|
<br />10
<br />
<br />TECHNIQUES OF WATER-RESOUI<<;ES INVESTIGATrONS
<br />
<br />Roughness Coefficients
<br />
<br />Roughness coefficients for use in the Manning
<br />equation should be selected in the field for both
<br />the approach section and the culvert at the timc
<br />of the field survey.
<br />
<br />Approach section
<br />
<br />Select roughness coefficients for the approach
<br />section as outlined in the discussion of "Field
<br />Data," These coefficients will usually be in the
<br />range between 0,030 and 0.060 at culverts, be-
<br />cause stream channels are usuaily kept cleared
<br />in the vicinity of the culvert entrance, At times
<br />the approach roughness coefficient may be
<br />lower than 0,030 when the culvert "pron and
<br />wing-walls extend upstre,am t.o, or throllg-h, t.he
<br />ll.pproH.ch sect.ion.
<br />SeleeL points of ~nbdivisioll of t.he cross :-\lW-
<br />tion in the field and assign va.llles of n to the
<br />various parts. For crest-:-;t,age g'uges where vari-
<br />ous headwater elevations are used, n nnd the
<br />points of subdivision may ehange. For these
<br />sections, note the elevations at which the
<br />change, take place,
<br />
<br />Culvert
<br />
<br />Field inspection is always necessary before "II
<br />values are assigned to any culvert. The condi-
<br />tion of the material, the type of joint, lwd the
<br />kind of bottom, whether natural or constructed,
<br />all influence the selection of rOllg'hncss co-
<br />efficients.
<br />
<br />Corrugated metal
<br />
<br />.A number of laboratory tests hnsc been fUll
<br />to det.ermine the roughness coefficient for eOf-
<br />rugllted-metal pipes of llll sizes,
<br />
<br />Standard riveted seclion
<br />The corrugated metal used in the mnoufac-
<br />t lire of standard pipes llnd pipe-nrches has It
<br />2%-inch pitch with II rise of % inch. According
<br />(0 laboratory tests (Neill, 1962), 11 mlues for
<br />full pipe flow vary from 0,0266 for II 1-foot-
<br />diamete,' pipe to 0,0224 for all 8-foot-dillmeter
<br />pipe for the \'elocities normally encountered in
<br />CUh'(ll'tS. Tests indicate tlmt. n is slightly smaller
<br />
<br />for pipes flowing- part. full than for full pipe How.
<br />l'he following- Ill'e the result.s of test~ by Neill
<br />(1962), :wel these nllues lllUY be used:
<br />
<br />]'ipedillTnetu
<br />(root)
<br />L_
<br />
<br />11
<br />0.027
<br />.1)2'-,
<br />,024
<br />,02:J
<br />,022
<br />
<br />~---"
<br />
<br />:1-4. __
<br />;')-7__
<br />8___,
<br />
<br />A single value of 0.024 is considered satisfactory
<br />for both Pllrtly full anrl full pipe flow for most
<br />computations. This llPplies to nIl rivet.ed pipe:-:.
<br />and pipe-arche..~ of standard size......
<br />
<br />Multiplole section
<br />In multiplate construction the corrugations
<br />,are much larger, having a 6-inch pitch with a
<br />2-inch rise. Tests show n vulues to he some-
<br />what higher than for riveted-pipe construction.
<br />Avcru,ge n values froIIl various experiment.....
<br />rang-e from 0.034 for u. 5-foot-cliameter pipe
<br />to 0.027 for " 22-foot pipe. A straight lille
<br />relationship of n values is assumed to exist for
<br />diameters between 5 and 22 feet, Use the follow-
<br />ing ronghness coefficients:
<br />
<br />Pipe diam(trr
<br />(leet)
<br />5-6_________________ ____________
<br />7-8_ _ h_' _______ _ ___, ___________
<br />9-IL__ '______, '________________
<br />12-13_______ _________,_,_________
<br />14-1.'-________________ _______,___
<br />16-18__ _ __ ___ _ __
<br />19-20_________________ ___________
<br />21-22_______ _____________'_______
<br />
<br />"
<br />
<br />0.034
<br />,033
<br />,032
<br />.031
<br />.030
<br />.029
<br />.028
<br />.027
<br />
<br />A corruguted pipe with eorrugations half the
<br />size of those in Ilmltiplate eonstruction, 3-inch
<br />pitch with It I-inch rise, is being made in both
<br />standard and multiplate sections, Until llctllal
<br />tests are run to obtain n values, use average
<br />roughness coefficients between equal sizes of
<br />standard "nd mllltiplate sections~for example,
<br />use an n value of 0,028 for a 7-foot diameter
<br />pipe.
<br />
<br />Paved inverts
<br />In many instances the bottom parts of cor-
<br />rugated pipe and pipe-arch cllh~erts are paved,
<br />usually with a bituminous mllteriaJ. This re-
<br />dllces the roughness coefficient to II value
<br />between that normlllly used llnd 0,012. The
<br />reduction is directly proportionlll to the amollnt
<br />of paved surface llrea in contact with the wllter,
<br />
<br />e
<br />
<br />,
<br />
<br />e
<br />
<br />,
<br />
<br />e
<br />
|