'AD-AlES L16 THE EFFECT OF SUBNERGED NORFOLK BAR ON NAVE CONDITIONS 1/i(U) ARNY ENGINEER NRTERHRYS EXPERIMENT STATIONVICKSBURG HS S A HUGHES 31 NAY 83

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'JAME OF PERFORMING ORGANIZATION Ib OFFICE SYMBOL 7a. NAME OF MONITORING ORGANIZATION. U.S. Army Engieer Waterways (If applicable) U.S. Army Corps of Engineers,

( Experiment Station, C.E.R.C. CERRE-CO Norfolk District,DOPESS (Cry, Stare, and ZIPCode) 7b. ADDRESS (City, State, and ZIP Code)

IC Vicksburg. Norfolk, Virginia 23510-1096'OJAME OF FUNDINGISPONSORING 6b. OFFICE SYMBOL 9. PROCUREMENT INSTRUMENT IDENTIFICATION NUMBER* _ )RGAZATION U.S. Army Corps of ( appIECJb/c)

bizineers. Norfolk Disftriet NAOPT./NAOnFs ,,,_ _-_-_-04ORESS (City, State. and ZIP Code) 10. SOURCE OF FUNOING NUMBERS

PROGRAM PROJECT TASK WORK UNITELEMENT NO. NO. NO ACCESSION NO.

" The Effect of Submerged Norfolk Bar on Wave Conditions

12. PERSONAL AUTHOR(S)

Hughes, tevpn A-

13a. TYPE OF REPORT -13b. T1ME COVERED 114. DATE OF REPORT (Year, Mon, 0y)hS. PAGE COUNT -Final ________1983. May 31-- 9___"-"_,

17. COSATI CODES 1S. SUBJECT TERMS (Continue oA revor if nectuaty and identr by block numb.,)FIELD 'ROUP SUB-GROUP . :

" wave conditions, submerged offshore bar, wave climate,mathematical model, wave refraction, storm waves, sediment

,9 AiSTRACT (Continue on .ever.e if n.cena.y and identify by block numb.. r.l he purpose of this study was to examine, through use of a mathematical model, the altera-

tions in the wave climate caused by a submerged offshore bar (offshore Virginia Beach/Norfolk). Conclusion was that in all cases a phase lag developed as the wave passed over th4bar, resulting in wave refraction around the ends of the bar. The refraction appeared to berelatively minor with little noticable effect on the adjacent shoreline. The estimatedbottom velocities at the bar crest are well above those necessary for the initiation of sandtransport, and a reworking of the bar material can be expected.~DT1 C

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00 FORM 1473, 34 MAR 63 APR edition may be sed It. euMm$utesd SECURITY CLASSIFICATION OF THIS PAGEJ ~All Other .dtonl are ob~o'etg."";. .S4 3 t */DqS

- %

CERRE-CO 31 May 1983

MEMORANDUM FOR RECORD

SUBJECT: The Effect of the Submerged Norfolk Bar on Wave Conditions

1. PURPOSE. The purpose of this memorandum is to convey the results ofstudies carried out by the Coastal Oceanography Branch into the alterations I ,,in wave climate caused by a submerged bar at Norfolk.

2. METHOD. The DHI System 21, Mark 8 model was used in the short wavemode to examine the wave conditions with and without the submerged bar.This numerical model solves the vertically integrated equations of conser-vation of volume and momentum (the Saint Venant equation) by implicitfinite difference techniques with variables defined on a uniform rectangulargrid. In the short wave simulation corrections for the nonhydrostatic pres-sure distribution due to vertical accelerations are accounted for by includ-ing the Boussinesq terms. A regular cnoidal wave was used as input to themodel, and no attempt was made to simulate the tidal current flow whichmight be present near the mouth of the Chesapeake.

