Co-evolution of wetland landscapes, flooding, and human settlement in the Mississippi River Delta Plain

Basic information

• Case ID: INT-063-1
• Intervention type: Management
• Intervention description:

  (Intervention) a major connection to river sediment supply was maintained in Atchafalaya Bay, where an outlet with Mississippi River was constructed, known as the Old River Control Structure. These flood control structures were designed to provide a floodway to the coast as an ‘‘outlet’’ that would help to protect urban centers downstream at Baton Rouge and New Orleans from flooding conditions. In turn natural wetland delta vegetation has expanded in this basin. This is the only region of coastal Louisiana that is building deltaic wetlands and confirms the ability of the river to sustain delta landscape if sediment delivery is allowed to occur across the coastal floodplain. Wetlands have colonized the emerging lands of the Atchafalaya and Wax Lake Deltas, including Sagitarria platyphylla as the dominant vegetation in the summer and fall. Older lobes of the Wax Lake Delta have a mixed community composed of Colocasia esculenta, Phragmites australis, Polygonum punctatum, Typha spp., Schoenoplectus spp., and Zizaniopsis miliacea. Salix nigra is the dominant vegetation present at levees of the older lobes, with an understory of C. esculenta and P. punctatum (Johnson et al. 1985; Shaffer et al. 1992; Holm and Sasser 2001). Marine and estuarine ecosystems have become less prominent as salinities decrease, and freshwater ecosystems expanded in this coastal deltaic basin, as has been observed in Fourleague Bay over the last several decades (Madden et al. 1988)

• Landscape/sea scape ecosystem management: Yes

• Climate change impacts	Effect of Nbs on CCI	Effect measures
  Coastal erosion	Positive	1. change in location of 50% L:W isopleths (Land-water) using satellite imagery We developed a technique to more clearly define the location of the 50 % L:W isopleths using query methods of images available in 1932, 1973, 1999, and 2010. We extended our analysis of the 50 % L:W isopleth across the Atchafalaya and Terrebonne Basins to compare landscape behavior over the last six decades with and without a significant sediment input. comparing the Atchafalaya Basin, with continued sediment delivery, with Terrebonne Basin, with reduced river inputs We can estimate a landward migration rate of the 50 % L:W isopleths in Terrebonne Basin of about 218 m/year (0.22 km/year) compared to \0.5 m/year in Atchafalaya Basin (Table 1; Fig. 6). 2. estimates of landward migration of sea (based on field transect data) between 1932 and 2010 Based on L:W isopleth changes over time (indicating landward migration) they derived, using equations, changes impacts on wind fetch, wave power, and storm surge L:W (land-water ratio) decrease increases flood risk (derived from model) L:W decrease increases wind fetch (derived from model) L:W decrease increases wave power and storm surge height (derived from model) I did not code for wind fetch however in climatic impacts because that is not described in relation to High winds damage HWD
  Storm surge	Positive	1. change in location of 50% L:W isopleths (Land-water) using satellite imagery We developed a technique to more clearly define the location of the 50 % L:W isopleths using query methods of images available in 1932, 1973, 1999, and 2010. We extended our analysis of the 50 % L:W isopleth across the Atchafalaya and Terrebonne Basins to compare landscape behavior over the last six decades with and without a significant sediment input. comparing the Atchafalaya Basin, with continued sediment delivery, with Terrebonne Basin, with reduced river inputs We can estimate a landward migration rate of the 50 % L:W isopleths in Terrebonne Basin of about 218 m/year (0.22 km/year) compared to \0.5 m/year in Atchafalaya Basin (Table 1; Fig. 6). 2. estimates of landward migration of sea (based on field transect data) between 1932 and 2010 Based on L:W isopleth changes over time (indicating landward migration) they derived, using equations, changes impacts on wind fetch, wave power, and storm surge L:W (land-water ratio) decrease increases flood risk (derived from model) L:W decrease increases wind fetch (derived from model) L:W decrease increases wave power and storm surge height (derived from model) I did not code for wind fetch however in climatic impacts because that is not described in relation to High winds damage HWD
  Coastal inundation	Positive	1. change in location of 50% L:W isopleths (Land-water) using satellite imagery We developed a technique to more clearly define the location of the 50 % L:W isopleths using query methods of images available in 1932, 1973, 1999, and 2010. We extended our analysis of the 50 % L:W isopleth across the Atchafalaya and Terrebonne Basins to compare landscape behavior over the last six decades with and without a significant sediment input. comparing the Atchafalaya Basin, with continued sediment delivery, with Terrebonne Basin, with reduced river inputs We can estimate a landward migration rate of the 50 % L:W isopleths in Terrebonne Basin of about 218 m/year (0.22 km/year) compared to \0.5 m/year in Atchafalaya Basin (Table 1; Fig. 6). 2. estimates of landward migration of sea (based on field transect data) between 1932 and 2010 Based on L:W isopleth changes over time (indicating landward migration) they derived, using equations, changes impacts on wind fetch, wave power, and storm surge L:W (land-water ratio) decrease increases flood risk (derived from model) L:W decrease increases wind fetch (derived from model) L:W decrease increases wave power and storm surge height (derived from model) I did not code for wind fetch however in climatic impacts because that is not described in relation to High winds damage HWD

