Reprinted from WAVES Proceedings of the Third International Symposium on Ocean Wave Measurement and Analysis Virginia Beach VA Nov Exp erimental Mo deling of a Rip Current System Merrick C Haller R A Dalrymple and I A Svendsen Abstract Results from an exp erimental investigation of a rip current system in a lab o ratory wave basin are presented The mo deled system includes a planar b each with a sup erimp osed longshore bar containing two rip channels Dense mea surements of water surface elevation crossshore and longshore currents are presented The exp erimental results indicate that in addition to the steady mean water level gradients which drive the steady nearshore circulation time varying pressure gradients are generated in the rip current system and these gradients are directly related to measured current oscillations in the rips Os cillations in the rip currents are shown to have multiple scales A simple calculation shows that classical jet instability theory can predict the order of magnitude of the measured short time scale rip current oscillations Background Shepard rst intro duced the concept of the rip current In the obser vations of Shepard et al and Shepard and Inman those authors describ ed three characteristics of rip currents they are driven by longshore variations in wave height they exhibit p erio dic uctuations in time and are often p erio dically spaced in the longshore direction and they increase in strength with increasing wave height Later observers also noted that many rip currents are not stationary McKenzie Short The oshore end of the rips the rip head can op around like an untended garden hose and the entire rip current can migrate in the longshore direction generation is nearshore One commonly observed mechanism of rip current wave height variations induced by longshore varying b ottom bathymetry In this pap er we will investigate in the lab oratory the waves and currents gen erated in a nearshore system characterized by a longshore varying bathymetry all at Center for Applied Coastal Research Ocean Engineering Lab University of Delaware Newark DE USA Corresp ondence email merrickcoastaludeledu y 3.6 m 1.8 m 18.2 m 7.3 m 1.8 m 3.6 m x 20 m 6 cm 1:30 1:5 Figure Plan view and crosssection of the exp erimental basin and p erio dic rip currents A limited set of eld measurements of rip current systems do es exist Sonu Bowman et al Tang and Dalrymple however due to their transient nature rip currents tend to elude eld in vestigators intent on measuring them with stationary instrumentdeployments In contrast the lab oratory has proven to b e very conducive to the study of rip currents since the environment is more easily controlled Preliminary results from this exp erimentwere given in Haller et al the presentwork utilizes all the data obtained during this exp eriment therefore providing more detailed rip current measurements than rep orted previously and provides an indepth discussion of the low frequency motion observed Physical Mo del t was conducted in the m m directional wave basin at The exp erimen the Center for Applied Coastal Research at the University of Delaware The basin Figure contains a planar concrete b each of slop e along with a steep er to e structure A discontinuous longshore bar made of molded plastic was attached directly onto the b eachslope The crest of the bar is cm ab ove the planar b each and it has a parab olic shap e in the crossshore The discontinuities result from the two gaps in the bar which act as rip channels and tend to x the lo cation of oshore directed rip currents dep ending on the wave conditions These rip channels are lo cated at and of the width of the basin and are each m wide with slop ed sides Separate arrays of ten capacitance wave gages and acousticdoppler velo city meters ADV were used to measure the generated wave elds mean water levels and circulation patterns u u v Details of the exp erimental pro cedure can b e found in Haller et al Most of the exp erimental runs lasted for minutes after the onset of wave generation However a limited set of additional tests were conducted with longer runs of minutes and these will also be discussed here a) Wave Gages b) Current Meters 20 20 15 15 10 10 x (m) x (m) x 5 5 x 0 0 15 10 5 0 15 10 5 0 y (m) y (m) Figure Sampling lo cations for awave gages and b current meters Figure shows the measurement lo cations for the measuring runs describ ed in this pap er The measurement lo cations were densely concentrated around one rip channel in order to resolve the strong pressure and velo city gradients near the rip current However measurements did span a large area of the basin and covered all regions of interest The ADV measurements taken shoreward of x m were taken cm from