STREAMFLOW CHARACTERISTICS OF THE LOWER PASCAGOULA RIVER, MISSISSIPPI

Transcription

1 STREAMFLOW CHARACTERISTICS OF THE LOWER PASCAGOULA RIVER MISSISSIPPI D. Phl Turnpseed and John B. Storm U.S. Geologcal Survey Jackson Msssspp INTRODUCTION Fresh surface-water dscharge s an abundant and mponant nalurai resource n the lower Pascagoula Rver Basn n Jackson County. Msssspp. but much s unknown of the real-tme flow condtons (dynamc channel geometry. velocty dstrbuton and drecton. and other hydraulc characterstcs) n complex estuarne and tde-affected rverne systems. The approprate desgn of hydrodynamc flow models requres adequate understandng of the physcal attrbutes of these complex flow phenomena. In November the U.S. Geologcal Survey (USGS) constructed a contnuous-recordng stage and velocty streamgage at the Pascagoula Rver at Graham Ferry to gan a better understandng of the effect tdes from the Gulf of Mexco have on freshwater dscharge n the lower Pascagoula Rver Basn. The Msssspp Department of Envnonmental Qualty. Offce of Land and Water Resources (OLWR). currently (1995) s nvolved n ntensve envronmental studes of the lower Pascagoula Rver Basn n cooperaton wth the USGS. the Msssspp Department of Wldlfe and Fsheres. the Msssspp Depanment of Marne Resources. and other State and county agences. In accordance wth the goals of these studes. the USGS surveyed channel-bed cross sectons. thalweg (lowest pont of flow) profles. water-surface profles. and geographc postons on the lower Pascagoula Rver Basn n Aprl and May Usng state-of-the-scence sgnal processng technology known as pulse-to-pulse coherent sonar (RD Instruments 1993). the USGS. n cooperaton wth the OLWR. profj.led three-dmensonal veloctes usng an Acoustc Doppler Current Profler (ADCP) [the use of trnde or product names n ths repon s for dentfcaton purposes only and does not consttute endorsement by the USGS) at fve locatons n about an 18-m (mle) reach of the lower Pascagoula Rver n October Pulse-to-pulse coherent sonar technology greatly enhances the calbraton. performance. evaluaton. and desgn of hydrodynamc flow models used to smulate the complex flow n rverne and esluarne systems. Velocty measurements made usng ths technology were also smultaneously compared to measurements obtaned usng standard USGS methods of measurng velocty (Rantz and others 1982). Purpose and Scope Ths repon descrbes and presents streamflow data collected by the USGS n for the lower Pascagoula Rver. These data are correlated wth data collected usng conventonal streamgagng methods. an ADCP to measure three-dmensonal velocty profles. and a contnuous-recordng acoustc velocty meter (AVM). Examples of contnuous stage. velocty. and dscharge relatons computed for data obtaned from the USGS contnuous recordng stage and acoustc velocty streamgage at the Pascagoula Rver at Graham Ferry are presented. Descrpton of tbe Study Area The study area encompasses the lower 35 m of the Pascagoula Rver n Jackson County. Msssspp (Fgure I). The dranage area at rver mle 0.0 (second per upstream from the Msssspp Sound and 0.8 m upstream of!he mouth on the left (east) bank) of the Pascagoula Rver s 9498 m2 (square mles). The dranage area at the USGS contnuous recordng stage and velocty gage at the Pascagoula Rver at Graham Ferry (staton no ) at rver mle 34.6 s m2. The length of the channel from the gage to the basn dvde s about 230 m. and the average slope of the channel between ponts located at 10 and 85 percent of the length upstream of the gage s about 1.6 ftlm (feet per mle). Average channel and valley slopes n the vcnty of the gage are about 0.6 ftlm;. The dranage area of the Escatawpa Rver at ts confluence wth the East Pascagoula Rver at rver mle 6.8 s 1037 m2. DATA COLLECTION Data descrbng streamflow characterstcs were collected on the lower Pascagoula Rver durng shon-term sludes n Aprl and October 1994 and collected contnuously snce November 1993 at the USGS streamgage at the Pascagoula Rver at Graham Ferry. These data were used by the USGS. other government agences and prvate nterests to descrbe and document flow n the lower Pascagoula Rver durng ths perod. The data-collecton phase of ths study s documented to detal not only what lypes of data were 258

