1. Introduction
Deltas occupy only 5% of the Earth’s surface, but nourish over a half billion people around the world. This leads to an average population density of about 500/km
2 along deltaic coasts, more than 10 times of the world average [
1]. Many river deltas worldwide are disappearing, leading to significant threats to our natural, economic and social systems [
2]. This is mainly due to the combined effects of anthropogenic changes to sediment supply and river flow, subsidence, and global sea level rise. Sinking deltaic coasts pose an immediate threat to millions of residents who live in coastal megacities [
3], and scientists have been trying to find strategies dealing with the challenge of “building land with rising sea” [
4,
5].
Being home of over two million people, Louisiana′s deltaic coast supports the largest commercial fishery for the lower 48 U.S. states, supplies 90% of the nation′s outer continental shelf oil and gas, and facilitates about 20% of the nation′s annual waterborne commerce. Louisiana wetlands play a number of important roles in the environment, primarily life habitat, flood control and sediment retention; the wetlands also buffer the storm surge and protect the coast from severe damage during hurricanes. These wetlands, however, are in peril as Louisiana is currently responsible for about 90% of the nation′s coastal wetland loss [
6]. Since the 1930s, coastal Louisiana has lost over 4660 km
2 of land, diminishing wetland habitats, increasing flood risk, and endangering coastal environment.
This land loss is primarily associated with decreased sediment discharge from the Mississippi and Atchafalaya Rivers, relative sea level rise, levee construction, sediment compaction, withdrawals of water, oil and gas, as well as other natural and human activities [
7,
8,
9,
10,
11,
12]. Thus, stabilizing disappearing wetlands and maintaining them as one of the most productive natural areas in the world are critical to the nation′s economy. In 2012, Louisiana Coastal Protection & Restoration Authority (CPRA) issued Louisiana′s Comprehensive Master Plan for a Sustainable Coast [
13]. One of the recommended restoration tools is the diversion of sediment-laden water from the Mississippi and Atchafalaya Rivers into adjacent receiving basins to build new land. Diversions reconnect the river to the deltaic plain via river reintroductions, the reopening of old distributaries, and crevasse-splay development [
7]. In the next 50 years, about $50 billion is planned to be spent on marsh creation, sediment diversion and other types of projects along the Louisiana coast. For instance, between 2012 and 2031, the estimated total cost of sediment diversions at Atchafalaya River, middle Barataria Bay and middle Breton Sound (
Figure 1) will exceed $2.5 billion.
Sediment diversions are impacted by biological, chemical, geological and physical processes which interact with human activities. There is, however, a considerable argument on whether sediment diversions can create significant land. Some research groups believe that these diversions are a key tool to restore the shrinking land and protect the coast when they are designed effectively and used properly [
7,
10,
14,
15]. Turner
et al. [
16] argued that the major source of mineral sediment to coastal marshes is from hurricanes, not river floods; a more recent detailed study finds that fluvial sediment supply is more important than hurricanes over decadal timescales and longer [
17]. Blum and Roberts [
9] even suggested that the significant drowning of the Louisiana coast is inevitable because of insufficient sediment supply, rapid compaction of young sediment and faster global sea level rise in the coming century.
Figure 1.
The study area in the Louisiana coast as well as the Mississippi and Atchafalaya Rivers. Green arrows are future large diversions proposed in Louisiana′s Master Plan (CPRA, 2012). Baton Rouge, Belle Chasse and Caernarvon are three stations in which water discharge was measured. Shell Beach is the National Oceanic and Atmospheric Administration’s National Data Buoy Center (NDBC) station for wind speed measurement. Black dots on Louisiana shelf are the stations for an erodibility study by Xu
et al. [
18]. Bathymetric contours are in 10, 20, 50, 100 and 300 m. BS = Breton Sound; BB = Barataria Bay. See
Figure 2A,B for details of two study areas.
Figure 1.
