36, D£SIGH I-IURRIOAIIE ELOCD LEVELS 



The computation of storm-tide potentials, including a description 

 of the method used, is contained in a report dated March 1956, "Dynamic 

 Storm-Tide Potentials," prepared by the Department of Oceanograpl'^r of 

 the Texas A <Sc M Research Foundation in connection vjith research for 

 the Beach Erosion Board, In accordance with procedures outlined in 

 this report, the 19hk design hurricane stonn-tide potential off Newport 

 liarbor at the entrance to Narragansett Bay was calculated to be 12,8 

 feet above mean sea level or approximately tvxo feet above the level 

 of the 1938 hurricane. By a combination of analytical methods and 

 model tests, the level was determined to be 18,7 feet above mean sea 

 level at Providence or about three feet above the actual 1938 flood 

 elevation. The full development of the design hurricane is given in 

 Appendix B, 



39,' DESIGN VJAVES A^ID VIAVE RUN-UP 



Wave heights with the design wind velocity of 76 miles per hour 

 are estimated to be 2$ feet from crest to tro\igh at the entrance to 

 the bay and from 6 feet to 9 feet within the bay depending on the 

 fetch and depth of water (see Table 3)« These are "significant" 

 wave heights and would be exceeded by individual waves by as much 

 as 60 percent. 



The amount of wave run-up (the height above still-water level 

 reached by the rush of water up a struct-ure on the breald.ng of a 

 wave) on the seaward side of protective works wo\ald exceed the wave 

 heights shown in Table 3» 



liO, STEEAII TlOtJ COINCIDENT WITH IIURRIGA1>IE TIDAL FLOODING 



Heavy rainfall is normally associated with great hurricanes. 

 Warm, moist\ire-laden air is carried in over the land from the ocean 

 by northeasterly winds in advance of a hurricpjie causing heavy preci- 

 pitation before and at the time the hurricane strikes. Consequently, 

 Idgh river runoff and severe flooding are likely to occur at the same 

 time as a hurricane tidal flood. An example of this situation was 

 provided by the hurricane flood of September 1938 when many of the 

 rivers in New England overflowed their banks and caused extreme 

 flood losses one day in advance of the hurricane, while on other 

 rivers, such as the Thamrs River above New London, Connecticut, 

 fresh-xrater flooding occurred two hours before salt-water flooding. 

 See Appendix C for a detailed discussion of stream flow. 



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