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of the value of the contraction ratio M, all field structures investigated in the State of Mississippi operated well within the subcritical range, It was thus necessary to revise the former backwater bllSe curve, figure 6, and some others, Where changes in the formcr design curves have been made, mention is made of this fact in the appropriate chapter and explanations and data supporting these changes are included in appendix B. To maintain continuity and brevity in the design procedure, extraneous material has been reserved for the appendixes, The changes incorporated in this edition are in the backwater coefficient curve (fig. 6), the distance to maximum backwater curve (fig, 13), and dual bridges (figs, 14 and 15). Figure 10 for skewed crossings and figure 12 for differential level across embankments have been changed only in format to facilitate their use, New sections have been added on partially inundated bridges and flow over road- way (ch. VIII), spur dikes (ch. IX), and back- water coefficients for type II flow (ch, X), 1.7 Definition of symbols. Most of the symbols used in this publication are recorded here for refer- ence, Symbols not found here are defined where first mentioned, A. = Area of flow including backwater at sec- tion 1 (figs. 2B and 3B) (sq, ft,). A.. = Area of flow below normal water surface at section 1 (figs, 2B and 3B) (sq, ft.), A.. Gross area of flow in constriction below normal water surface at section 2 (figs, 2C and 3C) (sq, ft.), A. Area of flow at section 4 at which normal water surface is reestablished (fig, 2A) (sq, ft,), A, = Projeeted area of piers normal to flow (between normal water surface and streambed) (sq. ft,), A. = Area of seour measured on downstream side of bridge (sq, ft.), a = Area of flow in a subsection of approach channel (sq, ft.), B = Width of test flume or AdO for field structures, b = Width of constriction (figs, 2C, 3C, and sec. 1.8) (ft,), b. = Width of constriction of a skew crossing measured along centerline of roadway (fig, 9) (ft.), C = hI.' I h.' = Correction factor for back- water wiiih scour. C. = Backwater coefficient for flow type II, CI = Freeflow coefficient for flow over road- way embankment. C. = Submergence factor for flow over roadway, D. ""''':;h,' = Differential level ratio, e = Eccentricity = (I-Q'/Q.) where Q, < Q., or (l-Q.IQ,) where Q, > Q., g = Acceleration of gravity = 32,2 (ft./see,'), lvr = Total energy loss between sections 1 and 4 (figs, 2A and 3A) (ft.), h. = lvr- SoL'-4 = Energy loss caused by con- striction (figs, 2A and 3A) (ft,). h,' = Total backwater or rise above normal stage at section 1 (figs, 2A and SA) (ft.), hI.' = Backwater with scour (ft,), "'" = Backwater computed from base curve (fig, 6) (ft,). hd' = Backwater produced by dual bridges, measured at section I (fig, 14), 1<.,' = Vertical distance from water surface on downstream side of embankment to nor- ma! water surface at section 3 (figs. 2A and 3A) (ft,). Ah = h.'+h,'+S.L._. = Difference in water surface elevation across roadway em- bankment (figs, 2A and 3A) (ft.), J = A,I A., = Ratio of area obstructed by piers to groS8 area of bridge waterway below normal water surface at section 2 (fig, 7), K. = Backwater coefficient from base curve (fig, 6), AK, = Incremental backwater coefficient for piers (fig. 7). AK. = Incremental backwater coefficient for ec. centricity (fig, 8), AK. = Incremental backwater coefficient for skew (fig. 10), K' = K.+AK,+AK.+AK. = Total backwater coefficient for subcritical flow, k = Conveyance in subsection of approach channel. Ie. = Conveyance of portion of channel within projected length of bridge at section 1 (figs, 2B and 3B and see, 1.9). . k., k, = Conveyance of that portion of the natural flood plain obstructed by the roadway em- bankments (subscripts refer to left and 6 J