Announcements. Project 2 due Nov 7 th Topics for today: Big waves. Tsunamis, seiches and tidal waves. Tsunamis and seiches

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1 Announcements Project 2 due Nov 7 th Topics for today: Big waves Tsunamis, seiches and tidal waves Tsunamis and seiches Seiche: standing wave that rocks back and forth within an enclosed or semi-enclosed area Tsunami: long-wavelength shallow-water wave caused by a large water displacement (e.g. seismic sea waves) wavelengths often > 100 km height in open ocean < 1 m Tsunami facts: Shallow-water wave velocity: C = 3.1 sqrt(d), where d = 4500 m, = 200 m/sec (or, 750 km/h!) Sumatra Tsunami Wave generation and propagation Upon reaching the coast, water shallows and the leading edge of the tsunami slows first. Wave height can increase to > 30m. Dec 26 th, 2004 tsunami illustrates these phenomena What you d see from the beach Consequences of long wavelength 1

2 2011 Tohoku earthquake and tsunami, Japan Tsunamis in Washington State Cascadia subduction zone is similar to the Sumatra subduction zone. How could you protect yourself from a Tsunami produced at the Cascadia subduction zone? Modeled tsunami hazard map of Bellingham Bay Green: m Yellow: m Red: 2 5 m Tsunamis vs. tidal waves Similarities: Long-wavelength shallow-water waves wavelengths often > 100 km height in open ocean < 1 m Differences: Tsunamis caused by sudden water displacements (e.g. seismic) and propagate freely. Tsunami height can increase to > 30 m in shallow water. Tidal waves are forced by gravity. Heights can reach > 10m (Bay of Fundy), but these heights occur regularly. 2

3 Saint John, New Brunswick, Reversing Falls Tides I. Tidal forces (Equilibrium Theory) A. Moon B. Sun C. Moon and Sun II. Effects of Basin Geometry and Coriolis (Dynamic Theory) III. Shoreline Patterns A. Patterns in time B. Patterns in Space 1. Gulf of Maine tides 2. Bay of Fundy tides VI. Predicting Tides VII. Tidal currents VIII. Effects on organisms 1. Zonation 2. Vertical migration and retention in estuaries Forces acting upon the earth-moon system Equilibrium theory of tides: Assumptions: 1. Tidal-potential can be determined from celestial mechanics 2. Oceans of uniform depth cover the earth 3. No Coriolis 4. No friction Equilibrium theory is a useful way to think about forces affecting tides. But, it clearly neglects many important factors F=G ( ) m 1 m 2 r 2 3

4 Resulting tidal bulges Spring and neap tides Dynamic Theory of tides - Shallow-water wave velocity: Earth surface velocity near equator: ~ 460 m/s Maximum shallow-water wave velocity: 200 m/sec Between 26 degrees N/S, the lag is ~ 90 degrees (indirect tide) N and S of 65 degrees the tide lag disappears (direct tide) -Changes in water depth and shapes of ocean basins steer tidal waves -Coriolis and pressure gradient forces act on waves: local acceleration + Coriolis acceleration = pressure gradient force + astronomical force Tides propagate as rotary waves 4

5 Things are not so simple: Dynamical Theory - Tides propagate around amphidromic nodes Formation of amphidromic points in large semi-enclosed basins Cotidal lines: all places along a line experience the same tidal phase Diurnal, semidiurnal, and mixed-semidiurnal tides Cotidal lines Corange lines: All places experience the same tidal range Many amphidromic points lead to complex tides 5

6 Distributions of diurnal, semidiurnal, and mixed-semidiurnal tides Why is the Bay of Fundy tide so large? 1: Time required for water to enter and exit the bay is nearly identical to the period of the semi-diurnal tide 2: The bay is funnel shaped, which increases the magnitude of the tide in the upper reaches Predicting complex tides: Harmonic analysis 6

7 Predicted tidal height Tide simulation, adding M 2, S 2, N 2, K 2, K 1, O 1, and P 1 tidal constituents to a simple sinusoidal tide model Time (days) Predicting tides: Fourier analysis 1: Measure the tides at a site for many years 2: Break down the measured tidal heights into components using fourier-series deconvolution (up to 65 components, both gravitational and non-gravitational) 3: Use the model to predict next year s tides. 4: Continue to measure the tides to correct the model Tidal currents Nearshore, Tidal waves act like waves on a beach, moving water back and forth - tidal currents Puget Sound Washington Burrows Bay Deception Pass Low tide at coast Ebbing tide in Seattle High tide in Shelton Agate Pass 7

8 Tidal wave amplitude change from Port Townsend to Shelton 1m offset in the moon s contribution to the tide (overall change is closer to 3m) (As channel narrows or shallows, wave amplitude increases) Tidal wave amplitude change from Crescent Bay (Strait of Juan de Fuca) to Olympia Spring tide range (ft) Crescent Bay Admiralty Inlet Distance from Crescent Bay (nm) Tacoma Narrows Bridge Olympia Modeled currents in Puget Sound Resulting tidal currents Strongest to the north Strongest at bottle necks UW Hydrodynamic model 8

9 Effects of tides on Organisms From Taylor et al. 2005, Estuaries (Emergence during first nighttime tidal deceleration) Tidal power in Puget Sound? Potential sites for in-stream tidal turbines Spieden Channel Guemes Channel Agate Passage San Juan Channel Deception Pass Rich Passage Point Wilson Marrowstone Point Tacoma Narrows Bush Point Large resource Strong currents Small resource Weaker currents 700+ MW of tidal resources identified 006, ,SNOPUD.ppt 9