a system shift in tidal choking due to the construction of yangshan harbour shanghai china abstract tidal choking is a geometric feature caused by a narrowed channel construction of the yangshan harbour shanghai china obstructed three key channels and intensively changed the local geometry and topography in this study nine numerical experiments based on the finite volume community ocean model are conducted to study the project s influence on tidal characteristics results show that stronger tidal choking happened at the east entrance after project mainly due to the jet induced water level drop forced by bernoulli law and the longer and narrower geometry the stronger tidal choking forces a faster flow and larger tidal energy flux at the choked channel while reducing the tidal amplitude in the inner harbour area iha the scouring on this channel reduces the choking effect but further enlarges tidal energy flux moreover damming the channels decrease the tidal amplitude at the lee side of tidal propagating direction while increasing the amplitude on the stoss side the dams also decrease the tidal current on both sides and meanwhile develop two patches with stronger current aside the dam the project induced changes in tidal characteristics are complex in space and perturbations in bathymetry increase this complexity yangshan harbour s construction induces little changes in the total tidal energy density in the iha but induces obvious changes in the spatial distribution of tidal energy although this study is site specific the findings may be applicable to tidal dynamics in land reclamation close to open seas such as the dramatic reclamation of islands in the south china sea

envelope curves for extreme flood events maximum flood experience in a region can be depicted by a graph on which maximum known floods are plotted against drainage area a smooth curve enveloping the plotted points provides the most concise description such curves have been developed for 17 regions in the conterminous united states and are described by equations the curves are not associated with specific probabilities or frequencies but within the region to which they apply they give evidence as to the magnitude of flow that was occurred given similar basin characteristics a peak lying close to the envelope curve might occur at other basins in the same region

the modern evolution of geomorphology binghamton and personal perspectives 1970 2019 and beyond abstract the annual binghamton geomorphology symposia bgs began in 1970 initiated by professors donald r coates and marie morisawa of the state university of new york at binghamton the 50 bgs meeting topics through 2019 can be organized into five general themes as follows 1 applications 2 methods 3 process and form 4 history philosophy and theory 5 systems my own geomorphological research can be divided among these themes though it has not always been in tune with any prevailing paradigm the experience of the bgs meetings suggests that the immediate future of geomorphology will follow current trends involving technological advances in such areas as geochronology geospatial analysis lidar mapping computer simulation and systems based predictive modeling for the longer term it may be that the research frontiers will lie in outer and inner space with the former involving the discovery and analysis of the surfaces of earth like planets within and beyond our own solar system the challenges of inner space may be even more profound as they are imposed against the background of rapidly accelerating advances in artificial intelligence

peak discharge of a pleistocene lava dam outburst flood in grand canyon arizona usa abstract the failure of a lava dam 165 000 yr ago produced the largest known flood on the colorado river in grand canyon the hyaloclastite dam was up to 366 m high and geochemical evidence linked this structure to outburst flood deposits that occurred for 32 km downstream using the hyaloclastite outburst flood deposits as paleostage indicators we used dam failure and unsteady flow modeling to estimate a peak discharge and flow hydrograph failure of the hyaloclastite dam released a maximum 11 109 m3 of water in 31 h peak discharges estimated from uncertainty in channel geometry dam height and hydraulic characteristics ranged from 2 3 to 5 3 105 m3 s 1 for the hyaloclastite outburst flood this discharge is an order of magnitude greater than the largest known discharge on the colorado river 1 4 104 m3 s 1 and the largest peak discharge resulting from failure of a constructed dam in the usa 6 5 104 m3 s 1 moreover the hyaloclastite outburst flood is the oldest documented quaternary flood and one of the largest to have occurred in the continental usa the peak discharge for this flood ranks in the top 30 floods 105 m3 s 1 known worldwide and in the top ten largest floods in north america

a 2000 year natural record of magnitudes and frequencies for the largest upper colorado river floods near moab utah using well established procedures for paleoflood hydrology and employing optically stimulated luminescence osl geochronology we analyzed a very well preserved natural record of 44 upper colorado river extreme floods with discharges ranging from 1800 to 9200 m3s 1 these are the largest floods occurring during the last 2140 220 years and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914 most of these large floods occurred during the last 500 years and the two largest floods in the record both exceeded the probable maximum flood pmf estimated at 8500 m3s 1 300 000 cfs for nearby moab utah another four floods with discharges greater than 7000 m3s 1 occurred during the last two millennia flood frequency analyses using the fldfrq3 model yields the following values depending on the manning n roughness coefficients 100 yr flood 4670 4990 m3s 1 500 yr flood 6675 7270 m3s 1 1000 yr flood 7680 8440 m3s 1 the presumed pmf discharge 8500 m3s 1 gets assigned a recurrence interval of about 1000 years and the largest historical 1884 flood 3540 m3s 1 a recurrence interval of 100 years flood frequency analysis for the moab valley based on the gaged record 1914 2012 yield 2730 m3s 1 for the 100 yr flood and 3185 m3s 1 for the 500 yr flood this underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture

paleohydrology of southwestern texas current statistical methods may be unable to accurately predict recurrence intervals of rare large magnitude floods especially in semiarid regions having positively skewed annual flood distributions great hydrologic variability and widely spaced gaging stations current approaches rely on historical data but catastrophic floods may have recurrence intervals far greater than the length of historical records in the lower pecos and devils rivers of southwestern texas paleoflood discharge and frequency estimates are extended over 10 000 years by the study of slack water flood sediments slack water deposits are typically fine grained sand and silt that accumulate during floods in areas where current velocity is reduced i e in back flooded tributary mouths channel expansions downstream from bedrock spurs and or slump blocks and in shallow caves along bedrock walls radiocarbon dating of organic detritus in slack water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope area techniques paleoflood information extracted from slack water sediments can greatly extend flood records these floods may be weighted like historical data in log pearson type 3 calculations of flood frequency our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency this technique offers an inexpensive and rapid way to assess catastrophic flood risk

adcp measurements of momentum balance and dynamic topography in a constricted tidal channel abstract the dynamics of tidal flow through inlets are not fully understood observations are scarce because of the small spatial scales over which the flow varies this paper gives the first detailed measurements of the 2d structure of tidal currents and the dynamical terms of the momentum equation within a tidal inlet leading to an improved understanding of the physics of tidal inlets in the 180 cm s 1 peak flow the near steady state momentum balance is dominated by horizontal advection and the pressure gradient with bottom friction playing a secondary role at slack water there is a balance between local acceleration and the pressure gradient numerical integration of the adcp measured terms in the momentum equation yields 60 m resolution dynamic topography that shows a 7 cm variation at peak flood consistent with bernoulli s equation the surface topography because of friction forms a linear ramp with a peak irreversible head loss of 2 cm over 600 m tidal velocities were extracted from the adcp

measurements of tidal flow around a headland with a shipboard acoustic doppler current profiler a shipboard acoustic doppler current profiler adcp and moored current meters were used to obtain detailed measurements of the spatial structure of the tidal flow around a headland in vineyard sound massachusetts where tidal currents typically range from 50 to 70 cm s 1 eight shipboard surveys were conducted each of which followed one of five trapezoidal tracks in the vicinity of the headland completing 11 12 circuits in the course of the tidal cycle 12 4 hours the measurements from the adcp compare favorably with moored velocity measurements at two locations rms deviations of 3 cm s 1 but the comparison showed more scatter at two other sites rms deviations of 7 cm s 1 the adcp measurements from separate cruises were merged to form a composite spatial representation of the tidal and residual currents providing resolution of the spatial structure of the flow around the headland at scales from several hundred meters to 10 km the semidiurnal tidal flow is relatively uniform in space roughly following the bathymetry around the headland while a residual current field consists of counterrotating eddies on either side of the headland with spatial scales of 5 8 km and velocities as high as 25 cm s 1 the instantaneous current field indicates the formation of transient eddies on the downstream side of the headland during flood and ebb with clear evidence of flow separation near the tip of the headland

ebb tide dynamics and spreading of a large river plume abstract momentum balances in the near field region of a large tidally pulsed river plume are examined the authors concentrate on a single ebb tide of the columbia river plume using the regional ocean modeling system roms configured to hindcast flow conditions on the washington and oregon shelves and in the columbia river estuary during ebb plume interior streamwise balances are largely between advection pressure gradient and frictional forces stream normal balances in this region reduce to centrifugal cross stream pressure gradient and coriolis terms i e the gradient wind balance commonly assumed in river plume bulge investigations temporal derivatives are most important at the plume front and as the ebb progresses winds were light and contributed little to the force balance midebb stress and vertical salt flux were largest at a midplume depth where stratification and vertical shear were also high consistent with shear induced mixing internal stress slows the spreading plume consi

