Lesley C. Ewing
Gary B. Griggs and Kiki Patsch
The intensively developed southern California coastline from Malibu to the Palos Verdes Peninsula can be divided into two littoral cells, which have both undergone significant but markedly different changes over the past century. The Zuma Cell, extending from Pt. Mugu to Pt. Dume, trends nearly east-west and has relatively little sand input. Continuous littoral transport and a general lack of barriers have led to limited beach development. Modest beach cottages built on the sand in the Broad Beach area of Malibu decades ago have been replaced by very large homes in recent years, but a reduction of littoral sand from the west is now threatening these homes. It appears that much of the sand moving through the Zuma Cell and that was responsible for the beaches was initially provided by leakage around the head of Mugu Submarine Canyon from the upcoast Santa Barbara Littoral Cell. Headward erosion of the canyon has now cut off this sand supply leading to progressive narrowing of the downcoast beaches. The adjacent 24 km of the western portion of the Santa Monica Cell has the same general east-west trend as the Zuma Cell and is also characterized by limited sand input and very narrow beaches. At Pacific Palisades the trend of the shoreline changes almost 90 degrees. The next 32 km of Santa Monica Bay shoreline is oriented almost north-south, nearly parallel to the wave approach, reducing littoral drift rates and allowing wider beaches to develop. Combined with the addition of approximately 23 million m3 of sand added to the Santa Monica Bay beaches from coastal construction and dredging projects over a 60-year span, and a series of sand retention structures, this 32 km stretch of shoreline and the 18.6 million people in the adjacent greater Los Angeles area, have benefitted from wide and generally stable beaches.
Syed M. Khalil, Angelina M. Freeman, and Richard C. Raynie
The Mississippi River Delta Plain (MRDP) in southeast coastal Louisiana exemplifies an ecocatastrophe which was triggered by geological and geophysical processes resulting in the present degradation of the coastal landscape. Between 1932 and 2016, Louisiana has lost approximately 4,800 km2 of coastal land, much of which was lost between 1932 and 1985 (~4,000 km2; ~76 km2/yr). Since 1985, cumulative wetland loss has continued at a rate of ~25 km2/yr (Couvillion et al. 2017). This rate of land loss threatens a range of key national assets and important communities. A Coastal Master Plan for coastal protection and restoration was developed by the Louisiana Coastal Protection and Restoration Authority (CPRA) to mitigate land loss and degradation of the ecosystem and reduce flood risk (CPRA 2017). This multi-decadal, multi-billion-dollar plan prescribes a portfolio of protection and restoration projects to reduce land loss, maintain and restore coastal environments, and sustain communities. The large-scale effort to restore coastal Louisiana is made more challenging by reduced sediment supply from the Mississippi River mostly due to separation of the delta plain from the river by levees, rising sea levels, subsidence, oil and gas activities, local flood-control levees, and navigation infrastructure. Sedimentological restoration to restore geomorphic form is critically important to offset land loss. Many of the environmental conditions that impact land loss are influenced by climate change. With climate change impacts (e.g. sea level rise and increased storm intensity), the quantity of sediment needed for sustainable restoration will increase, while at the same time the accessibility to the sediment resources will become more challenging. Thus, restoration of coastal Louisiana under changing environmental conditions is a two-pronged challenge: 1) restoration and protection projects must be robust enough to counter varying future environmental scenarios; and 2) the quantity of sediment resources available for critical restoration projects is likely to decrease, rendering the dredging and utilization of the sediment resources more difficult and costly. To address these challenges, the Louisiana Sediment Management Plan (LASMP) identifies and delineates potential sediment sources for restoration and provides a framework for managing sediment resources wisely, cost effectively, and in a systematic manner. To this end, there is consensus that replicating natural processes by diverting sediment into wetlands via sediment diversions will be the most effective sediment management practice. Evaluating project design, construction goals and objectives, project monitoring, adaptive management, and future project and policy innovations in all aspects of a coastal restoration program (planning, policy, and implementation) provides an efficient and effective basis for managing sediment resources.
Charles J. Killebrew and Syed M. Khalil
Louisiana’s Coastal Wetlands Protection and Restoration Program, which was designed to restore degrading deltaic wetlands and barrier islands, has matured from inception by early state conservationists decades ago to the present. Now a body of restoration law guides restoration and protection efforts that are manifested in Louisiana’s Coastal Master Plan and federal Water Resources Development Act (WRDA) documents. While developing these plans, managers took advantage of decades of restoration experience that included hydrologic basin-specific restoration measures while accessing modern research from coastal science and engineering. Contemporary research offers various analytical methods including a suite of modeling tools that were unavailable to coastal scientists, managers, and engineers in the past. Over time, these technical advances along with an increased understanding of coastal processes have improved the science and engineering of coastal restoration, and coastal policy and planning have evolved with various legislative initiatives. It should be appreciated that developments of plans/programs over the years have culminated in the development of a much reviewed and refined plan which is available for implementation. This paper reviews some historical aspects of these developments in Louisiana’s efforts to restore coastal Louisiana including the severely deteriorating Mississippi River Delta Plain (MRDP).
James Behrens, Eric Terrill, Julianna Thomas, David Castel, and Richard Seymour
The Coastal Data Information Program (CDIP) recorded detailed information about the waves generated by Hurricane Matthew in October 2016. The wave field generated by the storm was measured by fifteen Datawell Directional Waverider moored buoys in the CDIP system. Significant wave height records and maximum individual waves are the focus of this report. The complete quality-controlled spectral and displacement data sets are publicly available at http://cdip. ucsd.edu.
Jack C. Cox
This discussion presents insight into some of the most basic concepts and premises of coastal engineering which have lost physical significance – or worse, have been allocated undeserving attributes because of the pure application of mathematical calculations without regard to the associated physics. The most classic example is the misuse of the “significant wave height.” The discussion extends further into understanding how these false attributes can lead to serious errors in design. Breakwaters are designed using values of hindcasted significant wave heights substituted into stability formulas developed from monochromatic waves; sediment transport rates are computed for single value wave heights as if that value represents every wave in a storm; wave period sensitive phenomena such as refraction and diffraction, or even the response of floating objects are oversimplified because the effect of a single representative wave period is assumed to describe the full behavior of the sea. In this paper, these same common coastal engineering parameters and relationships are explored to discover how the physics of the nearshore actually work and how that understanding can be used to cure many coastal maladies.
Reviewed by Sumi Selvaraj