2 collected. bui when and how these data were gamered. Ihereby provdng detals that help document and better defne the methodology used n Ihe analyss phase of the study. Ths phase also establshed baselne nformaton for future studes n me area. November 1993 to Present (1995) Snce November me USGS has recorded stage. velocty. and other parameters at 30-mnute ntervals through a Data Collecton Platform (OCP) at the Pascagoula Rver al Graham Ferry. These data are transmlled by satellte to me USGS at 4-hour ntervals. Stage s measured usng a submersble transdueer attached to the rghl (west) man per of me brdge. The transducer measures water pressure n pounds per square nch and then eonvens me pressure 10 feet of water and transmts ths value to the OCP where t s recorded. Velocty s measured and transmlled to the DCP usng an AVM whch receves data from two transducers; one s mounted on Ihe thrd man per east of the rght (west) abulment. and the second s mounted on a ple eluster near the rght (wesl) bank aboul 220 fl downstream of Ihe brdge. Dscharge measurements are made at me ste al 6-week ntervals wm elher a standard current meter attached to a soundng weght from the downstream sde of the brdge or wth an AOCP mounted to a boat. Aprl 1994 On Aprl the USGS surveyed me lower 35 mles of the Pascagoula Rver to collect channel geometry data mat neluded channel-bed cross sectons. thalweg elevatons. and water-surface elevatons. Cross sectons were surveyed from a boal wm a graphcal fathometer and georeferenced wth a mappng-grade global postonng system (GPS). These data were then dgtzed for later use n a geographc nformaton system (GIS). Fony-seven cross sectons were surveyed ncludng: 8 cross sectons n the man channel of me Pascagoula Rver; 18 cross sectons n me East Pascagoula Rver; 18 cross sectons n Ihe West Pascagoula Rver; and 3 cross sectons n the Escatawpa Rver. Thalweg ponts were fathomed at near quaner-mle ntervals along the Pascagoula Rver. me East and West Pascagoula Rvers. and me Escatawpa Rver. A total of 185 thalweg ponts were surveyed and georeferenced wth mappng-grade GPS equpment. Durng the perod me channel geometry was surveyed from Aprl me rver at Graham Ferry (rver mle 34.6) ncreased n stage from 15.0 ft (1000 hrs (hours) central daylght lme. Aprl 19) to a peak of 15.4 ft (1400 hrs. Aprl 21) and decreased ft (1200 hrs. Aprl 23). Relatve changes n stage were smlar durng ths perod to changes n stage at Cumbest Bluff (rver mle 25.4) and to changes at me dvergence of the rver nto an easi and west channel (rver mle 17.3). May 1994 On May me USGS surveyed temporary benchmarks establshed durng the Aprl study usng survey-grade GPS equpment. Ths survey establshed accurate elevatons (wthn 2-cm (centmeter) accuracy) as well as horwntal coordnates geoleferenced to Ihe Unversal Transverse Mercator coordnate system for all temporary benchmarks used n me Aprl study. The surveyed channel geometry and water-surface slopes were measured n the followng rver reaches: (1) me moum of the East Paseagoula Rver at rver mle -0.8 to the dvergence at rver mle 17.3; (2) the rnoum of Ihe West Pascagoula Rver at rver mle 0.7 to me dvergence at rver mle 17.3; (3) the dvergence of the East and West Pascagoula Rver at rver mle 17.3 to the USGS comnuous recordng stage and velocty streamgage al rver mle 34.6; and (4) the mouth of the Escatawpa Rver at ts confluence wth me East Pascagoula Rver at rver mle 6.8 to rver mle 2.8 on the Escatawpa Rver. October 1994 On October 10-1 I the USGS measured dscharge usng an AOCP at fve locatons and a Prce AA standard currem meter at one locaton n about an 18-m reach of the Pascagoula Rver. Water-surfaces elevatons n the reach were also montored durng ms perod. Dscharge and water-surface elevatons were measured at the followng rver locatons: (I) the East Paseagoula Rver at rver mle 17.0; (2) the West Pascagoula Rver at rver mle 17.0; (3) the dvergence of the Pascagoula Rver at rver mle 17.5; (4) the Pascagoula Rver about 0.4 m downstream of Cumbest Bluff at rver mle 25.0; and (5) the Pascagoula Rver al Ihe USGS contnuous recordng stage and velocty slreamgage at Graham Ferry al rver mle A total of 18 dscharge measurements were made wthn the 18-m reach durng the perod October The rver at Graham Ferry (rver mle 34.6) was n a tde-affected low-water flow condton. but runoff from ranfall n the upper pans of basn on October 8 negated the tdal effect begnnng carlyon October I I. The stage ncreased steadly from 2.3 ft (1800 hrs. October 10) at the begnnng of the data-collecton perod [0 a stage of 3.6 ft (0830 hrs. Oclober 11) at the end of the data-colleclon perod. Records at ste ndcate the stage peaked at 6.5 ft (1700 hrs. October 12). 259

