22 research outputs found
Under-Reporting of Road Traffic Mortality in Developing Countries: Application of a Capture-Recapture Statistical Model to Refine Mortality Estimates
Road traffic injuries are a major cause of preventable death in sub-Saharan Africa. Accurate epidemiologic data are scarce and under-reporting from primary data sources is common. Our objectives were to estimate the incidence of road traffic deaths in Malawi using capture-recapture statistical analysis and determine what future efforts will best improve upon this estimate. Our capture-recapture model combined primary data from both police and hospital-based registries over a one year period (July 2008 to June 2009). The mortality incidences from the primary data sources were 0.075 and 0.051 deaths/1000 person-years, respectively. Using capture-recapture analysis, the combined incidence of road traffic deaths ranged 0.192β0.209 deaths/1000 person-years. Additionally, police data were more likely to include victims who were male, drivers or pedestrians, and victims from incidents with greater than one vehicle involved. We concluded that capture-recapture analysis is a good tool to estimate the incidence of road traffic deaths, and that capture-recapture analysis overcomes limitations of incomplete data sources. The World Health Organization estimated incidence of road traffic deaths for Malawi utilizing a binomial regression model and survey data and found a similar estimate despite strikingly different methods, suggesting both approaches are valid. Further research should seek to improve capture-recapture data through utilization of more than two data sources and improving accuracy of matches by minimizing missing data, application of geographic information systems, and use of names and civil registration numbers if available
Accretion and Vegetation Community Change in the Wax Lake Delta Following the Historic 2011 Mississippi River Flood
During the 2011 Mississippi River flood, discharge to the lower river exceeded that of the 1927 and 1937 floods and the lower river remained above flood stage for nearly 2 months. A combination of WorldView-2 and Land Satellite 5 Thematic Mapper (Landsat 5 TM) imagery was used to assess the impact of this flood event on the Wax Lake Delta, one of few areas where the river is building new land. Vegetation community change was mapped from 2010 to 2011 and related to elevation change using plant species elevation distributions calculated from light detection and ranging (LIDAR) data. Changes in the land area in the delta were also assessed by regressing land area against water level for a series of pre- and postflood Landsat 5 TM images. The results indicate a net growth of 6.5 km2 at mean water level and 4.90 km2 at mean sea level. Areal gains were greatest at high water levels, indicating substantial vertical accretion across the subaerial delta. At least 8.7 km2, or 31.8%, of the area studied converted to a higher-elevation species. The most change occurred at low elevations with conversion from fully submerged aquatic vegetation to Potamogeton nodosus and Nelumbo lutea. Conversion to lower-elevation species occurred across 3.4 km2, or 12.8% of the study area, while 55.5% remained unchanged. The results highlight the importance of infrequent, large flood events in the maintenance of river deltas and provide a reference for estimating the impact of proposed large-scale river diversions on the Mississippi River Delta
Accretion and Vegetation Community Change in the Wax Lake Delta Following the Historic 2011 Mississippi River Flood
During the 2011 Mississippi River flood, discharge to the lower river exceeded that of the 1927 and 1937 floods and the lower river remained above flood stage for nearly 2 months. A combination of WorldView-2 and Land Satellite 5 Thematic Mapper (Landsat 5 TM) imagery was used to assess the impact of this flood event on the Wax Lake Delta, one of few areas where the river is building new land. Vegetation community change was mapped from 2010 to 2011 and related to elevation change using plant species elevation distributions calculated from light detection and ranging (LIDAR) data. Changes in the land area in the delta were also assessed by regressing land area against water level for a series of pre- and postflood Landsat 5 TM images. The results indicate a net growth of 6.5 km2 at mean water level and 4.90 km2 at mean sea level. Areal gains were greatest at high water levels, indicating substantial vertical accretion across the subaerial delta. At least 8.7 km2, or 31.8%, of the area studied converted to a higher-elevation species. The most change occurred at low elevations with conversion from fully submerged aquatic vegetation to Potamogeton nodosus and Nelumbo lutea. Conversion to lower-elevation species occurred across 3.4 km2, or 12.8% of the study area, while 55.5% remained unchanged. The results highlight the importance of infrequent, large flood events in the maintenance of river deltas and provide a reference for estimating the impact of proposed large-scale river diversions on the Mississippi River Delta
Floods and cold front passages: Impacts on coastal marshes in a river diversion setting (Wax Lake Delta Area, Louisiana)
Diversion of the Mississippi River is considered the most effective way of offsetting Louisiana\u27s coastal plain land loss. The Wax Lake delta, a man-made diversion that represents mostly sand-rich distributary mouth bar deposits, is cited as a model for land building from a large river diversion. The fine-grained suspended load is mostly deposited outside of the delta. Numerical simulations of diversion delta building focus primarily on sand deposition. The fine-grained suspended load is only secondarily considered. Research results reported here suggest that natural processes associated with the synergistic relationship between floods and cold front passages can effectively distribute suspended sediments to maintain and rebuild wetlands outside the sand-rich delta. Measurements (fixed acoustic Doppler current profilers) of current speed, current direction, and water level in a mainland marsh channel, north of Atchafalaya Bay, indicate that as cold fronts approach, local water levels can be elevated by as much as 1 m, and normal discharge into the bay is reversed so that flow is into the coastal plain marshes. LIDAR bank elevations indicate that overbank flow is initiated by these events. Streamside sediment accretion (2-y monitoring period) using feldspar plots indicates up to 6.8 cm. Longer term Cs-137 data reveal an average sediment accretion rate of \u3e1 cm y over 50+ years. Marsh flooding results in denitrification of nitrate in river water plus nutrient uptake resulting in increased plant productivity, considerably greater than for similar coastal plain marshes free of riverine sediment input. Fine-grained suspended sediments are important for rebuilding Louisiana\u27s coastal plain, which is naturally constructed mostly of riverine silt and clay plus organic matter. -
Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain
River deltas all over the world are sinking beneath sea-level rise, causing significant threats to natural and social systems. This is due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and sea-level rise, posing an immediate threat to the 500-1,000 million residents, many in megacities that live on deltaic coasts. The Mississippi River Deltaic Plain (MRDP) provides examples for many of the functions and feedbacks, regarding how human river management has impacted source-sink processes in coastal deltaic basins, resulting in human settlements more at risk to coastal storms. The survival of human settlement on the MRDP is arguably coupled to a shifting mass balance between a deltaic landscape occupied by either land built by the Mississippi River or water occupied by the Gulf of Mexico. We developed an approach to compare 50 % isopleths ( is ratio of land to water) across the Atchafalaya and Terrebonne Basins to test landscape behavior over the last six decades to measure delta instability in coastal deltaic basins as a function of reduced sediment supply from river flooding. The Atchafalaya Basin, with continued sediment delivery, compared to Terrebonne Basin, with reduced river inputs, allow us to test assumptions of how coastal deltaic basins respond to river management over the last 75 years by analyzing landward migration rate of 50 % isopleths between 1932 and 2010. The average landward migration for Terrebonne Basin was nearly 17,000 m (17 km) compared to only 22 m in Atchafalaya Basin over the last 78 years ( \u3c 0.001), resulting in migration rates of 218 m/year (0.22 km/year) and \u3c0.5 m/year, respectively. In addition, freshwater vegetation expanded in Atchafalaya Basin since 1949 compared to migration of intermediate and brackish marshes landward in the Terrebonne Basin. Changes in salt marsh vegetation patterns were very distinct in these two basins with gain of 25 % in the Terrebonne Basin compared to 90 % decrease in the Atchafalaya Basin since 1949. These shifts in vegetation types as ratio decreases with reduced sediment input and increase in salinity also coincide with an increase in wind fetch in Terrebonne Bay. In the upper Terrebonne Bay, where the largest landward migration of the 50 % ratio isopleth occurred, we estimate that the wave power has increased by 50-100 % from 1932 to 2010, as the bathymetric and topographic conditions changed, and increase in maximum storm-surge height also increased owing to the landward migration of the ratio isopleth. We argue that this balance of land relative to water in this delta provides a much clearer understanding of increased flood risk from tropical cyclones rather than just estimates of areal land loss. We describe how coastal deltaic basins of the MRDP can be used as experimental landscapes to provide insights into how varying degrees of sediment delivery to coastal deltaic floodplains change flooding risks of a sinking delta using landward migrations of 50 % isopleths. The nonlinear response of migrating isopleths as wind fetch increases is a critical feedback effect that should influence human river-management decisions in deltaic coast. Changes in land area alone do not capture how corresponding landscape degradation and increased water area can lead to exponential increase in flood risk to human populations in low-lying coastal regions. Reduced land formation in coastal deltaic basins (measured by changes in the land:water ratio) can contribute significantly to increasing flood risks by removing the negative feedback of wetlands on wave and storm-surge that occur during extreme weather events. Increased flood risks will promote population migration as human risks associated with living in a deltaic landscape increase, as land is submerged and coastal inundation threats rise. These system linkages in dynamic deltaic coasts define a balance of river management and human settlement dependent on a certain level of land area within coastal deltaic basins ()
Floods and Cold Front Passages: Impacts on Coastal Marshes in a River Diversion Setting (Wax Lake Delta Area, Louisiana)
Diversion of the Mississippi River is considered the most effective way of offsetting Louisiana\u27s coastal plain land loss. The Wax Lake delta, a man-made diversion that represents mostly sand-rich distributary mouth bar deposits, is cited as a model for land building from a large river diversion. The fine-grained suspended load is mostly deposited outside of the delta. Numerical simulations of diversion delta building focus primarily on sand deposition. The fine-grained suspended load is only secondarily considered. Research results reported here suggest that natural processes associated with the synergistic relationship between floods and cold front passages can effectively distribute suspended sediments to maintain and rebuild wetlands outside the sand-rich delta. Measurements (fixed acoustic Doppler current profilers) of current speed, current direction, and water level in a mainland marsh channel, north of Atchafalaya Bay, indicate that as cold fronts approach, local water levels can be elevated by as much as 1 m, and normal discharge into the bay is reversed so that flow is into the coastal plain marshes. LIDAR bank elevations indicate that overbank flow is initiated by these events. Streamside sediment accretion (2-y monitoring period) using feldspar plots indicates up to 6.8 cm. Longer term Cs-137 data reveal an average sediment accretion rate of \u3e1 cm y over 50+ years. Marsh flooding results in denitrification of nitrate in river water plus nutrient uptake resulting in increased plant productivity, considerably greater than for similar coastal plain marshes free of riverine sediment input. Fine-grained suspended sediments are important for rebuilding Louisiana\u27s coastal plain, which is naturally constructed mostly of riverine silt and clay plus organic matter. -