Strategic Defense of the Mississippi
River Delta –
Dredging a New River outlet into
the Gulf
Nicholas De Gennaro PhD PE
D&D Civil & Marine Engineering
Research Basis
The research of this Presentation is based on work done in the Changing Course
competition this past year.
The body of work completed in the competition included a much more comprehensive
review than can be presented here.
This presentation will generally look at the Mississippi River Delta and the engineering
considerations of dredging and new river exit along with relocating a port facility
Overview
•
Driving Consideration
Marsh & land loss
•
Land building Processes Considered
– Diversion capacities & operaton regiems
New River Outlet Location
– Constraints
•
Engineering design process
– Drivers
– Decisions on navigation channel and diversion sites
•
Navigation channel
•
Cost & Maintenance comparisons
- Land building potential
Not Discussed
Socioeconomic conditions
Land Loss
• During the last 50 years coastal Louisiana has
lost roughly 1,900 square miles of land.
• Land loss projections of the USGS indicate a
further loss of potentially 1,750 square miles
for the next 50 years.
• This translates to a football field every hour
and a near total loss of the Delta by 2070.
IMPORTANCE OF LOUISIANA
WETLANDS-
Long taken for granted the ecosystems of Mississippi River Delta
are estimated to provide between $12 and $47 billion worth of benefits annually
If no action is taken, annual damage from flooding would increase tenfold over the next
50 years from an average of $2.4 billion to $23 billion. (WaterMarks 5-2012)
Sediment Transport Load
The average annual sediment load of the lower Mississippi River (below Old River Control Complex) is estimated
to be 131 million tons.
Because the natural delta building processes have been drastically altered, a very large portion of this sediment
goes off the edge of the continental shelf and is deposited in the deep waters of the Gulf of Mexico.
Cause of Loss
• The causes of sediment losses are many, this
paper will focus of the loss of sediment off the
shelf and maintenance of the Southwest pass for
navigation as comparison to a new shorter more
hydraulically efficient outlet to the Gulf.
• Then consider a position for strategic defense of
the coastline.
The idea here is we cannot save it all but we
should concentrate on what we can save.
The Concept
A hydraulically efficient channel
• When the river switched channels to a more
hydraulically efficient channel, the abandoned
delta becomes transgressive.
• Holding the river in its present alignment means
that much of the sediment load of the Mississippi
River is deposited in deep water off the edge of
the continental shelf. Thus, this sediment does
not go toward building land.
• This results in large land loss rates.
The Natural Course River Evolution
This kind of switching has occurred roughly every 1,500 years, geologists say, and since about
1950 the river has been ready for a change — to the Atchafalaya. The Corps of Engineers prevents
that from happening with an enormous installation of locks, dams and power stations near
Lettsworth, north of Baton Rouge and about 100 miles northwest of New Orleans.
Sediment Loss
Bold Ideas
•
In the time since the Mississippi River was leveed in the 1800's, Louisiana has lost
much of its coastline, which is normally replenished by river sediment.
•
Scientists have long said the only way to restore Louisiana’s vanishing wetlands is
to undo the elaborate levee system that controls the Mississippi River, not with the
small projects that have been tried here and there, but with a massive diversion
that would send the muddy river flooding wholesale into the state’s sedimentstarved marshes.
•
Many scientists advocate diverting the Mississippi River below New Orleans,
•
Many people have long dismissed the idea as impractical, unaffordable and lethal
to the region’s economy. Now, they are reconsidering.
•
In fact, when a group of researchers convened last year to consider the fate of the
Louisiana coast, their recommendation was unanimous: divert the river.
River Constrained by Levees
It is unrealistic to think in terms of removing existing levees to allow
the river to naturally flood and deposit sediment to adjacent wetlands.