3 METHODOLOGY Q= VA (I) The procedure used to process and analyze the data collecled for ths repon was accomplshed n three steps: ( I) Channel geometry computatons; (2) Unsteady flow computatons; (3) Steady flow computatons. All three steps requred Ihe use of both conventonal and unque computatonal processes 10 analyze Ihe data. CHANNEL GEOMETRY COMPUTATION Channel depths from channel cross sectons and Ihalweg elevalons measured on Aprl were dgtzed from the graphcal fathomeler sttp chans and entered nto a data base. Vencal controls of water surfaces at the varous temporary gages used n the Aprl 1994 study were compled and stage hydrographs at each gage were consuucted for the perod. Water surface slopes were computed from near smultaneous readngs of water-surface elevatons along the lower 35 m study reach of Ihe rver. Water surface elevatons al each channel cross secton and Ihalweg pont were computed by lnear nterpolaton of water surfaee elevatons between approprate temporary gages upstream and downstream of each cross secton and Ihalweg ponl. Channel bed elevatons were then obtaned by subtractng deplhs from water surface elevalons at all channel cross sectons and thalweg locatons. Channel-bed cross sectons were computed at the Pascagoula Rver at the Roy O. Cumbesl brdge at Graham Ferry from a Msssspp Depanment of Transportaton (MOOT) survey and dscharge measurements made at the ste between 1958 and 1994 (Fgure 2). A comparson of these cross sectons ndcates sgnfcant scour has occurred snce conslructon of the brdge n about Thalweg elevatons n and 1994 are and 26.1 ft. respectvely. These elevalons ndcate that the channel bed has lowered a maxmum of fl. although scour near the rght (west) per appears to be approachng about 25 fl. UNSTEADY FLOW COMPUTATION Accordng to Harvey and others (l965). durng perods of low flow the Pascagoula Rver s affected by normal tdes n Ihe Msssspp Sound to at least rver mle 42. but noi upstream of rver mle 53. Data collected at the Pascagoula Rver al Graham Ferry (rver mle 34.6) ndcated a maxmum ldal fluctualon of 0.67 fl for the 1994 water year occurred durng a ldal perod from August 31 to September I Chow (l959) documented that dscharge (Q) n cubc feet per second for steady or unsleady flow at a gven channel secton can be expressed as; where. for Ihe purposes of ths repon. V s the mean velocty n feet per second. and A s the cross-sectonal flow area n square feel normal to the drecton of the flow. The law of contnuty for unsteady flow requres consderaton of tme. To gage velocty and compute contnuous dscharge al the Pascagoula Rver at Graham Ferry durng perods of low flow n whch unsteady flow condtons exst as a result of tdal nfluence on stage and velocty. two relatons were developed. Frst. acoustc transmttng and recevng transducers were placed al an elevaton of about -4 ft on Ihe downstream sde of the thrd man per left (east) of the rght (wesl) brdge seat and on a ple cluster near the rght (west) bank about 220 ft downstream of the brdge and skewed aboul 47 degrees from normal to the flow (Fgure 3). Transducers are devces whch conven electtcal sgnals to sound and vce versa. The AVM measures travel lme of sound through water usng a dfferental travel tme. The dfferental travel tme s determned by makng two successve travel tme measurements on a gven path n each drecton. The forward drecton measurement s made by measurng the travel tme of sound from the upstream transducer to the downstream transducer. whereas the reverse drecton measurement s the travel tme of the sound from Ihe downstream transducer to the upslream transducer. Therefore. the arrval tme of the sound sgnal from the forward drecton measurement wll be shonened n proponon to the water velocty. When the velocty s zero. the forward and reverse travel tmes are equal. The AVM n use at Ihs gage suppons both postve and negalve drectons of now. The relaton can be best defned as; TF = LI(C + l'i:osa} TR = LI(C - l'i:osa} where: TF s travel tme n the forward (downstream) drecton. n seconds; TR s traveltme n the reverse (upstream) drecton. n seconds; L s the path length between the two transducers. n feet; C s the speed of sound. n feet per second; v s the velocty of Ihe water along the path. n feel per second; and cos A s the cosne of the path angle from the drecton of flow. (2) (3) 260

