Chapter 17 Engineering Projects in Coastal Lagoons

Coastal Lagoon Processes edited by B. Kjerfve (Elsevier Oceanography Series, 60) 0 1994 Elsevier Science Publishers B.V.All rights reserved

507

Chapter 17

Engineering Projects in Coastal Lagoons Per Bruun 34 Baynard Cove Road, Hilton Head Island, South Carolina 29928, USA

The development of coastal lagoons for practical purposes is an old field. Its start was in China and India where, thousands of years ago, canals were built with primitive tools to serve navigation between lagoons. The lagoons were developed for fisheries and salt extraction by dams and gates regulating water tables and flow. The development of lagoons for engineering purposes accelerated after engineering tools became available for major engineering works involving management of water tables, dredging of canals and basins, establishment of fishing and later pleasure craft harbors and fish farms. With man’s increased activities followed the possibility of certain adverse effects on the natural environment. This chapter deals with all of these subjects. In conclusion, I recommend proper planning, considering all pertinent environmental aspects associated with engineering in coastal lagoons.

Introduction

A coastal lagoon is a water body located inside a barrier, a barrier island, a spit or similar coastal feature. It often receives freshwater discharge from the mainland and it is almost always drained to the sea through a tidal inlet or entrance (Kjerfve, 1986). Coastal lagoons occur singularly or in connection with other longshore lagoons. Most lagoons are natural, although man-made lagoons may have been dredged to serve development purposes, e.g. as canals, basins, marinas and perhaps aquaculture projects. A coastal lagoon is subjected to forces by winds, a variety of currents, waves, and sediment transports. Its water table fluctuates in relation to inflows of freshwater, salt water, sewage discharges and wind stresses. Natural lagoons on barrier shores which occasionally may be overrun by storm tides, e.g. hurricanes, may be subjected to short-term major inflows of devastating effects to the hydraulic stability, including salinity and the accompanying adverse effects on biological life. In addition, overflow-fans of sand may destroy marsh grass.

508

Engineering Projects in Coastal Lagoons

Water Table Management of Coastal Lagoons

A coastal lagoon receives inflows from rivers, drainage canals, sewer outlets, and precipitation. Tidal fluctuations through inlets and entrances bring salt water into and out of the lagoon. Water is also lost from the lagoon by evaporation, which is considerable in tropical and sub-tropical climates. Control of Freshwater Inflows In arid areas where freshwater inflow and precipitation are limited, high concentrations of dissolved minerals may develop. Many coastal salt ponds or lagoons, e.g. on Australian, Caribbean, and Indian shores, are examples of this phenomenon. Engineering controls of freshwater discharges into coastal lagoons are found in a number of dams or dike structures that are supplied with gates which regulate the outflow of water creating the desired balance between water tables on either side of the gate. The hydraulic literature has many examples of such regulating works often administered by Flood Control Districts. An example is the Southern and Central Florida Flood Control District, which handles the drainage of Lake Okeechobee and the Everglades. Control of Saltwater Inflows The flow through coastal inlets is naturally-controlled by the establishment of equilibrium cross sectional areas of the gorge channel. The equilibrium sectional area is a function of the discharge and maximum flow velocities within a narrow range of 1 m s-1 (Bruun and Genitsen, 1960, Bruun et al., 1966, Bruun et al., 1978, and Bruun, 1989). The following is a brief representation of tidal hydraulics at coastal inlets in alluvial materials (Mehta and Joshi, 1988). Tidal Hydraulics Figure 17.1 shows a simple inlet-lagoon system. It consists of a tidal inlet connecting the lagoon with the ocean. Tributary inflow leaves the inlet as freshwater outflow. The lagoon may be a distinct physiographic feature or, as in the case of many river mouths, it may constitute the seaward end of the river influenced by tide. Most inlets have a rather well-defined throat section, i.e., flow cross section of minimum area, which is analogous to the ‘vena contracta’ of such flow-measuring devices as the Venturi meter. Inlets may therefore be calibrated by determining a coefficient that relates the discharge to the tidal head difference between the ocean and the lagoon. As a result of the high degree of variability in inlet physiography, such commonly used terms as channel, gorge, entrance, and mouth require

P.Bruun

509

Inflow

1:

or

Sea or Ocean

Fig. 17.1. Simple inlet-lagoon system with tributary inflow (after Mehta and Joshi, 1988).

clarification. The term channel is considered here to include the entire inlet length between the lagoon and the seaward ends of the land barrier, including jetties, where applicable. The gorge is the segment of the channel between the throat and the lagoon end of the channel. The entrance, or the mouth, is the region seaward of the throat up to and including the outer bar or ebb shoal. Thus the channel segment seaward of the throat is a part of the mouth. The term near-field is the region encompassing inlet-influenced waters in the lagoon and the ocean, not including the channel. Hydraulic information of typical interest includes temporal and spatial variations of currents and water level in the channel and vicinity. Depending on the degree of accuracy of the answer desired, several predictive approaches are available. Relatively simple analytical approaches, even though approximate, yield quick answers and are used quite extensively. Channel and near-field hydraulics are described in the sequel, with reference to analytic solutions followed by physical and numerical modelling.

Channel Hydraulics An idealized inlet is considered to be a relatively short and geometrically narrow, but hydraulically wide, open channel with mean cross-sectional area A,, mean depth h,, and length L,. The ocean tide represents the boundary condition, or the forcing function, at one end of the channel. The lagoon, representing storage, imposes the boundary condition at the other end. The one dimensional, depth- and width-averaged shallow (long) water wave equation for the channel is

au au

-at+ - =ax- g - - g

aq

ax

n2ulul

h;l3

(17.1)

510

Engineering Projects in Coastal Lagoons

Seo u.0

I

h,

Chonnel

U-

Boy u = o

Fig. 17.2. Contributions to total head loss in idealized channel (after Mehta and Joshi, 1988).

where u(x,t) = the cross-section averaged flow velocity in thex-direction, i.e., along the length of the channel; t = time, y ( x , t ) = the tidal elevation with respect to mean water level; and n = Manning's bed resistance coefficient. The term n2u 1 u I /ht'3 is the slope of the energy grade line in the channel. Given that y&) and y&) are the tidal elevations in the sea and in the lagoodbay, respectively, Eq. 17.1, upon integration over length, L,, yields q0

