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sr-012373-10a1~Ifl&;F'ATT ~C NICIIflZ,, EtiGI\EEI2S LONG BEACH, GALfPORNtA PORTLAN O, OAEGON November 6, 1972 €L.. _. i 1, i ~_ Mr. Perry Scott City Manager,. City of Santa Monica 1685 Main Street Santa Monica, California 90401 Subject: Report Our L-1470 Dear Mr. Scott: Enclosed are six {6) copies of our Report to you of anticipated shoreline effects of the proposed island shoreward- of the Santa T+Inica breakwater, ide are pleased to have had ths- opportunity to be of service zo .you, and look forward to any future service we may rendere Sincerely, MOFFATT & NICHOL, ENGINEERS ;- i / ~~' ~ ,_ John G. T~loffatt, President - me GTH enels. _ .° _. ., ~ ~~~ - U P. O. BOX 7707 250 WeST WAROLOW ROAD LONG EEACH CALIFORNIA 90607 (2:3) 426-9551, 7?4-5650 i_3 ABSTRACT _{ E _! ,_ In order to create the 35 acre island behind the Santa P4onica Breakwater and maintain the beach south of Colorado Avenue during construction approximately 2,500,000 cubic yards of sand will have. to"be removed from the existing salient behind the breakcaater. This will essentially .remove the salient and straighten the exist- t_ ing alinement of the beach. Approximately 250,000 cubic yards of littoral sand will be trapped each year in the. lee of the island and will have to be bypassed to the beach south of Colorado Avenue '_ in order to prevent serious erosion of this beach. The most desirable sand bypass program is continuous sand bypassing by a floating pipeline dredge. However, this program j must be modified as it is not desirable to interfere ~~rith usage of the beach during the sur2mer period nor is it desirable to i..... .operate floating equipment during the winter months of storm i~raves .and strong winds. Ideally the bypass operation should be aceom- ~ _, plished each year between October and January. This will result in a reasonably uniform alinement of the beach behind and to the ;- :_ north of the island. This semi-continuous, or annual sand ~, bypassing will result in a stable beach to the south with fluctua- tions in width comparable to present seasonal changes. This pro- gram, by a City owned and operated dredge is the most economical-means of bypassing littoral sand movement and, based on 1872 prices, would cost about X200,000 per. year. If the sand bypass program is to`be done by contracting with a commercial dredging company, it should be done by a larger dredge on a biennial bass. The annual cost to the City would be about $250,000 per year and there would be a corresponding larger fluctuation in widths of adjacent beaches, although not to such an extent as to en- i' L, danger structures or severely interfere with recreatior_al use off' ~ the beaches. ~~.z, __ _ ~a. PRELIMINARY REPORT ON ' SHORELINE EFFECTS OF PROPOSED ISLAND SHORELTARD OF SANTA MONICA BREAKS+IATER INTRODUCTION It is proposed to build an offshore island at Santa P4onica. The area will be about 35 acres and the present 2000 foot offshore breakwater will be incorporated into the seaward face of the island. '. ` While the island perimeter will be curvilinear. it will, in general, ~- ~, extend about 800 feet shoreward of the breakwater (see Plate I). i" It is planned to dredge from 2.0 to 2.5 million yards of sand from i_ the existing beach salient behind the breakwater which will be used i to create the island and counteract shoreline effects. This will `" straighten the alinement of the beach and create an initial channel I some 700 to 800 feet in width and 20 feet or more deep betcreen the l~ proposed island and the shore. -- j_ The purpose of this study is: to develop the best pattern for excavation to create the island; to develop the natural rate of littoral, or longshore, sand movement in this area; to develop the probable effects of the island construc£ion on the upcoast area, the area behind the island and the do~rrncoast area; to determine the r- ~__: E feasibility of bypassing sand by several alternative methods and ~~ to estimate t'rie probable quantity and costs.. `. HISTORY ~__ In its natural state, the entire shoreline of Santa P2oniea Bay t_ consisted of a rather narrow beach backed by soft sedimentary bluffs. !