Dredging and other human-induced increases in turbidity levels and sedimentation rates may impact coastal ecosystems such as coral reefs and seagrass meadows. The Interactive Dredge Planning Tool (IDPT) is able to perform a rapid assessment of the expected, initial ecological effects caused by interactively defined dredging operations. For this, the IDPT makes use of rapid assessment dredge plume modelling, a database with computed hydrodynamic background conditions and a database with ecological information, i.e. locations, species and species tolerance information. The effect of this increased turbidity and sedimentation on the ecosystems is site and species specific, but has been addressed for certain species in Singapore in different projects within the Building with Nature programme.
Dredging operations can be optimised in terms of environmental impact by assessing these effects before commencement of the work. In particular during the tender phase of a project, however, time hardly allows for a detailed (modelling) study to assess the environmental effects of different work methods. The Interactive Dredge Planning Tool (IDPT) enables the user to make a rapid quantitative assessment including the newly developed knowledge on ecological response to (dredging-related) sediment stresses. The IDPT is developed based on the Interactive group modelling - MapTable concept.
The Interactive Dredge Planning Tool enables (initial) assessments of the spreading of dredging-induced turbidity plumes and their environmental effects. The results of this assessment are clearly visualised, along with other project-relevant data, on an interactive map which can be used for communication with stakeholders and other interested parties. The tool in its present form has been developed for Singapore, but can serve as an example for applications elsewhere.
In addition, the IDPT can create so-called dredging suitability maps, which indicate the advised maximum turbidity production given a specific maximum allowable ecological effect. Thus it facilitates the transition from the current emission-based practice to a more impact-based approach.
Building with Nature interest
The IDPT consolidates knowledge and tools related to dredging-induced turbidity and ecological effects into a (prototype) rapid-assessment tool for the effects of dredging on ecology. By making BwN knowledge available to practice, it contributes to spreading the Building with Nature philosophy on ecosystem-based norms and rules. The tool is especially useful for projects in ecologically sensitive areas where hydraulic engineering works are planned or executed and where different stakeholders are involved in the project. It can serve as a means to communicate with stakeholders about the effects to be expected and to involve them in the project process.
This tool is easy to use for rapid assessments of the expected direct environmental effects of dredging operations. The user interface is easy to use and enables defining and evaluating different dredging scenarios. Clearly, the proper interpretation of results requires a certain level of knowledge on dredging operations, ecology and the models and databases underlying the tool. This sections provides more details on the following:
The IDPT is available as a stand-alone tool locally installed on the user's PC, in order to be used for the planning of dredging operations, for instance. Installation of Python, Pylons, the Matlab wrapper and Matlab is required. A tutorial of installing these components can be found here. The open-source code of the IDPT is included in the OpenEarth tools repository and can be downloaded from there.
2. Working principles
The working principles of the IDPT can be described with the steps of the workflow. The technical workflow diagram in Figure 2 shows the relations between the different modules.
Figure 2: Workflow diagram of IDPT tool modules
Note that different types of ecological criteria can be part of the IDPT, such as the mentioned SRCs. Also other ecological can be used, or non-ecological ones such as criteria related to territorial waters between countries. Criteria can simply be added to the database and assessment modules described above.
3. Functioning of the IDPT prototype version
This section describes how in general the user can work with the tool.
Figure 3: Mapping of model results and ecological species response curves to effect regimes
4. Developing a case with the IDPT
The set-up of the IDPT is generic, such that this software can be used for any site or location. In the current research program of Building with Nature the tool has been applied to the Singapore coastal waters. Application at another location requires site-specific data, like:
The IDPT-code is easily adaptable by experienced Matlab users, because of its modular set-up. If desired, modules can be replaced or added to the workflow and the behaviour and appearance of the tool can be changed. This flexibility is used, for instance, when making dredging suitability maps (based on a maximum specified effect) instead of assessing the effects of a specific dredging operation
Advice and recommendations
The IDPT is intended for rapid assessment of the effects of dredging activities on vulnerable coastal ecosystems. The first-order estimate of ecological effects it produces cannot replace more advanced analyses or detailed modelling. The usefulness of the results depends, among others, on the accuracy of the ecological database, the hydrodynamic and sediment background conditions and the dredge plume dispersion model. This means that the computed ecological effects need to be considered carefully, taking into account the possible uncertainty ranges associated with the complex nature of the chain of operations leading to the estimated ecological effects. Expert interpretation and explanation are essential for proper use.
Tips and Tricks
5. SWOT analysis
These lessons are derived from a workshop session on 22 December 2011 involving Ecoshape Partners. The overview has been updated on 1 March 2012 and 21 June 2012.