-q

B =-g at

+ k,, +-

(17.2)

where u and y are functions of time only. The quantities Iten and It,, are the head loss coefficients associated with channel entrance and exit flows, respectively. Three noteworthy assumptions inherent in Eq. 17.2 are: (1) current velocities in the lagoon and the ocean are negligible compared to those in the channel; (2) the tidal amplitude is small compared to the mean depth; and (3) change of water volume in the channel due to tidal variation is negligible compared to mean volume in the channel. The total head, yo - y ~ is , the sum of four contributions indicated by the distributed loss dashed lines in Fig. 17.2. These are: entrance loss, kenu2/2g; due to bed friction, 2gn2L,/htt3;head due to inertial, (Ldg)& / a t , and exit loss, k,,u2/ 2g. By analogy with steady-state electrical or acoustical problems, the term within parentheses in Eq. 17.2 has been referred to as impedance, F. Application of Eq. 17.2 additionally requires a continuity relationship for the bay storage volume V, i.e., Q = Qf+ dV/dt, where the channel discharge Q is equal to UA, and Qf= the freshwater discharge. At this point, an assumption is introduced concerning the lagoon, which is considered to be relatively small in surface area as well as deep, such that the tide propagates rapidly through the bay waters. As a consequence, spatial gradients in the lagoon water surface at any instance may be ignored. The condition has been referred to as hydraulic filling. Continuity may therefore be expressed in terms of velocity u according to

P.Bruun

511

AB ~ T B Qr u = --+ -( 17.3) A, dt A, where AB= the lagoon or bay area. Analytical Solutions Solving the governing equations, Eqs. 17.2 and 17.3, analytically requires some additional simplifylng assumptions. The ocean tide is usually considered to be sinusoidal, i.e., yo = a0 sin(ot - z) where a0 = the tidal amplitude, o = the tidal frequency; and z = the angular measure of the lag of slack water in the channel after mid-tide in the sea. The parameters Ag and Qfare assumed to be independent of time. Then, by eliminating u between Eqs. 17.2 and 17.3, the following is obtained:

(17.4) Solutions to Eqs. 17.3 and 17.4 that have appeared in the literature fall broadly into two categories: (1) those in which both the freshwater inflow Qf and the inertia term d2y$dt2have been ignored; and (2) those in which the middle term on the left hand side of Eq. 17.4 is essentially linearized or simplified (Bruun, 1978). A s a consequence of their relative simplicity, simplified solutions are commonly used, and it is worthwhile stating the assumptions under which they are obtained. The assumptions are: (1) the inlet and lagoon banks are vertical; (2) the range of tide is small compared with the depth of water everywhere and, as a corollary, the time variation of water volume in the channel is small compared to the mean channel volume; (3) the lagoon water surface remains horizontal at all times; (4) the mean water level in the lagoon equals that in the ocean; ( 5 ) flow acceleration in the channel is negligible; (6) there is no freshwater discharge; and (7) the tide in the ocean is sinusoidal. Thus the head difference, yo - y ~ is , due to bed frictional dissipation plus entrance and exit losses. Under these conditions, Eqs. 17.3 and 17.4 are simplified and can be solved for the channel current and the lagoon tide, both of which can be related uniquely to the dimensionless parameter, K = (A,,bAB)(2g/Fu0)'/2.This parameter is referred to as the coefficient of filling or repletion, since lagoon filling increases with increasing K. A definition sketch for the time-variations of yo, y~ and u is shown in Fig. 17.3. Principal parameters that define these curves are ao, U B , z or e (lag of slack water after high water or low water in the ocean), and the maximum

512

Engineering Projects in Coastal Lagoons

Slack Water in Channel

C I

Fig. 17.3. Sea tide, lagoon tide, and current through channel as functions of dimensionless time (radians)(after Mehta and Joshi, 1988).

velocity urn.A characteristic feature of Keulegan's (1967) result is that slack water (urn = 0) occurs when the lagoon elevation is at its maximum (or minimum) value, i.e., UB. Furthermore, as the lag E (= 0.5 n) - z increases, the lagoon tide becomes smaller until E approaches go", when there is no tidal variation in the lagoon. This limiting situation arises as K -+ 0, which B 0, or when the impedance F becomes can occur, for example, when A ~ I A+ very large as the inlet cross section diminishes. Another limiting situation occurs as K + 00, when E + 0. This is the case of a very wide inlet with large AJABor very small F. In this case U B approaches UO,and the lagoon essentially becomes a part of the ocean (as in the case of leaky lagoons) without the constructing influence of the inlet. Finally, the maximum velocity urn occurs when yo - y~ is a maximum at mid-tide in the lagoon. Equations 17.3 and 17.4 can be solved in a different way without excluding the inertia term, and with linearization of the equations themselves, but by ignoring the generated higher harmonics in obtaining a first-order solution, as shown by Ozsoy (Dilorenzo, 1986). This procedure, coupled with a sinusoidal variation of the flow velocity u , leads t o the results plotted in Figs. 17.4 and 17.5.The following dimensionless quantities are represented: lagoon amplitude, f i =~ uduo; channel velocity Q, = u,,,AJuo AB;tidal frequency, a1 = a(LAdgA,)Y2;and bed dissipation coefficient, p = a$'Ad2L~l,. Significant features (not predicted by Keulegan's results) are lagoon water level amplification ( f i ~> 1) under a certain range of conditions specified by a1 and p (Fig. 17.4) and lag E, greater than 90" (Fig. 17.5).Both features are exhibited by several inlet-lagoon systems (Dorrestein, 1961; Sorensen and

513

P.Bruun

Fig. 17.4. Dimensionless lagoon tide amplitude U B or channel velocity, urn,a8 a function of dimensionlessfrequency (after Mehta and Joshi, 1988).

i05

01

05

10

Dimensionless Frequency,

ui

50

Fig. 17.5. Log E as function of dimensionless frequency, a (after Mehta and Joshi, 1988).

Seelig, 1976). Another consequence of the retention of the inertia term is that unlike the Keulegan case, the time of slack water does not necessarily coincide with high or low tide in the lagoon. It can be easily shown that at slack, the lagoon and the ocean tide elevations differ by an amount equal to the contribution to the head from flow inertia. If inertia is ignored, ~ z s o y ' s solutions become similar in form to those obtained by Brown (1928).

