ML19309E079
| ML19309E079 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 04/15/1980 |
| From: | Andognini G BOSTON EDISON CO. |
| To: | Ippolito T Office of Nuclear Reactor Regulation |
| References | |
| 80-66, NUDOCS 8004180363 | |
| Download: ML19309E079 (26) | |
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BOSTON EDISDN COMPANY oENEna6 orriCEs soo sov6sTon sTnECT sosTON. MAESACMuBETTS o2199
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supKm4NTENDENT MuCLEAN OPENATIONS DEpamTMENT April 15,1980 BECo. Ltr. #80-66 Mr. Thomas A. Ippolito, Chief Operating Reactors Branch #3 Division of Cperating Reactors Office of Naclear Reactor Regulation U.S. Nuclesr Regulatory Commission Washingto*4, D. C.
20555 License No. DPR-35 Docket No. 50-293 Ref. (a) BECo Letter (G. C. Andognini) to NRC (T. A. Ippolito) titled "Information on the Pilgrim Station Breakwater" dated September 28, 1979
Dear Sir:
Reference (a) provided you with infotbation on the breakwater at Pilgrim Station.
Your letter dated December 11, 1979 requested further information in order to clarify the adequacy of the breakwater relative to its designed safety function.
Accordingly, this letter provides that supplemental information.
addresses the specific concerns of your December 11, 1979 letter, and Enclosure 2 provides the results of a detailed survey and inspection of the breakwater as committed in Reference (a), which we consider to be an integral part of our overall study on the ability of the breakwater to protect the Pilgrim shorefront and facilities.
We trust this letter adequately addresses all your concerns on the adequacy of our breakwater design; however, should you have any further questions, please contact us at jour convenience.
Very truly yours, 1
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Enclosures r,oo *.
fi eocu8o3G3 Woeo-qqae
t ENCLOSURE (1)0F RESPONSE TO NRC LETTER Dated December ll,1979 General l
The breakwater stability tests performed at the University of Califsrnia indicated that the maximum susceptibility of the breakwater to damage occurred when the still water level was approximately two feet below the crest of the breakwater (elevation 14.0' f1LW) and waves were of such a height that they broke from two to seven wave heights in front of the structure. Testing was then continued at that level primarily to determine what armor stone sizes and side slopes were stable for different brea1cwater heights, or water depths, the height being two feet greater than the depth, at that water level.
This test water depth
- was exceeded in the 1978 storm by approximately half a foot and the water level was above 13.0' MLW for te different periods totaling approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.
In 1979 the water depth probably exceeded 13.0 feet for a short period of time in January and 12.3 feet in February.
The hindcasts show that wave heights during the 1978 storm were great enough that a large proportion of the waves broke in front of the structure during coincidental periods of high tide and peak surge, such that the maximum potentially damaging conditions probably lasted for two hours or so during two periods.
It is unlikely that this " maximum condition" happened in February 1979 because the surge ** was not high, but in January 1979 the wind was sustained and the surge lasted long enough so that conditions close to the maximum occurred.
The breakwater layout (orientation and location with respect to tb.e shoreline) was first tested at Alden Research Laboratories in a 1:50 scale m; del with a number of variations to determine the degree of protection that would be provided at the shorefront just north of and to the east of Unit 1.
The design i
of the breakwater layout was based on these tests. The design of the break-water characteristics affecting stability, stone sizes, side siopes and crest height, was based on the stability tests done at the University of California.
The redesign discussed in response to question (b) was primarily a layout change, namely the shortening of both breakwaters.
It was Mr. Eaton's judgment that this change would not significantly reduce the amount of protection afforded to the shorefront close of Unit 1.
This was later confirmed by further model tests at Alden, which were completed before extensive breakwater construc-tion work was done. This shortening did result in some changes to the break-water, primarily due to the relocation'of the breakwater head.
It was also Mr. Eaton's judgment that the capstone and drystone layers could be combined as long as the larger stone in the size category specified were placed toward the top of the structure. The changes made did not. violate the stability criteria established on the basis of the stability model tests.
1
- Test water depth is analogous to still water elevation, i.e. the water depth in the absence of wave activity.
