Shore protection and coastal change on the Lake Michigan shore: Duck Lake, Orchard Beach State Park, and Onekama, Michigan.
A high resolution beach profile change monitoring program was conducted during generally high water in the 1990s at three sites along the eastern shore of Lake Michigan in a variety of coastal geological settings to evaluate an experimental shore protection technology called the Undercurrent Stabilizer System[TM]. Shore protection structures at all three sites produced minimal negative impact at and immediately adjacent to the study sites and generally resulted in significant net accretion of near-shore sediment in and around the structures during the study period compared to control sites. Local complexities and variations in coastal processes and conditions (especially local long-shore transport, coastal substrates and geological setting, and other existent engineered structures) play a critical role in the specific performance characteristics of these small (property owner) scale shore protection structures in the Great Lakes region. A fundamental factor in the evaluation of coastal change associated with shore protection on eastern Lake Michigan (as well as many other areas of the Great Lakes) is that net transport of sand is offshore, especially during periods of prolonged high water levels. Maintenance of long-term sediment budgets through input of beach-grade sand is dependent on the erosion of upland (bluff/dune materials) or nourishment. The use of coastal monitoring results for coastal planning, development, and permitting decisions requires careful consideration of several factors: the intent of the monitoring project in terms of spatial scale and time frame of influence of the structures that are monitored and the appropriateness of extrapolating monitoring results to other areas with substantially different coastal setting. Although the overall results of the study indicate that no substantial negative impact occurs within the experimental structure study sites compared to control sites, we believe that the isolation and protection of back-shore coastal sediments from wave action through the use of any shore protection technology will ultimately result in increasing offshore loss of near-shore sand throughout the eastern Lake Michigan coastal system.
INTRODUCTION AND PURPOSE
Hard shore protection is considered with increasing skepticism by the coastal management community in the U.S. Permits for shore protection are very critically reviewed by state and federal coastal managers in Michigan, especially shore-perpendicular structures projecting on to "state owned bottom land." Much of lower Michigan Great Lakes shoreline comprise erodible coastal substrates. Periodically high water levels in the recent past have resulted in accelerated bluff/dune recession rates, and future high water level periods will undoubtedly result in a public outcry for "protection."
This paper presents the final results of coastal change monitoring studies of small-scale, experimental shore protection technology marketed as the Undercurrent Stabilizer System[TM]. The studies were conducted in three study areas along the eastern Lake Michigan shore: near Duck Lake State park north of Muskegon, near Orchard Beach State Park north of Manistee, and North of Onekama, MI (Figure 1). Coastal change monitoring was conducted from 1991 through 1998 (see time line, Table 1). The purpose of these studies is to document the overall impact of the experimental shore protection structures and assess the effectiveness of this approach to Great Lakes coastal engineering practices relative to other shore protection alternatives, including no action.
A complete discussion of general study background is presented in Duck Lake Outlet Demonstration Project Undercurrent Stabilizer System[TM] Shore Stabilization/Accretion Program Final Report of 1991-1996 Surveys (Barnes 1998; available from the author, see references).
EXISTING POLICIES RELATED TO SHORE PROTECTION IN MICHIGAN
Coastal management policy in Michigan (in accordance with federal policies) has evolved over the last three decades away from "hard" engineering intervention in the coastal zone to emphasis on setback (initial construction of permanent structures a safe distance from receding shore lines or landward relocation of pre-existing permanent structures). In areas subject to high rates of coastal land loss, permanent structures must be either moved landward (where possible), protected by shore protection, or lost. Permanent structures built in erosion hazard areas prior to enforcement of setback-oriented management policies, and that cannot be relocated, remain a problem for property owners and the management community. The hope of "permanent engineering solutions" to accelerated bluff recession, especially during high lake levels, springs eternal. Short-term "protection" from erosion hazards is of paramount importance to coastal property owners. Long-term effectiveness in terms of the "public trust" is paramount in the policies of the management community.
[FIGURE 1 OMITTED]
Coastal management policies in Michigan allow installation of new shore protection within 1,000 feet of existing structures. This policy acknowledges the probable negative impact of most pre-existing, shore protection structures. Michigan statutes mandate that shore protection constructed on state-owned bottom land (below 580.5 feet ASL, the "Ordinary High Water Mark of Lake Michigan") may be considered as an option only in "high risk erosion areas"; shorelands of the Great Lakes and connecting waters where recession (landward movement) of the zone of active erosion has occurred at a long-term average rate of one foot or more per year. Shore protection intended to mitigate documented erosion hazards must satisfy environmental considerations which include (1) minimal adverse effects to the environment, public trust, and riparian interests of adjacent owners; and (2) that there are no feasible or prudent alternatives to shore protection.
The extent of adverse effects to adjacent shore reaches resulting from existing shore protection structures, on all scales from small sea walls to large navigation channels, is often difficult to establish. However, superimposed negative impact on a shore reach due to ever-increasing coastal engineering structures can only be completely averted by avoiding engineering approaches at the outset. The question remains: what coastal engineering "solutions" really result in "adverse effects"? Ultimately, interference with littoral drift, other near-shore processes, and enhanced erosion hazard to immediately adjacent shore reaches constitutes an important acid test for any shore protection strategy. Unfortunately, this approach does not value the "protection" afforded a particular reach of the shore line by small-scale shore protection technology.
In the broader context of responsible and sustainable coastal development, the Federal Emergency Management Agency (FEMA) vigorously promotes an approach to shore protection, nationwide, that emphasizes the dangers associated with any engineering approach to coastal erosion hazards. FEMA warns of additional and denser development that may be attracted to the coastal zone as a result of short-term engineering "solutions" to coastal erosion/recession hazards. Continued unwise site selection and development on the coast resulting from the perceived engineering solution to long-term coastal erosion hazards "relegates future options to a perpetual and expensive fight against the sea with (additional) engineering solutions" (Davidson et al. 1996).
BACK GROUND AND METHODS
The main objective of this study is to evaluate the influence on coastal change of the experimental Undercurrent Stabilizer Systems[TM] shore protection structure. These structures consist of low profile, flattened, tubular, mortarfilled, geotextile bags that extend and taper (vertically through a step-down design) lakeward and perpendicular to the beach (Figure 2). The structures are typically installed to project 1-4 feet above the native beach profile and extend approximately 50-175 offshore. The general design of the structures resembles anchor groins, structures intended to stabilize a length of shoreline by retaining a portion of long-shore drift material. The general design intentions of such structures are to allow beach grade sediment bypassing, either over or around the structure, in order to maintain long-shore drift patterns.
