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Handbook of Steel Drainage & Highway Construction …

INTRODUCTIONMany millions of dollars are spent annually on culverts, storm drains and subdrains,all vital to the protection of streets, highways and railroads. If inadequately sized,they can jeopardize the roadway and cause excessive property damage and loss oflife. Over design means extravagance. Engineering can find an economical , soil and climate are extremely variable, so Drainage sites should bedesigned individually from reasonably adequate data for each particular site. Inaddition, the designer is advised to consult with those responsible for maintainingdrainage structures in the area. One Highway engineer comments:"With the exception of the riding qualities of the traveled way, no other singleitem requires as much attention on the part of maintenance personnel as highwayculverts.

130 STEEL DRAINAGE AND HIGHWAY CONSTRUCTION PRODUCTS The design engineer with needs beyond the scope of this handbook may refer to the CSPI publication, “Modern Sewer Design” and AISI “Design Charts for Open

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Transcription of Handbook of Steel Drainage & Highway Construction …

1 INTRODUCTIONMany millions of dollars are spent annually on culverts, storm drains and subdrains,all vital to the protection of streets, highways and railroads. If inadequately sized,they can jeopardize the roadway and cause excessive property damage and loss oflife. Over design means extravagance. Engineering can find an economical , soil and climate are extremely variable, so Drainage sites should bedesigned individually from reasonably adequate data for each particular site. Inaddition, the designer is advised to consult with those responsible for maintainingdrainage structures in the area. One Highway engineer comments:"With the exception of the riding qualities of the traveled way, no other singleitem requires as much attention on the part of maintenance personnel as highwayculverts.

2 Many of the problems of culvert maintenance stem from the fact thatdesigners in all too many instances consider that culverts will be required to transportonly clear is a condition hardly ever realized in practice, and in manyinstances storm waters may be carrying as much as 50 percent detrimental change in grade line at the culvert entrance can cause complete blockage ofthe culvert. This results in overflow across the Highway and in some cases,especially where high fills are involved, the intense static pressure results in loss ofthe embankment." HYDRAULICS OF OPEN Drainage CHANNELSG eneralBefore designing culverts, storm sewers and other Drainage structures, one shouldconsider the design of ditches, gutters, chutes, median swales, and other channelsleading to these structures.

3 (See Figure ).129 HYDRAULICSCHAPTER 4 Figure of roadside Drainage channels. 130 Steel Drainage AND Highway Construction PRODUCTSThe design engineer with needs beyond the scope of this Handbook may refer tothe CSPI publication, Modern Sewer design and AISI design Charts for OpenChannel Flow . These include numerous examples of calculations and references onall aspects of the subject. Rainfall and runoff, once calculated, are followed by the design of suitablechannels to handle the peak discharge with minimum erosion, maintenance andhazard to AASHTO publication "A Policy on Geometric design of Highway andStreets" states: "The depth of channels should be sufficient to remove the waterwithout saturation of the pavement subgrade.

4 The depth of water that can betolerated, particularly on flat channel slopes, depends upon the soil open country, channel side slopes of 5:1 or 6:1 are preferable in order to reducesnow drifts."Systematic maintenance is recognized as essential to any Drainage channel andtherefore should be considered in the design of those EquationChezy developed a basic hydraulic formula for determining the flow of water,particularly in open channels. It is as follows:Q =AVif:V = c then: Q = Ac where: Q = discharge, m3/sA=cross-sectional area of flow, m2V=mean velocity of flow, m/sc=coefficient of roughness, depending upon the surface over which water is flowing, m1/2/sR = hydraulic radius, m =WP= wetted perimeter (length of wetted contact between water andits containing channel), mS=slope, or grade, m/mThis fundamental formula is the basis of most capacity s EquationManning s equation, published in 1890, gives the value of c in the Chezy formulaas:c= where:n = coefficient of roughness (see Tables and )AWPR1/6nRSRS4.

5 HYDRAULICS131 Type of channel and descriptionn1. LINED OR BUILT-UPA. Concrete - Trowel Finish.. Concrete - Float Finish .. Concrete - Unfinished.. Gunite - Good Section .. Gravel Bottom with sides of:1) Formed Concrete.. ) Random Stone in Mortar .. ) Dry Rubble or Rip Rap .. EXCAVATED OR DREDGED - EARTHA. Straight and Uniform1) Clean, Recently Completed.. ) Clean, After Weathering.. ) Gravel, Uniform Section, Clean.. ) With Short Grass, Few Weeds .. Winding and Sluggish1)No Vegetation .. ) Grass, Some Weeds .. ) Dense Weeds, Deep Channels .. ) Earth Bottom and Rubble Sides .. ) Stony Bottom and Weedy Banks.. ) Cobble Bottom and Clean Sides .. CHANNELS NOT MAINTAINED, WEEDS & BRUSH UNCUTA.

