CMS/DredgeModule

In 2013, work began development of a dredging module for the Coastal Modeling System through Reed & Reed Consulting. This document references some of their descriptions and initial findings.

Introduction

Dredging of federal navigation channels is one of the primary missions of USACE. The Coastal Inlets Research Program (CIRP) has determined the need to implement a dredging module within the Coastal Modeling System (CMS). This module will greatly enhance the capability of the model to support the USACE districts in their planning efforts.

A FORTRAN code module has been developed in the CMS to simulate dredge activities, herein referred to as the Dredge Module. The Dredge Module encompasses the specifications of dredging areas, dredge disposal sites and operational rules governing the dredging. The module is based on the dredge module currently documented in the Delft3D-Flow User’s Manual dated September 2010 and provides the same simulation capabilities. The module has been developed using the current CMS-Flow code structure with a single file containing a variable definition (FORTRAN Module), card reading routine, initialization routine and routines necessary to implement the dredge simulations. The Dredge Module provides specific output to record the dredging rates and volumes occurring during the simulation. The Dredge Module has been implemented and tested in both the explicit and implicit solvers. The Dredge Module was designed so that it can be implemented in the current SMS 11.1+ version without GUI modifications. The module will make use of the existing advanced card capabilities and the data set creation capabilities for specifying dredge source and dredge placement areas. The module was developed in two phases. First, to accommodate single grain size simulations and, second, to extend the dredge module for use with multiple sediment grain sizes.

A dredging module interface was developed for SMS version 13.0 and some modifications to the initial CMS Dredge Module code were made to accommodate the new interface, but is backward compatible to the initial CMS file inputs and data files.

Overview

The dredge module is implemented in the CMS as a subroutine call from the explicit or implicit flow solver routines. The dredge simulation's primary interaction with the CMS is a modification of the morphology by adjusting the bottom elevation of cells that are in the user defined dredge areas or placement areas. When certain criteria are met, the bottom elevations of cells in the dredge area are lowered to simulate the removal of sediment and the bottom elevations of cells in the placement areas are increased to simulate the placement of the dredge material. A mass balance is maintained and it is also possible to represent conditions in which the dredge material is placed in upland areas or in areas not represented in the CMS grid domain. The simulation can also be configured to simulate the construction of dredge placement islands.

Implementing a dredge simulation in CMS consists of two primary setup steps:

1. Define the dredge source and placement areas
2. Define the dredge scenarios and operations

Dredge and Placement

Area Definition

The dredge and placement areas are defined by creating input arrays identifying the cells that are in each source or dredge area. These input arrays are similar to a bottom friction or hard bottom input file. The input arrays are each stored in XMDF binary files. ASCII format is not presently accepted, but will be implemented in the near future. Within each input array, the source or placement areas are defined as those cells with a non-zero value.

Triggering

There are presently four options for triggering dredging for a given scenario.

1. If any cell in the source are has a bottom elevation above a user-specified threshold.
2. If the volume of sediment above the dredge depth is above a user-specified threshold.
3. If a certain percentage of the source area's bottom elevation above the dredge depth exceeds a user-specified threshold.
4. Specification of a time window during which dredging can occur. Any dredge areas with elevations above the dredge depth will be dredged during the specified time period. Multiple time periods can be specified and each is defined by a start and end time by the user.

Order of Dredging within Dredge Area

Figure 1. Example of Dredge Approach 1.

The options of dredging define the order in which cells are dredged to the specified dredge depth:

1. The volume dredged during each interval is taken from the highest point in the source area first. This approach is depicted in Figure 1.
2. A dredge starting cell is defined and the volume dredge during each interval is taken from the cell closest to the starting point first, and then dredging progresses to cells that are further away from the starting point. The starting point is user-prescribed by specifying the cell ID that contains the starting point.

Order of Placement within each Placement Area

Figure 2. Example of Placement Approach 1.

There are two options for dredge material placement during each interval.

1. The volume dredged during each interval is placed uniformly across the placement area. Each cell is filled to the specified thickness, or to the placement water depth limit (aka placement limit). This approach is depicted in Figure 2.
2. A placement area starting point is defined, and the volume dredged during each interval is placed in the cell closest to the starting point first. Placement then progresses to cells that are further away from the starting point. The starting point is prescribed by the user by specifying the cell ID that contains the starting point. Each cell is filled to the specified thickness, or to the placement water depth limit (aka placement limit).

Distribution among defined Placement Areas

When multiple placement areas are defined, there are two options for specifying how the dredge material is allocated to the placement areas:

1. The placements are filled sequentially in the order as listed, completely filling one before the next placement area will start to be filled.
2. A percentage of the dredge material is assigned for each placement area. The percentage of all placement areas must sum up to 100.

Phase 1 Verification tests

In Phase 1, A total of eight dredge simulations were developed and executed to test the various dredge simulation options. The basic test template consists of a rectangular grid domain with a typical offshore profile increasing in depth from north to south. Two dredge scenarios were defined and were named "CapitolDredge" and "MaintDredge" respectively.

Information on this phase can be found on the following page: CMS/DredgeModule/Phase1

Phase 2 Verification tests

In Phase 2, a total of three dredge simulations were developed and executed to test the implementation of the Dredge Module with multiple grain sizes. The basic test template consists of a rectangular grid domain with a typical offshore profile increasing in depth from north to south.

Information on this phase can be found on the following page: CMS/DredgeModule/Phase2