- Surge does not include wave activity.
1.
edel tests did show that flatter slopes would be more stable as might fected.
But the model test did show also that a cross section of 28 tructure height (Model test report, figure 11) would meet the 6% damage ia with 2 to 1 sea and 1.5 to 1 lee slopes with twelve to thirteen ton nes and capstones of more than 18 tons.
For this reason the drawing ed hrmor of not less than 12 ton size with 50% exceeding 15 ton, and ecification required the placement of the larger stones, in the size at the top or crest of the structure. Generally the capstones are
,rgest stones along the breakwater,in the 18 ton range,and thus the i intent has been a:hieved.
' cussed in Section 3.6 of the Model Test Report, three kinds of stone ent were defined; hat.dpacked, fitted and keyed, in ascending order of grea of care taken in placing the stones. The placement used for the was " fitted" for the c00 stones and "handpacked" for the wet and ines. Because the prctotype armor stones were individually placed and ans l
reset to achieve a more stable position, the constructed breakwater is 1-sly closer to the keyed condition. A great deal of care was exercised cing the stones. Some settlement of the structure was expected as it l
ed to attack by waves.
For most of its length there has been little settlement as pointed out in Enclosure (2) to this reply.
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2.
(5)
In January 1979 the waves reached significant wave heights in excess of 20 feet but these may not have coincided with the peri:d of maximum water elevation so the maximum design condition may not have been reached. Water levels were great enough however, that the action of the waves on both the sea and lee slopes should be characterized as severe.
l (6)
The surge in February 1979 did not exceed 1.5 feet.
EveTi though waves of significant height were present (e.g.17 ft.) the water depth in front of the braakwater did not induce critical breaking.
b.
" Provide your design bases..."
Response
The original design was based upon the main breakwater fulfilling three basic requirements:
1)
Providing protection of the plant shoreline and particularly the cooling water intake system against ocean storms; (2) preventing the recirculation of warm water discharging from the discharge canal to the intake structure; and (3) preventing excessive shoaling, due to siltation, of the dredged intake channel.
Initial layouts of the shorefront protection were tested in August 1967, l
at Alden Research Laboratories and further testing of revised layouts including temperature considerations was started in March 1968. The revisions to the breakwater consisted of realignment closer to the shore and somewhat reduced length to reduce the water depth along the structure and accordingly reduce armor stone sizes.
In the fall of 1968 it was decided to perform stability model tests of the breakwater because little was known concerning the maximum stresses involved in the stability of structures subject to still water levels approaching and reaching peak stages well above the structure crest.
These tests were conducted at the University of California from October 1968 to January 1969, under the direction of Professor J. W. Johnson, eminently known in coastal engineering research.
The breakwater stability tests were perfonned to determine what armour stone sizes and sideslopes are stable for various breakwater heights and water depths. The changes in breakwater length and resulting rock downsizing did not invalidate the criteria derived from these tests, since a number of cross sections with various heights, slopes, and rock sizes were tested including a cross section corresponding to that of the constructed breakwater.
I The adopted breakwater design was issued for bids in early February 1969 and bids were received in mid March. The quotations indicated that stone of the required size and quantity would be exceedingly difficult to obtain from local rock quarries.
For this reason the breakwater was shortened to reduce both armor size and quantity.
Contract drawings were revised and a contract signed for the work.
The actual construction started on May 28, 1969. The judgment that it would satisfactorily protect the shoreline and facilities of Unit 1 was confirmed by additional i
tests at Alden Laboratories in July and August 1969.
4.
_ = _, -...
There was no redesign of the section other than a combining of the capstone and drystone into a single armorstone layer, with the specification requirement that the larger size stones in the class be placed toward the top of the slope.
There is always some judgment that goes into the interpretation of model test results.
This is l
l discussed in section 5 of the model test report.
c.
"How do you justi fy...."
Response
Although there is a clear correlation between stability and flatness of slope, there is also a correlation between armor stone size and structure sl. opes that has long been established in design practice.
Steep slopes may be employed on a breakwater provided that the armor size is increased to compensate.
