UFL/COEL-90/002

WATER AND EROSION DAMAGE TO COASTALSTRUCTURES - SOUTH CAROLINA COAST,HURRICANE HUGO, 1989

by

Hsiang Wang

March 1990

WATER AND EROSION DAMAGE TO COASTAL STRUCTURES-SOUTH CAROLINA COAST, HURRICANE HUGO, 1989

ABSTRACT

Hurricane Hugo hit U.S. Mainland on September 21, 1989just north of Charleston, South Carolina. It was billedas the most costly hurricane on record. The loss on themainland alone exceeded 7 billion dollars, more than15,000 homes were destroyed and the loss of livesexceeded forty.

This article documents one aspect of the multi-destructions caused by the hurricane - the water anderosion damage on water front or near water frontproperties. A general damage survey was given first,followed by assessment on the performance of variousengineered and non-engineering structures, on the majorfactors contributing to failures. Conclusions were thendrawn with recommendations for future improvement.

INTRODUCTION

One of the facets that sets Hugo apart from otherhurricanes in recent years was its severity of inflictingwater damages on coastal constructions. It could easilybe ranked one of the worst, comparable to HurricaneCamille in 1969 which was the only category 5 hurricanethat hit U.S. mainland (along the low land region ofLouisiana and Mississippi) with full force in thiscentury. A number of factors contributed to the greatdestruction:

1. The extremely high storm surge level, secondonly to Hurricane Camille in the history.

2. The high density of old structures constructedbefore adequate building code and code enforcement.

3. The high chronicle background erosion alongthese barrier islands (approximately 6' per year).

Damage reconnaissance was carried cut at selectedlocations from Seabrook Island to North Myrtle Beach, anarc of about 120 miles coastal belt affected by Hugo.

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Based upon visual inspection, the severity of waterdamages was subjectively determined, shown graphicallyin Figure 1.

WATER DAMAGE RESULTS IN SOUTH CAROLINA

FLORENCE

COLUMBIAN. MYRTLE BEACH

MYRTLE BEACH

SURFSIDE BEACHGARDEN CiY- 13FIf )

ORANGEBURG PAWLEYS ISLAND.#GEORGETOWN

' 13 F1 3

SUMMERVILLE McCLELLANVILLE

WALTERBORO 10Fi 17 F 0

' A 17Ri CHARLESTON ISLE OF PALMS

14Ft f1 1\0

FOLLY BEACH

12Ff .F. "_____________" 1""L w,• LEGEND• BEAUFORT LEGEND

1 _._ I Severe Water Damage

\ Considerable Water Damaget --- • ^ Light Water Damage

Figure 1: A General Assessment of Water Damages AlongCoastal Region Under the Influence of Hugo (Surge andWind Information From Federal Insurance Administration,1989).

Table 1 tabulates the percentage of destructed beachfront structures along the barrier coast. The destructedstructure is defined as the structural damage is greaterthan 66.67%.

Table 1 Percentage of Destructed Beach Front Structures

Surfside 7%Garden City 43%Pawleys Island 23%Folleys 20%Others 7%

* Source of Information, South Carolina Coastal Council

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The descriptions of these damages are given in thefollowing sections.

NARRATIVES

Seabrook Island

The heavily developed portion of this island is a plannedresort community of town houses and multi-unitcondominiums. The Island is obviously under erosionalstress and portion of it is heavily armored with newlyplaced revetment. Both wind and water damages werelight. Structure damages were limited to a few brokenwindows, missing roof shingles and up-rooted chimneys.The revetment structure showed differential settlementat a few locations with torn filters, thus, exposing thebank soil.

The damage was light in despite of its exposed location.This was mainly because the Island is located at thesouthern fringe of the Hurricane path. The storm surgenever reached the structures. Wave overtopping was alsominimal.

Folly Beach

Folly Beach is on the barrier island south of Charleston.It is mainly a residential community. The onlysubstantial commercial building fronting the beach is theHoliday Inn at the middle section of the island. Thebeach is under very heavy erosional stress for lack ofsand supply from the north. In fact, part of the streetsystem, West Arctic Avenue, crumbled into the sea withinthe last decade. A restaurant built 20 years ago onArctic Ave was hanging over water before Hurricane Hugoand it was subsequently completely demolished by theHurricane. Many houses on the west side of the townstill have addresses of West Arctic street that no longerexisted. The beach is narrow and low and is almostentirely covered with groins and revetment of marginalquality.

