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Modeling Tsunami Waves and Ocean Storm Surges with FLO

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Modeling Tsunami Waves and Ocean Storm Surges with FLO Powered By Docstoc
					                   MODELING TSUNAMI WAVES AND OCEAN STORM SURGES WITH FLO-2D®

                                                        J. S. O’Brien*

ABSTRACT: Overland floodwave progression of ocean storm surges from hurricanes and fast rising tsunami waves can be
simulated with the FLO-2D® two-dimensional flood routing model. Ocean surge flooding can be simulated by assigning
water surface stage and duration to the coastline grid elements. FLO-2D® is a volume conservation model that is effective for
analyzing riverine or unconfined alluvial fan flooding, but it can also simulate storm surges through coastal urban areas with
detailed resolution. Various ocean storm surges were simulated for the City of Waikiki, Oahu, Hawaii using an existing FLO-
2D® watershed model. The results illustrate that the area of inundation is a function of both wave height and duration as they
progress through the downtown Waikiki area and into the Ala Wai Canal that bisects the city.
KEY TERMS: tsunami, storm surge, two-dimensional flood routing, hydraulic modeling, FLO-2D® model


                                                     INTRODUCTION

     Identifying coastal flood hazard areas for storm surges, wave run-up and tsunamis has been undertaken by agencies and
consultants worldwide. Each study has a unique focus ranging from replication of recent flooding to hazard delineation and
mitigation design. From a FEMA perspective, the delineation of coastal flood hazard areas due to storm surges and wave
action from large open water bodies is based on water surface and wave elevations associated with the 100-year storm.
FEMA has identified methods for determining coastal area base flood elevations (BFE’s) and hazard areas for the 100-year
storm that include the analysis of wave height and run-up. New storm surge and tsunami models are being developed in
response to last years’ hurricane disasters in Florida and the recent tsunami in the Indian Ocean. Most existing coastal
hydraulic models focus on generating wave height. The FLO-2D® model can be used to create detailed overland mapping for
ocean storm surge or tsunami extreme hazards and is particularly effective in urban areas where buildings, obstructions,
streets and channels can affect the floodwave progression. The modeling detail provided by FLO-2D® exceeds that of other
hydrodynamic models and the results include predicted flow depths, velocities, discharge hydrographs, dynamic and static
pressure, specific energy, and area of inundation. The specific input data required for a FLO-2D® ocean storm surge model is
wave height or water surface elevation as a function of time (time – stage data pairs) for the coastal grid elements.


                                      Study Area and Original FLO-2D® Watershed Model

     The storm surge application of FLO-2D® to the City of Waikiki is based on an existing watershed model developed by
Hawaii District Corps of Engineers to delineate the rainfall runoff flood hazard for the Ala Wai Canal drainage basin. The
Ala Wai basin consists of three subbasins and the lower Ala Wai Canal passes through the city. The entire Ala Wai Canal
watershed encompasses about 17 square miles. Two significant storms that caused flooding in the basin occurred on
November 14-15, 1965 and December 17-18, 1967. The 1965 storm resulted in damages of approximately $700,000 and
floodwaters were 2 feet deep in some areas of the city. The goal of the Corps’ investigation was to delineate the flood hazard
for various return period flood events and use the resulting flood hazard maps to quantify economic benefits to justify federal
participation in the construction of an Ala Wai Canal flood damage reduction project.
     The 100-year flood from various watersheds on the slopes of Diamondhead and surrounding hills was predicted to
exceed the channel and Ala Wai Canal conveyance capacity and inundate downtown Waikiki. The extent of urban flooding
is primarily dependent on street flooding and flow obstruction. The loss flood storage due to structures is a model
component. The mild canal slope and flat topography near the ocean exacerbates the flooding. In addition, the ocean water
surface boundary condition creates backwater in the canal. The canal opens to the ocean with below-sea-level channel bed
elevations. The channels and canals were represented by surveyed cross sections. The preliminary results for the area of
inundation based on the predicted maximum flow depths for the 100-year Ala Wai watershed flood is shown in Figure 1.

*
 Senior Hydraulic Engineer, Tetra Tech, Inc., P.O. Box 66, Nutrioso, Arizona 85932, Phone and Fax: (928) 339-1935, email:
flo2d@frontiernet.net


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     Figure 1. City of Waikiki 100-yr Area of Inundation from the Ala Wai Canal Watershed Flooding (preliminary results)