[FIGURE 2 OMITTED]
Coastal Change and Erosion Hazards in the Great Lakes Region
The main factors controlling coastal change and erosion hazards in typical Great Lakes settings are (1) wave climate and wave-induced currents (cross-and long-shore currents); (2) lake level fluctuations; (3) geological setting and coastal substrates including ground/surface water conditions; and (4) coastal engineering structures.
It is critical to distinguish between two important concepts related to Great Lakes coastal change analysis:
1. "Shore Erosion/Accretion" is the net loss/gain of sediment volume from the active portion of the beach profile (to the depth of significant, wave-induced, beach grade sediment transport). Shore erosion/accretion, along any shore-perpendicular beach profile, may result from the interruption of long-shore sand transport by engineering structures as well as numerous other natural and anthropogenic factors.
2. "Shore Recession/Progradation" is the landward/lakeward migration of the "shoreline," or the shoreward/lakeward translation of geomorphic features related to the water line: fore dunes, dune, or bluff crest, etc.
Shoreline recession/progradation may result simply from a rise/fall in lake level with no net loss/gain of coastal sediment along a given beach profile. The true day-to-day, month-to-month, and year-to-year environmental hazard in the Great Lakes Region is shoreline recession. The success of shore protection is measured, not in terms of directly preventing shore erosion (since long-term coastal change patterns in typical Great Lakes shorelines are almost certainly erosional, see "Wave Induced Currents," below) but in the mitigation of shoreline recession and the loss of dry land to the lake. Lake level rise, for example, may not result in direct shore erosion but certainly accelerates shoreline recession.
We now know that negative beach profile volume change (shore erosion) in the active beach profile will ultimately result in additional wave energy brought to bear on the above-water ("dry") portion of the beach profile (Nairn 1992; Meadows et al. 1997). Additional wave energy and wave-induced scour on the "dry" beach, toe of the bluff, or foredune will typically cause offshore movement of coastal sediment and eventual shoreline recession. Immediate mitigation of shoreline recession and coastal land loss resulting from the installation of shore protection is typically emphasized in evaluating the "success" of a shore protection strategy. However, long-term (years to decades) response to coastal engineering in light of possible shore erosion and short-term damage to adjacent shore reaches must also be carefully considered.
Wave Climate and Wave-Induced Currents
Meadows et al. (1991) suggest that larger waves generated by local sustained winds in excess of 10 mph are the major force responsible for coastal sediment transport and coastal change in the tide-free Great Lakes. Davis et al. (1975) presented a long-term analysis of Muskegon weather station data for 1960 through 1970 and found that 92% of 24 hour periods with 20 mph winds occurred from November through March. No directly measured wave data are available for the November through April period.
The wave climate responsible for near-shore sediment transport on the eastern shore of Lake Michigan is well documented in Hubertz et al. (1991). Typical wave climate is also discussed in USAEDD (1975, 1976). The wave climate in the 3 study areas is generally similar, with predominant wave energy produced by winds from the westerly two quadrants during local storms within the lake basin. Significant sediment transport by wave-induced currents is accomplished by short period (<11 seconds), steep waves during and shortly after these storm events. The strongest winds and largest waves occur during local high wind events most typically associated with the west to east passage of frontal storm systems. The storm track through the lake basin varies seasonally with a more southerly track, south of the lake, most common during the late fall, winter, and early spring (October-March, "winter" pattern) months and a more northerly track, at or north of the lake, during the late spring, summer, and early fall (April-Sept "summer" pattern). The winter storm track typically produces strong northerly winds and waves from the north. The typical summer storm track produces winds and waves from the south in advance of the storm's passage and winds and waves from the north after the front has passed through the basin.
In order to examine differences between the Muskegon area and the two northern sites (Orchard Beach Park and Onekema), mean significant wave heights, peak periods, and wave direction were evaluated from 1956-1987 using the data of Hubertz et al. (1991). The Muskegon area has one of the most energetic wave climates along the eastern Lake Michigan shore with mean significant wave height of 0.9 meter, a maximum significant wave height of 7.7 meters, a mean peak wave period of 4.1 seconds, and a maximum peak wave period of 11.0 seconds. There does not appear to be a predominant annual wave direction in the Muskegon area perhaps due to the approximately equal fetch distances from all westerly quadrants. In contrast, for the northern study area the mean significant wave height is 0.6-0.7 meter, the maximum significant wave height is 6.5-6.9 meters, the mean peak wave period is 3.8-4.1 seconds, and the maximum peak wave period is 10.0-11.0 seconds. The most frequent wave direction band in these sites is from the southwest (180[degrees] to 202.5[degrees] combined, 39% to 50% of observations). For all three study areas mean significant wave heights are greatest from October through March although wave energy and resultant wave-induced currents are strongly influenced by the extent and timing of shore-fast ice.
Cross-Shore Sediment Transport
Most sediment transport in the study area probably occurs during local "storms" or periods of sustained local winds. Local wind-induced waves, or "seas," with high steepness are thought to produce a long-term, net offshore transport of coastal sediment and produce "dry beach" erosion. Limited fetch distances in Lake Michigan preclude significant wave activity due to far traveled, lower steepness "swell" that might produce long-term, net onshore transport and accretion (USAEDD 1975, 1976). Furthermore, the typical association of larger, more energetic, and steeper waves with onshore winds is also likely to enhance offshore transport of littoral sediments by offshore-directed, pressure-gradient-induced undercurrents (USAEDD 1975, 1976). This study concludes that "a steady amount of erosion [offshore directed sediment transport] is taking place under natural processes" and that coastal "sediment is continually lost offshore."