The measured storm surge levels were around 12 feet,slightly lower towards the two ends. The east andcentral sections of the island sustained severe waterdamage. Many single family homes fronting the beach werecompletely destroyed. First floor flooding was commonin the second tier and to a much less extent in the thirdrow and beyond. Damages due to dynamic force and staticbuoyancy were both evident as some structures werecrushed and the others were simply floated off theirfoundation. The most devastated section was between West

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2nd Street and West 7th Street. This was because manyold structures built 20 years ago were clustered in thissection. -his section was also eroded heavily in thepast years and,as mentioned earlier, had lost roads tothe ocean. Amid this devastation, however, wellconstructed dwellings did survive and survived well. Thehouse sh-wn in Figure 2, for instance, escaped withhardly any visible damage among a row of completelydestructed houses.

-. .. -

m

Ficure 2: A Concrete Residential S-ructure on FollyIsland Survived With No Damage Amongst HeavyDestructions.

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It is a frame structure with four concrete columns seatedon four 14" square piles. By discussing with the designarchitect and the County building inspector, it isconcluded that this structure fared so well for a numberof reasons:

1. Sufficient elevation (bottom of support beam atelv.15 feet MWL).

2. Deep and strong pilings (22 feet plus 5 feettip).

3. Simple and strong connections.

4. Low profile and low roof line.

5. Very rigid main columns.

Further west of West 7th Street, the water damage becameprogressively lighter but noticeable. The structures inthis section were usually newer with higher elevation andbetter construction. The west end of Folly Beach is theCounty Park where only wind damage was noticeable.

The Holiday Inn is the only substantial commercialbuilding along the open coast. It is a relatively newconstruction, built about 5 years ago. The building isat an exposed location with receding shorelines on bothsides. The front is protected by a retaining wall andheavy revetment. The building, nevertheless, sustainedheavy water damage. Practically all the ground floorfacilities were destroyed (Figure 3).

Erosion behind the retaining wall was significant,causing concrete deck to collapse and undermining theswimming pool. The seawall cap was partially destroyeddue to wave impact. The water damage was certainlyheavier than one would expect for structures of this typebuilt in such a recent date. A number of contributingfactors are listed here:

1. The elevations of the structure as well as theprotective seawall are inadequate for the surge level.

2. There is no fronting beach to dissipate waveenergy, causing heavy wave runup and overtopping.

3. The structure protrudes beyond the adjacentshorelines and there is no return wall to protect itsflanks.

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I- A

Figure 3: Photo Shown Water Damages to the Ground LevelUnits and Water Front Amenity Facilities at Holiday Inn,Folly Island, a Typical Damage Mode Among High RiseHotels and Condominiums.

Charleston and its Vicinity

City of Charleston and its surrounding areas includingMt. Pleasant and James Island, water damage to structuresrelated to flooding was minor compared with wind relateddamage(including rain water damage due to broken windowsand open roofs. The battery along the southern tip oftown provided adequate protection of city streets andriver front buildings. Hurricane Hugo was a relativelydry hurricane; heavy rain fall did not occur until twodays later. Thus, river flooding was not a major factor.High water level in the creeks, intercoastal water waysand rivers combined with high wind did cause extensiveboat damages. Numerous boats were thrown off waterlanded on shore and streets.

Sullivan's Island and Isle of Palms

Sullivan's Island and Isle of Palms belong to the samebarrier island chain separated by Breach Inlet. Theyhave a combined length of about 11 miles with the majoraxis oriented in the east-west direction. The onlyaccess to the mainland is Highway 703 via Ben SawyerMemorial Bridge. This bridge has a rotating section inthe center span for boat traffic. This section wasseverely damaged by wind during Hurricane Hugo that thebridge was closed for more than 10 days.

