The great lakes groin performance experiment, Phase II: Effects of a groin field

The Great Lakes Groin Performance Experiment,

Phase II: Effects of a Groin Field

BY

Guy A. Meadows, LoreIle Meadows, Donald D. Carpenter,

Hans Van Sumeren and Brandy Kuebel

Ocean Engineering Lab University of Michigan

William L. Wood* and Brian Caufield

Great Lakes Coastal Research Laboratory

Purdue University

IN

THEORY, GROINS ARE CONSTRUCTED in an attempt

to stabilize a length of beach where the primary erosion mechanism is due to a net longshore transport (National Research Council, 1990). In practice, groins are intended to increase beach width, extend the performance life of beach nourishment projects, and prevent the loss of sand from

beach-SP OP

Figure 1. Sits location and experimental set-up. Total length of iniensive study area Is 700 m.

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es to longshore sinks such as inlets and navigation channels.

Over time, a great deal of controversy has arisen, both in

litera-ture and in practice, over the use of groins as shoreline protec-tion/stabilization suuctures. This controversy stems from the fact that many groin deployments have resulted in significant

negative impacts on the downdrift shoreline. This has led to the

use of stringent regulations in the coastal management poli-cy of many states restricting

the CORStrUCti011 of groins.

In an effort to better

understand and quantify the

short-term response of the

beach and nearshore region. the Michigan Department of

Environmental Quality

(MDEQ) initiated this project to attempt to understand the performance characteristics of

groin design. Kraus et

al.

(1994) argues that the

response of the coastal zone to

groins is expressed by a

func-tional relation involving a

large number of parameters

for which no guidance on

functional design is available.

This leads to the potential

rnis-design and loss of valu-able shore protection struc-tures which could function effectively and economically under certain conditions. Ills within this context that the

Great Lakes Groin

Performance Experiment

(GLGPE) was conceived and carried out. The two primary goals of the GLGPE were to

quantitatively examine the

impacts of groins on the adja-cent shoreline and nearshore environment, and to evaluate two functional groin design paruneters: length and height.

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Figure 2.Deep water wave time series (from NDBC45007). Hollowcircles represent

waves from a northern component. Filled circles represent waves from a southern The GLGPE was a two-part field experiment supported by

the U.S. National Oceanic and Atmospheric Administration (NOAA) Coastal Zone Management Program through the MDEQ. It was a collaborative effort involving the University of

Michigan's Ocean Engineering Laboratory and Purdue

University's Great Lakes Coastal Research Laboratory. The first phase took place in the spring of 1996 and involved the evalua-tion of four individual impermeable groins of varying height and length (Meadows, et al., 1997; Caufield, 1997). The second phase took place in the spring of 1997 and was used to further evaluate individual groin behavior as well as that of a groin field. It is the purpose of this paper to present the findings of the second phase of this experiment.

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Figure 3.Pre-construction (April 23) and post-haul-out (June5)shoreline locations

---19EXPERIMENTAL

SITE

The site selected for the GLGPE was Ludington State Park. approxi-mately 17 kilometers north of the city of Ludington. Michigan on the

eastern shore of Lake Michigan

(Figure 1). The study site, approxi-mately 1 km in length, resides on a 20 km stretch of natural beach free of man-made barriers to sediment transport. The nearshore

bathyme-try consists of a barred profile

which is relatively uniform in the longshore direction. The site has direct maximum fetch distances of about 280 km to the south-south-west and 100 km to the northsouth-south-west. It is, however, potentially affected by waves which refract from the northern basin with a 200 km fetch. The maximum significant waves incident at this site are on the order of 7 meters. While the site is tide-less, it is subject to annual hydro-logic cycles that cause water level variations as large as 0.5 meters. In addition, storm setup can cause short-term water level changes on the order of 0.5 meters. The net longshore transport of the region is from south to north with short peri-odic reversals due to less frequent storms which produce waves that propagate from a northern quadrant.

The second phase of the GLGPE was designed to evaluate the effect of a groin field. In order to verify and enhance the

results of phase I, a single long groin was also deployed (Figure

1). The groin field consisted of three 15 meter high profile groins spaced at 22.5 meters (45 meter field width). The groins were constructed of steel I-beams and were "instantaneously" installed during a 6 hour calm period. Similarly, at the conclu-sion of the experiment, the entire installation was "instanta-neously" removed. The post-experiment recovery of the beach was also monitored.

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FIELD DATA COLLECTION AND ENVIRONMENT CONDITIONS

Phase 11 of the GLGPE was approximately 6 weeks in duration with field data acquisition consisting of periodic nearshore and offshore bathymetric surveys, daily offset mea-surements, daily still photography, and periodic video photog-raphy. The nearshore bathymetric surveys were performed approximately twice weekly and extended from the back beach through wading depth. The surveys were performed along reg-ularly spaced longshore intervals using a total station and prism rod. Survey range spacing varied along the site with closer spac-ing near the structures as shown in Figure 1. Offshore bathy-metric surveys were performed four times during the experi-ment and consisted of precision hydrographic surveys at four longshore locations extending to 10 meters depth. Environmental data were obtained from on-site observations, the National Data Buoy Center (NDBC) buoy 45007 located approximately 65 kin offshore in the southern basin of Lake Michigan and the National Ocean Service (NOS) water level station 9087023 located 17 km to the south.