Empirical coefficients employed in design formula have been adjusted from time to time to reflect knowledge gained by research based upon both laboratory investigation and field experience.
No part of the project breakwater was designed with slopes steeper than 1 on lh and no part of the structure was built with such slopes.
It is normal practice for surveyors measuring completed cross sections to measure projections above and depressions below the basic cross section in order to determine whether specified tolerance limits have been met.
In preparing Exhibit 2 of BEco's response to NRC's questions in letter
- 50-293, the surveyor has apparently done this with the result that a number of cross sections appear to be steeper than designed.
This is easy to do with rubble mound construction.
Further detailed cross sectioning which are furnished with the breakwater survey report $nclo-sure (2)) show the slopes to be within design limits with few exceptions.
As pointed out in the general section of this report, the lee slope design of 1 on lh for most of the structure is in conformity with the armor stone sizes selected for the design criteria adopted from the model test. The flatter (1 on 2) slope adopted for most of the seaward face was in conformity with usual practice predicated upon expectation of more i
frequent exposure of the seaward face than the lee slope to high stress of the fully breaking wave. This is particularly so on lower slope segments at water stages below those which result in critical stages of overtopping.
I 1
5.
1
d.
" Discuss how repairs were made to the damaged areas of the breakwater.... "
Repairs in 1978 and 1979 to the damaged areas of the break-water were made in accordance with applicable sections of the original Construction Specification 6498-C-20 and Drawings C-401 through C-423. Quality controls applied to assure the acceptability of breakwater repairs comprised continuous direct field supervision provided by the contractor and the issuance of a Compliance Inspec-l tion Report by the contractor upon completion of each repair.
On September 26, 1978, the contractor reported:
The main breakwater from Station 5+40 to 23+50 had no stones out of position. The loose armor stone that was not interlocked and visible around the toe of slope at Station 23+50 was not part of the original breakwater design.
It was placed in that location during original construction to give added protection against erosion.
The slopes were almost entirely denuded of chink stone in the openings between the heavy amor. The chinking stone was placed during original construction merely to facilitate access and was also not part of the original breakwater design.
In the repair process, no additional amor stone was required due to the dislocated stones being in close proximity to their original locations. The amor stones were reset and inter-locked with the surrounding amor.
Additional chink stone was purchased to create a level roadway for the setting crane.
The groins of the discharge canal from Station 0+0 to 8+80 were found to be in good condition with the amor secure and voids chinked. The revetment area adjacent to the barge slip has been restored and is consistent with the adjoining revetment areas.
On August 23, 1979, the contractor reported:
The repairs involved resetting of individual stones from Station 10+00 to 23+00, plus the resetting of one section of cap stones (approximately 30 ft.).
The breakwater has been restored to its pre-stom condition with all stone properly interlocked. No new stone was required.
/mjs -
ENCLOSURE (2) 0F RESPONSE TO NRC LETTER DATED DECEMBER 11, 1979 I.
Introduction
~
A survey of the breakwater, constructed during 1969 and 1970 as part of the Pilgrim Nuclear Power Station, was undertaken in late 1979 to assess its present condition. This was done to provide additional data with which to answer a series of questions asked by the NRC in letters, (Docket No. 50-293) l Ippolito to Andognini of July 31, 1979 and December 11, 1979.
A commitment was made in the answer to the July 31, 1979 questions to effect this survey and to make follow-up surveys as necessary to monitor the condition of the breakwater.
II.
Description of Survey As part of the survey aerial photographs of the breakwater were taken and enlarged so that individual armor stones could be seen.
(Exhibit 3)
Base lines and location points were
(
established on the breakwater to provide frames of reference for the photography and the field reconnaissance. A series of photographs were taken from a boat of the ocean side and the j
lee side of the breakwater and a number of photographs were taken l
of the slopes from fixed positions at the inshore ends of the breakwater.
l A field reconnaissance of the breakwater was made in November 1979.
l Its purpose was; l) to check the breakwater for armor stone that might be undersized, 2) to check areas where the ocean and inshore slopes might be steeper than called for on the design drawings,
- 3) to check for evidence of foundation instability and 4) to observe the placement and keying of the armor stone. The program of photography and cross sectioning was established during this reconnaissance.
