The use of large water and sediment diversions in the lower Mississippi River (Louisiana) for coastal restoration

Journal of Hydrology 387 (2010) 346–360

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Review paper

The use of large water and sediment diversions in the lower Mississippi River (Louisiana) for coastal restoration Mead A. Allison a,*, Ehab A. Meselhe b a b

University of Texas Institute for Geophysics, University of Texas, 10100 Burnet Road (R2200) Austin, TX 78758-4445, USA Department of Civil Engineering, University of Louisiana, Lafayette, LA 70504, USA

a r t i c l e

i n f o

Article history: Received 26 October 2009 Received in revised form 19 January 2010 Accepted 1 April 2010 This manuscript was handled by G. Syme, Editor-in-Chief Keywords: Rivers Surface water quality Water resources Particle-laden flows Hydrologic cycles and budgets

s u m m a r y This study examines the use of large sediment and water diversions in the lower Mississippi River (e.g., South Louisiana) as a tool for coastal restoration. Herein we provide a review, new analysis and synthesis of existing work, much of it previously only available in government reports, and integrate our recent research on the topic. We outline critical knowledge gaps that need to be addressed by the time that construction begins on any future large diversions. The focus of this study is on ‘‘river side” issues and the policy considerations that arise from them. The study includes a quantitative examination of the sediment budget of the Lower Mississippi River as a region of potential diversion construction in South Louisiana, due to its critical control on any future management plans that include large diversions. We conclude that development of a coordinated system of diversions and other restoration strategies for land-building will require parceling this sediment budget out between individual projects. However, this is only possible if the input function and its variability is well understood. It is clear that numerical simulations are the only way to adequately predict the combined effects of multiple diversions and other restoration projects, such as dredge-fed pipelines, on the river channel for navigation, flood control and sediment regime. Numerical models also provide the only way to properly examine diversion structures to maximize their sediment capture and minimize any negative impacts. The status of these models and their application to lower Mississippi River channel hydrodynamics and sediment transport is examined herein. Ó 2010 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

Introduction and background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The tidal-estuarine reach of the Mississippi River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sediment input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Sediment transport processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Suspended load cycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Bedload transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Numerical modeling of hydrodynamics and sediment transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Overview and present capabilities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Modeling uncertainties and performance assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The case for large water–sediment diversions in the tidal-estuarine River . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Lessons from existing diversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Optimum location(s) and operation strategy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Engineering and design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Stakeholder issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions: future directions and critical observational and modeling needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author. Tel.: +1 512 471 8453; fax: +1 512 471 0348. E-mail address: [email protected] (M.A. Allison). 0022-1694/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2010.04.001

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1. Introduction and background The world’s deltas are among the most sensitive features on the Earth’s surface to climate change, given that these vast and agriculturally rich regions are located at or near sea level, and their reliance for maintaining elevation on water and sediment collected from large inland catchments. Over the past 50 y increases in the human population have had severe deleterious effects on the hydrologic operation of deltaic systems through enhanced fertilizer usage, damming, deforestation, and many other land-use changes (Bianchi and Allison, 2009 and references therein). Globally, water and/or sediment discharge to deltas and the coastal ocean has been decreasing, despite an increase soil erosion rates due to human activities in drainage basins (Vörösmarty et al., 2004; Syvitski et al., 2005). This is mainly due to dam and reservoir construction in the catchment: these reductions directly influence deltaic coastal retreat, in areas where a large fraction of the human population lives. Future climate change is forecast to have significant further impact on the health of the world’s deltas due to changes in: (1) precipitation/denudation/runoff patterns, (2) eustatic sea level rise rates, and (3) frequency and intensity of cyclonic storms. The Mississippi River Delta is one of the most anthropogenically altered deltaic systems on Earth, as well as one of the best studied, and hence, serves as a representative example of the impact of hydrologic alterations in deltaic systems and their potential solutions. The entire system is 25,000 km2, and consists of wetlands, bayous, shallow estuarine bays and emergent ridges formed during the late Holocene (6000 yBP to present) progradation of delta plain distributaries (Coleman et al., 1998). In the 1960s it was first recognized that the Mississippi delta region of south Louisiana was experiencing coastal wetland land loss rates that were among the highest on Earth (Gagliano et al., 1981; Day et al., 2000). Rates reached a maximum of 102 km2/y in the 1970s (Barras et al., 2003), and although rates have decreased somewhat since then (61 km2 from 1990 to 2000; Barras et al., 2003), Hurricanes Katrina and Rita in 2005 showed that individual cyclonic storms could account for a significant proportion of this wetland degradation (the two storms caused a combined 562 km2 of land loss in South Louisiana; Barras, 2006). Extensive studies of the causes of wetland loss in south Louisiana have determined that it is a combination of anthropogenic (e.g., artificial channel cutting and subsequent expansion, pond creation, urbanization, reduction in sediment supply, oil and gas withdrawal, etc.) and natural mechanisms including subsidence, salt water intrusion, sediment toxicity towards marsh plants, along with wave and storm surge related erosion (Britsch and Kemp, 1990; Penland et al., 1992; Turner, 1997; Day et al., 2000, 2007; Reed, 2002; Morton et al., 2003, 2006; Barras, 2006). The preponderance of natural wetland loss has been attributed to the effects of compaction-induced sediment subsidence exacerbated by a starvation of new sediment to wetland surfaces that resulted from levee construction along the lower Mississippi River (Baumann et al., 1984; Walker et al., 1987; Boesch et al., 1994; Penland and Ramsey, 1990). This makes the delta one of the most exposed sites to relative sea level rise (RSLR) on Earth (Blum and Roberts, 2009). The vulnerability of Mississippi delta wetlands is potentially even greater in the near future given that projections for global (eustatic) sea level rise for the 21st century range from 20 to 60 cm (multiple climate model means; IPCC, 2007) to as much as 1 m (Rahmstorf, 2007). Coastal wetlands maintain their viability and resist tidal inundation during times of rising sea level by having combined organic and mineral accumulation rates that meet or exceed the rate of RSLR (Delaune et al., 1983; Nyman et al., 1990; Cahoon and Reed,

