Seawall! 

Description:  a man-made wall that has a purpose to keep sea waves from getting up onto land.

 Disadvantages: The waves hit the seawall upwards and downwards. The downwards hit moves the sand and the water moves down the seawall, causing it to create the area before the seawall to have a downwards slope. 

 Citation: Learner's Dictionary. 2013. Merriam-Webster, Incorporated. 9 Jan. 2013 <http://www.learnersdictionary.com/search/seawall>.