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Larry Routten and Erica Crosby -9- November 3, 1993
<br />Specific channel section boundaries shown on Figure 6 are approximate, and will be adjusted in
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<br />i accordance with field construction constraints.
<br />Channel designs are provided for the 100-year, 24-hour storm prediction, as summarized in
<br />' Tables 4 and 5. posed designs for Sections.B,_D, and E ate_provi~e~using both a bottom
<br />~~' , ~ width of 15 feet, and a_b_ottom width of. 20 feet. Total channel dimensions are the dimensions
<br />at the riprap surface. The calculations indicate that either configuration will function properly.
<br />Ultimately, I recommerd the-configuration-which-will.provide transition areas between the
<br />constructed, and existing channels, with the least width difference between those sections.
<br />An attempt should be made to avoid channel junctions with acute angles larger than 45 to 55
<br />degrees (Figure 7). This will be especially important at the junction of Section E with Magpie
<br />Creek. As with all riprap installations, all transitions must be properly keyed to avoid erosion at
<br />the channel junctions, and at the upper and lower ends of the riprap installations (Figure 81.
<br />Riprap installation at the upper end of Section B must extend a minimum of 15 feet along the
<br />_native channel. Likewise, the junction between Sections_B, C, and D should provide riprap to
<br />length of 15 feet along Channel C. Finally, Section E should be riprapped such that riprap
<br />extends into the Magpie Channel arid upstream along the Magpie Charinel for distances of
<br />~, (~- 15 feet, respectively. It is my understanding that additional and unrelated work may be done
<br />within the Magpie Channel in this area, so you may wish to implement a lesser and temporary
<br />design within the Magpie Channel. If this option is implemented, I recommend that specific
<br />commitments be secured for the other work. Riprap must be installed over the entire channel
<br />cross section.
<br />Riprap should be installed to a minimum total depth which is 1.25 (D,,,,,,1 (Simons et al.; 1982).
<br />Thus,
<br />Minimum Total Depth =(1.25)(2.2 feet) = 2.75 feet.
<br />A filter layer_of gravel should be installed beneath the riprap layer (Simons et al., 1982). The
<br />layer should be a minimum of 9 inches deep (Simons et al., 1982), and a thickness of 10 to
<br />12 inches is recommended. The filter layer gradation should be approximately as follows
<br />(Urban Drainage Flood Control District, 1969):
<br />~,-~~-„ t~~ ~16 ~ (,ic, D,,,~„ = 3 inches
<br />~,_,,,,, ~~~,; ((~-f5 Dzo-so = 3/4 inch
<br />D20 = #4 mesh
<br />Some contractors use geotextile filter blankets with success, but a gravel filter is considered to
<br />be a better option (Simons et al., 1982).
<br />All riprap must have specific gravity no less than 2.5, and 2.65 is preferred (Urban Drainage
<br />and Flood Control District, 1969). Rock must be angular in shape, and of excellent durability;
<br />sandstones and shales should be avoided (Simons et al., 1982).
<br />In conclusion, the success of any installation such as the one proposed above, is dependent
<br />upon the proper installation of durable rock. The designs provided are based upon all available
<br />information and upon accepted modelling methodologies. Each variable has been carefully
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