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1995). Mineral accumulation is sourced from suspended sediment particles composed mainly of flocculated fines (clay and silt) and are trapped during high water sheet-flow river events when the flow is slowed by bed friction and above ground structures (stems, trunks, pneumatophores, burrows, etc.; Wolanski, 1995; Young and Harvey, 1996; Woodruffe, 2002). Hence, input of turbid Mississippi River water can aid in maintenance of marsh elevation in the face of rising RSLR by direct supply of mineral particles and allocthonous organic matter, and by supplying dissolved nutrients (N, P, Si, trace elements) that stimulate organic production rates. The combined threats of climate-induced eustatic sea level rise and a potential increase in the frequency or potency of cyclonic storms to wetlands and their associated ecosystem services have led a few influential voices to suggest that restoration, or even preservation of Mississippi delta coastal wetlands is a lost cause. Despite these views and through a series of State-Federal partnerships, various examinations of coastal restoration issues and potential solutions have been conducted in recent years, including Coast2050 (http://www.coast2050.gov), the Louisiana Coastal Area Ecosystem Restoration (http://lca.gov/) plan, and post-Hurricane Katrina, State Master Plan (http://www.lacpra.org) and US Army Corps of Engineers (http://lacpr.usace.army.mil/) planning documents that tie coastal, hurricane, and flood protection in South Louisiana (LACPR). LACPR has been recently reviewed by the National Academy of Sciences (NAS, 2008). Significantly, none of these initiatives have agreed upon specific recommendations for the use or location of large water diversions from the Mississippi channel for land building. Two main mechanisms have been suggested for rebuilding marsh areas: water and sediment diversions from the Mississippi and its Atchafalaya distributary, and long-distance pipelines to spoil dredged materials from the river beds as well as inland and offshore deposits. It is not our focus in this study to compare these various land building approached. Rather, we focus on large-scale Mississippi River diversions. We also explore the tradeoffs in the operation of large Mississippi River diversions as part of our examination of their relative location and operational strategy. While no large river diversions, defined here as >50,000 cubic feet per second (cfs) or 1420 cubic meters per second (cms) water exits (with or without gates to control the discharge) that are utilized annually, have been constructed or even approved to date in the lower Mississippi River (defined here as the section of the river influenced by tides at lower water discharges—about Baton Rouge to the Gulf of Mexico). However, the US Army Corps of Engineers (USACE) has conducted a series of examinations of large structure design and operation as part of project-specific and holistic coastal restoration studies (ABFS, 1982; USACE, 1984; MRSNFR, 2000; LCA, 2004; LACPR, 2009). Also, the Bonnet Carre spillway is an existing gated water exit with a capacity of 250,000 cfs or 7080 cms for protection of New Orleans and areas lower on the river from extreme river floods. It differs from large water and sediment diversions that are the focus of this discussion in that it is not used annually (only opened during years when flows exceed 1.25 million cfs), cannot be operated below a certain river stage, and is not managed to optimize either water and sediment delivery for beneficial coastal restoration or preservation. Two relatively small gated freshwater diversions have also been constructed on the lower Mississippi River (Fig. 1) at Caernarvon (3500 cfs or 99 cms, operational since 1991) and Davis Pond (10,650 cfs or 302 cms, operational since 2002) designed to limit salinity intrusion with minimal sediment capture. Another smaller and non-gated diversion was designed for sediment capture (West Bay) immediately above the Head of Passes (20,000 cfs or 566 cms with a potential to enlarge to 50,000 cfs (1420 cms), operational since 2004). The

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Fig. 1. An overview map showing the Lower Mississippi River and major points discussed in the text. The arrows refer to the three existing diversion projects (Davis Pond, Caernarvon and West Bay) and the Belle Chasse designation is the location of the USGS monitoring station.

State Master Plan distinguishes between land sustaining diversions, such as Caernarvon and Davis Pond, and land building diversions, with no definitive water volume to characterize the latter (>50,000 cfs is used herein). Our objectives of the present study are to outline the scientific and engineering underpinning of river channel hydrology on the construction of land building diversions on the Mississippi River. Specifically: (1) what is the present state of knowledge (and critical knowledge gaps) in support of constructing large diversions in the lower Mississippi River on the availability and accessibility of sediments, and the status of sediment transport modeling and (2) what can be learned from this knowledge base and from the operation of existing land sustaining diversions on the river with respect to design, location and socio-economic implications. We will not focus upon possible use of the Atchafalaya River above Morgan City, LA for diversions that would support coastal restoration in the Terrebonne Basin. The Atchafalaya receives a mandated 30% of the combined Mississippi + Red River discharges through the control structures at Old River (Fig. 1). Although not dealt with directly, much of the general information presented below about diversion engineering and science would hold for Atchafalaya diversions as well and they are mentioned in both the State and LACPR master plans. 2. The tidal-estuarine reach of the Mississippi River 2.1. Sediment input Among world rivers, the Mississippi, draining approximately 47% of the conterminous US, has the seventh largest water dis-

charge and suspended load (Milliman and Meade, 1983; Meade, 1996). The quantity and timing of the sediment and water discharge cycle is critical to the design and operation of large diversions. The Mississippi River (MR) water and sediment discharge hydrograph exhibits a large seasonal and annual variability, with high discharge (>30,000 cms) typically occurring in spring (February–May) with a series of individual peaks that last 1–2 weeks. In nearly all years, along the length of the MR, mean discharges during the high-water months can be expected to be about three times the discharges during the low water months (Meade, 1995). Tides penetrate roughly 350–400 km into the river during low flow (September–October) and <50 km during high flow (February–June). Tidal currents are generally too weak to reverse the downstream flow. As Meade (1996) points out, the ‘‘delivery to the oceans” of large rivers like the Mississippi can be misleading because in most cases the riverine input is quantified at a point on the river that is landward of the influence of tides and salinity. The furthest downstream long-term (>50 y) gauging station on the Mississippi utilized to calculate both water and sediment discharge above the tidal reach is located at river kilometer (RK) 492 (Tarbert Landing, MS, USACE operated). Tarbert Landing is located immediately downriver of the three river control structures (RK 508.9, 506.3, 501.4) at Old River, where water flow into the Atchafalaya pathway is controlled on a daily basis at 30% of the combined Red and Mississippi discharge (Fig. 1). Tarbert Landing has been operated at three sites (RK 377 in 1949–1958, RK 486 in 1958–1963, RK 492 since 1963) but is considered as a continuous record given that there are no significant gains or losses in water discharge over this reach (Meade and Moo-

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dy, 2010). Over the record interval, suspended sediment concentrations were measured biweekly (26 times/y) using isokinetic (intake velocity = flow velocity) point-integrative samplers, with sampling density of 4–8 vertical stations along a cross-river transects and 2–5 individual samples on each vertical (intervals between 0.15 and 0.95 total water depth at each vertical), with velocity (water discharge) measured along the transect using acoustic doppler current profiling (ADCP since 1994) (see Thorne et al., 2000 for details of the sampling). Daily sediment discharges are interpolated using water a sediment ratings curve. Suspended sediments (not water discharge) have also been measured 12–14 times/y by the US Geological Survey (USGS) since 1978 at St. Francisville, LA (RK 428) using depth-integrative (single sample integrated over about 0–0.9 total water depth; for methods see Edwards and Glysson, 1988). The timing of sediment and water discharge maxima is relatively synchronous at St. Francisville (Mossa, 1996). However, the quantity and timing of sediment discharge at non-tidal stations like Tarbert Landing and St. Francisville can only be viewed as an annual sediment input function to the lower MR—the suspended sediment loads, bedload transport rates and timing of sediment discharge is distinct in the tidal and estuarine MR reach due to the processes outlined in Section 2.2. A number of studies of interannual variability in these water and sediment records indicate that the suspended sediment input at these stations (and comparison with older records from further upstream) dropped sharply (40–70%) in the latter half of the 20th century. Given that water discharge has no trend in variability during this period (Poore et al., 2001), this decline in suspended sediment concentration has been attributed to the construction of dams on major tributaries, artificial levees, river straightening, wingdams, bank revetments, and soil conservation practices (Keown et al., 1986; Kesel, 1988; Milliman and Syvitski, 1992; Smith and Winkley, 1996; Horowitz, 2010; Meade and Moody, 2010). An independent confirmation of this trend and timing is observations of a factor of 2–3 reductions in mass (sediment) accumulation rate on the continental shelf seaward of the birds-foot delta in beginning in 1946 ± 8 y (Allison et al., 2007). As shown in Fig. 2A and B, the Tarbert Landing data (from Meade and Moody, 2010) show that after the sharp drop in sediment load in the 1950s (both in tons/y and flow-weighted suspended sediment concentration), there was a continued slow decline to near asymptotic values after about 1990. After 1981, the St. Francisville data shows fairly good agreement in tons/y with

