Duncan M. FitzGerald

Hurricane-induced failure of low salinity wetlands

During the 2005 hurricane season, the storm surge and wave field associated with Hurricanes Katrina and Rita eroded 527 km2 of wetlands within the Louisiana coastal plain. Low salinity wetlands were preferentially eroded, while higher salinity wetlands remained robust and largely unchanged. Here we highlight geotechnical differences between the soil profiles of high and low salinity regimes, which are controlled by vegetation and result in differential erosion. In low salinity wetlands, a weak zone (shear strength 500–1450 Pa) was observed ∼30 cm below the marsh surface, coinciding with the base of rooting. High salinity wetlands had no such zone (shear strengths > 4500 Pa) and contained deeper rooting. Storm waves during Hurricane Katrina produced shear stresses between 425–3600 Pa, sufficient to cause widespread erosion of the low salinity wetlands. Vegetation in low salinity marshes is subject to shallower rooting and is susceptible to erosion during large magnitude storms; these conditions may be exacerbated by low inorganic sediment content and high nutrient inputs. The dramatic difference in resiliency of fresh versus more saline marshes suggests that the introduction of freshwater to marshes as part of restoration efforts may therefore weaken existing wetlands rendering them vulnerable to hurricanes.

Control of barrier island shape by inlet sediment bypassing: East Frisian Islands, West Germany

Abstract A study of the East Frisian Islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing. This island chain is located on a high wave energy, high tide range shoreline where the average deep-water significant wave height exceeds 1.0 m and the spring tidal range varies from 2.7 m at Juist to 2.9 m at Wangerooge. An abundant sediment supply and a strong eastward component of wave power (4.4 × 103 W m−1) have caused a persistent eastward growth of the barrier islands. The eastward extension of the barriers has been accommodated more by inlet narrowing, than by inlet migration. It is estimated from morphological evidence that a minimum of 2.7 × 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system. Much of this sand ultimately bypasses the inlets in the form of large, migrating swash bars. The location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb-tidal delta along the downdrift inlet shoreline. The configuration of the ebbtidal delta, in turn, is a function of inlet size and position of the main ebb channel. The swash bar welding process has caused preferential beach nourishment and historical shoreline progradation. Along the East Frisian Islands this process has produced barrier islands with humpbacked, bulbous updrift and bulbous downdrift shapes. The model of barrier island development presented in this paper not only explains well the configuration of the German barriers but also the morphology of barriers along many other mixed energy coasts.

Interactions between the ebb-tidal delta and landward shoreline; Price Inlet, South Carolina

ABSTRACT Patterns of erosion and deposition along the shoreline of Price Inlet, South Carolina, are strongly influenced by ebb-tidal delta processes. The ebb delta goes through cycles of growth and decay (15-20% change in volume) which last from 4 to 7 years. In the growth phase, large intertidal bar complexes are formed on both sides of the main ebb channel by the coalescence of landward-migrating swash bars. During this stage, waves refracting around the southern bar complex produce a reversal in the longshore-transport direction on the downdrift shoreline. This results in little sediment escaping the inlet and delta system. The sand-trapping mechanism ceases and the delta reduces in volume when the bar complexes migrate onshore and attach to the beach. The inlet shoreline also goes through cycles of 7- to 42-years duration in which its offset configuration changes. The shift in shoreline alignment is controlled by the orientation of the main ebb channel and the resulting ebb-tidal delta morphology. When the inlet has a downdrift, offset configuration, the ebb-tidal delta is skewed along the updrift shoreline. In this position bar complexes preferentially accrete along the updrift beach causing a progradation of the shoreline. At the same time, the downdrift shoreline erodes due to little sediment bypassing the inlet. This pattern of sedimentation eventually produces an updrift, offset inlet. The erosional-depositional processes are reversed when the ebb flow in the channel thalweg is deflected off the prograding updrift, inlet shore ine. This causes a clockwise rotation of the main ebb channel and a shift in the ebb delta to a position along the downdrift shoreline. In this configuration the updrift shoreline erodes and the downdrift shoreline accretes. This study documents the close relationship that exists at Price Inlet, South Carolina, between the ebb-tidal delta and landward shoreline. Evidence is presented to show that this relationship may hold for other mixed-energy tidal inlets as well.

