Water Quality at Marinas: Flushing
The demand for marinas is on the rise in Florida and the
Caribbean. Waterfront properties are continually being developed
at a fast pace. Either by reducing available land for marina
and docking facilities. Furthermore, the trend of waterfront
development is increasingly private which limits public access.
There are few remaining natural marina sites and stringent
regulatory environmental policies preclude marina development.
Additionally, strict regulatory policies on ownership of submerged
lands need to be addressed and may introduce further complexity
to such projects. To overcome these difficulties, developers
and engineers are seeking alternatives for marina development.
One of the alternatives is to create a marina basin and navigation
channel by excavating upland properties.
A major concern for these new basins as well as existing
facilities is water quality. Proper flushing is required to
ensure that the water quality is maintained and to avoid debris
accumulation. For any proposed marina development, professional
engineering assistance must be retained to thoroughly study
the flushing characteristics of the proposed marina and to
demonstrate minimal environmental impact. Very often, the
analysis of tidal hydraulics in conjunction with complex geometries
of basins with docks requires advanced numerical analysis
that are beyond desktop engineering solutions. In such cases,
sophisticated computer generated numerical models are used
to study flushing characteristics through numerical simulation
of tidal hydrodynamics. Through a calibration procedure against
field tide/current measurement, the advanced numerical models
can further be used to optimize flushing characteristics by
evaluating alternative configurations. This perspective will
discuss hydrodynamic numerical models that can be used to
study flushing characteristics of a proposed basin, and present
three case studies of flushing studies completed for marina
To simulate tidal hydraulics and to evaluate flushing characteristics
of a marina basin, sophisticated engineering software is required.
Among several advanced two-dimensional hydrodynamic numerical
models, the state-of-the-art MIKE21 HD Model, developed by
the Danish Hydraulics Institute, is commercially available.
The MIKE21 HD Module simulates the water level variations
and flows in response to a variety of forcing functions in
lakes, estuaries and coastal areas. The water levels and flows
are resolved on a rectangular grid covering the area of interest
when provided with the bathymetry, bed resistance coefficients,
wind field, hydrographic boundary conditions, etc. The numerical
model solves the full time-dependent non-linear equations
of continuity and conservation of momentum. The solution is
obtained using an implicit finite difference scheme of second-order
Calibration of the model is essential to simulate tidal hydrodynamic
processes accurately. Generally, the model is calibrated for
existing conditions using the field data, and the calibrated
model is further modified to account for proposed basin geometries.
To calibrate a numerical model, extensive field data collection
is required such as tidal current and tidal elevation measurements
at the project site. The field data can be collected by deploying
remote sensing equipment in the form of electronic gauges
that measure both tide elevation and current velocity.
Case Study 1: Cap Cana,Dominican Republic
Cap Cana is located on the east coast of the Dominican Republic
near Punta Cana, just across the strait from Puerto Rico.
The 29,000-acre development includes five miles of shoreline.
The developer intends to excavate an entrance channel to create
a large marina basin with 1.2 miles of navigation canals to
accommodate 1,000 marina slips. This will be the largest marina
in the Caribbean and the only one between Nassau and Puerto
Rico capable of docking vessels over 150 feet in length. Flushing
and water quality were a major concern based on master plan
concepts developed for the canal/marina basin, primarily due
to the small tidal range (1.5 feet) and length of canals large
The initial marina concept that involved a single entrance
channel did not provide sufficient flushing for the marina.
Alternatives including cutting a flushing channel at various
locations were evaluated. Comprehensive numerical model simulations
were conducted to assist in the flushing analysis and optimization
of a flushing channel configuration. Further basin geometry
was refined based on the HD modeling results to avoid "no
circulation areas." The resulting simulation and refined
design of the Cap Cana Marina will enhance water quality and
assure proper flushing. The added flushing channel (Figure
2) presented the developer with valuable real estate by providing
oceanfront lots with a navigation channel for yachts.
Case Study 2: Harbour Isle, Ft. Pierce,
The developer envisioned a marina basin for this 108-acre
residential condominium development on Hutchinson Island,
in Ft. Pierce, Florida.
The basin would create additional waterfront property for
the condominiums and safe harbor for a 63-slip marina. Initial
concept designs for the basin did not provide sufficient flushing
to meet regulatory flushing criteria due to the small tidal
fluctuation of 2 feet from the Intercostal Waterway and the
proposed length of the entrance channel. As an alternative,
a flushing culvert was designed to access the Fort Pierce
Inlet and increase tidal flushing with the tidal phase difference
between the inlet and the waterway. The flushing of the basin
was significantly improved allowing the basin to flush in
1.6 days or 3 tidal cycles, thereby meeting the flushing criteria
established by the environmental regulatory agencies.
Case Study 3: Hurricane Cove, Miami, Florida
A new condominium development is proposed for this site to
convert the existing boat yard facilities to a residential
condominium development on the Miami River. The existing 150-slip
marina will be rehabilitated, and an existing barge basin
will be filled to create additional real estate. The Miami
River has a combination of revine flow and tidal flow, requiring
the use of the MIKE 21 HD model to simulate the site hydraulics.
To justify the environmental impact of filling of the basin
to regulatory agencies, a numerical study using MIKE21 HD
model was conducted. The model was initially set up and calibrated
against measured tide/current data for the existing conditions.
The bathymetric information of the calibrated model was then
modified to simulate various basin fill geometries. Flushing
analysis were then conducted to evaluate the impact of various
fill alternatives, and the results were submitted to the regulatory
Based on the results of the numerical model, environmental
permits were secured for the filing of the basin.
Water quality in marina basins is a major concern for
developers and regulatory agencies. Sufficient flushing must
be provided to ensure water quality in accordance with specified
standards, as well as to minimize debris collection. Advanced
numerical models are commercially available to be utilized
to simulate flow patterns of proposed/existing marina basins,
to optimize basin geometry and to evaluate flushing characteristics
for various alternatives to meet regulatory flushing criteria.
By: Coastal Systems International, Inc.
|Miamarina: Miami, Florida (In Use)
|Miamarina: Miami, Florida (Under Construction)
|Cap Cana: Initial Concept
|Cap Cana: Numerical Model of Revised Marina
|Harbour Isle Master Plan
Pre-construction Harbour Isle
|Hurricane Cove: Tidal Hydrodynamic Model
Coastal Engineering Manual
Marinas and Small Craft Harbors
MIKE 21HD Users Guide
Disclaimer: The material presented in
this perspective is for general information only. The
information should not be used without first securing
engineering advice from qualified personnel with respect
to its suitability for any application. Utilization of
this information assumes all liability arising from such