Fish and Wildlife. Service in the Delta. National Wildlife. Refuge (DNWR), and added data from the well-established crevasse Brant's Splay within the Cubits Gap.
Do constructed crevasses obey delta laws? Implications for the restoration of the Mississippi River delta Tara
Ioannis Y.
1,2 Georgiou
Department of Earth and Environmental Sciences, University of New Orleans; 2 Pontchartrain Institute for Environmental Sciences, University of New Orleans, New Orleans LA
Hypotheses
Results and Analysis
River deltas are among the most important interfaces on Earth. They are located within the fluvialmarine transition (FMT) and are accumulations of sedimentary deposits (mouth bars, levees, wetlands, inter-distributary bays) resulting from sediment delivered by rivers via a network of distributary channels. Deltas worldwide are threatened by environmental changes, including high rates of subsidence, global sea level rise, reduced sediment inputs, altered nutrient budgets and a suite of other local factors (Syvitski and Saito, 2007). The Mississippi River Delta (MRD) typifies these impacts with recorded land loss of over 4,800 km2 in the past century, threatening the population, infrastructure, and Barataria Bay Modern Delta reducing ecosystem services (Day et al., 2007; Couvillion et al., 2011).
(H): Crevasses within the MRD follow the same growth laws as large deltas, following metrics suggested by Edmonds and Slingerland (2007) – hereafter ES2007 (Wolinsky et al., 2010).
Fig 4 below: shows crevasse areas and growth rates over time, demonstrating the saturation in growth within a few years (asymptotic behavior). The growth rates are variable – initially high, especially following maintenance.
Methods
MR-09_06
800,000
MR-09_09
700,000
Image analysis
MR-09_11
Area (m²)
MR-09_20
400,000
60
MR-09_31 MR-09_38
300,000
MR-09_51
200,000
Log. (MR-09_06)
100,000
Log. (MR-09_11)
0 10,000
Log. (MR-09_31)
𝒎𝟐 ( ) 𝒚𝒆𝒂𝒓
Log. (MR-09_15)
Crevasses were maintained
1,000
Rate of Change
Crevasse metrics were obtained using a combination of geospatial tools such as ArcGIS® and Google Earth®. Bifurcation length (L) is the distance between two bifurcation points along the channels center line. Width (W) is the across-stream distance from water edge to water edge. Bifurcation order is recorded every time one channel splits into two (see Fig 2 – Motivation). These metrics are expected to be a function of delta growth.
measured Modern
MR-09_15
500,000
crevasses Wolinsky 0.80
mossy Numerical experimental
0.60
Fractal Tree 0.40
0.20
0.00 0.00
0.20
0.40
Crevasses
Boyers
40
HDDA Dredged material 18 years
20 8 years 14 years 14 years 0
100
MR-09_31
Brant's
1ha= 10,000 m2
10
0.60
0.80
1.00
Awet/Amax
8 years
MR-09_12
600,000
1.00
Lwet/Lmax
Introduction
Fig 7 right: Delta area and delta perimeter metrics (Wolinsky et al., 2010) demonstrate further that crevasses obey delta laws, as shown in normalized measurements. ha/years of growth
1
1,2 Yocum ,
Fig 8 left: Growth rates are highest after the initial construction and diminish exponentially with time. Compared to direct placement from dredging operations from the HDDA project.