514

Engineering Projects in Coastal Lagoons

Equations 17.3 and 17.4 can be solved inclusive of freshwater discharge,

as shown by Escoffier and Walton (1979).This analytic solution is achieved

by replacing the middle term on the left hand side of Eq. 17.4 with a linear term and then minimizing the integral of the difference between the linear and the nonlinear terms over a tidal period. In the absence of freshwater discharge, the resulting equations become identical to those given in Fig. 17.4 and 17.5. Applications of Eq. 17.3 (with Q = 0)and 17.4 (usually in the linearized form) to multiple inlet-bayllagoon systems by obtaining analytic solutions have been considered (Eiser, 1986). Shallow wetland areas surrounding lagoons assimilate water. This will cause some delays in the tidal outflow as well as the inflow, but the so-called “sheet flow” is usually a negligible fraction of the total water balance. It may increase considerably during storm tides (Eiser and Kjerfve, 1986). The stability of tidal entrances in relation to discharges at spring tide flows is dealt with extensively by Bruun and Gerritsen (1960) Bruun et al. (1966, 19781, and Bruun (1986,1989), the latter explaining the new developments in effective and economic bypassing methods to mitigate or solve the problem of the inevitable downdrift erosion. Such erosion is often very severe. In fact, it has been established that about 80% of the erosion of the Florida shores is caused by tidal inlets due to non-existing or inadequate bypassing procedures. Most of them only cure about 1/3 of the actual problem (Bruun, 1989). The State of Florida no longer allows material dredged in tidal entrance to be dumped offshore. If the sediment is suitable for beach nourishment, it shall be deposited on or just off of the downdrift beaches.

Control of Storm Surges in Lagoons High wind velocities may cause severe wind pile-ups by wind shear in shallow water lagoons. The first country t o realize this was undoubtedly the Netherlands. A simple formula for the calculation of the height of wind tides is the so-called “Zuider Zee” formula (Bruun, 1959):H = (WF)/(1,400 d ) cos a.This equation expresses the set-up of water by the wind (in feet above the original still-water elevation) of the leeward end of the lagoon or bay, after a stationary condition has been established. F = miles of free fetch of water over which the wind at velocity U miles per hour is blowing, d = water depth in feet, a = the angle between the wind and the fetch. Today mathematical surge models are available from a number of research institutes. Control structures such as dikes or dams have been erected where possibilities for overwashes by wind pile ups on shore exist (example: Lake Okeechobee, Florida) and built-in gate structures through which water is released to canals or basins for temporary storage or outflow to major recipients, like the ocean.

P.Bruun

Fig. 17.6. The gated white sands (Hvide Sande) inlet on the Danish North Sea Coast.

515

516

Engineering Projects in Coastal Lagoons

Large storm surge barriers have been built in the Thames and Scheldt rivers. The Dutch Deltawerken is the world’s largest project. The planned Venice barriers are mentioned in a following paragraph. Gated Tidal Entrances

Gates may be placed in tidal entrances to coastal lagoons for two different reasons: (1) to control the salinity in the lagoon or bay keeping it within rather narrow limits out of consideration to biological life. Freshwater supply may be very seasonal. (2) t o control the water table in the lagoon and to prevent storm surges from penetrating into the lagoon or bay or conversely to allow discharges of water from the lagoon during an adverse wind pile-up situation. There are many examples of massive gate structures for control of flows and water levels. They are found in rivers, estuaries and coastal lagoons all over the world. They have sometimes been combined with navigation locks. This is true for the relatively small lagoons on the Danish North Sea Coast at Torsminde and Hvide Sande (White Sands). Figure 17.6a is an aerial photo of the entrance of the dam at Hvide Sande provided with flow as well as navigation gates and locks (Fig. 17.6b). A Case Study: Venice Lagoon, Italy

The Venice Lagoon system is one of the best known in the world - for good reasons, the sinking of the City of Venice. The lagoon is separated from the Adriatic Sea by a narrow barrier system penetrated by inlets, which for reasons of navigation are jetty protected (Fig. 17.7). The Lagoon of Venice in its present state is artificial. During the past 500 years, it has been subjected to many man-made changes, the most important of which has been the diversion of several large rivers, such as the Brenta, Piave, and Sile so that they now flow directly into the Adriatic sea rather than into the lagoon. These 16th and 17th century works were carried out to assuage the Venetians’ paranoia about siltation in the lagoon which would have threatened its military and commercial shipping lanes. Many laws were passed to prevent siltation, and at one time people were even forbidden to beat carpets because of the dust created. Three hundred years later, man is facing the threat from the sea’s movement in and out of the lagoon that is destroying the natural channels and islands within it. And the lack of freshwater entering the lagoon from the land added to the increase of waste water is causing serious water quality problems.

P. Bruun

0

h

cI1

'5 n

d

;a

E

cc;

517

5 18

Engineering Projects in Coastal Lagoons

The City of Venice has always been aware of the flood threat from the sea. In the 18th century, the murazzi or seawalls were built to protect the dunes and perform this function to this day, though they were overtopped in the 1966 flood. In the 19th and 20th centuries, breakwaters have been built to prevent siltation in the three channels linking the sea and the lagoon. The effect of these measures, added to general subsidence, has been to increase the exchange flow between the sea and the lagoon and to produce a negative balance of sediments as sediments in larger quantities from the sea could no longer enter the lagoon which had therefore lost many millions of cubic meters (Prof. di Silvio, personal communication). An additional and important effect of these measures has been to speed up tides in the channels within the lagoon, leading to noticeable physical changes, which were accentuated even further by the two large shipping lanes dug this century to carry vessels, even tankers, to the industrial port of Marghera. The lagoon is characterized by deep channels, continuously excavated over the years, and many small islands or mudflats. They fall into two types: barene, which are submerged only in very high tides and are covered with vegetation; and velme, which are covered by every tide. The increasing velocity of the tides in the channels has considerably reduced the extent of the barene (Prof. di Silvio, personal communication). The result has been a flatter deeper lagoon more vulnerable to erosion. One effect of this physical change is deterioration in water quality. Increasing the depth of water reduces the flushing effect because the fast currents rush in and out of a few deep channels and many shallower areas remain almost stagnant. The project ‘Yenezia Nuova” is currently carrying out studies of this problem close to the Lido shore and one of the three lagoon entrances. Pollution coming into the lagoon from the land side is yet another problem which will have to be resolved. For instance, a large agricultural runoff from the Padua region actually reaches the lagoon by passing beneath the Brenta river channel. This could be diverted into the river, but then in high summer this would pollute the beaches. The 1981proposals for the installation of mobile gates set in fixed barriers at the port openings would reduce the effects of high tides and control floods, but they would also reduce the exchange flow between the sea and the lagoon with a negative effect on pollution control. The program of studies and experiments undertaken by ‘Yenezia Nuova” has made possible a rethink of the 1981proposals and to substitute for them an integrated program of work involving the entire lagoon which can be applied with flexibility. Such a program was outlined by “Venezia Nuova” in a document presented to the Comitato di Indirizzo, Coordinamento e controllo and to the Magistrato all Acque in March 1987. “Venezia Nuova” has already produced an initial list of management possibilities towards which it will be working:

P. Bruun

5 19

(a) The mobile gates when open will not reduce the lagoon-ocean water exchange (unlike previous proposals). (b) Sequential operation of the gates can be used to alter the tidal flow and control erosive currents. (c) Control entry and exit flows to avoid periodic and dangerous low water levels with consequent risk of water stagnation. (d) Construction of locks and attention to port organization will allow efficient action to be taken when water levels are higher than a meter above normal, without interfering with port traffic. (el A series of works in the low-lying parts of the city will reduce the areas affected by average high tides so that they only start to invade the city when water level is 0.9-0.95 m higher than normal. It will be possible to decide at which water levels to intervene, so that, on each occasion, aspects of shipping, population, ecology, etc. can be allocated more importance, depending on the situation. Meanwhile, some work is already under way. New barene are being built in the lagoon inside wooden barriers constructed from piles and branches and filled with material dredged from some of the minor channels. The idea is to recreate a network of inter-tidal channels to improve the flushing of the lagoon. A semi-mathematical model has been created to test conditions of navigation into channels. Structured like a video game, the model has been tested using an actual boat captain reacting to situations on a video screen. Now a similar simulation is being devised to choose different layouts within the lagoon. Work on the gates is continuing but the problems are many, including fouling and waves. The latest estimate for the whole Venice works is US$ 2.5 billion with no firm completion date. Dredging in Lagoons

Dredging in coastal lagoons may be undertaken for reasons of navigation, development purposes to generate land areas, to cut into surrounding land areas, to establish harbor and marina facilities, and to remove polluted bottom materials. Causes of Shoaling

Shoaling occurs when the supply of depositable sediment exceeds the flow’s capability to transport it. Thus, a n area of waterway in sedimentation equilibrium is one in which the flow is just able to prevent quasi-permanent deposition of the available supply of sediment. A change in shoaling behavior at a site can be the result of a n increase in depositable sediment or by a decrease in transport capability or both. Fills in coastal lagoons decrease the tidal prism which may cause entrance shoaling (Bruun, 1989).

520

Engineering Projects in Coastal Lugoons

Depositable sediment consists of those particles (single or aggregate) with a settling velocity great enough to approach or remain near the bed under given flow conditions. A particle’s settling velocity is a function of its size, shape, density, water density, viscosity, and turbulence level. The depositable sediment supply to an area is determined by the amount of sediment (depositable or not) that is delivered to the area, the amount of sediment available on the bed nearby, plus, in the case of cohesive sediments, the intensity of aggregation processes. For instance, the depositable sediment supply will be increased by introduction of sediment-laden water, erosion of banks or bottom, trapping supplied sediment (i.e., turbidity maxima), or by an increase in aggregation of cohesive sediments already present. Aggregation is increased by increased rate of particle collisions due to more internal flow shear, by obstacle-induced turbulence, or by water quality changes that facilitate physicochemical bonding. Flow transport capacity is a function of the strength of the current and its turbulent fluctuations. Noncohesive sediment transport functions use the flow velocity raised to a power to compute transport rates and many use bed shear stress (proportional to velocity squared) to determine conditions of incipient motion. For cohesive sediments, bed shear stress is used to compute the rate of deposition and erosion, as described in the preceding section. Thus, for both cohesive and noncohesive sediments, flow transport capability can be related to a characteristic current velocity. In estuaries, the magnitude, direction, and duration of current velocities determine the flow’s ability to transport sediment. Changes in geometry, discharge, or pressure gradient can alter flow intensity and, thus, shoaling patterns. Sites of low flow intensity (relative to the supply of sediment) include points at which an estuary widens, sheltered basins that experience small or no net through flow, and depressions in the bed. These occur naturally but are often manmade navigation facilities such as harbor or pier slips adjacent to the main flow lanes and anchorage basins or channels that increase the waterways cross-sectional area. Deepening and Extending Channels Historically, natural channels have been extended at desirable navigation depths to lengthen and improve harbor access. In recent years, most of the harbors in the United States have been deepened to accommodate the increase in draft of the world shipping fleet. Extension of the navigation channels upstream and seaward usually has accompanied deepening. The effects of deepening and extending navigation channels, or both, have been determined by hydrographic surveys of prototypes and by model studies. Both methods of monitoring have shown that deepening and extending channels, or both, cause the null point(s) for the penetration of salt water to move (usually upstream), thereby changing the location and configuration

P. Bruun

521

of shoals. These changes in shoaling patterns are due to several factors. Enlargement of the waterway and associated shift in null point(s) may cause current speed to decrease, thereby reducing the flow’s capacity to transport sediment through the area. Experience with maintaining harbordeepening projects has shown that the volume of annual maintenance usually increases after deepening (Bruun, 1989).

Structural Modifications Structures, such as dikes, added to direct the flow or increase current speeds by narrowing the waterway, create turbulent wakes that increase the clay particle aggregation rate. Decreases in waterway cross sections due to structural installations also cause null points to shift and velocity to increase at the construction point. Any change in the friction coefficient of the waterway channel, such as lining with concrete, affects velocity and stage. The response of a particular system to structural modifications is a complex combination of these and other, less obvious, consequences.