Net cross-shore sediment transport is also well documented in typical Great Lake settings (Hands 1980) as a result of lake level changes. "Brunn's Rule" (Brunn 1962) predicts that as lake level rises the equilibrium beach profile will migrate upward and landward. This results from removal of beach sediment from the upper portions of the beach and transport of this material offshore to be deposited on the near-shore bottom. In this way the elevation of the near-shore area is equal to the rise in water level such that constant water depths are maintained in the near-shore area subsequent to the water level rise. These relationships were tested by Hands (1980). The reverse is theoretically true; the beach profile will migrate downwards and lakeward in response to accretion of beach grade sediment in the backshore through landward transport of near-shore bottom sediment. This relationship has not been quantitatively tested in the Great Lakes but would appear to be the case on the basis of anecdotal observation.
Long-shore Sediment Transport
USAEDD (1975, 1976) tests were conducted in the vicinity of several federal navigation structures along the eastern Lake Michigan Shore including Muskegon and Frankfort Harbor (north of the northern study sites). These studies provide traditional engineering assessment of littoral processes along the Lake Michigan shore. Using the energy flux method (U.S. Army Corp of Engineers, CERC 1984) estimates of long-term average, net and gross long-shore transport (from 1962 through 1973) were calculated and are relevant to the study areas.
The best estimates of gross, annual long-shore transport during ice-free periods from 1962 through 1973 in the Muskegon area are between 300,000 to 600,000 [yd.sup.3] (USAEDD 1975). This study also estimates net annual drift of less than 5,000 [yd.sup.3] to the south (Figure 3). In general, net drift in the study area is to the south in the ice-free fall-winter-spring period and to the north in the late spring-summer-early fall period (Figure 3). These estimates are highly variable, however, and subject to variations in ice-fast periods and wave climate. Other studies (Hands 1980) generally verify the seasonally bimodal long-shore transport pattern and annual net transport trends along the central Lake Michigan shore. The ratio of net to gross long-shore transport is very small: much sediment is in motion, relatively small amounts of sand move southward annually.
The best estimates of gross, annual long-shore transport during ice-free periods from 1962 through 1973 are between 600,000 to 700,000 [yd.sup.3] (USAEDD 1976) at Frankfort, Michigan, (within 25 mi north of the northern study areas). Net drift is probably less than 15,000 [yd.sup.3] per annum to the north (Figure 4). As at Muskegon there are frequent reversals of long-shore drift with complex and variable seasonal trends. The net to gross long-shore drift at Frankfort is also quite small.
[FIGURE 3 OMITTED]
Climate and hydrologic variations cause significant water level fluctuations in the Great Lakes. Lake level variations since the initiation of the monitoring programs (USACDD, http://www.lre.usace.army.mil/index.cfm?chn_ID=1383 & lake_id=2) are summarized in Figure 5. Since late fall of 1992 and through the study period lake levels in Lake Michigan-Huron were above long-term (100-year) average.
Work in sand-dominated environments in Lake Michigan during the 1970s (Hands 1980) showed that coastal recession resulted from increased lake levels and was associated with long-shore bar migration landward. These responses are predictable based on equilibrium beach profile theories, which characterize near-shore coastal morphologic change under the influence of wind waves and water level changes (Bruun 1962).
Geological Setting and Coastal Substrates
The surficial geology and geomorphology of the eastern shore of Lake Michigan is quite variable although glacial drift and recent dunes in the coastal zone is predominant. Recent studies (Nairn 1992) clearly indicate the importance of offshore substrates in the coastal change response in the Great Lakes. For the purposes of this study two main shore types are recognized: (1) sand dominated and dune backed and (2) thin sand cover over clay-rich till and till bluff dominated.
[FIGURE 4 OMITTED]
The Duck Lake study area is characterized by moderate to high, generally vegetated sand dunes and sand-rich near-shore substrates. These features overlie Quaternary glacial deposits at depths of a few hundred to a few tens of feet. This depth of sand cover over glacial drift is sufficient to produce a shore response that is "sand dominated" and equivalent to an "infinite thickness" of sand. Lacustrine sand and offshore sand bars characterize the recreational beach in the vicinity. A limited sample set indicates that beach sands are medium-grained, well-sorted, and are size graded to fine-grained sand offshore. These characteristics are similar to the findings of Hands (1983) in other sand-dominated shore types on the eastern shore of Lake Michigan.
The Orchard Beach study area is characterized by high (75-100 feet), clayrich till bluffs and a cobble-bearing sandy beach (Figure 6). The till is close to the surface in the near-shore zone and sand cover offshore is less than 10-15 feet (unpublished ground-penetrating radar survey data). This shore type is significant because of its possible susceptibility to "irreversible lake bed down cutting" (Nairn 1992) and the loss of near-shore, clay-rich substrate to wave-induced erosion. Downcutting of fine-grained lake bed substrates may accelerate formation of deeper water offshore and result in increased wave energy to the near shore. Although accelerated bluff recession is well documented in Great Lakes settings where cohesive substrates are exposed at the lake bed or are covered by only an ephemeral sand cover (Morang et al. 2002), a significant cobble lag at the Orchard Beach site may provide resistance to lake bed down cutting.
[FIGURE 5 OMITTED]
The Onekama study area is a combination of the above-mentioned coastal substrate types; the southern portion of the installation site is sand dominated and dune-backed while the northern portion is backed by a high, vegetated, clay-rich till bluff. Dual control sites were chosen to match each of these coastal substrate types.
Noteworthy in the analysis of the Onekema study site, in the transition zone between the sand-dominated (south, see Figure 1) and till-bluff-backed (north) coastal substrate types, is a topographically low area underlain by fine-grained, organic-material-rich sediment of probable wetland origin. This weakly consolidated substrate is periodically exposed in the swash zone and was probably deposited in a swamp or wetland area in a pre-modern lake level setting.
Coastal Engineering Structures
Various small-scale shore protection structures are present in the vicinity of each of the study sites. In general the influence of these structures, typically small steel or rock revetments, were not deemed significant to measurable coastal change. The Duck Lake study area is distinct, however, in that large 1600-2000 foot, engineered revetments dominate the vicinity of both the installation and control sites. These structures are important in the consideration of coastal change.
