ABSTRACT

The Caminada-Moreau Coast of Louisiana is located within the abandoned Lafourche distributary system of the Mississippi River. This outlet was abandoned about 600 years ago, and marine processes reworked the deltaic materials to form marginal islands and beaches. The morphology of this coastline reflects both the processes effecting erosion and the nature of the pre-existing ancient deltaic environments that now form the coastal margin. Basically, the geomorphological factors of the coastal margin consist of beach face erosion, littoral drift, washover, and aeolian sand transport. In the first half of 1979, a field study examined sand encroachment on a variety of landward surfaces and the extent to which the elevation and nature of the surfaces modified the washover and aeolian transport processes.

Two scales of washover features are identified along this coastline: 1) large washover forms generated by a major depositional event such as a hurricane storm surge; and 2) smaller forms resulting from frontal passage and localized storm activity. Only small scale washover processes were observed, and field measurements indicate that washover takes place when onshore wind and wave energy combine with the tides to produce a water level set-up of approximately 2.0 ft above average. Elevation of the coastal edge is critical to the amount of washover taking place. The pre-existing surface exerts control over the form of washover deposition. In areas of low coastal edge elevation extensive washover occurs; but in areas of high coastal edge elevation, the coastline erodes, producing a small scarp or washover at points of weakness, producing narrow tongue or fan-shaped penetrations.

Aeolian sediment transport and dune development are functions of proximity and size of sediment source. Dune fields are associated with large washovers and areas where old cheniers are being eroded. Well developed dune ridges occur on the spit adjacent to Caminada Pass where littoral drift nourishes the beach and dune areas.

GENERAL SETTING AND ENVIRONMENTAL FACTORS

Previous surveys and reports (Harper, 1977; Dement, 1972 and 1979; Ritchie, 1972; Morgan and Larimore, 1957) have demonstrated the considerable retreat of the south Louisiana coastline between Caminada Pass and Belle Pass some 12 mi to the southwest. Measures of this retreat are given in figure 1, which also attempts to illustrate the fact that different parts of the coastline have retreated at different rates. In general, there is a tendency for the coastline to retreat less in the direction of Caminada Pass.

The basic reason for retreat is probably related to persistent land subsidence and to the absence of substantial sediment sources along the coastal zone. About 1,200 years ago the Mississippi River occupied the Lafourche course. This outlet was abandoned about 600 years ago, and marine processes reworked the deltaic materials to form marginal islands and beaches. Sediment sources are confined to the reworking of existing beaches, and, as the coastline retreats, old beach ridges and cheniers may provide additional inputs. Other studies (Harper, 1977; Dement, 1979) suggest a negative sand budget for the coastline, in so far as the rate of sand replenishment is less than the net loss of beach materials.

Against this background of continuous retreat, the changing coastal morphology reflects the processes effecting erosion and the nature of the pre-existing ancient deltaic environments that now form the coastal margin.

In their simplest form,the geomorphological factors of the coastal margin consist of beach face erosion, littoral drift, washover and aeolian sand transport. To a lesser degree, there is localized anthropogenetic impact. The role of vegetation is also of considerable importance as a control of sand dune formation and stabilization.

In relation to washover, the main forces are tidal change and wave set-up. The tidal range is 1.2 to 1.9 ft (0.37 to 0.6 m); but in every year since records are available, several days of elevated water levels occur as a result of the passage of fronts and strong onshore winds. Normally this elevation is

Figure 1. Location map of study area showing measures of coastal retreat.

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of the order of 4 ft (1.2 m) above the predicted tidal level. Since substantial lengths of this coastline are only one or two feet above mean sea level, the effect of such high sea levels is to flood most of the coastal barrier. Areas that remain above flood level are the higher dune ridges and some ancient levee formations.

During the study period, January to June 1979, field observation confirmed that minor washover took place when tide and wind waves combined to elevate the water surface to about 4.0 ft (1.2 m) above Mean Sea Level as defined by a Corps of Engineers bench mark; a level that was exceeded at least 10% of the time. A sea elevation to this 4.0 ft (1.2 m) level is thus locally equivalent to a wind and pressure generated rise of about 2.0 ft (0.6 m) above predicted high tide conditions.