Tarbert Landing data, albeit most years are slightly lower than at Tarbert Landing. The St. Francisville flow-weighted concentrations (Fig. 2B from Horowitz, 2010) exhibit much lower interannual variability. The causes of these differences with Tarbert Landing are unresolved but may relate to differences in the sampler used, maximum depth of sampling, or real differences in load between the two sites. There is also a temporal offset in the two datasets—Tarbert Landing is based on the flood year (1 October–30 September), while St. Francisville is based on the calendar year. The St. Francisville data, which in Fig. 2 shows little evidence of declining sediment loads, has been reduced differently in Fig. 3A and B; data from Horowitz (2010). In Fig. 2, sediment discharges at both sites were calculated using a single sediment ratings curve that utilized data from all years. In Fig. 3, the St. Francisville data has been calculated using independent annual ratings curves, based on just 12–14 samples collected in a calendar year, reflecting the fact that the concentration–discharge relationship may vary interannually. The multiple curve approach show clear evidence of declining sediment loads and flow-weighted concentrations over the period of record, although most of the slope is generated by pre-1990 higher values. It should be pointed out that Horowitz et al. (2001) and Horowitz (2006) also suggest there has been a significant stepped decline in sediment loads in the years (calculated through 2004) following the large flood of 1993 (including a 29% decline at St. Francisville using the data presented in Fig. 3) that is particularly well-defined at monitoring stations further up the drainage such as at Thebes, Illinois. This behavior may be attributed to downcutting of the floodplain in the upper basin, hence depleting it as a sediment source for the lower MR. We have chosen to focus on the fact that both stations show relatively stable values after about 1990 regardless of data reduction methodology. The present estimates of post-1990 annual sediment input to the lower river are presented in Table 1. Several caveats exist for this sediment discharge data. Thorne et al. (2000) recognized that there is a poor correlation coefficient for suspended sediment concentration versus water discharge at St. Francisville (0.19 for all sediment) and at Tarbert Landing (0.21). Although a significant portion of this scatter is a function of sediment load peaking on the rising limb. Examination of data from even a single discharge year shows large scatter that induces error in the total suspended load calculation. Such large scatter is

Fig. 2. Annual sediment loads (A) and flow-weighted sediment concentrations (B) in the lower Mississippi River showing inputs to the lower rivers as measured at gages at Tarbert Landing, MS and St. Francisville, LA. Data synthesized from Meade and Moody (2010) and Horowitz (2010).

Fig. 3. Annual sediment loads (A) and flow-weighted suspended sediment concentrations (B) at St. Francisville, LA calculated based on individual (annual) ratings curves versus the combined curves presented in Fig. 2. Data synthesized from Horowitz (2010).

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Table 1 Summary of annual sediment input to the lower Mississippi River. Station

1981–1990 106 Tons/y

1990–2004 106 Tons/y

1981–1990 Flow-weighted mg/l

1990–2004 Flow-weighted mg/l

Tarbert Landing (single ratings curve) St. Francisville (single ratings curve) St. Francisville (multiple ratings curves)

133.7 ± 38.4 103.8 ± 24.6 122.3 ± 23.6

123.5 ± 28.8 110.5 ± 23.7 91.1 ± 28.1

300.1 ± 39.7 222.2 ± 12.3 271.4 ± 51.7

246.3 ± 39.3 224.5 ± 11.5 193.3 ± 40.4

likely due to sampling errors and variations in measurement methods over the decades. Further, suspended load measurements by isokinetic sampler (either point-integrative or depth-integrative; see Edwards and Glysson, 1988 for description of types) generally sample to 0.9 of total water depth in MR stations. This yields a discharge-weighted suspended sediment concentration along the vertical, since isokinetic samplers equilibrate with flow velocity. Nonlinear increases in sediment concentration in the near-bottom (see Section 2.2) might lead to underestimation in this ‘‘unsampled fraction” as has been predicted by modeling (Wright and Parker, 2004), and, hence, in the total suspended load calculation. As bedload transport is not measured at these stations, the additional contribution of bedload sand input is not included in the annual sediment input function to the lower river, leading to a further underestimation. It has also been observed in rivers that this bedload transport rate may also vary interannually (delivered in pulses) even in times of steady annual water discharge (Gomez et al., 1989). To what extent can these loads be relied upon for modeling of future diversion yields assuming no major changes in upriver anthropogenic impact (e.g., dam removal, shifts in agriculture, etc.)? The effects of the 1993 MR flood (Horowitz, 2006) suggest that basin denudation in major floods can decrease suspended load for periods of more than a decade. Miller and Russell (1992) posit that increased global warming in the future will increase runoff for 75% of the world’s major rivers. However, recent detailed predictive modeling of basin precipitation patterns associated with warming suggest that mid-latitude systems like the Mississippi are likely to see little net change in water (and hence, sediment) discharge (Nohara et al., 2006). It should be noted that these trends are based on annual averages: given the non-linearity of suspended sediment discharge (e.g., large floods deliver proportionately greater sediment loads), future changes in the frequency or magnitude of large floods could be masked in this type of data analysis. However, no available evidence suggests we can expect loads larger than those we synthesize in Table 1. 2.2. Sediment transport processes 2.2.1. Suspended load cycling A plot of stage elevations relative to national geodetic vertical datum (NGVD) mean sea level for lower MR stations (Fig. 4) demonstrates why suspended load varies significantly over the discharge cycle relative to St. Francisville and Tarbert Landing. During the 2008 flood (highest discharge since 1997; 38,500 cms), water surface elevations show a constant slope of about 1 m per 20 river km all the way to Head of Passes (RK 0). In this case, water velocity is slope-driven and one would expect the lower river velocities to be similar to those at Tarbert Landing. However, at a discharge that corresponds to the average annual low at Tarbert Landing (6340 cms), significant slopes are absent (1.3 m elevation at RK 250) below about RK 350, which corresponds approximately to the maximum penetration of tides. Flow is inertially driven, and the resulting reduction in velocities have been shown by Galler (2004) to result in deposition of muds (<63 lm fraction) in water depths of less than 20 m, where bed friction further reduces flow velocities and flow reversals are pres-

Fig. 4. Water surface elevations (above NGVD sea level) for gaging stations on the lower Mississippi River at a low and high discharge showing the slope-related control on water and sediment velocities in the lower river (data provided by the US Army Corps of Engineers from http://www.mvn.usace.army.mil/eng/edhd/ watercon.asp).

ent associated with bend eddies. These observations are supported by Demas and Curwick (1987) who showed that suspended sediment concentrations and bottom-material size in the lower MR varies with discharge and is finest during low discharge. Dagg et al. (2005) also observed a downstream linear trend of decreasing suspended sediment concentration in a Lagrangian study of the lower river in June, 2003 that they attributed to rainout of flocculated particles. The flocculation aspect is critical to this tidal reach storage. Flocculation studies in the freshwater MR by Rees and Ranville (1990) and Galler and Allison (2008) suggest that a significant fraction of the suspended mud is in aggregate form, with settling velocities much faster than the disaggregated particle distribution. Galler (2004) estimated that about 14  106 tons of the annual suspended sediment load may be stored seasonally in the lower river (RK 290-13), on the basis of low discharge (November 2002) side-scan sonar mapping and sediment coring studies that extended from RK 290 to 127. During the rising water discharge limb, Demas and Curwick (1987) and Galler (2004) demonstrated that this stored mud is remobilized, altering suspended sediment concentrations in the lower MR relative to Tarbert Landing or St. Francisville. This storage-remobilization cycle can be observed (Fig. 5) by comparing lower river water and sediment gauging stations at Baton Rouge (RK 369.3, installed 2004) and Belle Chasse (RK 123.3, active 1977–1997, reinstalled 2007) collected by USGS 2008 (data from http://waterdata.usgs.gov/la/nwis/qw) in the large flood of 2008. At water discharges below about 10,000 cms (December–January), samples from the tidal reach (Belle Chasse and a dataset collected in 2008 by the authors near Empire (RK 35–56) have a reduced suspended sediment load and concentration relative to the upriver stations, reflecting fallout of fines. During the initial rising discharge phase (to about 25,000 cms in early March), suspended loads and concentrations were higher at Belle Chasse, likely caused by the remobilization of this bed-stored sediment. All values were approximately equal in late March, despite further increases in dis-

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of this frontal zone, the position of which is well-constrained and dominantly controlled by volume of water discharge; although flow duration, wind velocity and direction, tides and water temperature are all contributing factors (Soileau et al., 1989). Frontal zones are effective sediment traps, due to the convergence of flux at the salt front (Postma, 1967). Settled mud deposits as deep as 2 m (based on the depth of the radiotracer 7Be [53-day half-life]) have been found below the MR bottom salinity front, confined to the channel thalweg, with deposition rates of up to 1 cm/day (Galler and Allison, 2008), indicating localized zones of very intense trapping. These deposits remain on the bed for up to several months until discharges rise above 8500 cms: during the 1997– 2007 period the duration of discharge less than 8500 cms ranged from 34 days (2004) to 197 days (1999) with a mean of 96 days. Galler and Allison (2008) found seasonal trapping associated with the frontal zone accounted for about 10  106 tons of sediment quantified for the reach from SW Pass to RK 21). In extreme low discharge (<3000 cms) events in historic times, the salt wedge has migrated above New Orleans (RK 165), leading the USACE in recent events (once in 1988 and another in 1999) to place an earthen thalweg sill at RK 116 to halt contamination of New Orleans drinking water intakes. The bottom salinity front reaches this sill location at a discharge of about 4200 cms (according to the plots of Soileau et al., 1989).

