Problem Background
Shoaling at the Port of Anchorage during the summer months has required
annual dredging that averages between 152,911 and 305,821.9 cu m/year
(200,000 and 400,000 cu yd/year) with occasional larger deposition quantities of
between 611,643.9 and 764,554.9 cu m (800,000 and 1,000,000 cu yd). The
deposited material is mostly fine silts. Because there is no way to predict the
larger deposition episodes, the Alaska District budgets for typical dredging
cycles, and then has to perform emergency dredging during years when greater
deposition occurs. Emergency dredging involves substantial mobilization costs,
which must be covered by additional appropriation and by shifting funds within
the operations and maintenance budget.
The Alaska District is undertaking studies to examine several questions
related to the operation and maintenance of the Port of Anchorage. In particular,
the District is interested in:
a. Determining the source(s) of fine sediment being deposited in Anchorage
Harbor.
b. Devising some means of predicting in advance when increased sediment
deposition is expected so adequate funds can be allocated a priori.
c. Examining potential harbor modifications that might decrease deposition
within the harbor.
A key component to answering these questions is better understanding of
the hydrodynamic regime. Tidal flows with maximum currents above 2 m/sec
(6.56 ft.sec) (4 knots) result from the 10-m (33-ft) tide range. These currents
are capable of transporting huge quantities of fine sediment eroded from the
extensive tidal flats upstream and downstream of Anchorage. Prominent
headlands such as Point MacKenzie, Point Woronzof, and Cairn Point cause
separation of the tidal flow along with generation of gyres and formation of
reduced flow areas in the lee of the headlands. The whole process of flow
separation, flow entrainment, and lateral shearing of the currents leads to
significant flow variations both through the water column and laterally across
the inlet. If these three-dimensional (3-D) hydrodynamic processes contribute
substantially to shoaling of the Port of Anchorage, then it is critical that
simulation tools used to study the problem include 3-D capabilities.
Study Purpose and Components
The study performed at the U.S. Army Engineer Research and Development
Center's (ERDC) Coastal and Hydraulics Laboratory (CHL) had two principal
components which are discussed in the following paragraphs.
Flow table physical models
The first component was to use small-scale physical models to examine the
complex flow conditions in upper Cook Inlet. Under steady flow conditions
1
Units of measurement in the text of this report are shown in SI units followed by non-SI units in
parenthesis. A table of factors for converting non-SI to SI units of measusrement used in figures
and tables is presented on page xiii.
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Chapter 1 Introduction
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