The North Carolina State University Coastal and Estuary Storm Surge and Flood Prediction System

The North Carolina State University coastal and estuary storm surge and flood prediction system

L. J. Pietrafesa, L. Xie, D. A. Dickey, M. C. Peng & S. Yan

College of Physical & Mathematical Sciences, North Carolina State University, Raleigh, NC 27691, USA

Abstract

The North Carolina State University Coastal and Estuary Marine & Environmental Prediction System (CEMEPS) is a coupled system of mathematical models. CEMEPS contains a suite of interactively linked atmospheric, oceanic, estuary and river model components. The model architecture couples mesoscale atmospheric models or event models such as hurricanes or a suite of atmospheric variable measurements, wind-fields and precipitation, to ocean basin, continental margin, and estuary hydrodynamic models to a river discharge-interaction model. So, winds and precipitation are both observed and modeled and water waves and currents and water levels are predicted. Thus, storm surge and estuary flooding can be accurately determined well in advance of a storm. CEMEPS output is routinely used by the North Carolina Office of the

National Weather Service to make forecasts of coastal and estuary flooding during the passage of Tropical and Extra-Tropical Cyclones. While the model system is currently focused on the coasts of the Carolinas, CEMEPS could be ported to all coasts. The goal of CEMEPS is to improve the capacity of coastal communities to reduce flood impacts.

l Introduction

In a coastal location, flooding occurs when the actual water level in the adjacent ocean, estuary or inland water body significantly exceeds spring tidal levels or the banks of the containment barrier and intrudes onto the adjacent land. Thus,

in order to predict coastal and inland flooding during a storm event, one must have accurate information of local land elevation and actual water level prior to and during the event. Along a coastline and in estuaries the total water level height is determined by 6 main factors: wind and pressure induced sea level rise or fall; wind induced waves; astronomical tides; seasonal modulation of the steric state of the ocean basins; freshwater input; and the estuary's water level. Here, we define a model scenario to develop a prognostic capability to accurately predict coastal and inland flooding in all of its aspects during the passages of high energy atmospheric events such as Tropical Cyclones. The NCSU coastal flooding model has been used to make real-time coastal flooding forecasts in the North Carolina coastal lagoon sound system since the 1993 hurricane season. The prediction has been generally accurate (within 10% of observations). The very first real-time test case was Hurricane Emily of 1993

[l]. For that case, the maximum water level along the Outer Banks, NC on the sound side predicted by NCSU model 12 hours in advance was not only within 0.5 feet of actual observations (of > 11 feet) but also 2 feet closer to observations than the National Hurricane Center's SLOSH model [2].

While NOAA's SLOSH model has been used for guidance by the NOAA National Weather Service (NWS), the NCSU CEMEPS has demonstrated an improved capability to predict coastal flooding on the NC coast both offshore and inshore. CEMEPS was intended to contain a suite of interactively linked oceanic and atmospheric model components as described in Xie [3]. Albeit, the passage of Hurricane Floyd in 1999 showed the need for an even more advanced predictive modeling capability. First, we should consider what the National Hurricane Center (NHC) presently uses for coastal flooding forecasts, the SLOSH model and its difference with CEMEPS.

2 SLOSH versus CEMEPS

First, the SLOSH model is 2-D. It estimates the average motion of a water column and then computes the surface water level. However, it cannot explicitly reveal the water flow speed and direction near the bottom or any specific depth. Since an important factor determining the storm surge is bottom friction, 2-D storm surge models cannot properly estimate bottom stress and thus storm surge.