- The- exception is the Marina-del Rey-Ballona Creek area, w'riich at 1. l;k I ~.3 times was the mouth of the Los Angeles River. There have been numerous man-made structures and fills created along this beach since the early 1900's and it is very difficult to develop a clear-cut picture of littoral processes throughout the bay. It has been developed in previous Corps of Engineers studies that the littoral drift is predominantly from north to 's outki,although there is limited upcoast movement in the summer that must be taken into account. They, established a net. rate of movement of 270,000 cubic yards per year to the south-. They also developed that primary sources of littoral beach sand are from upcoast, mostly from upcoast beaches and to a lesser extent from the numerous small local drainage features. For the purposes of this study, only the area from the Light House Cafe southward to the Ocean Park. Pier will be given detailed consideration. ANALYSIS OF LSTTORAL SAND P~IOVENLETdT The rate of littoral sand movement is a function of wave energy and wave direction when it reaches the beach. In the Santa Nonica area the long term condition of the beach is related to the winter and spring waves from the North Pacific Ocean areas as compared to summer and fall .waves from ocean areas to the south extending as far as Peru and Australia. Ldhile day to day wave action is very important, the severity and frequency of local or off-shore storms also make major contributions to the total wave energy-sand movement relationship. On the sand supply side, the rainfall rates and severity of storms such as those in 1936, 1938, 1843 and 19og contribute to local sand supply as idell as influencing the rate of i. 2 . ~~ sand movement from the upcoast, or updrift beaches. In view of ' these variables, it is remarkable how consistent the average annual rate of littoral sand movement is along the shoreline df southern California. Frequent field measurements have been made for ever 35 year s. at Santa Barbara, 30 years at Channel Island, and 25 years in north Orange County, and it has been found that the annual rate of sand movement does not vary more than about + 15 percent. The most reliable method of measuring littoral sand movement is actual field measurements at an obstruction to littoral drift. In order to determine the total amount of drift, measurements are only reliable so long as the structure is a total obstacle to longshore sand movement. During this period either accretion on the updrift side or erosion on the downdrift side may be a measure of movement. The exception is an area like Channel Island and Santa Barbara where a measured amount of impounded sand is removed periodically and placed in the littoral stream sufficiently-far down- coast to not return to the trap. and confuse the measurement. ~ At Santa Monica a relative short period of time i~ras available for this type of measurement. i~lhile there were a fe~~r open pile piers in the Santa P~Tonica area, such as the Santa Monica pier built in 1907, they appeared to have little effect or_ the shoreline and i. the surveyed mean high tide line in 1921 sho~,red a generally straight -_ ': alinement with a narrow beach through the City of Santa Monica. L. In a situation such as that at Santa Monica, with an offshore breakwater parallel to the shore and open at both ends, the sand moving along the shoreline is influenced by 3 different factors: _ 3 (1) The-sand moving downcoast is deposited in the shelter of the breakwater causing large scale accretion; (2) at the southerly end of the breakwater a limited amount of sand is moved upcoast sufficiently to be sheltered from the pre- dominantly dotancoast wave action; and (3)-the lotr profile of of the breakwater and the voids in the cap rock permit a sufficient amount of *rrave energy to pass through the structure and move sand along the beach in the lee of the breakvrater. (a) Downeoast-Sand Movement:. This is the predominant cause of shoreline accretion and development of the large salient behind the existing Santa Monica breakwater. Between the start of construction of the breakt•.rater in ?riarch 1933 and the survey of 1946, over 3,500,000 cubic yards of sand were impounded behind the breakvrater. S+?ost of this resulted from the interference with dot~rncoast littoral drift. This beach sand is moved by wave induced littoral. currents. The breakwater influences this sand movement in tyro trays: first it eliminates, or greatly reduces, the amount of wave energy and thus long shore sand moving capability along the shoreline behind the breakvrater. Second, the waves at the ends of the structure ere changed in height and direction as they. round the north end of. the breakwater by the process knot~m as grave diffraction. The