SAND BOTTOM EROSION AND CHANGES OF AN ACTIVE LAYER THICKNESS IN THE SURF ZONE OF THE NORDERNEY ISLAND

Transcription

1 SAND BOTTOM EROSION AND CHANGES OF AN ACTIVE LAYER THICKNESS IN THE SURF ZONE OF THE NORDERNEY ISLAND Kos'yan R. 1, Kunz H. 2, Podymov l. 3 1 Prof.Dr.,The Southern Branch of the P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences Gelendzhik-7, Russia. Fax kosyan@sdios.sea.ru 2 Prof.Dr., Coastal Research Station of the Lower Saxonian Central Stat Board for Ecology. An der Muehle 5, D Norderney. Germany. Fax kunz.crs@t-online.de 3 Dr., The Southern Branch of the P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences Gelendzhik-7, Russia. Fax ipodymov@inbox.ru Abstract On the basis of a field experiment "Norderney-94" there are examined peculiarities of bottom deformations in the wave breaking zone, when there is a modulation of wind waves under the influence of tidal processes. Introduction Sediment transport in tideless seas depends mainly on wind waves and wave-induced currents in the surf zone. In tidal conditions water level and currents effect considerably on the sediment transport too. Direction and intensity of cross-shore and longshore sediment transport depends on the current velocity, which usually is presented as a vector sum of time-mean current velocity depending on wind waves and periodic component of a tidal current. Near the shore in the surf zone velocity of tidal currents is usually few centimeters per a second. The net sediment transport depends basically on longshore currents and timemean currents in the cross-shore direction. On the other hand the tidal water level oscillations result in displacing of the surf zone in the course of storm, changing of both kinds of velocity and draining of a part of the profile with the tidal periodicity. These factors determines the final bottom deformations along the submerged slope profile. In this paper on the basis of field data there are examined peculiarities of the bottom relief deformations during the storm under changing water level and wind wave modulation with the frequency of tidal oscillations in the nearshore zone in the northwestern part of the Norderney Island. Instrumentation and measurements Investigations of bottom deformations were fulfilled in October, 1994 on the beach section situated on the north-western coast of the Norderney Island in the North Sea (Figure 1a, 1b). A beach profile relative to a mean annual water level (MAWL), disposition of measuring points on it and measuring instrument are given in Figure 2. Special steel pins with mobile plates placed in 15 points of submerged slope were used for the determination of bottom deformations and thickness of an active sedimentary layer averaged for 24 hours (2 tidal cycles) (Figure 2). Sand level gauges were used to research high-frequency fluctuations of the bottom level (Podymov, Kos'yan, 1997; Kos'yan, Kunz, Podymov, 1995).They were installed in three points within a drained part of profile (Figure 2). A general view of the sand level gauge is presented in Figure 3. Kos'yan R., Kunz H., Podymov I. 189

2 Figure 1a. Site of the field experiment. m1c Groin Е1 m1b g1 O1 m1a O2 2.0 Groin D1 W1 W2 W3 m1 m2 m3 O3 EMBANKMENT Figure 1b. Bottom topography at the area of measuring in the beginning of the experiment. Kos'yan R., Kunz H., Podymov I. 190

3 Synchronously with the measuring of the bottom level fluctuations in the same points there were recorded longshore and cross-shore velocities, free surface elevations and suspended sediment concentration. There were used, successively, electromagnetic current meters, pressure gauges and optical turbidimeters (Kos'yan et al., 1994). 34 series of recording were done during the experiment. The duration of every recording was 90 minutes with sampling rate being 2 Hz Distance from the MAWL, m bottom profile steel pins with mobile plates pressure gauges sand level gauges electromagnetic current meters optical turbidimeters Cross-shore distance, m Figure 2. A scheme of positioning of measuring devices at the testing site. Figure 3. Sand level gauge. Kos'yan R., Kunz H., Podymov I. 191

4 Measurements were fulfilled on a sandy beach where a mean diameter of particles was 0.21 mm. Grain size distribution of sand in the measuring point, where the sand level gauge was used, is given in Figure 4. % Norderney beach sand Dmean=0.212mm ϕ d, mm Figure 4. Grain-size distribution for Norderney beach sands. Wave conditions Measuring was performed during two storms from September, 25 to October, 07. The main part of measurements was done during the second storm lasting from the 3rd to 7th of October, when wind waves with the period, being 7-12 sec., were approaching the shore at a small angle from the north-north-west. Figure 5d shows a change of the wave height averaged for 15 minute intervals in the point where the depth was 5 m relative to MAWL. Measurements were carried out during the second storm. Wave height modulation with the period close to tidal one is vividly traced on the diagram. The largest wave height (about 1.3 m) was observed during the high tide. During the ebb the wave height reduced roughly half and was m. So strongly pronounced influence of tidal oscillations upon the height of wind waves approaching the beach can be explained by the presence of a shoal at the distance of one kilometer from the shore. There the first wave breaking takes place. Then, when going to the shore, the wave height undergoes an additional transformation. After the fall the wave height becomes roughly equal to the depth above the shoal. Therefore in the surf zone there were observed the changes of the wave height proportional to the change of the tidal water level, that determined the water depth above the shoal. Kos'yan R., Kunz H., Podymov I. 192

5 Active bottom layer thickness, m September - October Sep 1-Oct 2-Oct 3-Oct 4-Oct 5-Oct 6-Oct 7-Oct a Distance from the MSL, m b m1a Tide level, m Mean wave height, m d Sep 1-Oct 2-Oct 3-Oct 4-Oct 5-Oct 6-Oct 7-Oct Figure 5. Fluctuations of an active layer thickness ( a ), bottom level ( b ), tidal level ( c ) and a mean wave height ( d ) in the course of storm. c Results Fluctuations of an active layer thickness, bottom level, tidal level and a mean wave height in the course of storm, measured in the point m1a are presented in Figure 5. Blue lines in the upper part of the diagram show the fluctuations of the thickness of an active layer, which have been measured with the help of mobile plates. Red dashed line demonstrates bottom level changes measured on pins, and brown line presents a process of Kos'yan R., Kunz H., Podymov I. 193

6 continuous bottom level fluctuations during the storm according to the data obtained with the help of sand level gauge. Middle and low diagrams show the changes of water tidal level and wave height, successively, measured in the point where the depth was 5 m relative to MAWL. There are vividly seen bottom level fluctuations with amplitude being 5-8 cm and with frequency of tidal oscillations. A local erosion of the bottom is timed to the phase of a high water and the accumulation - to the phase of low water. The analysis of these data has shown that local maxima of the washout takes place minutes later the maximum level of tide. This interval increases to minutes during the storm attenuation. Local erosion of the bottom during a high tide is caused by an intensive suspension and offshore sediment transport. The data on suspended sediment discharge during these periods in the measuring points indicate this (Kos'yan et al., 1996). In Figure 5 there are also seen separate fluctuations of the bottom level with the amplitude of 1 cm. They are not of periodic character and most likely are connected with local short changes of the wind wave regime in the periods of high and low water. Cross-shore distance, m Time, hours Figure 6a. Isolines of the bottom deformation along the submerged slope on the central line during the experiment. Kos'yan R., Kunz H., Podymov I. 194

7 Figure 6a shows isolines of bottom deformations along the drained part of submerged slope in the course of the experiment obtained according to measuring data of benchmark pins during the ebb. Here the distance from the shore is given on the Y-axis, and the time of the measuring start - on the abscissa axis. Negative values mean the bottom erosion, and positive ones - its accretion. Figure 6b demonstrates three-dimensional drawing of the bottom deformations along drained part of submerged slope during the whole experiment, but the bottom view is given from the sea side Figure 6b. Three-dimensional drawing of the bottom deformations along drained part of submerged slope during the whole experiment (bottom view is given from the sea side). The same deformations as the thickness of an active layer are presented in Figures 7a,b. Maximum 24 hour deformations of the bottom within 100 m far from the coastline, being cm were recorded on September, (during the first storm) and on October, 4-5 (during the second storm), when the height of approaching waves was the largest one. The bottom erosion prevailed during the whole period of observations. A short period of sediment accumulation took place only during the phase of the storm damping. A maximum thickness of an active layer during the first storm was 33 cm at the distance of m from the shore (Figure 7a). During the second storm high values of an active Kos'yan R., Kunz H., Podymov I. 195

8 layer thickness (up to 35 cm) prevailed along the whole length of the profile. As a rule, an active layer thickness exceeded absolute values of bottom deformations. Cross-shore distance, m Time, hours Figure 7a. Isolines of the active layer thickness along the submerged slope on the central line during the experiment. Kos'yan R., Kunz H., Podymov I. 196

9 Figure 7b. Three-dimensional drawing of the active layer thickness along the submerged slope on the central line during the whole experiment (bottom view is given from the sea side). A comparison of the beach profiles measured before the experiment and after the second storm is shown in Figure 8. Figure 9 shows diagrams of the bottom profile before the second storm (October, 3-7) and after it. A resultant bottom erosion near the coastline was cm and it reduced gradually to 1-3 cm in the offshore part of the profile. At the average, during two storms the bottom level along the profile reduced for cm. Sediments are transported from the erodible zone offshore, outside the boundary of the wave breaking, and there their accumulation is observed. In tideless seas with fine sandy bottom and a small inclination of relief there are secondary bars behind the boundary of the wave breaking. They appear when the wave height is 1 m. On the observed profiles of the Norderney test-ground submerged bar was absent. Most likely, this is connected with the migration of the surf zone boundary owing to water level fluctuations and height modulation of waves approaching the shore with the periodicity of 12 hours. Kos'yan R., Kunz H., Podymov I. 197

10 Depth from the MAWL, m MAWL Cross-shore distance, m Figure 8. Profiles of the submerged slope relative to a mean national level at the beginning and at the end of the experiment. Depth from the MAWL, m beginning beach profile beach profile after the storm MAWL Cross-shore distance, m Figure 9. The diagrams of the bottom profile before the second storm (October, 3-7) and after it. Conclusion Obtained data show that in conditions of modulation of the height of waves approaching the shore with frequency of tidal change of level, the bottom erosion during the storm prevails on the drained part of the beach profile. Most likely, the prevalence of bottom erosion depends on migration of the boundary of wave breaking zone owing to modulation of height of waves approaching the shore with tidal cycles. In its turn, such boundary migration promotes intensive outflow of water with suspended sediments offshore. The process of an intensive erosion and suspended sediment drift is typical for the wave breaking zone. Maximum bottom deformations are timed to the periods of Kos'yan R., Kunz H., Podymov I. 198

11 influence of the highest waves during the storm. During the period of tidal cycles they were not larger than cm. The largest thickness of an active sedimentary layer, being 35 cm, was observed during the same time intervals. Measurements with the help of sand level gauges have shown in some points of the beach profile the bottom level fluctuations with a tidal frequency and amplitude being 5-8 cm, and high-frequency fluctuations with amplitude of 1 cm with non-pronounced periodicity. References Kos'yan R., Kunz H., Podymov I. (1995). Employment of electronic sand level gauges for measurements of beach slope deformation on Norderney island. Proc. of the Second International Conference "Coastal Dynamics'95". ASCE, New York. P Kos'yan R., Kuznetsov S., Podymov I., Pushkarev O., Pykhov N., Grishin N., Harizomenov D. (1994). Nearshore suspended sediment concentration measuring during storm. Proc. of the Second International Symposium "LITTORAL'94", v.1. Lisbon, Portugal. P Kos'yan R., Kunz H., Kuznetsov S., Pykhov N. (1996). Suspended sediment transport in the surf zone of the Norderney island. Proc. of the Second International Conference on Hydrodynamics. Hong Kong. P Kunz H., Kos'yan R. (1997). German-Russian nearshore dynamics experiment on Norderney island. Proc. of MEDCOAST'97. Podymov I., Kos'yan R. (1997). Sand level gauge. Patent on Invention of the Russian Federation # (in Russian). Kos'yan R., Kunz H., Podymov I. 199