7789 - Laserfiche WebLink

Home Browse Search

Starting with the n-value of 0.029, water surface elevations were predicted with multiple HEC-2 runs decreasing the n-values by 0.001 from 0.029 to 0.026. The average absolute difference in the surveyed versus the predicted water surface for all the cross sections was minimized for an n-value of 0.027. This was essentially the same average n-value estimated in the June HEC-2 analysis (0.028) indicating the cross section changes did not appreciably effect the average n-value. The largest difference in the measured versus the predicted water surface elevation was 0.60 ft at cross section E-1. The results are presented in Table 4. A printout of the HEC-2 results is presented in Appendix B. A diskette of the data files, output, and HEC-2 program is also included with the report. Using an n-value of 0.027, water surfaces were computed for a range of discharges. The results were examined for cross sections E-14.5 and E-15 to determine the potential for diverting flows from the river to the wetlands at these locations. This data is presented in Table 5. The average wetlands water surface elevation was approximately 4829 when surveyed in June and 4827 in August. A decline of about 2 ft in the wetlands water surface occurred over this period. The existing channel depressions along the bluff from cross section E-14.5 to the wetlands, however, are higher (4831 to 4833) than the wetlands water surface and would require a river water surface elevation of about 4833.5 to insure inflow to the wetlands from a proposed inlet near E-14.5. The river discharge required for this water surface would be approximately 13,500 cfs (Table 5). If the channel way to the wetlands was excavated, then it would be possible to reduce the required river discharge to flood the wetlands as shown according to the elevations in Table 5. The original banks have been modified by levee construction and by riprap material in the wetlands reach. On the east bank, overbank flow would not occur until the levee was overtopped. The small levee is one to three feet higher than the natural river bank. The top of levee elevations were not considered in the computations or in the analysis of flow to the wetlands from the river. The bank elevation varies along this reach because of the location and elevation of the old abandoned meander channels. Estimates of the natural bank elevations were surveyed and used to calculate bankfuli discharge with HEC-2. The natural bankfull discharge along this reach is projected to be 25,000 (E-16) to 30,000 cfs (E-14.5). Additional discussion on proposed hydraulic control of inflow to the wetlands is presented in section C of the Summary and Conclusions. 21