Natural Processes in Delta Restoration: Application to the Mississippi Delta (original) (raw)
Related papers
Limnology and Oceanography, 2012
The Mississippi River basin was developed for agriculture at an unprecedented scale and intensity within the last 200 yr. These changes favored erosion and sediment transport, which were subsequently reduced by soil conservation practices, the trapping of sediment behind large reservoirs, and channel engineering. We discuss the relationship between sediment loading and the growth, loss, and stability of wetlands in the Mississippi River birdfoot delta (MRBD). We report an anthropogenically driven increase in mean suspended sediment concentrations in the Mississippi River below New Orleans, from a modeled 350 mg L 21 (174 3 10 9 kg yr 21 ) at the end of the 18th century to a measured maximum of 675 mg L 21 (348 3 10 9 kg yr 21 ) in the late 19th century, followed by a sharp reduction, and then a period of stabilization at 157 mg L 21 (91 3 10 9 kg yr 21 ) after 1962. Changes in wetland area of the MRBD over the past two centuries reflect variations in sediment supply during periods of increasing, decreasing, and stable sediment loading, and are distinct from wetland loss patterns in other areas of coastal Louisiana. The rapid growth of the MRBD until the 1930s, which has been used as a reference for one of the world's largest wetland restoration efforts, may not be a suitable archetype for the majority of the coast. Additionally, future variation in sediment supply to the MRBD would likely change the total wetland area of the MRBD.
Sediment Distribution, Retention and Morphodynamic Analysis of a River-Dominated Deltaic System
Water, 2021
River deltas have received considerable attention due to coastal land loss issues caused by subsidence, storms, and sea level rise. Improved understanding of deltaic processes and dynamics is vital to coastal restoration efforts. This paper describes the application of process-based morphodynamic models to a prograding river delta. The analysis focuses on the flow and sediment dynamics amongst the interconnected channel network of the delta. The models were validated against observations of velocity and sediment concentrations for the Wax Lake Delta (WLD) of the Atchafalaya River system in Louisiana, USA. The WLD provides an opportunity as a natural laboratory for studying the processes associated with river dominated deltaic growth. It includes a network of bifurcated channels that self-organize and dynamically adjust, as the delta grows seaward to the Gulf of Mexico. The model results for a flood event show that 47% of the flow exits the system as channelized flow and the remainin...
Large infrequently operated river diversions for Mississippi delta restoration
Estuarine, Coastal and Shelf Science, 2016
Currently the Mississippi delta stands as a highly degraded and threatened coastal ecosystem having lost about 25% of coastal wetlands during the 20th century. To address this problem, a $50 billion, 50-year restoration program is underway. A central component of this program is reintroduction of river water back into the deltaic plain to mimic natural functioning of the delta. However, opposition to diversions has developed based on a number of perceived threats. These include over-freshening of coastal estuaries, displacement of fisheries, perceived water quality problems, and assertions that nutrients in river water leads to wetland deterioration. In addition, growing climate impacts and increasing scarcity and cost of energy will make coastal restoration more challenging and limit restoration options. We address these issues in the context of an analysis of natural and artificial diversions, crevasse splays, and small sub-delta lobes. We suggest that episodic large diversions and crevasses (>5000 m 3 s À1) can build land quickly while having transient impacts on the estuarine system. Small diversions (<200 m 3 s À1) that are more or less continuously operated build land slowly and can lead to over-freshening and water level stress. We use land building rates for different sized diversions and impacts of large periodic inputs of river water to coastal systems in the Mississippi delta to conclude that high discharge diversions operated episodically will lead to rapid coastal restoration and alleviate concerns about diversions. Single diversion events have deposited sediments up to 40 cm in depth over areas up to 130e180 km 2. This approach should have broad applicability to deltas globally.
