OS23C-1273
Elevation Uncertainty in Sea-Level Rise Assessments: How Much Land is Really at Risk?
The need for high-quality sea-level rise vulnerability and impact assessments is clear and has been well documented. Elevation is one of the most important parameters that determine the vulnerability of coastal lands to inundation due to storm surge flooding and longer term sea-level rise. In many sea-level rise impact assessments, elevation is often the primary variable that is analyzed to determine vulnerability to adverse effects of rising water levels. The accuracy and resolution with which coastal elevations have been mapped directly affect the reliability and usefulness of sea-level rise impact assessments that rely upon elevation as the foremost geophysical characteristic for determining vulnerability. Maps visualizing coastal areas subject to potential inundation or other adverse effects of sea-level rise have great appeal to planners and land managers who are charged with adapting to or reducing the risks. Likewise, sea-level rise assessments often include statistical summaries of population, infrastructure, and economic activity in the mapped impact zone because this information is critical for the mitigation efforts. Many studies have necessarily used elevation data to delineate potential impact zones and quantify effects. These studies generally have not considered the inherent vertical uncertainty of the elevation data used to delineate vulnerable lands. An analysis of four different elevation datasets, including lidar, that have been commonly used for mapping vulnerable coastal lands demonstrates that vertical uncertainty (elevation error) must be considered quantitatively to derive reliable estimates of land areas subject to potential inundation. Maps that include a spatial representation of the uncertainty associated with a given sea-level rise projected onto the land can then be produced. Statistical summaries of impacted population and other socioeconomic variables within the mapped inundation zone also benefit by incorporating the elevation uncertainty information. Because the coast is a highly dynamic environment subject to complex interactions among numerous physical processes and parameters, inundation will be the primary response to sea-level rise in only some coastal locations. For those areas, elevation is the most important factor in assessing vulnerability. However, the challenge remains to be able to quantify the various effects of sea-level rise and to identify the areas and settings along the coast in which inundation will be the dominant coastal change process.
OS23C-1274
California Shoreline Sand Retention: Existing Structure Performance and Future Potential
Amidst rising sea level, climate change and expanding coastal populations, sandy beaches are frequently exposed to erosional processes. Effective sea level rise will lead to recreational beach loss as a result of coastal inundation. Beach nourishment is growing in popularity as a mitigation approach to meet the increasing need to protect coastal resources. The practice of beach nourishment along high energy shorelines, such as in California, is often improved by the construction of sediment retention structures (groins) to enhance project lifespans. However, our current ability to design effective littoral barriers is extremely limited. An underutilized and cost-effective resource for critically analyzing engineered retention structure performance is the record of existing structures within California. The impacts of 205 structures along California's 1700 km shoreline have been systematically explored though measurements collected from aerial imagery and historic shoreline positions. The findings of this study suggest that approximately 30 million m3 of sand and 18% of California's total exposed sandy beach area is presently retained in fillet and salient beaches associated with man-made structures such as groins, breakwaters, piers and jetties. Preliminary results suggest statistically significant correlations between structure effectiveness and key characteristics such as orientation, littoral cell position and construction materials. The central product of this study is a complete and robust GIS catalog of retention structures along California's coastline. A detailed analysis of historic structure performance combined with a systematically measured record of structure characteristics for the entire state results in a useful product to help coastal planners use the lessons of the past to plan future beach management.
