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Tides & Water Levels

Tides & Water Levels. Mazen Abualtayef Associate Prof., IUG, Palestine. Introduction.

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Tides & Water Levels

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  1. Tides & Water Levels Mazen AbualtayefAssociate Prof., IUG, Palestine

  2. Introduction Although coastal design is normally considered to be a function of wave conditions, it is primarily a function of water levels. It is water level that control both flooding and wave exposure. Imagine a simple structure close to shore that is subject to waves. When the water level rises, the structure will be exposed to larger waves because the water depth determines where waves break and loose most of their energy. High water levels cause: • retreat of sandy shores, • allow larger waves to come closer into shore, • waves will erode the dunes and upper beach and deposit the sandoffshore.

  3. 6.1 Introduction There are several types of water level fluctuations and they can be classified according to their return period as: 1. Short term • Tides • Storm surge and barometric surge 2. Seasonal 3. Long term - Climatic fluctuations - Eustatic (Sea) level rise - Isostatic (Land) emergence and subsidence - Climate change

  4. 6.2 Tides Astronomic tides are often the defining water motion in coastal areas. They cause the water levels to rise and fall and cause large-scale currents patterns, sometimes with large velocities. Tides directly affect coastal morphology, navigation, fisheries, habitat and recreational activity. The tides are the result of a combination of forces acting on individual water particles. These are: - gravitational attraction of the earth, - centrifugal force generated by the rotation of the earth - moon combination, - gravitational attraction of the moon, - gravitational attraction of the sun.

  5. What is the Tidal Period? Time between successive High and Low Tides (~12 hrs) What is the Tidal Day? One complete revolution of Earth beneath tidal bulges What is the Tidal Range? high tide mark (2.0 m) - low tide mark (-1.5 m) = 3.5 m Tidal Range

  6. Variations in Height/ Time ~ involve MOON and Sun Both create tidal bulge via tidal forces Moon = M2 tide Sun = S2 tide M2 S2

  7. Newton’s Law of Gravitation

  8. Types of Tides • Diurnal • only one high and one low tide/day • Semi-diurnal • ~ 2 equivalent High Tides, 2 low tides/ day • Mixed Semi-diurnal • Unequal pattern of 2 high and low tides

  9. Locations of the Occurring 3 Tide Types Worldwide

  10. Spring and Neap Tides Spring Tide: when the tidal Range reaches a Maximum NeapTide: when the Tidal Range reaches a Minimum

  11. Common Tidal Elevations

  12. 6.2.5 Tide Analysis and Prediction Tide Analysis consists of separating a measured ride into as many of its constituents as can be identified from the length of record available. The tide is assumed to be represented by the harmonic summation where hT(t) is the tidal water level at time t. ai and ai, are the amplitudes and phase angles of the tidal constituents. wi are their angular frequencies. For example, the semi-diurnal lunar constituent, usually identified as M2, has a period of 12.42 hours and therefore wM2 = 2π/(3600x12.42) =1.405xI0-4 sec-1

  13. 6.2.6 Tidal Currents The velocity of currents: C = √(gd) The length of wave: Ｌ= CT Example 1: in deep ocean For d = 4km  C = 200m/s  L = 9000km for M2 (T = 12.42 hrs) Example 2: in shallow water For d = 10m  C = 10m/s  L = 450km For d = 3m  C = 5.5m/s  L = 245km

  14. 6.3 Storm Surge The water level fluctuation of greatest concern in design is storm surge, which is an increase in water level resulting from shear stress by onshore wind over the water surface (Fig. 6.14). Parts of Bangladesh are flooded regularly by storm surge generated by passing cyclones, resulting in the loss of thousands of lives. The shorelines along the southern borders of the North Sea were flooded in 1953 because storm surge caused dike breaches. Property damage was very extensive and 1835 lives were lost in the Netherlands.

  15. 6.3 Storm Surge Computations of storm surge are carried out using the equations of motion and continuity that are used for tidal computations. In this case wind-generated shear stress is the main driving force. For simple problems, the equations can be reduced to a one-dimensional computation S: storm surge x: distance over which S is calculated U: wind speed, f: is the angle between the wind direction and the x-axis D: new water depth (= d+S)

  16. Example 6.1 One-Dimensional Surge Calculation The following table presents S for a 10 km long offshore profile, divided into 6 sections for which the depth is assumed to be constant. For U = 20 m/sec and f=0, the storm surge at the shore is shown to be0.29 m. Dx: section length part of 10km d: given water depth D = d+S DS: calculated from eq. 6.4 S: cumulative storm from deep sea to shore

  17. 6.4 Barometric Surge Suppose there is a difference in barometric pressure Dp. Water level rise will be generated where r is the density of water. Equation 6.5 results in a water level rise of about 0.1m for each kPa of pressure difference. A major depression can easily generate a pressure difference of 5 kPa, resulting in a potential barometric surge of 0.5m.

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