UNDERSTANDING COASTAL GEOLOGIC HAZARDS ON A MICROTIDAL, MIXED WAVE/TIDE DOMINATED SHORE: RHODE ISLAND

When contemplating Rhode Island coastal geologic hazards, one must consider: 1) hurricanes (tropical cyclones), 2) extratropical cyclones (“Nor'easters”), and 3) sea-level rise. The hazards give rise to these geologic processes: 1) frontal erosion from breaking waves and swash run up, 2) storm-surge overwash, and 3) an elevated level of mean-higher high water into the future. Scale of these processes is: 1) breaking waves: 1 to 3+ meters at the shoreline, 2) storm-surge overwash: 0.5 to 4 m water depth across the shore zone, and 3) sea-level rise: 3 mm per year at present.

The south shore of Rhode Island is a microtidal (1.05 m mean, 1.17 m spring range) mixed wave/tide dominated shore; Narragansett Bay also is microtidal (1.05 m mean Newport, 1.34 m Providence; spring range- 1.17 m, 1.47 m respectively). Geologic shore zone types and percentages are:

1) Beach plain/barrier spit – 25%,

2) Glacial stratified material bluff – 8%

3) Till bluff - 23%

4) Meta-sedimentary bedrock - 8%

5) Igneous/other Meta bedrock – 5

6) Discontinuous bedrock – 1%

7) Shore protection structure – 30%

The entire system is storm driven, thus size and intensity, forward speed, path, tidal phase, and time between storms (Hayes and Boothroyd, 1969) control the resulting changes. Storm surges range from 2.9 m above MHHW (1938 category 3 hurricane) to 0.9 m (Patriots Day 2007 extratropical). Sustained southeast winds may cause extratropical surges to extend over 5-8 tidal cycles. Barrier spits and bluffs of stratified material have retreated up to a net 75 m between 1939 and 2006, although non-storm periods have allowed intermediate term temporary recovery.

Relative sea-level rise of 25.8 cm per 100 yr has resulted in a 22 cm rise since 1930 and 17 cm since 1938. Continuing frontal erosion combined with a possible accelerated sea-level rise of 1-1.5 m by 2100, and perhaps by 2050, will allow storm surges from to penetrate further inland and result in deeper water depths than present obsolete maps suggest. However, sediment budget considerations suggest that the barriers spits will continue to migrate landward with sea-level rise and not disintegrate to shoreface shoals, although they will narrow and become increasingly washover fan dominated.