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JL\l DI - z Gu oc. • lv-vv . ., .-.... ..._- <br /> AMAZON CHANNEL MAINTENANCE STUDY <br /> all the roadway and bikeway crossings. Manning's roughness coefficients used in the hydraulic <br /> analysis were estimated from these observations. <br /> Photographs taken along the Amazon Channel in February, 1987 as part of the Amazon Channel <br /> Study(Reference 2) were obtained from the project file. Manning's roughness coefficients were <br /> estimated from the 1987 photographs to provide some historical perspective. Table 1 presents the <br /> estimated Manning's roughness coefficients for both the main channel and overbank based on 1987 <br /> and 1992 conditions. <br /> It is interesting to note that the roughness conditions on the lower channel from the diversion to <br /> Beltline Road did not dramatically increase from 1987 to the present 1992 conditions, although <br /> maintenance occurred in 1984 and 1985. However the upper channel, especially upstream of West <br /> 11th Avenue, was dramatically different reflecting the maintenance that occurred in 1990 and 1991. <br /> This historical perspective seems to indicate that the lower channel may not have to be leaned as <br /> frequently as the upper channel since the Manning's 'n" value in the lower channel appeared to <br /> increase by only 0.002 per year over the five year period. <br /> It should also be noted that existing maintenance practices appear to lower Manning's 'n" values to <br /> around 0.025, and the time it takes for the 'n' value to increase to 0.035 varies from 2 to 3 or more <br /> _ years. A simple relationship between Manning's "n"value and the length of time following the <br /> application of existing maintenance practices is not apparent from the observations noted in Table 1. <br /> EXISTING CHANNEL MAINTENANCE PRACTICES <br /> Since the topic of existing channel maintenance practices was introduced above, these practices will <br /> be defined for those readers who may not be familiar with them. The description is taken directly <br /> from a December 4, 1992 letter to the U.S. Army Corp of Engineers (COE) from the City's Public <br /> Works Maintenance Director. <br /> "The City's current channel maintenance practice is to mow the upper portion of the <br /> banks and slopes during the dry summer months. This not only helps prevent <br /> excessive rutting by the mowing equipment but also allows easier access to the bottom <br /> of the channel. In addition, a hydraulic excavator (Gradall) is used to clean sediments <br /> from the bottom of the channel and remove restrictive vegetation from the slopes. <br /> The Gradall has a limited reach so most areas must be cleaned from both sides of the <br /> bank. Needed maintenance that is beyond the capability of the Gradall is sometimes <br /> contracted and a crane with a dredging bucket is used to clean sediment and <br /> restrictive vegetation. At the end of each summer the scraped slopes where <br /> vegetation has been totally removed are reseeded to reduce erosion. Periodic removal <br /> of refuse and debris from the channel is also performed." <br /> Once again, these practices are effective in reducing the Manning's roughness coefficient down to <br /> 0.025 or lower. <br /> I <br /> 2 <br />