To supplement the field reconnaissance, cross sections were measured at a number of locations where the breakwater appeared to have steeper slopes than designed or to have missing armor stones.
(Exhibit 6)
In February 1980 a second field survey was made to compare the photographs and cross sections with the breakwater.
l
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l III. Office Investigation of Breakwater i
The office investigation of the breakwater consisted of careful study of the breakwater cross sections measured in November 1979 showing individual armor stones, the photographs taken at the same time, and photographs taken in October 1972 and after' the 1978 storm. This investigation concentrated on the lee side of the main breakwater because it was the only area that has suffered significant storm damage since its completion in 1970.
In order to compare the three sets of photographs, to identify individual armor stones and determine which stones had been moved by the 1978 and 1979 storms, all three sets of photographs were enlarged to an approximate scale of 1-inch equals 15 feet.
The recent photographs taken in November 1979 (dual annor stones.
Exhibits 1 & 2 )
were taken specifically to identify indivi They also serve as a record of the present condition and form the basis for further monitoring of the breakwater.
l The October 1972 photograph (Exhibit 5) was an oblique aerial photo of the power plant area and shows the full length of the breakwater across the photo. From this photo a series of enlargements to approximately the same scale as the recent photos was made for i
comparison.
Due to th~e quality of the photo and the level of enlargement, the enlargements lose usable definition beyond i
station 21+00.
(photograph location point 15) The stationing i
is that shown on Drawing 6498-C-417, Revision 5.
f The photo showing the 1978 storm damage (Exhibit 4) was an enlargemer.t from a Polaroid photo taken from the shorefront across the intake channel from the breakwater. This photograph covers from l
about station 15+30 to station 19+00, and shows the 1978 damage located i
1n the vicinity of station 17+60, (photograph location point 9)
In the comparison of the three sets of photographs each of the armor stones discernible on any two of the three photos was numbered. This allowed the identification of those stones, within i
the limits of photographic coverage, which were moved during the periods 1972 to 1978 and 1978 to 1979. The one area of 1978 i
damage was clearly shown on the 1978 photo taken to specifically show it. The other area of 1978 damage at station 15+00 is not i
readily apparent on the 1978 photo.
In other areas some stones were moved.
Since the area of displaced armor stones coincided with the descriptions cf the 1979 damage, it was assumed that the displacement resulted from that storm..
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IV.
Findings A.
Field Reconnaissance Two site visits were made to examine the breakwater.
The first visit was made in November 1979 to establish the fomal survey and photography programs. At that time brea,kwater sections which appeared worthy of further stucy were identified.
Detailed cross sections were measured at those locations. The cross sections showed individual armor stone outlines rather than the general overall outline or envelope. This detail was included to detemine how the amor stones fit the design cross sections. A tolerance of one foot above or below the design cross section was allowed by the construction specification, provided there was no continuous under building.
A second site visit was made in February 1980 to compare the photographs and cross sections with the breakwater to confirm conclusions. Unfortunately due to the weather and ice on the breakwater, it was difficult or impossible to examine all areas of interest along the breakwater. Based on this later site visit the accuracy of the cross sections was confirmed. Also areas which showed small amor stones in the photos were confirmed.
I B.
Breakwater Stability 1.
Foundation The breakwater foundation is satisfactory.
There is i
no evidence of overall subsidence of the breakwater and the crest elevation has not changed measurably.
2.
Main Breakwater Lee Side Slopes The following are descriptions of areas along the breakwater where there are stones which fall below the design slope as indicated on the cross sections.
A few stones near the low water line on the lee side between stations 16+40 and 16+70 appear to be about 2 feet below the design section. However, there is t
no evidence of stone displacement in that area from the photograph comparison.
Similarly in the area on the lee side from station 19+60 to 19+70 there are a few stones at the water line which l
apppear to be about 3 feet below the design cross section, however, again there is no evidence of armor stone displacement from the 1972 photo to the 1979 survey photos. -
Th2 slope also appears steep in th2. vicinity of station 21+20 on the lee side, however this appears to be due to t
an overbuilding of the crest width and the armor stones near the top of the slope by several feet.
