Risk Analysis and Uncertainty in Flood Damage Reduction Studies (2000) / Chapter Skim
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Case Studies
Case Studies
Pages 71-113
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From page 71... ...
The Red River of the North case study focuses on the reliability of the levee system in Grand Forks, which suffered a devastating failure in April 1997 that resulted in more than $1 billion in flood damages and related emergency services. The Corps of Engineers has used risk analysis methods in several flood damage reduction studies across the nation, any of which could have been chosen for detailed investigation.
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From page 72... ...
at which experience accumulated since 1991 in risk analysis for flood damage reduction studies was reviewed. O'Leary (1997)
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From page 73... ...
Flood Damage Reduction Measures Beargrass Creek has several flood damage reduction structures, the most notable of which is a very large levee at its outlet on the Ohio River (Figure 5.2a)
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From page 74... ...
The proposed flood damage reduction measures for Beargrass Creek form an interesting contrast to traditional approaches. The emphasis of the proposed measures is on altering the natural channel as little as possible and detaining the floodwaters with detention basins.
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From page 75... ...
(a) Levee on the Ohio River (b)
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From page 76... ...
Beargrass Creek at Me detention pond FIGURE 5.2 Images of Beargrass Creek at various locations: (a) the levee on the Ohio River, (b)
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From page 77... ...
Longer damage reaches are used downstream on Beargrass Creek where less damage occurs. The highest expected annual flood damage is on Reach SF-9 on the upper portion of the South Fork of Beargrass Creek.
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From page 78... ...
. In each damage reach, and for each alternative plan considered, the risk analysis procedure for flood damage assessment requires a floodfrequency curve defining the annual maximum flood discharge at that location which is equaled or exceeded in any given year with a given probability.
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From page 79... ...
There are eight standard annual exceedance probabilities normally used to define this frequency curve: p = 0.5, 0.2, 0.l, 0.04, 0.02, 0.01, 0.004, and 0.002, corresponding to return periods of 2, 5, 10, 25, 50, 100, 250, and 500 years, respectively. In this study, because even small floods cause damage, a 1-year return period event was included in the analysis and assigned an exceedance probability of 0.999.
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From page 80... ...
There is a Tong flood record of 56 years of data (1940-i996) available in the study area (USGS gage on the South Fork of Beargrass Creek at Trevallian Way)
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From page 81... ...
~ this case the graphical method was used with an equivalent record length of 56 years of data, the length of the flood record of the USGS gage station at Trevallian Way at the time of the study. Figure 5.4 shows the flood-frequency curve for damage reach SF-9 on the South Fork of Beargrass Creek, with corresponding confidence limits based on ~ 2 standard deviations about the mean curve.
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From page 82... ...
Maps with a scale of 1 inch = 100 feet with contour intervals of 2 feet were used to define cross sections elsewhere on the stream reaches and were used for measuring the distance between cross sections on the channel and in the left and right overbank areas. Manning's n values for roughness were based on field inspection, on reproduction of known high-water marks from the March 1964 flood on Beargrass Creek, and on reproduction of the rating curve of the USGS gage at Trevallian Way.
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From page 83... ...
Flood damage reduction measures are most effective when they are located close to damage reaches with s~gnificant numbers of structures, and they are least effective when they are distant Tom such reaches.
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From page 84... ...
One damage reach on Beargrass Creek was subdivided into three subreaches to make this assumption more nearly correct. A spatial distribution of buildings over the damage reach is thus converted
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From page 86... ...
The various lines shown in Figure 5.8 are drawn as the expected water surface elevation ~ ~ or 2 standard deviations determined in this manner. Economic Analysis The Corps's analysis of a flood damage reduction project's economic costs and benefits is guided by the Principles and Guidelines (Box 1.1 provides details on the P&G's application to flood damage reduction
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From page 87... ...
. According to the P&G, the economic analysis of damages avoided to floodplain structures because of a flood damage reduction project is restricted to existing structures (i.e., federal policy does not allow damages avoided to prospective future structures to be counted as benefits)
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From page 88... ...
Types of flood damages beyond those to structures were also considered. For instance, there are several automobile sales lots in the floodplain, and prospective damages to cars parked there during a flood were estimated.
