Sea Defences Jessica Krauza

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  • Seawalls are located where the cost of further erosion is unacceptable. Where they are used, the toe should be carried down below the long term scour level, and this scour protection should complement the reflection characteristics of the main seawall. The use of highly reflective scour protection [e.g. vertical piles] is a failure of concept, unless this is fronted by a suitable sloping and absorbent scour apron.

    But whatever you do, nothing lasts forever. Whenever you can, make sure the materials you use are reclaimable for re-use.

    Many seawalls that I have designed or been involved with are buried at the back of the beach, at what I call the Capital Protection line, ie, where the Benefit/Cost curve rises rapidly. In some cases, these walls were originally associated with a beach nourishment programme [e.g. Balmoral and Cronulla Beaches, NSW and Lincolnshire Beaches in UK]. When the beach replenishment policy is abandonned, then the seawall design must be revisited before the consequent erosion causes problems. Rock and concrete unit [Seabees Xbloks etc] are all reclaimable, whereas solid walls are not. Gabions should be regarded as a consumable, with a life expectancy limited to only a few design storm exposures. [See galvin park, Towradgi Beach, Cronulla & Bondi Beach installations in NSW]. Regrettably, the Cronulla gabion toe protection was replaced after 20 years with vertical piles, instead of being replaced with a similar sloping non-reflective toe.

    Does any one ever do a full CDM design evaluation on seawalls?
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Sea Defences Jessica Krauza

  1. 1. People Have Tried To Control Flooding And Erosion For Hundreds Of Years. There Are A Number Of Options Available When There Is A Flooding Or Erosion Problem. There Are Many Difficult And Complex Choices About How Best To Reduce Property Damage In The Shore Lines. Here Are A Few Major Ways To reduce Costal Erosion.
  2. 2. Groynes Can Be Made Out Of A Variety Of Different Materials Depending On The Surroundings. They Are Designed To Stop Long Shore Drift And Act to Build Up Beach Material Which Then Helps Protect The Base Of The Cliff. Groynes May Also Stop Materials Travelling To Certain Parts Of The Coast By Preventing Long Shore Drift, Meaning There Is An Increase In Erosion In Some Areas. Wooden Groynes Are Also Made From Old Railway Sleepers. This Particular Type Of Groyne Prevents The Beach Being Carried Away By Long Shore Drift.
  3. 3. Gabions Are Used For Erosion Control. They Are Large Wire Netted Blocks Of Medium Sized Pieces Of Hard Rock. They Are Expensive And Can Be Ugly. Gabion Baskets Have Some Advantages Over Loose Rip Rap Because Of Their Modularity And Ability To Be Stacked In Various Shapes. They Also Have Advantages Over More Rigid Structures Because They Can Conform To Ground Movement, Dissipate Energy From Flowing Water And Drain Freely.
  4. 4. Rip Rap Is Rock Or Other Material Used To Amour Shorelines And Streambeds Against Water, Erosion And Other Coastal Processes Caused By The Sea. It Is Normally Made From Hard Rock Such As Granite Or Concrete Rubble. This Is A Very Expensive Option To The Wooden Defence, Hard Rock Boulders Are Placed On the Coast To Prevent Rapid Erosion. Rip Rap Normally Works By Absorbing The Impact Of A Wave Before It Reaches The Cliff Or Sea Defence And So Minimizes The Erosion Caused By The Wave.
  5. 5. A Sea Wall Is A Form Of Hard And Strong Coastal Defence Constructed On The In Land Part Of A Coast To Reduce The Effects Of Strong Waves. Sea Walls Maybe Constructed From A Variety Of Materials Most Commonly Reinforced Concrete, Boulders, Steel Or Gabions . Additional Sea Wall Construction Materials May Include Vinyl, Wood, Aluminium And Fibreglass Composite. Modern Concretes Tend To Be Curved To Reflect The Wave Energy Back Out To Sea. Poor Designs Require Constant Maintenance As Waves Erode The Base Of The Sea Wall.
  6. 6. Off Shore Break Waters A Structures Constructed On Coasts As Another Form Of Coastal Defence Or To Protect An Anchorage From The Effects Of Weather And Longshore Drift. Off Shore Break Waters Also Called Bulk Heads Reduce The Intensity Of Wave Action In Inshore Waters And There By Reduce Coastal Erosion . They Are Constructed Some Distance Away From The Coast Or Built With One End Linked To The Coast. The Breakwaters Are Small Structures Placed In Relatively Shallow Water. When Waves Hit These Breakwaters There Erosive Power Is Concentrated On These Structures Some Distance Away From The Coast. Rock Or Concrete Armour Units On The Outside Of the Structure Absorb Most Of The Energy While The Wave Energy Is Prevented Continuing Through The Breakwater Core By Gravels Or Sand.
  7. 7. Absorbs wave action. May need replacing £3500 per m Beach nourishment Trap sand if there is any to trap. Steal sand 100m apart from beaches downdraft increasing erosion £5000 each Groynes Protects the base of the cliff. Must be large rocks or will be moved by storms £5000 per m Offshore bar Absorbs the energy of the waves. Over time the rocks break up. Unsightly £4500 per m Rock revetment Very strong, but may reflect waves, causing turbulence and undercutting . Very expensive £6000 per m Sea wall ADVANTAGES and DISADVANTAGES COST DEFENCE

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