[FIGURE 6 OMITTED]
Beach Profile Analysis
Beach profile survey data is used to assess coastal change resulting from all possible sources. These data were typically collected twice a year in the late spring and mid/late-summer. Surveys were conducted using a total station: a microprocessor-equipped electronic theodolite and electromagnetic distance measurement (EDM) device in conjunction with walking/wading near-shore and boat-mounted echo sounder offshore depth measurements. Comparison of near-shore (walking) survey data with offshore (boat) data at overlap points was used to calibrate digitized fathometer strip chart data in the Y and Z coordinates. The degree of spatial overlap for sets of survey data was a function of wave and current conditions during data acquisition. Survey data were usually acquired during essentially flat water conditions although this was not always the case. Less than ideal survey conditions, waves in excess of 1-2 feet, result in significant increase in possible survey error.
Temperature corrections, based on lead line measurements or sea sled calibration profiles (Barnes et al. 1995), were applied to all fathometer survey data and assessed amongst all survey data at or beyond probable depth of closure (25 feet to 28 feet). These corrections are significant (and hence a possible source of error) only for profile data at depths in excess of 10 feet to 12 feet (600 feet to 800 feet offshore) in the study area.
Beach profile change analysis was conducted using final, corrected survey coordinate data (and the volume change utility of Golden's Surfer for Windows software). Differences in the volume calculated as a result of change in elevation along the profile between initial surveys and subsequent surveys are reported as volume change (in cubic yards) per linear foot of shoreline (a conventional format for reporting beach profile change).
An assessment of quantitative beach profile change measurement accuracy/precision is important in the evaluation of the significance of coastal change presented in the "Quantitative Results of Coastal Monitoring" section. A detailed discussion of error analysis procedures and results is presented in Barnes (1998) although a brief summary of salient points is presented here (because this reference is not widely available).
1. Total station controlled echo sounder beach profiling techniques are demonstrably more accurate for near-shore sediment volume change measurements in Great Lakes conditions than is reported in the literature for open ocean coastal settings (see Grosskopf and Kraus 1994); however, field technique is critical, especially the ability to reproduce straight, overlapping profile lines during each survey event. This increased accuracy is mainly the result of common flat water (no/minimal wave activity) conditions extant during survey data collection on the Great Lakes.
2. Echo sounder beach profiling must be conducted in conjunction with sea sled or lead line direct depth measurement to determine accurate temperature correction during data reduction. Our measurements routinely provided beach profile change resolution in the range of 6 [yd.sup.3]/foot to 15 [yd.sup.3]/foot of beach at 1500 feet to 2500 feet offshore and substantially less than 5 [yd.sup.3]/foot at 500 feet offshore under ideal conditions (flat water) and dependant on the care with which field techniques are conducted (straight, overlapping profile lines; Barnes et al. 1995).
QUANTITATIVE RESULTS OF COASTAL MONITORING
Quantitative, beach profile volume change is used to assess the combined effects of coastal processes on the geomorphic changes in a reach of shoreline between preinstallation surveys and subsequent postinstallation surveys. Nearby control sites for each of the study areas were chosen to closely match coastal attributes and processes at the installation sites. The comparison of beach profile volume change measured at the installation sites of experimental shore protection with change measured at control sites can, therefore, be assumed to represent coastal change attributable to the experimental structures themselves. The graphical representation and summary statistics for beach profile volume change are presented in a series of charts (Figures 7, 9, and 11). Charts of beach profile volume change are presented for two different offshore profile extents of approximately 500 feet and 1000 feet. These profile distances are probably not beyond the "depth of closure," the theoretical offshore extent of significant current-induced sediment transport, but provide a high degree of precision for change measurements in short beach profiles most subject to changes effected by the experimental structures.
Duck Lake Study Area
Figures 7a and b show graphic representations of beach profile volume change at the Duck Lake Installation study site (Figure 8) determined from profile extents of approximately 500 feet and 1000 feet offshore. Overall average beach profile volume change to approximately 500 feet offshore for all measured profiles during the study period in the vicinity of the structure is about -2 [yd.sup.3]/feet (see Figure 7a "Summer '96 Mean Change"). Overall, average beach profile volume change to approximately 1000 feet offshore for all measured profiles during the study period is -9 [yd.sup.3]/foot (see Figure 7b "Summer '96 Mean Change").
The most noteworthy relationship shown in both sets of profile volume change data is the general loss of profile volume in front of the Duck Lake outlet revetment and minimal net profile volume loss to virtual "no change" in the down drift area of the experimental structure, profiles 5 through 10. This change is associated with pronounced profile volume gains on the north side (up drift) of the revetment. Furthermore, the late spring surveys (light colored bars in 1994 and 1995) indicate beach profile volume gain at each profile line in the spring relative to late summer (dark colored bars) surveys north of the revetment. Net long-shore transport at the study site during the summer is north while net transport between November and April is to the south (Figure 2). Net annual (hence long-term average) long-shore drift is to the south but the net to gross long-shore drift is very small.
Net annual accretion to virtual no change occurs on the south end of the experimental structure in profile 5 through 10 in the short profile extents. More significant profile volume loss is shown in longer profiles in this area (Figure 7b). The data suggest an antithetic relationship compared to the profile volume change described north of the revetment; the experimental structure apparently creates an accretionary fillet to the south during the summer months. Note that early summer profile volumes are generally greater than in late summer in this area, generally opposite the relationship to the north of the revetment.
Memorial Drive control site volume changes for profiles to 500 feet offshore are almost all negative (Figure 7c), while they are variable but generally positive for profiles to 1000 feet offshore. This pattern indicates that the Memorial Drive revetment has had the effect of moving sand offshore. The near-shore volume losses (especially in the dry beach and fore dune areas) at the control site contrast strongly with the volume gains to virtually no change in the analogous locations at the installation site. However a number of factors confound a direct comparison between the two sites. These include (1) the reconstruction of the control site revetment and introduction up drift of beach fill in midstudy, (2) the proximity of the down-drift sector of the control site to the accretionary fillet associated with the Muskegon Harbor structure, and (3) the presence of the Duck Lake outlet channel on the north end of the installation study site.
