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<br />MEASUREMENT OF PEAK DISCHARGE AT CULVERTS BY INDIRECT METHODS II
<br />
<br />or wetted perimeter. The composite value of n
<br />for standard pipes and pipe-arches may be
<br />computed by the equation,
<br />
<br />n, 0.012P.+0:24(P-P.), (12)
<br />
<br />where
<br />
<br />P .=Iength of wetted perimeter that IS
<br />paved, and
<br />P=totallength of wetted perimeter.
<br />
<br />For multiplate construction the value of
<br />0.024 must be replaced with the correct value
<br />corresponding to the size of the pipe,
<br />
<br />Concrete
<br />
<br />The roughness coefficient of concrete is de-
<br />pendent upon the condition of the concrete and
<br />the irregularities of the surface resulting from
<br />construction. Suggested values of n for general
<br />use are:
<br />
<br />Con.dUlon of CMU::1d~
<br />Very smooth (spun pipe)u____
<br />Smooth (cast or tamped pipe)__
<br />Ordinary field constructioD_____
<br />Badly ,palledm __ _ _ _ __ _ _
<br />
<br />n
<br />
<br />0,010
<br />.011-0,015
<br />, 012- , 015
<br />, 015- , 020
<br />
<br />At times, sections of concrete pipe become
<br />displaced either vertically or laterally, resulting
<br />in a much rougher interior surface than normal.
<br />Where this occurs, increase n commensurate
<br />with the degree of displacement of the culvert
<br />sections. Laboratory tests have shown that the
<br />displacement must be considerable before the
<br />roughness coefficient is very much affected.
<br />Slight bends or changes in alinement of the
<br />culvert will not affect the roughness coefficient.
<br />However, the effects of fairly sharp bends or
<br />angles can be compensated for by raising the n
<br />value, as is done in slope-area measureIJ1ents.
<br />Russell (1935) showed that for extremely sharp
<br />bends (900) the head loss may vary from 0.2 to
<br />to 1,0 times the velocity head, depending on the
<br />radius of the bend and the velocity, The lower
<br />value applies to velocities of 2 or 3 feet per
<br />second and radii of 1-8 feet, and the higher
<br />value to velocities of 15-20 feet per second and
<br />radii of 40-60 feet, King (1954) stated that the
<br />losses in a 450 bend may be about %: as great,
<br />and for a 22~0 bend about ~ as great as those
<br />of a 900 bend.
<br />
<br />Other materials
<br />
<br />Occasionally culverts will be constructed of
<br />sorne material other than concrete or corrug'llted
<br />metal. Mannin!('s coefficients (Kin!(. 1954) fol'
<br />some of these materials are:
<br />
<br />Material
<br />Welded steeL_______ ________
<br />Wood stave__________
<br />Cast iron_un___ _ ___
<br />Vitrified clay __ __ _ _ __ _ _ __ _ _ _
<br />Riveted steeL _ _ _ _ _ __
<br />
<br />n
<br />0,012
<br />,012
<br />, 01~
<br />, 01~
<br />.015
<br />
<br />Culverts made from cement rubble or I'ock
<br />may have roughness coefficients ranging from
<br />0,020 to 0,030, depending on the type of
<br />material and the care with which it is laid,
<br />
<br />Naturol bottom
<br />
<br />Many culverts, especially the large arch type,
<br />are constructed with the natural channel "'
<br />the bottom, The bottom I'oughness usually
<br />wei!(hts the composite roughness coefficient
<br />quite heavily, especially when the bottom is
<br />composed of cobbles and large angular rock,
<br />The formula used for paved inverts can be used
<br />here if the correct n values are substituted
<br />therein,
<br />
<br />Computation of Discharge
<br />
<br />The first step in the computation of discharge
<br />is to determine the type of flow, Under low
<br />heads, headwater-diameter ratios less than 1.5,
<br />type 3 flow will occur if the elevation of the
<br />downstream water surface is higher than the wa-
<br />ter-surface elevation at critical depth. If the tail-
<br />water elevation is lower than the water-surface
<br />elevation at critical depth, type 1 flow will OCCUI'
<br />with the bed slope of the culvert greater than
<br />the critical slope, or type 2 flow will occur with
<br />the bed slope less than the critical slope. Type
<br />5 or 6 flow will occur with high heads, head-
<br />water-diameter ratios greater than 1.5, de-
<br />pending on the steepness of the culvert and the
<br />entrance conditions.
<br />Discharge coefficien ts are a vital part of each
<br />culvert computation, These are discussed in
<br />detail on pages 37-45.
<br />Tables 2-4 have information that applies to
<br />circular sections, riveted pipe-arches, and mul-
<br />tiplate pipe-arches, Figures 5-8 are graphs
<br />
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