Tsunami / Storm Surge

Modeling Overland Progression of Tsunami Waves and Ocean Storm Surges with FLO-2D

The FLO-2D model can be used to create detailed overland inundation mapping for ocean storm surge or tsunami 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.

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 canal through the heart of the city opens to the ocean with below-sea-level channel bed elevations. The mild canal slope and ocean backwater exacerbates the flooding. The extent of urban flooding is dependent on street flooding and flow obstruction. 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.

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

Modeling the overland progression of ocean storm surges is accomplished using a time-stage data 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 to all accurate FLO-2D flood simulations and is observed with storm surge modeling using the time-stage input option. The time-stage data can be as simple as a triangular function:

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

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

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. A series of time-stage scenarios were developed for ocean surge/tsunami models. Table 1 indicates the maximum wave height and duration assigned to the coastal time-stage grid elements.

Table 1. Simulated Storm Surge and Tsunami Wave Heights and Durations

The following figures were generated by a FLO-2D post-processor program (MAPPER) with imported *.jpg aerial photograph images prepared by the Corps. The predicted area of inundation is revealed in shaded color contours ranging from blue (shallow flow depths) to red (high maximum flow depths).

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.

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 surge 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 on the ocean side of Ala Wai Canal.

NOTE: 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.

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

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

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

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

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

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 flood inundation mapping associated with ocean storm surges or tsunamis. By adding urban detail such as levees, channels, streets and buildings or other obstructions, detail flood inundation maps can be prepared. 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.