|
<br />SIZING HYDRAULIC STRUCTUREl~
<br />
<br />There are three ba~ic types of hydraulic stmctures that wiU be touched on in l;his handout. This handout is
<br />intended to be a supplement, not a replac,:ment for a textbook on hydrauli,;s, Several widely published
<br />charts and tables have been reproduced from other sources, Reference; are provided,
<br />
<br />The goal of this paper is to provide a groundwork for basic lmderstanding of hydraulic principles as they
<br />relate to;
<br />
<br />L Open Channels
<br />2; Storm Sewers, and
<br />3; Culvens,
<br />
<br />OVERVIEW OF FUNDAMENTAL CONCEPTS
<br />
<br />One of several equations for solving various parameters of flow and design cf thl: above structures wiU be
<br />presented and developed herein, The most commonly used equation for determining flow in an open chan-
<br />nel and storm seWllrs is Manning's Equation, which is well documented for provi(ling reliable results when
<br />properly applied,
<br />
<br />2 1
<br />1.486 A R' S'
<br />0=-
<br />, n "
<br />
<br />(1)
<br />
<br />Where;
<br />Q = Flow (cubic feet per second)
<br />n = Manning's Coefficient
<br />A = Area of Flow
<br />R = Hydraulic Radius
<br />S, = Slope (ftIft)
<br />
<br />To solve for flow using equation (I), you must know a variety of parmneters regardiug the channeL Vir-
<br />tually an infinite variety of shapes and materials are used in channel construction, Earth lined channels,
<br />rock channels, concrete pipe, corrugated metal pipe, log flumes, curb and guUer, and on and on. Figure I
<br />on page 2 shows the channel parameters that are required,
<br />
<br />The velocity of flow, V, normally refers to the average veloci'~, of flow in the chaImel. The absolute veloc-
<br />ity of flow varies widely within the channel. The velocity is necessarily zero at the flow boundary (the
<br />channel wall) and varies to its maximum at the water surfae<:. This is not necessarily true in the case of
<br />storm sewers, however, The absolute velocity at the water surface will vary in respect to below the surface
<br />as the depth of flow in the sewer varies, This is because as the flow re~ches the lOp of the pipe, the surface
<br />velocity approaches zero and the maximum velocity win be near the center of the Ipipe, The average veloc-
<br />ity is found by dividing the flow by the area of flow; V = Q/A, This will be discussed in greater detail in
<br />other sections.
<br />
<br />Manning's coefficient, n, is a resistance factor that is influenced by a numlxlr of variables, The primary
<br />influence on n is the roughness of the Channel. Other factors that play a role in selection of n include
<br />bends, depth of flow, expected vegetation growth, and others, Selection of a roughness coefficient is criti-
<br />cal to accurate results, When selecting a value for concrete or other man-made materials, a fairly accurate
<br />value can be readily obtained, but in the = of natural channels, judgement and e:xperience must be called
<br />in to play, and the selected value may provide results that are Q,uite inar:curate, Table 1 gives representative
<br />values of n commonly used for various surfaces and situations,
<br />
<br />The Area of flow, A, is the cross sectional area through which the fluid passes. For example, in a
<br />rectangular channel 3 feet wide, 3 feet deep flowing haJffuU, the area offiow would b<: 1,5x3 or 4.5 ft'. In
<br />
<br />Page 2
<br />Greenhorne & O'Mara, Inc.
<br />
|