OS23C-1275
Two Sea-Level Challenges
"No place on the sandy ocean shores of the world has been shown to be eroding because of sea level rise." This statement appeared nearly 19 years ago in bold print at the top of the page in a brief article published in Shore and Beach (Galvin,1990). The term "sea level rise" was defined in 1990 as follows: "In this statement, "sea level rise" has the meaning that the average person on the street usually attaches to that term. That is, sea level is rising; not, as in some places like the Mississippi River delta, land level is sinking." While still a subject of controversy, it is now (2008) increasingly plausible (Tornqvist et al,2008) that damage from Hurricane Katrina was significantly worse on the Mississippi River delta because floodwaters exploited wetlands and levees whose elevations had been lowered by decades of compaction in the underlying soil. (1) "Sea level" commonly appears in the literature as "relative sea level rise", occurring that way in 711 publications between 1980 and 2009 (GeoRef database on 8 Sep 08). "Relative sea level rise" does not appear in the 2005 AGI Glossary. The nearest Glossary term is "relative change in sea level", but that term occurs in only 12 publications between 1980 and 2009. The Glossary defines this term in a sequence stratigraphy sense, which infers that "relative sea level rise" is the sum of bottom subsidence and eustatic sea level rise. In plain English, "relative sea level rise" means "water depth increase". For present day coastal environments, "relative sea level rise" is commonly used where eustatic sea level rise is less than subsidence, that is, where the magnitude of actual sea level rise is smaller than the magnitude of subsidence. In that situation, "relative sea level rise" misleads both the average person and the scientist who is not a coastal geologist. Thus, the first challenge is to abandon "relative sea level rise" in favor of "water depth increase", in order that the words accurately descibe what happens. It would further clarify popular understanding if the term "actual sea level rise" were used in place of "eustatic sea level rise". (2)Geologists have approximated the the practice of paleontologists and biologists in establishing type examples of important geological features. This is a useful practice. A graduate geologist holds in mind clear conceptions of "beach cusps", "drumlin fields", "birdfoot deltas", and "igneous sills" based on seeing field examples accepted by professional geologists as representative of these features. However, although publications frequently report that sea level rise erodes a particular beach, no one identifies a type beach where that cause has been proven to produce the alleged effect. At the type beach, it is necessary to show that sea level is rising, and that the beach erodes primarily from this sea level rise, rather than from interrupted longshore transport. Thus, the second challenge is to identify a type ocean beach proven to erode because of sea level rise.
OS23C-1276
Protecting Coastal Areas from Flooding by Injecting Solids into the Subsurface
Subsidence and sea level rise conspire to increase the risk of flooding in coastal cities throughout the world, and these processes were key contributors to the devastation of New Orleans by hurricane Katrina. Constructing levees and placing fill to raise ground elevations are currently the main options for reducing flooding risks in coastal areas, and both of these options have drawbacks. We suggest that hydromechanical injection of solid compounds suspended in liquid can be used to lift the ground surface and thereby expand the options for protecting such coastal cities as New Orleans, Venice, and Shanghai from flooding. These techniques are broadly related to hydraulic fracturing and compensation grouting, where solid compounds are injected as slurries and cause upward displacements at the ground surface. The equipment and logistics required for hydromechanical solid injection and ground lifting are readily available from current geotechnical and petroleum operations. Hydraulic fractures are routinely created in the upper tens of meters of sediments, where they are filled with a wide range of different proppants for environmental applications. At shallow depths, many of these fractures are sub-parallel to the ground surface and lift their overburden by a few mm to cm, although lifting is not the objective of these fractures. Much larger, vertical displacements, of the order of several meters, could be created in low-cohesion sediments over areas as large as square kilometers. This would be achieved as a result of multiple injections. Injecting solid particulates provides the benefits of a permanent displacement supported by the solids. We have demonstrated that hydraulic fractures will lift the ground surface at shallow depths in Texas near the Sabine River, where the geological setting is generally similar to that of New Orleans (and where, incidentally, hurricane Rita landed in 2005). In these regions, the soft surficial sediments are underlain by relatively stiff Pleistocene deposits, which create in-situ stress conditions favorable for sub-horizontal orientation of hydraulic fractures. Based on the poroelastic effect, these conditions can further be improved by subsurface manipulations of pore fluid. Also, there are many geological examples of natural, sub- horizontal hydraulic fractures. These include multiple igneous sills (e.g., Henry Mountains, Utah) and sand- filled sills intruded into sedimentary formations (e.g., Shetland-Faroe Islands). Techniques that are currently used, or planned, for protecting coastal cities from flood are typically based on the concept of a barrier to the seawater (e.g., levees or water gates). However, the failure of any barrier to flood waters can be catastrophic when the city it protects is below sea level. Hydromechanical injection of solid compounds could permanently lift elevations above a Category 5 hurricane surge, so the risk of a catastrophic failure and subsequent flooding becomes insignificant. We envision that the hydromechanical method can be used in combination with other strategies. For example, in some areas it may be efficient to let most of a city retreat and only lift localized regions of particularly high value, such as airports, port facilities, refineries, historical areas, military bases, etc. In other cases, the protecting equipment itself may begin subsiding (e.g., massive, metal water gates on a soft-sediment foundation). Then, hydromechanical injections could be used to lift the region supporting this equipment.