• Approach implemented in the field: Yes
• Specific location:

  Atchafalaya basin, Louisiana

• Country: United States of America
• Habitat/Biome type: Coastal | Streams, rivers, riparian |
• Issue specific term: Not applicable
  

Evidence

• Notes on intervention effectivness: Conclusions on how managing to retain natural basin processes (which maintain sedimentation provisioning) are primarily based on the measure of L:W changes (which they input in models to derive increase in flood risk, storm surge, wind fetch). Key summary (theory of change) root driver (due to loss of sediment delivery) -> Reduced land formation in coastal deltaic basins (measured by changes in the land:water ratio) -> increase in flood risk/storm surge (including via intermediate effects on vegetation/wetlands) The nonlinear response of migrating L:W isopleths as wind fetch increases is a critical feedback effect that should influence human river-management decisions in deltaic coast How vegetation mediates impacts – Environmental succession of coastal deltaic basins linked to river sediment supply describes a gradient in ecosystem services that occur during distinct stages of delta cycle. The succession of these ecosystem services is linked to the ecosystem sequence that occurs as landform, salinity, and elevation change, and relative L:W ratio shifts with sediment input. Under these conditions, ecosystem services, such as stormsurge reduction is high. in contrast, a coastal deltaic basin with decreasing L:W ratio as result of wetland loss and salinity increase is more susceptible to coastal inundation, as forested wetland area declines and water area increases.
• Is the assessment original?: Yes
• Broadtype of intervention considered: Engineered approach(s)
• Compare effectivness?: Yes
• Compared to the non-NBS approach: More effective
• Report greenhouse gas mitigation?: No
• Impacts on GHG: Not applicable
• Assess outcomes of the intervention on natural ecosystems: Yes
• Impacts for the ecosystem: Positive
• Ecosystem measures: existing wetland vegetation surveys and land use/land cover data sets were compiled and analyzed using geoprocessing tools provided in the ArcMap 10.2 software. they analyzed shift in vegetation types over time in both river basins existing wetland vegetation surveys and land use/land cover data sets were compiled and analyzed using geoprocessing tools provided in the ArcMap 10.2 software
• Assess outcomes of the intervention on people: Yes
• Impacts for people: Unclear
• People measures: There is evidence in the MRDP that humans migrate away from deltaic landscapes experiencing land loss due to increased flooding risks with cyclonic storm-surges (Fig. 10) there is evidence that changes in populations in central MRDP are presently occurring (both increases and decreases), and appear to be correlated to rates of land loss and gain note - that doesn’t apply in terrebone basin, only in atchalaya basin in relationship to increases in land area “It is possible that Terrebonne and Timbalier regions are influenced by factors not present in the other parish statistics displaying population loss with land loss.” However in Terrebone bay increasing storm surge to due decreasing L:W ratio has led to population decrease
• Considers economic costs: No
• Economic appraisal conducted: No
• Economic appraisal described:
• Economic costs of alternative considered: No
• Compared to an alternative: Not reported

Evaluation methodology

• Type of data: Quantitative
• Is it experimental: No
• Experimental evalution done: Not applicable
• Non-experimental evalution done: Empirical case study
• Study is systematic: