Swash zone processes: An examination of water motion and the relations between water motion and foreshore response on some mixed sand and shingle beaches, Kaikoura, New Zealand (1970)
AuthorsKirk, R. M.show all
While much work in coastal geomorphology has been concerned with the analysis and description of morphological and sedimentological changes occurring on the sub-aerial beach face, relatively little effort has been applied to the detailed investigation of the processes responsible for the production and modification of these features. Only scattered, fragmentary observations of such fundamental properties as swash velocity, backwash velocity, flow depths and durations exist in the literature and these data have been poorly co-ordinated with other information relating to input wave parameters, grain size, slope and profile dimensions which is necessary to a complete understanding of the forces governing swash zone morphology. In an effort to bridge this gap an instrument system has been designed and constructed for the measurement of flow speeds, pressures, asymmetries, depths, levels of groundwater storage and outflow, and of flow durations. In association with the observations of these and other process factors several responses to the flow regime were also sampled. These included alterations to grain size parameters of bed sediments, transport rates of solids in the swash and backwash, vertical distributions of sediment in both swash and backwash, and changes in bed elevation. An electromechanical force-plate dynamometer was employed for the measurement of flow speed and duration and a small parallel-wire gauge sensed depth fluctuations in swash and backwash flows. The output from both of these units was recorded on a strip chart for later analysis. A total of twenty one data sets were derived in this way from four profiles located on mixed sand/shingle beaches at Kaikoura, East Coast, New Zealand. Two profile stations have steep, well sorted shingle foreshores while the other two are flatter and are composed of mixed sand and shingle. A wide range of breaker heights, periods and types is received at all four stations. More than 3,000 individual measurements of swash/backwash flow conditions were obtained from these four stations under varying wave, tide and foreshore conditions. Analysis of this and associated data relating to sediment transport indicates that the flow/sediment system of the swash zone on the study beaches is bounded approximately by current speeds of 100 to 250 cm/sec. and by grain diameters of 1.0 to 50.0 mm. Because of the high energy nature of the system and the large range of grain sizes available for transport, the conditions for initiation of motion are rapidly and frequently exceeded and particles are transported at high rates. Sorting processes are thus dependent upon the net rates of transport of individual size fractions in the flows rather than on critical selection of individual sizes from the bed. The latter situation applies only to very large particles and these are quantitatively infrequent on the study beaches. The concentration profiles of entrained sediments in the water column vary with differences in turbulent structure so that mean current speed is a poor estimator of flow sediment load. Between 50 and 95% of the sediment load in both the swash and backwash occurs in the lower one tenth of the water column so that transport is dominantly in the bedload phase. This feature is especially pronounced in the backwash. Bedload sediment motion in the swash zone occurs mainly in the form of sheet flow, though antidunes may be formed in the backwash and saltation is locally important near the swash limit. Energy levels in both the swash and backwash rise with increasing wave height and period, the backwash becoming dominant at higher energy levels. The chief determinant of the flow structures and morphological results of given flow regimes is the phase ratio of swash period to breaker period. For low values of the ratio there is little interference between incoming and outgoing flows, up to one third of the swash volume may be stored in the beach, the flow regime is dominantly tranquil, and the foreshore accretes. Circulation of sediments through the breakers is intermittent and would appear to occur mainly in rip currents. For values of the ratio near unity (transitional conditions) a scour zone is developed by the backwash owing to alterations in flow structure during downslope passage of the water and some erosion occurs, thus offsetting swash deposition to some extent. At high values of the ratio interference between incoming and outgoing flows is continual. The backwash scour zone is very wide and seaward circulation of sediments through the breaker is continuously developed. Flow turbulence and asymmetries are at maximum levels and suspended load transport accounts for a quantitatively significant proportion of total load transport. The foreshore erodes intensively. Characteristic sediment sorting processes are associated with these flow regimes. Tranquil, low energy conditions result in truncation and mixing of the finer fractions. Polymodal or bimodal size-frequency distributions may be produced by deposition at the swash limit. For higher energy flow regimes transport of whole bed size ranges occurs, though individual component fractions may move at differing net rates and in net opposite directions. Where wide ranges of sizes occur on the bed bimodal distributions are produced by backwash scour owing to selection and preferential erosion of a mid-range fraction. Owing to the retarding effect of groundwater storage all of these processes have a strong tidal aspect with maximum swash and backwash energy levels occurring some time after high water during the ebb tide phase. High rates of groundwater return to the backwash may result in significant fluidisation of bed sediments. Residual groundwater flow in the form of thin surface sheets results in the formation of rills and rhomboidal ripple marks.