Fig. 5. USGS gaging station data on water and suspended sediment discharge at stations (St. Francisville, Baton Rouge, Belle Chasse) in the lower river during the flood of 2008 (data from http://waterdata.usgs.gov/la/nwis/qw). Data collected by the authors in 2008 at Empire (RK 35–56) is also included.

charge, indicating that this bottom source is now exhausted. The cause of a second increase in the suspended load and concentration at the tidal stations in the April high discharge phase is unknown, but may relate to inundation of land areas inside the levees (e.g., batture) as an additional source of fines. The behavior of suspended sand (>63 lm) loads differ from that of the fine (<63 lm) fraction: little or no sand is in suspension in all stations at discharges below about 15,000 cms (Fig. 5). In higher discharges, although all stations show a non-linear increase in sand load in suspension, the highest values are at Baton Rouge, and the lowest are at St. Francisville. Although the source of the Baton Rouge peak is unresolved, the increases in the lower river relative to St. Francisville do indicate that this reach is a source of additional sand, likely again from remobilization of the finest sand fractions (<125 lm) stored on the channel floor as bed load during the waning discharge phase. A second storage mechanism that alters suspended sediment concentrations with discharge is associated with a strong estuarine frontal zone. This ‘‘salt wedge” is found in Southwest Pass (below RK 0) at discharges of about 8500–25,000 cms (Soileau et al., 1989). Southwest (SW) Pass (Fig. 1) has a dredged channel depth of 14 m and now forms the dominant conduit for salt wedge penetration over the shallower Mississippi passes to the east (e.g., South Pass, Pass a Loutre, Main Pass). When the water discharge drops below about 8500 cms (Soileau et al., 1989), the salt–water wedge penetrates through SW pass and moves upstream into the main channel of the MR above the Head of Passes. The thalweg of the MR channel exceeds the depth of the surface of the Gulf of Mexico for a distance of greater than 560 km above the Head of Passes, and therefore is conducive to the upstream migration

2.2.2. Bedload transport Spatial and temporal limitations on sampler design have hampered development of accurate mass flux measurement of bed load in large rivers like the Mississippi (see discussion in Gomez, 1991). Recently, however, Nittrouer et al. (2008) utilized a newly developed repeat multibeam bathymetric mapping methodology to arrive at bedform transport rates for the lower MR. This methodology measures that fraction of the sand transport near bed that results in the migration of dunes. These measurements, from New Orleans (RK 162–167), English Turn (RK 132–140), and Venice (RK 7–12) from 2003–2006, combined with more recent measurements (Fig. 6), show a relatively spatially invariant mass flux in the tidal MR reach that has a logarithmic relationship (r2 = 0.73) with water discharge. To date, the only measurements by this method of bedload transport rate where suspended load data was also obtained from the same site, were surveys the authors conducted in 2008 and 2009 (see Fig. 6). A comparison of low versus high discharge sediment load calculations made in 2008 by the authors at Empire, LA (RK 47) give an estimate of the range of contribution of bedload (sand) transport to the overall sediment load of the tidal

Fig. 6. Bedload transport rates calculated in the lower Mississippi River utilizing repeat multibeam bathymetric mapping from Nittrouer et al. (2008) for 2003–2006 data and 2008–2009 data (Empire, Magnolia, and Myrtle Grove) collected by the authors. Empire data was collected at RK 47, Magnolia at RK 79, and Myrtle Grove at RK 95 and RK 102.

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MR. In the lower discharge survey (January 2008, 11,180 cms), bedload transport (2054 tons/d) was less than 4% of the overall sediment load (suspended + bedload), but made up 100% of the sand in transport (e.g., no sand in suspension). At high discharge (April 2008, 38,220 cms), bedload transport (104,297 tons/d) was 11% of the overall sediment load and 35% of the sand in transport. The grain size character of lower MR sand in bedload transport was first examined in systematic downriver channel bed surveys in 1932–1938 and 1989 (Nordin and Queen, 1992). This study indicated that mean grain size of the bed declined between 1932 and 1989 above Baton Rouge, but observed no significant change in either mean or median grain size in the tidal reach. Several issues complicate the use of these data, including that while both data sets were collected at low discharge (a period when there is no suspended sand in the river that might contribute to the character of bed sand), the 1932–1938 data were collected at over a relatively long time period (weeks) such that significant variations in river discharge were likely encountered. These low discharge figures also do not indicate how bed grain size character changes over high discharge. Finally, no attempt was made to restrict sampling to channel bed areas covered by a sand sheet in active bedload transport: other bottom types include shallow areas of mud storage (see Section 2.2.1) or relict incised sediments (e.g., no modern sediment cover). To address these issues, Allison and Nittrouer (2004) collected a low-discharge bed grain size dataset of 240 samples surficial samples over only 5 days in November 2003 that extended from English Turn RK 126 to RK 0 (Fig. 7). These samples were collected in league with multibeam bathymetric and subbottom seismic data that allowed sampling to be carried out exclusively in sandy (bedform) areas. These data indicate that sand in bedload transport fines throughout the tidal reach from a median (D50) of 265 lm to about 180 lm at Head of Passes. There is a coarsening of sand grain size at water depths greater than about 24 m (Fig. 7B), reflecting increased flow velocities near the channel thalweg. Finally, a repeat of a subset of the stations at high discharge in May 2008 (Fig. 7A), showed no significant difference in grain size with the low discharge data, although sample-to-sample standard deviation was higher, perhaps reflecting differences in samples collected from dune troughs versus crests at a time of larger bedforms. The major conclusion of this recent survey is that the available bedload sand, either for capture in diversions or long-distance pipeline dredging, is of declining quality downriver, if viewed from the point of view that larger grain sizes are less likely to be remobilized by waves, tides and currents after placement (e.g., barrier island or wetland construction). 2.3. Numerical modeling of hydrodynamics and sediment transport 2.3.1. Overview and present capabilities In the field of environmental hydraulics, computer flow models have been used successfully to analyze complex engineering problems and examine innovative solutions in an efficient and economical manner. These models can provide accurate information ranging from global and integrated parameters such as stage and total discharge to detailed information of complex three dimensional flow patterns. Currently available models are fully capable of providing accurate information regarding vertical and horizontal flow velocities, secondary motion and eddies, flow acceleration, turbulent fluctuations, etc. However, the current capabilities of modeling sediment fate and transport are not as advanced. There are models that provide adequate information regarding global and local erosion and deposition patterns. However our ability to model specific sediment processes such as entrainment mechanisms of bed sediment into suspension, migration of bed-forms, impact of bed-forms on bed-roughness and overall hydrodynamics, and sediment concentration distribution over water column is lim-