Figure 2 summarizes the wave conditions from the

NDBC buoy showing the wave heights incient from the

northern and southern quadrants. In general, the majority of the wave energy originated from the north with the largest deep water wave heights reaching about 4 meters. During the first two weeks of the experiment, the site experienced storm waves of approximately equal magnitude from both the north and south quadrants. This storm period was followed by con-structive waves primarily from the north. Through this exper-iment there was a water level increase of about 0.12 meters, characteristic of this month's normal annual mean water level rise in the Great Lakes basin.

A conscious choice was made to conduct this experiment on a natural open coast beach, since these structures are placed for performance in this environment This choice resulted in the introduction of a wide range of environmental conditions into the data set. Part of this is the natural variability in incident wind and wave conditions at the site. Also, the occurrence of small scale morphological features is introduced into the data set. In particular, this study site was host to an ephemeral bar which welded to shore early in the experimental process. This accretionary feature welded to shore north of the individual long groin shortly after deployment and migrated to the south

forming a large accretion fillet. Eventually this fillet overtopped the structure and proceeded to fill the erosion scour on the south side of the structure.

DATA ANALYSIS

The data collected during the experiment were analyzed to determine the impact of the structures on the nearshore envi-ronment. The response of the entire nearshore bathymetry was examined by tracking the movement of individual isobaths and

through volumetric calculations.

As a baseline for comparison, Figure 3 shows the pre-con-struction and post-haul-out shoreline positions as a function of longshore distance. The general retreat of the shoreline south of the structures exceeds that of simple encroachment due to the increase in water levels during the experiment. The large fillet surrounding the individual groin is the subaerial expression of

the welded ephemeral bar mentioned previously.

The shoreline evolution at a site is a complex response of the bathymetric contours to the incident environmental forces. In an attempt to separate these effects, Work and Dean (1990) developed an even/odd analysis technique. In their work, the shoreline response is separated into even (symmetric) and odd

Figure 4.Evenand oddparameter time series forboththe groin fieldandthe individual longgroin.

16 SHORE & BEACH

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Figure 5. Overall topographic change, April 23 through June5, 1997.

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Figure 6. Net(a) andgross (b) cross-shore contour movement, April 23 through June 5, 1997.

(anti-symmetric) changes centered about the structure axis. The even component of shoreline change reflects the influences of phenomena which would be expected to impact either side of the structure in a similar fashion, such as water level changes. The odd component reflects phenomena expected to impact the region on either side of the structure inversely, such as the

inter-APRIL, 1998

ruption of longshore sediment transport.

For the

present

study, a simple shoreline change parameter was used: that of the waterline position on each side of the structure.

Figure 4a and b shows the

even and odd analyses for the groin field. The figures show the time history of the shore-line change parameter imme-diately adjacent to the struc-tures as well as 30,60 and 90 meters alongshore. Ninety meters from the groin field,

the even shoreline change

parameter exhibits a relatively linear recession of the shore-line. This is directly attribut-able to the steady water level rise of 0.12 meters. The most significant shoreline recession

occurs 30 meters from the

groin field where there is an additional 2 meters of shore-line displacement beyond that

due to encroachment.

Immediately adjacent to the groin field, the mean displace-ment of the shoreline recedes under the earlier high energy wave conditions and subse-quently recovers to match the background encroachment value. The high energy events induced comparable overall recession in the region extending

from the groin field to at least 60 meters up and down drift, with

the shoreline responding most rapidly at the 30 meter offset. During the quiescent period which followed, the shoreline 'recovered at all but the 30 meter offset.

17

125 250 375 500

Long-shore Distance (m)

Pr-

The odd parameter analysis shows the most dramatic shoreline changes also occurring within 30 meters of the struc-ture as offsets change with incident wave direction. The

vari-ability of the odd parameter decreases with distance from tho structure, delineating the rapid response of the shoreline imme-diately adjacent to the structure. There is less variability at 60 meters and there is little change 90 meters from the field.

Figures 4c and d provide the even and odd parameter time history for the single long groin. As described previously, this site was subject to the welding of an ephemeral bar north of the structure. This feature shows up in the analysis as an abnormal growth in the even and odd parameters on May 3. Through the even analysis, it is possible to see that the welding bar influ-ences the 0 and 30 meter sites most significantly. Downdrift erosion due to a storm from the north serves to balance the bar weld in the even parameter at the 60 and 90 meter sites at this

time. The odd parameter values for the single groin are

extreme-ly large as a result of the abnormalextreme-ly large accretion of the bar weld. However, as can be viewed in Figure 3 in the absence of the bar, an offset to the north would still exist due to the large amount of downdrift erosion resulting from the structure.

