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Methodology for Risk-Based Flood Damage Assessment

Methodology for Risk-Based Flood Damage Assessment. David R. Maidment CE 394K.2, Spring 1999. Annual maximum flood has exceedance probability, p, 0 < p < 1, which measures flood severity Return period, T, of an event is given by. Expected Annual Damage, D e.

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Methodology for Risk-Based Flood Damage Assessment

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  1. Methodology for Risk-Based Flood Damage Assessment David R. Maidment CE 394K.2, Spring 1999

  2. Annual maximum flood has exceedance probability, p, 0< p <1, which measures flood severity Return period, T, of an event is given by Expected Annual Damage, De • Expected annual damage, De,is expected value of the damage which occurs considering floods of all possible severity, p:

  3. The flood frequency curve, Q(p), is described by the Log Pearson Type III distribution which has parameters, m, s, Cs(m, s are uncertain) The flow frequency curve can be generated by flood frequency analysis or by rainfall-runoff modeling There is uncertainty in Q|p which can be specified by f1(Q|p) Q f1(Q|p) p Annual Flood Discharge, Q Realization • This uncertainty can also be expressed by alternative “realizations” of the flow frequency curve which could exist based on uncertainty in m,s of Q

  4. Annual flood stage, H, is the highest water surface elevation occurring during the annual maximum flood Flood stage is estimated with flow program, HEC-RAS rating curve at gage site Stage height has uncertainty, expressed by probability density, f2(H|Q) Selection of a probability level on f2, yields an alternative realization of the rating curve H f2(H|Q) H datum Q Annual Flood Stage, H

  5. Annual Damage, D, is the flood damage which occurs during the annual maximum flood event as a function of flood stage, H Errors occur in structure values and elevations Uncertainty in the damage is described by the probability density, f3(D|H) Selection of a probability level on f3, yields an alternative realization of the damage-stage curve D f3(D|H) H Annual Damage

  6. Expected annual damage H H* Q* Q f2(H|Q) f1(Q|p) dp Q p p* Q* D f3(D|H) H Computation of Expected Annual Damage D* H* • For interval,dp, find damage D(H*(Q*(p*))) • Integrate over p with (1) • Repeat this with sufficient realizations to get stable statistics for mean De and 25% and 75% values (1) • Generate a realization • 2 random numbers for m,s for f1(Q|p) • 1 number for “shift” in f2(H|Q) • 1 number for “shift” in f3(D|H)

  7. Total annual damage, DT, for a project is found by summing the expected annual damages, in each reach, Di, i=1,2,…,I i=1 i=3 i=2 i=4 i=5 Total Expected Annual Damage for a Project

  8. Summing over reaches, then integrating over the flood realizations, the total annual damage is given by: Integrating over flood realizations, then summing over the reaches, the total annual damage is given by: Summing over Floods or Reaches Corps Method Should this method be used? Currently, the D25, D50 and D75 statistics for a plan are obtained by summing these statistics for each reach

  9. H H* f2(H|Q) Q Q* Improvement using GIS Q Q* f1(Q|p) p dp p* • GIS-based calculation of flood likelihood and damage at each location • Risk-based flood plain boundary zone

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