The wave climate for this coastline has been analyzed by the Corps of Engineers (Dement, 1972). The main conclusion is that "since the waves appraoching from the predominant direction, the southeast, do not have a significant littoral direction, the waves from the southwest and south are the primary cause of littoral transport along the shoreline ... essentially towards Caminada Pass and Grand Isle" (pp. B-7, 8). The same report gives the average surf conditions as waves of 6 to 7 second periods and 1 to 3 ft high. The maximum range is 3 to 9 seconds and heights of 1 to 9 ft.

Average wind directions as provided by the U.S. Coast Guard Station on Grand Isle show that the most frequent and the strongest winds are from the southeast for about 20% of the time in any average year. An analysis of winds in the 1960 to 1963 period is given below to indicate the importance of other directions (Grand Isle Station):

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   Direction of Average Surgace Wind Direction at Grand Isle
   in Percentage of Time.
             N 12.42                         S 13.94
            NE 16.86                        SW 10.91
             E 11.7                          W  6.22
            SE 18.66                        NW  9.41
------------------------------------------------------------

There is no significant difference between the pattern of wind frequency and that of wind strengths. Adding these quantities as vectors gives a resultant direction of 282°. These values are significant in that in any given year it would take only a small shift in wind patterns to alter the energy vectors so that the directions of aeolian and littoral transport would alter.

Against this background of persistent erosion and shoreline retreat, this paper seeks to describe observations and measurements, made in the first half of 1979, which were designed to examine how sand encroached on a variety of landward surfaces and the extent to which the elevation and nature of these surfaces modified the washover and aeolian transport processes. Additional evidence was provided by the interpretation and analysis of several sets of high quality aerial photographs.

FIELD AND OTHER METHODS

In January 1979, lines for profiles were established between Belle Pass and Caminada Pass. We had hoped to re-occupy profiles established by the Corps of Engineers and other investigators (Harper, 1977), but with one exception, these lines could not be found with sufficient precision. Moreover, since the morphological situation was the factor of most interest, selection of survey sites was based on obtaining representative zones such as open sand bar, old deltaic marsh, lagoon, old distributary channel, etc., rather than equally spaced, arbitrary divisions. The locations of these lines are shown on figure 2, which also shows the characterization of the coastline tract. This classification is based on a combination of field mapping and aerial photographic interpretation. Many morphological and surface distributions which are obvious on aerial photographs are almost impossible to discern at ground level, due to the low relief amplitude and preponderance of surface water in this coastal zone.

The profiles were surveyed with a Wild automatic level using tacheometry to establish distances. Accretion or removal of sand can be deduced from changes in the profile as well as from direct measurement against stakes. The surveying interval was approximately every four weeks. All the survey lines have been reduced to the datum of Mean Sea Level by levelling to a common bench mark established near Elmer's Road (fig. 2) by the Corps of Engineers (RG14PT1). At the time of the surveys in 1979, actual local mean sea level appears to be around 2.5 ft above the Corps of Engineers Zero Datum.

Direct observations of active processes were also being made during these field visits and the results were correlated with simultaneous wind and water level records from the Coast Guard Station at Grand Isle.

RESULTS

A selection of profiles (lettered to correspond with fig. 2) is shown on figure 3. Although more than 14 profiles were used and 10 types of coastal tracts are identified, only five type-situations are described below, but these are considered to exemplify the general nature of the washover and other

Figure 2. Map of study area showing coastal tract types and locations for profiles in figure 3.

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processes operating on this retreating coastline.