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On Sullivan's Island, the storm surge level was estimatedto be about 13 feet above mean sea level whereas on Isleof Palms, the level was about two feet higher reaching15 feet. The all over water damage was extensive on bothislands but the sDacial distribution was rather uneven.As the dominant wind direction at the height of thehurricane was northeasterly the damage was less severeon the western end of Sullivan's Island. This sectionwas also benefited by the wide beach. The sand overwashonto the streets was substantial. The damage east of 703was very extensive and became progressively worse towardsthe Breach Inlet. Older houses on Marshall Blvd. werepractically all destructed, some crushed by waves and theothers floated off their foundation. These older houseswere mostly constructed on shallow piles, piers or slabswith elevation around 12 feet or less. After theHurricane, many of the damaged houses were raised to 14feet or higher. A significant number of houses on thesecond row across the Marshal were also have to becondemned due to water damage. The bridge across theBreach Inlet sustained erosion at the west end but thebridge was not threatened. The jetties fared well.

Some of the newer constructions with adequate elevationsand pilings did survive even at very exposed locationseast of 29th Street. The types of damages typical to newconstructions are:

1. Erosion and scouring causing decks, pools andslabs to collapse.

2. Deck structures with access ladders or ramps tothe beach front. These type of structures wouldinvariably collapse, slamming onto the mainstructure, causing extensive damage.

On Isle of Palms, severe damages were concentrated in tworegions; one was in the commercial strip between 10thAvenue and 14th Avenue and the other was east of 42ndAvenue to 57th Avenue. The sections west of 9th andbetween 14th and 42nd, the damage was less severe. Thisappeared to be directly related to the beach width infront of the structures.

The Sea Cabin Condominiums at Ocean Blvd. and 14th Avenuesustained heavy water damage. The entire complex wasprefabricated modular units seated on pile foundation.The ground units and the end unit were completelydamaged. Inadequate elevation and poor structural memberconnections were the main reasons of failure. Twostructures on the exposed Ocean Blvd. (#126 and #912)stood out with little visible damage, wind or water(Figure 4 shows the post storm condition of #912).

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Figure 4: A WellConstructed Pile Supported- r(Wooden) Frame Residential ______--_-__

Structure On Isle of PalmFared Very Well Even In AnExposed Location.

Both structures were built by the same builder and theconstructions appeared to exceed the current buildingcode requirement. Class "B" piles (12" Dia. from buttin and 14" Dia. above ground) with 3/4" steel diagonalbracings were used. Frames were connected throughoutwith 1/2" and 3/4" plywood using #16 Galvanized pennynails at 4" intervals around corners to insure shearrigidity. Hurricane clips were properly installed atcritical connection points around roof frame and at pile-beam junctions. Roof tiles were nailed properly to 5/8"roof plywood. Ridge vent was installed to reliefpressure difference. As a consequence, only a minimalnumber of roof tiles were missing, uncharacteristic tothe general extensive roof damages in this region.Members that are subject to lift force were connected bygalvanized screws instead of nails.

Between 14th Avenue and 41st Avenue, there is no spurroad south of Hwy. 703. Therefore, all the structureswere set way back from the water line with exceptionallywide beach and dunes. Water damage to houses has beenminimal in this reach. Washover over highway wasevident.

East of 42nd Avenue, roads perpendicular to Hwy. 703extend to the south close to the beach. Houses at theend, sometimes to second and third rows were mostlydevastated due to the combined effects of high surgelevel and the close distance to the ocen water. Unlikesome of the devastated areas mentioned earlier, most ofthe houses here were up-scale newer structures. Damageswere mainly due to the dynamic forces as oppose toflooding. This section provided a classic example ofdiminishing damage as a function of increasing distanceto the shoreline. On the 50th Avenue, there were 5

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houses in a row. The first two were completely leveledto the ground; the third, and fourth houses sustainedprogressively less damage, some due to the debris of thedestructed houses, and the fifth house which was thefarthermost from the shore had no evident water damage.

At the eastern tip of Isle of Palms is the Wild Duneluxurious condominium complex and the only high risecommunity on the island. Water damage was limited towashed away walk ways, local scourings near foundationand ground level utilities. Since this area bore thefront assault of hurricane winds, tree, roof, window andchimney damages were extensive.

McClellanville and Vicinity

McClellanville is a fishing village dominated by modestwood frame houses. Mobile homes are also common in thisregion. Since the estimated landfall of the hurricanecenter was Bulls Bay just south of the town the stormsurge was the highest reaching 20 feet or higher. Waterdamage was very extensive. Unlike on the barrier island,however, damage was mainly due to flooding.