Fig. 7. Bed material grain size in sand reaches of the lower Mississippi River with river kilometer and station number (A) in 2003 (low discharge as a line plot) and 2008 (high discharge as triangles) from a report to the State of Louisiana by Allison and Nittrouer (2004). Median (D50), D90 and D10 for the low discharge data is plotted as best fit linear regression lines. The 2003 low discharge data is presented as a function of water depth in graph (B), grouping samples in 3–6 m water depth bins.

ited. Thus the need to improve our ability to simulate the various physical process of sediment transport is obvious. We emphasize here the importance of gathering high-quality and extensive field measurements of sediment especially during and after the construction of diversions. This will provide us with valuable understanding of the Riverine and Bay systems response to diversions. Such knowledge would certainly enhance our ability to numerically mimic the physical sediment transport processes. 2.3.2. Modeling uncertainties and performance assessment It is often a challenge to thoroughly evaluate the performance of numerical modeling tools, and provide decision makers a ‘‘level of confidence” in the model predictions. In the practical sense, the objective of a performance assessment and uncertainty analysis is to determine the range and likelihood of model predictions for existing conditions and under future proposed restoration scenarios. It is recognized (Habib et al., 2007, 2008) that models will provide only an approximate representation of what actually takes place in the physical riverine system. Uncertainty analysis is a broad subject that encompasses limitations on field observations, limiting assumptions on physical and numerical formulations, and imprecise knowledge of physical and numerical parameters. However, it is critical to quantify the uncertainty associated with the predicted response of the riverine

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system under future proposed diversion strategies. The model performance can be assess through a detailed sensitivity analysis of the model response to uncertainties of the boundary conditions (measured flow discharge, sediment load, and bathymetry), calibration data (e.g. velocities, sediment concentrations, etc.) as well as physical and numerical parameters (e.g. roughness coefficients, diffusion coefficient, choice of turbulent closure model, etc.). Specifically, model predictions of water level, flow velocities, sediment concentrations and load as well as erosion and deposition patters should be thoroughly analyzed to address and provide formal evidence and assessment of the model performance. Such analysis should be carefully designed to specifically address questions such as: (1) what are the physical/statistical characteristics of the model deviations from the field measurements? (2) is the model performance consistent in predicting average as well as extreme conditions? (3) is the level of confidence in the model predictions suggests ability to discern differences among alternative designs of diversion scenarios? (4) are the uncertainties in the model predictions high enough, such that it renders the model unusable for practical applications? and (5) what is the confidence associated with the predicted response of the system under proposed diversion scenarios? 3. The case for large water–sediment diversions in the tidalestuarine River 3.1. Lessons from existing diversions The only large (>1420 cms) water ‘‘diversion” from the tidalestuarine section of the Mississippi River to date is the Bonnet Carre Spillway (RK 206), authorized in 1928 and built in 1929–1936 as part of the post-1927 flood control system for the lower river. The speed of its design and construction are testament to the importance of political will in creating large diversions in the lower river. The structure is a 7000 ft long (2134 m) concrete weir with 350 bays, each sealed with 20 timber ‘‘needles” that can be raised by a crane running along the weir top to allow river water to pass into an earthen leveed spillway: total capacity is 250,000 cfs (7079 cms). The advantages of this relatively simple structure are ease of operation and precise control of flow into the spillway, which flows into Lake Pontchartrain. However, water only reaches the weir entrance channel at high discharges and hence it captures water from the relatively sediment-poor uppermost water column (see below). Opened on nine occasions since 1937 (triggered by a flow at New Orleans above 1.25 million cfs [35,396 cms]), detailed monitoring of suspended sediment behavior was conducted by the USGS and USACE in the last two openings (1997 and 2008), which have the advantage of being records of sediment-rich high flow events. Figs. 8 and 9 shows a compilation of USGS/USACE data of suspended loads passing through the structure into the spillway in 1997 and 2008. The 1997 data constrain the daily sediment loads that can be expected to pass through a structure in a large flood: 340,000 metric tons/d at 5985 cms (0.66 kg/m3, 62% sand) late in the rising limb of the flood (Fig. 8). Flow-weighted total loads at the structure (spillway at Forebay in Fig. 8) decline thereafter during the falling limb of the flood (due to hysteresis) to 0.24 kg/m3 (38% sand). Fallout of particulates in the spillway in 1997 is also rapid and weighted toward the more rapidly settling coarse (sand) fraction. An average of 88.1 ± 2.8% of the mud (<62.5 l) fraction, but only 12.4 ± 11.1% of the sand, reached the Airline Highway bridge 3.3 km beyond the structure Little additional fallout was observed at the I-10 bridge (8.8 km beyond the structure) in 1997, suggesting that most of the fallout takes place within the first 1–2 km of the spillway. This data demonstrates the relatively rapid buildup of a near-range sand platform that is possible in large sediment diversion structures. Given that we are

Fig. 8. Bonnet Carre diversion data from the 1997 opening of the structure (data provided by C. Demas, USGS Lousiana Water Sciences Center). Samples were collected (by the USGS) at the Spillway exit from the river, at the Airline Highway bridge 3.3 km from the exit, and at the Interstate 10 bridge (8.8 km from the exit).

decanting the upper water column at Bonnet Carre, it should represent a relative minimum for sand diversion when compared to structures designed for sediment capture. In the smaller diversion of 2008 that was only sampled at the Airline Highway bridge (Fig. 9), suspended sediment concentrations were somewhat lower than in 1997 (peak at Airline of 217 vs. 186 mg/l), which may be a function of the longer rising water discharge limb in 2008 and/or the decline in overall suspended sediment concentration carried by the lower MR as discussed in Section 2.1. The Bonnet Carre structure is not operated for land-building purposes: operating it as such is complicated by possible detrimental ecological effects on Lake Pontchartrain flora and fauna from the nutrient-rich freshwater influx (see Brammer et al., 2007 and references therein). The three diversion structures at Old River that reroute 30% of the combined Mississippi–Red water into the Atchafalaya River constitute the only other large diversion structures constructed on the MR in Louisiana. The Low Sill-Overbank, Auxiliary Structure, and Murray Hydroelectric Power Station are designed to together divert up to 20,390 cms during major floods. Construction of the hydroelectric power station at the site is possible due to the average 6 m head differential between the Mississippi and Atchafalaya. Given the much lower stage elevation change with discharge in the lower MR, structure and outflow channel design will necessarily be distinct for any future sediment diversions. The Auxiliary Structure is located furthest downriver along the inside of a meander bend and has a river-side access channel (152 m wide, 3.1 km long) cut into the batture. Annual flushing of sediment accumulating in this access channel is necessary, given that it is only operated in

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Fig. 9. Bonnet Carre diversion data from the 2008 opening of the structure (data provided by C. Demas, USGS Lousiana Water Sciences Center). Data was only collected at the Airline Highway bridge (3.3 km from the river exit) during this event.