The study area in the Louisiana coast as well as the Mississippi and Atchafalaya Rivers. Green arrows are future large diversions proposed in Louisiana′s Master Plan (CPRA, 2012). Baton Rouge, Belle Chasse and Caernarvon are three stations in which water discharge was measured. Shell Beach is the National Oceanic and Atmospheric Administration’s National Data Buoy Center (NDBC) station for wind speed measurement. Black dots on Louisiana shelf are the stations for an erodibility study by Xu
et al. [
18]. Bathymetric contours are in 10, 20, 50, 100 and 300 m. BS = Breton Sound; BB = Barataria Bay. See
Figure 2A,B for details of two study areas.
Based on comprehensive synthesis, Paola
et al. [
19] proposed that the area of a delta plain
Aw in a receiving basin for sediment diversion is primarily controlled by an Equation:
where
Qs is the sediment supply via diversion;
fr is the sediment retention rate;
ro is the volume ratio of organic matter to mineral sediment;
C0 is the overall solids fraction in the sediment column (1-porosity);
σ is subsidence rate; and
H is the rate of global sea-level rise.
A critical, but elusive, parameter is sediment retention rate
fr,
i.e., the fraction of sediment retained in the subaerial and subaqueous parts of delta to help build and sustain land. This will, at least partially, determine whether many Louisiana sediment diversion projects will be successful in the next century. The retention rate is controlled by many factors, including texture, sediment concentration, waves, tides, sediment erodibility, sediment consolidation, bioturbation, plant-sediment interaction, river discharge, relative sea level change, storm activities, and many others. For instance, comparing with unconsolidated mud, sand is harder to resuspend and tends to settle quickly to facilitate land building. Waves can easily resuspend muddy sediment for transport by tidal currents, which move sediment in and out of coastal bays and estuaries. Erodibility is defined as the measured propensity for sediment to be resuspended from the sediment surface [
20]; normally a higher erodibility leads to a lower sediment retention rate.
Shallow-water deltas on the Louisiana coast, such as the relatively high-energy distributary channels of Wax Lake Delta [
21] inside of Atchafalaya Bay (
Figure 1), tend to be sand-dominated, because muddy sediment is prone to resuspension (or non-deposition) and export away from the receiving basins before sufficient consolidation can occur to impede erosion. However, mud and sand represent, respectively, >80% and <20% of sediment load in the Mississippi and Atchafalaya Rivers [
14], so the loss of mud represents a substantial issue in the land-building process. The mechanism of sand transport in aquatic systems is widely understood [
22]. Muddy sediment dynamics, however, are much more complicated and are widely recognized as nonlinear processes operating at rates highly dependent on local conditions [
23], which must be evaluated on an individual basis.
Studies of mud erodibility on the Mississippi Delta have commenced only recently, and have addressed some of the wide variability of delta sediments. Xu
et al. [
18] and Mickey
et al. [
24] collected a total of 106 sediment cores on Louisiana shelf and quantified critical shear stress and eroded mass based on field experiments in early spring and late summer seasons. Lo
et al. [
25] collected sediment from Lake Lery which is downstream of Big Mar that receives discharge from Caernarvon freshwater diversion (
Figure 1), and did
ex-situ sediment erodibility experiments in a lab to quantify the erodibility changes one, two and four weeks after initial settling. However, there is currently a paucity of data of field measurement of sediment erodibility in Louisiana estuaries and bays. The lack of field erodibility data poses a challenge to the ongoing modeling work of Louisiana CPRA to predict land growth and sediment retention in receiving basins for future large diversions. Although the Mississippi River deltaic plain has been the subject of abundant research over recent decades [
12], few studies have quantified erodibility and high-resolution grain size distribution, both of which control the sediment retention rate in receiving basins.