Sample Profile A

The type-situation illustrated by this profile is a barrier beach encroaching on an extensive delta distributary lake (Bay Champagne). During 1979, this beach was continuously washed over whenever the water level reached approximately 4.0 ft (1.2 m) (with respect to Corps of Engineers Datum). Two stages of washover were observed. At exceptional levels of wave energy and sea elevation (approximately 5.0 ft, [1.5 m]), the entire beach and sand bar was in a state of continuous sheet-type movement. At lower levels (around 4.0 ft [1.2 m]), the water level was approximately equal to the height of the highest beach crest and overwash occurred as a series of fan-shaped penetrations with narrow run-up "throats" about 20 to 30 m apart. On the landward (lake) side the water formed small channels carrying sediment towards the lake; many flows soaked into the sand less than halfway across the bar. On succeeding days, beach morphology consisted of low barchan-like forms about 25 m wide and 40 m deep, with the horns pointing towards the lake. The profile surveys tend to show a buildup in the elevation of the backshore beach crest and a lowering of the mid and lakeside backslope. The beach profile changed rapidly and in May showed marked accretion. There is also some intermittent creation of small embryo dunes which are more persistent near the lake margin.

As shown in the small sketch maps of 1887, 1954 and 1978, the retreat has been remarkable and the planmetric detail has varied considerably. There is little subsurface control, and the nature of the overwash process is a simple function of wave strength and sea elevation. The beach is migrating across earlier overwash formations, especially the major depositional episode of hurricane Carmen. There are a few exceptional areas near the west margin of the beach where there are more permanent low mobile sand dune ridges, and near the east margin where remnant Mangrove and marsh "islands" have provided nuclei around which wind blown sand has accumulated and is stabilized by dune and Salicornia type vegetation.

Sample Profile B

Sample Profile B is taken across a section that corresponds to the old levee bank of Bayou Moreau. This profile is the shortest distance between the channel and the coast. The area is higher than the average elevations of the coastline, and this protects it from overwash other than during exceptionally high water levels, such as are produced by tropical storms and frontal passages. Accordingly, the profile reveals the other mechanism of coastline retreat: bank erosion to form a low coastal scarp. More than 7 m · of erosion took place between January and March 1979, with a further 5 m of retreat in April and May when a gently sloping beach slope was also replaced by a 0.5 m high sand cliff. This erosion is probably the main source of sediment supply to the beach system. This crest was also lowered, presumably by a rare overwash event. Overwash sand raised the backslope by a few inches. Because of the general elevation of this coastal section, overwash takes the form of small fans with a narrow throat at points where the crest of the coastal edge is lower or discontinuous. There is also some local concentration of wave action. Such concentration may be caused by variations in the width of the organic soil layer which outcrops on the lower beach and has a distinctive irregular outline. These peat outcrops are themselves good indicators of coastline retreat. This type of washover is described as small scale channel washover and is often marked by two marginal, shallow run-off trenches. This zone thus provides an example where higher pre-existing topography restricts washover to rare storm occasions and the energy of the waves is used to erode the sandy coastal edge to produce the characteristic

Figure 3. Coastline profiles A-E for locations A-E on figure 2.

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micro-cliff profile with sand, shell and debris accumulation on the edge of the scarp. The washover feature is always a narrow penetration at a point where a line of weakness or lower coastal elevation exists.

Sample Profile C

Sample Profile C is chosen as a contrast to Profile B in that it corresponds to a low former backswamp area within the meander bend of Bayou Moreau (fig. 2). Overwash penetration inland is deeper and on a wider front. There is little if any subsurface control. The general elevation is lower and the zone is best described as a bare washover flat which is large enough to generate small aeolian deposits, especially near the inner and lateral margins of the sand plain. There are numerous shallow run-off channels on the backslope which carry sediment landwards and tend to make the middle part of the profile flatter. During 1979, extensive sheet-type washover was not observed, but there was persistent spilling of water over the crest onto the backslope zone. Since the inner margin of the sand surface is lower than the beach and leads to a series of shallow marsh lakes, this area is a smaller scale version of the situation where a sand bar is encroaching onto a pre-existing lake or lagoon.

This zone also provided the best examples of the control exerted by the position and amount of beach debris, including large logs, and oil-platform jetsam in concentrating wave impact on specific parts of the coastal edge to produce small scale crenulations which would be exploited subsequently during more powerful wave and washover episodes.

