Analysis and Lessons Learned from Mardi Gras Pass within the

Analysis and Lessons Learned from Mardi Gras Pass
within the Bohemia Spillway, SE Louisiana
Paul Connor, John Lopez, Theryn Henkel, Andreas Moshogianis,
Andrew Baker, Ezra Boyd, Eva Hillmann, David Baker
To
CEER 2014
Thursday July 31, 2014
Bohemia Spillway Area
American Bay
Mississippi River flood protection and “back” Levees
Mardi Gras Pass
Breton Sound
Bayou Lamoque
Mississippi River flood protection and “back” levees
Mardi Gras Pass
Bohemia Spillway Area
Breton Sound
Bayou Lamoque
A naturally recovered or re-claimed canal: Back Levee Canal (old route) in the Bohemia Spillway near Nester
Canal (view looking north). Oak trees are on the original spoil bank of the canal. The Back levee Canal was
probably in-filled by sediment deposited by early discharges (1926 to 1940) through the Bohemia
Spillway. Photograph taken February 14, 2013.
High water surveys (LPBF
& UNO) May 17 through
June 7, 2011
Pre-High water LPBF
survey March 3, 2011
Post-High Surveys
July 10 17,and 24,
2011 with lower
water but active
breach
Cut July 2011
Cut February 2012
Mardi Gras Pass
Reach 1
Back levee Canal
Mardi Gras Pass May 5, 2012
picture by Frank Cole OCM
Reach 4b
Reach 4
Reach 4a
Road
Reach 3
Reach 2
Reach 1
Mississippi River
February 24, 2012
Mardi Gras Pass March 10, 2012
Mississippi River
5 ft water depth or -2.5 ft elevation 3/10/2012
10 feet water depth or -8 ft elevation 4/23/2012
~38 ft wide
Survey water level 2.6 feet NAVD 1988
Mississippi River at Pointe a la Hache 2.48 ft
8
2011
2012
2013
2014
7
6
2,300
3,840
River Stage (Ft.)
5
3,530
626
4
2,095
3
2,354
2,170
100-200
2
475
1
971
436
621
0
J FMAM J J A S O N D J FMAM J J A S O N D J FMAM J J A S O N D J FMAM J J A
Pointe a la Hache Stage
Bank Surveys
ADCP Discharge (CFS)
Bathymetry Surveys
Sediment Survey
Biological Surveys
Be7 Survey
NOAA
River Stage Pointe a la Hache Feet NAVD88
River Stage vs. Mardi Gras Pass Discharge for 2012 (black) and 2013 (red)
8
4200 cfs
Maximum Expected Stage 7.6 feet @ Pointe a la Hache
7
6
5200 cfs
Extrapolated rating curves (dashed)
Unidirectional inbank & Across River
Bar at Reach 1
5/14/13
3/28/12
6/26/13
5
2/1/13
4
3
2
1
0
-2000
Unidirectional
3/14/12
in-bank
5/3/12
7/3/12
Bi-directional
in-bank
6/11/14
7/31/13
4/18/12
2014 rating curve
1/16/13
11/11/13
Flow subject to reversal by tides
-1000
0
1000
2000
Discharge (CFS)
3000
4000
5000
6000
All CSA change 756 sq ft 1/13 to 8/13 2013 ~143% increase
Reach 4a CSA change 55 sq ft 1/13 to 8/13 2013 13% increase
January 2013
Approx. Cross
Sectional Area (ft2)
528
500
1111
882
689
Table 1: Summary statistics by reach for the transects andAugust
polygons used
to assess the width and depth of Mardi Gras Pass based on the
2013
August 2013 survey.
Reach
Number of
Width
Transects
Avg. Width
(ft)
Min. Width
(ft)
Max. Width (ft)
Number of
Depth Polygons
Avg. Depth
(ft) (*)
Min. Depth (ft)
(*)
Max. Depth
(ft) (*)
Avg. Thalweg
Depth (ft) (**)
1–4
50
95.8
59
185
139
13.4
6.3
30.3
16.5
1
5
105
87
113
13
9.8
6.9
12.8
12.3
2
6
115
94
152
17
15.5
11.1
21.5
20.6
3
6
107
86
158
18
12.1
6.3
17.9
16.2
4
33
89
59
185
91
13.8
8.0
30.3
16.3
Approx. Cross
Sectional Area (ft2)
(***)
1,284
1,284
1,029
1,783
1,249
1,228
Back levee Canal
Reach 4b Ave. CSA 1,434 sq ft
Reach 4
Mardi Gras Pass May 5, 2012
picture by Frank Cole OCM
Cross-sectional areas August 2013
Reach 4a Ave. CSA 489 sq ft
Reach 3 CSA 2,100 sq at road ft
Road
Reach 2 Ave. CSA 1,783 sq ft
Reach 1 Ave. CSA 1,029 sq ft
Mississippi River
Cross Sectional Areas Mardi Gras Pass Aug 2013
4000
3500
3000
2500
2000
1500
1000
500
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
4b
4a
3
2
1
Aquatic Biological Survey Results in Mardi Gras Pass
Time
Stage
Flood 5
Head limits
Flood 1
Flood 4
Flood 2
Flood 3
Discharge
“geomorphic limit”, i.e. mature crevasse
Time
Stage
Discharge
How the River Built the Coastal Louisiana
Landscape
“geomorphic limit” i.e. mature crevasse”
“Acceptable
discharge” less
“Acceptable
discharge” greater
Stage
Flood 1
Flood 3
Flood 2
Discharge
Necessary conditions for a
meandering-river avulsion
Slingerland and Smith, 1998
Avulsion limit is determined
by increasing sediment load
captured from main channel
and decreasing gradient of
crevasse over time
Mardi Gras Pass
Theoretical Maximum Potential Discharge
4,200
end of 2012 flood
5,200 cfs
end of 2013 flood
6,000
end of 2014 flood (estimate)
Lower Breton Diversion 50,000 cfs in Approved
State Master Plan cost $220 M , 2014 Annual Plan
Design budget $2m 2014, $6 m 2015, $10 m 2016
Bohemia Mississippi River Reintroduction
BS-15
10,000 cfs CWPPRA PPL 17 $10-20M
De-Authorized 2013 due to MGP
Engineered conveyance channel
Naturally developed conveyance channel
Saveourlake.org
Summary
Over 18 months of monitoring with ten ADCP surveys, the flow through MGP ranged from 436
cfs cfs to 3,840 cfs.
Temporal interpretation of the rating curves and geomorphic changes suggest MGP is enlarging
and increasing its flow capacity primarily during higher river stages.
The % increase in discharge in 2013 is significantly less than the % increase in the average
channel dimensions
Increase is discharge seems to be moderated by the narrow dimensions and slow rate of
enlargement of Reach 4a. It is likely that the lithologic properties of the natural ridge at reach 4a
are more durable and less erodible.
In the 2014 flood, the MGP channels appears to be stabilizing
The combined effects of limited channel growth at 4a and accumulating downstream deposition
may have tipped the balance toward stabilization of MGP.
MGP is contributing to the science of natural river processes such as creavassing and riverine
ecology.

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