STANDARD STEP METHOD 



101 



a different tentative water-surface elevation in column (2). The 

 unsuccessful trial should not be discarded, for it will serve as a guide in 

 re-estimating the unknov/n elevation and will provide a partial check, 

 by comparison, on the subsequent computations. 



If the value obtained does agree with that entered in column (5), and 

 there are no errors in the computation, the trial water-surface elevation 

 assumed in column (2) was correct, and a value for the water-surface 

 elevation at the next section can be assumed and tested in the next line 

 of computations. 



Fort Loudoun Dam -First Construction Stage Cofferdam 



Backwater for Q = /60. OOO c. f.s. Igy.OOlS'^ _ 



•"Manning Formula with w=C!CJ5 1~486 



■■ 0.0357, 



Si =0.03 57 -^-^R"!^ 

 2g 



1 



10 



11 



12 



13 



Station 



Water 

 Surface 

 Elevation 



Area 



Velocity 

 Head 



Elevation of 



Total Head 



Line 



Hydraulic 



Radius 



R 



R'A 



Friction 

 Slope 



Average 

 S( thru 

 Reach 



Length 



of 

 Reach 



Friction 

 Loss 



Eddy 

 Loss 



Elevation of 



Total Head 



Line 



0*00 

 10*80 

 bridge 



766.0 



765.0 

 765.0 

 -766^ 



l%700 

 11.500 

 *9.600 

 fZ.lOO 



1.03 

 3.01 

 431 

 -2f^ 



76703 

 768. 01 

 769. 31 

 709 52 



30.7 



30.6 



14.4 



-3h»- 



961 

 95.6 

 350 



-tew- 



00038 

 .OOIIZ 

 0044 



.00075 



m^h 



1080 

 



-hoe- 



0.81 

 000 



-a27- 



0.17 

 1.30 



-em- 



7G703 

 768 01 

 76931 

 7 695 8 



IhSO 

 19*50 



766.9 

 76^.3 

 -776S- 



12.150 

 22 J 00 

 31.200 



2.70 

 082 

 -04+- 



769 60 

 770. 12 

 770. 91 



31.8 

 21.7 

 -i5S-^ 



1010 

 604 

 -74^ 



00095 

 00048 

 00020 



0027 

 .00071 

 0003 4 



100 

 770 



-me- 



0.27 

 055 

 -045- 



0.00 

 0.00 

 ^&0- 



76958 

 770.13 

 - 770.59 - 



32*80 



-S4^S0-\- 

 54*50 



770.2 



30700 



042 



770.6 



30.500 

 30.600 



-943- 



0.42 



770.62 

 77 . 93 



25.0 



-3he- 



31.1 



73.0 

 -99:5- 

 97.8 



00021 



mei6- 



00015 



00034 

 00018 

 .00018 



1330 

 2170 



044> 



039 



0.00 

 0.00 



770.59 

 770. 9 9 

 770.98 



771. 02 



*Effechve area fbrough bridge of 10*80 assumed fo be 90% of ocfual area of consfricHon. 



Fig. 902, Typical Backwater Computation, Standard Step Method. 



Based on simple area-elevation and hydraulic radius curves. If curves like those shown in Fig. 903 

 have been prepared columns (3), (6), and (7) of this table may be omitted. Multiplying the net area 

 under the bridge by 0.9 is equivalent to using K = 0.9 in D'Aubuisson's formula, equation (1012). 



An example of backwater curve computation by the method just 

 described is shown in Fig. 902. When the Kutter formula is to be used 

 instead of the Manning formula, column (7) should be omitted and a 

 column added for the average velocity, following the one for the area. 



If the roughness varies with the water-surface elevation, if there is 

 considerable overbank flow which must be considered separately, or if 

 several backwater curves are to be obtained, a variant of the method 

 just described is more convenient. According to the Manning formula, 

 the friction slope is given by 



5/ = 



n^(T 



2.21 A^R^'^ 



[905] 



Except for the discharge Q, the variables on the right side of equation 