The sequence of change of the profiles from January to June shows a general lowering between January and March of the entire profile, followed by a retreat of about 6 m per month of the highest beach crest. Although the limit of sand encroachment on the landward part of the profile is conspicuous, the landward part of the profile did not increase in elevation, and the evidence of extension of the sand surface is inconclusive. Aeolian movement was observed in May, 1979, where surface movement of sand was measured as far inland as point A on the profile. The thickness of the drifting layer was only a few cm and the sand was being removed from the backshore beach crest. Embryonic dune forms were noted as forming around stranded debris on the overwash plain, but most accumulation was on the margins of the bare sand surface where the sand lodged against the higher Spartina and pre-existing dune-type long grasses. This area is thus an active example of the manner by which higher, larger dune ridges form along the sides and inner limit of washover fans and other formations. Years later, these dunes become stabilized and the overwash platform vegetated. Many examples of "inner" dunes are found along this coastline, especially east of Elmer's Road (fig. 2) and near Bay Marchand. These older, inner dunes can normally be identified as being on the margin of the washover fans or flats produced by hurricane Carmen (as detected on special post-hurricane aerial photographs taken by the Corps of Engineers in September 1974).

Sample Profile D

Profile D is located across a low dune terrace where a small tongue-shaped washover sand deposit spills onto an area of Mangrove swamp and small lakes. There is a small outcrop of marsh peat strata near low water mark, but this is not a conspicuous feature. The area is relatively higher than average and corresponds to the older chenier marsh and submerged ridge coastal tract (Ritchie, 1972). This is one of the narrowest areas of sand deposition along the entire coastline. As shown on the profile, this section represents sand encroachment onto a small marshy depression which appears to be an old overwash channel (marked C on the profile). There is no real dune development, simply a gentle vegetated sand slope. Ridge B is part of a tongue of bare sand, which is a single overwash penetration curving around behind the channel (as shown in the sketch accompanying the profiles).

The most interesting aspect of the profiles, however, is a complete lack of beach, coastal edge, or dune terrace change between January and April, 1979. Precise profiling revealed no changes in distance or elevation. Even the backshore profile was unaltered. After the storm of April, 1979 (when the water level was measured at 5.7 to 6.2 ft [approximately 1.8 m] above the Corps of Engineers Datum), the tongue of washover sand (area B) was reactivated, and sand was deposited in the margins of the channel (C). The May survey revealed this change as well as some modification of the coastal edge. Retreat and undercutting, as well as some edge accumulation of sand and beach debris, produced the second profile. The elevation of the profile is such that this is a marginal situation in that only the highest phase of wave elevation can overtop the coastal edge; at lower levels erosion by undercutting is the dominant process. Nevertheless, at adjacent parts of the same edge there are slightly lower edge heights or different beach run-up slopes where narrow tongue-shaped washover penetrations took place and briefly broke through to the low Spartina and Mangrove marsh areas which lie only 70 m from the coastal edge.

Sample Profile E

This profile corresponds to Range No. 14, as established and surveyed by the Corps of Engineers in 1973 and by Harper (1977). This is an area of stability and coastal edge oscillation. The main dune ridge is about 5 ft above mean intertidal beach and is only reached by exceptional storm wave levels. The profiles show a low sand platform with embryonic dune forms in front of the main vegetated dune ridge. This platform shows considerable profile variability with a trend to accretion. The 1974 post-hurricane aerial photographs show that this area was an extensive sand washover plain. The present line of coastal dunes has developed on top of the extensive sand platform, a platform that is now fully vegetated and can be traced 600 ft inland to a low, rounded ridge which marks the furthest penetration of hurricane washover. The entire section is based on a sand spit, which has developed northeastward since the 1950's to join marsh remnants into a continuous beach system stretching as far as Caminada Pass (cf. inset map). These marsh remnants can still be seen on the lagoonal side of the coastal formation. Careful levelling shows no change in the landward part of the profile in 1979; geomorphological activity is confined to the coastal edge and upper beach. Harper (1977) calculated a net linear retreat of about 3 m for this range in the period 1973 to 1976, but in three of his six surveys, there was progradation of up to 10 m (p. 288). The 1979 survey suggests a phase of progradation and the possibility of dune development seaward-of the present line. Stability and possible accretion in this zone accords with the

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general trend, as illustrated by figure 1 where longshore drift towards Caminada Pass has created a wide beach and reduced the tendency to shoreline retreat.

