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I I I I I I I I I I I I I I I I IV. EROSION CONTROL AND BANK STABILIZATION The majority of East Plum Creek studied in this report flows in a northerly direction with the exception of one reach which meanders sharply to the east. If development is to occur in this area, erosion control is recommended along the banks of the creek in this vicinity to keep the creek from undercutting its banks and eventually changing its path. Figure 4 shows the locations where erosion control is recommended. Erosion control, in this report, refers to protection of the stream channel banks from~he erosive forces of flowing water during times of flooding. Bank stabilization refers to measures to mitigate the sloughing of the high bank or bluff on the north side of the creek from Station 56300 to Station 57300. This is the part of the bank that is above the 100-year flood level. Erosion Control: The alternatives for erosion control evaluated in this report are riprap and soil cement. If riprap were used as a means of erosion control, approximately 4600 CY would be required to provide the necessary protection along the channel banks as shown on Figure 4. Riprap with a mean diameter of 18" would be required to provide erosion protection against velocities of up to 11 fps. Based on the HEC-2 analysis, this ;s the velocity that could be expected along the banks during a 100-year flood. The estimated cost for riprap erosion protection in the areas shown is $230,000. If the soil cement alternative were used, approximately 8100 CY would be required at an estimated cost of $240,000. Eleven feet per second is about the highest velocity which can be expected in the channel overbanks during the 100-year storm in this reach of channel. However, in the main stream channel, (see figure 2) velocities of up to 14 fps can be expected to occur during a 100-year storm. If soil cement were used, erosion protection against velocities of up to 14 fps could be achieved. This would be advantageious if encroachment on the channel (and resulting increased velocity) is planned. Placement of 18" riprap along the channel banks may be easier than the construction of soil cement channel banks and could be done with less of a disturbance to the channel by providing greater flexibility in the placement of the material. Bank Stabilization: There is a near vertical bank to East Plum Creek which in some places extends as high as 30 feet. This steep bank has actually been under cut in some places by large storms. If development is to occur in this vicinity (see figure 4) it is recommended that this bank be stabilized to prevent further erosion. Several alternatives exist. The soils report by Woodward Clyde Consultants, suggests laying the slope back at 3H to IV. If this method were used, approximately 15500 CY of earth would need to be cut and hauled away. By doing so, approximately 1.8 acres of developable land would be lost to the 3 to 1 slope. The estimated cost for the required earthwork is $31,000.