To:

Olivia Dorothy
Associate Director
Upper Mississippi River Basin
American Rivers

March 9, 2018

From: Jonathan W.F. Remo Ph.D.
Big Muddy Consulting
Re: Assessment of flood surcharge related to differences in existing and authorized levee
elevations along the Mississippi River between Keokuk, IA and Thebes, IL using the U.S. Army
Corps of Engineers Upper Mississippi River Flood Risk Management Model
This letter report documents the procedures and findings of a flood surcharge assessment using
the U.S. Army Corps of Engineers (USACE) Upper Mississippi River Flood Risk Management
Model (UMR FRM; USACE, 2018a). This assessment consisted of running the UMR FRM
model with its two model geometries, one which contained the existing levee elevations, and the
second that contained the federally-authorized levee design elevations along the Upper
Mississippi River between Keokuk, IA and Thebes, IL. Four flood scenarios were evaluated to
assess the impact the differing levee elevations had on water-surface elevations and maximum
flood depths. A description of the UMR FRM model, surcharge assessment methods, results,
discussion, and conclusions are reported below.
Model Description: The UMR FRM model geometry was developed using the best available
topographic and bathymetric data. Topography for the floodplain was developed using LiDAR
based terrain data collected for the Upper Mississippi River Restoration Program Long-term
Resource Monitoring element between 2008 and 2011(USACE, 2016). Holes within this data set
were filled with state collected LiDAR terrain data. The bathymetric data consisted of a
combination of single and multi-beam sonar surveys collected between 1999 and 2015. These
topographic data were combined through interpolation to create a topobathy digital elevation
model (DEM) with a 2-meter resolution. Levee elevations for levee systems which participate in
the USACE’s PL 84-99 levee safety program were compiled from the latest National Levee
Database (NLD) surveys. Land cover from the U.S. Geological Survey’s (USGS) National
Landcover Database (USGS, 2011) were used to guide the selection of roughness coefficient
applied in the hydraulic model. Information on bridges, navigation dams, inline structures, and
river training structures were compiled for parameterization within the hydraulic model. River
discharges and water-surface elevations used in the development of the UMR FRM model were
compiled from USGS and USACE hydrologic monitoring stations, respectively. In addition, the
National Weather Service’s River Forecast models were used to estimate inflows from ungauged
tributaries. All of these data were compiled using the tools within the USACE’s Hydrologic
Engineering Center’s River Analysis System (HEC-RAS v. 5.0.3) and its associated GIS tools,
HEC-GeoRAS, to create a 1-D/2-D hydraulic model for the investigated river segments. The
channel of the Mississippi River and its substantial tributaries were modeled as 1-D elements
using the full momentum equation. Areas behind the main-stem and some of the major tributary
levees were modeled as 2-D flow areas. Modeling the levee protected floodplain as 2-D flow
areas allowed for a more realistic representation of flow into and through these portions of the
floodplain. The 2-D element of the UMR FRM model uses the diffusion wave equation to
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 simulate flow within the levee protected floodplains of the Mississippi and its major tributaries.
Discharge hydrographs were used for the upstream boundary condition at Keokuk, IA and for
lateral inflow locations along the modeled river segments. Stage hydrographs or a rating curve
were used for the downstream boundary condition at Thebes, IL (USACE, 2018b).
Model scenarios and calibration: The UMR FRM model contains five flood scenarios. Four
flood events, May-June 2008, April 2013, June-July 2014, and April-May2017, were the floods
used to calibrate the UMR FRM model. Calibration results revealed a reasonably well
performing model with root-mean-squared and mean-absolute error values for differences
between predicted and observed water-surface elevations ranged from 0.01 up to 2.09 ft with an
average error of ~1.0 ft. For all the flood scenarios included within the UMR FRM model, only
levee breaches during the 2008 flood within the Rock Island District were assessed. For all the
other model scenarios, levees that are overtopped are assumed not to breach. Breach analysis
was not performed for the other flood scenarios because the data to model the breaches was not
available. A fifth flood, not used for model calibration, was the 1993 gaged inflow scenario.
This flood scenario was included because modeling potential outcomes for such a large flood
event is useful for evaluating impacts of flood-risk reduction efforts such as changes in levee
height or configuration. However, this scenario is not a realistic recreation of the 1993 flood.
This is because no effort was made to account for the impacts of flood fighting or levee
breaching on water levels or discharges in order to make it a realistic recreation of the 1993 flood
(USACE, 2018b).
Authorized Levee Geometry: In addition to the existing condition geometry, an authorized levee
elevation geometry was provided with the UMR FRM model. The authorized levee elevations
were determined from as-constructed or design drawings from general design memorandums or