6 Dense Weeds, High as Flow Depth .. Clean Bottom, Brush on Sides .. Same, Highest Stage of Flow.. Dense Brush, High Stage.. s nfor constructed channelsManning snfor natural stream channelsSurface width at flood stage less than 30 mTable regular section:a. Some grass and weeds, little or no brush.. growth of weeds, depth of flow materially greater than weed height.. Some weeds, light brush on banks.. Some weeds, heavy brush on banks .. Some weeds, dense willows on banks .. For trees within channel, with branches submerged at high stage, increase allabove values by .. Irregular sections, with pools, slight channel meander; increase values given above about.. Mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stage:a.

7 Bottom of gravel, cobbles, and few boulders .. b. Bottom of cobbles, with large boulders .. Drainage AND Highway Construction PRODUCTSThe complete Manning equation is:V= Combining this with the Chezy Equation results in the equation:Q =In many calculations, it is convenient to group the channel cross sectionproperties in one term called conveyance, K, so that:K =Then:Q =KS1/2 Uniform flow of clean water in a straight unobstructed channel would be a simpleproblem but is rarely attained. Manning s formula gives reliable results if thechannel cross section, roughness, and slope are fairly constant over a sufficientdistance to establish uniform Use of Charts and TablesWhile design charts for open-channel flow reduce computational effort, they cannotreplace engineering judgment and a knowledge of the hydraulics of open-channelflow and flow through conduits with a free water charts contain the channel properties (area and hydraulic radius) of manychannel sections and tables of velocity for various combinations of slope andhydraulic radius.

8 Their use is explained in the following 1 Given: A trapezoidal channel of straight alignment and uniform cross section inearth with a bottom width of m, side slopes at 1:1, a channel slope m/m, and a normal depth of water of : Velocity and on Table , for an excavated channel in ordinary earth, n is taken as Cross-sectional area, A, is m2[ * ( + 1 * )].3. Wetted perimeter, WP, is m [ + 2 * * (12+1)1/2].4. Hydraulic radius, R, is m [ / ].5. Using the nomograph in Figure , lay a straight edge between the outerscales at the values of S = and n = Mark where the straight edgeintersects the turning Place the straight edge to line up the point on the turning line and thehydraulic radius of Read the velocity, V, of m/s on the velocity Discharge, Q, is m3/s [ * ].

9 R2/3S1/2nAR2/3S1/2nAR2/3n4. HYDRAULICS133 Figure Nomograph for solution of Manning s Drainage AND Highway Construction PRODUCTSF igure provides the means to calculate a trapezoidal channel capacity for aspecific bottom width, channel slope, side slope, n value and a variety of flow a given Drainage project, these variables are known or determined using knownsite parameters through trial and error. The flow rate, Q, can then be Capacity of trapezoidal nExample 2 Given: Bottom width, b = mSide slopes @ 2:1, so z = 2 Roughness coefficient, n = (from Table for grass and weeds, no brush)Channel slope, S = m/mDepth to width ratio, = (flood stage depth)Find: Depth of flow, d, and flow rate, :Depth, d = ( ) = mFrom Figure := := :Q = m3/sIf the resulting design is not satisfactory, the channel parameters are adjusted andthe design calculations are VelocitiesThe ideal situation is one where the velocity will cause neither silt deposition norerosion.

10 For the design of a channel, the approximate grade can be determined froma topographic map, from the plan profiles, or from prevent the deposition of sediment, the minimum gradient for earth and grass-lined channels should be about percent and that for smooth paved channels guidelines for permissible velocities are provided in Tables More comprehensive design data may be found in the FHWA s HEC 15( design of Stable Channels with Flexible Linings).Channel ProtectionCorrugated Steel flumes or chutes (and pipe spillways) are favored solutions forchannel protection especially in wet, unstable or frost susceptible soils. They shouldbe anchored to prevent undue shifting. This will also protect against buoyancy anduplift, which can occur especially when empty.


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