Inflow Changes The volume of sediment entering the estuary from upstream increases or decreases with respective increase or decrease in inflow but, perhaps more importantly, the null point(s) usually move in response to the inflow changes. These changes in location can be many kilometers when the inflow change is large. Velocity patterns in the estuary are altered in the vertical profile and in plan by inflow changes. The predominant tide phase affecting sediment movement can change from ebb tide to flood tide, or vice versa, depending upon the magnitude of the inflow increase or decrease. The combined effect of these phenomena on the estuary shoaling regimen is difficult to predict. Even annual shoaling statistics from field measurements cannot be used to evaluate shoaling patterns without consideration of several years of record that may be biased by unusual hydrodynamic events or manmade events, such as dredging.

Water Quality Changes Lagoons are frequently the sites of harbors with dense populations and industries, contributing significant amounts of sediment to the shoaling areas. Harbor dredging and shipping facility construction also tend to improve the sediment trapping efficiency. In the major shoal area in Delaware Estuary, the U.S. Army Corps of Engineers estimates that of the 911 x lo6 kg of annual shoaling materials to this site the major portion is clay and silt from the watershed 73 x lo6 kg consist of proteinaceous organic matter and subordinate amounts of fatty acids and hydrocarbons from

522

Engineering Projects in Coastal Lagoons

sewage outfalls and industrial effluents, while approximately 164 x 106 kg are diatom fmsticules resulting from eutrophication of the estuary by nutrients from sewage, industrial effluents, and fertilizers from agricultural sources; another 49 x lo6 kg of silt-size anthracite coal, also organic originates from the coal fields in the watershed and a large portion of 31 x lo6 kg of amorphous iron oxide is attributed to industrial effluents. The cation concentrations in the suspending waters may also be altered by industrial effluents. Introduction of high valence cations promotes aggregation and, consequently, may cause shoaling. Organic material may act as a binding agent with similar effects. Changes in water temperature due to power plant discharges and the construction of dams can also affect the sediment structure and transport rates. Resuspension of Deposited Sediments

Man’s activities often add to shoaling problems by placing sediments directly in suspension. Dredging and disposal operations and construction in or adjacent t o the waterway are obvious ways that sediment is added to the supply available for deposition. Vessels can also suspend sediment, either by propwash acting on the channel bottom or by bow waves eroding banklines. Fishing operations, such as trawling, can be a source of some sediment re-suspension, though it is usually a minor source. Shoaling problems can often be addressed hydraulic models. In the case of weak vertical stratification, two-dimensional models usually work well, In addition to conventional study approaches with moored instruments and hydrographic survey vessels, remote sensing techniques (c.f. Gierloff-Emden, 1976) are likely to yield useful results in coastal lagoons because of the large surface areas, lack of significant vertical stratification, and sharp density and turbidity fronts. For example, the satellite-sensed turbidity distribution in a coastal lagoon can provide a good synoptic clue to the circulation (c.f. Herz, 1977). In recent years, developments in numerical modelling have improved chances of obtaining quick answers to specific problems. Examples of hydraulic modelling of tidal inlets and lagoons are given in Bruun et al. (1966, 1989) and ASCE (1975). Navigation Channels

Channels for navigation may be dredged to accommodate ocean vessels through a tidal entrance and a lagoon or bay to a port in the lagoon. There are a great number of examples on that all over the world where lagoons separate a barrier shore from the mainland, such as in Australia, India, in Europe on the Adriatic Shores, the Bay of Biscaya, the Dutch, German and Danish North Sea Shores, in the United States on the entire Eastern

P. Bruun

523

Seaboard, the Gulf of Mexico and part of the Pacific, in South America, the Caribbean shores, some shores in Argentina and furthermore, and not least on the Mexican Pacific and Gulf Shores. The cross sections of such navigation channels may vary from as little as 3-20 m in depth and from 30 to 500 m in width (Bruun, 1989). Dredging may generate huge quantities of materials which may be deposited in shoal banks if not used for other practical purposes like the generation of land fills, islands, or for artificial nourishment of beaches, if the material is suitable. Earlier practices of just dumping the material offshore now meet strong opposition. The State of Florida will no longer permit dumping of dredged materials from navigation channels at sea, if the material is suitable for beach nourishment. Other states plan to follow suit. Some lagoons are interconnected to generate a n intra coastal waterway, like the water along the Atlantic and Gulf barrier shores in the United States. The navigation depth and width of such canals range from 2.5 x 20 m to 4-5 x 100 m.

Dredging for Development of Fills During recent decades the dredging industry has faced many new tasks, some of which are: the increased demand for dredge and fill of large submerged areas for housing developments, e.g. in Florida, industrial and harbor developments, e.g. Port Elizabeth, airports, e.g. Kennedy Airport, New York, Hong Kong, and Tokyo. The Kennedy International Airport is one of the largest airport projects. From 1944 to 1949 dredges pumped over 30 million m3 to reclaim this tideland area. In 1958, long after the field was in operation, a n additional 3 million m3 were placed and later 7 million m3 of fill were used to extend a runway for use by jet planes. Examples of other airport projects are: Aruba, N.W.I. - 2 million m3; Boston, Mass. - 14 million m3; Bermuda - 6.5 million m3; Newark, N J - 11 million m3; and Philadelphia, PA - 1.1 million m3 (Bruun, 1989). In general, large fill projects, e.g. the Tokyo Airport expansion, are strongly opposed by environmental groups (Bruun, 1989).

Establishment of Marinas in Coastal Lagoons Lagoons are usually centers for comprehensive recreational developments in the form of marinas and small boat harbors. Dredging and filling may then be undertaken to produce a balance in quantities. Environmental considerations, however, often make such projects very difficult to permit due to concerns of pollution in and by the marina operations. A few marinas, e.g. the Windmill Harbor on Hilton Head Island, SC, are provided with navigation locks to keep the exchange of waters between the marina basins and the lagoon down to a minimum. Local environmental standards deter-

