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.04 8920 Legend 8918 . WS 1 800 cfs 8916 WS 100 yr ~ . c 8914 WS 50 yr 0 . ~ WS 25 yr > 8912 WS 1 0 yr (]) [jJ . 8910 Ground 0 8908 Levee . Bank Sta 8906 0 50 100 150 200 250 300 350 Station (ft) Figure 2.2.7. Flood elevations at weir Figure 2.2.8. Path of weir overbank flooding Large volumes of sediment are deposited behind the weir due to reduced velocities and ponding. Sediment must be removed almost yearly from behind the weir, although some accumulation of sediment above the weir may tend to protect the downstream channel and flume somewhat from additional deposition or damage. Avulsion at Cross Section 8.5 A 3- foot timber drop at cross-section 9 is shown in the photo to the right. Just downstream of the weir (XS8.5), the west bank has been eroded and the levee is absent. The 1800cfs flood will overflow the bank at this location. During overtopping of the weir at cross-section 14 during the 100-yr flood, the remaining flow in the main channel will not overtop cross-section 18 and some overbank flow should actually return at this location. However, if overtopping at the XS 14 weir is fixed, the full 100-year flood would overtop the bank at this location. Figure 2.2.9. Timber drop at cross-section 9 Figure 2.2.10. Timber drop at cross-section 6 Avulsion at Cross Section 6.1 The stream has eroded around portions of an old timber drop at cross-section 6. The stream is still constricted at this location as shown at right. Mine drainage fills a pond just upstream of this location that outlets to the stream channel at cross-section 6.1 (a photo also follows). High-flow events will back water into this pond. A low point in the road is also in line with cross-section 6.1 and the fire department tunnel (also shown in a photo). 2-24