A 1500 yr record of North Atlantic storm activity based on optically dated relict beach scarps

Understanding of long-term dynamics of intense coastal storms is important for determining the frequency and impact of these events on sandy coasts. We use optically stimulated luminescence (OSL) dates on relict scarps within a prograded barrier sequence to reconstruct the chronology of large-magnitude erosional events in the western Gulf of Maine. OSL dates obtained on quartz-rich sediments immediately overlying relict scarps indicate severe beach erosion and retreat due to erosional events ca. 1550, 390, 290, and 150 cal yr B.P. Our data provide new evidence of increased storm activity (most likely frequency and/or intensity of extratropical storms) during the past 500 yr, which was preceded by a relatively calm period lasting ~1000 yr. The width of the coastal sequence preserved between successive paleoscarps shows strong correlation with the time interval elapsed between storms. Our fi ndings indicate that diagnostic geophysical and sedimentological signatures of severe erosional events offer new opportunities for assessing the impact and timing of major storms along sandy coasts.

New England tidal inlets with special reference to riverine-associated inlet systems

Abstract Tidal inlets along the glaciated coast of New England exhibit a diverse morphology due to widely different physical and geological settings and sediment abundance. Except for the glacial sediment coasts of Cape Cod, Nantucket Island, and Marthas Vineyard, most regions of New England are rocky, and barrier and tidal inlet development is related to isolated glacial and riverine sediment supplies. Inlet-connected bays and marshes encompass many different origins, including drowned river valleys, glacial lake discharge channels, indented bedrock and glacial sediment coasts, kettles, and groundwater sapping channels. Generally, inlet size is correlated to tidal range; the largest inlets (width >300 m) occur along mesotidal coasts, whereas most inlets along microtidal coasts are small (width In spite of their diversity, New England tidal inlets can be grouped into three broad categories based on their morphology, hydrographic regime, and sediment transport characteristics. These classes include the wave-dominated and mixed-energy tidal inlets, and riverine-associated tidal inlets. Wave-dominated inlets tend to occur in microtidal settings and are found primarily in southern New England. They contain flood-tidal deltas of variable morphology and poorly developed or absent ebb-tidal deltas. They have histories of migration, closure, and reopening during storms. Mixed-energy tidal inlets occur mostly along the mesotidal shorelines of central and northern New England. These inlets exhibit well-developed ebb- and flood-tidal deltas. They are backed by extensive marshes and tidal creeks and, in some cases, by a broad system of tidal flats. Riverine-associated tidal inlets coincide with major rivers that have significant freshwater discharge, especially during late winter and early spring. The throat cross-sections are large in comparison to other inlets in New England and are a function of their tidal prism. They contain well-developed subtidal ebb-tidal deltas and variable developed flood-tidal deltas. Ebb-oriented bedforms, grain size, and mineralogical analyses of channel sediments, and hydraulic data have been used to demonstrate that the estuaries are exporting sand to the nearshore, a process that has likely been ongoing since deglaciation. The mechanism for downstream movement of sand in the estuaries is moderate to large spring freshets and major floods when freshwater discharge supplants the saltwater tidal prism. During these periods, there are unidirectional seaward currents in the estuaries that may last for several days or weeks.

