Session of the wmo-ioc joint Technical Commission for Oceanography and Marine Meteorology (jcomm) agreed that it would be logical to transform the wmo wave Programme into the jcomm wind Wave and Storm Surge Programme



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Foreword

The National Meteorological and Hydrological Services of an increasing number of maritime countries have, for many years now, been engaged in the provision of storm surge forecast services in support of the requirements of users in the whole range of maritime and coastal activities (shipping, fisheries, offshore mining, commerce, coastal engineering, construction, recreation, and so on). In recognition of this, the 1st session of the WMO-IOC Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM) agreed that it would be logical to transform the WMO Wave Programme into the JCOMM Wind Wave and Storm Surge Programme. Most important considerations were that storm surge prediction support was included in the new expanded terms of reference of JCOMM and that there were many commonalties between systems providing wind wave and storm surge prediction. The Commission therefore agreed to establish an Expert Team on Wind Waves and Storm Surges (ETWS), building on the success of the former WMO Commission on Marine Meteorology (CMM) Subgroup on Wave Modelling and Forecasting.


The Second Session of the Commission (JCOMM-II, Halifax, Canada, 2005) noted that (1) storm surges, both tropical and extra-tropical, represent a major marine hazard, and result in the loss of life and property in many parts of the world on a regular basis, (2) that accurate and timely forecasts and warnings would contribute substantially to mitigating the threat to life and property from storm surges, (3) that the preparation and issuing of such forecasts and warnings is the responsibility of National Meteorological Services and/or oceanographic agencies in many countries, and (4) that many such services and agencies would benefit substantially from enhanced technical guidance and support in the preparation of forecasts and warnings of storm surges. The Commission recognized the potential value to Members/Member States of a guide to storm surge analysis and forecasting, and urged the Expert Team on Wind Waves and Storm Surges to provide technical advice and guidance in the preparation of such a guide, while also noting that the guide should raise attention to the need to address the vulnerability of coastal areas exposed to storm surges, and to forecasting not only hazards but risks, which result from the combination of a hazard with a vulnerability.

To this end, the Expert Team on Wind Waves and Storm Surges established an ad hoc group of storm surge experts, under the chairmanship of V. Swail (Canada), to undertake the preparation of the Guide.

This international team of experts individually prepared the different chapters of the Guide. These individual contributions were subsequently coordinated, assembled and edited by Drs Horsburgh and de Vries into a draft, which was then submitted to storm surge experts for review and comment. Reviewers' comments were incorporated to the extent possible and a final editing of the first Guide to Storm Surge Forecasting was made by Drs Horsburgh and de Vries.

No publication such as this can ever be perfect, particularly in such a continuously developing field of science and technology, and further additions and modifications will undoubtedly be required in the future. Nevertheless, it is firmly believed that this first ever Guide to storm surge forecasting will continue to prove a very valuable publication in support of the marine services provided by WMO and IOC's maritime Members/Member States. It is also believed that it will continue to meet very well its two-fold objectives: to provide introductory but self-sufficient guidance material for use in the provision of basic storm surge forecast services, while at the same time acting as a source text and a guide to further reading on the subject. In addition, a companion dynamic component of the Guide will be implemented online order to keep material in the Guide updated as appropriate.

Detailed acknowledgements to authors are given with each chapter as appropriate, but I should like here, on behalf of the World Meteorological Organization, to express my sincere appreciation to all the experts (authors, reviewers and particularly Drs Horsburgh and de Vries) who have contributed so much to this important and valuable publication.