In this area the 1972 photo has potr definition, hcwever from what can 3
be seen, there has bee.n no armor stone movement.
3.
Main Breakwater Lee Slope Armor Size
- There are some areas where the armor stones appear small l
compared to.the required 12 ton amor stone with a nominal dimension of 6 feet. The 12 ton amor stone, according to i
the design drawings, starts.at station 14+00.
Before that station the breakwater is in shallower water and the i
design armor stone size is 8 ton or less. The following are areas where some amor stone appear to be undersize:
l Station 14+20 to 14+40, 6 stones along crest Station 15+20 to 15+40, 6 stones on slope Station 16+00 to 16+20, 4 stones along crest l
Station 16+50 to 16+60, 4 stones along crest This area appears to have been either rework d after i
the 1978 stem or damaged in the 1979. stom.
Station 17+30 to 17+70, many along crest and on slope.
This includes area of 1978 damage, station 17+40 to 17+70.
Station 18+00 to 18+30, 8 stones along crest.
From examination of the breakwater it is quite difficult j
to determine which of these smaller stones are actually 1
functioning as armor stones; that is, their displacement could deprive a larger armor stone of support. Many smaller stones appear to have been placed on top of the breakwater i
to smooth the cross section (chinking), and from the stand-point of stability of the breakwater serve no purpose.
\\
4.
Main Breakwater Lee Side Displaced Armor The comparison of the 1972, 1978 and 1979 photos shows l
that there have been very few stones on the lee side which i
have moved in spite of areas with some stones below the design cross section or undersized armor stone. The following are descriptions of areas of armor stone dis-placement on the lee side of the breakwater.
At station 15+00, the apparent location of the lesser 1978 damage, the comparison of photos shows that one or two amor stones had been dislodged and are now replaced with other armor stones.
At station 15+30 there appears a very larga stona (about 10 feet by 6 fest) on tha 1979 photos which is not identifiable on either of the earlier photos.
However the 1972 and 1978 photos are similar to each other, indicating the large stone was placed after the 1978 storm.
(probably by the repair contractor)
Between stations 16+30 and 16+40 there are two armor stones at the crest of the breakwater identifiable on the 1972 and
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1978 photos which are not visible on the 1979 photos. Also between stations 16+50 and 16+60 there are an additional two armor stones identifiable on the 1972 and 1978 photos but not on the 1979 photo. From the qualitative descriptions of the 1979 damage and its location, it appears that these armor stones may have been displaced by that stom.
Another area where there is visible difference between the photographs is at the location of the 1978 damage, between stations 17+40 and 17+70.
In that 30 foot section it appears that most or all of the annor stones on the lee side were displaced. However, on either side of the 30 foot damaged area all armor stones are identifiable and none appear to l
have been disturbed. The indentation or break in the crest I
i of the breakwater at the damaged area appears to be no more i
than about one armor stone or 5 to 10 feet deep.
In sunnary, it has been determined that very few armor stones on the lee side have been displaced.
Even the stones which appear to be undersized have not moved in most areas. However, the comparison of photographs also indicates that a possible l
reason for the major displacement of the armor stones in the area from station 17+40 to 17+70 may have resulted from the concentration of small armor stones in that area visible in f
the 1972 photo. Small stones are also visible in this area l
in the 1979 photos. When the breakwater was repaired, no new armor stone was brought in, but the original armor was picked-j up and replaced in the damaged area.
l F
In examining the photographs it appears that the 1978 damage l
was a very local phenomenon resulting from a local concentra-tion of undersize armor stones which either did not have sufficient support or interlocking or somehow lost their support during the storm.
If the damage had resulted from an overall or general inadequacy of the breakwater design or L
construction one would have expected damage of at least minor nature along the entire length of the breakwater. This was not the case. The remainder of the breakwater other than the two damaged areas of the 1978 storm appears to be completely undamaged.
5.
Main Breakwater Sea Side The sea side of the main breakwater has not experienced notice-able displacement of armor stone. Consequently the investigation of the sea side was less vigorous than that of the lee side.
The surveyed cross sections show that the breakwater sea side slope is according to the design in all sections measured.