SEDIMENT BYPASSING AT MIXED ENERGY TIDAL INLETS

The cost of many coastal projects is often increased by the expensive beach repair and maintenance required to remedy the destabilising effects of structures on the adjoining coastline. Physical and/or mathematical models have been developed for use in planning these projects in order to predict and quantify the effects of marine sediment transport on the coastal topography. Such models need to be calibrated against prototype data and one method of gauging volumetric sediment movement is by successive bathymetric/ topographic profiting surveys which are performed seasonally and annually. Since large quantities of sediment are related to small changes in bed elevation it is clear that this profiling needs to be done with the utmost precision* The areas most affected extend from the beach through the surf zone to water depths of about 25 metres. The surf zone in particular is a dynamic and hostile area which falls outside the traditional activities of both the hydrographic and land surveyors. Consequently innovative methods, deficient in sound survey principle and practice, have often been pursued in this area without any attempt being made to assess the tolerance on the data. This paper attempts to show that it is possible to produce reliable and verifiable results to the required accuracy by using conventional survey equipment and techniques, also by taking the necessary precautions against the many possible sources of survey error. The procedures and techniques described have evolved from NRIOs involvement over the past decade in major projects at Richards Bay, Durban, Koeberg and in False Bay. The results of a recent verification investigation are fully reported in this paper.Besides wave impact forces, erosion of the inner side of a sea dike is a serious cause of destruction. Therefore, wave run-up and overtopping effects have to be considered with respect to the safety of a dike. Strong relations were found between both these influences (TAUTENHAIN et.al., 1980, 1981, 1982), based on experiments in a wave flume and using an energy conservation concept. However, under natural conditions, an oblique wave approach has to be considered. This paper deals with the influence of wave direction on wave runup on a smooth dike slope in order to provide a basis for calculating the overtopping rates for both regular and irregular waves.This paper describes a study carried out at Port Taranaki, New Plymouth, New Zealand to determine ships track and motions at the port entrance. The results of the study being used to establish the extent (plan area) and optimum depth of proposed capital dredging works. The time lapse photographic technique, incorporating reference levels and bearings in each frame, used to record vessels entering and leaving the port is described, Maximum increases in ships draft due to sea conditions are given.N.S.L. program is a finite-difference code for two dimensionnal flows with a free surface in a vertical plane. Basic equations are Navier-Stokes Equations with a simple simulation of turbulent effects by an eddy viscosity coefficient related to the mixing length and the mean velocity gradient. Theses equations are solved in a variable domain in time. The main features of the numerical method are presented. Some comparisons with theoretical solutions give a good validation of the code both in linear and non linear cases. Other examples of application are given.The design of the coal unloading terminal in an unprotected environment created the need to develop design criteria as well as operational and structural systems that will respond toproblems arising from unloading bulk in the open sea. As there was no known precedent of an unprotected unloading terminal, the criteria was based on experience in existing offshore loadina terminals, laboratory tests and engineering judgement.A large number of man-made tidal swimming pools (two examples are shown in Figure 1) exist along the South African coast. They are usually situated on rocky outcrops in the close vicinity of popular sandy bathing beaches to provide protected bathing conditions in these areas mainly for children and elderly people. Some tidal pools, especially along rocky coast, provide the only safe bathing facilities. Besides affording protection against waves and surfzone currents the pools provide protection from sharks. A large number of tidal pools were built during the early 1950s along the Natal South Coast after the occurrence of a relatively large number of shark attacks on bathers on that coast. The semi-diurnal tide with a range of about 1,5 m along the South African coast makes it possible for pools to be built such that water replenishment can occur during every high-water (approximately every 12 hours) during both neap and spring tide periods. Presently, there exists a great need for more tidal pools as part of the demand for more recreational facilities along the South African coast. However, no information on design criteria could be found in the literature.The objective of this research is to study the prediction method of hurricane waves around this island, especially in the Taiwan Strait. The paper describes the prediction of hurricane waves used by Bretchneiders (1976) Method and finds out the predicted waves are different from measured waves, therefore the Bretchneider predicted model is modified by the authors and then the modified model is applied to predict waves again. It is found out that predicted waves match well with the measured waves. The results of the modified Bretchneider model are compared with those of the Ijima tracing method and find out the former is better than the latter. The second part is to apply the modified model to predict the extreme value of wave heights and compute the worse hurricane wave condition of the surrounding sea area around island, within recent score year (1959-1978). The calculated sites are Chu-Wei, Nan-Liaw, Ta-Shih, Cheng-Kung, Pu-Tai, Tung-Kang, Nan-Wan as shown in Fig.1 and Shiau Liu- Chieu totally 8 stations. Then use the Gumbel Distribution TYPE 1 to predict the extreme wave height of each returned period.In the design and construction of waterfront bulkhead systems, it is essential to consider the coastal effects of tides, waves, boat wakes, currents, bottom sediment movement and bottom scour. Many improperly designed bulkhead systems experience severe loss of backfill and toe materials with the bulkhead eventually failing if it is not corrected in time. Inadequate drainage, joint connections, and/or inadequate toe protection are typically the causes of failure. This paper describes an investigation of a bulkhead system supporting a large waterfront development in southern California which was experiencing widespread sinkhole development in the bulkheads backfill and was on the verge of losing toe material. The objective of this investigation was to determine the extent and cause of ongoing subsurface erosion, to evaluate its effect on the bulkhead stability, and to recommend and design mitigative measures. The cause of the erosion was determined to be piping of fine grained soils due to inadequate backfill drainage. A remedial drainage scheme was designed and field-tested, and several structural repair schemes were suggested for portions of the bulkhead where accumulated damage affected the integrity of the structure.

Response Characteristics of an Ebb-Dominated Tidal Inlet Channel

ABSTRACT A three-year study (July 1974 to July 1977) of Price Inlet, South Carolina, monitored inlet hydraulics and channel morphology. The inlet is strongly ebb-dominant. Both the peak and mean current velocities during ebb exceed the corresponding velocities during flood. The velocity asymmetry is caused by changes in inlet efficiency during the tidal cycle. The intertidal marsh of the Price Inlet drainage area experiences a change in open water surface area of about 670 percent during an average tidal cycle. At high tide, the large water surface of the marsh is responsible for inefficient exchange of water through the inlet. This causes a long time lag between ocean and bay tides. The small surface area of open water in the marsh at low tide, on the other hand, yields nearly no time lag. These differential time lags cause longer flood durations than ebb durations in the inlet throat and, consequently, stronger ebb elocities. Monitoring of the cross-sectional flow area of the inlet throat demonstrates rapid adjustment to changing flow conditions. The 1159 m2 cross sectional area of the inlet throat varied as much as 8.3 percent during one tidal cycle. The cross sectional area varied in phase with the tidal range. Long-term changes in the dimensions of the channel appear to be a direct result of changes in the size of ebb-tidal delta shoals. During the study period, the growth of the linear bars in the channel margin gradually reduced wave energy along the inlet shoreline and on the inner ebb-tidal delta platform. This in turn reduced sediment transport into the inlet, which resulted in a gradual increase in the cross-sectional area of the channel. Seasonal fluctuations in mean sea level appear t have little effect on the inlet channel.