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

high resolution observations of the intensity of secondary circulation along a curved tidal channel 1 high horizontal resolution moving vessel acoustic doppler current profiler adcp observations of the spatial pattern of cross stream velocities in a curved tidal channel show radially outward surface velocities up to 10 cm s 1 which are maximum midchannel consistent with helical secondary flow in a vertical plane normal to the depth averaged velocity the 30 m cross and 150 m along channel resolution observations are from a 2700 m long section of a 350 m wide horizontally and vertically well mixed tidal channel with a radius of curvature 1 5 km the along channel resolution allows the intensity of the curvature induced secondary flow to be estimated from the linear correlation between the observed cross channel component of vertical shear and the shear estimated from the streamwise velocity and its varying curvature using an existing analytic model the two shears are highly correlated and the regression line slope demonstrates that the observed curvature induced secondary flow is 30 more intense than that predicted by the model for a typical bottom drag coefficient the secondary flow is 50 more intense than that predicted using the drag coefficient which best fits the streamwise velocity profile numerical solutions demonstrate that the intensity of the secondary flow is sensitive to small changes in the shape of the eddy viscosity profile hence intensity may be sensitive to the way turbulence is modeled lagged correlation of the observations showed that the secondary flow adapts to changes in curvature and primary flow over a 300 m length scale or 20 water depths consistent with the existing model and laboratory studies

palaeoflood hydrology of the salt river arizona geomorphic evidence along bedrock confined reaches of the salt river in east central arizona provides a record of the river s largest flood events fine grained flood slackwater deposits accumulated at channel margin irregularities several metres above the low flow channel discharges associated with flow events responsible for the deposits were estimated by computer flow modelling these estimates document flood magnitudes in excess of gauged historic streamflows relative and radiocarbon dating suggest that a flood record in excess of 600 y is preserved in the slackwater sequences a prominent flood scar cut into grussy hillslope soils allows the extension of the prehistoric flood record to several thousand years a maximum discharge estimate of 4600 m3s 1 affixed to the flood scar represents the largest flood event in the record and is given a minimum recurrence interval of 1000 2000 y the 1952 flood is the largest historic flow event experienced along the study reach and is estimated at 2900 m3s 1 two palaeoflood events preserved in the slackwater stratigraphy exceed the 1952 event and are given recurrence intervals of 300 and 600 y the latter flood event had an estimated discharge of 3200 m3s 1 it is apparent that discharge estimates affixed to these infrequent large magnitude flood events approach a maximum with decreased probabilities large recurrence intervals this suggests that a physical limit on discharge may exist within the salt river drainage basin and is perhaps directly related to drainage basin size

