Date of Award

Spring 5-1999

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Civil/Environmental Engineering

Program/Concentration

Civil Engineering

Committee Director

David R. Basco

Committee Member

Jaewan Yoon

Committee Member

George F. Oertel

Call Number for Print

Special Collections LD4331.E54 L66

Abstract

Coastal communities rely on the beaches to attract tourism, but these sources of revenue require maintenance, such as beach nourishment. In an effort to maintain these beaches without causing any detrimental results, research is conducted to determine what effects, if any, exist when mining for sand in offshore areas.

The U.S. Government is permitting the search for beach quality sands offshore but is also ensuring that such mining will not have adverse results to the nearshore wave climate. This study was conducted using a coarse grid bathymetric map to calibrate a numerical model developed by the Danish Hydraulic Institute (DHI). Upon establishing calibration coefficients, a fine grid bathymetric map of Virginia Beach, Virginia was obtained from the Virginia Institute of Marine Science (VIMS) and subsequently used as a case study to evaluate effects of offshore mining to nearshore wave climate. Hindcast wave information from a 20-year time series was obtained from the Wave Information Study (WIS) of the Coastal Engineering Research Center (CERC) of the U.S. Army Corps of Engineers. This time series was statistically categorized in seven wave period and angle bands by CERC's computer program, WHEREWA V (Gravens, 1992). The resulting bands were then modeled with the DHI software, using the bathymetric map from VIMS.

A fixed reference for the change in wave climate from before and after dredging, was establish with a reference line parallel to the shoreline. This reference line was found by using the more significant wave events from the WIS wave hindcast information. A trial and error method was employed to determine the location of this line along the study area where seaward of this line, waves would not break. The line was located at a depth of 6.5 meters where 99.1 % of all waves would not break seaward of this line.

In modeling the affects of offshore mining for a before and after comparison, a unit wave height was used to establish wave height multipliers for the area prior to and after 50 years of mining. The WIS time series was analyzed to determine the number of events for each wave height, at each wave direction, for each of the wave periods. The weighted average wave energy density was then calculated for each of 100 points along the reference line for before and after dredging and the percent change was determined.

Test of a two-sided, and subsequently a one-sided hypothesis was conducted on the results to determine if a potential borrow area should be dredged. For this case study, there was insufficient evidence to support the null hypothesis that there would be no change in the nearshore wave climate as a result of 50 years of mining from that shoal. In summary, the wave climate will experience significant change as a result of offshore mining, established at the 95% confidence level ( a.=0.05) with a P-value of 0.0079 from the t-statistics.

Additional study was conducted to generalize if the slope and/or median grain size diameter would have an effect on the results. Equilibrium profiles were created for the input bathymetry in the numerical models by closely matching the mean slope of a very shallow coastline and a relatively steep coastline. The wave climate from the Gulf of Mexico and Atlantic Ocean was used as input parameters in the numerical modeling for mild and more active wave climate, respectively. Wave period and angle bands were derived from WHEREW AV and the numerical modeling was conducted with the same method as the case study. The results of the generalized study were similar to the case study, with a level of confidence for rejecting the null hypothesis, at 95%.

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DOI

10.25777/p98y-rm66

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