the b ottom The oshore ADV measure ment lines at x m and m were taken cm and cm from the b ottom resp ectively The still water line was at x m the wave generator at x m and the water depth at the bar crest x m was cm Results Steady circulation The waves were generated normally incident to the shoreline and the oshore wave heightwas cm at x m y m with a frequency ofHz The spatial variations of wave height and water level timeaveraged over the last half of the data collection p erio d s are shown in Figure The mea surements show that there is little variation in the mean quantities H rms oshore but near x m the waveheight decays sharply due to strong break ing induced by the bar Corresp ondingly the intense wave breaking over the bar drives a sharp increase in the mean water level in this region and a wave induced setup of mm at the shoreline in the center of the basin y m In the rip channel wave breaking is less intense and mostly o ccurs shore ward of x m Therefore there is a lo cal elevation hill of wave height in the channel and the contours of in the rip channel show a lo cal depression valley trending in the oshore direction a) b) 14 14 13 13 12 12 x (m) 11 x (m) 11 10 10 9 9 8 8 18 16 14 12 10 8 18 16 14 12 10 8 y (m) y (m) Figure Contours of a H contour interval cm and b contour rms interval cm H is computed from bandpassed f Hz rms water surface elevation records Figure compares the crossshore variations of H and measured through rms the rip channel and in the center of the basin Notice the largest measured longshore gradients are near x m which is just shoreward of the bar in what would be considered the bar trough It is also interesting to note that close to the shoreline at x m the longshore gradients are reversed from those in the trough The time average of the mean ows are shown in Figure It can b e seen that the longshore setup gradient induces strong longshore currents in the troughs and exit oshore as rip which are driven towards the rip where they converge currents However due to the reversal of the longshore setup gradient near the shoreline the ow at the shoreline is driven away from the rip channels Figure ab shows crossshore proles of the longshore currents measured near the rip channel and in the center of the basin areas enclosed by dashed lines in c resp ectively There is very little longshore ow in the surf zone at the basin center which implies the dynamics are dominated by the crossshore momentum balance in this region Near the rip channel the longshore currents a) 6 4 cm s m 2 r H 0 4 6 8 10 12 14 14.9 b) 0.4 0.2 cm 0 −0.2 4 6 8 10 12 14 14.9 c) 0.8 0.6 h (m) 0.4 0.2 4 6 8 10 12 14 14.9 x (m) Figure Crossshore variation of a H b measured in rip channel rms dashed line and in center of basin solid line c crossshore prole of bathymetry in center of basin are considerably stronger Also the reversal of longshore ow at the shoreline induces a pronounced shear on the shoreward face of the prole The entire measured circulation is shown in Figure c The current vectors indicate that the dominant feature of the nearshore circulation is the strong oshore directed jet in the rip channel In addition there are two separate h circulation patterns The rst is the classical rip current circulation whic encompasses the longshore feeder currents at the base of the rip the narrow rip neck where the currents are strongest and the rip head where the current spreads out and diminishes Oshore of the rip head the ow diverges and returns shorewards over the bars The secondary circulation is the reverse ows just shoreward of the base of the rips Here the waves which have shoaled through the rip channels break again at the shoreline driving ows is opp osite from the primary circulation away from the rip channels which Figure shows the spatial variation of the timeaveraged ows measured in the rip channel At the entrance to the rip channel abthe rip current is a narrow oshore directed jet which is being fed by converging feeder currents a) b) 14 14 13 13 12 12 x (m) 11 x (m) 11 10 10 9 9 10 0 −10 10 0 −10 v (cm/s) v (cm/s) c) 14 13 12 x (m) 11 10 9 18 16 14 12 10 8 6 4 2 0 y (m) Figure Crossshore proles of longshore current measured near a the rip channel y m b the basin center y m Measuring lo cations for a and b corresp ond to those enclosed by dashed lines in c cvectors of time averaged measured currentvelo cities Dashed line at x m represents still water line as shown by the opp osite sign of the longshore velo city on each side of the rip Figure cd shows that as the rip current exits the channel the sign of the v velo cities has reversed which indicates the ow is starting to diverge oshore comp onent of the ow u has a triangular longshore In addition the prole Results