In general, the armor stone sizes also are in accordance with the design. There are a few scattered smaller stones along the breakwater, however these are not functio'ning as armor stones.
A cursory photographic comparison of :he sea side of the breakwater shows little or no armor stone movement.
6.
Main Breakwater Crest The crest of the main breakwater was constructed with a very flat surface in order to accommodate construction equipment.
In general the cap stones are of specified size. The width of the breakwater crest is 16 feet to station 21+50 at which point it widens to 20 feet at station 23+15, In most sections j
in the 16 foot wide areas the crest is two or three cap stones l
wide, indicating the stones are of sufficient size. There are no areas where there is a concentration of undersized cap stones.
However, the interstices between the cap stones are filled with smaller stones. These smaller stones serve as chinking to provide the smooth upper surface.
It is difficult to determine whether the root of the 1978 damage was in the inadequacy of the armor stone on the slope or the cap stones.
From the photographs it appears that the damage probably started in the lee side slope which deprived the cap l
stones in the area of their necessary support resulting in the displacement of both the cap stones and armor stones within the thirty foot damaged length of breakwater.
V.
Conclusions l
The survey of the breakwater shows that there has been surprisingly l
little movement of armor stone.except in thase areas where damage occurred during the 1978 and 1979 storms. The comparison of the photographs taken soon after construction, right after the February 1978 l
storm and in 1979 after the repairs shows that the. stones have remained in their original positions with few exceptions. One exception is the displacement of one or two large armor stones on the lee side of the main breakwater head.
The crest of the breakwater has remained level and at its design elevation. This indicates that the core material has remained in place and the foundations are stable.
There are some stones in the armor stone layers which do not meet the weight requirements of the design drawings. Where these stones are functioning as armor stones and are keyed into the structure this l
lack of size and weight may cause a problem under design storm conditions.
l Some small stones which are not keyed in~ appear to have been placed l
where the line of the slope intersects the line of the crest to achieve a smooth and regular cross section. The loss of this stone would result in a less attractive cross section but would not i
decrease the overall stability.
i l.
The study of the photograph taken soon after the breakwater construction (Exhibit 5) indicates that the main damage in the 1978 stom centered on an area where there was a number of apparently smaller stones on the lee side.
It is very likely that this concentration of smaller stones was the reason for,the damage at this location. Specific reasons for the breakwater distress at the other point of damage during the 1978 storm and the location of the damage in the 1979 stom are not readily apparent, other than because of random movement of the armor stone as is expected in rubble mound breakwaters.
But, the 1979 damage could also have been the result of small movement of the annor stone in 1978 which was not detected and repaired.
t The breakwater has performed well to date.
It has been exposed to water levels and waves equal to the conditions for which it was designed on two or possibly three occasions without significant damage, other than the one area in the 1978 storm. This indicates that the design was good.
It also indicates that the construction l
was satisfactory, except where concentrations of small stone exist such as the area where 1978 damage occured.
l The breakwater experienced no damage during the winter of 1980.
l Although 1980 is considered a reletively mild winter, there were a ntsnber of times when the breakwater experienced high water levels and heavy wave impact.
j l
VI.
Recommendations It is recomended that no alteration or further repair work be done at this time.
Should the breakwater suffer additional damage, similar to that of the 1978 and 1979 storms, it should be repaired at the earliest opportunity, preferably during the summer season following the storm.
A regular series of breakwater inspections following the winter storm season should establish whether there was damage that requires repair.
In making the repairs care should be taken to be sure that annor stone meets the criteria of the armor stone layer being repaired.
Additional new armor of the proper size should be brought in to i
assure that no undersize armor stones are replaced in the armor 1ayer. There should be no filling of cracks and voids (chinking) with smaller material. The repaired amor should be. keyed into the existing breakwater to effect a homogeneous interlocking structure.
l l -
- *n Appendix A to Enclosure (1) i HINDCASTING OF WATER LEVEIS AND WAVE CONDITIONS AT THE PII4 RIM I SIIE DURING 1978 AND 1979 STORM EVEITIS OBJECTIVES The purpose of this study was to hindcast the still-water levels and associated wave heights at the Pilgrim I breakwater for the following storm periods:
February 6-7, 1978 l
January 24-25, 1979 February 25-26, 1979 j
I The main breakwater of the Pilgrim Station sustained some damage during l
the stom of February 6-7, 1978. Repair of the breakwater was undertaken l
in the summer of 1978. However, portions of the breakwater were found damaged after the storms of January 24-25 and February 25-26, 1979 l
The still water levels and c.ssociated wave heights determined from this study can serve as a basis for the evaluation of the breakwater design.