the operation and maintenance manuals for each levees system. The authorized levee elevations
are the net grade the levee system was intended to be after any post construction settlement or
compaction. The net-levee grade elevations were edited to incorporate the approved Section 408
alteration requests for the Rock Island District levees. Net levee grade elevations were then
converted from the original datum to NAVD 1988. Authorized levee elevations are not available
for non-federal levee systems and therefore the NLD information was used for non-federal
levees in the authorized levee geometry. For non-federal levees for which NLD information was
not available, the elevation from the Topobathy DEM were used to establish top of levee
elevations in the authorized levee geometry file (USACE, 2018b).
Assessment of Surcharge Between Existing and Authorized Levee Geometries:
To assess for differences in water-surface elevations related to changes in levee heights, the 1993
gaged inflow, 2008, 2013, and 2017 flood scenarios were run in HEC-RAS v. 5.0.3 using both
the existing and authorized levee geometries. The 2014 flood scenario was not run because the
magnitude of this flood was insufficient to substantially inundate any of the levees systems along
the modeled river segment. For the existing levee geometry, plans for the flood scenarios were
provided and were run without any changes to the input parameters. To run the four flood
scenarios with the authorized levee elevations, the associated geometry file was imported into
HEC-RAS. However, during the HEC-RAS software’s pre-check for input errors, eight issues
were detected. These errors were associated with new connections in the authorized levee
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 elevation geometry. These connections were located between the 2D flow and storage areas in
which a mesh face was overlapping between two model elements. To overcome these issues, the
original model boundaries from the existing condition geometry were used instead of the new
boundaries contained within the authorized levee geometry. Using the existing condition
boundaries between the 2D flow and storage areas allowed for successful completion of the
model runs using the authorized levee geometry. Comparison of the model boundaries between
the existing and authorized levee geometries did not reveal any differences, except there was no
overlap between the 2D flow and storage areas. This indicates using the original working 2D
flow and storage area boundaries should not substantially affect the results for the authorizedlevee elevation model runs.
Upon completion of the model runs, tabular discharge and water-surface elevation data were
exported into Microsoft Excel to assess changes in these parameters between the existing and
authorized levee height geometries for each of the assessed flood scenarios. Raster layers of the
maximum water-surface elevations for the 1993 gaged inflow and 2008 flood scenarios were
exported from HEC-RAS into ArcMap (v. 10.4.1) GIS software. The water-surface elevation
raster layers for these flood scenarios for both the existing and authorized levee elevation model
runs were subtracted from the topobathy DEM to calculate and map flood depths. The existing
and authorized levee elevation flood-depth maps were subtracted from one another to calculate
changes in flood depths.
Results:
Comparison of the UMR FRM model results using the existing and authorized levee elevation
geometries for the 1993 inflow scenario revealed changes in maximum water-surface elevations
from -0.38 up to 1.67 ft with an average increase of 0.52 ft. The most substantial increases in
maximum water-surface elevation (>1.0 ft) occurred from La Grange to near Hannibal, MO.
Other substantial increases in maximum water-surface elevations (>0.5 ft to 1.0 ft) were also
noted between Keokuk, IA to La Grange, MO, Hannibal to Clarksville, MO, and Jefferson
Barracks to Cape Girardeau, MO (Figure 1). The largest increases in the maximum water-surface
elevations occurred within and just upstream of where the USACE (2017) levee surveys
identified six levee systems which raised substantial portions of their levees two to four feet
above the authorized design. Figure 2 shows the comparison of flood depths for the maximum
water-surface elevations between the existing and authorized levee elevations for the 1993 gaged
inflow scenario. The modeling results suggest under current levee conditions, the Sny Island
Levees systems (reaches 1, 2, and 3) would not be substantially flooded during this scenario.
However, at the authorized levee elevations the Sny Island Levees would be substantially
flooded with depths exceeding 16.0 ft. in some areas. Fabius and South River levee systems,
which also substantially raised their levees, saw large reductions in flood depths (6-12 ft).
Elsewhere along the modeled river segment, substantial decreases in flood depths (5-16 ft) were
noted within the Des Moines and Mississippi and the Harrisonville / Fort Chartres levee systems
(Figures 2 and 3).
For the 2008 flood scenario, comparison of the UMR FRM model runs using existing and
authorized levee geometries revealed changes in maximum water-surface elevations ranged from
0.0 up to 1.5 ft with an average increase of 0.72 ft. The most substantial increases in maximum
3