separation of tidal and subtidal currents in ship mounted acoustic doppler current profiler observations a simple method is developed to analyze current measurements obtained from a moving platform the need for this is motivated by the now common use of the ship mounted acoustic doppler current profiler adcp to acquire absolute velocity data during an oceanographic survey of a given region the full potential of this new measurement technique is severely hindered when the presence of high frequency phenomena e g tidal or inertial motions prevents a clear visualization of the lower frequency current structure of interest our analysis method is based on a spatial interpolation scheme using arbitrary functions that allows for the tidal current time variability which then permits the tide induced motions to be subtracted from the adcp data to yield the subtidal current field the method also allows nearby moored and drifter current measurements if available to be combined with the shipboard adcp observations in a single analysis to obtain the best description of the tidal and subtidal currents to illustrate this method we present results from the analysis of adcp data taken during oceanographic surveys in two different continental shelf regions the east china sea and the amazon shelf a 5 day conductivity temperature depth ctd and adcp survey was made in the east china sea near the mouth of the yellow sea during january 1986 there the currents were essentially barotropic and dominated by the semidiurnal tide the adcp derived cotidal chart for the m2 12 42 hours component agrees well with existing charts derived empirically from sea level observations or from regional numerical models the adcp derived steady flow is also consistent with the observed density field and indicates little flow in or out of the yellow sea and a transport of about 1 sv toward the tsushima strait two ctd adcp surveys lasting 21 and 23 days were conducted over the amazon shelf during march and may 1990 simultaneously moored current observations were also obtained at three locations along a cross shelf array 200 km north of the amazon river mouth for a common period of at least 2 months february 12 to april 13 over the shelf tidal and subtidal currents were comparable in magnitude our analysis indicates that the tidal currents were essentially barotropic semidiurnal and polarized in the cross shelf direction increasing in magnitude toward the inner shelf where current values of more than 2 m s are common the adcp derived steady currents were aligned in the along shelf direction and strongly influenced by the north brazil current nbc during both adcp surveys the northwestward flowing nbc was transporting more than 2 sv over the shelf at depths shallower than 100 m

ecogeomorphology in the australian drylands and the role of biota in mediating the effects of climate change on landscape processes and evolution

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

paleoflood evidence for a natural upper bound to flood magnitudes in the colorado river basin the existence of an upper limit to the magnitude of floods in a region is a long standing and controversial hypothesis in flood hydrology regional envelope curves encompassing maximum flood magnitudes stabilize with progressive increases in the areal coverage and period of observation wolman and costa 1984 however the short lengths of conventional gaging records limit substantial advances in testing whether this stabilization is evidence of an upper limit in the colorado river basin there are 32 120 station years of gage data but the average period at a gaging station is only 20 years with most stations having less than 70 years of observation paleoflood magnitudes derived from sediments of large prehistoric floods from 25 sites on rivers in arizona and utah provide additional data to extend the records of the largest floods the paleoflood data identify the maximum flood discharges that have occurred on individual rivers over the last several hundred to several thousand years even with this increase in the observational period the largest paleoflood discharges do not exceed the upper bound of maximum peak discharges delineated by the envelope curve derived from the available gaged and historical records this result accords with the hypothesis of an upper physical limit for flood magnitudes and suggests that for the colorado river basin the upper limit can be approximated by existing systematic and historical data for large floods similar relationships also hold when paleofloods and gaged records are presented for the subregion of southern arizona

transient tidal circulation and momentum balances at a shallow inlet an analysis of transient momentum balances is carried out to elucidate circulation dynamics and exchange mechanisms at shallow barotropic tidal inlets circulation is computed using a depth integrated fully nonlinear time stepping finite element model with variably spaced grids having horizontal resolution down to 50 m velocity and elevation fields from the model are used to directly evaluate the contribution of each term in the momentum equations to the overall momentum balance a transformation of the x y momentum terms into an s n coordinate system is used to simplify the interpretation of the dynamics and provide vivid illustrations of the forces and resulting accelerations in the flow the analysis is conducted for an idealized inlet and contrasted with a highly detailed model of beaufort inlet north carolina results show that momentum balances in the immediate vicinity of these inlets vary significantly in time and space and oscillate between two dynamical states near maximum ebb or flood the alongstream momentum balances are dominated by advective acceleration pressure gradient and bottom friction cross stream balances are dominated by centrifugal acceleration and pressure gradients near slack balances more closely follow linear wave dynamics with local accelerations balancing pressure gradients and to a lesser degree coriolis comparisons between the idealized inlet and beaufort inlet show broad similarities in these momentum balances however natural inlet geometry and bottom topography as well as the tidal transmission characteristics of the sounds behind beaufort inlet produce strong asymmetries moreover momentum balances are highly localized often with subkilometer length scales the dynamics are used to explain the physical mechanisms for inlet exchange in particular the results indicate that the cross stream dynamics generate a wall along the length of an inlet during the stronger phases of the tide the wall is established by opposing cross inlet pressure gradients and centrifugal forces and it poses a significant barrier to cross inlet exchange during the stronger phases of the tide but is absent near slack