METHODOLOGY Northeaster Storm Simulation The February 1978 and January 1979 storms can unequivocally be categorized as northeaster storms. The February 1979 high wind period was characterized
'cy winds from the northeast but the driving force in this latter case originated with a massive high pressure system centered over Quebec. The first two storms were amenable to analysis using the Stone & Webster North-easter Model which relies upon such input from weather charts as storm center location, radius of maximum winds, central pressure, and peripheral pressure.
A grid system was set up for the purposes of the finite-difference numerical l
modeling.
In the present case a four-nautical-mile square grid was over-laid on the area of interest. This horizontal grid system was oriented north-south, east-west. Its southern boundary was Cape Cod: its northern boundary extended into southern Maine. The coastline formed a natural barrier on the western boundary and the eastern boundary was seaward at approximately the location of the 600-ft depth contour. Enmeshed in the four-nautical-mile grid system was a finer one-nautical-mile square grid system covering the area from Boston Harbor to Cape Ann. The complexity of the bathymetry and coastline in this region warranted the use of the finer grid.
The Northeaster Model subsequently generated over the grid a time history of the sea level pressure and wind for the duration of the storm events.
,-.,,e w
Storm Tide Simulation The time-varying pressure field as obtained from the Northeaster Model, and actual wind data from Logan Airport, Boston served as input to the Stone & Vebster Storm Surge Model which employed the same grid mystem.
This model produces a time history of the surge over the entire grid and particularly at points of interest along the coastline e.g., Boston and the Pilgrim I site; the time of maximum surge is highlighted. This information is coupled with a prediction for the astronomical tide to i
produce a time history of the total still-water elevation. Given the predicted surge at Boston, a location where a National Ocean Survey tide gage has been maintained over a long period, the predicted values were compared with those actually observed and an appropriate adjustment made.
The predicted surge values at Pilgrim I were adjusted to reflect this difference between the predicted and observed values of surge at Boston.
The surge value at Pilgrim I was multiplied by a factor equal to the ratio between the observed and predicted values at Boston at hourly increments for the duration of the February,1978 and January,1979 storms.
The results thus generated were used to predict the maximum storm tide level experienced during the February 1979 storm because it was not appropriate to apply the Northeaster Model in this case: a correlation was established between maximum predicted surge in Boston and maximum predicted surge at Pilgrim for the two northeasters studied above and this information was used with data from the Boston gage for February 1979 to yield a predicted storm tide level at Pilgrim I.
Wave Height Simulation Coincident wave activity was hindcasted using the same wind field information as that used to predict the stom tide still-water levels.
Stone & Webster's Shallow Vater Vave Generation program, which accounts for wave generation due to a time-varying wind field, was employed.
Three selected transects radiating from the Pilgrim I site outward towards the northeast were chosen as the potential paths for =w4=2m vave gener-ation. All three transects fell within the northeast qua ant. The three i
tracks which were investigated were N60 E, N50 E, and N40, respectively.
