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I ~ \ ( ~ ( \ I , I ( ~ \ ,. i I ': " ( \ ( ( ,. , '. i " , I I ( , , , \ ( , I. i ( I r f ( '. i , \ ( t I C ( i ( \ I ( f i i .. ( r I ( f ! FHF less than 8.0 there can be considerable difference in damage depending upon the FHF and type of structure. In this range the FHF is a sensitive parameter. Figures A-5 through A-8 also show a significant variation in damage relative to the event at the first floor of the structure. This can be illustrated better by taking a cross-section through each family of curves at a given FHF. Figures A-9, A-10 and A-11 show this for FHF's of 2.0, 4.0 and 8.0. Thus, for a given flood hazard factor the curves show that there is a point (location in the flood plain beyond which expected annual damage is relatively unchanged. For an FHF = 2.0 this point is about the 15 year event, for an FHF = 4.0 it's about the 20 year, and for an FHF = 8.0 it's more nearly the 25 year event. To illustrate that this progression does not continue beyond the 25 year event, Figure A-12 shows a plot for a FHF = 20.0. When events less than 15-25 years are located at the first floor, expected annual damage increases significantly, especially below 10 years. This is true for all flood hazard factors and all type structures. Figur~s A-9, A-10 and A-11 also show the difference in expected annual damage between different type structures, At the 25 year event (the relatively stable range) the difference between damage for a one story structure with basement and the same structure without were 7.0 and 1.2 percent for a FHF = 2.0; 3.1 and 2.0 for a FHf = 8.0; and 2.9 and 2.6 with a FHF = 20.0. Thus, structure type makes a large difference in expected annual damage with FHF less than 8.0, but considerably less difference when the FHF is greater than 8.0. Before leaving this discussion on FHF and event at the first floor an explanation of the underlying interrelationship between the two may help to explain some trends in the data. Figure A-5 shows for a structure without basement that expected annuai damage increases with increasing FHF for all events at the first floor. However, Figure A-7 shows, for a structure with basement, that expected annual damage increases with increasing FHF for the 2 year and 5 year event, but decreases with the 10 year and 25 year event. This difference is also evident in Figures A-39 and A-41 which represent total damage. Figure A-39 shows increaSing damage with increasing flood hazard factor for all events at the first floor. Figure A-41 (one story with basement) shows the same trend for events less than about the 7 year, but increasing damage with decreasing flood hazard factors for events greater than 7 year. Why the difference? The answer lies in an understanding of how the three principal variables which determine expected annual damage interrelate. Since expected annual damage is computed by weighting each damage value by its probability of occurrence, and since more frequent events weight more than less frequent ones, the different distributions of damage and frequencies and their relative relationship cause different trends in the data. Figure A-H illustrates how for a given event at the first floor an increase in FHF results in an increase in the frequency of the event at every elevation above the first .floor. An increase in FHF results in a decrease in the frequency of event at every elevation below the first floor. These statements can be illustrated by referring to Figure A-H. It shows frequency curves for flood hazard factors of 4.0 and 8.0 located with their 25 year event at the first floor (-2.4 feet). With the 25 year event fixed relative to' the first floor and because a frequency curve with a FHF = 8.0 has a greater slope than a curve with a FHF = 4.0, the exceedance interval for the larger FHF must be less above the first floor and greater below. As the event at the first floor changes, the frequency of events above and below the first floor changes. If instead of the 25 year event at the first floor it was the 2 year then there would be more frequent events occurring above the first floor. This is illustrated in Figure A-H. The reverse is also true: As the event at the first floor becomes less frequent the events above the first floor become less frequent. A-8