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From page 89... ...
that accumulates the damage to all multifamily structures in this damage reach for various water surface elevations at the index location, denoted by stage on the horizontal axis. This curve is prepared by first dividing the range of the stage (476~86 feet)
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From page 90... ...
In carrying out project planning, the spatial location of the principal damage reaches is important because flood damage reduction measures located just upstream of or within such reaches have greater economic impact than do flood damage reduction measures located in areas of low flood damage. Project planning also involves a great deal of interaction with local and state agencies, in this case principally the Jefferson County Metropolitan Sewer District.
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From page 91... ...
in the detailed design phase. Evaluation of Project Alternatives Expected annual flood damages in Beargrass Creek under existing conditions are estimated to be $3 million.
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From page 92... ...
In the downstream region, flood damage reduction measures on the upper South Fork and Buechel Branch compete for project benefits by reducing flood damages. The result of these complications is that the plan was built up incrementally by separately considering the three sections of the region.
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From page 93... ...
The conditional nonexceedance probability estimate divides these two uncertainties, because it is conditional on the severity of the natural event and thus represents only the knowledge uncertainty component. In this sense, the conditional nonexceedance probability corresponds most closely to the traditional idea of adding ~ foot or 3 feet on the 100-year base flood elevation, while the annual exceedance probability corresponds more closely to the goal of ensuring that the chance of being flooded is less than a given value, such as 1 percent, considering all sources of uncertainty.
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From page 94... ...
Probability Median Expected 10 25 0.0920 0.0920 0.6182 0.9099 SF-9 SF-10 SF-1 1 SF-12 SF-13 SF-14 SF-15 477.2 0.3570 0.3640 0.9892 1.0000 482.2 0.4240 0.4550 0.9977 1.0000 487.7 0.0720 0.0780 0.5566 0.8691 491.6 0.1220 0.1230 0.7310 0.9625 500.8 0.0990 0.0950 0.6237 0.9182 509.4 0.1030 0.1010 0.6561 0.9306 Note: Without project base year performance target criteria: event exceedance probability = 0.04 and residual damage = 5.00%.
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From page 95... ...
Case Studies 95 Conditional Exceedance Probability by Events 50 10% 4% 2% 1 % .4% .2% 0.9919 0.5686 0.1939 0.0146 0.0027 0.0009 0.0005 1.0000 0.0027 0.0006 0.0000 0.0000 0.0000 0.0000 1.0000 0.0004 0.0000 0 0000 0 0000 0 0000 0~0000 0.9829 0.6489 0.3206 0.0903 0.0317 0.0082 0.0031 0.9986 0.3106 0.0718 0.0078 0.0011 0.0004 0.0001 0.9984 0.3127 0.0733 0.0093 0.0021 0.0004 0.0002 0.9933 0.5190 0.1978 0.0384 0.0100 0.0020 0.0008 0.9952 0.4919 0.1361 0.0294 0.0092 0.0029 0.0014
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From page 96... ...
It is evident that including risk and uncertainty increases the expected annual damage both with and without flood damage reduction plans. The net effect of their inclusion on the Beargrass Creek project is to increase the annual flood damage reduction benefits from $2.078 million to $2.314 million.
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From page 97... ...
All these characteristics point to the fact that the distributions of flood damages and of expected annual benefits are positively skewed when uncertainties in project hydrology, hydraulics, and economics are considered. This is why the project benefits increase when these uncertainties are considered.
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From page 98... ...
Net Benefits Benefit to Cost Ratio 2.314 0.810 1.504 2.86 0.810 0.555 1.69 0.810 1.261 2.56 RED RIVER OF THE NORTH AT EAST GRAND FORKS, MINNESOTA, AND GRAND FORKS, NORTH DAKOTA A devastating flood occurred at East Grand Forks, Minnesota, and Grand Forks, North Dakota, in April 1997. After the flood, flood damage reduction studies previously done for the two cities were combined into a joint study, and risk analysis was performed to evaluate the reliability of the proposed alternatives and to evaluate their economic impacts.
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From page 99... ...
and Red Lake River (RLR) at the East Grand Forks, Minnesota and Grand Forks, North Dakota study area.
