Advice

To design the coastal defense a couple of decisions need to be made. In general, coastal defense can consist of the construction of a seawall or a natural sand reef in front of the island. Both are viable options in to defend the island, yet the decision has been made to focus the research solely on the seawall. For more, the decision has been made to exclusively look at the wind head on the island and the design of 1 profile to suit these conditions, this means there will be no optimization of the seawall around the island to allow for a reduction is production costs.

In this piece the design process will be explained, first the requirements will be stated, then we will look at how the height of the seawall corelates to the slope of the outer wall and the material of which the outer wall is constructed. With this as a base line the next main question is asked:

With what data does the seawall of the NSWPH need to be designed, and what profile will be suitable in this situation to successfully defend the Island with as low as costs as possible?

With this as the main question, the next 4 side questions are asked to aid with the research:

  1. What are the factors which will affect the design the seawall?
  2. What is the optimum material to use for the outer seawall, considering the slope and height of the seawall?
  3. What is the optimal slope for the seawall, considering the material and the height of the seawall?
  4. What is the optimal height for the seawall, considering the material and the slope of the seawall?

As visible above side question 2 to 4 are very related to each other, as such, in this piece they will be looked at together, after they are touched briefly.

The more detailed prescription of this research can be found in supplement X.

Requirements and assumptions

As stated above the coast defense consist of a seawall. The design of the seawall will be made with the following requirements and assumptions. All assumptions are made following a literature research or with consultation of the team and the supervisor. These assumptions are:

  • HAT = +1,2m NAP (1,19m), (RAI, 2019),
  • Island height is +5m NAP, (RAI, 2019),
  • Sea-level rise (ZSS) = 1,55m (2020 to 2100), (Deltares, 2018),
  • Waveheight is 6,5m, (RAI, 2019) ,
  • Wave steepnes is 0,08 [-], (Deltares, 2015) ,
  • A berm will be constructed on the mean sea-level of 2100,
  • The seawall will be constructed using xblocplus,
  • The Υf of xblocplus is 0,45, (Jansen, 2018),
  • The Costs of 1 m3 sand is € 5, -, (van Oord, 2019),
  • The Costs of 1 m2 xblocplus is € 230, -,
  • The slope and profile of the seawall is homogeneous.

The detailed prescription of how these assumptions were made or found can be found in supplement X.

Slope

The slope of the outer wall will change the runup of the waves crashing on the seawall, with a steeper slope producing higher wave-runups. Because the material used for the outer wall being very rough, a steeper slope will be more economical because the wave doesn’t run up very high. A lower roughness of the material used for the outer wall may have meant a shallower slope would have been more economical, but this is not the case. The most economical slope for the seawall is 1 to 3, increasing 1 meter in height every 3 meters its deep. This is shown in the calculations producing an exponential curve which get shallow the steeper the slopes get. This curve is shown in figure X1.

Hight

The height of the seawall is determined by calculating the height which 2% of the waves with a height of 6,5m reach. At this level, it is believed a wall will be high enough to protect the inner slope of the seawall. The height calculation also includes the rising sea level and high tide. Interesting enough, the height of the seawall also follows the same curve visualized above, as seen in figure X2.

Berm

In addition to just a simple seawall, the construction of a berm will also be researched, with a berm having a positive influence on the height of the seawall, yet not the costs. Through research it is found that adding 10 meters of berm in length decreases the height needed by 1 meter(if the slope is 1:3), meaning adding 10 meters of land reduces the depth of the slope by only 3 meters. This means that it will always be more economical to not construct a berm, if the seawall is homogenous, meaning the slope in front of the berm is the same as behind and the slope also being the same, this effect is show in figure X3.

Because of the information found above it is advised to construct the seawall using Xblocsplus as a protective layer, with a slope of 1:3 and with crest at the height of 10,84m +NAP. This will produce the cut-through section as shown in figure X4.

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