[FIGURE 7A OMITTED]
[FIGURE 7B OMITTED]
[FIGURE 7C OMITTED]
[FIGURE 7D OMITTED]
[FIGURE 8 OMITTED]
Discussion: Duck Lake Study Area
Profile volume change data, in context with the seasonal long-shore transport relationships, suggest that the Duck Lake outlet revetment and the experimental structure both act as barriers to sediment transported along shore from both north and south. The impact of barriers to long-shore sediment transport resulting from both the revetment and the experimental structure dominate coastal change in this sand-dominated reach. Significant net accretion occurs north and adjacent to the revetment in the winter/early spring while net erosion occurs in the summer/early fall. The net annual change to the north of the revetment is accretionary. The net annual southerly drift is probably interrupted in the vicinity of the Duck Lake outlet revetment and the revetment probably contributes to local, net annual down-drift volume loss to virtual no change in the south adjacent reach. Net northerly sediment transport in the summer months is interrupted by the experimental structure. Small profile volume loss to virtual no change occurs in the southern extent of the installation site. These relationships are consistent with the seasonal changes in long-shore sediment transport directions and typically small, annual net transport to the south.
The presence of the Duck Lake outlet revetment may also contribute to down drift profile volume loss due to the effective protection of onshore sand supply from wave erosion and the introduction of this sand into the long-shore drift system. These interpretations are consistent with interpretations presented in previous studies (Daviset al. 1975).
Orchard Beach Park Study Area
Figures 9a and b show graphic representations of beach profile volume change at the Orchard Beach Park Installation study site determined from profile extents of approximately 500 feet and 1000 feet offshore. Overall average beach profile volume change to approximately 500 feet is about 6 [yd.sup.3]/foot (see Figure 9a "Summer 1997 Mean Change").
Beach profile volume change data in up-drift profiles (south) show volume loss and minimal shoreline recession in the most distant profiles (profiles 1 and 3) beyond the along shore distance conventionally expected to be influenced by small, shore-perpendicular structures. In closer proximity to the structures, increasing profile volume gain to significant gain and shoreline progradation (growth of dry beach) occurs immediately adjacent and within the extent of the experimental structure (profiles 5-17). The location of profile volume gain along the beach profiles is variable, but most profiles show accretion between the back shore and the first permanent bar (about 500 feet offshore) including areas well beyond the lakeward extent of the structures (approximately 75 feet).
Beach profile volume change to approximately 500 feet offshore (Figure 9a) during the study period in net annual down-drift profiles (north of the experimental structure, profiles 19-27) show small profile volume gains and progradation increasing away (to a distance in excess of 1000 feet along shore) of the experimental structure well in excess of our analytical resolution. The most distant, down-drift profile shows negligible profile volume change.
Beach profile volume change measured to approximately 1000 feet is shown in Figure 9b. Overall average beach profile volume change is 20 [yd.sup.3]/foot (see Figure 9b "Summer 1997 Mean Change"). Profile volume change as a result of small-scale structures is not normally expected to extend this distance offshore although clear, quantitative profile volume gain in "long" profiles in the vicinity of the experimental structure is shown.
[FIGURE 9A OMITTED]
[FIGURE 9B OMITTED]
[FIGURE 9C OMITTED]
[FIGURE 9D OMITTED]
Profile volume change at the Orchard Beach Park Control Site during the study period is shown in Figures 9c and d. Overall average beach profile volume change for profile extents to 500 feet offshore at the control site is almost -4 [yd.sup.3]/foot (see "Summer 1997 Mean Change," Figure 9c). Overall average beach profile volume loss for profile extents to 1000 feet offshore at the control site is almost -24 [yd.sup.3]/foot (see "Summer 1997 Mean Change," Figure 9d). Four control site profiles show profile volume loss near or well in excess of analytical resolution; and one profile shows volume gain well in excess of our analytical resolution.
Individual beach profiles in the southern portion of the control site, lines 2 and 4, all show significant shore recession and substantial profile volume loss in the near-shore area to the first permanent bar at 400 feet to 500 feet (see Figure 10). Individual beach profiles in the northern portion of the control site, lines 6, 8, and 10, range from minimal or no shore recession and no profile volume change to substantial profile volume gain in line 10. These gains result from the elevation of both the ephemeral and the first permanent bars within 500 feet of shore.
Discussion: Orchard Beach Park Study Area
Individual beach profile volume change data both up-and down-net annual long-shore drift at the installation site show profile volume gains (net beach accretion) either at or in excess of analytical resolution at both 500 feet and 1000 feet offshore. Net beach profile volume loss occurs at the study site only in the most distant profiles, profile 1 updrift and profile 29 down drift, in excess of 1000 feet on either side of the experimental structure. The profile volume gain is in accordance with local net annual long-shore drift; greater profile volume accretion occurs to the south. The overall profile volume gain down drift of the experimental structure is noteworthy, albeit close to the resolution of our survey techniques.
Net profile volume gain in and around the experimental structure and within the profile extents measured may also be influenced by the migration of the second permanent bar, at approximately 800 feet to 1000 feet along most profiles, landward. Landward migration of all bars probably results from elevated lake levels in 1997 relative to the preinstallation surveys in 1993. This profile response may not reflect a significant influence of the experimental structure at these distances offshore. The comparison of profile volume change between the installation site and control site in the Orchard Beach Park study area, however, clearly indicates significant net accretion of beach grade sediment in and around the experimental structure compared to the control site.
[FIGURE 10 OMITTED]
Onekama (Florman) Study Area
Figures 11a and b show graphic representations of beach profile volume change at the Onekama (Florman) Installation site determined from profile extents of approximately 500 feet and 1000 feet offshore. Overall average beach profile volume change to approximately 500 feet offshore is in excess of 3 cu-yds/foot (see Figure 11a "Summer 1998 Mean Change"). Overall average beach profile volume change to approximately 1000 feet is 2 cu-yds/foot (see Figure 11b "Summer 1998 Mean Change").
Comparison of individual beach profile volume change data in up-drift profiles (south) shows significant profile volume loss or minimal gain (below survey resolution) immediately adjacent to the experimental structure within a distance expected to be influenced by small, shore-perpendicular structures (less than approximately 500 feet). There is substantial volume gain in profiles in excess of 700 feet up-drift of the structure.
Individual, up-drift beach profiles (profiles 0 through 14) show variable, but minimal shoreline recession except profiles 8 and 10. These profiles lost nearly 30 feet of dry beach in 1997. Recovery of a portion of this profile volume in the back shore and near shore occurred between 1997 and 1998. The location of profile volume gain along the up-drift beach profiles is variable, but most profiles show accretion between the first ephemeral trough (about 100 feet offshore) and the first permanent bar (about 400 feet offshore) including areas well beyond the lakeward extent of the structures. Most profile volume loss occurs between the back shore and the first ephemeral trough.