3. PARAMETER SELECTION.a) Bathymetry: The bathymetry for the model was approximated using

charts of the region in the vicinity of the bar placement. Over the areabeing modeled (900 meters by 1200 meters, see Encl. 1) the bottom can betaken as a composite of two uniform slopes (Encl. 2). Based on informationfurnished by Dr. Weishar of Coastal Processes Branch, the submerged barwas approximated as shown in Encl. 2. The top of the bar was about 7.5

0 meters below mean low water and the onshore foot of the bar began at thei. -9m (MLW) contour. The sides of the bar had a slope of.1/lll which was

thought to be close to the expected angle of response of the -placed dredgematerial. This slope was maintained at the head of the bar as well (Encl 1).

b) Wave Conditions: The model was run for three sets of wave condi-* tions representing swell, storm seas, and extreme storm- seas. %e1WS• a swell represented with height (H) - 0.5 meters and period (T) - 14.4

seconds. These are long waves which occur at Norfolk at a freguency of1.2 percent of the time. The second case ves typlca of a northeasterwith H - 3'5 and T8 sec. This is a steep wave which has about o.6 Rercentoccurrence. The frinal case represents the extreme event v th He 4m'ind T. 12-seconds. Frequency of occurrence of this wave is 0.03 percent (Encl. 3).In all instances the incoming wave crest approacbed piarallel o the barcrest and bottom contours.

Of EROlneer

Report a- 4

- - -. -. I

-CERRE-O 31 May 1983

SUBJECT: The Effect of the Submerged Norfolk Bar on Wave Conditions

"c) Model Parameters: In order to resolve the selected wave conditions

* the model grid spacing was selected as AX - AY = 8 meters. This providedalmost 9 points per wavelength in the shortest wave case. Similarly, thetime step was chosen to be AT=0.8 seconds in order to properly simulatethe wave translation. The model was run for 200 times steps, the timenecessary for all transients and the first full wave to pass through the

.model.

4. SWELL WAVE RESULTS. Encl. 4 presents the results of the swell wavetests. The wave trains shown can be likened to an instantaneous onshore-offshore cross-section. The presence of the bar in this case causes asteepening of the waves as they traverse the bar with a resultant phaseshift of about 0.15 wavelength or about 2.2 second lag. Any attenuationof wave height is not discernible after the passage of the wave over thebar. In a two-dimensional sense, the presence of the bar merely steepens thewaves temporarily before they continue shoreward, phase shifted. Refractioneffects are discussed later. Calculated maximum bottom velcities bystream function theory at the crest of the submerged bar are on the orderof 50 cm/s. 16. .,

5. STORM WAVE RESULTS. Encl. 5 gives the results for the case of H = 3m andT-8 seconds. As in the swell wave case, the bar steepens the waves andcauses a phase shift on the order of 2.2 seconds. This temporar- shorteningof the wavelength causes the wave to heighten to conserve energy. Thishigher wave then propagates shoreward with a phase shift as mentioned. Theincreased wave height in the barred case is approximately 0.4 meters or 13%of the _ave height. Actually the same process occurs during the swell wavecase, but it is not really large enough to be noticeable on the plot. Esti-mated bottom velocities using stream function theory are on the order of 190 em/.

6. EXTREME CASE. Encl. 6 details the results of waves having H - 4 m, T = 12s.As can be seen a similar phase shifting has occurred as was evidenced in theearlier cases. However, the results also indicate that this case is pushing

the stability limits of the numerical scheme, and hence, cannot be consideredreliable. Estimated maximum horizontal bottom velocities at the top of thebar in this case are around 230 cm/s. .

7. REFRACTION. The phosedifference between waves passing over the bar andthose passing by either end will produce some refraction of the wave intothe region in the lee of the submerged bar. Model simulation results showthis refraction for the storm wave case. but the effect does not app@a tpbe very significant. Since the model didn't continue to the shoreline, itis difficult to surmise if these refraction effects will have any impact onthe shoreline. It is intuitively felt that any effect will be minor andwithout conseauence.

86 3 11: 048S1h 0 1-

CERRE-CO 31 May 1983SUBJECT: The Effect of the Submerged Norfolk Bar on Wave Conditions

8. CONCLUSIONS. The selected wave cases run in the Model vere chosen asbeing most likely'to be affected by the presence of a submerged offshore bar.In all cases a phase lag develops as the wave passes over the bar, resultingin vave refraction around the ends of the bar. This refraction appears tobe relatively minor-vith little noticeable effect the acent shoreline.e estimated bottom velocities at the bar crest are yl. above that necessary

for the initiation of sand transport -and a revorking of the bar materia1 . .-'.*should be expected.

A. NTB YEN A. HUGHES

Hydraulic EngineerCoastal Oceanography Branch

CF:EvertsParker w**Whalin

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