THE ASYMMETRY OF COASTAL WATER LEVEL RESPONSE TO LANDFALLING HURRICANES SIMULATED BY A THREE-DIMENSIONAL STORM SURGE MODEL

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1 THE ASYMMETRY OF COASTAL WATER LEVEL RESPONSE TO LANDFALLING HURRICANES SIMULATED BY A THREE-DIMENSIONAL STORM SURGE MODEL Mhun Peng *, Lin Xie nd Leonrd J. Pietrfes Deprtment of Mrine, Erth nd Atmospheri Sienes, North Crolin Stte University College of Physil & Mthemtil Sienes, North Crolin Stte University. INTRODUCTION Any storm surge model depends ritilly on the urte input of the hurrine s wind fields. In idel experiments, symmetri prmetri wind model is frequently used to drive the storm surge model. However, ny mistke of prmeteriztion my led to serious prolems. For exmple, when the Hollnd model (98) ws pplied to storm surge hindst of Hurrine Isel in 3 (Fig. ), the model lulted mximum storm surge, generlly, gree well with the oservtions t the ville wter level sttions (Fig. ). However, pprent overestimtion of se level fll ourred fter Mximum Storm Tide (Meter ove N.G.V.D.) Storm Tide (m) Simultion S S S3 S S5* S* S7 S8 Chespeke Bridge (S) Simultion Storm Tide (m) Simultion Duk Pier (S3) - 7th-Z Z 8th-Z Z 9th-Z Z Fig. Upper: oserved nd simulted mximum storm tide. Lowers: time series t S3, nd S. the mxim, nd this pseudo negtive surge my e lrgely due to the tendeny of more se level drop for n offshore wind. NUMERICAL MODEL Fig. Trk of Hurrine Isel nd the lotions of se level sttions Corresponding uthor ddress: Dr. Mhun Peng, Deprtment of Mrine, Erth & Atmospheri Sienes, North Crolin Stte University, Rleigh, NC 795 Emil: mpeng@nsu.edu The storm surge model employed in this study is desried in detils in Xie et l () nd Peng et l (). The hydrodynmi omponent of the modeling system is sed on the POM Model (Mellor, 99).

2 THE ASYMMETRY OF SURGE AND FALL UNDER STEADY WIND FORCES To investigte the extent of suh symmetry under different symmetri wind fores, stedy onshore nd offshore winds with the sme mgnitude pprently offer perfet nd simple symmetri wind fore. To mke things esy, n symmetry index is introdued s: AI = ( fllmx surgemx ) / surgemx %, where fll mx nd surge mx re respetively the mximum se level fll in the offshore-wind se nd the mximum surge in the onshorewind se. A squre region of points with grid size of 5km in oth diretions is hosen s the study domin. The wter depth linerly inreses from the ost to the oen t slope of /. Suh slope, whih is muh smller thn typil ontinentl shelf, is hosen to emphsize the shllow wter effet nd mke storm surge more striking. In the experiments,,, 3 nd 5m re ritrrily hosen s the minimum wter depth to redjust the originl thymetry. These four ses re respetively nmed Cse, Cse, Cse3 nd Cse. In Cse, the wter depth is ssumed to e homogenously m for the region where the originl vlue is less thn it. Similr re the other ses. A stedy onshore or offshore wind with speed of 3m s - is the driving fore in the experiments. As indited in Fig. 3, there is no pprent symmetry of se level surge nd fll under the ssumed wind for Cse3 nd Cse (AI is less thn %). But for the shllow wter ses (Cses, ), n offshore wind depresses se level fr more effiiently thn n onshore wind indues surge. To investigte how se level responds to different wind fores in shllow region, wind Se Level (m) Se Level (m) Cse Cse Cse 3 Cse hour hour 3 hour hour 5 hour Fig. 3 ) The spin-up proess t ostl lotionwith the stedy 3m s - onshore wind. ) The spin-down proess with the stedy 3m s - offshore wind. speeds of, 5, 3 nd 35m s - re hosen for the numeril experiments. It is found tht the extent of the symmetry inreses distintly with the wind speed. This is why pprent symmetry of the se level surge nd fll my exist under most hurrine wind fores (e.g. the minimum wind speed for Ctegory hurrine is 3m s - ). However, stronger hurrine does not neessrily indue lrger AI sine wind durtion of typil hurrine is not long enough for pressure grdient t given lotion to reh finl lne with wind stress. THE ASYMMETRY OF SURGE AND FALL UNDER HURRICANE WINDS The prmetri wind model Hollnd model is the prmetri wind model in this study to generte symmetry wind