Down-drift profiles (north of the experimental structure, Figure 11a and b) and profiles within the along shore extent of the structure show small to substantial profile volume gains in the near shore area (within 500 feet) mostly in excess of our analytical resolution. Minimal profile volume gain to significant loss was measured in the longer profiles (Figure 11b). The most distant, down-drift profiles show negligible profile volume change to significant loss. All down-drift beach profiles (profiles 18 through 28) show modest to significant shoreline recession with more significant recession in profiles immediately adjacent and down-drift of the experimental structure.
Beach profile volume change data for profile extents to 500 feet and 1000 feet offshore at the Onekema control sites are shown in Figure 11c and d. The control sites for the Florman study area consist of two separate reaches: a "clay substrate" backed by a high till bluff and a "sandy substrate" backed by dunes. This control site configuration was necessary because the installation site consists of both of these shore types, a "sandy substrate" to the south and a "clay substrate" to the north. Overall average beach profile volume loss at the north ("clay substrate") control site ranges from -5 [yd.sup.3]/foot in the near-shore profiles (Figure 11c, "Summer '98 Mean Change") to -3 [yd.sup.3]/foot in the longer profile analysis. The greater average profile volume loss recorded in the near shore (500 feet) profiles indicates that most profile volume change has resulted from sediment transport offshore during the study period. This profile volume change was most probably influenced by lake level rise (see Figure 4) and the predictable landward migration of beach profiles.
Overall, average beach profile volume loss at the south (sand dominated) control site is substantial, -15 [yd.sup.3]/foot recorded in the short profile analysis (Figure 11c) and -7 [yd.sup.3]/foot in the long profile analysis (Figure 11d). Individual beach profiles in the south control site show significant shore recession of between 20 feet to more than 75 feet, including pronounced scarping of the fore dune area measured in the 1997 survey. (Note: We were very fortunate to visit the site early in the spring prior to survey work in 1997 and "rescue" the temporary survey bench mark which was exposed at the edge of a rapidly receding, 6 foot high fore dune scarp, tens of feet from the previous summer's shoreline.)
[FIGURE 11A OMITTED]
[FIGURE 11B OMITTED]
[FIGURE 11C OMITTED]
[FIGURE 11D OMITTED]
Profile volume loss in the near shore area (to 500 feet, see above) in excess of volume loss measured in longer profiles (to 1000 feet) indicates that sediment from the foreshore/backshore area moved offshore along with the landward translation of beach profiles during a period of rapid lake level rise (see Figure 4). Elevation of bar crests in both the ephemeral and the first permanent bars within 400 feet of shore are probably associated with lake level rise from the preinstallation survey in 1995 to the most recent survey of spring 1998.
Individual beach profiles in the north control site all show modest shore recession, from 5 feet to nearly 10 feet of shore retreat, and profile volume loss in the near-shore area to the first permanent bar at 300 feet to 400 feet. These profile volume changes occur in conjunction with the elevation of both the ephemeral and the first permanent bars within 400 feet of shore, probably associated with lake level rise from the pre-installation survey in 1995 to the of Fall 1998 survey. Rapid lake level drop from 1997 to 1998 survey dates has resulted in modest shoreward transport of near-shore sediment although this coastal response may have lagged the rate of lake level change and may not have reached equilibrium at the time of our survey.
Discussion: Onekama (Florman) Installation Site
The profile volume loss during the study period in the immediately adjacent updrift area relative to the experimental structure is noteworthy. This observed coastal change response relative to most small-scale, shore-perpendicular structures is almost certainly influenced by the presence of a distinct coastal substrate in the area of profile lines 6-12. Observations in this area during the monitoring period indicate that a highly friable substrate is periodically exposed in the swash zone. This organic-rich material is a classic "cohesive substrate" in that it is easily scoured and consists mostly of fine-grained and organic material. No consideration was given to the unusual substrate in this portion of the installation site because this relationship was not known prior to configuration of the control sites. When the local beach sand cover is thin or not present (common when the long-shore drift is southward on the south flank of the experimental structure) rapid scour of this substrate probably occurs. The profile volume loss and shore recession in profile lines 6-12 is an expression of the effects of high degree of erodibility of this local substrate and the presence of a shore-perpendicular, engineered structure that periodically produces a significant erosional fillet and exposure of this erodible, cohesive substrate.
Performance of the Undercurrent Stabilizer System[TM]
The interpretation of coastal dynamics associated with the experimental Undercurrent Stabilizer System[TM] shore protection structure calls upon a conventional understanding of coastal processes, including the following local coastal processes and conditions:
1. Lake level rise in 1996-97 was near record highs, well in excess of 581 feet (IGLD); more than 1.5 feet above long term average and above lake level at the time of the pre-installation surveys. These lake level changes have resulted in a general landward and upward translation of equilibrium beach profiles including the ephemeral and permanent offshore bars and a general increase in the relief of bars in the study areas. The result is significant recession in the dry beach in most areas through simple inundation as well as offshore sediment transport in accordance with the Bruun Rule (Bruun 1962).
2. Low net/gross long-shore drift occurs at the experimental structure installation sites. At each of the study sites much sediment is in motion but there is a small net drift. The gross long-shore drift along the eastern Lake Michigan shore is estimated at several hundred thousand cubic yards annually, while net drift is probably less than 15,000 cubic yards per annum in the northern sites and less than 5,000 cubic yards near Muskegon. There are frequent reversals of long-shore drift along this reach of the Lake Michigan shore annually with complex and variable seasonal trends. These local long-shore drift relationships tend to mute the long-term, down-drift erosion hazards associated with most shore-perpendicular structures.
3. Conventional wisdom regarding right-angle shore protection is that "[right angle shore protection structures] do not attract to an area any sand which would not have otherwise passed" (USACE, CERC 1984).
4. There is no evidence for volumetrically significant sources of beach grade sediment along the Lake Michigan shore in the study areas other than that moving in the littoral drift and derived, by erosion, from the upland bluffs and dunes. If a long-term, net offshore transport of coastal sediment occurs in the study area due to the wave climate characteristic of the Great Lakes (USAEDD 1975, 1976), then the isolation and protection of back-shore coastal sediments through the use of any shore protection technology will ultimately result in an increasing loss of near-shore sand, systemwide.