Considerations for the lower
Mississippi River Delta
•
•
•
•
Natural river course
Build upstream reinforcement
Allow coastline retreat
Beneficial dredge
• And continual maintenance
• Abandon bird’s foot
• River diversions -• New navigation entry --
Currently accepted method to gain
sediment for wetland preservation:
DIVERSIONS
• Based on studies from the Louisiana CPRA the
most cost efficient mechanism to retain and
accrete wetlands is river diversions.
• Therefore any consideration to add a new
navigation channel must account for diversions
that are in the CPRA master plan.
Diversion as a Partial Solution
• Multiple controlled diversions being reviewed to be
constructed adjacent to the river, conveying water and
sediment through (controlled) structures to receiving
basins in the wetlands. Some have already been consructed
• In this study the consideration is 6 large diversions. These
range from 50,000 cfs to 100,000 cfs opened during river
high flows.
• The diversions are operated in combination, using an
adaptive management scheme that sets guidelines for
deployment of individual diversions.
Preferred Plan
6 High Capacity Diversions
Diversion Concepts
Diversion quantities
• Overall diversion quantities (sites combined):
River Flow
1,000k cfs
700k cfs
250k cfs
Divert %
30%
30%
8%
Divert cfs
500k cfs
240k cfs
20k cfs
• Indicative capacities per site
Des Allemands:
Davis:
Lower Brenton Control:
max 100k
max 100k
max 100k
U. Breton:
M. Breton:
Myrtle Grove:
max 100k
max 100k
max 60k
Mississippi River Discharge
Shorten the Rivera new outlet to the Gulf
• Another controversial concept to save the delta is to
shorten the river and dredge a new outlet to the Gulf.
• This concept requires a consideration for the diversions
that will probably be in place by the time a new outlet
is approved, designed and constructed.
• Most people have long dismissed the idea as
impractical, unaffordable and lethal to the region’s
economy.
Strategic defense of the Mississippi River
Delta
• Criteria: The selected site must consider where
the new channel can deliver remaining sediment
to shallower waters were land building has a
chance of development.
• The new outlet must also be located where the
existing channel is sufficiently deep such that
navigation dredging is not increased but with the
added benefit that all the dredge material will be
used for beneficial marsh creation or barrier
island nourishment.
Location of the New Head of Pass (HOP)
• One of the early considerations to develop a new
outlet is determining a location of the new HOP
• Locations were reviewed from Bell Chase to
Venice.
• It came down to Port Sulfur or Empire. The
locations were based on Social & Economic
considerations. (How many people and businesses would be displaced
or defended, however these issues will not be reviewed in this presentation)
Port Sulphur to Open Water
Empire to Open Water
A navigation channel closer to Empire would reduce the positive hydraulic and land building
effects, while an outlet closer to New Orleans would increase construction costs and reduce
the effectiveness of the diversions
Empire to the 55’ Contour
On New River Outlet
Empire to the Gulf
Via Southwest Pass
Navigation Channel at Empire
• Reduces navigation channel from 50 miles out
SW Pass to 14 Miles from Empire through
proposed new channel.
Dredging Quantities Current
• Currently approximately 15-25 million CY/yr of
sediment is dredged at HOP.
• If a similar load would be dredged from the
new HOP in the preferred plan and deployed
in the areas of need nearby, approximately
3,000 acres/yr additional land could be built.
Locks vs Natural
Conventional navigation locks were considered to
prevent the river flow from going down the navigation
channel.
Some proposals include the concept of sail through
locks where the gates are separated by a sufficient
distance so that gates can be opened and closed
without requiring the ship to stop and tie off.
– Either way these concepts are fatally flawed because,
without riverine (or tidal) flow, a deep draft channel from
deep water through the estuary is vulnerable to being
filled in by occasional strong tropical events.
The new channel must allow the River water to flow
unobstructed
Hydraulic Gradient and Navigaiton
• By locating the channel at RM50 up River water levels
will be reduced, resulting in reduced flooding risks
• Also a steeper gradient will enhanced flow velocities.
• Navigational concerns were modeled and analyzed.
The models show that water levels throughout all
conditions (low, flood, and high);
– the navigation channel for deep drafts will remain consistent.