(M. Jarraud) Secretary-General




ACKNOWLEDGEMENTS
The development of this Guide to Storm Surge Forecasting, produced under the guidance of the JCOMM Expert Team on Wind Waves and Storm Surges (ETWS), has been very much a team effort, involving a number of experts from several countries in various aspects of storm surge modelling and forecasting, including many who are members of ETWS.
The overall responsibility for the final version of the Guide, including the final synthesis and editing, has been undertaken by Dr Kevin Horsburgh (National Oceanography Centre, United Kingdom) and Dr Hans de Vries (Royal Netherlands Meteorological Institute). Individual chapters were produced from contributions from one or more co-authors listed below:



  1. Kevin Horsburgh (UK)

  2. Hans de Vries (Netherlands)

  3. Paula Etala (Argentina)

  4. Tad Murty (Canada)

  5. Jang-Won Seo (Republic of Korea);

  6. Shishir Dube (India)

  7. the late Igor Lavrenov (Russia)

  8. Martin Holt (UK)

  9. Pierre Daniel (France)

  10. Masakazu Higaki (Japan)

  11. Graham Warren (Australia)

  12. Regina Cabrera (US)

  13. N. Nirupama (Canada)

  14. Denis Paradis (France)

  15. Philippe Dandin (France)

Illustrations have been acknowledged in the captions. Otherwise they have been specially prepared for this edition.


Contact with contributors can be made through the Ocean Affairs Division of the WMO Secretariat. The Secretariat of both WMO and IOC, in particular Boram Lee and Alice Soares, assisted greatly in the guidance and preparation of this publication. Mikhail Entel and Peter Otto (Australia) provided a very thorough peer review of the manuscript prior to its final publication.


Storm Surge Guide
Final Draft

March 2011


ACRONYMS (WMO SECRETARIAT WILL PROVIDE THIS)


1. INTRODUCTION AND GENERAL CONSIDERATIONS

    1. Oceanographical Aspects of Storm Surges

Storm surges are oscillations of the water level in a coastal or inland water body in the period range of a few minutes to a few days, resulting from forcing from atmospheric weather systems. By this definition, the so-called wind waves, which have periods of the order of several seconds, are excluded (Murty, 1984).


The fact that storm surges could have short periods of the order of few minutes is generally well understood and recognized. However, the situations in which high water levels associated with storm surge events could last up to 2 to 3 days is generally not well recognized. Figure (1.1) shows the storm surges at Sagar Island and Pussur River entrance in the Bay of Bengal during a major storm surge event on 13th November 1970.

Figure 1.1: Calculated water level (tide plus surge) at (a) Sagar Island and at (b) the Pussur River entrance in the Bay of Bengal for a Hypothetical storm modeled after the November 1970 storm.


It can be seen from figure (1.1) that the tidal oscillations are superimposed on the elevated water levels due to the storm surge. It may be noted that the contribution from the storm surge is several meters and the surge event lasted 2 to 3 days.
The ocean wave spectrum is shown in Figure (1.2). Tides, storm surges and tsunamis belong to t

he class of long gravity waves (Gonnert et. al, 2001).

Figure 1.2: Frequencies of oceanic wave’s motion in cycles per second (cps). (Platzman, 1971).


Storm surges are centred at about 10-4 cycles per second (cps or Hz), which gives a period of about 3 h. However, depending mainly on the topography of the water body and secondarily on other parameters, such as the direction of movement of the storm, strength of the storm, stratification of the water body, presence or absence of ice cover, nature of tidal motion in the water body, etc., the periods in the water level oscillations may vary considerably. Even in the same water body, storm surge records at different locations can exhibit different periods.
Storm surges occur due to meteorological forcing fields from synoptic (large) scale weather systems (cyclones) and also from meso (medium) scale systems (squall lines). Tides arise due to the gravitational attraction of the moon and the sun on the ocean waters. Tsunamis are generated mainly from under ocean earthquakes (Murty, 1977), but other sources of tsunami generation include volcanic island eruptions, submarine land slides, nuclear or large chemical explosions in the oceans and asteroid strikes on the ocean surface.
The main characteristic of a long gravity wave is that its wavelength is much greater than the depth of the water over which it is traveling. For all practical purposes, one can use the following simple formula for the speed of a long gravity wave.
(1.1)
c = Speed of a long gravity wave

g = Acceleration due to gravity = 9.8 m/sec2



D = Water depth
However, it should be noted that there are some slight corrections to the above equation if we take dispersion into account.
Although storm surges belong to the same class known as long waves, as do astronomical tides and tsunamis, there are at least two important differences. First, whereas tides and tsunamis occur on the oceanic scale, storm surges are predominantly a coastal phenomenon. Second, significant tsunamis and tides cannot occur in a completely closed small coastal or inland water body, but storm surges can occur even in completely enclosed lakes, or in canals and rivers.
Figure (1.3) shows how a storm surge is built up in the Bay of Bengal during the highly destructive event of November 1970.