4 Solvng both equatons for the speed of sound results n the followng equalon: at an elevaton of about -4 ft by the AVM transducers. Ths equaton may be expressed as LI(TF -.cosa) = U(TR + l'cosa) (4) v = (6) The equaton s funher smplfed by solvng for v: = Ll2cosA (litf - litr) (5) Because the speed of sound can be altered by temperature. pressure. and salnty. the elmnaton of the speed of sound both smplfes the computaton of an nslanlaneous pont velocty and negates the need to measure other varables to account for changes n the speed of sound. Compensaton was also made for sgnal delays due to cable length. the two transducers. and electronc hardware wth the AVM. The transducers measure nslanlaneous pont velocty and transmt ths value to the AVM whch then transmts the data to a data logger on 3D-mnute ntervals. Secondly. dscharge measurements by conventonal methods (Rantz et ai. 1982) and from an ADCP. whch transmts acoustc pulses from a transducer assembly along four beams. were used to develop a relaton between the nstantaneous pont velocty measured by the AVM and average velocty computed from these dscharge measurements. The ADCP has four transducers whch Iransml pulses of sound nto the water. The transducers Ihen measure the return sound as t s echoed from small pancles floatng ambenlly wth the water currents. Usng Ihe Doppler prncple. the ADCP convens the returned sound nto components of water-current velocty. The ADCP measures both the speed and drecton of the water currents at multple locatons n the water column. It s beyond the scope of Ihs repon to detal all the vector mechancs nvolved n how the ADCP computes ts poston through tme. Comparson of near smullaneous dscharge measurements by personnel of the USGS have shown the ADCP to measure dscharge wthn acceptable lmts (wthn 5 percenl) of the measurement obtaned usng a slandard current meter. Comparson of the typcal curves of equal velocty n a naturaj. rregular' channel cross secton from Chow (1959) and a cross secton showng smlar velocty curves generated from the ADCP data at Ihe Pascagoula Rver al Graham Ferry. on October II show good correlaton (Fgure 4). A relaton was developed between average velocty computed by dvdng Ihe measured dscharge by the area and nslanlaneous pont velocty measured by the AVM (Fgure 5). Dscharge measurements made al 6-week ntervals. as well as those measurements made durng the October 1994 sludy. were used to develop ths relaton between average velocly across the measured channel-bed cross secton and the nstantaneous pont velocty measured where V s the average velocty for the cross secton. 0.0 I s the y-offset. n feet s a dmensonless slope coeffcent and v s nslanlaneous pont velocty. n feet per second. For the purposes of ths repon. t was assumed that when nstanlaneous pont velocty was 0 fl/s. average velocty was 0 fl/s. so the 0.0 I ft offset was gnored. Because average velocty s nversely proportonal to the area for a gven conslant dscharge. f area changes. then velocty also changes. Gven ths and the unsteady nature of tdal fluctuatons at the gage. slope and offset may change over tme. For the purposes of ths repon. ths equaton to defne average velocty s assumed vald for the perod of the study. To complete the frsl relaton. a stage/area relaton was developed usng a fathomed cross secton surveyed on Aprl Area was computed at I-ft elevaton ntervals to develop a stage/area curve for stages nfluenced by tdal fluctuaton (Fgure 5). Therefore. to compute dscharge durng perods of tdal fluctuaton. the velocty relaton was used to compute mean velocty from nslanlaneous pont velocty whch was then multpled by the area determned from the stage/area curve. The product of these two varables produced an nslanlaneous dscharge. STEADY FLOW COMPUTATION Upon observatons of both the shape of the stage hydrograph and relatve change of nslanlaneous pont velocty to stage. a slandard stage/dscharge ratng curve was determned effectve above a stage of about 7 ft. although the exact stage at whch tdal fluctuaton becomes nsgnfcant depends on the effect of the tde at the gage and the flow characterstcs approachng from upstream. Dscharge measurements made from 1958 to 1994 were used to develop a stage/dscharge ratng for perods of stage above 7 fl. Because the effect of tde on stage and velocty at the ste depends upon flow characterstcs near the gage. a I-ft range of overlap was developed to provde a correlaton between the slandard stage/dscharge ratng and the tde-affected dscharges. Between 7 and 8 ft of stage. nslanlaneous dscharge was computed usng both methods to nsure a smooth transton n dscharge hydrographs compuled by the AVM and the slandard stage/dscharge ralng. RESULTS Contnuous streamflow data as well as data for other channel characterstcs have been collected. processed. and 261

