Flexible Seawalls: Modified Bulkhead Approach

Seawalls played an important role in protecting harbors and multiple beach properties along the country’s East and West Coasts, coast line of the Gulf of Mexico, and the Great Lakes. Engineers successfully used different types of seawalls, ranging from simple gravity walls to flexible sheet pile walls. Until recently industry interchangeably used the term bulkhead or seawall. The majority of bulkheads can be built as simple flexible sheet pile soil retaining structures, whereas seawalls have to be designed for an additional set of loads that includes large hydrodynamic forces. The difference between two structures becomes apparent in toll seawalls designed for large magnitude wave-generated pressures produced by storm or seismic events. The “Great Storm” that pounded the West Coast in January 1988, was one such events. Significant wave height ($H_s$) along the beach during that event was recorded to be in excess of 16 ft. Waves of such magnitude may carry high energies that can be destructive to the seawall itself. A large percentage of seawalls built in Great Britain required replacement only after 25 years of service. Evolution of seawall design and analysis is based on observation and lessons learned from some past and recent catastrophic events and failures. Another lesson relates to the requirement for wall ductility: Severely distressed areas may develop within the wall length during maximum predicted catastrophic events, but failure will be ductile and prevent the loss of life and loss of protected property. Obviously, wall ductility has a major impact on ease of wall restoration. Given that the desired life span of a major seawall is usually in a range of 50 years, events such as the Great Storm or possibility of tsunami wave during the same life span will be strongly considered for seawall design. The following discussion reviews loads and modified bulkhead approach for design of flexible seawalls.