 water-surface elevations (>1.0 ft) generally occurred in the same locations as the 1993 gaged
inflow scenario. Other substantial increases in maximum water-surface elevations (>0.5 to 1.0 ft)
were noted between Gregory to La Grange, MO and from the Cuivre River to the Big Muddy
River confluences (Figure 4). Comparison of flood depths for the maximum water-surface
elevations between the existing and authorized levee heights also suggest under current levee
elevations the Sny Island Levee System (reaches 1, 2, and 3) would not be substantially flooded.
However, model results using the authorized levee elevations indicated Reach 1 of the Sny
Island Levee System would be substantially inundated with flood depths >16.0 ft. in some areas.
Five other levee districts, the Des Moines and Mississippi, Indian Grave North, Fabius, Marion
County, and South River were substantially less flooded in the model run with existing levee
system elevations (Figure 5).
Discussion: Comparison of modeled water-surface elevations between the existing and
authorized levee elevations for the 1993 gaged inflow and 2008 flood scenarios indicated nine
levees systems have increased their levee heights. The modeling results also show the increased
levee heights would reduce flood inundation and consequently flood risk within these systems.
However, the modeling results also indicated raising these nine levee systems above the
federally-authorized levee elevations increased water-surface elevations and consequently flood
risk for unprotected floodplain areas and levees systems which have not increased their levee
heights (Figures 2, 3, and 5). Levee surveys by USACE (2017) indicate the Fabius, Marion,
Southern River and Sny Island Levee Districts have raised substantial portions of their levees by
2 to 4 ft. above the federally-authorized elevations. The modeling results presented here also
suggest three other levee systems have raised their levees above the levee elevations contained
within the UMR FRM model’s federally-authorized levee geometry. These levee systems include
the Des Moines and Mississippi, Harrisonville / Ft. Chartres, and Indian Grave North levee
systems (Figures 2, 3, and 5). The analysis performed here could not determine if 14 of the levee
systems which were not overtopped in the flood scenarios provided in the UMR FRM model
were raised above the federally-authorized levee elevations. These 14 levee systems are located
in the St. Louis District and were not measured in the 2017 Rock Island District’s levee surveys
(USACE, 2017; Appendix A). Hence, a comparison of levee heights between the existing and
authorized levee elevations needs to be under taken to assess which, if any, of these 14 levees
systems may also have been substantially raised.
Differences in the pattern of inundation between the 1993 gaged inflow and 2008 flood scenarios
are attributed to hydrologic differences between the floods, changes in levee system geometry,
and methods in which levee system inundation were modeled (i.e., levee breaching in 2008 flood
verses only levee overtopping in the 1993 gaged inflow scenarios). While the magnitudes of the
1993 gaged inflows and 2008 flood scenarios were similar above the Illinois River confluence,
the duration of the 1993 flood was much longer than the 2008 flood (i.e., one month verses one
week for discharges exceeding the100-year event). The longer duration of the 1993 flood
generally allowed for more filling of the impacted levees systems. For example, under the 1993
gaged inflow scenario with the authorized levee elevations, three of the four Sny Levees were
substantially inundated where under the 2008 flood scenario only the Reach 1 levee was
substantially flooded (Figures 2 and 5).