OS23C-1277
Dutch Perspective on Coastal Louisiana Flood Risk Reduction and Landscape Stabilization
Strategies were analyzed for long-term flood risk reduction in coastal Louisiana and for strengthening the natural ecosystem functions of the Mississippi Delta, aimed at stabilizing the landscape. This was done as an independent, external contribution to the ongoing planning in the Louisiana Coastal Protection and Restoration Project (LACPR). A cost-benefit analysis was carried out and led to the conclusion that it is economically justified to provide flood protection to the city of New Orleans against water levels with a probability of 1/1,000 per year, which is considerably higher than the existing protection level. Regarding landscape stabilization, a series of options were identified to not only stabilize the remaining wetlands in the Mississippi Delta, but also to create new wetlands. The role of wetlands in hurricane surge level reduction and wave attenuation provides a link between the issues of flood risk reduction and the degradation of the delta ecosystem. Several alternative strategies were designed to illustrate the available options. These strategies include an open system, a semi-open system and a closed system, with gates that can be closed during hurricanes. Based on the characteristics and impacts of these strategies the project team formulated a Preferred Strategy, with total costs estimated at $20 billion
OS23C-1278
Improvement of Armor Stone Performance for Protection of Great Lakes Coastal Navigation Areas
Evaluating long-term performance and deterioration of armor stones are essential for maritime structures to protect harbors or navigable areas. Armor rocks are impacted by the natural elements such as seasonal weather, and repeated cycles of temperature (e.g., flowing water, wetting and drying, wave action, freeze and thaw, etc.). The rock's behavior in the field may vary greatly from the laboratory test results. The design process for the determination of armor stone sizes is complex and various factors must be considered in order to fully understand how the design parameters have an indirect effect on stone performance. Numerous investigators have studied to develop relationships for the minimum stable weight of a rubble-mound armor unit for given wave conditions. The main objective of this study has been to evaluate major factors affecting the armor stone durability. The effects of scaling on the test results of various samples of rock types used in Great Lakes coastal projects have been investigated. To consider the combined effects of environmental stresses on armor stone, testing have been done to evaluate the performance of stone subjected to both freezing and thawing and wetting and drying. The stone quarries and sites were evaluated and sampled to determine the stone sources, and their surrounding environments. Long-term performance or deterioration of armor stones have been quantitatively monitored and characterized by the changes in dimensions measured. A degradation numerical model has been developed that relates the laboratory test results to the modification of the mass distribution and reduction at the project site. The paper presentation will describe and illustrate the latest results and developed tools for the armor stone evaluations. We will introduce new approaches that may be used to evaluate the quality and durability with reference to breakage and integrity.
OS23C-1279
The Study on Coastal Buffer Zone for Conception of Coastal Disaster and Protection
The purpose of this study is to establish basic principles for setting up coastal buffer zones. Following concepts of hydrodynamics, nearshore and tidal currents, and sand drifting, the coastal buffer zones on ocean and land interfacial areas can be defined. Numerical models for coastal waves, current, and topography are used to simulate coastal phenomena and determine parameters describing buffer zones. The proposed coastal buffer zone is applied to Haomeiliao coast in Chiayi. Using the SBEACH model together with conditions of 50-year recurrence period of Typhoon waves and average maximum tides, variations of coastal profiles are calculated. Dynamic simulations for various coastal profiles and analysis of physical quantities related to beach changes, together with calculations from run-up model, the coastal buffer zones on ocean and land interfacial area are then determined. The results are also referred to current coastal resources and environmental affecting parameters for overall discussions. The proposed definition, methodology, and procedure for coastal buffer zone can be practical for related hydraulic institutions in applying the concept in the future.