Northern Coast of South Carolina

The northern coast known as the "Grand Strand" area hasdeveloped rapidly in recent years. Non conformitystructures also flourished. Until the last two years orso, building code enforcement was rather lax and reliedon individual communities. Now, Horry County which hasthe jurisdiction over most of the affected areas hasadopted the Southern Standard Building Code and isintended to strengthen the enforcement and inspectionprocedures.

Pawleys Island is a relatively old residential community.Houses are very closely spaced. Quality of construction,in general, was marginal at best. The storm surge levelwas about 13 feet. Beach was very narrow during posthurricane survey. Emergency measure was taken bytrucking sand from the south end near North Inlet torepair the beach on the north. Water damage was verysevere due to the combined factors stated above.

Garden City is also primarily a residential communitywith a few condominiums and a controlled-access plannedcommunity at the south end. The main thoroughfare is theWaccamaw Ave which is also the first street parallel tothe beach. The road surface is about 8 to 11 feet aboveMSL. The measured storm surge was about 13 feet.However, local residence claimed the water level to bemuch higher along the coastal front of over 20 feet.Salt spray was estimated to reach 200 to 300 feet right

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after the passage of the Hurricane. This was the firstobservation of this kind and was based on the observationof Mr. Carlos Fredes who is also a county buildingofficial and lives on the island. He made his estimateon the mist level on a transmitting tower that has astring of stroboscopic lights.

The water damage and the sand washover were among theworst of all the areas inspected. Water damages due toflooding can be traced as far back as 1500 feet, or fouror five rows deed from the first street; the sandwashover was measured to top 1 foot on the second row ofhouses (north of Waccamaw Avenue see Figure 5). Rightafter the hurricane, debris were piled 20 feet high atintersections, made passage to the second row impossiblefrom either side.

------ --- ME

Figure 5: Sand Washover on Second Row of Houses (Northof Waccamaw Avenue, Garden City).

On the north end of Garden City, south of AtlanticAvenue, the destructions on the first row were almosttotal for a stretch of 4 or 5 blocks. Completedestructicns on second row were also common in thisreach. Two fishing piers, each 1000 feet long alsodisappeared completely. Most of the structures here,commercial or residential, were constructed 10 years ago,some more than 20 or 30 years old. Most of them were onshallow pier footings and some were on slabs. Judgingby the total crushing pattern and by the form of pierbreakage, waves might have overtopped the structures.

The water damages were still very heavy south from thislocation till the Waccamaw Avenue took a dog-lagged turn

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towards the north. In this reach, the constructions werea mixture of old and new. Some of the old ones wereconstructed about 20 years ago and the new ones were from5 tc 1 years old. Like the north reach, the oldstructures were practically wiped out completely. Thenew ones, particularly those built within the last twoyears, appeared to have largely survived with varyingdegrees of damage. However, not all the newconstructions made it. A concrete two story structuresbuilt 5 years ago by an architect collapsed completely(Figure 6). This structure had heavy reinforced concreteroof beams and precasted concrete walls. But it wassupported by cinder-block piers and bearing walls on apoured shallow footing. Waves must have hit thestructure causing the concrete front walls to collapseand the piers on the back to buckle. This concretestructure indeed provided a drastic contrast to theconcrete structure on Folly Beach discussed earlier.

(Gar--n City) .

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Ficcre 6: A Two StoryConcrete ResidentialStructure CollapsedUnder Its Own WeihtDue to Weak Foundation(Garden City).

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The loss of beach and scouring around foundations werealso among the severest we have witnessed along theentire coast affected by Hugo. Beach loss up to 6 feetof sand around structural piles were common. Anadditional 1 to 1 1/2 feet of scouring were measuredaround some large piles.

Further south on Waccamaw Avenue, water damage becameconsiderably lighter. South of #336 (the junction of adog-legged turn), only three houses were destroyed. Thehouses are all newer here. The beach is also wider thannorth because of the dog-legged turn towards inland.

Surfside is connected to the Garden City on the north.Unlike Garden City, it is full of commercial structuresalong the beach, mainly condominiums and high rises.Some older commercial establishment just north ofAtlantic Avenue sustained considerable damage. The 1stfloor of Ocean View Motel, for instance, was a totalloss. Water damage became less severe further northalthough a few condos sustained considerable wind damage.The nature of water damage was also quite different fromother regions discussed. Septic tanks, drain field andsewer lines were the most common components receivingdamage because of erosion. Swimming pools, spas, decksand other water front auxiliary structures protected byseawalls and/or revetment also suffered heavy damage dueto undermining. Seawalls, new or old, were found to beof insufficient elevation. Most of the return walls weretoo short and structurally inadequate.