CEMEPS is fully three-dimensional and the water column at each location is divided into levels. Water flow at each level is computed allowing the model to more accurately estimate the bottom stress by using the near-bottom flow instead of depth-averaged flow. Next, the SLOSH model does not consider the effect of fresh water. During "wet storms", runoff and river discharges can cause significant water level fluctuations near the river mouths and around the peripheries of the estuaries. CEMEPS has a mechanism to incorporate land and river runoff effects. Finally, unlike SLOSH, CEMEPS conserves water and has a physically correct lateral wetting and drying (inundation and retreat) boundary condition which allows the model to accurately estimate inland flooding by allowing water to move horizontally in equilibration with each ensuing time step. Unprecedented levels of flooding and 56 human fatalities followed the rapid

passage of Hurricane Floyd across North Carolina (NC), 16-17 September, 1999. The net damage ascribed to Humcane Floyd, a Cat 2, was estimated to be in excess of $6B. Why? Well, circumstantially, Hurricane Dennis, a Cat l, had been present on the NC coast over the period 30 August- 06 September. Paths of

Dennis and Floyd are in Figure 1. This is what physically occurred.

Figure 1: 1999 North Atlantic Tropical Cyclone Map for the paths of Hurricanes Dennis and Royd derived from NOAA National Hurricane Center

information.

Dennis' winds mechanically drove coastal waters towards the coast, and simultaneously drove inshore sound waters from the northeast end of Parnlico

Sound towards the southwest end. The offshore rise of water and inshore drop of water resulted in a hydraulic head along the axes of the barrier island inlets, particularly Oregon Inlet. This head drove a persistent non-tidal current of up to more than 2 knots for -6 days, into the sound.

The amount of coastal ocean water which entered the sound system during Dennis' presence (1.4x109m3) was equivalent to 75% of the amount of water already present in the sound (1.86x109m3)prior to the hurricanes arrival so the total volume of water in the sound expanded to 3.26x109m3of water (Fig. 3).

Both Dennis and Floyd had heavy precipitation (Fig. 2). The enormous amount of water in Parnlico Sound dammed up or blocked the flows at the mouths of all tributaries into Pamlico Sound, causing the rivers to go into relative storage modes and thus backed waters up towards the heads of the rivers (Fig. 3). Following Dennis' departure, the waters in the sound began to discharge through the three inlets (outlets) but within a week and a half, along came Floyd (Fig. l).

River waters were still blocked at their mouths and riverlestuary waters expanded explosively over the banks. The subsequent flooding was directly related to existing water levels prior to the arrival of Floyd (Figs. 4,s).

a. Hurricane Dennis Rainfall September 4-5,1999 North Carolina

Precipitation in Inches Based on Prelimfnary Data

b. Hurricane Floyd Rainfall September 14-16,1999 North Carolina

Figure 2: Precipitation associated with Dennis and Floyd.

The message here is that during the passage of the two very wet hurricanes, the coastal ocean and estuary and rivers coupled in a way that produced massive lateral flooding. No modeling architecture or combination of data and models existed at the time to have properly predicted the massive flooding that ensued. CEMEPS was designed to overcome these shortcomings.

Wnshingmn Oregon Duck a. 3 1

1. 9, Dxk-Oregon - - Washington-Oregon

Year Day

Figure 3: (a) 40 hour low pass time series of sea level at Duck, Oregon Inlet, and Washington North Carolina for the period 8125-9/20, 1999 (b) 40 hour low pass of the difference in sea level from outside Oregon Inlet

and inside Oregon Inlet(Duck minus Oregon) and from Oregon Inlet to Little Washington (Washington minus Oregon) for the period 8/25 9/20, 1999 (c) Northeastward(+)/Southwestward(-) component of the Wind Stress vector time series at Cape Lookout during the period 8/25-9120, 1999 (d) Volumetric flux of water through Oregon Inlet

during 30 August - 20 September, 1999 (e) Cumulative volumetric flux through Oregon Inlet from 30 August- 20 September, 1999.

CORRELATION BETWEEN SURGE AND FLOOD 80.

60- r7md Tendency (KmA2) -SurgeAnmaly (mch)

do.

-80. . . . , . . 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 IS 16 17 18 19 20

ExhHng Rmd Lcvcl (m)

Figure 4: Surge and lateral inundation vs. existing water level in Parnlico Sound.

Figure 5: SeaWifs image 23 September.

