Probable maximum storms

The storm generating the probable maximum storm surge, depending on the location of the site and the characteristics of the region, can be the probable maximum tropical cyclone or the probable maximum extra-tropical storm. For each site the generating storm for the probable maximum storm surge 

The probable maximum tropical cyclone is the hypothetical tropical cyclone. It is characterized as a rapidly revolving storm having that combination of characteristics that will make it the most severe, in terms of flooding, that can reasonably be expected to occur in the region, and which approaches the point under study along the critical path and at a rate of movement that will result in the most adverse flooding. 

The analysis consists in selecting those appropriate storm parameters and other relevant parameters (e.g. maximum wind velocity, atmospheric pressure differential, bottom friction and wind stress coefficients) to be used as inputs to a one or two dimensional storm surge model which maximizes the flooding potential. All parameters should be conservatively evaluated and adequately substantiated. 

The storm surge analysis gives the following as outputs  

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Sections 5 to 12 deal with the derivation of the probable maximum flood, probable maximum seiche from runoff, probable maximum storm surge and probable maximum tsunami at the site, after simulation of the effects on the site and the presentation of the possible combinations. 

The flood resulting from the probable maximum storm surge (see Ref. [4] for guidance on the associated meteorological hazard) ... 

It should be borne in mind that, in spite of the accepted terminology for the probable maximum storm surge, probable maximum tsunami, probable maximum seiche and probable maximum dam break, such events cannot always be characterized in a purely probabilistic framework. However, the terminology emphasizes that an estimate should always be made of the probability of exceedance associated with the design basis scenarios, even when they are investigated by means of deterministic approaches. 

A preliminary estimate of the height of the probable maximum storm surge should then be made by using the values of parameters representing the characteristics listed as inputs to empirical relationships. Whenever possible, these results should be compared with historical records of storm surges to check the suitability of the method used. A method that results in a calculated extreme event that is lower in magnitude than any event that was recorded is unacceptable. 

The reference water level in an enclosed body of water which is not subject to human control should be taken as the mean value of all data on the water level for a certain time period. Surge and seiche effects cause changes in the transient water level only and do not significantly change the mean water level. The reference water level upon which the computed probable maximum storm surge or probable maximum seiche is superimposed should be seleaed so that the probability of its being exceeded over the lifetime of the plant is sufficiently low. ... 

A site where the drainage area of the river is in a tropical cyclone area such that the probable maximum tropical cyclone not only causes a surge but can also cause a flood on the river. This offers the possibility of a coincidence of both the design basis flood due to precipitation and the probable maximum storm surge on the coast. 

For sites on rivers or estuaries that flow into large bodies of water, the computed probable maximum storm surge will require empirical or mathematical routing upstream of the point of interest. Sites on large enclosed bodies of water should be analysed for surges by the use of one or two dimensional surge models. 

DETERMINISTIC EVALUATION OF PROBABLE MAXIMUM STORM SURGE... 

For enclosed bodies of water the probable maximum storm surge should be derived by using validated one or two dimensional mathematical models. The critical portion of the wind field, after being adjusted for any overland effects, is used as input for this analysis. The selection of coefficients and boundary conditions should be based on conservative assumptions. When one dimensional models are used, the transverse or crosswind set-up or a transverse seiche component is calculated separately and added to the longitudinal wind set-up. If the water body is sensitive to resonance, the transient responses should also be considered separately in a one dimensional model. If the water body is considered to be relatively insensitive to resonance, an analysis should be performed to substantiate this. The two dimensional transient mathematical models automatically take into account the transverse components and resonance effects. Components of the probable maximum still water levels are the longitudinal wind set-up, the transverse or cross-wind set-up and the reference water level. 

K the potential for seiches derives from the action of the probable maximum storm surge on a body of water, this surge is the input for evaluating the probable maximum seiche. 

If the potential for seiches is associated with the action of the wind or a pressure field on a body of water, a probable maximum storm whose parameters are maximized for the production of seiches should be established... 

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