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Environmental Issues - Coastal Land Loss Contributors: Penland, Beall, Britsch, Williams Introduction
The dramatic loss of Louisiana's coastal wetlands and barrier shorelines is well recognized by government agencies, industries, universities and the public. Between 1932 and 1990, the deltaic plain of the Mississippi River lost over 680,000 acres of land due to a complex suite of causes. Controversy and debate continue about the causes of coastal land loss in Louisiana. Human contributions to the land loss problem is estimated to be between 10 percent and 90 percent (Britsch and Kemp, 1990; Penland et al., 1990; Penland et al., 1992; Turner, 1997). Several government agencies and industries have been targeted as the primary cause of coastal land loss, including the U.S. Army Corps of Engineers (USACE) and the oil and gas industry. Multiple causality, including natural processes, has been largely overlooked (Boesch et al., 1994). In an effort to further our understanding and knowledge of the coastal land loss problem in Louisiana, the Gas Research Institute (GRI) sponsored a research project through the Argonne National Laboratory (ANL) entitled Natural and Human Causes of Coastal Land Loss in Louisiana. The study team consisted of scientists from GRI, ANL, Louisiana State University (LSU), UNO, USGS, USACE and the Louisiana Universities Marine Consortium (LUMCON). This study focused on three major land loss research tasks: 1) Geologic Processes, 2) Vegetative Processes and 3) Geographic Information System (GIS) Analysis.
Through these research tasks, the objectives of this study were to quantify and rank the causes of coastal land loss within the Mississippi River delta plain in southeastern Louisiana. This study took advantage of continuing research by the USGS and USACE in framework geology and subsidence processes, GIS analysis, framework geology and subsidence processes (Dunbar et al., 1990; Dunbar et al., 1992; Britsch and Dunbar, 1993; Williams et al., 1993). The geological process task focused on the Holocene evolution of the Mississippi River delta plain in an effort to identify the regional geological controls on coastal land loss over the last 18,000 years. The vegetative process task conducted field investigations into the role of saltwater intrusion and soil inundation in plant dieback. The GIS analysis task focused on quantifying the geomorphic forms and processes of coastal land loss using the USACE coastal land loss database. The following two pages present the results of the GIS geomorphic classification and process classification of coastal land loss for the LPB. Geographic Information Systems Analysis
The GIS analysis captured the local types and causes of coastal land loss interwoven with regional land loss processes like subsidence. The GIS analysis highlighted coastal land loss hot spots and change trends in the land loss pattern. Processes such as flood control, diversion control, subsidence, and eustacy generally lack spatial attributes that can be mapped and used in a GIS analysis. As a result, the GIS analysis allowed the quantification of site-specific processes and it did not capture the regional effects of subsidence, eustacy, and river control.
Much of the coastal land loss controversy can be attributed to a lack of spatial quantitative land loss data. Recent land loss data collection efforts undertaken by the USACE have served to address this need for information by providing maps and statistics that can be used to characterize baseline conditions of land loss in Louisiana. The purpose of the GIS analysis task was to expand upon baseline data collection efforts by providing quantitative information about land loss geomorphology and process. We have developed a classification scheme capable of isolating the geomorphologically distinct forms of land loss (Wayne et al., 1993). The land loss data used within this GIS task provided by the USACE were first published in an atlas entitled Geological Investigation of the Mississippi River Deltaic Plain: Land Loss and Land Accretion. Three subsequent technical reports used the data to establish rates of land loss in (May and Britsch, 1987). The data were provided to the classification research team in digital format and include the following information for the Mississippi River deltaic plain: 1) 1932 land/water interface base map compiled from National Ocean Service (formerly U.S. Coast and Geodetic Survey) topographic sheets (NOS T-sheets) and 1:62,500 USGS topographic quadrangle maps; 2) areas that converted from land to water in each of four time periods (1932-1956/8, 1956/8 - 1974, 1974-1983 and 1983-1990); 3) coding, which discriminates features for each time period.
The data were originally developed by the USACE to: 1) map the location of land loss in coastal Louisiana; 2) quantify the spatial and temporal magnitude of land loss between 1932 and 1990; 3) identify significant historical trends in Louisiana land loss rates. The mapping was accomplished by comparing 1:62,500 scale aerial photography from each study period with the land loss base developed for the previous time period. Land loss was defined as the convergence of land in the base map to water on the photography. NOS T-sheets served as the primary base. However, early USGS1:62,500 topographic maps were used for those areas where T-sheet coverage was unavailable. Mapping was performed for each quadrangle map unit within the Mississippi River delta plain. Land loss statistics were generated for each map then compiled to produce a land loss rate curve for the entire deltaic plain. The USACE study of land loss rates resulted in the generation of a large, detailed land loss data set. To achieve the objectives of the GIS task, a single time period of data for classification was utilized. The cumulative time period (1932-1990) was selected for two primary reasons: 1) it contained the most diverse land loss conditions and therefore provided the best means of evaluating the range of applicability of the classification schemes; 2) the interim data could be used to understand the processes affecting the loss, and enable researchers to better refine the classification for complex loss scenarios.
