Kevin C. Hanegan and Ioannis Y. GeorgiouDepartment of Earth and Environmental Sciences / Pontchartrain Institute for Environmental Sciences,

University of New Orleans, 2000 Lakeshore Dr., New Orleans, LA 70148, khanegan@uno.edu

USGS 07381590

USGS 073815925

Legend Measurement GagesOpen Boundaries

Initial Bathymetry (m)2.68 - 4.561.29 - 2.670.67 - 1.280.43 - 0.660.18 - 0.42-0.17 - 0.17-0.65 - -0.18-1.20 - -0.66-1.68 - -1.21-2.42 - -1.69-4.88 - -2.43-9.71 - -4.89-15.12 - -9.72-24.99 - -15.13

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a) full spring/neap tidal cycle MN_03GD_01MN_04

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LegendProfile Output Line

Cross-sectionsMN_03GD_01MN_04

0 2.51.25km

Delta Apex

Start of Major Bifurcations

Basin

Hydrodynamics and Transport along Profile

Conclusions

References

Sand Flux through Distributary Channels

Methods - Delft3D Model Development

IntroductionThe Wax Lake Delta (WLD) has prograded into the Atchafalaya Bay

receiving basin through seaward channel extension, subaqueous river mouth bar formation, and channel bifurcation, building new land area in the form of sandy delta lobe deposits. With sediment supplied to the delta through the constructed Wax Lake Outlet (WLO) channel, the WLD is frequently cited as a natural analogue for the land-building potential of large sediment diversions from the Mississippi River.

Though traditionally viewed as river-dominant where delta progradationoccurs through deposition during floods, recent work by Shaw & Mohrig (2013) documents erosive channel extension at the most distal portion of a WLD distributary channel during low flows and points to tidal modulation of flow velocities as the causative mechanism. The present study examines the hydrodynamics and sediment transport within the WLD during low flows in greater detail to both corroborate the findings of Shaw & Mohrig (2013) and gain greater insight into the potential sediment reworking in deltas during non-flood events.

• Delft3D simulates hydrodynamics, sediment transport, and morphology• Depth-averaged hydrodynamics• Upstream flow boundary forced with USGS gage data, offshore boundary forced

with tidal constituents extracted from tidal databases• Two sediment fractions: fine sand and cohesive mud

Wax Lake Delta Delft3D model domain and initial bathymetry. Model open boundaries are indicated by thick red lines.

Model Calibration

Station O1 η (m)

O1 norm. amp. ε

K1 η (m)

K1 norm. amp. ε

M2 η (m)

M2 norm. amp. ε

S2 η (m)

S2 norm. amp. ε

07381590 0.039 -8 % 0.036 4 % 0.020 -33 % 0.001 -43 %073815925 0.069 -8 % 0.064 3 % 0.050 -59 % 0.017 -28 %η, measured constituent amplitude; norm. amp. ε, normalized error between measured and calculated constituent amplitudes

Table 1 Tidal constituent calibration results at water level gage locations

Table 2 Velocity and suspended sediment concentration calibration at transects from DuMars (2002)

At low spring tide, sand flux only occurs in distal reaches of distributary channels and increases downstream. Distal ends are supply-limited such that downstream-increasing flux erodes the bed. Conversely, sand transport during the rising tide completely ceases.

Transect cs_8 cs_15 cs_17 cs_18 cs_21V norm. ε -10 % 19 % 1 % 10 % -9 %C norm. ε 1 % -24 % -26 % -16 % -21 %V norm. ε, error between measured and calculated channel-averaged velocity; C norm. ε, error between measured and calcuatedchannel-averaged suspended sediment concentration

• Full spring-neap tidal cycle at four different flow levels• This study focuses on results from low flow case: Q = 1149.7 m3/s

Probability of exceedance, ~82%

Sand flux variation with the tidal cycle is evident in the figurebelow, showing total sand flux through time at cross-sections upstream and downstream of a channel bifurcation. Across the full tidal cycle, flux through cross-section MN_03 upstream of the bifurcation is much lower than flux through the downstream cross-sections(GD_01 and MN_04). The flux through the downstream cross-sections peaks during each spring low tide and ceases during the rising, high tide, and falling portions of the tide. Additionally, peak sand fluxes gradually diminish to zero as the tide cycles from spring to neap.

Plots of parameters along a streamwise profile (see map to left – delta apex to basin) at low tide and rising tide reveal the mechanism of erosive channel extension. For the low tide profile, water surface drawdown towards the receiving basin forms a convex, M2-type backwater profile which accelerates flow towards the channel mouth,increasing bed shear stress sufficiently to entrain sand in suspension. During the risingtide, the flat water surface profile results in agradual deceleration of flow towards the mouth.

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M2-type gradually-varied flow profile –Accelerating flow

Low Spring Tide

• Basinward-increasing sand transport throughout delta• Microtidal environment – still sufficient to affect sand transport• Drawdown at low tide – M2 (A2) profile accelerates flow up to channel mouth• Supports Shaw and Mohrig’s observations of erosive channel extension at low Q• Erosive channel-extension can be an important process, with mechanisms acting

during non-flood periods• Sand deposited in delta during floods can be significantly reworked by tides• Delta growth not solely a result of flood deposition

DuMars, A. J. (2002). Distributary Mouth Bar Formation and Channel Bifurcation in the Wax Lake Delta, Atchafalaya Bay, Louisiana. Hanegan, K. C. (2011). Modeling the Evolution of the Wax Lake Delta in Atchafalaya Bay, Louisiana. Delft University of Technology. Kim, W., et al. (2009). Is It Feasible to Build New Land in the Mississippi River Delta? Lamb, M. P., et al. (2012). Backwater and river plume controls on scour upstream of river mouths…Lesser, G. R., et al. (2004). Development and validation of a three-dimensional morphological model. Mukai, A. Y., et al. (2002). Eastcoast 2001: A tidal constituent database for the western North Atlantic, Gulf of Mexico and Caribbean. Paola, C., et al. (2011). Natural processes in delta restoration: application to the Mississippi Delta. Parker, G., & Sequeiros, O. (2006). Large Scale River Morphodynamics: Application to the Mississippi Delta.Partheniades, E. (1965). Erosion and deposition of cohesive soils.Pawlowicz, R., et al. (2002). Classical tidal harmonic analysis including error estimates in MATLAB using T_TIDE. Roberts, H. H. (1998). Delta Switching: Early Responses to the Atchafalaya River Diversion. Roberts, H. H., et al. (1997). Evolution of Sedimentary Architecture and Surface Morphology: Atchafalaya and Wax Lake Deltas.Shaw, J. B., & Mohrig, D. (2013). The importance of erosion in distributary channel network growth, Wax Lake Delta, Louisiana. U.S. Army Corps of Engineers. (1999). Atchafalaya River hydrographic survey, 1998-1999.U.S. Geological Survey. (2014). USGS Water Data for USA. National Water Information System: Web Interface. Van Rijn, L. C. (1993). Principles of Sediment Transport in Rivers, Estuaries and Coastal Seas.Wellner, R., et al. (2005). Jet-Plume Depositional Bodies—The Primary Building Blocks of Wax Lake Delta.

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[kg/

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[kg/

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Low Spring Tide Rising Spring TideDelta Apex Basin Delta Apex Basin