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|Title:||Frequency of coastal flooding at Roughans Point, Broad Sound, Lynn Harbor, and the Saugus-Pines River system|
|Authors:||United States. Army. Corps of Engineers. New England Division.|
Hardy, Thomas A.
Crawford, Peter L.
Roughans Point, Massachusetts
|Publisher:||Coastal Engineering Research Center (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical report (U.S. Army Engineer Waterways Experiment Station) ; CERC-86-8.|
Abstract: This report describes the establishment of frequency curves for water levels caused by the combination of tide, storm surge, and waves for a coastal area just north of Boston. The project procedure involves the conjunctive use of five modeling components, including numerical storm surge, numerical wave propagation, physical wave overtopping, flood routing, and probability models. At Roughans Point where flooding is caused by the overtopping of seawalls by storm waves, all five models were necessary. Multiple combinations of possible seawall-revetment structures were modeled. Major differences among the combinations were evident at the lower return periods with the combinations of a wide berm revetment and a cap on the existing seawall for the east wall of Roughans Point providing the greatest protection. At higher return periods the protection differential offered by the various structure combinations tended to diminish. For still-water levels and wave conditions of a Standard Project Northeaster all structure combinations tested would be ineffective at protecting the interior of Roughans Point. Tests were conducted to determine a structure height for the north wall. These tests indicated that significant overtopping did not begin until the north wall structure was lowered below 13 ft National Geodetic Vertical Datum (NGVD). Since the existing height of the north wall is above this level at several sections, it is recommended that the revetment height be set at 13 ft NGVD with the wall height set so that there is a transition between the existing wall heights. For areas where stage-frequency curves are presented for the still-water level resulting from the combination of storm surge and astronomical tide, only the storm surge and probability models were necessary. These areas include both open coast and estuarine locations. For areas flooded by the still-water level, results of the modeling indicated that the whole study area floods to approximately the same level. Flood levels are efficiently conveyed through the inlet and throughout the flood plain of the Saugus-Pines River system. Inside the inlet, there is a small gradient in the still-water level, rising from north to south, which results from local setup caused by north to northeast wind directions which predominate during storm conditions. This local wind setup results in flood levels inside the inlet which differ by one-half to three-fourths of a foot during the more severe storm events. Outside the river system in Broad Sound a smaller north-south gradient exists with differences of only a few tenths of a foot resulting. Data collected by the US Army Engineer Division, New England, after completion of the modeling indicated that losses do occur as flood levels propagate upstream of the Fox Hill Drawbridge on the Saugus River and upstream of the Highway embankment on the Pines River. Stage-frequency curves for these areas were adjusted to accommodate these additional data. The curves were lowered 0.3 and 0.5 ft at the lower return periods for upstream Saugus River and Pines River locations, respectively. Reductions were reduced for higher return periods because higher flood levels would provide greater access of floodwaters to these areas. The setup and operation of all models, except the physical model, are described. The method of constructing stage-frequency curves is explained, and estimates of the error involved in each of the processes are discussed. The final products are curves which relate flood stage to frequency of occurrence for several possible structures at Roughans Point as well as for several coastal and river areas.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Technical Report|
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|TR-CERC-86-8.pdf||18 MB||Adobe PDF|