large floods. This procedure suggests that future large diversions in the lower MR might be designed with access channels to facilitate this trapping, such that river sediment (particularly sands) may be trapped and stored during months between operation of the structure (typically at high discharge—see Section 3.2) to maximize coarse sediment outflow when operating. The only diversion to date designed specifically for land-building is the diversion at West Bay (RK 7.6) which began operating in November 2003. A dredged channel was cut in the west bank MR levee 7.6 m deep and 60 m at a 120° angle of diversion, with a project goal of discharging 566 cms of water at 50% river stage duration. The initial design calls for widening the channel to allow for 1420 cms discharge at 50% stage duration following a initial monitoring phase (which ended in 2008) to assess the likelihood of thalweg capture or shoaling in the adjacent MR navigation channels and anchorages. Project goals were to create, nourish, and maintain 9831 ac (3978 ha) of emergent marsh within the project area over the 20 y project life by enhancing the natural process of delta growth and through the beneficial placement of material dredged during construction and maintenance (data from Project Fact Sheet at http://www.lacoast.gov). The initial study also recommended eventual emplacement of a Sediment Retention Enhancement Device (SRED) within West Bay (earthen dike with low-level weirs located at 305 m intervals) to maximize wetland creation. Early attempts by the academic community to monitor sediment accretion within West Bay (62 m deep in the project area) in 2004–2006 showed net bay floor erosion, which was attributed to the passage of Hurricane Katrina in August 2005 (Andrus, 2007). Further, shoaling immediately upstream of the diver-

sion in the Pilottown Anchorage Area is requiring maintenance dredging. The main difficulties of the West Bay site relative to potential diversions further upriver is: (1) the more rapid compactional subsidence (increased relative sea level rise) in this thick (up to 100 m) recent (late Holocene) sedimentary section adjacent to the modern birds-foot delta (Penland and Ramsey, 1990), which would require additional sediment volumes per unit area to build and maintain wetlands, and (2) the less hydraulic head available to move sediment out of the channel. Furthermore, the river reach downstream of Venice (RK 20) and above Head of Passes experiences significant flow loss through large passes (e.g., Baptiste Collete, Grand Pass, Main Pass). The significant diversion of water discharge results in a decreased capacity for sediment transport. The tidal and estuarine storage-remobilization cycle described in Section 2.2 can also be expected to concentrate seasonal sediment storage in this reach. We conclude that annualized sediment transport (particularly sand) through potential diversions below Venice would be reduced over areas further upstream and the bulk of sediment discharge through such diversions would be limited to very brief intervals at high discharge phase, when sufficient energies exist to flush stored sediments in the reach. This is evident in the fact that nearly all maintenance dredging in the river reach between New Orleans and Head of Passes takes place downstream of Venice. The relatively small lower MR diversions at Davis Pond and Caernarvon are designed to limit saline intrusion and their value for land-building is hampered by small initial input (estimated 0.26 million tons/y at Caernarvon; Lane et al., 2006) and the design of ponding areas at the end of the basin-side access channel, which serve as sediment traps and limit escape (particularly of sand) into the basin beyond. In the case of Caernarvon, the initial design was modified to cut access points through the bayous that extend outward into the Breton Sound marshes. By a ‘‘pulsed” operation strategy, turbid river water escapes these cuts and sheet flows on top of the marsh surface, providing river nutrients and sediments (Snedden et al., 2007). Since construction the Caernarvon project area has shown evidence of increased sediment accumulation rates (Delaune et al., 2003), but there is no clear indication from the aforementioned studies about whether these rates exceed RSLR rates, and hence, will lead to land accretion. Further, marsh loss from Hurricane Katrina and Rita in 2005 is estimated at 108 km2 in the Breton Sound basin where the diversion discharges (Barras, 2006). 3.2. Optimum location(s) and operation strategy In a broad spatial sense, the location of large sediment diversions below New Orleans on the MR would have the greatest impact on slowing or reversing wetland loss: these are the areas that have experienced the most substantial incremental and tropical storm-induced decay. The majority of coastal communities in this reach are already protected by storm surge levees and the river levee, minimizing the need for construction of ring levees around communities caused by increasing water elevations in the proximal receiving basin when the diversion is operating. Diversions at the upper end of the New Orleans to Head of Passes (RK 0) reach are favored by: (1) thinner Holocene deltaic strata and presumably lower compactional subsidence rates which would cause less severe RSLR issues in the resulting subaerial splay lobe and (2) higher river velocities at a given discharge (e.g., hydraulic head) that presumably would lead to greater competence for suspended sediment transport (increased concentration and grain size). As Fig. 4, shows, however, the latter effect is not as consequential if the diversions are only operated at the highest discharges (see below) when river surface slopes are relatively constant in this reach. As mentioned above, this only holds to approximately Venice (RK 20): below Venice the loss of water through the passes upriver of Head of Passes may reduce competence for sediment transport in

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a given flow. Limited suspended sediment data is available below the USGS station at Belle Chasse to determine to what extent suspended sediment concentrations and grain size vary downriver, although annualized suspended sand flux declines from 22.6 to 17.6 million metric tons between St. Francisville and Belle Chasse (Nittrouer et al., 2008). Planning for the impact of subsidence also is hindered by the absence of a community consensus on the relative magnitude of sediment compaction, deep isostatic adjustment, and fault activation (see Tornqvist et al., 2008; Dokka, 2006; Meckel et al., 2006 for the issues) such that spatial maps of total subsidence can be generated. The reach below English Turn in the period 1750–1927 (prior to the installation of federal levees) was the location of 19 recurrent natural crevasse sites that built splay delta wetlands laterally out from the natural levee into Breton Sound (east bank) and Barataria Basin (west) (Davis, 2000). The potential efficacy of these natural crevasses for land-building is demonstrated by the artificial crevasse created by dynamiting the levee in the 1927 flood near the present Caernarvon freshwater diversion site: a clay layer up to 89 mm thick has been mapped in the adjacent wetlands, with diversion discharge estimated to have reached 9000 cms (Snedden et al., 2007). Sites of historical crevassing may provide an advantage for location of sediment diversions in that they likely contain a coarser, and hence, more compactionally stable, substrate upon which to load additional sediment. Dating of these historical crevasses suggest they operate on timescales of decades (Davis, 2000), experiencing rapid initial vertical aggradation of the splay and deepening of the entrance channel, followed by slowing of aggradation as splay elevation rises and wetland area growth transitions to a mainly progradational phase. These crevasses are ultimately abandoned as rising elevations of the splay reduce water flow, and crevassing is initiated elsewhere where there is a gradient advantage. This natural crevasse splay cycle suggests possible advantages for sediment diversions—both in terms of multiple locations and length of operation. There are several advantages to constructing multiple (at least one on each side of the river) large sediment diversions below New Orleans. From a cyclonic storm surge perspective, there is the example of Hurricane Katrina storm surges along the east bank in Plaquemines parishes exacerbated by water pile-up against the river levee. Opening diversions on both sides of the river would help reduce this surge intensification. Operating a series of large diversions on a rotating cycle of at least a decade, and alternating water diversion between basins, has the benefit of overcoming the issue of vertical aggradation of the splay, ultimately reducing competence of the basin-side delivery channel(s). During the static interim phase, RSLR (subsidence and sea level rise) would lead to elevation decay. It also has the benefit of creating a series of splay deltas in various stages of evolution to maximize ecological diversity. While sediment and water diversions of this volume will undoubtedly cause major ecological change to the existing basin ecology, such as shell and fin fisheries (see Section 3.4), ‘‘staging” will allow for basins with distinct purposes— wetland creation and establishment of low salinity marshes in active diversion areas, commercial and recreational estuarine fisheries in others, for instance. Precise control of water and sediment at multiple locations also allows for adaptive management for addressing ecological concerns and issues arising from channelside effects such as unforeseen shoaling. The complication of the multiple diversion strategy is the complexity of predicting their effect on the New Orleans to Head of Passes sediment dynamics: these effects may be non-linear. This argues strongly for a rigorous numerical modeling approach that tests multiple scenarios of location, number and volume of large diversions too best anticipate these effects, combined with adaptive management guiding their operation.