                                               FLO-2D® MODEL DESCRIPTION

      FLO-2D® is a volume conservation, two-dimensional flood routing model that distributes a flood hydrograph over a
system of square grid elements using a finite difference algorithm. The flood hydrograph is routed in channels or on the
unconfined floodplain using the full dynamic wave momentum equation. See the FLO-2D® User’s Manual for a discussion of
the model theory and attributes (FLO-2D Software, Inc, 2004; Note: the manual can be downloaded at the website www.flo-
2d.com). FLO-2D® can be a valuable tool for analyzing river overbank flows and floodwave attenuation, but it can also be
used to analyze unconventional flooding problems such as unconfined flows over complex alluvial fan topography and
roughness, split channel flows, mud/debris flows, and urban flooding. The model was initially conceived with FEMA support
of a mudflow project in 1989 and was developed over the years through application to a large number of Tetra Tech projects.
It is a FEMA approved flood insurance study (FIS) hydraulic model for both riverine and unconfined flooding.
      Starting with a basic overland flood simulation, flooding details can be added by turning on or off switches for various
components such as channel flow, street flow, rainfall, infiltration, loss of storage due to buildings, obstructions, levees and
levee failure, hydraulic structures (bridges and culverts), sediment transport and mudflow. Multiple flood hydrographs can be
introduced to the system at any number of inflow points either as a floodplain or channel flow. As the floodwave moves over
the dry floodplain or down channels, flow over adverse slopes, floodwave attenuation, ponding and backwater effects can be
simulated. With respect to coast flooding, FLO-2D® would be primarily applied to predict the area of inundation, maximum
flow depths, velocities and impact pressures. No model hot starts or special boundary conditions are necessary.
      The FLO-2D® software package includes a grid developer system (GDS) that will overlay a square grid system on
random digital terrain (DTM) points. The GDS will filter DTM points, interpolate the DTM data and assign elevations to grid
elements. Once the grid system has been created with assigned elevations, the GDS can be used to select the grid elements to
representing the coastline where the storm surge will occur. After the coastline grid elements are identified, the time-stage
data pairs can be assigned to all of the coastal elements at once. When the basic storm surge or tsunami has been simulated,
adding the flood details is relatively easy. Aerial photos such as *.jpg or *.tif files can be imported to the GDS as background
to the grid system to identify and assign data representing floodwalls, streets, buildings, and channels. The GDS data
assignment for these components has been automated and grid elements can be selected by polygon.
      Modeling the overland progression of ocean storm surges evolved from the need to assign time-stage hydraulic control
for river flood routing where the rivers entered estuaries or lakes. A number of projects have been completed using the time-
stage component to set hydraulic control. By setting the water surface higher than the ground surface, inflow to coastal areas
can be simulated without knowing the discharge. Volume conservation is the key element for all FLO-2D® flood simulations
and is observed with storm surge modeling using the time-stage input option. To simulate a simple overland ocean surge only

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two data bases are necessary: 1) Digital terrain model (DTM) of the land surface; and 2) Time-stage data pairs of the ocean
surge. The time-stage data can be as simple as a triangular function:

         Time (hrs)      Stage (ft or m)
          0.0                   0.0
          0.5                   10.0
          1.0                   0.0


                         FLO-2D® APPLICATION TO OCEAN SURGES IN THE CITY OF WAIKIKI

     An example FLO-2D® application of ocean storm surge/tsunamis modeling to the City of Waikiki was prepared for this
paper. The data files for physical features and components of the Ala Wai watershed model developed by the Corps were
used without modification. The model has 9205 grid elements, 100 ft square, 174 channel elements (79 channel elements for
the Ala Wai Canal), storage reduction factors for buildings, Ala Wai Canal bridge rating tables and some of the important
streets. The flood inflow hydrograph data file was replaced by a time-stage water surface pairs. The coastline time-stage grid
elements were located next to the outflow boundary elements along the coast. For the purpose of demonstrating the storm
surge and tsunami models, a series of time-stage scenarios were contrived. For future ocean surge model applications, the
time-stage data can be estimated by a wave height model. FEMA has list of approved coastal wave height models including
CHAMP, WHAFIS, RCPWAVE, RUNUP2, ACES 1.07 and others (FEMA, 2003). The following area of inundation results
are based on the predicted maximum flow depths regardless of the time of occurrence. Table 1 indicates the maximum wave
height and duration assigned to the coastal grid elements for the storm surge and tsunami models.

                            Table 1. Simulated Storm Surge and Tsunami Wave Heights and Durations
                                                                Maximum Wave Height
                          Storm Surge Model                      (elev. above m.s.l. ft)         Duration (hrs)
            Low wave height, long duration storm surge                      4                          6
            Moderate wave height, moderate duration surge                   6                          4
            Moderately high wave, short duration surge                      8                          2
            Moderately high wave, long duration surge                       8                          8
            High wave, short duration storm surge                          10                          1
            Tsunami, long duration                                         20                     20 minutes
            Tsunami, moderate duration                                     30                      5 minutes
            Tsunami, short duration                                        30                      1 minute


