STORM SURGE RESPONSE TO SEA LEVEL RISE UNDER VARYING MISSISSIPPI RIVER
FLOW RATES
Mary A. Cialone1, Alison Sleath2, Ty. V. Wamsley3
U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls
Ferry Road, Vicksburg, MS 39180, USA. Mary.A.Cialone@usace.army.mil
2
U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls
Ferry Road, Vicksburg, MS 39180, USA. Alison.S.Grzegorzewski@usace.army.mil
3
U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 3909 Halls
Ferry Road, Vicksburg, MS 39180, USA. Ty.V.Wamsley@usace.army.mil
1
A significant issue in the design of flood protection in Southern Louisiana is the consideration of relative
sea level rise (SLR) due to climate change and local subsidence. Relative sea level and its change over
time alter the extent and degree that surge generated by hurricanes impacts coastal areas. In the past,
relative SLR was included in coastal protection design by raising design water levels by an amount
equivalent to the relative SLR. However, surge generation and propagation are nonlinear processes and the
linear addition of relative SLR to design water levels can underestimate or overestimate the impact in many
areas. The Mississippi River levee design must also take into account the effect of the river flow rate at the
time of a particular storm on surge levels in the river. The combined effect of sea level rise and river flow
rates on surge levels in the Mississippi River was simulated in this study. The purpose of this paper is to
examine the effect of flow rate on the nonlinearity of surge response to sea level rise. This is accomplished
through numerical surge modeling of hypothetical storms for a base condition and with the inclusion of sea
level rise and various river flow rates.
The potential impact of SLR on surge due to storms in southeast Louisiana is evaluated using the numerical
model ADCIRC. A base condition consisted of existing water levels, post-Katrina bathymetry, and levee
elevations that were expected to be in place in 2011. Future conditions consisted of 0.3, 0.6, and 0.9 m of
additional water to represent future SLR scenarios. A river flow rate of 11.33 Km3/sec (400 cfs)
representing high flow conditions was simulated for the base and 3 SLR conditions and a river flow rate of
4.73x103 Km3/sec (167 cfs) representing average flow conditions was simulated for the base and 0.6-m
SLR condition (Table 1). For this evaluation, 17 hypothetical storms were simulated. These storms
generated approximately 100-yr water levels in areas of southeast Louisiana. For these simulations, the
bathymetry and topography were not modified to represent coastal erosion or wetland loss, but bottom
roughness values and frictional wind resistance were updated to reflect vegetation changes consistent with
the increased water levels. The 17 hypothetical storms selected for simulation have central pressure (Cp)
values of 90 or 93 KPa, radius of maximum winds (Rmax) of 23-48 km, and a forward speed (Vf) of 5.6 m/s.
A detailed analysis was made for four storms (Storm Numbers: 018, 053, 056, and 069). These storms span
the range of surge responses in the Mississippi River from all 17 storms simulated. By comparing Case 1
(SLR=0.0; Flow Rate=11.33 m3/s) to Case 4, (SLR 0.6 m and Flow Rate=11.33 Km3/sec) and Case 2
(SLR=0.0; Flow Rate=4.73 Km3/s) to Case 5, (SLR 0.6 m and Flow Rate=4.73 Km3/sec), it is evident that
the surge increase is not simply linearly proportional to the change in water level due to sea level rise.
Surge levels at Station 72 increase more than the sea level rise amount (Fig 1). Moreover, these increases
can vary depending upon the opposing river flow rate. Two of the storms (Storm 018 and Storm 056) show
the same non-linearity regardless of river flow rate. The other two storm surges (Storm 053 and Storm 069)
are increasingly non-linear when the opposing flow rate is reduced. The reason for the varying response of
storm surge to relative sea level rise under varying river flow rates will be presented.
Table 1. Sea Level Rise/Flow Rate Conditions
Condition
Relative Sea
Level Rise, m
Flow Rate x 103, m3/sec
1
0.0
11.33
2
0.0
4.73
3
0.3
11.33
4
0.6
11.33
5
0.6
4.73
6
0.9
11.33
Fig. 1. Surge increase above sea level rise amount for varying flow rates