DISCUSSION

Two scales of washover features are identified alone the Caminada-Moreau Coast of Louisiana: 1) large washover forms generated by a major depositional event such as a hurricane storm surge; and 2) smaller forms resulting from frontal passage and localized storm activity. During the field survey conducted in the first half of 1979, only small scale washover processes were observed. Field measurements indicate that when onshore wind and wave energy combine with the tides to produce a water level set-up of approximately 4 ft (1.2 m) (which compares with the recorded elevation of 9.0 ft [2.7 m] for hurricane conditions), washover will take place. The major morphological factors controlling the effects of washover processes are the elevation of the coastal edge above mean sea level and the topography of pre-existing surfaces. Factors that are of subsidiary importance are beach width and gradient.

Elevation of the coastal edge is critical to the amount of washover taking place, and thus the frequency of occurrence. The nature of the pre-existing surface exerts control over the form of washover deposition. In areas such as sample profiles A and C, the coastal edge is low and the pre-existing ancient deltaic surfaces inland are relatively smooth, offering little resistance to the landward transport of sediment by washover. This situation results in large, wide washover forms, characterized by extensive development of runoff channels on the landward slope. In contrast, sample profiles B and D, are higher sections of coastline, and in these areas washover occurs at weaknesses or low areas in the coastal edge. The pre-existing topography determines if the washover form is fan or tongue shaped. Typically, these forms have a narrow neck at the coastal edge, whereas the shape of the washover deposit on the backslope is variable, being controlled by vegetation and pre-existing topography. In sample profile E, the relative height of the coastal edge elevation excludes washover, except during extreme storm events. This coastal type is characterized by beach progradation and dune growth, related to the same processes that have given use to spit development at Caminada Pass.

Aeolian sediment transport and dune development is a function of proximity and size of a sediment source. Grain size is in the fine to medium category (FOOTNOTE 1) and field observation indicated that winds with a velocity greater than eight knots are sufficient to cause transport. In addition, the relative dryness of the sand surface is also critical. Dune fields are associated with the large fan and sheet-type washovers. There is also some correlation with areas where old cheniers are being eroded, creating a localized sediment source. The dunes associated with a washover source tend to be at the edges of the feature where the sand is trapped by adjacent vegetation. Smaller embryo forms are found on the surface of the main sand flat and are associated with beach debris and other obstacles. Well developed dune ridges occur between Elmer's Road and the spit adjacent to Caminada Pass (fig. 2) where littoral drift nourishes the beach and dune areas.

Coastal retreat along the Caminada-Moreau Coast takes place by two mechanisms; in areas of low coastal edge elevation extensive washover occurs, but in areas of high coastal edge elevation the coastline erodes, producing a small retreating scarp in the upper beach face. In areas of higher coastal edge elevation, washover takes the form of narrow tongue- or fan-shaped penetrations.

Inspection of meteorological records shows that, with the exception of the single storm of April, 1979 (Muller, 1979), weather and tide conditions were near normal for the first half of 1979. The results discussed in this paper thus illustrate the persistent, gradual reworking and modification of a coastline in retreat. Leveling of forms and processes shows that wind and waves need only raise the elevation of the sea by one or two feet above average to produce significant changes in the order and magnitude of geomorphological activity. Further examination of the existing coastal morphology and correlation with aerial photographs shows that most of the present coastal forms are a direct legacy of the single exceptional event of hurrican Carmen, and present processes are acting upon the stabilized surfaces produced by that event. Beneath these surfaces lie older formations of deltaic origin which continue to exert some control on the sequence of physiographic activity.