3 fields. The prmetri wind model, ssuming to the lowest order, gives irulr wind flow pttern round its enter. Trnsltion speed To investigte the symmetry of the hurrine indued se level surge nd fll, the idelized squre domin, gin, is employed with the hypothetil trk running long the ost from south to north with lnd on the left. As Fig. indites, for TCs with fixed entrl pressure, the slower the trnsltion speed, the greter the symmetry of the mximum surge nd fll m 9m 95m 9m 93m 9m 97m 9m 95m 9m 93m 9m 5km/h 5km/h hour hour hour 8 hour hour 97m 9m 95m 9m 93m 9m 97m 9m 95m 9m 93m 9m km/h km/h hour hour hour 8 hour hour Fig. The effet of hurrine s trnsltion speed on the symmetry of the surge nd fll. For those with the sme trnsltion speed, weker TC is expeted to gin higher AI vlue though its indued mximum surge nd fll re reltively smller. RMW Another importnt prmeter in the wind model is RMW (rdius of mximum wind) d tht n gretly influene hurrine indued storm surge. 97m 9m 95m 9m 93m 9m RMW=3km 97m 9m 95m 9m 93m 9m hour hour hour 8 hour hour RMW=7km 97m 9m 95m 9m 93m 9m RMW=5km 97m 9m 95m 9m 93m 9m d hour hour hour 8 hour hour RMW=9km Fig. 5 The effet of RMW on the symmetry of the surge nd fll For hurrines with fixed trnsltion speed, lrger RMW inreses not only durtion time for oth onshore nd offshore winds, ut lso wind feth. The effet of the ltter is importnt nd mkes peulir the reltionship etween RMW nd AI. As expeted, the extent of oth surge nd fll inreses s RMW is enlrged (Fig.5). The pprent orreltion etween RMW nd the mximum surge is due lrgely to wind feth vrition s RMW hnges. For those with fixed RMW, the reltionship etween entrl pressure nd AI depends on the vlue of RMW. Hurrine s inflow ngle Unlike trnsltion speed nd RMW, whih influene the mximum surge nd fll due to the vrition of durtion or feth, inflow ngle ffets the surge nd fll extent y hnging the symmetri nture of the lowest-