In this study, we focus on the fundamental sedimentary processes in seaward parts of receiving basins for diversions. We do not discuss the land growth or crevasse-splay development in the “proximal” parts of deltas. Rather, our work is focused on the relatively “distal” parts of subaqueous deltas in which diverted river flow is weak, wave resuspension is frequent, and volumetrically-dominant mud can escape out of the receiving basins. Specific objectives of this research are: (1) to quantify the high-resolution grain sizes of both surficial and down-core sediment in two existing diversion receiving basins: West Bay and Big Mar, and to compare with other grain size datasets from Louisiana coast; (2) to measure the erodibility of bed sediment in the field at West Bay and Big Mar; (3) to calculate wave-induced shear stresses in Louisiana bays and discuss the implication of texture and erodibility for sediment retention of Louisiana coastal diversions; and (4) to provide suggestions for the designing and implementation of receiving basins for future Louisiana sediment diversions.
2. Study Areas
There are two contrasting areas in our study: West Bay and Big Mar (
Figure 2A,B). West Bay represents a semi-enclosed bay which is under strong oceanographic influence and is located on top of the Mississippi River Delta (MRD) with a rapid subsidence rate of 15 mm/year. Big Mar is a more landward water body, surrounded by fresh to brackish wetlands, with a much slower subsidence rate of 2 mm/year and much less influence from the open ocean (
Table 1).
Figure 2.
(
A) Six stations (WB1–WB6) in West Bay study area. Sediment samples were collected and measured on 19–20 December 2014 and the satellite image was taken on 27 January 2015. (
B) Six stations (BM1–BM6) in Big Mar. Sediment samples were collected and measured on 6–7 March 2015 and the satellite image was taken on 31 October 2014. White arrows indicate overall flow directions. See
Figure 1 for the locations of two study areas. Background images are from Google Earth.
Figure 2.
(
A) Six stations (WB1–WB6) in West Bay study area. Sediment samples were collected and measured on 19–20 December 2014 and the satellite image was taken on 27 January 2015. (
B) Six stations (BM1–BM6) in Big Mar. Sediment samples were collected and measured on 6–7 March 2015 and the satellite image was taken on 31 October 2014. White arrows indicate overall flow directions. See
Figure 1 for the locations of two study areas. Background images are from Google Earth.
Table 1.
Comparison of two diversion receiving basins in West Bay and Big Mar.
Table 1.
Comparison of two diversion receiving basins in West Bay and Big Mar.
Study Area | Area before Diversion (km2) | Tidal Range (m) | Subsidence Rate (mm/year) | Connectivity to Open Ocean | Purpose of Diversion | Water Discharge (km3/year) | Sediment Discharge (Mt/year) |
---|
West Bay | 40 a | 0.3 m a | 15 b | semi-enclosed | sediment diversion and nourishing marsh | 33 c | 3.2 c |
Big Mar | 4 | negligible | 3 b | enclosed | water diversion for salinity control now. planned for sediment diversion in the future | 2 c | 0.2 c |
West Bay was selected as one of our study areas because it is the only operational artificial diversion to date designed specifically for land building in coastal Louisiana [
10]. The discharge of West Bay is also similar to that of future diversions at Breton Sound and Barataria Bay (
Figure 1). Physical settings of all three above bays are semi-enclosed, connecting to both open water and vegetated land, although seaward ends of the Barataria and Breton receiving basins are more sheltered than that of West Bay. Thus, West Bay is a good existing analog for the most energetic marine conditions likely for future major diversions. West Bay is one of the six subdelta complexes comprising the modern Mississippi bird-foot delta. Its subdelta started to develop around 1839 due to a flood break in the river levee and led to rapid development of land until 1932. After 1932, subsidence, sea-level rise, storms and reduced sediment deposition all contributed to land deterioration and formed the current open water body [
12,
15,
27]. In order to restore vegetated wetlands and create land, since 2003 water and sediment have been diverted from a non-gated crevasse at a 120° angle along the west bank of the Mississippi River 7.6 km upstream of the Head of Passes of MRD (
Figure 2A). This project was designed to divert sediment and water to create and nourish about 9831 acres of fresh to intermediate marsh. Earthen dike structures, called Sediment Retention Enhancement Devices (SREDs), were placed southwest of the crevasse to maximize the wetland creation.