     In this limited space, only selected results can be presented. Those figures that are selected indicate the range of the
predicted area of inundation for the possible storm surge and tsunamis listed in Table 1. The following figures were generated
by a FLO-2D® post-processor program (MAPPER) using imported *.jpg aerial photograph images prepared by the Corps.
The low wave height, long duration storm surge of 4 ft for 6 hours inundated only a small area of the coastline and is not
presented. Figure 2 shows the area of inundation for a moderate wave height, moderate duration surge of 6 ft for 4 hours. It
also displays the grid system. Figure 3 illustrates the area of inundation for a moderately high surge of short duration, 8 ft for
2 hours. The area of inundation for high wave short duration storm surge of 10 ft for 1 hour is show in Figure 4. Finally, the
area of inundation for the simulation of two tsunami waves, 20 ft for 20 minutes and 30 ft for 1 minute are display in Figures
5 and 6 respectively. The associated time-stage input data is presented next to each figure. The areas of inundation associated
with the moderately high wave, long duration storm surge and the moderate duration tsunami wave are not presented. Each
simulation took several hours to run on a 300 MHz PC computer.
     These simulations were conducted with the preliminary data files prepared in conjunction with the Corps of Engineers’
Ala Wai watershed FLO-2D® model. The final Corps watershed model with any additional detailed features and any
coastline or canal flood mitigation measures was not available. The Corps was not consulted in preparing these coastal flood
simulations and the FLO-2D® results are entirely fictitious from a storm surge and tsunami perspective.




                                                                3
           Time (hrs)    Stage (ft)
              0.0           0.0
              1.0           1.0
              2.0           6.0
              6.0           6.0
             12.0           0.0




Figure 2. City of Waikiki Area of Inundation for a 6 ft – 4 hr Storm Surge with Overlaid FLO-2D® Grid System




               Time (hrs)   Stage (ft)
                  0.0          0.0
                  1.0          1.0
                  2.0          8.0
                  4.0          8.0
                 12.0          0.0




                 Figure 3. City of Waikiki Area of Inundation for the 8 ft – 2 hr Storm Surge


                                                      4
Time (hrs)   Stage (ft)
   0.0          0.0
   1.0          2.0
   2.0         10.0
   3.0         10.0
   4.0          0.0
  12.0          0.0




        Figure 4. City of Waikiki Area of Inundation for the 10 ft – 1 hr Storm Surge




   Time (hrs)   Stage (ft)
      0.0          0.0
      2.0          2.0
     2.15         20.0
      2.3          4.0
      4.0          0.0
     12.0          0.0




    Figure 5. City of Waikiki Area of Inundation for the 20 ft – 20 minute Tsunami Wave

                                             5
                 Time (hrs)    Stage (ft)
                    0.0           0.0
                    1.0           2.0
                   1.008         30.0
                   1.017          5.0
                    2.0           0.0
                    12.0          0.0




                      Figure 6. City of Waikiki Area of Inundation for the 30 ft – 1 minute Tsunami Wave


     The predicted area of inundation is revealed in shaded color contours ranging from blue (shallow flow depths) to red
(high maximum flow depths). Using the Ala Wai Canal as a point of reference, the critical importance of the floodwave
duration (or flood volume) can be assessed. Both the 10 ft – 1 hr storm surge and the 20 ft – 20 minute storm surge
floodwaves cross the Ala Wai Canal and inundate areas in the north portion of the City. For the 8 ft – 2 hr storm surge, the
Ala Wai Canal forms an effective mitigation barrier. The 6ft – 4 hr storm is limited by the topography in the downtown
Waikiki area. The 30 ft – 1 minute tsunami wave (the wave rises and falls in 1 minute with the peak water surface of 30
above mean sea level at 30 seconds) has only limited volume and inundates only the downtown area south of Ala Wai Canal.


                                                        CONCLUSIONS

      With only a minimal data base consisting of a DTM model and ocean water surface as a function of time, FLO-2D® can
be used to generate detailed mapping associated with ocean storm surges or tsunamis. This is particularly valuable in urban
areas where buildings, obstructions, and flow in streets and channels can affect the area of inundation. The modeling detail
was demonstrated using the Ala Wai Canal watershed to simulate storm surges through the City of Waikiki where the Ala
Wai Canal could limit flooding in the northern part of city. By using an available wave height model to create the time-stage
data pairs, the FLO-2D® model can be applied to predict coastal flood hazard areas associated with storm surge or tsunamis
worldwide. Accurate delineation of coastal flood hazard areas for extreme storm surge and tsunami events will reduce loss of
                                                                                          ®
life and property damage if effective zoning regulations can be implemented. The FLO-2D results can also be applied to the
design of mitigation measures to limit to inland advance of surge floodwaves.


                                                      REFERENCES

FEMA, 2003. Appendix D Guidance for Coastal Flooding Analyses and Mapping in Guidelines and Specifications for
   Flood Hazard Mapping Partners, Federal Emergency Management Agency, Washington, D.C.
FLO-2D Software, Inc, 2004. FLO-2D® User’s Manual, Nutrioso, Arizona, www.flo-2d.com.



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