The pier or groin is interfering with the movement of sediment down current. As sand moves south, it is trapped by the groin. As a result, sand accumulates on the north side of the structure, but on the south side sand continues to move south. However, sand that would normally replace sediment eroding away to the south of the groin is trapped on the north side of the structure. Consequently, erosion causes the sediment or beach to be lost on the south side of the groin. In situations such as this, sand must be added on the down current side of the groin or transferred from the up current to the down current side of the structure in order to maintain the beach there. Owners of beach front property sometimes build groins to preserve their beach. As may now be apparent, protection of one property with groins will cause down current neighbors to have beach erosion problems. If the movement of sediment is halted, down current areas will be deprived of sediment and erosion will result unless “corrective action” (the addition of yet more structures or beach nourishment) is taken. As you can see, shorelines are dynamic, and that results in serious problems when people attempt to stabilize the shoreline with engineered structures without considering the consequences to neighboring properties.
Something else to consider
In reality, beach shorelines are not perfectly straight, but are irregular with bays, headlands, and river mouths. Waves and littoral transport help to straighten these shorelines in part by making peninsulas called “spits”, fingerlike projections of the beach that extend into open water (Figure 4). If a spit continues to grow and extends totally across a bay, a baymouth bar forms (Figure 5). As can be seen in the figures below, the orientation of the spit indicates the dominant direction of littoral transport. For example, in Figures 4 and 5 littoral transport is from north to south, so the spit builds out from north to south.