The MRGO Navigation Project: A Massive Human-Induced Environmental, Economic, and Storm Disaster

Abstract It is generally felt in the water resources community that the most significant twenty-first century public works projects will be those undertaken to correct environmental damage caused by twentieth century projects. A second axiom is that the switch from economic development to restoration and mitigation, what we call redemption, often will be precipitated by disaster. Finally, it must be expected that the repair project will cost far more than the initial public investment but also may have economic revitalization potential far exceeding anticipated environmental benefits. We examine this cycle for the federally funded Mississippi River Gulf Outlet (MRGO) navigation project east of New Orleans, beginning with its much heralded birth in 1963 as a 122 km long free-flowing tidal canal connecting New Orleans to the Gulf of Mexico and ending with its recent de-authorization and closure. We track the direct and indirect effects of the project through its commercial failure, and then on to the official denial, the pervasive environmental impacts, and finally exposure of its role in flooding New Orleans during Hurricane Betsy in 1965 and more seriously during Hurricane Katrina in 2005. Post de-authorization planning to curtail continuing environmental and economic damage now offers an opportunity to apply lessons that have been learned and to reinstate natural processes that were disrupted or interrupted by the MRGO during the half-century of its operation. One surprising outcome is that the restoration program may turn out to be more commercially successful than the original navigation project, which was conceived as an agent of economic transformation. The U.S. Army Core of Engineers still does not acknowledge, even in the face of compelling scientific evidence, that the MRGO project was a significant cause of early and catastrophic flooding of the Upper and Lower 9th Wards, St. Bernard Parish, and New Orleans East during Hurricane Katrina. A modeling effort that removed the MRGO from the landscape, and restored the cypress swamps and marshes killed by the MRGO, reduced flooding from Hurricane Katrina by 80%. We conclude that the MRGO spelled the difference between localized flooding, and the catastrophe that killed 1464 people and inflicted tens of billions of dollars of property damages. If the MRGO-caused economic damages associated with Hurricanes Betsy and Katrina are combined with those of construction, operation and maintenance, and wetlands destroyed, then the total economic cost of the MRGO is in the hundreds of billions of dollars.

Stability of giant sand waves in eastern Long Island Sound, U.S.A.

Abstract A combination of a highly accurate bathymetric surveying technique and in-situ submersible observations and measurements were used to assess the migrational trends and morphological changes of large sand waves (Ht ≤ 17 m) in eastern Long Island Sound. Although residing in a high-energy tidal environment characterized by a net westward sediment flux, the large bedforms are relatively stable over the short term. Over a 7 month period, 55.1% of a total 2942 m of sand wave crestline lengths migrated less than the horizontal accuracy limits of navigation (2 m). Approximately 35% of the remaining sand wave crests migrated less than 4 m. Net migration of the sand wave crests in the study area was 0.2 m. In addition, the bulk form (center of area in profile view) or the base of the sand waves showed little, if any, movement. These data, in conjunction with flow data within the sand wave field, suggest that net migration rates are greater than the time span of this study and/or the sand waves move in response to large residual flows created by high-energy, aperiodic storm events. The latter scenerio suggests that day to day processes only serve to rework and modify the sand waves.

Volumetric analysis of a New England barrier system using ground-penetrating-radar and coring techniques

Ground-penetrating-radar (GPR) profiles calibrated with core data allow accurate assessments of coastal barrier volumes. We applied this procedure successfully to the barrier system along Saco Bay, Maine (USA), as part of a sediment-budget study that focused on present-day sand volumes in various coastal, shoreface, and inner-shelf lithosomes, and on sand fluxes that have affected the volume or distribution of sand in these sediment bodies through time. On GPR profiles, the components of the barrier lithosome are readily differentiated from other facies, except where the radar signal is attenuated by brackish or salty groundwater. Significant differences between dielectric properties of the barrier lithosome and other units commonly result in strong boundary reflectors. The mostly sandy barrier sediments allow deep penetration of GPR waves, in contrast to finer-grained strata and till-covered bedrock. Within the Saco Bay barrier system,