Use of long groines as artificial headlands

CHAPTER 32

USE OF LONG GROINS AS AR TIFICIAL HEADLANDS James W. DWlham

Coastal Engineer, Moffatt & Nichol, Engineer s Long Beach, California

ABSTRACT

The construction of relatively short groins to trap fillets of sand along coastal reaches characterized by prevailing littoral transport has become a well-known practice in coastal engineering. Less common, but often equally important, is the use of long groins to form artificial headlands which trap sand more or less permanently in artificial pocket beaches. The

successful use of this long-groin technique at three Southern California beaches is described, the need for more research as to the effectiveness of such structures is suggested, and other possible uses of long groins are discussed.

INTRODUCTION

The primary purpose of a groin is to trap at least a certain arnoWlt of littoral drift along a coast. Customarily, groins are installed in series along segments of coast where littoral transport is predominantly in one direction in order to stabilize the position of the shore line. The purpose may be either to hold the existing shore line against progressive recession or to build up a sand beach where none existed before. Good practice in such cases requires that the groins not be too long, so that the littoral drift may pass aroWld the ends and thus continue downcoast to prevent erosion of the downdrift beaches. Sometimes an artificial fill is deposited in a groin field immediately following groin construction in order to prevent temporary denial of nourishment to the downdrift beach while the groin field is filling. Many groins of this type are kept low in height, generally following the profile of the updrift beach so that the excess sand is swept over the top of the

structure rather than aroWld its seaward end. Occasionally, the groins are made partially permeable so that the excess is carried through the structure. Occasionally, however, groins have been employed in an effort to trap all the littoral drift rather than just a small amount. This calls for a completely new set of ground rules. Such groins must be constructed only where there is no predominant direction drift or where it is desired to stop littoral transport altogether and thus prevent loss of sand from a given segment of coast. Such groins must be high enough to prevent overtopping, long enough to prevent

~OO,OCl 0 + c..V

10~'

LONG GROINS

757

Much of the natural littoral nourishment of the area was denied both

by flood-control reservoirs in the tributary streams and by littoral berriers

constructed upcoa:.;t. This resulted in erosion of the entire beach. When a

long groin was built at the downcoast end in 1936 to halt the reces sion and stabilize the entrance to Anaheim Bay, the adjacent beach to the northwest

began to widen. However, with continued denial of nourishment at the north

-west end, the shore line there receded to the bluffs. Construction of the Long Beach offshore breakwater to a point two miles southwest of the upcoast end of the beach by 1949 changed the wave climate to the extent that the accretion at the southeast end began shifting upcoast. Within three years the southeast accretion was gone and the northwest end of the beach was over 1000 feet wide.

Meanwhile, the Anaheim Bay groin had been extended over 2000 feet southward in 1945 as a breakwater protecting the entrance to what is now the

Seal Beach U. S. Naval Weapons Station. While this structure assured

complete retention of any sand in the City's littoral zone, it further aggravated

the erosion at the southeast end by reflecting waves in an upcoast direction. Continued erosion in that area threatened destruction of many beach homes and apartments, while the unwanted accretion at the northwest end of the beach continued to grow.

In a survey of this situation reported in 1954, the Corps of Engineers 1 recommended construction of a barrier groin in the center of the Seal Beach area to compartmentize the beach into two separate littoral segments. A concrete sheetpile groin 700 feet long was built in accordance with this recommendation in 1959, and the eroded southeast segment was artifically nourished with a deposition of about 200,000 cubic yards of material dredged from the Navy harbor. This material, supplemented by an unmeasured quantity of sand hauled down from the northwest accretion by the City before the work was done, has effectively stabilized the beach. Although the Corps estimated that the southeast fill would last only ten years because of expected

attrition, the shoreline has not receded appreciably in the seven years that

have elapsed since the groin was constructed and the fill completed. The crest of the beach berm has retained approximately a 200 foot offset at the groin throughout this period, varying from time to time as the beach fills

o

~

I

I

LONG GROINS 759

normal to the average direction of wave approach. 'This caused erosion at the south end of the beach and unwanted accretion at the north end opposite the river mouth. Obviously, any artificial fill placed in the eroded area would soon be distributed uniformly along the beach all the way northward to the Middle Jetty.

