Presentation on theme: "Crossing Structures Crossing structures are those constructed at intersections of: 1. Waterway-road (culvert or bridge). 2. Waterway-Waterway (syphon or."— Presentation transcript:
1Crossing StructuresCrossing structures are those constructed at intersections of:1. Waterway-road (culvert or bridge).2. Waterway-Waterway (syphon or aqueduct).3. Waterway ending at another waterway (tail escape).
2Culvert or BridgeRoadWaterwaySyphon or AqueductWaterwayTail escapeWaterway
4The culvert is a closed conduit (pipe or box section) constructed to carry the discharge of a waterway under a road.The selection of the type of the culvert is based on the discharge as follows:For Q ≤ 3.0 m3/sec use Pipe culvert.For Q ≤ 12.0 m3/sec use Box section.
6Hydraulic DesignThe purpose of the hydraulic design is to select the culvert’s dimensions.The dimensions are selected based on:Velocity through culvert between 1.0 to 2.0 m/sec.Head losses is less then 15 cm.Entrance submergence is at least 30 cm.
7The only equation available is the continuity equation: Q = A . VWhere Q is the discharge in m3/sec, A is the culvert’s cross section area in m2, and V is the water velocity through the culvert in m/sec. Since the discharge is known, the velocity must be assumed to determine the dimensions of the culvert. The velocity is usually assumed between 1.0 and 2.0 m/sec for practical reasons.
8Once the dimensions are determined, then head losses must be checked Once the dimensions are determined, then head losses must be checked. Head losses are calculated as follows,Where Hl is the head losses in mV is the velocity in m/secg is the acceleration of gravity
9Ce is the entrance coefficient of loss (= 0.5 )Cf is the friction coefficientCo is the outlet coefficient of losses( = 1.0 )Friction coefficient can be calculated as follows,
10Where f is coefficient depends on the culvert material and dimension, L is the length of the culvert and m is the hydraulic radius.The coefficient a and b are constant and depends on the material of the conduit.For Steel: a = , b =For Concrete: a = , b=
11Example 1:Design a culvert under a road. The discharge is 3 m3/sec and the water depth is 1.8 m. The culvert length is 20 m.Sol.Q = 3 m3/sec * use pipe culvertassume V=1.3 m/secA = Q/V = 2.31 m2A = ∏ D2/ * D = 1.7 mHowever, D ≤ 1.8 – … 1.5 m
12Then use 2 pipes …one pipe A = 1.155 m2 D = 1.3 m …. V = 1.13 m/sec … OKCheck of heading upm = D/4 = mf =Cf = 0.32Hu = (1.13)2/(2*9.81) *( )= 0.12 m … OK
13Example 2:design a culvert under a road. The discharge is 5 m3/sec and the water depth is 2.2 m. The culvert length is 20 m.Sol.Q = 5 m3/sec … Use Box culvertAssume V = 1.3 m/secA = Q/V = 3.85 m2H = 2.2 – 0.3 = 1.9 mS = 3.85/1.9 = 2.0 m
14S < 1.5 H … OKV = 5/(1.9*2) = 1.32 m/secCheck for Heading Upm = 2*1.9/(2+1.9)*2= 0.49 mf =Cf = 0.14Hu = m … OK