Andrus [
26] compared multiple-year bathymetric data and found that the deepening of West Bay since 2003 was probably caused by sediment erosion due to the large waves and surges generated by Hurricane Katrina. Allison
et al. [
14] reported that annual total sediment load into West Bay was about 3.2 million tons (Mt) but only 0.3 Mt of sand actually entered the bay (
Table 1). Kolker
et al. [
15] found that the maximum deposition in West Bay occurred at the seaward end of the diversion project boundary, contradictory to simple sedimentary models which predict that depositional center should be close to the river bank. Because of rapid relative sea level rising due to compaction of >100 m thick of Holocene sediment and less hydraulic head available to move coarse sediment, there was little growth of a delta in West Bay before the 2011 flood. Following the Mississippi River flood in 2011, however, a significant portion of West Bay shows growth of a subaqueous delta (
Figure 2A). As a result, the Louisiana Coastal Wetlands Planning, Protection and Restoration Act Task Force decided to rescind its previous decision to close the West Bay sediment diversion, and to allow it to remain open for at least another ten years.
Comparing with West Bay, Big Mar is shallower in depth (0.23 m in Big Mar
vs. 1.26 m in West Bay), smaller in size (4 km
2 in Big Mar
vs. 40 m
2 in West Bay) and is a more enclosed system (
Table 1 and
Table 2;
Figure 2A,B). Big Mar is an artificial pond caused by an agricultural impoundment [
28]. It is located south of the small gated Caernarvon freshwater diversion on the lower Mississippi River to limit salt water intrusion with minimal sediment capture [
10]. Allison
et al. [
14] reported that annual water and sediment discharge passing through Caernarvon diversion are 2 km
3/year and 0.2 Mt/year, respectively. Water passing through the Caernarvon diversion structure immediately enters Big Mar and Lake Lery, and then through the complex Breton Sound estuary system [
29,
30]. Often the Caernarvon diversion is not operated when sediment spikes are present and therefore does not maximize potential sediment retention. Despite this intermittent operation and the nature of freshwater diversion, there has been incidental sediment accumulation in Big Mar pond to permanently support emergent wetland plant on a new subdelta [
31] (
Figure 2B). Although smaller in size, the morphology of this new emerging subdelta is not unlike typical river-dominated bay-head deltas in West Bay and Wax Lake Delta. Since 2004, land gain and wetland growth in Big Mar has been significant. Lopez
et al. [
31] reported approximately 4 km
2 of new emerging land and about 201,800 m
3 of sediment retention in Big Mar pond.
6. Conclusions
(1) Based on our synthesis of grain size data of 1191 sediment samples, sand, silt and clay contents are, respectively, 24.7%, 54.9% and 20.4% in surficial and down-core samples in Louisiana bays and estuaries. Silt is the largest fraction of not only river sediment but also retained sediment in receiving basins.
(2) The average erodibility of Big Mar sediment is similar to that of the Louisiana shelf, but is higher than that of West Bay. When 0.45 Pa shear stress is applied, the average eroded mass is 0.044, 0.178 and 0.164 kg/m2 in West Bay, Big Mar, and Louisiana continental shelf, respectively.
(3) There seems to be an inverse relationship between river kilometer and the retention rate based on the synthesis of multiple studies. Since the retention rate is high in more landward receiving basins, preferential delivery of fine grained materials to more landward and protected receiving basins would likely enhance mud retention.
(4) The critical shear stress for sediment resuspension in Louisiana bays is around 0.2 Pa. Under the influence of a variety of fetches, depths and wind speeds, >0.2 Pa can be generated in many bays and estuaries. The fragmentation of large receiving basins can help decrease the fetch sizes and minimize wave-induced sediment resuspension.