projection of these tt~;o factors from the breakwater to the shore varies from day to day, depending on the direction of the graves and the result is a transition curve forming the sal;ents of the general shape shown on Plates 1 and 3. Additional accretion is 4 ,_, caused when the salient .advances a sufficient . distance seataard to act as a groin, so that at Santa D'fonica accretion has occurred along the beach for some three miles northward of the break- vrater. This effect does not change the rate of accretion but instead slo~r~s the seaward advance of the salient. This is shown on Plate 1 by the rapid growth of the salient bettueen 1933 and 1946 - as compared to later years. (b) Upcoast Sand Movement. While the net ahnual sand movement at Santa Monica is dotvncoast, there is an appreciable amount of upcoast movement through the summer and fall caused by i~raves generated by storms off the coast of Mexico or in the South Pacific Ocean. .This sand moves into the shelter ' behind the south end of the breakwater. It is .further influenced by variable wave directions and vrave diffraction around the south end of the i j; structure. This impounded sand tends to form the ~- south flank of the salient from which the Corps cf ~' Engineers estimate of 270,000 cubic yards per year !~ represents an average rate of accretion, more than ~_. average rate of littoral .drift. ~- i. ~_, (c) Wave Energy Through the Breakwater. While the Santa Monica Breakwater reduces the amount of wave 5. energy acting on the shoreline behind the structure, it is not a total barrier. The orieina7 hrFakUratar was of capstone with 35 to 40~ voids from elevation +10 to -10 feet P-?LLLd when completed in 1935. This elevation was reduced to +7 feet by 1957 and has further deteriorated since that date. Hence, an increasing amount of wave. energy is going through and over the structure. It is not within the scope of this study. to quantify this residual energy but, particularly during the higher tides, it has a capa- bility to move sand. Tiro effects can be noted: it .tends to further blend the ti~rinter and summer sand deposits on-the flanks of the salient making identifi- cation of source more difficult, and in recent years is assisting the predominant do.aneoast waves to move ~- ,_, sand beyond the salient and help substain the down- ~_ coast beaches. i Tn the early stages of development of the large sand salient behind the breakwater, this eras probably a negligible factor due to dispersion of this energy betT~reen the breakwater and the i~ beach. The Corps of Engineers findings that between 1935 and r, 1946 the average. annual rate of accretion was in the order of ~_ 270,000 cubic yards vaas undoubtedly correct. Between completion of the breakwater, in 1935, and 1959, some 1,800,000 cubic yards of sand was dredged from the salient ~ 6. L and placed on the beach dotirncoast of the Santa P~Tonica Pier. This, in combination vrith placement of the Hyperion Beach fill, has ~- prevented serious erosion of the downcoast beach. Examination of the historical mean high tide lines shows on Plate 1, shows a relatively stable beach alinement since 1946 indicating that !~ either a considerable portion of the littoral sand has been ,_ carried through the area to the down-coast beaches or that this - sand- or a portion of it- is impounded along the entire 3 mile sand fillet to the north and is not easily identified. Ordinarily examination of the doivncoast beach would show compensating erosion in the same manner as occurred between 1035 and. 1939 but the ~_, enlargement of the Pacific Ocean Park Pier and the placement of the Hyperion beach fill plus the. three dredging-sand episodes prevents a reliable accretion vs erosion analysis. At present, 'i _: it is probable that upti~rard of 100;000 cubic yards per year is ~_ being carried past the existing sand salient and, along with-the r_.. Hyperion beach fill, ;s reasonably sustaining the downcoast beaches. L., Construction of the proposed island. and the removal of most r of the existing beach salient will essentially recreate conditions ~. as they were in 1935• In addition, the breakvrater-island structure ~ will completely prevent waves passing .through or over the structure and the inshore side of the island will be-some 800 feet shore,va'rd ~_. of the 1935 structure thus insuring a complete obstacle to littoral ~' currents. we must assume that the rate of impoundment will be ~, the same as that experienced bet~nreen 1935 and 1846 and that scheduled sand-bypassing will be required to sustain the downeoast beaches. 