CMS/DredgeModule

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CMS Dredge Module

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 has been developed for SMS version 13.0 and some modifications to the initial CMS Dredge Module code were made to accomodate the new interfact, 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.

Dredge Area Setup

Figure 3. Test Simulation Grid Domain and Bathymetry.

Figure 4. Dredge Scenario Definitions.

The first dredge area, "CapitolDredge" consists of a dredge source area aligned to represent a navigation channel (north-south alignment). The dredge depth was set to 7.5 meters, which created a total dredge volume of approximately 153,000 m³. The dredge rate was set to 5000 m³/day, which yielded a total time of 30 days to complete dredging. Two placement areas were defined; both near the southern edge of the grid domain, one to each side of the channel.

The second dredge area, "MaintDredge" consisted of a dredge area in the northeast corner of the grid domain and resembles a sediment impoundment basin. The dredge depth was set to 8 meters yielding a dredge volume of 130,000 m³. The dredging rate was set to 6000 m³/day, yielding a dredge time of 22 days. A single placement area was defined to the south of the dredge source area.

The bathymetry of the simulation grid is shown in Figure 3. The location and extent of the dredge source and placement areas for each scenario are shown in Figure 4.

Test Cases

Table 1. Summary of Test Simulations

Test Number	Description
101	CapitalDredge: Dredge option 2 (starting location set to north end of dredge source area), Trigger Option 1 with Threshold set to 7 m. Placement option 1 with Limit of 3m. Placement Allocation 100% for #1 and 0 % for #2. MaintDredge: Dredge option 1 (shallowest), Trigger Option 1 with Threshold set to 7.5 m. Placement option 1 with Limit of 3.5 m.
102	CapitolDredge: Dredge Approach is changed to method 2 (shallowest cell)
103	CapitolDredge: Placement Allocation is changed form 100%/0% to 60%/40%.
104	CapitolDredge: Placement Approach is changed from 1 to 2. The CapitolDredge starting location is the northwest corner for placement area 1 and is at the northeast corner of placement area 2. The 60%/40% placement allocations used in Test Simulation 103 was applied.
105	CapitolDredge: Placement Areas were reduced to limit capacity and force an excess of dredge material during the simulation; placement allocation is sequential
106	CapitolDredge: Placement Areas were reduced to limit capacity and force an excess of dredge material during the simulation; placement allocation is 80%/20%
107	Capitol Dredge: trigger approach is changed to option 2 and the MaintDredge trigger approach is changed to option 3
108	CapitolDredge: trigger approach is changed to option 4

Test 101

Figure 5. Final Morphology Changes for Test Simulation 101.

Figure 6. Dredge Profile Evolution for Test Simulation 101 - CapitolDredge Scenario.

Figure 7. Dredge Profile Evolution for Test Simulation 101 - MaintDredge Scenario.

Figure 8. Test Simulation 101 - MaintDredge Scenario Dredge Source and Placement Area volume evolution.

Examples of the simulated dredging activities for Test 101 are shown in Figure 5 through Figure 8. Figure 5 shows the final morphology changes due to the simulated dredging activities. The CapitolDredge navigation channel is apparent, as is the placement in Area 1. The capacity of Placement Area 1 was not exceeded, so the second area was not used. The changes in morphology for the MaintDredge scenario are also evident in the figure.

Figure 6 shows the morphological evolution for the CapitolDredge scenario along a transect aligned with the navigation channel. The dredging begins at the specified starting location at the north end of the channel and proceeds southward. The evolution for the MaintDredge scenario is shown in Figure 7 along a north-south transect passing through both the dredge source and placement areas. The dredging for the source area starts at the northern end, which is the shallowest, but quickly proceeds uniformly over the dredge source area as all cells obtain an equal depth. The placement of the material builds up uniformly over the placement area until the specified placement depth limit (3.5 m) is reached in the shallower portion of the profile. At this time, more material is placed in the deeper areas.

A time series of the dredge volume for the MaintDredge Scenario is provided in Figure 8. The dredge volume starts at 130,000m3 and steadily decreases during the dredging activity at 6000 m3/day and is completed at approximately 22 days (528 hours). The dredge volume placed in the MaintDredge placement area increases steadily up to 220 hours at which time the placement area capacity of 54,000 m3 is reached. The placement area capacity is controlled by the specified placement depth limit of 3.5 m for this scenario. At 200 hours, the remaining dredge volume is assumed to be placed outside of the CMS grid domain and is tracked as the Unplaced Volume (excess).