USGS Coastal & Marine Geology
Wong, F.L., Hamer, M.R., Hampton, M.A., and Torresan, M.E., 1996, Bottom Characteristics of an Ocean Disposal Site off Honolulu, Hawaii: Time-based Navigational Trackline Data Managed by Routes and Events: Redlands, California, Environmental Systems Research Institute, 1996 ESRI Users Conference Proceedings (cdrom), approx. 15 p.

GIS Analysis
Results and Conclusions
External Links

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GIS Analysis

For this study, it may appear circuitous to start and end with point features, but a mix of the points and lines with the dynamic segmentation model provided the best tools to analyze the data. Points and lines do not preserve time-sequential data, but the dynamic segmentation model falters on query and display speed. The slow query speed probably is attributable to the sequential nature of the model: commands such as EVENTLINES and EVENTLINETEXT need to visit the whole trackline to produce the segments of interest listed in the event tables (Table 5, plotrt.aml). Routes also are disrupted if two consecutive times have the same location--which could happen if the survey vessel were stationary; the duplicate records were removed for these calculations. The format of the measure item in the section attribute table of the route coverage is limited to 4,12,F,3. We have worked around this limitation by using only the day, hour, and minute fields of the time record. However, this limitation needs to be addressed if we are to work with surveys that cross year and, soon, century boundaries for which we will need more significant digits in the time field.

Sea-bottom Sedimentary Processes

Sea bottom types. The small-scale irregularities in sea-floor morphology and current directions are easily displayed in the plots derived from the observation tables. The still photographs reveal that the floor of Mamala Bay is characterized by morphologies ranging from well-formed ripples to fairly disorganized sediment surfaces (no ripples evident). (Figure 6). There is a broad spectrum of ripple types, including both symmetrical and asymmetrical forms. In profile, crests commonly are sharp (although they can be rounded), and in plan, they are straight to curved and continuous to discontinuous (Figure 5). Some ripples appear highly degraded, and others have crests that are short and disorganized. Ripple type can change significantly over a short distance (1 minute of time or 20-40 m of distance, depending on ship speed).

Observed current directions. Ripples with discernible asymmmetrical form provide an inferred current direction; symmetrical ones suggest one of two possible current directions or an oscillatory current (for example, by waves) with no net translation. These observations provide a sense of the dominant current directions in Mamala Bay, but, as with the ripple type, the current direction may also vary greatly over small distances (Figure 7). The majority of asymmetrical ripples are short-crested, have a lunate or linguoid shape, and face upslope or along-slope in a westerly to northwesterly direction; few face downslope. There are a few areas over which ripples face consistently to the east.

Supporting data from other aspects of the Mamala Bay study suggest that the ripples (and larger bedforms) appear to have been formed by episodic bottom currents, perhaps internal waves (Hampton and others, 1995). The currents are oscillatory, but they transport dredged material and native sediment primarily in a net westerly to northwesterly direction.

The observations from the photographs in conjunction with grain-size analyses provide "ground-truthing" of the features evident in the sidescan-sonar image (Figure 2). As other types of data are analyzed and mapped with tools described here, a more thorough understanding of the sedimentary processes and, consequently, the fate of the dredged material in Mamala Bay should emerge.


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Last modified: 01 Oct 97.

U.S. Department of the Interior, U.S. Geological Survey, Western Coastal and Marine Geology