Mapping the Zone Improving Flood Map Accuracy (2009) / Chapter Skim
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4 Inland Flooding
4 Inland Flooding
Pages 41-66
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From page 41... ...
The base flood elevation is computed at each in riverine flood mapping: cross section using the design discharge and a channel roughness factor by applying a hydraulic model such 1. Hydrologic uncertainty about the magnitude of as HEC-RAS (Hydrologic Engineering Center-River the base flood discharge; Analysis System)
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From page 42... ...
It should be used in flood mapping. understood that the purpose of this exercise is to gain insight into the sampling variation of extreme water UNCERTAINTY OF THE BASE FLOOD surface elevations around a statistically determined ELEVATION AT STREAM GAGES expected value, not to statistically determine the base flood elevation.
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From page 43... ...
less flood discharge for a given drainage area, but once At each stream gage site, the historical record of the banks overflow, floodwaters spread over a broader both flood discharges and flood stage was analyzed floodplain. The relationship between the terrain slope using the U.S.
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From page 44... ...
5.46 72 03449000 North Fork Swannanoa River near Black Mountain, N.C. 23.8 32 Mecklenburg County 02142900a Long Creek near Paw Creek, N.C.
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From page 45... ...
Thus, the confidence limits calculated by this tainty exists in BFEs derived from measured annual method provide only an approximate estimate of the sampling error flood elevations at gages with long flood records. of the peak stage data.
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From page 46... ...
Finding. The sampling error of the base flood eleva This frequency analysis of stage heights has a tion estimated using flood frequency analysis of number of limitations: no regional skew estimates were annual maximum stage heights measured at 30 longincluded (none exist for stage height data)
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From page 47... ...
mapping methods are required to over- in about half of the studies and flood frequency analysis lay the computed flood elevation on the surrounding or regression equations in the others (Table 4.2)
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From page 48... ...
Knowledge uncertainty value of 100 years return period for flood discharge and is also sometimes referred to as epistemic, func- across to the vertical axis yields an equivalent return tional, internal, or subjective uncertainty. period of 50 years for the lower confidence interval discharge and 180 years for the upper confidence Estimation of flood peaks at return periods of interval discharge.
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From page 49... ...
. Knowledge uncertainty is associated with the structure of the model and USGS REGIONAL REGRESSION EQUATIONS its ability to capture the behavior of the studied system in part or as a whole, the model parameters used to USGS regional regression equations are used to quantify the relationships between the various compo- compute flood discharges in nearly all approximate nents of the system, and model input and output.
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From page 50... ...
Most states characteristics at gaged sites. This relation is then used have updated their regional regression equations since to estimate flood discharges at ungaged sites.
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From page 51... ...
The USGS is currently revising the sion coefficients. Catchment area, or the area draining regression equations for the Blue Ridge region using to a defined point on the stream system, is the single additional stream gages from adjacent states with most important independent variable.
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From page 52... ...
Four variants of hydrologic m ethods for determining the flood peak discharge were A typical result for the effect of these variations in examined: flood discharge on the BFE is shown in Figure 4.10. The water surface profiles for the rainfall-runoff, rural 1.
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From page 53... ...
landscape Figure 4-9.eps 4 bitmap images w/ vector arrows FIGURE 4.9 Location of the study reaches. Source NCFMP (2008)
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From page 54... ...
USGS regional regression equation method is estimat It seems surprising at first that there is so little dif- ing flood discharges with sufficient precision to support ference between the results from rainfall-runoff, flood FEMA flood mapping efforts. frequency analysis, and regional regression equations The average error of prediction for a 100-year flood because the regional regression equations are simple in the USGS regional regression equations differs by empirical expressions that do not involve the precision physiographic region in North Carolina: 47 percent in of rainfall-runoff modeling or flood frequency analysis.
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From page 55... ...
Inaccuracies in hydraulic modeling add to inac curacies associated with the base flood discharge Finding. Flood frequency analysis of stream gage and decrease the accuracy of the BFE.
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From page 56... ...
A single the water surface profile caused by the irregular shapes water surface elevation value is computed, and the of natural channels, which create flow resistance, and depth of water over all points in the cross section is (2) structures and flow impediments, which increase determined by extending a horizontal water surface the height of the water surface upstream and create a elevation line across the channel.
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From page 57... ...
Several equations that relate surface rough- flow of the 100-year event, so flood mapping analyses ness to flow characteristics are available, but the most are performed assuming steady flow. In steady flow popular in open-channel flow computation is the the water surface elevation is constant over time; in Manning equation.
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From page 58... ...
surface water elevation at main channel centerline produced by the one-dimensional model, (c) two-dimensional model setup with computational mesh, and (d)
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From page 59... ...
. Lidar data were used for Swannanoa River are plotted in Figure 4.16 for the five topography, field surveys for channel cross sections and The LDSNAT variant is specific to the NCFMP (2008)
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From page 60... ...
690 680 670 660 Elevation (feet) 650 640 630 DS LDSNC 620 LDSNAT APPROX APPROX-NED 610 STRUCTURE USGS Gage 600 19000 24000 29000 34000 39000 44000 49000 54000 Station FIGURE 4.15 Base flood elevation profiles for different hydraulic study types on Long Creek.
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From page 61... ...
30 DS 25 LDSNC LDSNAT APPROX 20 APPROX-NED STRUCTURE USGS Gage 15 29000 34000 39000 44000 49000 54000 59000 64000 69000 Station Figure 4-16 bottom.eps FIGURE 4.16 Water surface elevation profiles for different hydraulic study types on the Swannanoa River and Ahoskie Creek.
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From page 62... ...
The differences are striking, strate that backwater effects from structures increase particularly for Long Creek, where on average the BFE base flood elevations and that the distance these is more than 20 feet higher if calculated using the NED effects extend upstream is longest at Ahoskie Creek in rather than lidar. In the other two study reaches, the the coastal plain and shortest on the Swannanoa River NED BFE is, on average, fairly close to the lidar BFE, in the mountains of western North Carolina.
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From page 63... ...
, it follows that floodplain boundary stream gages. This is heartening for floodplain map- delineation is more uncertain in the coastal plain than ping because it suggests that there is a good deal more in the piedmont or mountains -- in fact, about four to similarity in stream flood processes across broad regions five times more uncertain, in proportion to the rise-run than might be expected.
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From page 64... ...
Used with permission. bitmap image TABLE 4.9 Differences in Inundated Area for Various Hydraulic Study Types Ahoskie Creek Swannanoa River Long Creek Area Percent Area Percent Area Percent Topographic Source (acre)
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From page 65... ...
the flow dynamics become similar to those in dendritic drainage landscapes. The committee's frequency analysis of stage heights pitted landscapes.
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From page 66... ...
studies than in detailed studies, the total areas they • Observed flood discharges at stream gages are encompass can be reasonably similar, provided the the most critical component for estimating the base stream and topographic data are properly aligned. flood discharge in the three study reaches because all hydrologic methods are calibrated using these data These conclusions were based on limited studies in and each stream reach contained a stream gage.
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