential mitigation through sand nourishment. Accretion Erosion Sedimentation ~Max. ~Min.

10 © CSIR 2009 4d. Potential impacts to structures located too near sea due to storm wave run-up (& erosion, underscouring). 250x250m North constr. site Storm run-up line 1 Sep. 09 50 m Extreme wave run-up levels for Namibian coast > + 5 m MSL Beach slope ~ 1:6 (0-2m MSL); berm at ~3.3MSL (overtopped 1Sep’08), road at ~5.5m MSL Set-back distance for structures > 100 m (include SLR effects)

11 N S N S Trench will potentially induce off-shore rip current during construction period when waves high or strong on-shore wind. Large open trench will disrupt natural sed. transp. patterns. Will trap part of local transport – potential temporary partial reduction in sed. feed to downdrift area. Sand dredged from trench during construction period should ideally be fed to adjacent downdrift area. Backfill trench after pipe-laying to prevent permanent risk. 10 ~3 m? 0 to 5 m N S N S 4e. Potential pipeline trench impacts Wave/wind induced current field Trench ~3x3m? Rip current

12 © CSIR 2009 5a. Further information required: Coastal topographic survey (incl. surf zone) Trench/structure dimensions Elevations of recorded storm wave runups 5b. Overlaps with other specialist studies: Marine Ecology Potential small overlaps with most other specialist studies Marine Modelling

13 6. Recommended monitoring Beach topographical survey Sediment sampling Aerial photography

14 Thank you Contact details: A K Theron Tel: 021 888 2511 / 2400 Email: Fax: 021 888 2693 P O Box 320 7599 Stellenbosch

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