5 computed snce November 1993 at the USGS contnuous recordng strearngage at the Pascagoula Rver at Graham Ferry. Msssspp. Analyses of the dam provde stage/dscharge nfonnaton through tme at the gage. as well as a varety of other characterstcs of the lower rver reaches of the Pascagoula Rver Basn (Fgure 5). Analyses ofthe dam also provde streamflow characterstcs at the gage durng perods of tdal fluctuaton as well as durng perods of hgh flow n whch tdal nfluence s nsgnfcant. Channel geometry dam (that s. channel cross sectons. water-surface elevatons. thalweg profles. and water-surface profles) were compled and processed for the lower 35 mles of the Pascagoula Rver from dam collected durng Aprl and May Channel-bed cross sectons were surveyed usng a graphcal fathometer. Each cross secton was georeferenced for entry nto a GIS. The channel-bed cross secton near rver mle 34.6 was used to develop a smgelarea relaton for use n computng dscharge at the Pascagoula Rver at Graham Ferry. Msssspp. durng perods of tdal fluctuaton. These dam were provded to the OLWR. Markers were placed at these cross sectons for use n later studes. Dscharge was measured for unsteady and steady flow condtons at the gage usng conventonal velocty measurng nstruments and an ADCP. Two ratngs were used to compute dscharge. The product of an average velocty for the entre channel cross secton (derved from an average velocty to nstantaneous pont velocty relaton) and an area for a gven range n gage heght from I to 8 ft was used to compute dscharge durng low flow perods that were affected by tdal fluctuaton (Fgure 5). The maxmum change n stage durng a tdal cycle at the gage snce November 1993 was about 0.7 ft. For perods of flow above a gage heght of 7 ft. a standard smgeldscharge ratng was developed from dscharge measurements made from 1958 through 1994 (Fgure 5). Because some uncertanty exsts as to at what stage tdal fluctuaton sgnfcantly affects dscharge. a I-ft overlap from a gage heght of 7 to 8 ft was created between the two ratngs. Dscharges computed by the two ratngs at these smges correlated very well. REFERENCES Chow. V.T Open channel hydraulcs. McGraw-Hl Publshng Company. Harvey. E.J.. H.G. Golden. and H.G. Jeffrey Water resources of the Pascagoula area Msssspp:-u:5. Geologcal Survey Water-Supply Paper Landers. M.L.. and K. V. Wlson. Jr Flood characterstcs ofmsssspp streams. U.S. Geologcal Survey Water-Resources Investgatons Repon RD Instruments Drect-readng broadband acoustc Doppler current profler techncal manual. RD Instruments. San Dego. Rantz. S.E. et ai Measurement and computaton of streamflow: Volume I. Measurement of stage and dscharge and Volume 2. Computaton of dscharge. U.S. Geologcal Survey Water-Supply Paper Tels. P.A Technques for estmatng 7-day. IQ-year low-flow characterstcs for ungaged stes on streams n Msssspp. U.S. Geologcal Survey Water-Resources Investgatons Repon A flood-frequency relaton (Table I) for the USGS gage at the Pascagoula Rver at Graham Ferry was developed usng procedures outlned by Landers and Wlson (1991). The USGS has nlennuently collected stage and dscharge data at the ste snce A stage/dscharge ratng for stages above 7.0 ft was developed usng dscharge measurements made from 1959 through Ths ratng represents non-tdal affected dscharge. 262