7. EFFECT OF PROPOSED ISLAAiD ON LITTORAL DRIFT The construction of the island by excavation of some 2,500,000 cubic yards of sand from the existing beach salient will result in conditions as sho~~rn on Plate 2. The construc- Lion of either the island or the excavation of the beach area j will cause major changes in the rates of accretion and/or erosion of sand to the beaches behind and adjacent to the project. These effects would become evident at about the 25 percent of completion point of either the breakwater rehabili- tation, construction of the island or excavation of the beach salient. EFFECT GJITHOUT SAND BYPASSING: .Sand bypassing must be an intergral part of this project but because the beach salient and the adjacent beaches have not been subjected to noticable changes since the last bypass dredging in 1958-59 this approach is discussed. ..Construction of the island and rehabilitation of the breakwater as shown on plate 2 grill constitute a complete barrier to waves impinging on the structure. Wave action around the ends .vill penetrate to a limited extent behind j the island due to the wave diffraction effect but as the ~..,: water gag will be only some 700 feet wide as compared to an ~-- i ~-- island-breaksrrater length of 2000 feet wave diffraction around ( each end of the island will form separate beach salients i`~ instead of the smooth combined salient presently existing. Most of the southerly impoundment caused by upcoast wave action can be intercepted by a land filled base L- g, f_ ` for the bridge to the island at Colorado Avenue, as shown on Plate 2, and thus remain in circulation. Excavation of the beach salient as shovrn on Plate 2 brill ,: i_ delay the formation of the nevr salient behind the north 1/3 of ~ the island until this dredged area is brought up to normal ~ beach slope. Then a tombola ti~rill form-similar to that shovrn on plate 3, connecting the island to the shore. This tiaould require some 5 to 7 years. {_ -. In the mean time, there would have been no supply of ~-- littoral sand to the beach south of the pier. and some 1,000.,000 1 to 2,000,000 cubic yards of sand would have moved on dotivncoast. This represents a loss of 20 to 35 acres of shoreline, including public beach, parking lots and private property. I_ betvreeri the beach and the promenade. (- EFF,FC^ ~dITH SAND BYPASSING E' j~Tith a properly planned sand bypass program, during and E _. ~ after construction, there will be no adverse effects on the t_.. hew alinement of beach behind the island or on the adjacent ~ beaches.. ~ The most desirable approach would be to establish a `-" continuous sand bypassing program but this is neither econo- ~- mical or desirable. The beach is very heavily used during t._ the summer season and between early April through September i. ~_ should not be cluttered T.aith pipelines and heavy equipment. ~- Also, it is not desirable to be placing sand on the south beach during periods of summer upcoast drift as the newly deposited sand would tend to move nori;h~rard into the lee of the island where it would ultimately have to be rehandled. Therefor the program should be to bypass the total year of sand accretion from behind the island to the southerly beach between October and February;. before the worst of the tivinter winds and storm waves would endanger the equipment. During Construction, a 22 to 26 month period, some . 2,500,000 cubic yards of sand will be dredged from the shore salient behind the island as shown on Plate 2. Approximately 2,000,000 cubic yards will be needed to create the nets island. The remainder will be needed to maintain a stable beach to the south during the construction period. A coordinated schedule will be needed in which, preferably during each fa.11 of the construction year 250,000 + cubic yards of sand will be placed south of Colorado Street to replace the natural littoral drift to the south. the"landside base!'for the bridge, extending from Colorado Avenue to the island should, in conjunction tivzth sealing of the offshore breakwater and dredging of the salient to fill the island area, be a first start construction item. The island is of adequate size to allotir filling the center area from -24 to -10 ft. MLLtd Optimum dredging conditions vrill be obtained if the breaks%rater rehabilitation is done from south to north and. dredging also be started near the