Tsunamis have a small wave height offshore, and a very long wavelength (often hundreds of kilometres long, whereas normal ocean waves have a wavelength of only 30 or 40 metres),Facts and figures: how tsunamis form , Australian Geographic, March 18, 2011. which is why they generally pass unnoticed at sea, forming only a slight swell usually about above the normal sea surface. They grow in height when they reach shallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state and even at low tide can still inundate coastal areas.
Tsunamis hAve a sMALL WaVE HeigHt offsHorE, AnD a vErY loNG WaveL,ength (ofTen HUNDrEDs OF kilOmeTrES lOng, whErEas normAl ocean WavES haVe A wavelEngTh of ONLy 30 or 40 metres).facts aNd FigUrEs: how TsunAmis fOrm , AuStrAlIan GeOgrapHIC, maRch 18, 2011. whIch is why theY generalLy pasS unnoTicEd at sEa, FOrming onlY a sLIght sWelL USUally aBout above thE NorMal sEA suRfAce. they groW in height whEn They rEach shallOWeR WaTer, iN a WaVe ShOaliNg Process desCribeD Below a tsUnAmi cAn occuR in any tidAl StAtE and even aT Low TIde caN still InUndAte cOasTAl ArEas.