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From page 100... ...
. Risk Analysis A risk analysis for the proposed flood damage reduction project for the Red River of the North at East Grand Forks, Minnesota, and Grand Forks, North Dakota, used a Latin Hypercube analysis to sample interactions among uncertain relationships associated with flood discharge and elevation estimation.
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From page 101... ...
The resulting flows were subtracted from the Red River at Grand Forks flows to obtain coincident discharges on the Red River upstream of Red Lake River. The two-station comparison method of Bulletin 17B was used to adjust the logarithmic mean and standard deviation of this short record (96 years)
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From page 102... ...
) · Median Plotting Positions ·~ - 51 aIld 95~ Conf idence Limits FREQUENCY STAT ~ ST I CS LOG TRANSFORM OF FLOW, CFS ~ MEAN 4.1803 HISTORIC EVENTS '~ STANDARD DEV .3918 HIGH OUTLIERS SKEW - .1684 LaW OUTLIERS REGIONAL MAP SKEW -- ZERO OR MISSING ADOPTED SKEW -.1684 SYSTEMATIC EVENTS HISTORIC PERIOD<1826-1997' NUMBER OF EVENTS i\ Loden CaLrn& - d 1% 87,400 Discharge n cfe 146,000 ~ 9096 Cantos Oval ~1 RED RIVER AT GRAND FORKS USGS GAGE DISCHARGE - FREQUENCY Annual Instantaneous Peak WATER YEARS IN RECORD 1826, 1852, 1861, 1882 - 1997 FIGURE 5.11 Flood (discharge)
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From page 103... ...
~. The Plan Comparison Letter Report developed in February 1998 for flood damage reduction studies for East Grand Forks, Minnesota, and Grand Forks, North Dakota, evaluated an alternative flood damage reduction plan that included a split-flow diversion channel along with permanent levees.
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From page 104... ...
The modified-condition discharge-frequency curve for the Red River downstream of Red Lake River was graphically computed based upon the operation of the diversion channel. The modified-condition Red River discharges upstream of Red River were added to the coincident flows on Red Lake River (column 4~.
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From page 105... ...
were transferred to the current gage site at river mile 297.65 based on the gage location adjustments presented in Table 5.9, which were computed from the water surface elevations in Table 5.~. These adjustments were plotted against the corresponding discharge below the Red Lake River, and curves were developed to obtain adjustments for other discharges.
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From page 108... ...
Based on the observed elevations plotted on the adopted rating curve, it appeared that there was greater uncertainty for discharges less than about 10% of annual exceedance probability event due to ice effects on flow. Therefore, the standard deviation was computed for discharges greater than between 22,000 cfs, which corresponds approximately to the zero damage elevation based on the adopted rating curve, and 44,000 cfs, which is slightly greater than the 10 percent annual exceedance probability.
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From page 109... ...
In an earlier risk analysis that was per foe for the Grand Forks Feasibility Study, a much lower standard deviation of 0.50 feet was used for discharges greater than 50,000 cfs. However, adding the 1997 flood to the analysis resulted in a standard deviation of 1.55 feet, which is similar to that computed for discharges less than 44,000 cfs.
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From page 110... ...
The top-of-levee elevations were computed based on a water surface elevation profile to ensure initial overtopping would occur at the least-critical location (here, cross section 7700~. The downstream top-of-levee elevations were selected with the intent of having 90 percent probability of containing the specified flood and were based on previous risk analysis for the Grand Forks Feasibility Study preliminarily updated to include the 1997 flood.
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From page 111... ...
CTop of levee for 0.47% levee is computed water surface elevation plus 2.7 feet. Downstream end of project.
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From page 112... ...
the discharge-frequency curve based on the approximate statistics starts to diverge from the graphical curve for extreme events and, (2) there was limited information available to develop the Red River to the North rating curves for the diversion alternative.
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From page 113... ...
Through legislation, the State of Minnesota has committed to provide financial support in the for of bonds and resumed sales taxes to the city of East Grand Forks. In verbal and written comments from its governor, the State of North Dakota has committed to provide financial assistance to the city of Grand Forks.
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