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b. How was the normal depth downstream friction slope of Sr= 0.0015 fUfl determined? Downstream <br />topography from USGS maps suggests a different slope, and the current model shows an Ml profile from <br />• Section 53240 and Section 52370. This profile is not likely -please revisit. <br />3. Ineffective Flow: <br />a. The ineffective flow elevation appears to be too low on the right overbank at Section 56414 -this section <br />is never overtopped by any of the profiles in the study, and should be raised in similar fashion to adjacent <br />sections. <br />b. The ineffective flow elevation may be too low at Sections 55330 and 53970 - if this ineffective flow area <br />is attributed to road fill, it should be overtopped on the left bank of the roadway at CR 13 in the 100-year <br />storm and be fully effective downstream at 55330 and 53970. If there is another feature creating this <br />ineffective flow, please note in the cross section description boxes. <br />4. Bridge at WCR 13: <br />a. Sections 56414 and 56386 are crowding the bridge. The bridge appears to be 28 ft from upstream face to <br />downstream face, and these sections aze 28 ft apart. They should be moved back to a location that <br />intercepts the toe of slope where the shoulders intercept natural topography. Please refer to Figure 5.1 <br />and Figure 5.2 (A) in the Hydraulic Reference Manual, Chapter 5, Page 5-3 and 5-6. <br />i. Please adjust the Distance for the deck and roadway embankment. <br />ii. Please adjust the width of your bridge - it should represent the distance from the upstream face to <br />the downstream face, or the out-to-out dimension. This is where the 28 ft should be used. <br />iii. Abandon internal bridge cross sections -there is no need for anything but the default BRU and <br />BRD sections, as described in Chapter 5 of the Hydraulic Reference Manual. <br />b. Side slopes of zero (vertical) aze not representative of existing conditions at WCR 13. Please correct to <br />represent the appropriate shoulder slopes after moving the bounding sections away from the bridge. <br />c. Why aze the channel bank stations perched so high above the surrounding topo at Sections 56414 and <br />56386? Remember, these sections are the bounding sections for the bridge, and should represent natural <br />topography without any road fill. All road fill data is entered into the Deck and Roadway Editor. <br />d. In the Bridge Modeling Approach window you have checked the boxes to calculate the Energy method <br />• and Momentum method for low flow, but have selected the button to use the Energy Only method for <br />reporting. Please change this to Highest Energy Answer and let the program report the appropriate <br />method. <br />e. Continuity Challenge (Complex7ssue) <br />i. The 100-year overtopping flow at the weir section of WCR 13 is about 2,000 cfs. The cross <br />section immediately upstream (56414) has a total discharge of 5,840 cfs in the left overbank <br />supplying the weir. There will be only 20 or 30 feet of distance between the upstream bridge face <br />and XS 56414. This flow distribution suggests 3,840 cfs abandons the left overbank and <br />squeezes through the bridge opening in less than 30 feet of longitudinal distance, and the <br />remaining 2,000 cfs flows over the weir. This discontinuity is a numerical anomaly of the <br />program, and is not physically possible at this bridge, and is likely to receive comment from <br />FEMA reviewers. <br />ii. The cross sections upstream of the bridge at Sections 56414 and 56555 need to be adjusted to <br />maintain continuity in HEC-RAS. The program authors at HEC recommend changing the n- <br />value in the overbank such that the total flow arriving to the weir section of WCR 13 is within <br />10% to 20% of the total flow over the weir. In this case, the weir is limiting how much flow is <br />conveyed in the left overbank ai Sections 56414 and 56555. The Manning's-tt values should be <br />increased in the left overbank at these sections [o decrease the total conveyance upstream of the <br />road. <br />iii. Be sure [o carefully document [he use of artificially-high n-values in your CLOMR report and in <br />the description boxes of the Cross Section Data Edi[or in HEC-RAS. <br />5. Weirs: <br />a. The weir coefficients for your lateral weirs are too high at side-spill weirs. These weirs are not as <br />efficient as inline weirs, and the C-value will not be 3.0. Please refer to the Hydraulic Reference Manual <br />Chapter 5 (2002), King (1963), Brater and King (1976), HEC-22 FHWA-NHI-01-021 (2001), or other <br />• hydraulic reference for an appropriate C-value. <br />Page 2 of 3 <br />