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<br />Examination of the computed water depths at the peaks of three events <br />(Tables 5.1 through 5.3) indicated significantly higher water surface <br />elevations through La Junta under both project #3 and project #4 conditions <br />than for existing channel conditions. At the peak of the SPF the project #4 <br />water depths at cross section 1198.4 (just upstream from the Hwy 109 bddge) <br />were 3.2 times greater or 16.6 feet deeper than would occur for natural <br />channel conditions. For project #3 conditions there may be 2.8 times or 13.6 <br />feet deeper f lows at the same cross sec tion. Deeper flow depths and greater <br />average channel velocities (as high as 17.6 feet per second) occur under both <br />project #3 and #4 conditions. This indicates that significant levee revetment <br />and stabilization works (e.g., rip rap protection) may be necessary to avoid <br />levee scouring. <br /> <br />5.5.3 Evaluation of Scour Potential UsinK Hand Computation Methods <br /> <br />This section presents some additional analyses for comparison to the <br />computed single event results from HEC-6. Based on the HEC-6 analysis, two <br />representative areas were chosen: a reach of scour (near river mile 1198.9) <br />and a reach of deposition (near river mile 1197.2). Four types of hand <br />computations were performed: (1) actual shear stress, (2) critical shear <br />stress, (3) sediment transport rates, and (4) critical velocity. Values <br />calculated in items (1) and (2) are used in (3) to compute the transport rate. <br /> <br />1) Actual Shear Stress - The actual shear stress on the river bed at a <br />particular flow rate can be calculated using the equation <br /> <br />T = yRSf <br /> <br />(5.1) <br /> <br />where: <br /> <br />T = shear stress in lb/ft2 <br />y = unit weight of water (= 62.4 lb/ft9) <br />R = hydraulic radius in ft. <br />Sf = slope of the energy grade line <br /> <br />The relationship between the actual shear stress and the critical shear <br />stress gives an idea of the amount of scour that can occur. <br /> <br />2) Critical Shear Stress - The shear stress for which a sediment particle <br />of a certain diameter is removed from the bed is known as the critical shear <br />stress T c' The preferred method for determining T c is through the use <br />of a Shields diagram as shown here (12, p. 96). <br /> <br /> 1.0 <br />_Ii 0.8 <br />0.6 <br />. , 0' <br /><" " <br />, <br />..' 03 <br />! 0.2 <br />. 01 <br />< <br /> 0.08 <br />. 006 <br />l 0.05 <br />. 0.04 <br />, 0-03 <br />is <br /> 0.02 <br /> 02 <br /> <br />.11.111 11111:11, ~;;::~ <br />Shields Diagram 0 Am'''1 1.06 <br />I" II ! ;,",!, :.!11,[[ ,,,'"',,' ~",,' "ofi" : ~~;;,: (5hi"',) :;: <br /> <br />III I -j I * Sand(Case,') 2.55 <br />+ Sand(K'Jme'> 2.65 <br />" Sand (U.S. WES.) 2.65 <br />t:. Sand(G,lbertj 265:: <br />-" I- Sand (Wl"ltte) 2.61 - <br />Tu,bule'lt boundary lay~t 0 Sand in air (Wh,te) 2.10 <br />I I I 1111.. Steel SIlOI (White) 7.9 <br />1 VaIUeQ!~Jo.l(~' I)gd, I III <br />I "I I II' I III <br />.'~ a 10 ;2 .' 6100 1 . /; 1~ <br /> <br />, <br />;< <br />T <br /> <br />* <br /> <br />* . <br />. I, <br />Shierl4SlCtUlrve <br /> <br />0.4 0.6 1,0 <br /> <br />4 6 8 lO 20 40 60 100 200 <br />Bounda'YReynOldsNumber,R.=~ <br /> <br />500 I 000 <br /> <br />58 <br />