524

Engineering Projects in Coastal Lagoons

mine under which circumstances the establishment of such marinas may be permitted and if so, the conditions under which they may be operated. A unique case of such development is the marina which was established by the Palmetto Dunes Development on Hilton Head Island, SC, where a canal development connected to the Calibogue Sound by tide gates was excavated and the fill from the canals was used for beach nourishment of a highly eroding beach in front of the development. A Case Study: Dredging and Reclaiming in South Carolina The Palmetto Dunes Development on Hilton Head Island, South Carolina in the United States is an example of a thoroughly planned canal (lagoon) and beach development, made possible by efficient, modern dredging equipment and procedures. The development covers a land area of about 600 hectares, and canals-lagoons-ponds of about 50 hectares. The dimensions of the canals and lagoons vary from 18 to 90 m in width and depth from 2.5 to 3.5 m at low water. The Palmetto Dunes Beach is about 5 km long. Before development started there was no beach at normal high tide. The mean tidal range is about 2 m, with spring range at 2.5-3 m and neap range 1.5-2 m. Due to a shoreline recession of 1-1.5 m per year, high tides washed up towards the low beach dunes and developed vertical erosion scarps which contributed further to erosion by reflection of waves. Erosion was also demonstrated by a great number of tree stumps and peats exposed on the beach. At low tide the beach was about 45 m wide but very wet, not only because of the receding tides but also due to ground seepage of water through the narrow dune zone, which in turn contributed further to erosion by run-down, as well as lift forces on the sand. To stabilize the beach there was apparently no other solution than replacement of eroded material raising the beach about 1.2-1.5 m and widening it by 40-50 m. Furthermore, in order to protect the dunes and the area behind the dunes against overwashing and flooding, it was necessary to build an artificial dune of 20 m crown width at elevation 3.3 m above mean sea level (MSL), slope 1in 7 seaward and 1in 3 landward. The total fill quantity was 1.1million m3 (220 m3 per meter of shore front). Fill for beach nourishment was obtained from dredging canals and lagoons in the development thereby creating a great number of canal and lagoon front lots for home-sites. For reasons of economy it was decided to limit the pumping distance to a maximum of 1km from the dredge to the discharge point. In addition some interior dredging and filling was undertaken. The work comprising dredging and dumping of 220 m3 m-1 was undertaken by an American dredging contractor, using a 14-inch cutter suction dredger, which was brought in on marsh side of the island and pulled across approximately 0.4 km of mainland, then across Highway 278, and launched

P.Bruun

525

Fig. 17.8. Lagoon scenery on Hilton Head Island, SC ( B N ~ 1981). ,

at an interior lake. Departure after completion of work was accomplished on a relatively calm day by rolling the dredge out of the lake over the beach dunes and into the ocean. The Palmetto Dunes project was carried out with ease and minimum cost compared to assembly and reassembly. Figure 17.8 gives an impression of the scenes lefi for h r t h e r development. The many advantages associated with the development include an effective flushing system of all canals and lagoons based on two major inlets and outlets with gated 1.5 m culverts, provided with slide and flap gates (Fig. 17.9) controlling the tides and water quality of the waterways of the development within wide ranges to meet all possibilities of rain falls and temperatures. During storms and heavy rains, the water table in the development waterways may be lowered to provide pressure from outside on the development beaches and shores, thereby decreasing erosion. A major marina with space for about 300 boats and a number of boat facilities was thoroughly tested in a hydraulic model study run in the development. Fig. 17.10 shows a layout of the marina. Dredging and excavation of the marina included about 250,000 m3 of material. The marina adds another contribution to one of the most thoroughly planned combined beach, canal, and marina developments ever built and a perhaps relatively modest, but quite convincing victory for the dredging industry, its flexibility and ability.

526

Engineering Projects in Coastal Lagoons

Fig. 17.9. The main flushing canal and tidal gates on Hilton Head Island, SC (Bruun, 1981).

The latest development in this particular lagoon project is the installation of tide gauge stations on either side of the controlling gates to record the water table continuously and display the results of either one of the records and the difference in head at a central station. As soon as water tables are equal a red light flashes announcing that now it is the time to change gate positions to obtain maximum efficiency for flushing, thereby optimizing water quality in the lagoon system. Problems Associated with Rising Relative Sea Level

The Dangers Involved in the Destruction of Coastal Wetlands by Bulkheads and Other Coastal Protective Structures Coastal wetlands are generally found between the highest tide of the year and mean sea level and are common surrounding many coastal lagoons. Wetlands have kept pace with the past rate of sea level rise because they collect sediment and produce peat upon which they can build; meanwhile they expanded inland as lowlands were inundated (Fig. 17.11). Wetlands accrete vertically and expand inland. Thus, the present area of wetlands is

P. Bruun

527

Fig. 17.10. Schematic of the South Channel and Marina Basin between Tidal Lagoon and Broad Creek on Hilton Head Island, SC (Bruun, 1981).

generally far greater than the area that would be available for new wetlands as sea level rises (Titus et al., 1984, 1987). In the United States, the potential loss would be the greatest in Louisiana. In many areas, bulkheads have been constructed just landward of marsh and mangrove wetlands. If sea level rises, the wetlands would be squeezed between the sea and the bulkheads (see Fig. 17.11). Previous studies have estimated that if the development in coastal areas is removed to allow new wetlands to form inland, a 1.5-2.0 m rise would destroy 30-70% of the U S . coastal wetlands. If levees and bulkheads are erected to protect today’s dry land, the loss could be 50-80% (Titus, 1984, 1987, 1988; Armentano et al., 1988). Such a loss would reduce the available habitat for birds and juvenile fish and would reduce the production of organic materials on which estuarine fish rely. The dry land within 2 m of high tide includes forests, farms, low parts of some port cities, cities that sank after they were built and are now protected with levees, and the lagoon sides of barrier islands. The low forests and farms are generally in the mid-Atlantic and Southeast regions; these would provide potential areas for new wetland formation. Major port cities with

528

Engineering Projects in Coastal Lagoons

Fig. 17.11. Shore evolution as sea level rises (after Titus, 1986).

low areas include Boston, New York, Charleston, and Miami. New Orleans is generally almost 3 m below sea level, and parts of Galveston, Texas City, and areas around the San Francisco Bay are also well below sea level. Because they are already protected by levees, these cities are more concerned with flooding than with inundation.