4

 Increases in water-surface elevations upstream and along reaches of river where levees have
been raised are primarily attributed to hydraulic impacts of increase levee heights. Increases in
water-surface elevations downstream of raised levees are chiefly attributed to increases in
discharge. Raising the heights of levees can cause downstream increases in discharge by
reducing the volume of flood waters that otherwise would have been stored within the levee
system whose elevations have been substantially raised. The increase in discharge would
consequently result in higher water-surface elevations downstream of the raised levees. The
increase in river discharge is the reason why there are notable increases in maximum watersurface elevations below the Missouri River confluence to Thebes, IL for the 2008 flood scenario
even though this was a minor flood (<20-year event) in which changes in levee heights along this
river reach would have little to no impact on water-surface elevations (Figures 1 and 4).
Summary: Comparison of water-surface elevations between UMR FRM model runs with
existing and authorized levee elevations for the 1993 gaged inflow and 2008 flood scenarios
indicate increases in levee heights along nine levee systems. The modeling results also suggest
the increased levee heights have resulted in higher water-surface elevations for large floods
(>50-year event) throughout a majority of the UMR modeled segment. While increased levee
elevations have likely reduced flood risk for areas protected by these nine levee systems, they
have increase flood heights and consequently flood risk for unprotected floodplain areas and for
levee systems which have maintained the federally-authorized levee elevations. The 14 levee
systems which were not overtopped by the flood scenarios modeled in this assessment were not
evaluated to determine if they exceed the federally-authorized levee elevations. In addition, these
14 levee systems are located within the St. Louis District and were not measured during the 2017
Rock Island District’s levee surveys (USACE, 2017; Appendix A). A comparison of levee
elevations between the existing and authorized geometries needs to be undertaken to assess
which, if any, of these 14 levee systems have been raised beyond the federally-authorized
elevations.

5

 Figure 1. Difference in maximum water-surface elevations for the 1993 gaged inflow scenario for model runs using existing and
authorized levee geometries within the Upper Mississippi River Flood Risk Management Model
6

 Figure 2. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management
Model existing and authorized levee elevations for the 1993 gaged inflow scenario between Warsaw, IL and Louisiana, MO. (A) maximum flood
depth for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between maximum flood
depth between existing and authorized design levee elevations.

7

 Figure 3. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management
Model existing and authorized levee elevations for the 1993 gaged inflow scenario between Missouri River Confluence and Chester, IL. (A)
maximum flood depth for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between
maximum flood depth between existing and authorized design levee elevations.

8

 Figure 4. Difference in maximum water-surface elevations for the 2008 flood scenario for model runs using existing and authorized
levee geometries within the Upper Mississippi River Flood Risk Management Model
9

 Figure 5. Comparison of maximum flood depths and difference in flood depths between the Upper Mississippi River Flood Risk Management
Model existing and authorized levee elevations for the 2008 flood scenario between Warsaw, IL and Louisiana, MO. (A) maximum flood depth
for the existing elevations; (B) maximum flood depth for the authorized design levee elevations; and (C) difference between maximum flood depth
between existing and authorized design levee elevations.

10

 References:
U.S. Army Corps of Engineers (USACE). 2004. Upper Mississippi River System flow frequency study.
Available: http://www.mvr.usace.army.mil/Missions/ Flood-Risk-Management/Upper-Mississippi-FlowFrequency-Study/
U.S. Army Corps of Engineers (USACE), 2016. Upper Mississippi River Restoration (UMRR) Program
Long Term Resource Monitoring (LTRM) element. 2016, UMRR Pool 19 through Open River South
Topobathy: La Crosse, WI, https://www.umesc.usgs.gov/
U.S. Army Corps of Engineers (USACE), 2017. Letter report on the Upper Mississippi River Levee
Elevations. Rock Island District. May 10, 2017 p. 5
U.S. Army Corps of Engineers (USACE), 2018a. Upper Mississippi River Flood Risk Management
Hydraulic Model. Rock Island District, February, 2018.
U.S. Army Corps of Engineers (USACE), 2018b. Upper Mississippi River Flood Risk Management
Existing Conditions Hydraulic Model Documentation Report. Rock Island District, February, 2018. p. 30.
U.S. Geological Survey (USGS), 2014. National Land Cover Dataset 2011 Land Cover National
Geospatial Data Asset (NGDA) Land Use Land Cover, U.S. Geological Survey, Sioux Falls, SD.

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 Appendix A - Levee height assessment for levees between Keokuk, IA and Thebes, IL
U.S. Army Corps
of Engineers
District
Rock Island

Levee System
Name
Canton LFPP

Rock Island

Des Moines and
Mississippi Levee
District

Rock Island
Rock Island
Rock Island

Fabius River LFP
Gregory D&LD
Hunt-Lima DD

Rock Island

Indian Grave
Drainage District South

Rock Island
Rock Island
Rock Island

Indian Grave
Drainage District North
Marion County DD
Mississippi - Fox
Drainage District
Lower