In 1955 the Corps of Engineers2 recommended construction of a

stone groin 530 feet long off the end of Cape May Ave. in Ocean Beach and placement of an artificial fill of 250,000 cubic yards along the beach from the groin southward to the headland bluff. The City of San Diego built this groin in 1959, and the Corps placed the fill concurrently with dredging operations in the Mission Bay entrance shortly thereafter. This fill has remained stable since that time, with a considerable offset in the position of the beach berm at the groin. Because of this success, plans are now under

way to build a new groin -contained beach on the rocky bottom opposite the

bluff farther to the south.

CABRILLO BEACH GROIN

A somewhat different situation existed at Cabrillo Beach. There, a breakwater extending southeastward from the San Pedro headland, which was constructed by the Corps of Engineers at the turn of the century to protect the entrance to Los Angeles Harbor, had retained a small pocket beach. This beach all but disappeared from time to time during the next half century, being filled occasionally with whatever material might be available from harbor dredging. A model study by H. C. Gee and C. M. McAfee while working for their Masters Degrees at the University of California in 1938 and 1939 showed how the refraction of waves in this area tended to control the beach alignment. Incidentally, this study included one of the first uses of refraction diagramming in the annals of coastal engineering. While somewhat crude by present techniques, these diagrams checked the model studies and were later proved to be quite accurate.

In a report by the Corps of Engineers in 1952,3 a barrier groin was recommended to hold a substantial pocket beach in the Cabrillo Beach area.

t--<~---

7

35-,---,

-~

I

'Z.,ooo,ooot eLl.YD, FI~"

,"OM "~C,\,I!:O MA"'C\o\ 1<;)",.

Figure 3.

LONG GROINS

In the eroding reach of shore between Seal Beach and Newport

Submarine Canyon, the Corps has considered reorientation of the shore

761

in a sawtooth pattern with a series of long groins each of which would be

virtually a complete barrier to littoral transport. In this case recirculation

of the drifting sand proved to be more economical, but in other cases, such

a plan might prove to be the best solution. So far, all barrier groins have

been designed with their crests above extreme high water throughout their

length. A wholly or partially submerged groin is normally considered a

menace to navigation. However, in the case of a long groin extending out

to depths well below the draft of small craft that normally ply the coastal

waters, it may be possible to extend the end with a low sill that rises but

a few feet above the bottom. This would permit re -establishment of

equili-brium profiles out to considerable depth, causing slight differences in bottom

elevation on either side of the groin. The sill depths, thus artifically created,

would probably have to be kept below the minus 15 -foot elevation for navigation

safety.

CONCLUSIONS

As in the case of short groins, a long groin does not always provide

a ready solution to every problem. A fairly flat offshore slope is required,

and the effectiveness of the groin as a barrier diminishes greatly in areas

with large tide ranges. The effectiveness also varies with wave exposure,

decreasing as the height of the prevailing waves increases. When a long

groin is being constructed in the presence of relatively high waves approaching

in a direction that is inclined to the axis of the structure, considerable scour

occurs at the working end. This occurs whether the structure is being

extended either seaward from shore or shoreward from the outer end, and

is apparent both in rubble mound and sheet -pi Ie construction. The seaward

end of the structure must be stabilized against this scouring action, and this

has customarily been accomplished by filling the scour hole with dumped

stone of such size that it remains in place in the presence of the most severe

type of wave action.

REFERENCES

1. House Docwnent No. 349, 83d Congress, 2nd Session

Anaheim Bay Harbor, Calif.

2. House Docwnent No. 344, 84th Congress, 2nd Session

Oceanside, Ocean Beach, Imperial Beach and Coronado San Diego County Beach Erosion Control Study

3. House Docwnent No. 277, 83d Congress, 2nd Session

Appendix II, Coast of California, Point Mugu to San Pedro Breakwater, Beach Erosion Control Study