Furthermore, ships would be able to bypass the areas south
of Empire where uncontrolled diversions, leakage and
consequent aggradation cause permanent need for dredging.
Head Differential
• Water levels in river
Discharge
New Orleans Baton Rouge
250,000
-0.2 ft
-0.1 ft
700,000
-2.7 ft
-1.9 ft
1,000,000
-3.7 ft
-1.6 ft
Outflow at New HOP
25K– 100,000 (East)Control – Lower Brenton
Opened when river flows are greter than 325,000 CFS
50K CFS Flow (South Old Channel)
250,000 CFS constant Naviation Channel
associated durations at Tarbert Landing
Regime
Discharge
Duration
range (cfs)
(for 30/70
split)
1
<230k
74 days
2
230k - 450k
125 days
3
450k – 850k
120 days
4
>850k
46 days
Minimum Flow at HOP 300,000 CFS Befeore Divrsion operaton—
Control Diversion Operatates River flow of 325,000 cfs (Lower Brenton)
Location of new head of passes could be based on the a
defendable shore line for the Mississippi River Delta
At the new Head of Passes (new HOP) a new coastline
will be developed.
The new HOP will naturally distribute the water as is
currently occurring at the Bird’s foot area.
New Major Port at Empire
To enhance storage and transhipment facilities a new port is projected having a new
navigation entry to the Gulf of Mexico
Beneficial Considerations
Addressing major concerns of marsh creation and protection
•
•
•
•
•
•
•
•
•
•
The River realignment and the associated river diversions would facilitate the
beneficial use of sediments that are currently disposed in unstainable areas.
The new channel located near the Western Barrier Islands is a beneficial location
where dredge material can be used.
The new outlet shortens the steam time up the river from open
water by over 35 miles and decreases the cost of major shipping
lines steaming to upstream river destinations.
It increases the slope of the hydraulic grade line as a result of the lowering the
water. This increases flood protection in New Orleans
The increased slope increases the hydraulic gradient thus the velocity of the river
will increase. The flow velocities the upper river will increase, this will improve the
sediment movement throughout the Lower Mississippi.
It allows greater flows and accommodates the (6) planned diversions.
Creates a new economic engine via a new bulk and container port along the new
channel.
Dredge material can be used to construct habitat islands that would act to reduce
fetch on fragile marsh.
Breton Sound basin, would get the benefit of a near continuous sediment diversion
( Lower Brenton) building wetland thereby reducing the erosion power of wave
action.
The realigned mouth of the river will serve as a means to improve land building
north of Empire thus developing a strategic defense line.
New Head of Pass
• To construct the new diversions and the new
HOP a large amount of dredged material will
become available.
• These resources can be deposited at
designated locations in the Lower Breton and
Barataria Bay areas, and be used as
nourishments on the Barrier islands.
South West Pass to Beneficial Dredge
Distance from New Channel to Beneficial
Use
Construction Quantities
Cross section area ;
marsh section = 150,000sf/ft
open water section=84,000sf/ft
Dredging :
Marsh to open water;
Open water to 55’contour
7 miles = 190 Million CY
7 miles = 110 Million CY
300Millon CY At $ 4/CY =$ 1.2 Billion
Low Crested Segmented Break Water:
Marsh to open water
7 miles = 50 Million CY
Open water to 35’ contour
5 miles = 85 Million CY
135 Million CY
at $100/CY= $135 Million
Dredged material to fill Proposed Port to +20:
2500 acres
80 Million CY
Side slope 6:1
Value of Dredged Material
Net dredge spoils 300 Million - 80 Million = 220 Million CY
for wetland creation and beach restoration.
