Page:Final Report - The Columbia River Interstate Bridge.pdf/35

 The embankments across Hayden Island and in the main approach to Union Avenue are 42 ft. wide on top. The embankment of the secondary approach to Derby Street is 40 ft. wide on top and on the Vancouver approaches the widths conform to the widths of the streets occupied. The embankments average about 24 ft. in height and have side slopes of 2 horizontal to 1 vertical.

The upstream side of the Union Avenue embankment is protected to the elevation of 20 ft. above low water by concrete slabs laid on the slope of the finished till. These slabs are 4 ins. thick, reinforced with wire mesh and in strips about 8 ft. wide, extending continuously up and down the slope. The overflow

water, which at times of high water inundates most of the river valley, is largely back water and has no perceptible current, except that caused by its outflow from the river over the land as the river rises and its return flow to the river as the river falls. The time of greatest current with corresponding danger of erosion is at the time when the water is slightly above the height of the river bank, and occurs near the banks. For this reason