Inundation and Erosion of Beaches, Barrier Islands and Wetlands Some of the most important vulnerable areas are the recreational barrier islands and spits (peninsulas) of the Atlantic and Gulf coasts. Coastal barriers are generally long narrow islands and spits with the ocean on one side and a lagoon on the other. Typically, the ocean-front block of an island ranges from 2 to 3 m above high tide, and the lagoon side is 1m above high water. Thus, even a 1m sea level rise would threaten much of this valuable land with inundation. Erosion threatens the high part of these islands and is generally viewed as a more immediate problem than the inundation of the lagoon sides. As Fig. 17.12 shows, a rise in sea level can cause an ocean beach to retreat considerably more than it would from the effects of inundation alone. The visible part of the beach is much steeper than the underwater portion, which comprises most of the active surf zone. While inundation alone is determined by the slope of the land just above the water, Bruun (1962) and others have shown that the total shoreline retreat from a sea level rise depends on the average slope of the entire beach profile. Previous studies suggest that a 0.3 m rise in sea level would generally cause beaches to erode 20-30 m from the Northeast to Maryland (e.g., Kyper and Sorensen, 1985; Everts, 1985); 70 m along the Carolinas (Kana et al.,

P.Bruun

529

Fig. 17.12. The Bruun Rule (Bruun, 1962; after Titus, 1986).

1984);30-300 m along the Florida coast (Bruun, 1962); 60-1 10 m along the California coast (Wilcoxen, 1986);and perhaps several kilometers in Louisiana. Because most U.S. recreational beaches are less than 30 m wide at high tide, even a 0.3 m rise in sea level would require a response. In many areas, undeveloped barrier islands do keep up with rising sea level by overwashing landward. In Louisiana, however, barrier islands are breaking up and exposing the wetlands behind them to Gulf waves because of land subsidence. Consequently, the Louisiana barrier islands have rapidly eroded. Wetlands surrounding coastal lagoons are subject to increased flooding if the tidal exchange, including storm surges, increases. Flooding Flooding would increase along the coast and in coastal lagoons if sea level rises for three reasons: (1)A higher sea level provides a higher base for storm surges to build upon. A 1-m sea level rise would enable a 15-year storm to flood many areas that today are flooded only by a 100-year storm (e.g., Kana et al., 1984; Leatherman, 1984). (2) Beach erosion also would

530

Engineering Projects in Coastal Lagoons

leave ocean-front properties more vulnerable to storm waves. (3) Higher water levels would reduce coastal drainage and thus would increase flooding attributable to rain storms. In artificially drained areas such as New Orleans, the increased need for pumping could exceed current capacities. Finally, (4) a rise in sea level would raise water tables and would flood basements and in cases where the groundwater is just below the surface perhaps leach it above the surface.

Protection against Flooding On ocean coasts, dunes and nourishment of beaches provide protection. In coastal lagoons, productive wetlands should not be destroyed by any structure or fill. Revetments, not bulkheads, may be established, where wetlands join higher grounds. Various countries and states exercise control over wetlands by laws, rules and regulations. Environmental impact statements are standard requirements for any project.

Saltwater Intrusion Finally, a rise in sea level would enable salt water to penetrate farther inland and upstream into rivers, bays, wetlands, and aquifers. Salinity increases would be harmful to some aquatic plants and animals, and would threaten human uses of water. For example, increased salinity already has been cited as a factor contributing to reduced oyster harvests in the Delaware and Chesapeake Bays, and to conversion of cypress swamps in Louisiana to open lakes. Moreover, New York, Philadelphia, and much of California’s Central Valley obtain their water from areas located just upstream from areas where the water is salty during droughts. Farmers in central New Jersey and the city of Camden rely on the Potomac-Raritan-Magothy aquifer, which could become salty if sea level rises (Hull and Titus, 1986). The South Florida Water Management District already spends millions of dollars every year to prevent Miami’s Biscayne Aquifer from becoming contaminated with sea water. Water Quality Problems in Coastal Lagoons

It was general practice in earlier days to discharge even raw sewage into coastal lagoons. This is still done in many countries particularly in the Far East and in South America. This has sometimes caused severe problems of diseases like Typhoid and Cholera. In Western countries, sewage now often receives a first treatment, by which solids and sediments are removed, while a more rational and effective second treatment that removes dissolved and colloid matters by activated-

P.Bruun

531

sludge process and filters still has a long way to go. In the USA, EPA requires a secondary treatment. Sedimentary pollution is common in coastal lagoons and has always been due to river discharges of suspended and bed load materials. Sewage adds to the problem. Many rivers discharging in lagoons have developed small deltas of materials which they carried. Fine materials of silt and clay size, however, do not settle in the coastal regions but in relatively calmer and deeper areas in the middle of the lagoon or bay. Some materials are of organic nature and develop gasses when they decompose. Siltation rates vary greatly depending upon local conditions of discharges of sediments and the effects of salt water intrusion (Bruun, 1989).Some of the sediments may be chemically or biologically polluted. The dredging operation in itself generates sedimentary pollution of concern to certain types of biological life like oyster and other shell organisms (Bruun, 1989).Removal of pollutants in bottom material is a newly developed field for which special equipment is available.

The Clean Water Act Environmental concerns in the United States in the 1960s led to a series of bills passed by the U.S. Congress. Some of them had a profound effect on the dredging industry. The Clean Water Act (CWA) of 1977 has as a purpose to restore and maintain the chemical, physical, and biological integrity of the waters of the United States. Section 404 of the CWA established a set of criteria for regulating the discharge of dredged or fill material into waters of the United States. Section 404(b)(l)guidelines require a thorough review of all alternatives to the dredging and disposal operations. Dredging considerations include analyzing channel locations, need for channel depths, and techniques for dredging. Included in the analysis of disposal site selection are the quality of materials to be disposed and the impacts on water quality, wetlands, and the benthic environment especially related to shell fisheries. For example, wetlands act as a primary recharge for much of the nation’s groundwater. They provide excellent erosion control, and they act as a pollution filtration system. Last, and of major importance, wetlands act as flood prevention buffers both by increasing sheet flow and by water storage. The guidelines specify conditions which must be met for any dredging project, including: (a) compliance with State water quality standards; (b) compliance with EPA’s toxic effluent standards; (c) no adverse effect through bioaccumulation of toxic substances; (d) no impact to threatened or endangered species; (e) no impacts to marine sanctuaries; (0 there must be no impacts which would cause or contribute to significant degradation of the waters of the United States.

Engineering Projects in Coastal Lagoons

532

The dredging of areas where contaminants are known or suspected to reside requires special care. Testing procedures under 404(b)(l), provide for categories of dredged material from clean, with no potential for harm, to very polluted requiring extensive bioassays to assess impacts.