The USACE land loss data set was carefully reviewed to derive initial concepts of loss geomorphology and processes. A mosaic of the fifty maps was created on a single wall of the laboratory and used as reference during a series of open discussions in which similarities in land loss configurations were identified and evaluated. Additional information was compiled about land loss processes and landscape activities (cultural and natural) associated with individual areas of loss. This information was used to generate process scenarios for highly expressive land loss formations. Once a familiarity with the regional data set was acquired, a series of examples was extracted to illustrate rough concepts of similarity and disparity with regard to land loss geomorphology and process.
These basic concepts were presented to a group of agencies, organizations, companies and experts. An advisory committee was created as part of the classification study and was composed of scientists from the university community, state and federal government and private business with backgrounds in sedimentology, marsh ecology, coastal geology, wildlife biology, vegetative dynamics and coastal management. The advisory committee provided regional and disciplinary insight and responded to the conceptual presentation by generating a list of terms that more specifically characterized differences in form and process. The terms were organized into logical groups of process and geomorphology and the groups were refined into initial classification schemes.
Once the initial classification schemes were derived, the advisory committee reviewed the schemes and provided critical comments. The comments were used to refine the classification schemes. Several land loss committee meetings were held prior to establishing the final land loss geomorphology and process classification schemes. Geomorphological Classification
The geomorphological classification is intended to capture information about the physical form of land loss areas. Development of the geomorphological classification scheme was based upon two fundamental observations: 1) areas of land loss are, by definition, water; 2) morphology cannot imply action or process. As a result, the derived scheme employed morphological parameters commonly associated with the description of water bodies while avoiding process-oriented qualifiers. For example, the term "erosional shadow" aptly describes the linear loss patterns that occur in the lee of engineering structures. However, the term also imparts specific information about the process that may have caused the land loss and therefore was not appropriate for the geomorphological classification. There were two levels to the geomorphological classification hierarchy. The first level loss type, called shoreline, applied to loss areas that occur relative to existing waterbodies. The second level, called interior, applied to loss areas occurring independent of existing waterbodies. Shoreline loss areas are typically curvilinear, mirror the morphology of the previous shoreline and have a large ratio of shoreline length to total waterbody area. Interior loss areas are new waterbodies that develop within the land mass and may vary in form from linear to rounded. Interior areas may also occur adjacent to existing waterbodies, but the ratio of shoreline length to total area is smaller than that of shoreline loss areas.
The next level of the hierarchy addressed the waterbody type most closely related to loss. For shoreline areas, this level depicted the type of waterbody physically related to the loss. Four types of shoreline loss were established: 1) Gulf: the outer shoreline facing the Gulf of Mexico; 2) Bay: semi-enclosed waterbody with direct contact to the Gulf of Mexico; 3) Lake: enclosed or semi-enclosed waterbody with no direct contact to the Gulf of Mexico; 4) Channel: linear waterbody that commonly connects other waterbodies.
For interior areas, this level of the classification depicted the waterbody type that was most similar to new interior loss. Two interior classes were established: 1) Pond: enclosed or semi-enclosed waterbody with minor connections to the existing drainage network; 2) Channel: narrow, linear waterbody. PROCESS CLASSIFICATION
Land loss is typically the result of complex interactions among natural and human activities upon the landscape. Therefore, it is difficult to isolate an activity as the singular cause of a specific area of land loss. However, general assumptions can be made for most areas regarding the primary physical process that removed or submerged the land, as well as the primary actions that initiated the process. By employing a classification scheme that graduates from general land loss process to specific cultural and natural landscape activities, each loss area was specifically classified as the available information and scientific consensus allow. The process classification scheme is described below.
The first level of the classification addresses the basic processes of land loss. For purposes of this classification scheme, the term land was defined as all subaerial materials including surface vegetation, sediments and organic soils. Three primary land loss processes were identified: 1) Erosion: physical removal and transport of land by water action; 2) Submergence: increase of water level relative to ground surface elevation: 3) Direct removal: physical removal of land by actions other than water.
The second level of the process classification scheme identifies the primary actions that were associated with each loss process. This level of the classification included both natural and cultural actions.
The actions of erosion included: 1) Natural Waves: wind generated waves; 2) Navigation Waves: waves generated by boat wakes; 3) Channel Flow: suspension and conveyances by water.
The actions of submergence included:
The actions of direct removal included:
- Altered Hydrology Impoundment: submergence due to an impoundment levees;
- Altered Hydrology Oil/Gas: submergence due to presence of oil/gas channels;
- Altered Hydrology Roads: submergence due to presence of roads;
- Altered Hydrology Navigation: submergence due to presence of navigation channels;
- Altered Hydrology Multiple: submergence due to multiple causes of hydrologic alteration;
- Faulting: submergence due to active faulting;
- Natural Water Logging: submergence due to natural subsidence;
- Failed Reclamation: submergence due to flooding of former reclamation projects which have subsided;
- Herbivory: submergence due to animals eating the marsh followed by substrate collapse.
This level of the process classification scheme identified a diverse category of information including natural and cultural events, activities, and structures. Natural actions included phenomena such as wind, subsidence, or faulting. Cultural actions included human activities such as navigation, channel dredging, building of impoundments, resource extraction, and excavation of ponds.
- Oil/Gas Channels: dredging and/or surface excavation;
- Navigation Channels: dredging and/or surface excavation;
- Drainage Channels: dredging and/or surface excavation;
- Sewage Ponds: surface excavation;
- Borrow Pits: surface excavation;
- Burned Areas: fire;
- Agricultural Ponds: surface excavation;
- Access Channels: dredging and/or surface excavation.
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