Coastal change response to the experimental Undercurrent Stabilizer System[TM] shore protection structure can be evaluated in three components: (1) protection provided to the back-shore area immediately behind the experimental structure: a revetment effect; (2) alternating accretion/erosion observed immediately adjacent to the experimental structure within approximately 150 feet offshore during reversals of long-shore drift: a conventional groin effect; and (3) down-drift and offshore accretion beyond the offshore extent of the experimental structure (between 150 feet and 500 feet offshore): an unconventional groin effect.
The shore parallel portion of the experimental structure (the Undercurrent Stabilizer System[TM] "anchor") has performed much like a small-scale revetment effectively protecting the back-shore ("dry" beach) landward and within the extent of the structure at the installation sites. During periods of low water, vegetated fore dunes have developed and were stabilized behind the structures. Accretion on the "dry" beach is mostly the result of wind-transported sand trapped by dune grasses. Decrease in near-shore (within 500 feet) beach profile volume occurs mainly in the area of the first ephemeral bar. This response may result from a revetment effect that prevents wave-induced cross-shore exchange of sand from the protected back-shore area and locally enhances wave energy due to wave reflection.
Conventional Groin Effect
Both quantitative and qualitative observations, in the course of the monitoring program, indicate substantial variation in beach profile volumes in the net annual, down-drift-adjacent reaches in the study areas less than 150 feet offshore. In the down-drift end of the installation sites both accretionary fillets and erosional fillets (Figure 12) are observed. This coastal response is consistent with a shore-perpendicular obstruction to long-shore drift and seasonally reversing long-shore transport patterns; a conventional groin effect. The up-drift propagation of a conventional groin effect, accretionary fillet regularly formed much further up drift than predicted by conventional coastal engineering literature of 2 to 3 times the off-shore extent of the structure. This conventional groin effect has magnified the negative impact on shore reaches that have thin sand cover over cohesive substrates due to irreversible lake bed scour. Monitoring results at the Onekema study site are clear indication of this concern.
Unconventional Groin Effect
A poorly known beach profile change apparently influenced by the experimental structure occurs within the near-shore area alongshore and adjacent to the experimental structure, both within the offshore extent of the structure (less than 150 feet) and beyond the offshore extent of the structure (greater than 150 feet but less than 500 feet) at the installation sites. Contrary to most conventional (and mostly anecdotal) observations of coastal response to small-scale shore-perpendicular structures, net accretion occurs periodically down drift of the annual average long-shore drift direction at all sites rather than simply up-drift and within the extent of the structures.
Coastal dynamics responsible for this unconventional groin effect are conjectural but are based on observations from all of our installation sites and conventional coastal processes wisdom. The up-drift, conventional groin effect accretionary fillet (formed during fluctuating long-shore drift periods) apparently creates a disequilibrium "feeder beach" to offshore bars as a result of subsequent periods of reversed long-shore transport, steep, high-energy waves, and offshore-directed currents. This is analogous to artificial beach nourishment in which the adjacent offshore bar is "feed" via cross-shore-directed currents during storm wave periods. In this case the conventional groin effect accretionary fillet adjacent to the experimental structure apparently feeds the adjacent offshore ephemeral bar when long-shore drift reverses.
During periods of reversing long-shore transport the up-drift accretionary fillet initially builds and an erosional fillet forms adjacent to the structure down drift. With subsequent drift direction reversal the accretionary fillet moves down-drift and offshore (given sufficiently energetic wave action and offshore directed currents). The elevated ephemeral bar formed due to this unconventional groin effect in the net annual down-drift direction at each site presumably provides enhanced protection from incoming waves, especially beyond the extent of conventional groin effect erosional fillets. As this bar persists through time, less pronounced conventional groin effect erosional fillets occur in the immediately adjacent down-drift reaches. Beyond the typical conventional groin effect scour shadow profile volume gain is further enhanced. The sand accreted adjacent to the experimental structure is apparently captured from the normal long-shore drift system. A net near-shore volume loss directly attributable to the volume gain adjacent to the experimental structure cannot be proven at the study sites, however.
Coastal Change Response to Undercurrent Stabilizer System[TM] at the Project Study Sites
Coastal change effected by the experimental structure in the three study sites is complex and varied but there is minimal negative to significant positive net accretion of near-shore sediment in and around the experimental structures during the study periods relative to control sites. In the Duck Lake area, near-shore sediment volume loss within the extent of the experimental structure and up-drift of it may be explained by an approximate up drift reach of 1600 feet of shore protected by a rubble mound revetment. Profile volume gain along profiles down-drift of the structures is explained due to the complex interplay between a conventional groin effect obstruction of reversing long-shore drift and an unconventional groin effect resulting in the enhanced offshore transport (to the first ephemeral bar) of nearshore sediment and the protection afforded to the back-shore area by that elevated bar. An unexpected updrift profile volume loss was documented at the Onekea study site as a result of a local cohesive coastal substrate and the effect of scour of this substrate during periods of long-shore drift reversal relative to net annual drift.
The stabilization of the back-shore (dry-beach), growth and proliferation of beach grass, and preservation of down-drift sediment volumes is given due consideration in this evaluation. An important consideration in this analysis is the assessment of groins in a study by Kraus and others (1994): the main purpose of a groin (or other related shore-perpendicular structures) installation is "... maintaining a beach...." The Undercurrent Stabilizer System[TM] permeability to long-shore transport, the high ratio of gross long-shore transport to net transport, and the relatively low ratio of groin compartment volume compared to gross sediment transport volume that characterizes local conditions at the study site may explain the atypical coastal response to the experimental structure compared to other, small-scale, right-angle shore protection structures.
There is no clear and predictable negative impact to adjacent shore reaches within the study areas resulting from the net accretion of near-shore (to approximately 500 feet offshore) sediment around the experimental structure during the monitoring period. The interpretation of coastal dynamics resulting from installation of the experimental structure presented here, however, calls upon a conventional understanding of coastal processes to explain this accretion and requires a net deficit of coastal sediment in the adjacent portions of the coastal cell comprising the study site. Conventional wisdom regarding rightangle shore protection must be applied here: "[right angle shore protection structures] do not attract to an area any sand which would not have otherwise passed" (USACE, CERC 1984).