The overall topographic change from April 23rd (one week pre-construction) to June 5th (one day post haul-out) is

present-ed in Figure 5 as a contour plot of elevation change. The dark

regions in this diagram, representing areas of elevation loss, are

concentrated around the groin field and south of the structures. A deleterious interaction between coastal processes and groins is evident in the scour in the field which totals about 600 cubic meters. This material was transport offshore and most likely contributed to the accretionary region immediately offshore of the structures. A scour of this magnitude was not evident in the region of the single groin. The scours south of the groin field

and long groin were approximately 475 cubic meters in volume.

The movement of individual bathymetric contours gives a more detailed evaluation of the aerial impact of the groin field and individual groin. Figures 6a and b show the net and gross displacement of three isobaths for the duration of the experi-ment. The most striking feature of these diagrams is the dis-placement of the 0.6 meter contour in the vicinity of the groin field. This region of large contour movement abruptly ends 60 meters (four groin lengths) north and 90 meters (six groin lengths) south of the groin field. There is another relatively large displacement of this isobath south of the individual long groin (about 120 meters or four groin lengths). Outside of these regions, diagram b shows that the three contours behave in a similar fashion moving towards shore on average about 5 meters through the course of the experiment. This "back-ground" movement represents the increase in water level. Immediately offshore of the structures, however, the deeper contours remain offshore despite the water level rise, indica-tive of the offshore displacement of sediment due to the inter-action of the structures with the longshore current. Figure 6b shows that the -1.2 meter contour did not vary to the same extent as the other isobaths. Also, as shown in Figure 5, the longshore bar outside of this contour was not significantly affected by the presence of the groins. This suggests that the pronounced cross-shore effect of the groins was approximate-ly one to two groin lengths.

These results indicate that closely spaced multiple

struc-tures, such as the groin field, increase both longshore and

cross-shore effects. Therefore, the longer the length of the cross-shoreline impacted by the structure, the father afield the impacts.

18

CONCLUSIONS

The results of Phase I of the GLGPE indicated that high profile groins have a more marked impact on the shoreline and respond more dramatically to changing environmental condi-tions. It was also determined that short groins respond quicker and more dramatically than their longer counterparts. The results of Phase 11 of the GLGPE confirm these observations. The short, high profile groin field responded more rapidly to changing wave conditions. The singular long low profile groin

exhibited a slow response to changing incident waves and had a similar range of influence.

Overall, the groin field was erosive with shoreline reces-sion and the development of large scour holes at the groin tips. Both the groin field and long groin exhibited wave sheltering effects on the immediately adjacent downdrift shoreline. The groin field impacted the nearshore bathymetry approximately 6 structure lengths dovaidrift, while the effects of the long groin were evident 4 structure lengths away. The significant offshore effects of the groin field were limited to 1 structure length past the groin tip, while the single groin influenced the bathymetry only 0.5 structure lengths beyond its offshore extent.

In summary, the results of Phase II of the GLGPE support the findings of Phase I regarding the influence of single groin structures on the nearshore bathymetry and shoreline. Phase It

also indicates that the region of influence of a groin field is

larg-er in extent than that of a comparable single groin in the long-shore direction. The cross-long-shore influence of the groin field exceeded that of the individual groin, however, it remains rela-tively small (within the inner bar) as compared with the long-shore effect. In general, the groin deployments resulted in an overall loss of shoreland and a steepening of the nearshore pro-file, despite an apparent building of material within the shadow zone of a few of the individual groin structures.

ACKNOWLEDGMENTS

We would like to thank the Michigan Department of Environmental Quality who funded this experiment through the

U.S. National Oceanic and Atmospheric Administration Coastal

Zone Management Program. We would also like to thank the Detroit District of the Army Corps of Engineers and the staff of Ludington State Park for their assistance and cooperation.

REFERENCES

I. National Research Council (1990) Managing Coastal Erosionn; National Academy Press, Washington, DC.,182 pp.

Kraus, N.C., Hanson, H., Blomgren. S.H. (1994) Modern Functional Design of Groin Systems: Proc. 24th Int. Con!. on Coastal Engineering, p.I327-1342.

Meadows, G.A., Wood, W.L, Canfield. B.. Meadows, L., Bennett.

T., Pato, M., VanSumeren. H., Carpenter, D.. Mach, R. (1997) A

Field Investigation of Functional Design Parameter Influence on Groin Performance: The Great Lakes Groin Performance

Experiment; Proceedings of the 1997 Coastal Dynamics Conference.

in print.

Canfield, B. (1997) A Field Investigation of Functional Design Parameter Influence on Groin Performance; M.S. Thesis. Purdue University, West Lafayette, IN, 69 pp.

Work. PA. and Dean. R.G. (1990a) Shoreline Changes Adjacent to

Florida's East Coast Tidal Inlets; Coastal and Oceanographic Engineering Department Publication. Univ. of Florida. Gainesville. FL.

Work. PA. and Dean. R.G. (1990b) Even/Odd Analysis of Shoreline Changes Adjacent to Florida's Tidal Inlets: Proc. 22nd Int. Con.

Coastal Engineering. p. 2522-2535.