Geomorphology, 2011
Crevasse splay environments provide a mesocosm for evaluating wetland formation and maintenance processes on a decadal time scale. Site elevation, water levels, vertical accretion, elevation change, shallow subsidence, and plant biomass were measured at five habitats along an elevation gradient to evaluate wetland formation and development in Brant Pass Splay; an active crevasse splay of the Balize delta of the Mississippi River. The processes of vertical development (vertical accretion, elevation change, and shallow subsidence) were measured with the surface elevation table-marker horizon method. There were three distinct stages to the accrual of elevation capital and wetland formation in the splay: sediment infilling, vegetative colonization, and development of a mature wetland community. Accretion, elevation gain, and shallow subsidence all decreased by an order of magnitude from the open water (lowest elevation) to the forest (highest elevation) habitats. Vegetative colonization occurred within the first growing season following emergence of the mud surface. An explosively high rate of below-ground production quickly stabilized the loosely consolidated subaerial sediments. After emergent vegetation colonization, vertical development slowed and maintenance of marsh elevation was driven both by sediment trapping by the vegetation and accumulation of plant organic matter in the soil. Continued vertical development and survival of the marsh then depended on the health and productivity of the plant community. The process of delta wetland formation is both complex and nonlinear. Determining the dynamics of wetland formation will help in understanding the processes driving the past building of the delta and in developing models for restoring degraded wetlands in the Mississippi River delta and other deltas around the world.
Remote Sensing of Environment
The Mississippi River Delta is one of the most rapidly changing area on Earth, with large areas experiencing land loss and smaller areas experiencing loss. While some of the drivers of these changes are well known (high rates of relative sea level rise, reduced sediment inputs, canal dredging), debate exists about other drivers. One area that has received substantial attention is the role of, "river diversions," areas where sediments and water are diverted from the Mississippi River into degrading wetlands with the hope of reinitiating deltaic land building processes. Some authors have argued that diversions lead to reduced shear strengths of wetland soils that make them more vulnerable to storm driven erosion, while other authors have argued that sediments from river diversions will develop stable land. This study examined this controversy in the Cubits Gap Subdelta, an analogue for a large (N 1420 m 3 s −1) river diversion by testing the hypothesis that areas of land gain, and/or resilience to erosion occurred in areas that actively received river sediments and as a result had mineral rich soils with high shear strength. To accomplish this, a Normalized Difference Water Index (NDWI) was developed for Landsat-7 Enhanced Thematic Mapper (ETM+) and Landsat-8 Operational Land Imager (OLI) images. The NDWI was calculated from (Blue − SWIR) / (Blue + SWIR), where SWIR is the shorter wavelength, and yielded land/water boundary maps with 30 m resolution. Results indicate that land gain occurred predominantly in the riverside section of this subdelta where sediments were imported from Mississippi River crevasses and/or dredging. Land loss typically occurred in the distal regions of the subdelta, which had lower levels of sediment supply and greater wave exposure. Sediment geotechnical analyses revealed land loss pixels generally correlated sediments with to high organic contents (9.0 ± 1.9%), water contents (54.8 ± 3.7%) and salinity (6.5 ± 2.0 PSU), with low shear strengths (5.7 ± 0.8 kN m −2) and low bulk density (0.6 ± 0.8 g cm −3), whereas land gain pixels generally correlate with low organic content (3.9 ± 0.6%), water content (38.1 ± 4.2%) high shear strength (10.9 ± 4.1 kN m −2) and bulk density (1.00 ± 0.1 g cm −3). This study suggests plans to restore the region by partially diverting the flow of the Mississippi River will be most successful if they carry high loads of sediment, and that concerns about the integrity of fresh marsh may be unwarranted if those marshes are sediment rich.