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

the alderney race general hydrodynamic and particular features this study presents an overview of the main hydrodynamic features of the alderney race strait based on in situ measurements and two dimensional hydrodynamic model simulations the strait encompasse

the dynamical balance of flow in the menai strait abstract observations of sea surface slope and along channel current from two experiments are used to explore the dynamical balance in the tidally energetic menai strait a regression analysis is used to deduce a new value for the sea bed frictional drag coefficient which achieves a close balance of the forces involved the regression results are consistent between the two experiments and highly significant over 90 of the variance is explained and indicate that the friction coefficient for the menai strait is significantly larger than that usually employed for shelf sea flows but consistent with values of the chezy coefficient for open channel flow the tidal cycle of along channel flow is highly distorted as a result of shallow water frictional effects and there is a net transport of water to the south west of the order of 25 30 million tonnes per tidal cycle the non linear advective acceleration term in the dynamical balance is shown to be small

oil migration in a major growth fault structural analysis of the pathfinder core south eugene island block 330 offshore louisiana the pathfinder core collected in the south eugene island block 330 field offshore louisiana provides an outstanding sample of structures associated with a major growth fault that abuts a giant oil field and that is thought to have acted as a conduit for hydrocarbon migration into the producing reservoirs where cored the growth fault zone cuts semiconsolidated pliocene pleistocene mudstone and is over 100 m wide the fault zone in the core consists of three structural domains each characterized by a distinct rock type distribution of fault dips and dip azimuths and distribution of spacing between adjacent faults and fractures although all of the domains contain oil bearing sands only faults and fractures in the deepest domain contain oil even though the oil barren fault domains contain numerous faults and fractures that are parallel to those containing oil in the deepest domain the deepest domain is also distinguished from the other two domains by a greater degree of structural complexity and by a well defined power law distribution of fault and fracture spacings sediments in this domain behaved as competent rock with respect to fault and fracture spacing whereas the departure from power law distribution of fault and fracture spacing in the other two domains may reflect deformation of unconsolidated sediment this departure from a power law spacing distribution in the upper two domains combined with stable isotope data that indicate low temperature water rock interaction within a gouge zone that separates these two fault domains indicates that the faults in those domains may have been active only early in the history of the growth fault zone when the sampled sediments were at shallow burial depths thus these faults may predate oil migration in contrast the faults in the oil bearing domain appear to have been active later in the fault zone s history when the sediments faulted as competent rock and when geologic and organic geochemical investigations indicate oil migrated into the block 330 reservoirs even though oil is present in sands throughout the core its restriction to faults and fractures in the youngest sampled portion of the fault zone implies that oil migrated only through that part of the fault that was active during the time when oil had access to it the absence of oil in fractures or faults in the other probably older fault domains indicates that the oil was never sufficiently pressured to flow up the fault zone on its own either by hydraulic fracture or by increased permeability as a result of decreased effective stress instead fluid migration along faults and fractures in the pathfinder core was enhanced by permeability created in response to relatively far field stresses related to minibasin subsidence

moving vessel acoustic doppler current profiler measurement of tidal stream function using radial basis functions 1 acoustic doppler current profiler adcp measurements from moving vessels have recently been used to obtain detailed observations of the spatial patterns of tidal flows as well as patterns of vorticity and dynamical terms developments in radial basis function rbf interpolation theory are demonstrated to significantly improve the quality of the tidal velocity field extracted from the measurements using thin plate splines these include placing centers at data locations enforcing side conditions on the solution and using higher order splines for tidal flows with a scale less than a few kilometers the differentiability of rbfs can be exploited to fit the stream function directly to the measurements this ensures the observed tidal velocity field satisfies mass continuity enforcing mass continuity is demonstrated to significantly improve the ability of the splines to interpolate across wide gaps between vessel tracks extraordinarily detailed adcp measurements of the tidal stream function within bluff harbour new zealand reveal the stagnation streamline that separates a 200 m wide flood tidal jet from an associated 400 m diameter eddy the stagnation streamline clearly shows that the eddy gains its fluid and vorticity from inshore of the jet s vorticity maximum the eddy forms at 04 45 hours before high tide grows in spiral fashion and becomes isolated from the jet 02 30 hours before high tide after which its vorticity rapidly decays

