|
<br />A recent VQlu~~ of the loc~l nultiplc listing service catalog
<br />was obtained to place a valuation on the structures within
<br />the floodplain, The structures in the floodplain were inspected
<br />in the field and any residences for sale thero, or in neigh-
<br />boring blocks, were identified. The listed price less 25
<br />percent for lot value was used as the structural value of the
<br />building, This step produced quite reliable results due to
<br />the uniformity of construction in each inundated subdivision
<br />and the fact that most reaches h~d one or more flood prone
<br />houses for sale.
<br />
<br />The local State Farm Insurance office was contacted as a source
<br />for content value, Their standard procedure is to estimate
<br />contents at 50 percent of structure value, An itemized content
<br />estimate in an imaginary 'typical' household was also
<br />prepared. Substantial agreement was found between the two
<br />estimates; and 50 percent of structural value, 37,5 percent
<br />of listed price, was used as total content value,
<br />
<br />previously were applied to determine the structural and
<br />content damage, Throughout the damage estimate, velocity
<br />effects were ignored as average velocities were typically
<br />less than four feet per second. Velocity effects have proven
<br />difficult to quantify and present recommendations are to add
<br />velocity heads to water surface elevation in estimating
<br />damages, At four feet per second the velocity head is 0.25
<br />feet and considered negligible, The depth damage relationships
<br />are based on depths relative to first floor elevation, Where
<br />topographic mapping proved inadequate to determine first floor
<br />elevations, these elevations were measured by fiold survey,
<br />
<br />VI.E.
<br />
<br />The reSUlting discharge-damage curves by reach are reproduced
<br />in the technical addendum, These curves were also used in
<br />the calculation of residual damages of the proposed construction
<br />alternutives, A discrete damage estimate gives a less smooth
<br />discharge-damage curve than a unit area damage estimate would.
<br />However, it was felt the results would be more reliable for
<br />a sparsely populated floodplain as is churacteristic of the
<br />:najority of the west Vine Basin,
<br />
<br />Bascline Damages
<br />
<br />Literuturc sugg~sts dpv~loping uni~ ~re3 stdgP namage curves
<br />by land use!8)& (9) Due to th", relatively small number of
<br />struGtures in Lhe flOOdplain, it ~~s m~t~~lly ~grccd upc~ with
<br />the Tcchnical committee to count the actual number and depth
<br />of structures inundated during any given storm flow event
<br />using the 100 scale orthophoto plan sheets. When the sampl~
<br />population is low, the statistical averaging implicit in
<br />determininq dumages by square foot of land use could prejudice
<br />the results, In order to eliminate this effect, discharge-damage
<br />
<br />VI.F.
<br />
<br />Results
<br />
<br />The numeric results of the baseline damage estimate are presented
<br /><,' 17 II
<br />on ,,'l.gure ,Annual Damage by Reach", Figure 18, "Basin-Wide
<br />Damage - Exceedance Probability Curve" and Table 6, "Frequency-
<br />Damage by Reach", The remainder of this text summarizes some
<br />of these results,
<br />
<br />curves were developcd for each reach,
<br />
<br />~~ expectcd, rri~h Greer. S~bdivisicn proved ~o be tho rc~ch
<br />most susceptible to flood damages. Irish Green accounts for
<br />roughly one third of all annual damages in the basin and
<br />receives substantial low flow damage. Another result that waS
<br />anticipated early in the study is the shape of the Basin-Wide
<br />Damage - Exceedance Probability Curve, 'i'he near total lack
<br />of a natural channel and the rcs~lting dcvclopmen~ on the thalweq
<br />account for significant damages being associated with minor
<br />storm events, On the other hund, a light overall
<br />
<br />FlOOdplains with one foot depth regimes were plotted for a
<br />variety of discharges, The floodplains plotted were generat..,d
<br />from the 2,5,25 and 100 year design stonns. In plotting these
<br />floodplains, backwater surfaces behind Taft Hill and at Hollywood
<br />were compared to the detention models of the S,W,M.M. output
<br />and <ldjusted accordingly. The number of houses in each flow
<br />regime was then determined and the appropriate factors disc~ssed
<br />
<br />-57-
<br />
<br />-%-
<br />
|