Real Time Current Measurement System:
As a result of increasing export/import shipping and cruise line activities, thePort of Miami (POM) is currently conducting a major expansion of the turning basin at the south side of the Dodge Island port facility. This project will resultin the need to relocate up to 20,000 cubic yards of dredge material,from the port, daily for several years. The approved site for placement of the dredge material is known as the Offshore Dredge Material Disposal Site (ODMDS). The ODMDS constitutes a 1 nautical mile (NM) square oriented along lines of latitude and longitude.

About one mile west of the western boundary of the site is an extensive area of environmentally and commercially valuable coral reefs. This sensitive area is under both State and Federal jurisdiction.

RSD, in cooperation with the Rosenstiel School of Marine and Atmospheric Science of the University of Miami, was called upon to design a discharge procedure which would allow port dredging to occur while protecting the aforementioned coral reefs. As a result, a novel discharge and permitting approach was developed. Discharge is not to occur at the site if the westerly component of the ambient current exceeds 12 cm/sec.

For the first time, near real time measurements of oceanic current speed and direction at the disposal site are being used to regulate the disposal of dredge material. This requirement has led to the need to place a Real Time Current Measurement System (RTCMS) on the ocean bottom near the disposal site. Discharge is not permitted to occur unless ambient currents do not transport material toward the coral reefs. A complex but highly successful linkage of scientific data and management decision has been achieved. The system utilizes a 150 kHz upward looking Acoustic Doppler Current Profiler (ADCP) to obtain water column current profiles every twenty minutes. A running one hour temporal average of the currents, averaged vertically over the top fifty meters of the water column is provided every twenty minutes. If the westerly component of the vertically and temporally averaged current exceeds 12 cm/sec an
advice of no-discharge is rendered to the dredging company. This project may serve as a prototype for port dredging projects nationally in the future.

The RTCMS consists of six distinct subsystems

• A Tethered ADCP
• 2 suspended eloctromechanical cables (1,250') moored at 2 points
• A deep water (60'-500') electromechanical bottom cable. (12,000')
• A shallow water (20'-60') electromechanical bottom cable. (6,000').
• A microprocessor controlled radio modem.
• A data recording and processing computer.

  Shown below are four sequential acoustic visualizations of a plume of coastal ocean discharged dredge material from the Port of Miami. These images show the distribution of plume material as a function of depth, distance, and time. The first image (a) was gathered by a ship towing an acoustical device over the discharge site about one minute
  after completion of discharge. Note that the plume has impacted the bottom (120 meters depth) and generated a bottom surge. Images (b), (c), and (d) were made at increasing times after discharge. The plume material remaining within the water column (called the "residual" plume) moves with ambient currents. The ambient currents are monitored, in real time, using the RTCMS system described above. This is an example of OAA/RSD efforts to move from a reactive to a preventive mode in support of environmentally sustainable economic development.
  

  
  

  Acoustic iso-concentration contours of four transects. The gap in concentration indicated in (a) at horizontal distance coordinate of 90 to 100 m is attributed to acoustic absorption at the frequency of 20 kHz by a cloud of bubbles in the water near the surface. Horizontal distances in (a) and (b) are greater than those of (c) and (d) in
  order to present the whole feature of the surge plume.

  City Of Miami
  Phod ADCP Data