Levee Height Assessment
Within 2 ft. of authorized elevation
USACE (2017) levee survey indicates levee
elevations are within 2 ft. of their authorized
Elevation. However, modeling results suggest
current levee is higher in elevation than above
the Upper Mississippi River Flood Risk
Management Model's Authorized Levee
Geometry
Portions exceeding 2 ft. of authorized elevation
Within 2 ft. of authorized elevation
Within 2 ft. of authorized elevation
USACE (2017) levee survey indicates levee
elevations are within 2 ft. of their authorized
Elevation. However, modeling results suggest
current levee is higher in elevation than above
the Upper Mississippi River Flood Risk
Management Model's Authorized Levee
Geometry

Levee Height
Assessment
Source
USACE, 2017

USACE, 2017;
This report

USACE, 2017
USACE, 2017
USACE, 2017

USACE, 2017;
This report

Within 2 ft. of authorized elevation

USACE, 2017

Portions exceeding 2 ft. of authorized elevation

USACE, 2017

Within 2 ft. of authorized elevation

USACE, 2017

Rock Island

Sny Island Levee
Drainage District Reach 1

Portions exceeding 2 ft. of authorized elevation

USACE, 2017

Rock Island

Sny Island Levee
Drainage District Reach 2

Portions exceeding 2 ft. of authorized elevation

USACE, 2017

Rock Island

Sny Island Levee
Drainage District Reach 3

Portions exceeding 2 ft. of authorized elevation

USACE, 2017

Within 2 ft. of authorized elevation

USACE, 2017

Within 2 ft. of authorized elevation

USACE, 2017

Portions exceeding 2 ft. of authorized elevation

USACE, 2017

Rock Island
Rock Island
Rock Island

Sny Island Levee
Drainage District Reach 4
South Quincy DD
South River
Drainage District

12

 U.S. Army Corps
of Engineers
District

Levee System
Name

Levee Height Assessment

Levee Height
Assessment
Source

Rock Island

Union Township
LFP

Within 2 ft. of authorized elevation

USACE, 2017

Levee elevation not assessed

This report

Levee elevation not assessed

This report

St. Louis

Brevator Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Chouteau Island /
Chain of Rocks
West Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Columbia Bottoms
Levee System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Columbia D&LD
No.3

Levee elevation not assessed

This report

St. Louis

Consolidated North
County Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Elm Point Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Elsberry/King's
Lake System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Foley, Cap Au Gris
Levee & Winfield
Main System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Grand Tower /
Degonia

Levee elevation not assessed

This report

St. Louis

Harrisonville / Ft
Chartres

Modeling results suggest current levee is higher
in elevation than above the Upper Mississippi
River Flood Risk Management Model's
Authorized Levee Geometry

This report

St. Louis
St. Louis

Big 5 Levee
System
Bois Brule L&DD

13

 U.S. Army Corps
of Engineers
District
St. Louis

Levee Height
Assessment
Source
This report

Levee System
Name

Levee Height Assessment

Kaskaskia

Levee elevation not assessed

St. Louis

Kissinger Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

Kuhs Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

Levee elevation not assessed

This report

Levee elevation not assessed

This report

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

Levee elevation not assessed

This report

Levee elevation not assessed

This report
This report

St. Louis
St. Louis

St. Louis

St. Louis
St. Louis

Lakeside 370
Levee System
Metro East/ Chain
of Rocks
Pike Grain Levee
No.4 Levee System
Prairie Du Pont &
Fish Lake
Prairie Du Rocher

St. Louis

Sandy Creek Levee
System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

St. Louis

St Louis Flood
Protection Project
System

Levee elevation not assessed

This report

St. Louis

St. Genevieve
Levee System No.
2

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

St. Genevieve No.
3 Levee System

Levee elevation not assessed

This report

St. Louis

St. Peters Missouri
Old Town Levee
System

Levee elevation not assessed

This report

Stone Murdoch
Levee System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

St. Louis

14

 U.S. Army Corps
of Engineers
District

Levee System
Name

Levee Height Assessment

Levee Height
Assessment
Source

St. Louis

Winfield Pin Oaks
Levee System

Modeling results suggest current levee heights
are consistent with Upper Mississippi River
Flood Risk Management Model's Authorized
Levee Geometry File

This report

Levee elevation not assessed

This report

Levee elevation not assessed

This report

St. Louis
St. Louis

Wood River
D&LD Lower
System
Wood River
D&LD Upper
System

15