3 The CEMEPS architecture

CEMEPS includes one of several mesoscale atmospheric models, a coastal ocean model, the Princeton University Ocean Model (POM), [4], coupled to a 3rd generation wave model WAM Cycle4, and the estuary model of Xie and Pietrafesa [3] based on the earlier version of Pietrafesa [l]. Tides are simulated

Transfer Rain - - > Flow Model inoludas p10 d effect (intervention) Jaalzco

DATE PLOT -Ferecaat: Pamlico Pamlico Flow -- Lover 950 Confxdenca Limit -- Upper 95% Confidence Limit

Figure 6: River discharge vs. precipitation: a statistical predictor. via specified lateral boundary conditions that contain tidal information derived from a combination of in-situ tidal records and TOPEX altimeter data. The circulation model solves the 3-D primitive Navier Stokes equations cast in horizontally and vertically staggered grids with a terrain-following coordinate in the vertical. Prognostic equations governing momentum, temperature, salinity and free surface elevation are solved with a finite difference scheme that uses an implicit technique in the vertical and a mode splitting technique in time. Spatial grids are adjustable. For estuary applications, the horizontal grid size is presently set to 300m and the vertical resolution is set to 6 levels. For the coastal applications, the horizontal grid size is presently set to 1 Km with 18 levels in the vertical. CEMEPS could be extended to any coastal region.

The input variables for the CEMEPS coupled modeling system include time series of 2D surface wind fields with options for the wind input data either as simulated by a mesoscale atmospheric model or by actual observations of

Rainfall

Wind

Humidity

Storm Surga Coastal Flaad

Figure 7: CEMEPS Architecture.

hurricane track, maximum wind, radius of maximum wind, and minimum pressure or real-time surface wind analysis, such as the NOAA's near real-time hurricane wind analysis and precipitation using the NOAA National Severe Storms Lab's (NSSL) QPESUMS.

CEMEPS also incorporates time series of river fluxes (runoff rate). The sources of these time series can be either from real time or historical records which can be used in a diagnostic or hind-cast mode or from river stream flow forecasts in a prognostic mode, such as the output from a statistical river forecast model (Fig. 6) component. CEMEPS couples the estuary and coastal models to ocean basin scale models at the open boundaries. These modeling components include the parallel global ocean model (MICOM), the paralIe1 global atmospheric circulation model (CCM3), the regional climate model (RegCM2). The entire CEMEPS architecture is presented conceptually in Fig. 7.

4 Conclusions

The ability to accurately predict coastal, estuary and inland flooding related to the passage of high energy and wet atmospheric events requires a new paradigm in coupled model architecture. No longer can just wind intensity or even wind intensity and direction suffice for proper forecasts to be made. To properly and accurately predict the temporal and spatial inundation of waters in coastal, estuary and inland areas, a model system which couples atmospheric information to fully 3 dimensional time dependent ocean basin, coastal and estuary hydrodynamic models coupled to an interactive river discharge model is required. The atmospheric information can be derived from a model or from data or some combination thereof. The river and estuary components must both be capable of going into modes of storage or accelerated discharge. Horizontal spatial scales must downscale from 1000's to 100's to 10's of kilometers to 100's to 10's of meters. Vertically, downscaling from 100's to 10,s of meters to 100's tol0's to units of centimeters must occur.

Acknowledgements

This work was supported by the National Oceanic and Atmospheric

Administration under Grant #NA060C0373-001 through the NOAA Charleston Coastal Services Center with Waterstone Enterprise Strategies and Tech Inc. Jim Epps is acknowledged for data processing and figure production and Michele Kephart for manuscript formatting.

References

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Model, Bulletin of the American Meteorological Society, 66, p. 1408-1411, 1985. Xie, L. & Pietrafesa, L.J. Systemwide Modeling of Wind and Density Driven Circulation in the Croatan-Albemarle-Parnlico Estuary System. Part

I: Model configuration and testing. Journal of Coastal Research, Vol. 15 (4), p. 1163-1177, 1999. Mellor G.L. User's guide for three-dimensional primitive equation numerical ocean model. Princeton University, Princeton, NJ, 1997.