4 order wind fields. This hnge nd the onsequent wind symmetry re essentilly different from the former two prmeters in the wy to use symmetry of storm surge nd fll. degree degree mesured nd doumented s other prmeters, it is singled out in the following hindst studies to investigte its effets on storm surge symmetry. Hurrine Chrley () nd Isel (3) re the two histori hurrines tht re studied. Hurrine Chrley m 9m 95m 9m 93m 9m 3 degree 97m 9m 95m 9m 93m 9m degree Hurrine Chrley pssed over the midestern ost of the U.S. on Aug,, s shown in Fig. 7. Only wind inflow ngle is onsidered s the vrile in the experiments while others, suh s trk, entrl pressure, m 9m 95m 9m 93m 9m hour hour hour 8 hour hour 97m 9m 95m 9m 93m 9m hour hour hour 8 hour hour d 3.N Trk of Hurrine Chrley () Sunset Beh Springmid Pier Fig. The effet of hurrine s inflow ngle on the symmetry of the surge nd fll. 33.N Chrleston 8//5Z 99 As, shown in Fig., onsidertion of inflow ngle fundmentlly hnges the symmetry of wind fields, nd provides the surge model n symmetril (with respet to its enter) exterior fore s in rel ses. In the Northern Hemisphere, if hurrine moves northwrd with lnd on its left, inflow ngle onsidertion is expeted to generte lrger surge nd smller fll, or, moderte the symmetry of surge nd fll tht would otherwise e overestimted. HISTORIC HURRICANE CASES Any mistke of prmeteriztion in the wind model my led to mistkes in surge simultion. The misrepresenttion s indited previously is more pprent in the negtive phse of the se level movement. As hurrine s inflow ngle is not intentionlly 3.N Fort Pulski 8//9Z 99 8//Z W 8.W 8.W 79.W Fig. 7 Trk of Hurrine Chrley nd lotions of se level sttions trnsltion speed, nd RMW, tke their interpolted vlues from oservtions t eh time. Se level dt re ville t sttions efore, during, nd fter lndfll, nd the lotions of the sttions re illustrted in Fig. 7.

5 Sunset Beh, NC S e level under the influne of Hurrine Chrley (m ) degree degree degree degree degree degree degree degree degree Springmid Pier, SC Chrleston, SC Fort Pulski, GA Mximum Storm Tide (m) Mximum Storm Tide (m) - - degree degree degree degree degree degree Chespeke Bridge (S) Duk Pier (S3) 7th-Z Z 8th-Z Z 9th-Z Z - degree degree degree /Z /Z /Z /8Z Fig. 8 Se level time series tking, nd s the wind inflow ngle re ompred with the oservtion for Hurrine Chrley. The results indite the mximum se level fll will e lrgely redued s inflow ngle is onsidered in the prmetri wind model. As result, the symmetry of the mximum se level fll nd surge is moderted. Hurrine Isel The trk of Hurrine Isel hs een shown in Fig.. As mentioned previously, the prmetri wind fields with no inflow ngle orretion led to n pprent se level distortion in the fll phse, though the overll mximum se level surge t most sttions greed well with the oservtions. Different inflow ngles re fed into the model to see how muh differene they n mke with regrd to se level surge nd fll nd their symmetry. The influene of the inflow ngle on the surge nd fll is illustrted in Fig. 9 Fig. 9 Se level time series tking, nd s the wind inflow ngle re ompred with the oservtion for Hurrine Isel With inflow ngle onsidertion, the pseudo negtive surge is lrgely moderted. The disrepny still existing in negtive wter phse my due to the ontriution of preipittion nd runoffs tht is not onsidered in the model. CONCLUSION It is found tht offshore winds re more effetive in suppressing ostl se level thn the orresponding onshore winds in induing surge. As result, the response of se level in the hurrine s offshore wind qudrnt is more sensitive to wind fore thn in the onshore wind qudrnt. Any mistkes of the model speified prmeters, suh s trnsltion speed, RMW, nd inflow ngle my led to inurte se level results, nd this is more pprent in the hurrine s offshore wind qudrnt. This symmetry implies tht n urte se level forest is more diffiult to hieve in negtive thn in positive phse.

6 REFERENCES: Hollnd, G.J. 98: An nlyti model of the wind nd pressure profiles in hurrines. Monthly Wether Review, 8,. Mellor, G. L. (99) User s guide for three dimensionl, primitive eqution, numeril oen model. Prineton University, Prineton, NJ. Peng, M., L. Xie. nd L. Pietrfes. : A Numeril Study of Storm Surge nd Inundtion the Crotn-Alemrle-Pmlio Estury System. Esturine, Costl Shelf Si. 59, -37 Xie, L. Pietrfes, L.J, nd M. Peng, : An integrted storm surge nd inundtion modeling system for lkes, esturies nd ostl oen. J. of Costl Reserh,, 9-3.