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Scientific Investigations Report 2006–5323

U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006–5323

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Flood Profiles

Flood profiles computed for Duck Creek and Jordan Creek downstream from Egan Drive consist of the water-surface elevation for the 10-, 50-, 100-, or 500-year floods (fig. 8A-E). The model assumes Duck and Jordan Creeks peak simultaneously, and flooding coincides with a 20 ft high tide. Sensitivity analysis indicates the accuracy of the water-surface elevations is within the required tolerance of 0.5 ft.

Actual flooding conditions can damage bridges, culverts, and roadways, and debris obstructions can increase local flooding. However, this study assumes that all structures included in the model maintain their integrity during floods, that no road failures occur, and that structures remain unobstructed. For most culverts, the model assumes sediment observed during field investigations is washed out during flooding.

The steady-state model used does not address the possible attenuating effects of the numerous ponds within the Duck and Jordan Creek corridors. The likely effect of an unsteady-flow analysis, which would address storage, would be a reduction in water-surface elevations.

Generally, the lateral extent of the water-surface for the cross sections adequately delineates the flood plain. However, cross sections do not extend to the full extent of the 500-year flood plain in areas where it was impractical to do so and the extension would not affect the model hydraulics. In particular, cross sections do not extend fully at locations where the channel flows over roadways and along Jordan Creek between Egan Drive and Glacier Highway, where water-surface elevations exceed the elevation of the surrounding land surface. Flood plains in these areas may be delineated on the basis of elevations generated in the model.

Simulated water-surface profiles generally are higher than those published previously (Federal Emergency Management Agency, 1990), showing a 1- to 3-ft discrepancy in elevation at the limits of the study areas for the 100-year flood (table 8). Possible reasons for this discrepancy include new hydrologic data, different assumptions regarding storage, different survey datum, and land-surface elevation change over time. A re-analysis of Jordan Creek upstream from Egan Drive will be necessary to match the profiles shown in this report. The profiles downstream from the underground pipe for the tributary named East Fork Duck Creek in the previous FIS will need adjustment because the simulated water surface at the confluence is about 3 ft higher than in the previous FIS.

Inter-Basin Flow

The Duck and Jordan Creek flood plains merge downstream from Egan Drive for the 50-, 100-, and 500-year floods. The LIDAR data show a low area along the east bank of Duck Creek about 500 ft downstream from Egan Drive (fig. 9). Floodwaters exceeding the elevation of this area (modeled as 29.0 ft) will not return to Duck Creek, but instead will flow across a residential landscape to Jordan Creek. The simulated 100-year flood conditions show a flow of 22 ft3/s exiting Duck Creek and entering Jordan Creek upstream from Jordan Avenue (table 5).

A topographic low between the base of the Mendenhall Valley wall and Egan Drive extends to the southeast from Jordan Creek for about 4,000 ft (fig. 9). Floodwaters exceeding the elevation of this low (simulated as 29.5 ft) will exit the Jordan Creek system entirely and drain into salt water. Model simulation results show a flow of 104 ft3/s would exit Jordan Creek during the 100-year flood.

Discussion

Combined flows at the mouths of Duck and Jordan Creeks for the 100-year flood are 467 ft3/s, with 106 ft3/s leaving the Jordan Creek network upstream from Egan Drive (table 5). This is considerably less than the 699 ft3/s simulated in the previous FIS (Federal Emergency Management Agency, 1990). However, simulated profiles generally are higher than those for the previous study. The effect of these differences on the areal extent of the flood plain depends on the topographic data used during flood plain delineation and cannot be predicted from this study alone.

The strong influence of culverts and bridges can be seen in the stair-step profiles for the 100-year flood on Duck Creek, which are influenced by backwater from near Berners Avenue upstream to Cinema Drive (fig. 8A-B). As viewed moving upstream over this 1.5 mi reach, the water-surface elevation increases at culverts and bridges, then remains flat until the next structure is reached. Under the influence of this backwater, the presence of some smaller structures has no noticeable effect on the water-surface profile. A similar effect can be seen at culverts at Glacier Highway and Egan Drive along Jordan Creek (fig. 8E).

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