6 Table I.-Elevalon. peak dscharge. and recurrence nterval of annual floods al the Pascagoula Rver at Graham Ferry. Msssspp Elevatons (feet above sea level) Peak dscharge (cubc Ceet per second) Recurrence nterval (years) Low-flow data were computed usng procedures outlned by Tets (l991). The 7-day. Io-year low-flow dscharge (7QIO) Cor the Pascagoula Rver at Graham Ferry s cubc Ceet per second. Ths 7QIO neglects any tdal nfluence at the brdge. 263

7 89'00'. _._._._._._._._._._._._._._1 _._._._.._.1 I.f I owm STONE _._._._.j T ' EXPLANATION 3f00... _ _ _ _ I L-._._. I 88"30'._._._._._._._._._1_._.- -r Merrll :! I I GEORGE Agrcola o JACKSONr---"r-_--. GAGING STATION 60 INDICATES DISTANCE. IN RIVER MILES ABOVE MOUTH STUDY AREA o IOMll..ES I I! I I I I j o Kll..OMETERS Msssspp Sowu Fgure 1. Study area locaton on the lower Pascagoula Rver. 264

8 ...l UJ >' 30 LEFT (EAST) BANK UJ...l 20 I <t: UJ CI) "-"::...' W '"'''''''''' >' IOL 0 "... <t:.-: UJ [E Z..._--_ '" - V> >' 1994 UJ... t tl -20 RIGHT (WEST) BANK -30 I I I I I I HIGHWAY STATIONING. IN FEET Fgure 2. Cross sectons from soundngs make durng hstorcal and recent dscharge measurements at the Pascagoula Rver at Graham Ferry. Msssspp.

9 WADE 5 MILES )OW j "t ''5. " «t- " ACOUSTIC VELOCITY (AVM) TRANSDUCER (f I - TREATED TIMBER PILE CLUSTER 220' GAGE HOUSE )ID _====)@ II PIER/ - RAMP BOAT I\ NUMBER TREATED TIMBERS PILES LOCATION OF SUBMERSIBLE STAGE o FEET I I I I I o METERS Fgure 3. Plan vew of US Geologcal Survey contnuous recordng stage and velocty streamgage at the Pascagoula Rver at Graham Ferry Msssspp showng locatons of submerged transducers

10 (A) "Rtpl'llleel from Chow. VT.. 19n. Optll dlalllltl h)'dr.ulla: McGr... HlII P\lblhtlll' ComPM P and pubhed wth ptnnuloll: (B) -10 I I --l ok I I V I' I.nll 17' Illl!; ' I!!! o Fgure 4. Typcal curves of equal velocty n feet per second n a natural rregular channel as (A) documented by Chow (1959) and (8) measured by an Acoustc Doppler Current Profler (ADCP) on October II at the Pascagoula Rver at Graham Ferry Msssspp. 150 DISTANCEIN FEET

11 ' ;:- I: UO I.'" CXXI IOO.CXXI I.CXXI.CXXI DISCHARGE IN CUBIC FEET PER SECOND w > ::I // If / w w 5 > 0.. "r ' 0 z.. I Il! > w 5 0 Z u 0 w 1 / > / / '''''' 6JlOO "'" """."'" CROSS SECTIONAL AREA. IN SQUARE FEET u u u.. ".. INSTANTANEOUS VELOCJTY. IN FEET PER SECOND Fgure S. Stage/dscharge relaton stage/area relaton nstantaneous pont/average velocty relaton used to compute dscharge at Graham Ferry Msssspp. 268