Myrtle Beach and North Myrtle Beach were on the northernfringe of Hurricane influence. Surge level was alsoestablished around 13 feet. Water damage, however, wasmuch lighter compared to Garden City or Surfside.Exposed septic tanks and sewer lines were visible at anumber of locations, particularly between buildings wherethe sand losses were severer than at structural front.Unlike Surfside, many waterfront swimming pools and spassurvived structurally, although the quality of protectiveseawall structures were found to be of similar qualityas those on Surfside. This might be partially attributedto the recent beach nourishment along the Myrtle Beach.

Performance Assessment

Based upon damage surveys, factors that critically affectthe structural performance against water loads arediscussed here.

Foundation

In assessing water damage to coastal construction, themain focal point is the foundation, that is, what is the

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type of foundation? where is it located? and how thesuper-structure is connected to the foundation?

In terms of foundation types, one found a great varietyof them along this coast. Generally, they fall into oneof the three categories - slab on grade and pouredfootings, piers of various material and construction andpiles.

Slab on grade and poured footings were shown over andover again unfit for coastal application. Hogo'sexperience only strengthens the verdict because of thehigh storm surge. Structures on such foundation havevery little chance to escape severe damage if water levelreached above the foundation. Two common modes offailures were observed, the structure being totallydemolished by the dynamic force as illustrated in Figure7 or the structure being picked up by the flooding waterand floated away as shown to be the case in Figure 8.Pier foundation is permitted by FEMA's guidelines forresidential buildings as well as most of the buildingcodes. Hugo's experience showed that this type offoundation is very vulnerable in the dynamic water forcezone. Failures were frequent. The inherent problem ofthis type of foundations is the shallowness of thefootings as most of them were dug and poured in-place.As the overburden material began to erode away, thefootings simply toppled as shown in Figure 9. Bulkyshallow footings fared the worst as they often promotelocal scouring up to 1 to 1.5 feet (Figure 10).

Figure 7: Structure With Foundation On Grade TotallyDemolished By Wave Force.

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Figure 8: (a) Structure Built On Slab Floated Away FromFoundation And Deposited 100 Feet Away Across the Street.(b) The Arrancement Inside the Kitchen RemainedUndisturbed in the House.

-- - - . - -

Figure 9: Shallow Pier Footings Up Rccted.

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Figure 10: Bulky Shallow Footing Promoted ScouringAround It.

In the above ground section, masonry pier is inferior towood or poured concrete piers. Masonry pier often faileddue to inadequate reinforcement, sloppy job on masonryfills and weak joints between blocks (Figure 11).

Figure 11: Masonry Pier Made of Concrete Blocks FailedDue to weak Joint and Inadequate Fill.

Failure of poured concrete piers were often due toinadequate or improper placement of reinforcing bars(Figure 12).

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---- *l-- y ^ • -B-•- °. • •

(a) (b)

Figure 12: (a) Poured Concrete Piers Buckled UnderLateral Load. (b) Broken Piers Revealed InadequatePlacement of Reinforcing Bars.

The joint between the pier and the footing was also acommon place of failure (Figure 13). Poor constructionpractice was prevalent among older structures.

Figure 13: Bad Joint Between Pier and Footing Was aCcmrmon Failure Made.

Pile foundation is becoming the standard for modernconstructions. Properly sized and installed piles

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performed invariably well. It should be the only typeof foundation allowed in dynamic zone. Failures weremainly due to rot. Failures attributed to inadequatepenetration and pile size were also found in a fewoccasions. Both concrete and wooden piles were found tobe effective. Concrete piles usually showed goodrigidity and require simpler connections. Corrosion ofre-bars and their subsequent expansion was the mainsource that weakened the pile and led to eventualfailures. Brittle failure was also evident in a numberof cases while the pile size might be inadequate. Woodenpiles require more lateral bracings and closer attentionto connections as they are more flexible. Steel rodbracing appeared to perform better than wood boardbracing for two main reasons: wood bracings weresusceptible to break or buckle under wave force and theytended to separate or rotate from the main members. Thelatter was because many of them were secured by nailinginstead of bolts and nuts commonly used in metalbracings. For wooden pile, round piles appeared toperform better than square piles although some may prefersquare pile for aesthetic reason. For square piles, thehardened outer layers were often partially sawed offexposing the younger inner fibers.