south boundry to obtain 10. maximum protection from wave action. The norther]_y 800 feet of the borrow area is exposed to wave action from winter _' ~__ .waves and this area should be scheduled for dredging; during the late summer and fall season. There Lvill be very little disruption of recreational use of the beach during the 2 year construction season as most of the dredge spoil will be placed on the island. The two sessions of sand placement on the beach south of Colorado Avenue can be scheduled. for late fall or early winter vahen recreational.use of the beach is minimal. This construction dredging of 2,500,000 cubic yards of sand in 2 years crith the complexities of building up a 35 acre island requires the use of a large commercial dredge (24 inch diameter or larger) operated by experienced contractor personnel. SAND BYPASSING PROCEDURE Construction of the island will create a complete barrier to the longshore movement of beach sand by wave action. This r- j will cause accretion of sand behind the north portion of the ', island and denial of replacement sand to the beach south of Colorado Avenue vahere the beach is not protected from normal ~- wave action with the resultant longshore movement of sand. Based on previous experience this interference will amount ~ to about 250,000 cubic yards pe'r year. Within one year ~' after completion of the island construction a system must ~_ be established for transfer of this sand. Even in nature the width of a sand beach will vary seasonally or there may be a temporary loss of beach after a severe storm so a limited amount 11. of erosion can be tolerated 4vithout undue interference with recreational use of the beach. However, 2 years would represent a loss of 500,000 cubic yards of sand or 10 to 12 acres of beach which is about as much as can be tolerated. The associated accretion of 500,000 cubic yards of sand .would also begin to seriously narrow the protected crater area as shown on plate 3 and cause other inconveniences to the beach users in that area. For these reasons the sand should be bypassed at no greater than 2 year intervals. There are several established methods of sand bypassing, varying from continuous bypassing to a complete mobilization and demobolization of equipment for a short highly productive effort every two years, Any of these tivill result in a i `' stable beach alinement in both the accreting and eroding r- zones with little more variation than is presently observed fror!m seasonal effects. Obviously the more frequent the E . ~,, dredging the more the beaches will vary in width in the same `- manner- as they no?-~r do from natural causes. However, this ` stability must be measured against cost, clutter of the: beach with eauipment that might be hazardous to bathers, and noise: and visual pollution during summer periods of-heavy beach usage. ItSETH0DS OF SAND BYPASSING Examples of sand bypass systems successfully in opera- bons are as follocas: 12. Santa Barbara: Between 1956 and 1970 this harbor was i_ maintained by a small City-owned continuously operated !' pipeline dredge, Approximately 300,000 cubic yards per year c~rere bypassed at an annual cost of around X0.40 a yard. It is not certain that this included all of the overhead and plant rental costs. In the future this project ~~rill be done by commercial dredge bypassing 300,000 cubic yards in about four months of dredging. A contract was ac~rarded in September 1972 to bypass 225,000 cubic yards at a contractor cost of y1.21 per cubic yard. Channel Island Harbor: The rate of littoral sand movement -here is in the order of 1,200,000 cubic yards per year. ~- i The sand trap can accumulate a 2-year. supply so recent eontractorr costs for biennial bypassing of 2 500 000 cubic , , ~- yards in a 6-month dredging season is in the order of $0.50 per cubic yard. Ventura varina. The rate of littoral drift here is in '~, the order of 400,000 cubic yards per year. The first t-- contract in 1972 for biennial bypassing was for 825,000 ~ cubic yards ata0.92 per cubic yard. ~ ~ Oceanside: P~laintenance here is on a biennial basis at _, about 1400,000 cubic yards per dredging episode at a cost r., f_, of X0.80 per cubic yard. ~_. T%ixed Sand Bypass Plants: (land based) Two such plants ~ operate off the ends of jetties at North and South Lake Ldorth Florida but neither are over 50t~ effective as their ~, lack of mobility prevents interception of all the littoral I ~ stream. 13. There have been attempts at use of draglines and shovels with truck haul but unit costs a-re high and visual, noise and traffic problems were e:ctremely troublesome. The surest and most successful bypass system to date is the establishment of a large sheltered or semi-sheltered impounding area from which the periodic removal and placement downcoast of sand by a floating .pipeline dredge can be accomplished. i. I _ RECOP+i1+IENDED SAiJD BYPASS SYSTEtii It is recommended that the original dredging of 2,500,OQ0 cubic yards be done by a large commercial dredge. This s~rork ~ _. i i, should be scheduled so that near the completion of the project ~: sufficient sand be placed on the beach downcoast of Colorado ~- Avenue to replace any losses incurred during the 2-year construction period. %_, A permanent sand bypassing program should be initiated vrithin one year after completion of construction dredging. At _, the present stage of knowledge the following tyro methods are recommended. (1) Continuous Sand Bypassing. The required annual bypassing of 250,000 cubic yards averages out at only 700 cubic yards per day. However, this downcoast sand -. movement occurs during about 9 to 10 months of the the year with reversals or no drift during the summer period. For 9 months at 21 working days per month the average i 11; . rate of bypassing would be 1,330 cubic yards per day. Theoretically this could be handled by an ~ or 10 inch diameter pipeline hydraulic dredge. However, a safety ', factor must be provided for in oase of breakdowns, strikes or very severe storms which may increase the rate of littoral drift. Also a pumping capability for a distance of 3,000 feet is required which taxes the efficiency of small dredges. A 1~4-inch diameter pipelin2 hydraulic dredge pumping 10 percent solids vrould have a capacity of about 250 cubic yards of sand per hour or 2500 cubic yards per 10 hour shift. Thus about 4 months, October to January inclusive, would be required for annual bypassing. ^lcvo vrays are available to accomplish this annual bypass operation: (a) By contract, and based on 1972 costs, the costs to the city would be about X1.25 per cubic yard or X315,000 per year. (b) If the City i?ere to purchase a dredge and operate it themselves, and could supply adequately skilled personnel from their public vaorks depart- ment for 'the 4 months of dredging, the unit costs could be reduced to about $0.75 per cubic yard. This approach would require an initial city expenditure of $400,000 to X500,000 for a 14-inch dredge, 3.000 feet of pipeline, a vrork boat and '- other miscellaneous equipment. Taking into account time for mobilization and demobilization the dredge would not be needed for about 7 months of the year. The dredge could be stored on site but, considering the character of development planned :for the island, ' it 4~rould probably be most desirable to store the ', dredge in a corrli-nercial yard in Los Angeles-Long Beach Harbor. (2) Biennial Sand Bypassing: A biennial program would require the bypassing of 500,000 cubic yards of sand ~' during the same 4 months fall period as before and would require operating about 20 hours per day. Triis quantity i ~, _ is large enough so that, with shore based electrical power, the larger dredges could be economically used and. - complete the bypass effort in a shorter time, Based on 1972 prices, the cost to the City would be about $1.00 pe'r cubic yard if done by a contractor, It v.ould be quite. difficult for the city public works department to provide sufficient man power for a 3 shift ~- operation and the main savings in cost by use of a city i `J owned. dredge would be the elimination of the contractor's profit, _ It is recommended that the city consider acquiring ~_ ,__ and operating their oti~rn hydraulic pipeline dredge on an annual basis of 4 months operating time. An additional 6 to 8 weeks will be required for mobilization and 16. demobilization, This will result in a more stable beach alinement both behind the breaYwater-island and to the south of Colorado Avenue and in fact changes in beach width would not greatly exceed present seasonal changes. Annual costs, based on 1972 prices would be in the order of X200,000 per. year, including depreciation, maintenance, supervision and administration. The alternative would be to establish a biennial program by contract dredging v`rhich would average about X250,000 per year and would result in a greater fluctua- tion in ti~ridth of the beach, particularly the eroding 17 .