If all was used to create marsh at 7,000 CY /A = 31,000 Acres
Value of Dredged material;
According to the 2017 Louisiana CPRA master plan
over $20 billion is allocated for wetland creation
Reprehensive Projects such as;
001MC.02 is designed to create 540 acres at total cost of $150 million
001MC.05 is designed to create 8500 acres at a total cost of $2.3 billion
001MC 07 is destined to create 4250 acres at a total cost of
$1.2 billion
Average cost of an acre of marsh created $3.65billion/13,240A = $280,000/A
Note The dredge spoils from the new River Pathway to the Gulf can develop
almost 3X as much marsh as the three project at less than half the cost!!!!
Time Value of Dredged Material
High Maintenance Dredge Area
MISSISSIPPI RIVER SOUTHWEST PASS
HOPPER, CUTTERHEAD & DUSTPAN DREDGING
FY
90
91
92
93
94
95
96
97
98
99
0
1
2
3
4
5
6
7
8
9
10
11
12
13
CUBIC YARDS
34,386,932
26,101,374
11,659,581
27,368,307
26,796,109
21,460,737
17,293,426
25,714,897
14,837,275
17,652,641
3,995,583
12,555,710
17,834,811
9,381,050
13,010,523
13,616,139
7,245,234
10,556,543
13,406,342
18,477,845
23,055,732
14,431,080
17,626,336
15,541,144
COST
$/CY
$18,650,282
$0.54
$25,135,570
$0.96
$18,422,286
$1.58
$33,312,432
$1.22
$36,162,446
$1.35
$28,650,500
$1.34
$22,195,223
$1.28
$41,373,580
$1.61
$30,560,370
$2.06
$23,223,486
$1.32
$6,333,865
$1.59
$21,691,455
$1.73
$31,148,104
$1.75
$17,212,870
$1.83
$16,152,194
$1.24
$30,355,934
$2.23
$22,517,345
$3.11
$25,260,068
$2.39
$51,704,901
$3.86
$48,114,762
$2.60
$74,109,526
$3.21
$43,110,873
$2.99
$60,279,012
$3.42
$42,234,972
$2.72
LAST 5 YEAR
TOTAL:
89,132,137 $267,849,145
LAST 5 YEAR
AVERAGE:
17,826,427 $53,569,829
$3.01 PER CUBIC YARD (LAST 5 YEARS)
$14.94
$3.01
Maintenance Dredging
Near (new HOP) & Over Flow
• Reduced or comparable dredging in main river
channel
• The soft new river channel dredged in the marsh
will continually overflow during times when the
diversion is in operation. The overflows will be
similar to a natural river over flow and bring
sediment along its reach. The material dredged to
construct the channel can have a beneficial use to
create additional wetlands
Relocation Consideration
Trade-offs & Concerns:
• Gradual abandonment of the downstream part of the
Mississippi River (Venice) as port/industry facility
• Need for significant spatial rearranging of Empire
• Potential moving of oil pipelines and abandonment of
existing wells
• Eventual moving of up to 3,000 people & businesses
from area South of Empire
• Eventual loss of the “Birds Foot”
• Politically difficult for Legislation
• Designing such a diversion would be complex and timeconsuming
The “Bird’s Foot”
• In a way, the bird-foot delta is an artifact of
engineering. Without the levees and other structures
that keep the river in place, it probably would have
taken another path.
• Because the bird-foot delta has grown so far into the
gulf, the river’s mouth is at the edge of the continental
shelf. As a result, the sediment it carries ends up in
deep water, where it is lost forever.
Losing the “Bird’s Foot”
• The new channel allows the opportunity to take advantage
of the areas south of Empire to the mouth of the river for a
massive enhanced
sports recreational area for a period of
.
time.
• As the bird-foot delta breaks up it would provide needed
sediment to the frail strings of barrier islands that line some
of the Louisiana coast.
Concept
Concept Overview
New River Outlet 50 Years After Construction
Is it practical? Yes,
Will it be expensive? Yes.
But when you look at the alternatives,
it’s very cost effective, particularly in an
era of rising sea levels.
Nicholas De Gennaro PhD PE
DD Civil & Marine Engineering
[email protected]
Strategic Defense of the Mississippi River Delta
– Dredging a New River Outlet into the Gulf