Concluding Remarks Coastal lagoons are important for coastal development and recreation. Their function as breeding grounds and nurseries for biological life is enormous. Lagoon eco-systems are in a delicate balance which may be disturbed and adversely affected by man’s activities and engineering projects. By proper planning, the use of sound engineering technology, and scientific principles it is possible to mitigate or change adverse effects to beneficial, and in the meantime, a great number of regulatory steps attempt to control and protect lagoons against degradation by adverse human activities. Any planning of engineering projects in coastal lagoons, therefore, must be preceded by adequate surveys to ensure sound technical and environmental solutions without adverse effects. References ASCE (American Society of Civil Engineers) 1975 Sedimentation Engineering. Manual #54. Brown, E.I. 1928 Inlets on Sandy Coasts. Proceedings of the American Society of Civil Engineers 54,505-553. Bruun, P. and Genitsen, F. 1960 Stability of Coastal Inlets. North Holland Publishing Co., Amsterdam, The Netherlands. Bruun, P.M., et al. 1966 Coastal Engineering Model Studies of Three Florida Coastal Inlets. Engineering Progress a t the University of Florida, Bull. No. 122, College of Engineering, University of Florida, Gainesville, Florida, 1-68. Bruun, P., Mehta, A.J., and Jonsson, I.G. 1978 Stability of Tidal inlets: Theory and Engineering. Elsevier Scientific Publishing Co., Amsterdam, The Netherlands. Bruun, P. 1986 Morphological and Navigational Aspects of Tidal Inlets on Littoral Drift Shores. Journal of Coastal Research 2,123-141. Bruun, P. 1962 Sea-Level Rise as a Cause of Shore Erosion. Journal of Waterway and Port Division 88, 117-130. Bruun, P. 1959 Bay Fill and Bulkhead Lines. Engineering Progress at the University of Florida, Leaflet No. 105 13 Bruun, P. 1989 Port Engineering N 2. The Gulf Publishing Co., Houston, TX. Dilorenzo, J.L. 1986 The Overtide and Filtering Response of ZnletlBay systems. Thesis presented to the State University of New York, at Stony Brook, New York. Dorrestein, R. 1961 Amplification of Long Waves in Bays. Engineering Progress at the University of Florida, Technical Paper No. 213 15.

P.Bruun

533

Eiser, W.C. and Kjerfve, B. 1986 Marsh Topography and Hypsometric Characteristics of a South Carolina Salt Marsh Basin. Estuarine, Coastal and Shelf Science 23,595-605. Escoffier, F.F. and Walter, T.L. 1979 Inlet Stability Solutions for Tributary Inflows. Journal Waterway, Port and Coastal Engineering Division 105,341-353. Everts, C.H. 1985 Effect of Sea Level Rise and Net Sand Volume Change on Shoreline Position a t Ocean City, Maryland. In Potential Impact of Sea Level Rise on the Beach at Ocean City, Maryland (Titus, J.G., ed). U.S. Environmental Protection Agency, Washington D.C.. Gierloff-Emden, H.G. 1976 Manual of Interpretation of Orbital Remote Sensing Satellite Photography and Zmagery for Coastal and OffshoreEnvironmental Features (Zncluding Lagoons, Estuaries, and Bays). Munchener Geographische Abhandlungen, Institute fur Geographie der Universitat Miinchen, Band 20, IOC-UNESCO contract SC/RP600/341. Hem, R. 1977 Circulapio das aguas de superficie da Lagoa dos Patos. Ph.D. Dissertation, Departmento de Geografia, Universidade de Sao Paulo, Brazil. Hull, C.H.J. and Titus, J.G. 1986 Responses to Salinity Increases. In Greenhouse Effect, Sea Level Rise, and Salinity in the Delaware Estuary (Hull, C.H.J. and Titus, J.G., eds). U.S. Environmental Protection Agency and Delaware River Basin Commission. Kana, T.W., Michel, J., Hayes, M.O., and Jensen, J.R. 1984 The Physical Impact of Sea Level Rise in the Area of Charleston, South Carolina. In Greenhouse Effect and Sea Level Rise: A Challenge for this Generation (Barth, M.C. and Titus, J.G., eds). Van Nostrand Reinhold, New York. Keulegan, G.H. 1967 Tidal Flow in Entrances: Water Level Fluctuations of Basins in Communication with Seas. US.Army Engineer Waterways Experiment Station Technical Bulletin No. 14, Committee on Tidal Hydraulics, Vicksburg, Mississippi. Kjerfve, B. 1986 Comparative Oceanography of Coastal Lagoons. Estuarine Variability, Academic Press, Inc. Kyper, T. and Sorenson, R. 1985 Potential Impacts of Sea Level Rise on the Beach and Coastal Structures a t Sea Bright, New Jersey. In Coastal Zone '85 (Magson, O.T., ed). American Society of Civil Engineers, New York. Leatherman, S.P. 1984 Coastal Geomorphic Responses to Sea Level Rise: Galveston Bay, Texas. In Greenhouse Effect and Sea Level Rise: A Challenge for this Generation (Barth, M.C. and Titus, J.G., eds). Nostrand Reinhold, New York. Mehta, A.J. and Joshi, P.B. 1988 Tidal Inlet Hydraulics. Journal of Hydraulic Engineering 114, 1321-1338. Sorensen, R.M. and Seelig, W.N. 1976 Hydraulics of Great Lakes Inlet-harbor Systems. Proceedings of the Fifteenth Coastal Engineering Conference, ASCE 2, 1646-1665. Titus, J.G., Henderson, T. and Teal, J.M. 1984 Sea Level Rise and Wetlands Loss in the United States. National Wetlands Newsletter Environmental Law Institute 6. Titus, J.G., Kuo, C.Y., Gibbs, M.J., LaRoche, T.B., Webb, M.K. and Waddell, J.O. 1987 Greenhouse Effect, Sea Level Rise, and Coastal Drainage Systems. Journal of Water Resources Planning and Management, ASCE 113,216-227. Titus, J.G. 1988 Sea Level Rise and Wetlands Loss: An Overview. In Greenhouse Effect, Sea Level Rise, and Coastal Wetlands (Titus,J.E., ed). U.S. Environmental Protection Agency, Washington D.C. Wilcoxen, P.J. 1986 Coastal Erosion and Sea Level Rise: Implications for Ocean Beach and San Francisco's Westside Transport Project. Coastal Zone Management Journal 14.