Elevation

Elevation undoubtedly was the dominant factor.Practically all residential type of structures sustainedmajor damage if not complete destruction if the elevationwas inadequate. Several modes of failures were observed:structures collapsed under wave force; structuresfloated away from foundations; water went through thestructures and took every thing with it. For multi-storybuildings, damage usually was associated with the thirdmode. The first two modes were more or less related tofoundation as discussed earlier.

Set Back

There was a definite correlation between the extent ofwater damage and the set back of the structure. Widerbeach clearly provided better protection by dissipatingwave energy and retarding erosion when the storm surgelevel was not excessive. When the storm surge level'significantly exceeded the dune, such beneficial effectsappeared to rapidly diminish.

Appurtenant Structures

A surprising amount of damage was caused by failures ofappurtenant structures. Deck structures with accessladders or ramps to the beach were a main source ofproblem. They were not designed to resist water forces,

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yet, most of them were secured to the main structures.Water forces would most certainly destruct them which inturn would Dunc:ure or tear apart that cortion of themain structure connected to them (Figure 14). Groundfloor garage doors, air conditioners, water tanks, non-break-away walls, etc., all contributed to additionaldamages to the main structures. Revetment armor unitssometimes behaved like missiles powered by waves asdiscussed earlier.

Figure 14: (a) House With Access Ramp to Beach (Priorto Hugo).

Fi-gre 14: (b) The Access Ramp Washed Into the House Dueto Wave Force.

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Cornections

Cocnections play a major role in wind damage but a lesserone in water damace. Improper connections between floorbeams and foundations caused the structures to floataway, to collapse and to lean or shift. Figure 15shcws an example of a modular unit completely separatedfrom the foundaticn piles. Improper nailing practice andinadequate sizes of nails were a prevailing practicecausing member separations under minor shift (Figure 16).Missing and inadequate hurricane clips were alsoprevalent. Mobile home tie downs were found to becomon. Very few mobile home units were found near thecoast. One small mobile home park sou:h of Myrtle Beachdid incur heavy damage, but was mainly due to inadequateelevation as opposed to inadeauate tie downs.

Figure 15: (a) Modular Unit Constructions With PoorConnections

Figure 15: (b) The End Unit of the Modular StructureSimply Disconnected From the Piles.

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Figure 16: Inadequate and Improper Nailings Caused theFloor Joist to Separate From Floor Beams.

Coastal Structures

1. Seawalls: Sea walls are numerous in the affectedregion. Most of them are actually just retaining wallsbuilt for the primary purpose of supporting amenityfacilities or lawns. And, most of them are grosslyinadequate in height to protect the upland structuresfrom water damage. Scouring behind seawalls due toovertopping was the most prevalent damage mode observedwhich then led to failures of decks and pools orcollapsing of seawalls themselves (Figure 17). Inaddition, most of the return walls were underdesignedresulting in numerous failures (Figure 18). Once thereturn walls failed, water quickly rushed in from behindlike a breached dam causing rapid losses of material.

... ii i

• --. I ! "

__

Figure 17: Overtopping Induced Seawall Failure.

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Figure 18: Underdesigned Return Wall Was CommonOccurrences.

2. Revetment, groins and Jetties: Like seawall,revetments are everywhere in the affected area. They areparticularly common on Folly Beach, Sullivan's Island andPawley Island. Most of them are not engineered work butconstructed by local contractors or even ownersthemselves. The quality was evidently poor withinsufficient height and inadequate armor size. Some weresingle layer piled on bare soil. Those structures shouldnot be expected to serve their intended function and theycertainly didn't. The armor units on the revetment slopeowing to insufficient height often became missileshitting structures behind, thus causing more harm thangood. Some did perform marginally in retarding erosionbehind the structure.

Along Folly Beach, Sullivan's Island, Isle of Palm,Pawley Island and Garden City, groins are interspersedwith few field of heavy density. The damage to them wassurprisingly light possibly because the high surge levelsimply submerged them. Jetties at a number of inletentrances also appeared to fared well possibly for thesame reason.