Dynamic Changes of the Holocene Mississippi River Delta Plain: The Delta Cycle
1997
Previous geologic resear ch on Holocene Mississippi River delt aic deposits has verified that the present delta plain and associa ted nea rshore bar rier isla nds and subma rine shoals are either direct or indirect produ cts of cyclic delta-bu ilding even ts tha t have operate d on a variety of te mporal and spatial scales. A major depositi onal element of the modern delta plain is the delt a complex, of which th ere are six: (1) Mar ingouin, (2) Teche, (3) St. Bernard, (4) Lafourche, (5) Bali ze, and (6) Atchafalaya. Major delta-building events have occurred at a frequency of one every 1-2 kyr. Deposit s associate d with the six major delt a complexes are fund amental constructional units of th e delta plain , which collectively covers an area of-30,000 km-. Sedimentary deposit s associated with these delta-bu ildin g events ra nge in thickness from about 10 to 100 m. Their construction is modulated by st rea m capt ure , which develops a new delta complex by way of a new river course. Delta complexes may be compr ised of one or more delta lobes. As a product of th is delta switching, th e depositional arc hitecture of the delta plain consists oflaterally offset and stac ked delta lobes. With in delta lobes are subdeltas and even sma ller crevasse-splays. These smaller scale delt as sedimentologically and geomorphically mimic their lar ger delt a lobe coun terparts, but th ey are considerab ly thinner, cover less area , and have a shorter period of development an d aban donme nt. Subdeltas ar e usu ally < 10 m thick and may fill sha llow bays that cover over 300 krn-. They build and deteriorat e on timescales of 150-200 years. Crevasse-splays or overbank splays are < 5 m thi ck, cover only a few square kilometers, and ar e aba ndoned afte r severa l decades of active growth. Each delt a evolves through a rapid regressional phase as wate r and sediment are capt ure d from an antecedent river course. If highstand conditions persist long enough, delt as may prograde to th e outer shelf to form wedges of deltaic sediment much thicker tha n their inn er shelf counterparts. The delt a-building process sta rts with the filling of int erior lakes (lacustrine deltas), which is followed by bayhead delta-buil ding at the coast , an d fina lly by progradation across the marine shelf (shelf delta). Delta complexes and delta lobes, as well as their sma ller counterparts, experie nce three phases of growt h and abandonment : (1) rapid growth with increasing-to-stable discharge, (2) relative stability during init ial stages of waning discharge, when sedi ment input balan ces th e collective effects of subsidence, an d (3) aba ndonment , followed by ra pid subside nce-driven subae ria l delta deter iorat ion. In th e rapid growth stage, formerly eroding-subsi ding coasta l environments experience delt a plain accretion and coastal progradation from renewed sediment input. On the aba ndonment side of the cycle, marin e processes overwhelm fluvial processes and rework th e delta perimeter. Forced by the combine d processes of subsidence, the delt a sur face und ergoes progressive submergence. Transgressive sand bodies created by wave reworking of th e delta evolve from headl and beaches and spits, to barr ier island s, and fina lly to submarine shoals as the abandonment phase is completed.
Mississippi River subaqueous delta is entering a stage of retrogradation
Marine Geology
The subaqueous delta of the Mississippi River, the largest river system in the conterminous U.S., has entered a stage of retrogradation caused by multiple natural and anthropogenic activities. Since the 1950s, the suspended sediment load of the Mississippi River has decreased by~50% due primarily to the construction of > 50,000 dams in the Mississippi basin. The impact of this decreased sediment load has been observed in subaerial environments, but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined. To identify historic trends in sedimentation patterns, we compiled bathymetric datasets, including historical charts, industry and academic surveys, and National Oceanic and Atmospheric Administration hydrographic data, collected between 1764 and 2009. The progradation rate (measured at the 10 m depth contour) of Southwest Pass, which receives 69% of the suspended sediment load reaching Head of Passes, has decreased from~67 m/yr between 1874 and 1940 to~26 m/yr between 1940 and 1979, with evidence of further deceleration from 1979 to 2009. At South Pass and Pass a Loutre, the delta front has entered the destructive phase, with the 10 m contour retreating at rates > 20 m/yr at both passes since 1979. Advancement of the delta front also decelerated in deeper water (in some areas out to~180 m depth). Except locally, where mudflow lobes are advancing, deeper contours show a pattern of decreasing progradation rate between 1874-1940 and 1979-2005 time periods. Furthermore, based on differences measured between available bathymetric datasets, the sediment accumulation rate across the delta front decreased by~73% for the same period. The retention rate of Mississippi River sediment on the delta front ranged from 67 to 81% for the time periods assessed, with total sediment load stored on the delta front equal to