FLOOD06873 - Laserfiche WebLink

Home Browse Search

. . 61R HYDRAULIC ENGINEERING ")4 describes datu-collection and anatylicallCchniqucs Lo maximize the utility of informalion obtained in a roughness-coefficient study. DESIlON OF STUDY The roughness-coefficient study was designed to obtain accuralc records or measure- ments of discharge and waler-surface elevations that would cover the rangt of discharge and stage thai might he expected during the study period at a given sileo l1lis design en- abled computation of roughness coefficients throughoutlhe full range of discharges and permitted analysis of lhe change in the n value with increasing llow depth. Sile Selection Study siles were selected on Ihe basis of criteria described by Dalrymple and Benson (1967) for indirect measurement of discharge by the slope-area mclhod. Jarrett and PcLsch (1985) give specific guidance forscleclion of reaches to be used for a roughnc~s-coemcicnl sludy. The reaches wcre selected 10 meet the following qualilicalions: relatively straight: unifonn shape and vegetation cover within and. upstream and downstream from. the n~"ch boundaries; fully effcctive now area through the reach: minimal overbank now; and ab. sente of major now-resisting factors other than boundary friction from bed material and strcambank vegetation. To avoid computational uncertainties Ihal arise from assumptions made in estimating energy losses due 10 e~pansion in the channel. unifonn or slightly con- tracting reaches are preferred over eltpanding reaches, Benson and Dalrymple (19h1) givc guidance on the recommended length of the study reach. In the New York study. siles were chosen at or near existing streamnow.monitoring stations wilh stable high-now stage-to- discharge relations to enable easy and accurate computation of roughness coefficients lor many discharges. If a strcamnow-monitoring station is not nearby. a site that is accessible along the study reach during high nows and has a means by which discharge measurements can safely be made would be suitable. Sile selection should not be made by visual inspection alone because conditions hclow the water surface (such as extreme changes in channel shape) can cause variability in the n value and difficully in the subsequent analysis of the computed n values. A preliminary slope-area computation of discharge for a recent nood or a step-backwater analysiS of channel hydraulics can avert analytical diOicuhies by identifying adverse conditions. such as nonuniform channel conditions. expansion losses within the reach. inadequate fall, un- foreseen causes of backwater, and substantial changes in a panicular now-rclarding faclor, such as vegetation type or density, If channel conditions arc significantly different within a reasonable distance of the dis. charge.measurcment location. a second reach can be selected. and two sets of n-value dala collected for the same discharge data. This procedure provides a means for assessing Ihe effect of the different now-resistanl factors between the twO sites. ^ similar comparison can be made at sites where the roughness faeton change significantly during the sludy period, These changes can result from a large nood or from routine channel.maintcnance work. such as dredging. brush clearing. or channelization. prior knowledge of channel- maintenance work can be optimized 10 obtain beTore and after n-value computations, Data Collection The components of data collection for a roughness-coefficient sludy include instrumen- tation of the reach, recording of water-surface profiles. measurement of discharge and streambed-panicle size. evaluation of major idenlifiable now.returding faclors. and photo- graphing the study reach. . ROUGHNESS-COEFFlCIENT STUDY-DESIGN 671) Reach Instrumenfilllon The study rcm.:hes were surveyed ilnd r' . standard (USGS) . d . . '. e o.'iS sectluns were localed in accordance 'm . proce ures outhned 10 Bemon and Oal m I WI resenl the channel geometry of the reach (the av r ,ry p e (1967),10 adequiltcly rep- quires at least Ihree cross sectioR.'i' fo r r cage Slle and shape of the channel) re- gages (Rantz and others' 1982 P'11)U or ~ve are preferable. Slandard USGS cresHUagc . . . .. were Inslalled at each . surlacc profiles of high lIows that Ot:curred be w '. ' cross secllon to record walcr- he needed to cover the expecled ran c . t cen mspccuons, More Ihan one gage may stage data is to install continuous aug. '"l~lage, Anln alternative melhod or collecling .he , ' oma IC reeo ers at each' " I . IOstrumenlation on one bank should be n'" sec IOn. n umronn reaches su IClent. If the h . . to-bank water-surface discrepancies are Clt ct ' . reae IS on a bend such thai bank- Ihe cross section. AI least two elevation rcr;:re~~~ a:,lge should be inslall~d al each cnd of gage hi ,:nablc stability checks during the I d A a;kS should. be eSlahllshed near each surface can be convcnicnlly and -al.cly 5 u Y'd fC. erence polOt. from which (he waler I. h ... measure dunng high n ho IS ed~ Gage elevations should be ch k d d ' OWS, 5 uld also be estab- ec e urlng or at the end of the sludy. Water-surface Profiles Waler-surface profites were ohtained f . crest-stage gages or measured from the rcf~:;ater~surface elevallons recorded on lhe measured during rising and receding I e pomls. If waler-surface e1cvalions arc an auempt should be made 10 do so ,.5 ageshal eac~ of the cross sections during high' Rows d ' n as s on a lime I"II"riod 'bl .. unng waler-surface mcasuremenls could . I. f"- as poSSI e. Slage changes turn would introduce variabilily inl~ the rcs~~~nll: erroncous slope ~~Iculalions. which in venllhc potenlial erroB associated with dela .' g va!ues. Aulomallc recorders would pre- elevations at the crest-slage gages because :s In Ihe tlm~ly me~~urcment of waler-surface water-surl'ace prolilc. ( cy can proVide an mstantaneous record of the Discharge Measuremenl The discharge associaled with each recorded Ihe discharge record of Ihe nearby slreamD w~ler.surfacc prol1le was oblained from were frequently measured (following stand~~.~~~~ring station, H~g~-wat~r discharges olhcB. 1982) at each site durin the stud . proced,u,res descnbcd In Ranlz and relation. If a discharge record ~ unavail~~lo c:nlrr;" the stability of Ihe slage-to.di~harge discharges measured al slages (hat c e, eve op a slage-Io-discharge relation from the cross seclio~s. or us~ the measu:~e~::hranged~f water-surface elevations recorded at lowed, the waler-surface elevations should .,:r~~ ~rectly. If the lauer procedu.rc is 1'01- measurement. and averaged values should be use a. ured before an~ after the discharge that any computation of the roughness em' d ffor the calculallon of the n value. Note ceding stages mighl renect a wiIter-surfa:: ~~~:"h~~ dd~~ COneCrled during .rising. or re- slope. f"- a IS IlIcrenl rom a ma:ltlmUm stage I' .. Measurement or Streambed-particle Size n many situations. streambed pani I' ., , simply by inspection of the channel bote e ~Il.e and size ~Istrlbutlo~ arc ol'tcn evaluillcd quired for a roughness-coenicient stUdyl~m. a more dCla.lled analYSIS of Ihese faclors is fC- which methods of dala collection ;0 u~ ~~ever. The sIze of Ihe bed material delennincs present methods for obtaining fCpreSC 't 't' 0 man :1954).and Kel~erhals and Bray (1910) bed material. Benson and Dalr m Ie nl~ Ivesamp es ofs~1.e and S17.e distribution of coarse covcrthe full range of particle~i1.~S ~Uy61( :9d6c9SC) rdi.be.sedlment.meaSUring techniques thai . ., ISCUSseS laboratory theory and methods