Summary and Future Research
1
Data Collection and Analysis
• Most crevasses in the MRD appear to obey delta Fig 5 Below: Non-dimensional distance to the river We populated a database with large deltas from growth laws (ES2007, Edmonds et al. 2011), and delta mouth bar as a function of jet momentum flux for all ES2007 and added twelve crevasses created by the U.S. area and perimeter metrics (Wolinsky et al., 2010) crevasses in the MRD. The L/D has some variability in Fish and Wildlife • The distance to the mouth bar versus bifurcation Gulf of Mexico crevasses compared to large deltas (y axis), the Service in the Delta order is a very reasonable first order estimate of delta momentum flux term (x-axis) is within the same range, National Wildlife footprint and hence land building (Fig 6) while delta Refuge (DNWR), and suggesting that most crevasses obey similar growth laws allometry (Fig. 7) can be reduced to simple scaling laws. Brant’s Splay added data from the to large deltas, with the exception of a few (grey circle). • Direct placement builds land rapidly (Fig 8) but is West Bay well-established expensive ($38,421/ha) compared to constructed HDDA crevasse Brant’s Splay crevasses ($1,344/ha), based on reanalysis of presentwithin the Cubits Gap day areas plus maintenance costs of two crevasses in subdelta (Fig 3 left). 1992, Boyer et al (1997). • Future research includes establishing relationships W Equations LW with strong correlations with physical data such as Metrics for comparing crevasses with deltas include Crevasse data distance from the river, sediment concentrations and Large deltas the position of the river mouth bar (Eq. 1), assuming grain size, flow distribution, velocity etc., building on river-dominated conditions apply, and the corresponding analysis of Boyer et al (1997). WU non-dimensional form of the equation (Eq. 2), plotted ( ) gD W against the jet momentum flux (Eq.3), which is a proxy While a river diversion is an efficient way to build 3000 Brant's Fig 6 Right: The Splay • Boyer, M.E., Harris, J.O., Turner, R.E. (1997). Constructed Crevasses and Land Gain in the Mississippi River Delta, Restoration land there are too few analogs to draw upon aside from for the required fluid inertia forces to mobilize and 2500 Ecology, 5 (1), 85-92, DOI: 10.1046/j.1526-100X.1997.09709.x • Couvillion, B.R., J. A. Barras, G. D. Steyer, W. Sleavin William, M. Fisher, H. Beck, N. Trahan, B. Griffin, and D. Heckman. 2011. transport sediment grains. We used velocity and channel distance to the mouth 2000 experimental laboratory deltas, field observations and Land Area Change in Coastal Louisi-ana from 1932 to 2010: USGS Scientific Investigations Report Map 3164, 12 p. Pamphlet. http://pubs.usgs.gov/sim/3164/downloads/SIM3164_Pamphlet.pdf. geometry from Esposito et al., (2013) and Clark et al., bar (L ) can be used average numerical modeling studies of large deltas. Therefore, RMB • Clark, R.B., Georgiou, I.Y., FitzGerald, D.M., (2013). An Evolutionary Model of a Retrograding Subdeltaic Distributary of a 1500 River-Dominated System, Assoc. for the Sciences of Limnology and Oceanography - ASLO, February 17 – 22, New Orleans, LA. Exponential (2013) for all crevasses without velocity measurements. to forecast land • Day, J. W., Boesch, D. F., Clairain, E. J., Kemp, G. P., Laska, S. B., Mitsch, W. J., & Whigham, D. F. (2007). Restoration of the this study focuses on the processes responsible for 1000 Mississippi Delta: lessons from hurricanes Katrina and Rita. Science, 315(5819), 1679-1684. (Eq. 1) building. With each (Eq. 3) • Edmonds, D. A., and R. L. Slingerland (2007), Mechanics of river mouth bar formation: Implications for the morphodynamics sediment deposition (land building) resulting from WU 500 D W U 1 of delta distributary networks, J. Geophys. Res., 112, F02034, doi:10.1029/2006JF000574. L 104 • Edmonds, D. A., C. Paola, D. C. J. D. Hoyal, and B. A. Sheets (2011), Quantitative metrics that describe river deltas and their bifurcation order the ( ) gD W ( ) gD W 2 2 2 crevasse construction, and tests whether these smaller 0 channel networks, J. Geophys. Res., 116, F04022, doi:10.1029/2010JF001955. 0 1 2 3 • Esposito, C.R., Georgiou, I.Y., Kolker, A.K., (2013) Hydrodynamic and Geomorphic controls on mouth bar evolution, distance diminishes deltas follow laws/metrics similar to those of large Geophysical Research Letters, 40 (8), 1540-1545, DOI: 10.1002/grl.50333 Bifurcation order (Eq. 2) • Syvitski, J. P., & Saito, Y. (2007). Morphodynamics of deltas under the influence of humans. Global and Planetary Change, U L exponentially, suggesting it is a reasonable first order 57(3), 261-282. deltas (Edmonds and Slingerland,2007; and Edmonds et 104 • Wolinsky, M., D. A. Edmonds, J. M. Martin, and C. Paola (2010), Delta allometry: Growth laws for river deltas, Geophys. Res. D ( ) gD W Lett., 37, L21403, doi:10.1029/2010GL044592. metric to forecast land building. al., 2011). Time
Motivation
100,000.00
MRD Crevasses
Basin Slope 0.0025
MR-09_12
Basin Slope 0.00025
MR-09_20
LRMB/D (m)
10,000.00
MR-09_20 MR-09_15
y = 12148x-0.486 R² = 0.5407
Power (Basin Slope 0.00025)
MR-09_12
y = 158.22x0.2337 R² = 0.7404
MR-09_11
1,000.00
100.00
10.00
1
10
100
1000
10000
2
L RMB
50
RMB
o kn
o bn
0.2278
2 o 2 mn
50
50
max
0.2278
2
RMB
50
max
2
max
max
References