Water depths along these tracks and the time history of the wind component parallel to them served as input to the model. The output of the model identified the significant waves appearing at the Pilgrim I breakwater for each track. As the waves moved into shallow water the program accounted i
for various shallow water effects, e.g., bottom friction and shoaling. Any refraction effects were accounted for using the Stone & Vebster Vave Refrac-
' tion Model. For this particular model a horizontal grid system which encom-passed the site area was employed and incoming waves were tracked from its open-ocean boundary to the vicinity of the breakwater. This grid system was based on bathymetric mapping performed in February 1%9 which extended out to approximately the -50 ft contour Mean Lov Water. The use of this program allowed for waves impinging on the head of the breakwater and the trunk of the breakwater to be distinguished. Therefore, separate wave heights were generated at these two portions of the breakwater 1
- 1 Maximum Vave Detemination The information on the still-water levels and incident wave heights was i
combined in order to detemine the maximum significant wave height allow-i able at the breakvater, given a specific water depth. The toe of the trunk section of the breakwater was taken to be at a depth of -13 ft Mean Lov Vater (these numbers are illustrative of the ambient situation but are not necesearily definitive and the final results can be modified to i
reflect a change in the bcttom elevation numbers). Given this quantity and the appropriate storm tide still-water level, the maximum allovable wave in that depth of water was computed. The traditional breaking criterion relating) breaker wave height (H ) to breaking depth (h ) was b
b used (Hb = 0.78 db. The breaking depth could be easily modified if ve vere interested in computing the wave which causes maximum damage to the breakvater, which has been taken to be that wave which breaks a dis-i tance offshore of the breakvater equal to a specified number (e.g., 2 to 7) of breaker heights.
DATA SOURCES A brief description of the data used as input is given below.
1.
Still-water elevations for the Boston gage vere obtained from the National Ocean Survey (NOAA) for all three storms.
' 2.
Synoptic veather charts were obtained from the National Yeather Service for the February 1978 and January 1979 storms. These were analyzed for input to the Northeaster Model.
3.
Hourly wind data vere obtained from Logan Airport for all three storm events. The atmospheric pressure field generated by the Northeaster Model and the actual vind data from Logan Airport were used in the stem surge runs. These vind data vere also used for the wave generation hindcast.
RESULTS The results are summarized in both, tabular and graphical fom. The history of the still-water levels during the storm events at the Pilgrim I site and at Boston is illustrated in Figures 1 to 5 Shown is the time history I
of the total still-water elevation and the time history of the astronomical tide, the storm surge simply being the difference between the two curves.
Figure 6 shows the correlation between maximum stom surge at Boston and Pilgrim and the extrapolation of this information to the February 1979_ storm event.
Two tables summarize the overall results. Table 1 gives the maximum surge and the total maximum stillwater elevation for each storm period in the vicinity of the Pilgrim I site. Notice that the maximum surge does not necessarily coincide with the maximum still-water elevation.
Table 2 cites the maximum incident wave at the toe of the trunk and heat. &tetions of the breakwater for each of the storm events.
Comparison with Design Basis Event The maximum probable water level that the Pilgrim I site would experience is given as 19.5 feet MLW (Pilgrim Nuclear Power Station #1 FSAR, Section 2.4).
The maximum water level hindcasted in the present study is 14.5 feet MLW.
These levels are, respectively. 3.5 feet above and 1.5 feet below the crest of the Pilgrim I breakwater (16.0 feet MLW).
Existing evidence is not con-clusive as to the critical water level leading to maximum damage of a break-water. The model tests performed at the Berkeley Labs for the design of the Pilgrim I breakwater indicated that maximum damage occurred when the water level was approximately 2 feet below the breakwater crest.
The wave height in the model tests which was found to inflict maximum damage was defined in terms'of breaker distance from the structure, the critical wave breaking from 2 to 7 breaker heights seaward of the structure.
An average value of 4 5 is cited in the FSAR. The exact value of the wave height, therefore, depends on the water depth at the toe of the structure and the bottom slope. As an example, at the trunk of the breakwater, with a toe elevation of -13 feet MLW, a storm tide elevation of 14.5 feet MLW (February 1978 storm) and a (roughly estimated) bottom slope of.02, the critical breaker wave height, 4 5 wave heights offshore of the breakwater, would be approximately 23 feet. The predicted maximum breaking wave, which happened to coincide with m nien= high water, according to Table 2 is the depth-controlled value of 21.5 feet at the toe of the structure.
It would appear that for the storm events of February 1978 and January 1979 the hindcasted combinations of still-water level and breaker wave height duplicate the design conditions fairly closely. This conclusion 2:ests upon the assumption that the results of the model test are applicable i.e.,
maximum breakwater damage is sustained when the still-water level falls slightly below the crest of the breakwater.
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