6. Lessons learned
Lessons Learned development process
Lessons Learned user experience
Technical Lessons Learned
Lessons learned setting up a MapTable tool
Lessons learned Delft3D-PART - Dredge Plume Dispersion Modelling
The practical application of the IDPT tool will be explained with a case study from Singapore:
1. Introduction to the case study Singapore
The objective of the IDPT Showcase is to demonstrate how the Interactive Dredge Planning Tool can be used to assess ecological responses to dredging operations (sediment plumes and sedimentation) in a specific dredging project. A fictitious dredging project in Singapore is described, including dredging scenarios and source term modeling. This showcase yields lessons learned on the tool and shows users how to apply this tool. The ecological results and their reliability are not further assessed and interpreted.
The showcase presents an initial assessment of the response of sensitive species to turbidity and sedimentation, starting from a basic background hydrodynamic model. This configuration improves with time, analogous to the situation in a real dredging project, where information is initially limited but progressively improves as the project develops.
Within the Singapore case, the East Coast Park (ECP) has been selected as a location for the fictitious dredging project. Sediment plumes cause by a Trailing Suction Hopper Dredger (TSHD) operation are assessed by using the Turbidity ASsessment Software (TASS). TThe resulting sediment source terms are subsequently used as inpust to IDPT. The results of IDPT give an indication of the response of coral and seagrass ecosystems in the vicinity to dredging-induced suspended sediment and sedimentation.
The following aspects are considered within this showcase:
2. Study site Southern Islands region and ECP
The showcase concerns fictitious land reclamation in East Coast Park (ECP). For this area the BwN innovation program has developed the EDD design pilot ECP. This design was used to define the reclamation project. For the sake of argument, the sand for this reclamation is assumed to be found in the Southern Islands region in Singapore, because this is the area with the most sensitive receptors.
East Coast Park (ECP) is the result of various reclamation projects since the 1970’s on the coast of Katong (Southern Singapore) and extending from Changi to Tanjong Rhu. Today, East Coast Park is the largest park in Singapore, fully built on reclaimed area. Due to wave action, lack of sediment supply, and presumably sea level rise, ECP has been reported to experience coastal erosion, causing beachfront areas to collapse.
The BwN-design selected for the showcase is the "Lagoon, unprotected corals " variant, the. The purpose of the reclamation project is to create additional land for recreational purposes while favouring the development of local habitats.
The design involves a seaward extension of the existing land and the construction of a hard substrate to enables coral reef formation (see Figure). The hard substrate in shallow water should favour coral colonization; the ridge allows reducing the amount of landfill (perched beach).
3. Reclamation and dredging methods
Although in reality landfill material is scarce in the Singapore area, we assume for the sake of argument that the borrow area for landfill material for the ECP is located in the Southern Island region. This area is rich in coral and seagrass and therefore suitable to investigate the effect of dredging plumes on this ecosystem.
The dredge parameters used are fictitious and not necessarily valid in practice.
The dredging and reclamation works for this showcase include:
The required volume of dredged material is 8.5 million cubic meters. It is hereby assumed that all the dredged material is suitable for reclamation purposes. Therefore, the approximate output per dredger of 5.400 m3 in a dredging cycle of 6.2 hours will be 16.200 m3/day, assuming 3 dredging cycles daily. The total time for completing the dredging works will be 8-9 months
This design has been prepared as part of the multidisciplinary work “Developing an eco-dynamic design for additional land in front of East Coast Park, Singapore” as part of the Building with Nature Singapore project (Innovative coastal protection).
Characteristics of dredging equipment: THSD with hull overflow and constant tonnage
Particle size distribution (PSD)
4. Schematisation of sediment source term
The source term which is determined here is the source for the far field plume, which may serve as input for dispersion modelling, see for more information Assessment of dredging-induced turbidity. As mentioned before, the borrow area for the ECP reclamation works is fictitious. Correspondingly, the sediment properties are specific for this study and should not be considered as a reference for sediment properties in other studies in this area.
TASS software was used to model the source term for overflow and dynamic plumes from the TSHD. Initial TSHD dimensioning and operating characteristics, site conditions and in-situ sediment characteristics are required as inputs.
The percentage of fines in the overflow amounts to 10%. In the computation the fine and the course sediment fractions were lumped, respectively, yielding to binary grain-size distribution, with a fine fraction (particle size up to 50 µm) and coarse fraction (particle size from 50 µm to 100 µm).