In the absence of any other proven source for beach-grade sand, the isolation of the back-shore area of a native beach from the on-shore/off-shore interchange of sand and the accretion of littoral drift in and around a shore protection structure ultimately depletes the adjacent shore of beach sand. There is no evidence for volumetrically significant sources of beach-grade sediment along the Lake Michigan shore in the study area other than that moving in the littoral drift and derived, by erosion, from the upland bluffs and dunes. If a long-term, net-offshore transport of coastal sediment occurs in the study area due to the wave climate characteristic of the Great Lakes (USAEDD 1975, 1976), then the isolation and protection of back-shore coastal sediments through the use of any shore protection technology will ultimately result in increasing loss of near-shore sand systemwide.
TABLE 1. Time Line of Coastal Change Monitoring. Location Installation Date Initial Surveys Final Surveys Duck Lake June 1991 June 1991*, Sept 1993 Aug 1996 Orchard Beach Park November 1993 Nov 1993 Aug 1997 Onekama October 1995 Aug 1995 Aug 1998 *Initial survey data unreliable
Installation of the experimental structures and this analysis was supported by State of Michigan Legislative initiative funds granted in the 1990s to Erosion Control System Inc. (Whitehall, MI <http://www.erosion.com/>) and Western Michigan University for the monitoring component. The study was initiated at the Duck Lake site in 1991 and includes the results of surveys completed in the summer of 1998 at the Florman site.
The authors thank several generations of students from the Department of Geosciences at Western Michigan University for their assistance with field-work during this study. Long days, cold water, complex tasks, equipment failures, bad field food, and endless hours of data reduction were accepted with tremendous good humor and tireless tenacity. We had a lot of fun as well.
BARNES, D. A., M. S. KOVACICH, S. LIMEZS, AND W. R. LATON. 1995. Error analysis in Great Lakes beach profile survey techniques: Proceedings of the 1995 Canadian Coastal Conference, vol. 2, 17-26. Dartmouth, Nova Scotia, Canada.
BARNES, D. A. 1998. Duck Lake outlet demonstration project: "Undercurrent Stabilizer Systems[TM]" shore stabilization/accretion program, final draft report 1991-1996 surveys. Report issued to the Shorelands Management Division of the Michigan DEQ. Unpublished; available from author.
BRUUN, P. 1962. Sea level rise as a cause of shore erosion. Journ. Waterways and Harbors, ASCE 88:117-30.
DAVIDSON, A. T., M. BUCKLEY, AND R. W. KRIMM. 1996. Position of the Federal Emergency Management Agency on the report "Beach Nourishment and Protection." Shore and Beach 64:5-6.
DAVIS R. A., W. G. FINGLETON, AND P. C. PRITCHETT. 1975. Beach profile change: East coast of Lake Michigan, 1970-72. Misc. Pap no. 10-75, U.S. Army Corp of Engineers, Coastal Engineering Research Center.
GROSSKOPF, W. G., AND N. C. KRAUS. 1994. Guidelines for surveying beach nourishment projects. Shore and Beach 62(2): 9-16.
HANDS, E. B. 1980. Prediction of shore retreat and nearshore profile adjustment to rising water levels on the Great Lakes. U.S. Army Coastal Eng. Res. Center Tech. Pap., TP 80-7.
_____. 1983. The Great Lakes as a test model for profile responses to sea level changes. In CRC Handbook of Coastal Processes and Erosion, ed. P. D. Komar. Boca Raton, FL: CRC Press.
HUBERTZ, J. M., D. B. DRIVER, AND R. D. REINHARD. 1991. Hindcast wave information for the Great Lakes: Lake Michigan. Wave Information Studies of U.S. Coastlines, WIS Rpt. 24, CERC.
KRAUS, N. C., H. HANSON, AND S. BLOMGREN. 1994. Modern functional design of groins. Proceedings, 24th Coastal Engineering Conference, ASCE, 1327-42.
MEADOWS, L., G. MEADOWS, B. HAUS, J. PAZZDALSKI, AND A. KOENGETER. 1991. Coastal monitoring program and shoreline evolution model year three: Report to the State of Michigan Department of Natural Resources. The University of Michigan Ocean Engineering Laboratory, Report # OEL.
MEADOWS, G. A., L. A. MEADOWS, W. L. WOOD, J. M. HUBERTZ, AND M. PERLIN. 1997. The relationship between Great Lakes water levels, wave energies and shoreline damage. Bulletin of the American Meteorological Society, Vol. 78, No. 4.
MORANG, A., AND L. PARSON. 2002. Coastal Morphodynamics. Chapter IV-3 in Coastal Engineering Manual, Part IV, Engineer Manual 1110-2-1100, edited by A. Morang, 4-1-4-55. Washington, DC: U.S. Army Corps of Engineers,
NAIRN, R. B. 1992. Erosion processes evaluation paper: Final report. Int. Joint Comm. Great Lakes-St. Lawrence River Levels Reference Study Board.
U.S. ARMY ENGINEER DISTRICT DETROIT, CORPS OF ENGINEERS. 1975. Section 111 detailed project report on shore damage at Muskegon Harbor, Michigan.
_____. 1976. Section 111 detailed project report on shore damage at Frankfort Harbor, Michigan.
U.S. ARMY CORPS OF ENGINEERS, CERC. 1984. Shore Protection Manual. 4th ed. Vicksburg MS: Coastal Engineering Research Center, Deapartment of the Army, Waterways Experiment Station.
DAVID A. BARNES
Western Michigan University
MICHAEL S. KOVACICH
GEOTRANS, Ann Arbor
Consultant, St. Joseph
|Printer friendly Cite/link Email Feedback|
|Author:||Barnes, David A.; Kovacich, Michael S.; Limesz, Santis|
|Date:||Jan 1, 2004|
|Previous Article:||Lake level variability in Silver Lake, Michigan: a response to fluctuations in lake levels of Lake Michigan.|
|Next Article:||Contemporary geomorphic processes and change on Lake Michigan coastal dunes: an example from Hoffmaster State Park, Michigan.|