flood frequency of the mojave river and the formation of late holocene playa lakes southern california usa perennial lakes lasting several decades occupied silver lake playa the terminus of the mojave river as recently as 390 90 and 3620 70 bp flood routings from the mojave river headwaters to its terminal playa and water balance calculations for the resulting lakes indicate that floods comparable in magnitude to the largest floods in the modern record must occur almost annually to generate and maintain perennial lakes in contrast the modern floods of this magnitude have recurrence intervals greater than 18 years such an increase in flood frequency is required even if lake evaporation is reduced by as much as 50 modern extreme floods of the mojave river are related to meridional atmospheric circulation and a southeastward shift in the winter low pressure center over the north pacific ocean causing vigorous storms over the headwaters in the san bernardino mountains of southern california this winter pattern probably occurred more frequently to induce the large number of storms and floods needed

paleofloods and a dam failure flood on the virgin river utah and arizona abstract flood deposits on the virgin river in southern utah and northern arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since ad 900 in the virgin river gorge several of the natural paleoflood discharges were comparable with or even greater than the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m3 s 1 in this reach of the virgin river to estimate the paleoflood discharges the heights of slackwater flood deposits in the lower bedrock gorge of the virgin river were compared with water surface elevations calculated with the hec 2 step backwater hydraulic model the timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits the estimates of both the 1989 flood and the paleofloods were carried out in the virgin river gorge about 60 km downstream of the dam break site no natural floods of this magnitude have occurred within the relatively short period of the gaged record this study illustrates how incorporating rare extreme events from the long term paleoflood record improves the database for flood frequency analysis the 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower virgin river

spatial variability of small basin paleoflood magnitudes for a southeastern arizona mountain range paleohydrological procedures were used to analyze spatial variations in flood magnitude in drainage basins of the santa catalina mountains maximum elevation 2791 m southeastern arizona paleoflood reconstructions in eight watersheds ranging from 10 to 110 km2 were obtained for the largest floods in stable canyon reaches yielding discharges ranging from 50 to 270 m3 s 1 although there is a high correlation between drainage area and flood magnitude it is also clear that basins located in the southern half of the santa catalina mountains have larger unit discharges peak discharge divided by drainage area and that these values decrease toward the northern part of the range this spatial pattern is probably the result of local hydroclimatological conditions whereby the largest floods are associated with the northward influx of moist tropical and subtropical air for this atmospheric circulation the northern part of the range experiences a rain shadow effect that decreases the unit discharges generated in those basins the unusually high unit discharge for one of the south facing basins is explained by its basin morphometry inverse problem solutions using a rainfall runoff model demonstrate that both this basin and another with a comparable drainage area but a much lower estimated peak paleoflood discharge can have their peak flood outputs produced by the same storm intensity the results demonstrate the potential of paleoflood data collection for understanding spatial variability in small basin flood hydrology

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern

acoustic doppler current profiler measurements of the velocity field of an ebb tidal jet a broadband acoustic doppler current profiler was used to measure the velocity field of the ebb tidal jet formed at the entrance to the otago harbour south island new zealand four separate surveys were conducted where various sets of transects were run continuously over a 13 hour period producing time series of velocity profiles with 1 hour sampling over a grid with an across jet resolution of 20 m and an along jet resolution of 150 m the velocity field of the ebb tidal jet is phase locked to the tidal response of the harbor making it possible to combine the data from separate tidal cycle surveys to produce synoptic maps of the depth averaged velocity field the spatial structure of the ebb tidal jet at the time of peak flow exhibits many features of a classical plane jet there is a strong asymmetry between the ebb and flood flow structures the ebb flow extends beyond 2 km from the entrance while the flood flow is limited to within 600 m of the coast the flow along the jet axis becomes pulse like in time with distance from the entrance the pulse that develops propagates way from the entrance at a speed slower than the advective speed of the ebb jet the formation of a propagating eddy observed along the eastern side of the jet suggests that this momentum pulse may be associated with the pair of separation eddies generated by the accelerating ebb tidal flow