Figure 1.3: Storm surge heights (m) in the northern part of the Bay of Bengal from a hypothetical storm modeled after the November 1970 storm.


As can be seen, in the deeper part of the bay, the amplitude of the storm surge is zero. Over the deep water, the storm surge, which is a long gravity wave, propagates over the water much faster than the speed with which the weather system travels in the atmosphere. However, in the gradually shallowing waters, as one approaches the head of the bay, the gravity wave slows down and its speed gradually matches the speed of movement of the weather system. When both these speeds match, resonance coupling takes place and energy is transferred from the weather system to the ocean surface, leading to the development of the storm surge.
In the water bodies in higher latitudes, an ice cover over the water body can have a substantial influence on the storm surge. Studies in Canadian water bodies showed that an ice cover can clip the crest of the storm surge, but leave the trough unaffected as shown in Figure (1.4)

Figure 1.4: Observed storm surge at Pointe-du-Chêne, Canada.


A more detailed analysis of the influence of ice cover on storm surges will be provided in later sections.


    1. Meteorological aspects of Storm Surges

Storm surge is an air-sea interaction problem, i.e. the atmosphere forces the water body, which responds by generating oscillations in the water body with periods ranging from a few minutes to a few days.


When a weather system is moving over a water body, there are essentially two forcing fields: The first is the atmospheric pressure gradient normal to the ocean surface. For every one hectopascal (h Pa) drop of pressure at the centre of the weather system, the sea level moves up temporarily by about one centimetre. This phenomenon is referred to as the “inverse barometer effect” and is also called “static amplification” or the static part of the storm surge. Usually the static part contributes 10 to 15 percent to the storm surge.
The dominant part of the storm surge is caused by the tangential wind stress (associated with the wind field of the weather system) acting over the ocean surface, which pushes the water towards the coast, thereby causing a pileup of water at the coast that becomes a storm surge.
The dynamic amplification AD can be related to the static amplification AS (described above) through the following simple relationship (Proudman, 1953)
AD = AS . (1.2)
where vw is the speed of movement of the weather system and c is the speed of propagation of the storm surge, as defined in Equation (1.1)
Over deep water, c >> vw, hence AD is only slightly greater than AS. On the other hand, in shallow water, where c is close to vw in value, the denominator in equation (1.2) becomes small, and AD becomes much greater than AS. Theoretically, at least, when c = vw, AD becomes infinite, but in practice, due to friction and some other parameters, AD has an upper limiting value.
It can be shown that
S = (1.3)
where S is the storm surge amplitude, w is the wind speed, D is the water depth and K is a constant, encompassing several other factors, such as bottom stress, stratification of the water body, atmospheric stability, nature of the ocean surface (rough versus smooth), angle at which the wind is blowing, etc.
However, the most important parameters in determining the storm surge amplitude are the wind speed and the water depth. The surge amplitude is directly proportional to the square of the wind speed (see Pugh, 1987). Hence, if the wind speed doubles, the surge height increases four fold.
The surge amplitude is inversely proportional to the water depth. Thus, the shallower the water, the greater is the surge amplitude. This is because, as one enters shallow waters, approximately the same energy is compressed into a shorter vertical column of water.
The following factors can also enhance the storm surge amplitude: interaction with tides, interaction with wind waves, interaction with river flow, and effects of precipitation on surges in rivers, lakes and estuaries.


    1. Methods of Storm Surge Prediction

Before the computer era, the techniques used for storm surge prediction were analytical, empirical, graphical (monograms) and statistical (regression relations). Even some electric analog methods were used. However, with the advent of computers, numerical models gradually took over and now, almost exclusively, only numerical methods are used.