3. Piers: Practically all the piers along this coastsustained severe damage, if not totally destroyed. Thefew surviving ones had their mid sections across the surfzcne missing (Figure 19), which seemed to indicate wherethe most destructive water force had occurred. Thesepiers, though, might not have savage value.

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Figure 19: Piers With Mid-Section Missing Revealed theLocation of Maximum Dynamic Force to be in Surf Zone.

CONCLUSIONS

Hurricane Hugo inflicted very severe water damage onstructures near the coast. The general consensus wasthat in the zones reached by water force, the overalldamage incurred by water far exceeded that due to wind.Under such circumstances, attempts to separate wind andwater damages were often superfluous. This was quitedifferent from some of the recent hurricanes such asAlicia and Diana where wind was clearly the dominantdestructive force.

Damage reconnaissance was carried out by visualinspection and personal interviews. The importantfindings are summarized here:

1. Water damage was extensive but damage distributionwas very uneven spatially.

2. Most of the well engineered and well constructedstructures survived, some with very li-tle damage. Bothconcrete and wood structures demonstrated theirsurvivability. Wood structures usually require much more

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attention in details, that is, there are more componentsand joints which could go wrong here than concretest rctures.

3. Wide beaches and high dunes are definitely beneficialprovided the surge level is not significantly higher thanthe dune elevation. For extremely high surge level, thebeneficial effect, if any, is unclear.

4. Elevation appeared to be a pre-requisite if thestructure were to escape severe water damage.

5. Of the variety of the foundation types only deeppilings of sufficient sizes (over 9 inches at least)performed consistently.

6. Appurtenant Structures were a significant factorcontributing to damage.

7. Most of the protective structures were ineffectivefor lack of proper engineering.

8. Structures, buildings and protective structures,appeared to retard local beach erosion.

RECOMMENDATIONS

Structures built on open coast should be designed toavoid water force rather than resisting it. Deep pileis the only structural element that could be used, withinreasonable cost, to resist hurricane induced water force.Other structural elements, if have to be exposed in thiswater force zone, should be designed to break away underloading. Structural set back is important for a numberof reasons; it reduces the cost of construction; itreduces the vulnerability of being exposed to dynamicwater forces and it reduces damage. Figureillustrates the importance of elevation and setback.Clearly, the closer to the water line the higher thestructure, the deeper the foundation and the stronger thefoundation are required. Both the water crest line andthe scouring line shown in Figure 20 should be respectedin coastal construction.

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20

Before Hurricane- Hugo . - Dynamic

15- Zone

- 8t---- -Storm Surge10- " > "- 0T --

• .~I 'iEroslon.

> After Hurricane.5 - Hugo

Scou>.< Line; .>

-5

Z-10

5-

-15

Figure 20: Sketch Shows the Beneficial Effect of Setbackon Structural Foundation.

Further research should be conducted to simplifystructural connections. In the meantime, connectionsshould be designed and constructed exceeding the StandardDesign Code to minimize repair cost.

Protective structures are special structures and shouldbe engineered by personnel with special expertise. Thesestructures owing to their important function should beregulated much the same as buildings in terms ofengineering, code enforcement and inspections. The largenumber of inadequate structures found along this coastwas the consequence of rampant unregulated activities.An inadequate, non-engineered structure not only createsa false sense of security to the upland owners but couldalso have adverse effects on the surrounding.Furthermore, those home-made structures with noengineering and environmental considerations only serveto reinforce the public's perception that coastalstructures are uniformly harmful. As a consequence,coastal structures with legitimate purposes are becomingincreasingly more difficult to be accepted.

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Finally, one must realize that the Standard Building Codeis a minimum standard. Certain acceptable risk and levelof damage are expected. Revision of building code is notneeded. Code enforcement and field inspection aredefinitely the weak links from the Hugo experience.

ACKNOWLEDGEMENT

This article grows out from a team report prepared by theHugo Disaster Survey Team organized by the Committee onNatural Disasters, National Academy of Engineering. Theauthor wishes to acknowledge Dr. Dean, Messrs Sam Houstonand Mill Dowd all from University of Florida for theirparticipation. The author also wishes to thank Mr.Carlos R. Fredes and Mr. Karl Simmons, Building Officialsof Horry County and Charleston County, respectively andMr. Ken Hancuff of Isle of Palm for their valuablediscussions.

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