the Army Corps of Engineers proposed building parallel to shore a breakwater of rocks是倒装,正过来就是the Army Corps of Engineers proposed building a breakwater of rocks that(定从)parallel to shore 这里是因为 breakwater of rocks that(定从)太长所以放到parallel to shore后面了。
In a plan ..., the Army Corps of Engineers(主语)proposed building(谓语)parallel to shore(定语前置,因为宾语太长){a breakwater of rocks 【thatwould rise six feet above the waterline and act as a buffer, (absorbing the energy of crashing waves and protecting the beaches.)】}