Sea water intrusion into a fresh water forebay due to wave action

Journal of Hydrology 6 (1968) 95-101; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

SEA WATER INTRUSION INTO A F R E S H WATER FOREBAY DUE T O WAVE A C T I O N * K E N N E T H L. DYER and JEROLD J. BEHNKE** Abstract: Sea water intrusion adversely affects the water quality of the Salinas River Lagoon near Castroville, California. Bench levels indicated that the forebay is above mean sea level; however, additional investigations showed that wave action maintains a water level beneath the beach which is above mean sea level and the forebay. Wave action produces a net landward gradient resulting in underflow from the sea into the forebay. The quantity of sea water added to the lagoon was calculated from salt balance and from gradient-transmissibility relationships. The amount of sea water underflow calculated by these two independent methods was 24.5 and 25.1 acre-feet, respectively. Tides, wave action, and density differences between fresh and salt water should be considered in the interpretation of hydraulic relationships between coastal lagoons and the sea.

Introduction

Sea water intrusion of coastal ground-water aquifers in California has been reported in numerous localities, State of California, Department of Water Resources 1). Little has been written concerning the interplay between oceans and fresh-water lagoons or forebays where rivers at high stage discharge into seas. Often, fresh-water lagoons form behind beaches when the river stage recedes. In the Salinas River Basin sea water has penetrated several miles inland in deep aquifers in response to increased pumping in recent decades. This study is restricted to near-surface sea water underflow into the Salinas River Lagoon. The Salinas River forebay (Fig. 1) is landlocked for about nine consecutive months each year, and is essentially a reservoir except during periods of intense runoff. As winter flows recede, spring storms and tides construct a sand bar which prevents additional surface outflow. Waves continually roll upon the sand bar and occasionally, during extremely high tides or storms, spill over into the lagoon. In the fall or winter when the river flow increases, * Contribution from Soil and Water Conservation Research Division, Agricultural Research Service, U.S. Department of Agriculture. ** Formerly Soil Scientist (Chemistry) and Geohydrologist, respectively, Fresno Field Station, Southwest Branch, Soil and Water Conservation Research Division, Fresno, California.

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the sand bar is artificially breached to prevent drowning adjacent croplands. The forebay level is controlled by a culvert when the sand bar prevents surface outflow. Water flows through the culvert into a drainage ditch which discharges through tide gates into Monterey Bay at Moss Landing. After surface outflow has ceased, the forebay salinity is approximately 1/10 that of sea water. In spring and early summer the forebay is freshened by continued river flow and all but its seaward edge is acceptable for agriculture. In late summer and fall most of the forebay becomes too saline for agricultural use. The purpose of this study was to determine the mechanism by which salts are contaminating the forebay, and to suggest remedial measures.

Field investigations By April 1, 1964, the sand bar had blocked surface outflow from the Salinas River into the Pacific Ocean. Eight piezometers were installed in May 1964, and read approximately every 4 h for 32 h. Fig. 2 shows that the

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lagoon was 2.3 feet above mean sea level. Fig. 2 also shows that in May 1964 average water levels on the ocean side were in piezometric equilibrium with the surface of the lagoon. The equilibrium lagoon level was about 2.1 feet above mean tide level. Tl~is equilibrium was responding to increased water elevations under the beach resulting from wave action. Chemical analyses of water taken from the piezometers showed that sea water extended beneath the dune to the vicinity of the lagoon edge. 5

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98

KENNETH

L. D Y E R A N D J E R O L D J. B E H N K E

tion for the same tide. The data indicated that the average water level under the beach was above both the tide and lagoon levels. A portion of the 0.85-foot discrepancy in mean sea level may be due to seasonal changes in sea level. Studies by La Fond '2) and Pattullo 3) indicated that the September sea level along the southern California coast is approximately 0.2 to 0.5 foot above the mean sea level. Such sea level anomalies are related to seasonal changes in ocean densities, which in turn are influenced by temperature changes and oceanic rainfall, evaporation, etc. The remaining discrepancy (0.35 to 0.65 foot) in mean sea level is possibly attributed to regional or local subsidence within the Salinas River Basin. The nearest bench mark is located on a railroad bridge abutment resting in waterlogged alluvium. In August 1964, a water-level recording well was installed near the crest of the dune separating the forebay and the ocean. This installation provided a continuous record of the water level changes beneath the dune. Fig. 3 shows the forebay elevation and water levels under the dune determined from the recording well charts. Forebay elevations and water levels beneath the dune are not directly comparable without a density correction, because the tip of the forebay is underlain by sea water. The density correction for 9.1 feet of fresh water (the approximate depth of the forebay at its seaward edge) is about 0.20 foot. Fig. 3 shows that the forebay and water level under the dune were approximately 2.3 feet above mean sea level in September. After

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September, higher fall tides and wind-storms pushed the waves further inland, resulting in considerable elevation changes in the lagoon and beneath the beach. Wiegel 4) and Isaacs and Bascom 5) found that ground-water levels beneath beaches were higher than their tidal counterparts. In most cases this observation could be attributed, in part, to fresh ground water flowing seaward. Isaacs and Bascom 5) observed that successive waves impinging upon the beach saturated the sand, giving an approximation of standing water at that elevation. Fresh-water underflow was probably negligible in the studies of Emery and Foster6). They reported water levels at the beach edge from 0.0 to 5.0 feet above the corresponding tide level, the greatest discrepancy occurring at low tide. Their E1 Segundo Beach data showed the effect of waves on ground-water elevations beneath beaches, but they did not comment on this anomaly. Forebay water samples taken near its seaward edge indicated that the salt-fresh water interface was approximately at the seaward edge of the forebay. Water samples were taken monthly at several locations in the forebay and analyzed for chlorides. The analyses showed a gradual chloride buildup in the forebay resulting from sea water intrusion. The average water storage in the forebay from September 15 to December 10, 1964 was 435 acre-feet. During this same period the chlorides in the forebay increased from 13 to 27 meq/1, an amount equivalent to that contained in 13.5 acre-feet of seawater. During this same interval, chlorides equivalent to about 3.2 acre-feet of sea-water entered the lagoon via the Salinas River and chlorides equivalent to 14.2 acre-feet of sea water left the lagoon via the culvert. Salt balance calculations from these data suggest that 24.5 acre-feet of sea water entered the lagoon during the period from September 15 to December 10, 1964. The sea water volume entering the forebay also can be calculated for any given hydraulic gradient between the beach and the forebay from Eq. (1): (1)

Q = TIL

where, Q is the discharge in gallons per day, T is the coefficient of transmissibility in gallons per day per foot, I is the hydraulic gradient in feet per foot, and L is the width in feet of the cross section through which discharge occurs. The coefficient of transmissibility for the aquifer between the recording well and the beach was calculated using the sinusoidal head fluctuation equation developed by Ferris e t al.7):

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(2)

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KENNETH L. DYER AND JEROLD J. BEHNKE

Where, t o is the p e r i o d o f stage fluctuation in days, S is the coefficient o f storage, X is the distance f r o m the o b s e r v a t i o n well to the surface-water c o n t a c t with the aquifer a n d t t is the time-lag o f a given m a x i m u m or mini m u m g r o u n d - w a t e r stage, following a similar surface-water event. The coefficient o f transmissibility equaled 38000 gallons p e r d a y per f o o t for the zone m e a s u r e d , a n d it was a s s u m e d this value held for the entire beach at the edge o f the forebay. A c c o r d i n g to Ferris et al. 7) Eq. (2) will be satisfactory for water table (unconfined) c o n d i t i o n if (a) the o b s e r v a t i o n wells are far e n o u g h f r o m the s u b - o u t c r o p so t h a t they are unaffected b y vertical flow c o m p o n e n t s a n d (b) the range in fluctuations is o n l y a small fraction o f the s a t u r a t e d thickness o f the aquifer. C o n d i t i o n (a) is satisfied because tidal a n d wave effects are nil near the l a g o o n edge, a distance o f only 321 feet. I n t e r m s o f h y d r o l o g i c b o u n d a r y analysis, m e a n tide level represents a line source a n d the l a g o o n edge an a p p r o x i m a t e infinite distance. C o n d i t i o n (b) is satisfied because the s a t u r a t e d thickness o f the entire aquifer is a p p r o x i m a t e l y 400 feet; therefore, the r e q u i r e d b o u n d a r i e s are satisfied a n d Eq. (2) provides an a c c e p t a b l e solution. T h e 25.1 acre-feet o f sea-water entering the l a g o o n between S e p t e m b e r 15 a n d D e c e m b e r 10, 1964 as calculated f r o m Eqs. (1) a n d (2) closely agree with the 24.5 acre-feet d e t e r m i n e d b y the salt b a l a n c e m e t h o d . D u r i n g except i o n a l l y high tides, in late fall, some waves washed over the s a n d dune, directly a d d i n g some sea-water to the forebay. T h e a m o u n t o f sea-water a d d e d in this m a n n e r was p r o b a b l y negligible in c o m p a r i s o n with the int r u s i o n o f sea-water t h r o u g h a n d u n d e r the dune.

Summary and conclusions The effects of wave action and tidal fluctuations need to be considered in the hydrologic study of coastal, fresh-water forebays. The hydrologic system inland from the beach may often be responding to an equilibrium water level several feet above mean sea level. Where the required boundary conditions are met the time-lag equation of Ferris et al. 7) is useful in estimating water transfer between the sea and fresh-water forebays. In this study the volume of sea water entering the forebay as determined by the time-lag equation was similar to the amount of intrusion determined from salinity data. Fresh water may be maintained in a lagoon having hydrologic conditions similar to those of the Salinas Lagoon by blocking surface outflow as the river flow recedes, thus preventing a tide-induced interplay of river and sea water and maintaining the lagoon level at a sufficient elevation to prevent a landward gradient. A tile drain open to the sea placed under the center of the barrier dune, parallel to the river axis would lower the mean piezometric surface to approximately mean sea level. Sealing the landward end of the drain would return a portion of the wave-built ground-water mound to the sea, effectively preventing the landward dissipation of the mound.

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Acknowledgments The a u t h o r s wish to t h a n k Vince Shally, H o m e r M a r i o n , a n d Leon Pearce (Soil C o n s e r v a t i o n Service, Watsonville, California) for their c o n t i n u i n g assistance t h r o u g h o u t all phases of the study. W e w o u l d also like to t h a n k D e a n C. Muckel (Agricultural Research Service) for his c o n t i n u i n g guidance and encouragement.

References 1) State of California, Department of Water Resources, Division of Resources Planning, Bulletin No. 63, Sea-water intrusion in California (November, 1958) 91 pp. 2) EUGEr4EC. LA FONt), Variations of sea level on the Pacific Coast of the United States. J. Marine Res. 2 (1939) 17-29 3) JUNEC. PATTULLO,Seasonal variation in sea level in the Pacific Ocean during the international geophysical year, 1957-1958. J. Marine Res. 18 (1960) 168-184 4) ROBERT C. WIEGEL,Oceanographical engineering. (Prentice Hall, Englewood Cliffs, N.J., 1964) 532 pp. (Previously unpublished data by D. A. PATRICK(1950) given on p. 352) 5) J. D. ISAACSand W. W. BASCOM,Water table elevations in some Pacific Coast Beaches. Trans. Amer. Geophys. Union, 30 (1949) 293-294 6) K. O. EMERYand J. F. FOSTER,Water table elevations in marine beaches. J. Marine Res. 7 (1948) 644-654 7) J. G. FERRIS,D. B. K~OWLES,R. H. BROWNand R. W. SVALLMAN,Theory of aquifer tests. Geological Survey Water-Supply Paper 1536-E, pp. 132-135, and 73-74 (1962)