However, for the sake of simplicity, simple analytical and graphical methods are still used occasionally. For site-specific purposes, empirical and statistical methods are also used.
Until recently, the bulk of the numerical models for storm surge prediction consisted of vertically integrated two-dimensional models, with three independent variables, namely, the two horizontal coordinates (east-west and north-south) and time. The dependent variables usually are the surge amplitude and the x and y components of the vertically averaged horizontal current.


    1. Empirical Methods for Surge Prediction

Empirical methods are those techniques that are derived from simple analytical theories and experience and those that are usable more or less directly for practical situations.


Silvester (1971) considered an enclosed body of water such as a rectangular lake. Let d be the (uniform) depth of the lake, U10 the wind speed 10m above the water surface, L the length of the lake (or wind fetch), and S the surge amplitude at the downwind end. Then
(1.4)
where K10 = 3.3 x 10-6 (wind drag coefficient). Note that this equation is dimensionless. For nonrectangular shapes, S / d must be multiplied by a form factor N.
For storm surges on the continental shelf, Silvester (1971) assumed a uniform variation from a depth d1 at the shelf edge to d2 near the coast. Let L be the width of the shelf and F the fetch length. The depth ratio d2 / d1 can be expressed in terms of L / x where x is the distance inland where the plane of bed meets the mean water level. For extra-tropical cyclones, F > L generally; however, we must take F = L, since only the portion of the fetch over the shallow zone is effective in producing the surge. On the other hand, for tropical cyclones, usually, F < L. Let V be the speed of movement of the wind field; if V = 0, the wind field is referred to as a static wind field.
For the static wind field case, the surge S is given by Bretschneider (1966) as
(1.5)
Reid (1956) expressed the surge amplitude as
(1.6)
When a wind field is moving across the shelf towards the shore, the forward part of the surge wave system is being reflected as the later waves are still approaching the shore. Reid (1956) considered the interaction between these two wave systems and included graphs for the ratio, R, of the maximum surge, Smax, to that of static storm situations. Silvester (1971) gave the following formula for the surge, Sa, due to reduction in the atmospheric pressure:
(1.7)
where Sa is the surge amplitude in feet and Pc is the pressure at the storm centre in millibars. This relation is also known as the inverse barometer effect and, as usually expressed, indicates that a decrease of 1 mb in the atmospheric pressure gives rise to an increase of 1 cm in the water level.
Rao and Mazumdar (1966) expressed the storm surge S as
S = B + P + X + F (1.8)
where B is the static rise due to atmospheric pressure deficiency towards the centre of the storm, P is the rise due to piling up of the water against the coast by offshore winds, X is the height of crests of individual waves (wind-generated waves) superimposed on the general rise of the water level, and F is the effect of forerunners.
Of these, P and X are the most important. These authors combined the effects of B, P, and X, and gave the formula for the surge:
10-3 (1.9)
where W is the sustained speed of the onshore component of the wind, is the horizontal length of a section, d is the depth of the water column, and is the pressure deficiency (in millibars) at the point under consideration.
Bretschneider (1966) provided a convenient classification of water bodies for developing empirical methods for storm surge prediction that is shown below as table (1.1).
Table 1.1: A convenient classification of water bodies for developing empirical relations for engineering design problems. (Bretschneider 1966)


A.

Enclosed lakes and reservoirs




1. Rectangular channel, constant depth




2. Regular in shape




3. Somewhat irregular in shape




4. Very irregular in shape







B.

On coast or continental shelf




1. Bottom of constant depth




2. Bottom of constant slope




3. Slightly irregular bottom profile




4. Irregular bottom profile







C.

Coastline




1. Smooth coastline




2. Coastline somewhat irregular




3. Jagged coastline







D.

Behind coastline




1. Low natural barriers




2. Medium-high natural barriers




3. High natural barriers







E.

Open bays and estuaries




  1. Entrance backed by long estuaries and with tidal flow moving freely past entrance




  1. Entrance backed by short estuary and with tidal flow moving freely past entrance




  1. Entrance obstructed sufficiently to prevent free movement of tidal flow past entrance




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