Table G10. Mean Maximum Vertical Erosion Frequency due to Tropical and Extratropical Storms Impacting 0.4-mm Sand-Capped Mounds

events occurring during a single storm season. For example, although an average

of only 2.4 events occur per year at the Mud Dump site, results from the program

EST generates a simulated 200-year sequence of extratropical storm activity

during which it is possible to have eight or nine events in a single season. If

eight or nine severe events were to occur during a single winter season, the sum-

mation of maximum erosion magnitudes for each event may be large enough for

that season to be ranked as a 100-year season.

The erosion versus frequency-of-occurrence relationships for tropical and

extratropical events were combined to generate a single curve and table of fre-

quencies for each of the design configurations. The combined frequency-of-

occurrence is computed by adding the frequencies associated with tropical and

extratropical events for a given magnitude of erosion. For example, consider the

8-ft mound located in 83 ft of water. An erosion of 1.0 ft corresponds to a return

period of 83 years for hurricanes but only 10 years for extratropical events. The

combined frequency is equal to 1/83 + 1/10 or 0.11, corresponding to a return

period of just 9 years. A comparison of the combined event, Table G10, shown

below, and Tables G8 and G9, shows that extratropical events are the dominant

storm type in the New York Bight. This dominance is evidenced by the fact that

the combined event frequency relationships are very similar to the extratropical

relationships. This is not surprising considering that on the average, 15 extra-

tropical storms occur for every hurricane. Also, vertical erosion due to extra-

tropical events is generally more severe than for tropical events due to the longer

duration of extratropical storms.

Table G10

Mean Maximum Vertical Erosion Frequency due to Tropical and

Extratropical Storms Impacting 0.4-mm Sand-Capped Mounds

Mound Con-

figuration Base

Combined Hurricane/Northeaster Single-Year Erosion Frequency, ft

Depth/Mound

Height/Crest

Depth, ft

10 year

25 year

50 year

100 year

63/10/50

2.4

3.0

3.4

3.9

63/08/55

1.6

2.0

2.3

2.6

73/13/60

1.5

1.8

2.0

2.3

73/08/65

1.0

1.3

1.5

1.7

83/13/70

0.9

1.2

1.3

1.6

83/08/75

0.7

0.8

0.9

1.1

A summary of results for the Mud Dump site, shown in Table G10, was

prepared to provide both episodic and cumulative erosion estimates for each

design option. The episodic values are provided at return periods of 10, 25, 50,

and 100 years.

G30

Appendix G Procedures for Conducting Frequency-of-Erosion Studies