The TASS Dynamic Model computes the fate of the (near-field) plume and the rate of fine sediment release from the plume. The model calculates separately the sediment flux from the dynamic plume into suspension near the bed (density current) and in the water column (dilute suspension) and the sedimentation on the bed.
For the sediment source computation of the IDPT-showcase the flux into the near-bed density current and into the dilute suspension are used. The sediment source term resulting from the TASS model is shown in the diagram.
Source term schematisation
5. Schematisation of dredging operations and site conditions
The figure above schematically shows the dredging operations, between sand sourcing in the area of Southern Islands and the reclamation area near East Coast Park. The dredging scenario consists of five different phases, each with its own input into the IDPT. These five phases are:
1 Loading of TSHD until the overflow level is reached
The overflow level is reached after 27 minutes of dredging. The table shows the input for the IDPT as long as there is no overflow discharge.
From the moment that the overflow level is exceeded until the maximum TSHD’s tonnage is reached, sediment-laden overflow will be released into the water column. The maximum load is reached 180 minutes later after the overflow level is exceeded (27 minutes from start). The sediment fluxes into the near-bed density current and the dilute suspension in the water column are used to calculate the depth-averaged discharge rate (see table for the IDPT input in this phase).
Input IDT loading of TSHD until maximum permissible load is attained
The travelling time from the dredging site to ECP is assumed to take approximately 72 minutes, assuming a laden vessel speed of 13 knots and a vessel speed approaching the coastline of ECP of 3 knots. During this period no overflow is assumed.
4 Discharge operation
Discharging the sediment from the TSHD in the reclamation area will take approximately 30 minutes, assuming a discharge rate of 3.18 m3/s.
5 Sailing back to dredging site
The travelling time from ECP to the Southern Islands borrowing area is approximately 65 minutes, assuming an average sailing speed of 14 knots.
6. Description of hydrodynamic models and tests
When incorporating the hydrodynamic forcing and the species response indicators into the model, several factors need to be taken into consideration, such as:
Three nested hydrodynamic models (different in model domain and resolution, bathymetry, background suspended sediment concentration) were implemented and tested. The largest-scale model was obtained from Deltares; it uses the Singapore Regional Model SRM with detail in the Johor Estuary. The second model and third model were built using Delft Dashboard, which enables generating a model from available on-line data such as bathymetry and tidal datasets. It also facilitates a quick schematisation of the model by using tools to create and modify the model grid, to nest different models or to create domain decompositions. Tidal stations from on-line data bases can also be used to create Time-series of model results can be obtained in preset stations, in order to compare them with on-line tidal data, for instance. This sequence of models is used as a background hydrodynamic model for IDPT to predict the dispersion of the dredging plume.
Critical factors for the hydrodynamic model configuration are:
Model domain: important to accurately include the relevant large scale phenomena in the region, such as tidal forcing. For the Singapore region, the model domain needs to include the transition between the dominantly diurnal tidal regime from the South China Sea and the semi-diurnal regime from the Indian Ocean.
Grid resolution: determined according to the required detail of output data; nesting of models can be used for efficiency reasons.
Bathymetry: implementation of detailed bathymetry data may increase the accuracy of the simulations, but may also induce additional computational time.
Tidal forcing: tidal models are available to be incorporated via Delft Dashboard; the tidal stations in these models are used as observation stations.
7. Evaluation of IDPT results
Model 1: Singapore Regional Model SRM. Grid Johor Estuary (Source: Deltares)
Model 2: Configured in Delft Dashboard; large grid covering the area of the Malacca and Karimata Straits
Model 3: Nested model (Delft Dashboard); inner fine grid covering the Singapore area
In order to improve the results of the previous models, a nested model was built with detail in the area of the Southern Islands. The grid size was refined by a factor four. At the time of writing, however, this model required further examination of the hydrodynamic boundary conditions derived from Model 2 and the nesting process.
Analysis of results
The results of Model 1 (SRM Grid Johor Estuary) show that there is a well-defined control volume in Delft3D-PART for each computational grid. This may be the result of the relatively small grid size of the model at the Southern Islands location, when compared with the large grid of Model 2. Results of IDPT for Model 2 could be improved modifying the amount of available particles to be eroded for each computational grid in Delft3D PART.
The bathymetry dataset used in Model 1 is unknown, whereas for Model 2 and Model 3 GEBCO_08 was used (with a resolution of 30 arc-seconds). When preparing a hydrodynamic model for IDPT different bathymetric datasets can be used. Depending on model domain and resolution, the results may differ.
8. Lessons learned
Suggestions for further reading about the Interactive Dredging Tool: