COASTAL AQUIFER MANAGEMENT: monitoring, modeling, and case studies - Chapter 7

CHAPTER 7 Determination of the Temporal and Spatial Distribution of Beach Face Seepage D.W. Urish 1. INTRODUCTION Man is a creature closely linked to the coastal areas for many reasons. Some 70% of the earth’s population live within coastal zones, with the large portion of that population within a few kilometers of saltwater. Historically, as well as today, the saltwater seas are the main access to both the products of seas, as well as the lands beyond, a natural location for the development of commerce, habitation, and industrialization. This heavy concentration of mankind and his activities creates many anthropogenic products detrimental to the environment and to man himself. Much of this environmental impact moves into the groundwater system as a natural consequence of the hydrologic cycle. The impact of civilization is most keenly recognized in the more confined and poorly flushed estuaries, bays, and coastal lagoons. Within the larger concept of global water budgets, all freshwater falling on the terrestrial components of the earth eventually returns to the “mother of waters,” the saltwater seas. The path of a molecule of water may be long and tenuous following varying hydraulic gradients until it finally reaches its original source and the hydrologic cycle repeats. The meeting of freshwater with saltwater may be a glacier caving its icebergs into the sea, mighty rivers, or in our area of interest the more subtle, but constant discharge of coastal fresh groundwater. The time of transient through the ground may range from many years for coastal plains and large peninsulas to days for small islands and near-shore recharge. But eventually it reaches the saltwater, carrying with it many terrestrial derived components, both natural and anthropogenic. The increased recognition of the importance of the coastal groundwater discharge zone, and the greatly increased capabilities for ` 2004 by CRC Press LLC Figure 1: Fresh groundwater flow and discharge pattern (after Glover [1964]). data collection and analysis, have encouraged the study of the dynamic aspects of tidal effects for coastal groundwater seepage analysis [Gilbin and Gaines, 1990; Millham and Howes, 1994; Portnoy et al., 1998]. The objective of this discussion is to describe the dynamic concept of the coastal freshwater–saltwater relationship and the techniques that can be used to determine coastal fresh groundwater seepage in a quantitative and qualitative form. The descriptions and methods described are primarily directed to the more quiescent shores of the relatively sheltered bays and lagoons, and generally the source of most critical environmental concerns. It is further most applicable to the sandy seashore, influenced by the changing water levels of the ocean tides. In many cases a sandy beach or cove, even on the rock bound coast, is the zone of primary fresh groundwater discharge. 2. CONCEPTS 2.1 Freshwater-Saltwater Relationships Where freshwater meets saltwater in a permeable landmass, the freshwater will tend to float on the more dense saltwater according to the Ghyben-Herzberg Principle [Drabbe and Ghyben, 1889; Herzberg, 1901]. In ` 2004 by CRC Press LLC Figure 2: The sequence of coastal groundwater discharge through a sandy beach during the tidal cycle. an insular landmass, such as an island or peninsula, this configuration of body of freshwater will approximate a lens, bounded by and underlain by saltwater. The coastal manifestation of this lens is a pinching out of the lens at the coastal boundary to discharge through a narrow zone at the tidal margin described in a steady state theoretical case by Glover [1964], and as further illustrated for a coastal margin in Figure 1. Delineation of coastal discharge is a much more elusive problem when one considers the changing groundwater conditions in the inter-tidal zone incorporating the complexities of a boundary which changes cyclically twice a day both laterally and vertically, highly variable salinity, fluctuating hydraulic heads, and a geologically heterogeneous beach [Turner, 1993a; Baird and Horn, 1996; Robinson and Gallagher, 1999; Li et al., 2000]. 2.2 The Moving Boundary In tidally influenced coastlines both the freshwater lens and the discharge patterns are greatly changed from a static condition, depending on the topography and geologic nature of the beach inter-tidal zone. The water table in the coastal groundwater moves up and down with the tide; concurrently the boundary on a sloping beach surges shoreward and seaward; the beach is flooded with saltwater twice a day, and in many cases the hydraulic discharge gradient itself changes direction, an extremely complex and dynamic situation. The basic process of coastal groundwater discharge ` 2004 by CRC Press LLC through an idealized homogeneous sandy beach during a tidal cycle is illustrated in Figure 2. During high tide, groundwater flow is hydraulically blocked, with a reverse hydraulic gradient toward the land imposed by the tide, which is higher than the near-shore water table; additionally, saltwater will infiltrate into the land surface adding to and mixing with the fresh groundwater in the beach. As the tide ebbs the hydraulic gradient reverses and groundwater flow consisting of both salt and freshwater moves toward the lower beach. As low tide approaches groundwater discharge occurs, both as beach face seepage and lower beach submarine discharge. With the rising tide a reverse hydraulic gradient is again established and the groundwater discharge ceases. The cycle then repeats. Field sampling of coastal groundwater discharge is greatly complicated by the transient nature of the tidally induced changing boundary. The timing and location of the quality of groundwater in three dimensions becomes critical for groundwater sampling. This is further complicated by the indistinct and changing salinity of the beach groundwater and discharge. The earliest freshwater lens models made no attempt to discretely character the hydraulic and chemical nature of the seepage, treating it as a fixed sharp line in time and space. A significant advancement was the theoretical formulation of the discharge gap representation to describe coastal seepage by Glover [1959] and further described by Cooper [1965] under steady state conditions. This, however, failed to take into account anything other than the assumed discharge without regard for the salinity of the discharge. The distribution of the discharge as a decreasing exponential pattern was first examined in a field setting on the shores of Long Island by Bokuniewicz [1980, 1992], referencing earlier freshwater lake seepage studies by McBride and Pfannkuch [1975]. These field observations, however, were under essentially tideless conditions. Because of the laterally moving boundary on a sloping beach, there is a much wider outflow gap as well as major changes in the flow pattern of the discharge, including in many cases a complete reversal of flow and salinity. A beach face model, SEEP, was developed by Turner [1993a] to analyze and predict the exit dynamics of groundwater seepage with a falling tide. Turner further describes the role of the capillary fringe in the total water content of the beach. 2.3 Beach Slope Effect While the determination of mean sea level (MSL) in the open coastal water system is a necessary base line, it should be recognized that in a ` 2004 by CRC Press LLC sloping beach there is a dynamic phenomenon caused by the tide movement which can create an “effective mean sea level” (EMSL) in the beach considerably above open water measured MSL [Urish and Ozbilgin, 1989]. This was later elaborated on by Nielsen [1990] and Hegge and Masselink [1991]. The seawater is mounded in the upper beach by the dynamic movement of tide and consequent infiltration of saltwater as it moves up the beach face. There is, in effect, a pumping action caused by rapid infiltration of the seawater in the upper beach during high tide and much slower drainage of the seawater through the lower beach at low tide. This results in a super elevation of the apparent sea level boundary condition, which has been measured as much as 0.5 feet above open water MSL for a 5 foot tide range on a 0.05 beach slope [Urish, 1980]. This becomes important in modeling coastal boundary conditions. The inter-tidal beach is subjected to seawater flooding and infiltration from the rising tide, which is then a substantial component of the beach discharge. The rising edge of the incoming tide advances shoreward faster than the discharging freshwater can rise. Thus, the seawater quickly fills the available pore space in the sands of the upper beach, sometimes rising rapidly enough to trap air under the surface. The quantity of infiltrated saltwater in the beach which becomes seepage depends on the residual water content from the previous saturation episode, as well as the downward directed hydraulic gradient. The residual water in the upper portion of the inter-tidal zone is usually a layered mixture of saltwater over freshwater with some mixing, depending on the magnitude of the freshwater discharge and the antecedent drainage characteristics of the beach. As Bokuniewicz [1992] points out, however, saline pore water overlying fresh pore water has an inherently unstable density gradient, causing “fingering” of the different densities of water to occur; this leads to greater uncertainty in any attempts at determining the volume of infiltrated saltwater directly. The presence, however, of a substantial layer of infiltrated saltwater overlying freshwater in the inter-tidal zone is well established by both direct water table sampling [Portnoy et al., 1998] and by indirect surface electrical resistivity soundings in the inter-tidal beach [Frohlich, 2001]. 3. METHODOLOGY 3.1 Elevation Measurements 3.1.1 Elevation Control and Datums In order to relate water levels to the beach and near-shore surfaces it is essential that beach topographic profiles be made and referenced to a fixed ` 2004 by CRC Press LLC ... - tailieumienphi.vn