2013-10-30_REVISION - M1977310 (3)

Home Browse Search

W/hetkp"e AssociatesS Technical Memorandum performed based on the 3:1 design slope for the UDS. The results indicate that the channels will be stable under the design flow rates, which is consistent with the stable configuration achieved over time in the actual channels. Cotter commits to reconstructing the UDS with 3:1 side slopes on both banks. The reconstruction will fit within the existing footprint of the channel disturbance, and can be performed as part of regular ongoing maintenance at the site. 3. Page 6, Cowan's formula. The formula at the top of page 6 uses "nb" and "m'; Table 3 uses "no" and "m5 ". Please confirm the no is used for nb and m5 is used for m. Cotter response: The "no" variable listed in Table 3 corresponds to "nb" in the formula. The "1115" variable listed in Table 3 corresponds to "m" in the formula. The variable notations have been corrected in the revised DDP. 4. Cowan's method results in higher Manning's values when compared to the more traditional values published in "Open Channel Hydraulics" (Chow, 1959) and others (e.g., Van Haveren; 1986, Mays, 2001). Also, note that channel roughness is seldom uniform. Therefore, the DRMS requires channels be evaluated for both stability and capacity, i.e., minimum and maximum expected roughness. For example, an excavated earth channel, after weathering would be expected to have a minimum n = 0.018 (use to evaluate stability or maximum expected velocity); and a maximum n = 0.025 (use to evaluate capacity). The DRMS will accept roughness- values obtained, using Cowan's method for capacity, but an alternative method should be selected to evaluate channel stability. Please provide a complete summary analysis of expected channel performance for both stability and capacity for the redesigned channels. Cotter response: Additional hydraulic modeling has been performed in the revised DDP. Channel stability was evaluated using momentum and energy equations. Using channel profiles measured for the site, the Thorne and Zevenbergen (1985) method was used to calculate resistance to flow and mean velocity at the channel cross section. The roughness coefficient ( Manning's n) for each cross section was calculated based on the D84 particle size (i.e., the 84th - percentile particle size) and the relative roughness was adjusted based on changes in flow depth. Increased stage (higher water level) results in decreased roughness (lower Manning's n). The results are presented in the October 2013 revised DDP. 5. Page 7, section 2.3.4. This section states HEC 15 was used to evaluate diversion ditches to determine if channel protection is required. No constructed channel evaluation results were provided with the drainage Design Plan. a. Please provide criteria for the referenced evaluation (Note the DRMS requires channel protection for channel velocities that exceed five feet per second under design flow conditions). 4148A.131030 2