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The previously mentioned high and low discharge surveys of suspended load (plotted in Fig. 5) collected by the authors in 2008 and the Bonnet Carre openings (Figs. 8 and 9) provide a preliminary parameterization of sediment load and character that could be expected to be captured in a large sediment diversion below the monitoring station at Belle Chasse. The rising-to-high discharge phase, because of the remobilization of fines stored in the lower river (as shown in Fig. 5), is even more crucial in this reach for sediment capture than sites upstream of the tidal-estuarine reach. Suspended loads at high discharge in April 2008 (maximum water discharge allowed in the channel below Bonnet Carre; Fig. 9) were 11.6 times higher than in January when discharge was near the long-term annual average low flow. Further, no sand is present in the water column in January: it makes up 19% of the suspended load in the April high flow. Although these data do not allow the discharge at which sand becomes a suspended load component to be ascertained, Nittrouer et al. (2008) compiled data from the Belle Chasse station and determined that it occurred at about 16,000 cms: higher discharges occur on average about 127 days/y (in February–June) based on the 1930–2008 flow at Tarbert Landing. The suspended sediment load also has a strong relation to water depth (as well as cross-sectional and longitudinal variations). Near-surface concentrations of mud (<63 lm fraction) likely captured by a diversion average 48% (low flow) and 83% (high) value at mid-depth (Fig. 11) using a point-integrative sampler. With sand, the near surface depletion is even more severe, with less than 10% of the mid-level value. This mid-level value plotted in Fig. 10 is reflective of what a USGS depth-integrative sampler used at the upriver monitoring stations would record, suggesting that monitoring station concentrations significantly overestimate the loads available to a diversion that decants the uppermost water column. Fig. 11 demonstrates that sand grain size also shows a reduction of coarser sand components in the uppermost water column. Note also the much coarser grain size of the bedload sand. The length and timing of the high water discharge in the lower Mississippi varies from year-to-year and often shows as many as 3–5 individual peak events between December and June that last for 1–2 weeks. Maximizing sediment capture would require almost daily alteration of the water volume passing through the structure(s) to capture periods of the highest suspended sediment concentration, in order to minimize the freshening of the receiving basin (assuming that was a goal for ecological reasons). This ‘‘pulsing” of the diversion is presently being utilized at the existing Caernarvon diversion (Lane et al., 2006; Snedden et al., 2007) to create a temporary water elevation rise (e.g., sheet flow) in the receiving basin that floods inland marsh areas as well as the slightly higher, streamside-levee marshes. This helps to overcome the issue of interior marshes usually being less productive due to low sediment delivery, waterlogging, and lower dissolved nutrient input (Lane et al., 2006). We also recognize that large structures designed for maximizing sediment capture during high flow, might also be operated at lower flow rates (similar magnitudes to Davis Pond and Caernarvon) during the remainder of the year, to function as salinity control structures. 3.3. Engineering and design criteria Existing diversions in the Mississippi River have been ‘‘engineered”, meaning a concrete-gated gap in the artificial levee is constructed where flow is controlled by gate openings as well as the natural rise-and-fall of the river surface. The only exception is the West Bay Sediment Diversion, which is a ‘‘natural” diversion that directs river flow through an earthen cut in the levee, with flow constrained only by the dimensions (width and depth) of the cut. The advantage of ‘‘natural” diversions is their lower con-

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Fig. 10. Suspended sediment loads near Empire, LA (RK 47) collected by the authors at low (January) and high discharge (April) in 2008 using an isokinetic point sampler (P-63 USGS standard) showing the depth relationship of sand and mud concentration. The dotted line reflects what a USGS depth-integrative sampler would observe based upon depth-averaging these point values.

struction and maintenance costs. Although they also more closely resemble a natural crevasse and would likely evolve in a similar fashion (as described in Section 3.2), it is precisely this ‘‘uncontrolled” evolution that presents complications for its operation. Early results from West Bay (see MR-03 project at http://www.lacoast.gov/) suggest the cut has enlarged more rapidly than anticipated from the basic modeling studies conducted prior to construction, perhaps accelerated by the large storm surge in the lower river during Hurricane Katrina in 2005. Further questions are outstanding about to what extent this enlargement has caused greater-than-predicted shoaling in nearby vessel anchorages. Larger diversions installed further upriver where there is additional head elevation range, would likely be even more difficult to predict their evolution. In addition, as discussed above, if a multiple diversion scheme were developed, precise control of the timing of flows into the receiving basin would be key to an adaptive management plan, and can only be accomplished with engineered diversions. If an engineered large sediment diversion(s) are constructed, likely they will be similar in some design aspects to the Bonnet Carre spillway: low head differential between river and basin, large number of rapidly controlled gates. However, as mentioned earlier, the Bonnet Carre is constructed to decant only the relatively sediment-poor uppermost water column. We believe that design

should be optimized for maximizing wetland creation, rather than preservation of existing wetland areas (also a worthy goal). To do this, design must address the issue of establishing a stable substrate (e.g., sand subject to relatively low compaction) upon which marshes can colonize. We reach this conclusion based on observations of the Wax Lake and the Lower Atchafalaya bayhead deltas, which have a 2–4 m thick sand substrate upon which the high porosity mud-peat marsh is built (Van Heerden and Roberts, 1988). Further, despite increasing magnitude of the West Bay diversion, there is no evidence of any wetland creation to date from the mainly mud suspension delivered by the structure (Andrus, 2007). Given the issues of rapid fallout of coarse suspended particles (and elevation rise by sediment accumulation) in the low slope receiving channel—as observed at Bonnet Carre—delivery of sand well into the receiving basin would require a deep, energetic diversion cut: the entrance channel to the Wax Lake delta is about 15 m deep. A channel this deep functions at Wax Lake to deliver significant sand, because it extends into the deeper, sediment-rich suspension, and likely captures bedload as well. A large engineered diversion gated to these depths is costly: the most recent >1420 cms diversion structure constructed on the Mississippi River was the Auxiliary Control structure at Old River, which cost $300 million and was completed in 1986. Although some lowering of sill

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Fig. 11. High discharge suspended sand concentration collected near Empire, LA (RK 47) by the authors in April 2008 showing the depth relationship of grain size fractions.

elevations would increase sediment capture; Fig. 10 and 11) without resorting to a deep diversion structure, the incremental gain provided by lowering alone suggests other methods must be considered to capture significant sand. One method is an ‘‘augmented” diversion, where the river-side approach channel is designed to capture bedload and sand suspended in the lower water column: this sand is then dredged and pumped through the structure when it is operating at maximum flow velocities. This river-side ‘‘sediment trap” keeps dredging costs for the material low due to its proximity to the structure, and has the added benefit of potentially reducing shoaling in the nearby anchorages by ‘‘focusing” the sediment effects of the flow diversion. It is also possible to build a marsh platform in the receiving basin artificially (rather than injecting sand directly into the diversion flow), using dredged and pipelined river sand supplemented by compacted relict fluvio-deltaic and artificial levee material excavated to create the diversion. Once the platform is in place at, or slightly above mean sea level, frictional slowing of the diversion flows should promote deposition of fines, particularly after the substrate is colonized by marsh vegetation. One final design issue is how large should the diversion(s) be? From a purely land-building perspective, larger water and sediment capture results in more rapid and laterally extensive land-building. However, the web of stakeholder issues mentioned below likely will limit both the maximum capacity of the diversion, as well as the duration that it operates at that capacity. Given that this is a political, rather than scientific decision, the modeling community’s input should be confined to answering the question of how much land area can be created for a given diversion size, location, design, and operating parameters. Our best method for providing highquality estimates are utilizing morphological models build using parameters derived from the Wax Lake delta (Kim et al., 2009). 3.4. Stakeholder issues We focus here on stakeholder issues that are critical environmental and socio-economic factors that must be addressed prior to the construction of large sediment diversions on the New Or-

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leans to Head of Passes reach of the Mississippi River. In our opinion, the chief river-side issue relates to maintaining a deep-water navigation channel for the shipping industry and minimizing diversion impacts on shoaling in anchorages as well as the channel thalweg. While such effects may be unavoidable (witness the recent issues surrounding the West Bay diversion and anchorage shoaling) and are compounded if multiple large diversions are constructed, we believe they can be minimized by: (1) the development of reach-scale 1D and 3D numerical models that anticipate sediment effects and (2) the use of aforementioned ‘‘sediment traps” associated with diversions. Proper prediction is critical to establishing the level of State/Federal escrowing of funds necessary for maintenance dredging associated with diversion impacts. There are also issues with construction of the structure itself and receiving basin access channel. These include obtaining right-of-way and outright land acquisition and compensating landowners whose property is adversely affected by the diversion: an example would be land areas proximal to the structure that experience erosion even while more distal areas are experiencing accretion (as has been observed at Caernarvon; Lane et al., 2006). Given that these issues have been addressed previously by existing diversion projects, none of these issues are major impediments in our opinion. Population impact should be relatively minor (compared to other areas in coastal Louisiana) given that the area is relatively undeveloped and existing communities and industrial infrastructure in lower Plaquemines Parish (on both sides of the river) are already protected by the equivalent of earthen ring levees formed by the river and storm protection systems. In fact, marsh creation projects located near coastal communities may have a greater long-term socio-economic value (e.g., storm protection, fisheries) than in more remote areas. Perhaps the most contentious issues relate to ecological impacts on the receiving basin(s). Examination of ecological issues is beyond the scope of this study, but the major issues surround impact on: (1) estuarine fisheries, (2) introduction of sediment-hosted toxins, (3) wetland productivity, (4) coastal hypoxia, and (5) harmful algal blooms (HABs). However, regardless of limitations to the scientific communities ability to quantify and model ecological impacts caused by large diversions, it is clear that adaptive management will have to be practiced and that the diversions will have to be overseen by committees composed of State and Federal managers and local stakeholders operating according to an Environmental Impact Statement (as are existing projects such as Davis Pond).

4. Conclusions: future directions and critical observational and modeling needs We define ‘‘critical” as that research necessary either for selection of final sites for large diversions, their size, operation strategy, or the post-emplacement monitoring of their effect. While ecological observations and modeling are beyond the scope of the present study, it is clear that ecological research relative to the impact of large diversion on the receiving basin are lagging studies of the river channel. These studies in support of robust ecological models are critical to determine: (1) maximum flow volume and operational strategy of individual diversions, (2) to what extent engineered wetland substrates are necessary in concert with delivery of riverine sediment) for creating healthy wetlands and (3) longer-term ecological impact of the diversion(s) on the basin. In terms of river-side observational needs we suggest a critical need for study of: (A) Meander-bend effects on suspended sediment concentrations relevant to diversion siting. Do meander bends aug-

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(B)

(C)

(D)

(E)

(F)

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ment suspended sediment concentrations due to lift-off of bed sand in the accelerated flows of the turn? If so, this may make bends superior locations for such a diversion. However, if this lift-off takes place in the near bottom, it may be difficult to capture in a diversion. How long is the window of opportunity during which coarse material (sand) is available in suspension, and how much total volume can be captured during these windows, what is the interannual variability of this window, and finally how much land can be built with such sediment volume in 50 y? These issues will be critical for sizing diversions. Use of geomorphological modeling based on the Wax Lake subaerial delta evolution to provide realistic scenarios of units of land-building versus water and sediment volume supplied on an annual basis through a diversion. This is also critical for diversion scaling and for education of stakeholders about the pace of land creation. An understanding of the total volume and spatial distribution of sand stored in lateral and point bars in the New Orleans to Head of Passes reach. This is critical for pipeline augmentation of diversions, locating sediment ‘‘traps”, and for defining the magnitude of the sand resource. An examination and alteration of the USGS-NASQAN (National Stream Quality Accounting Network; see http:// water.usgs.gov/nasqan/) water quality monitoring strategy in the lower river that supplies suspended sediment information critical to diversion planning. An optimized strategy will also be critical for monitoring post-construction of large diversion(s) to allow for adaptive management. The community must begin to treat the total (suspended and bedload) sediment stored in, and transferred through the river, as a resource to be properly distributed for various land-building and maintenance projects. This will not be possible unless it is properly quantified. Possible improvements include: E1. More monitoring on main stem, less on sub-basins such as the White and Tennessee. Additional stations would be most useful immediately above Old River (to examine the sediment fraction passing through the Old River structures), and near Venice to capture the Belle Chasse to the Gulf reach. E2. Relocation of the Red River site, which is located in a backwater, to aid in the understanding of the Mississippi–Atchafalaya–Red River partitioning. E3. Collection of bedload data (by bedform migration rate or other channel-wide methodology) at some existing stations to examine that poorly quantified portion of the sediment budget. E4. Analysis of the evolution of suspended sediment sampling methodologies over the last 100+ y (e.g., Humphreys/Abbott, Vertical, Straub, Isokinetic) to allow for proper cross-calibration of historical results to minimize errors in the suspended load record from long-term stations. This may require finding or recreating the old samplers such that they can be tested alongside modern isokinetic samplers. E5. Change from depth-integrative to the more labor-intensive (and expensive) point-integrative suspended sediment monitoring at certain stations to allow vertical stratification of concentration to be better understood. An examination of the USACE decadal navigation survey methodology for the Mississippi and Atchafalaya channels. Such surveys are still mainly conducted by single-beam acoustic fathometer (three cross-sections per river mile). Significantly more information about bed evolution and for constructing accurate base maps for numerical modeling could be gained from conducting future surveys utilizing

multibeam (swath) bathymetry. This change to multibeam technology is underway for seafloor and coastal mapping conducted by NOAA. In terms of river-side numerical modeling needs, we suggest the following: (A) Perform extensive calibration of numerical models of the lower MR against high-quality field measurements. Sufficient data should be collected such that the ability to simulate separate physical processes can be assessed. (B) Investigate how will large water diversions impact potential shoaling above and below this reach—will it be larger in meandering reaches than along a straight reach? Numerical modeling supplemented by high-quality field measurements will be needed to answer this question. (C) Quantify the confidence level for the ability of the various numerical models to simulate variables such as velocities, secondary motion, and spatial variability of sediment concentration. (D) Conduct further field research to improve our ability to model multiple size-class sediment mixture and provide viable prediction of the temporal and spatial variability of sediment load. (E) Conduct further analytical research to improve our understanding of the interaction between salinity and sediment settling. (F) Adapt multi-modeling approach to establish adequate level of confidence in the predictions provided. This approach is adopted in hurricane predictions and provides an envelope or band around a given prediction, allowing decision makers to ascertain the level of risk or uncertainty involved. Acknowledgments We would like to thank the Environmental Defense Fund for financially supporting the creation of this report and, specifically Jim Tripp, Angelina Freeman and Paul Harrison for guiding its production. We would also like to acknowledge Dr. Chip Groat, Director of the University of Texas Center for International Energy & Environmental Policy, which served as the main contractor for this report production. Major contributions to this report came through face-to-face interviews conducted with experts on these issues in government and academia. The following individuals are due great appreciation for freely giving of their expertise. They are: David Biedenharn (Biedenharn and Associates), Troy Constance (USACE New Orleans District), John Day (Louisiana State University), Charles Demas (USGS), Arthur Horowitz (USGS), Chris Knotts (Louisiana Office of Coastal Protection and Restoration), Robert Meade (USGS), Chris Paola (National Center for Earth Surface Dynamics and University of Minnesota), Nancy Powell (USACE New Orleans District). We have also benefited from many informal conversations with our academic, government, and industrial colleagues, who are too numerous to single out, but who aided enormously in the preparation of this report. References ABFS, 1982. Atchafalaya Basin Floodway System, Louisiana. Feasibility Study, vol. 4. US Army Corps of Engineers, New Orleans District. Allison, M.A., Nittrouer, J.A., 2004. Assessing Quantity and Quality of Sand Available in the Lower Mississippi River Channel for Coastal Marsh and Barrier Island Restoration in Louisiana. Final Technical Report for Subcontract C-162523, Governor’s Applied Coastal Research and Development Program, Baton Rouge, 55 p. Allison, M.A., Bianchi, T.S., McKee, B.A., Sampere, T.P., 2007. Carbon burial on riverdominated continental shelves: impact of historical changes in sediment

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