THE CO2 BALANCE OF DREDGING PROJECTS, AN ECOSYSTEM APPROACH
27-28 October 2016, Lisbon, Portugal
Jasper Fiselier, RoyalHaskoningDHV, The Netherlands
Dredging produces vast amount of CO2 emissions. So far reducing the carbon footprint of dredging projects mainly looked into the possibilities of reducing the emissions by the equipment used, with a focus on dredging schemes that are more efficient, or using dredging vessels that are more fuel efficient. A reduction in the order of 10 to 20% may be within reach, most of it is a win-win since fuel reduction will also reduce costs.
The carbon foot print of marine projects depends however also on the impact a project has on the primary production and sedimentation processes in the coastal system. Primary production produces organic carbon that can be sequestered with the sedimentation of fines in sand pits and in blue carbon wetlands, such as salt marshes. These ecosystem based impacts can be a substantial or even overriding factor in the carbon footprint, especially on the longer term. A further reduction in the carbon footprint of a marine project can therefore be achieved by integrating blue carbon wetlands into the design, for example by using them as part of a protection schemes, or by furthering conditions that will stimulate their growth. Also the form and location of the sand pit may influence its potential to sequester (organic) carbon due to sedimentation of finer particles.
However, coastal systems are essentially open systems, and marine projects do not only impact and alter the area of the sand pit and the project area, but will influence a far larger part of the coastal system. Organic carbon that is produced by algae may be sequestered in the sand pit or the salt marsh that forms part of the project, but may, without the project taken place, have been sequestered in another salt marsh or other part of the coastal system. So the contribution of a salt marsh made possible by a project, is never 100% of the (organic) carbon that is sequestered by a growing salt marsh.
In order to assess the carbon balance between a coastal system with and without the project one may need to consider a myriad of factors that influence primary production and the formation of organic carbon as well as regional sedimentation patterns. In order to simplify the related complexity, we focus on a limited number of most relevant factors. The first is the availability of (mineral) fines that are needed to safely store organic carbon in sediments, since sand alone allows too much oxygen and the decay of organic matter. Fines are in principle limited, in an absolute sense, since only so many tons flow down rivers or are produced by coastal erosion. Fines are also limited in a relative sense, since various sedimentation areas compete with one another. The second limiting factor is phosphorus which is needed for primary production. There are other nutrients, such as nitrogen, and minerals needed, such as potassium and iron, but phosphorus is in most coastal seas the most limiting.
The carbon balance therefore depends on the most efficient use of phosphorus to produce organic carbon and of the most efficient use of fines to sequester organic carbon. If a salt marsh or a sand pit is the most efficient in this sense, their use as part of a marine engineering project, will contribute positively to the carbon footprint and the carbon balance. The coastal sea can in principle be seen as a combination of sedimentation zones that have different ability in sequestering organic carbon in an efficient way. Unfortunately many studies into CO2 footprints of blue carbon wetlands seldom look into sedimentation processes in relation to fines, TOC and phosphorus. This relation is also often an unknown for sand pits and other parts of the coastal system and wider open sea.
So, summarizing, a carbon footprint can be assessed on three different levels. The first level is that of combustion related CO2 emissions by dredging equipment. The CO2 emission of dredging equipment is well-known. In the long term maintenance may play an important role, especially where soft defenses that need nourishment form part of a marine project. In this case the long-term nourishment needs may form the largest uncertainty.
The second level is including the (organic) carbon that is sequestered by the sand pit and blue carbon wetlands, made possible or perhaps also directly impacted by the project. The carbon sequestration of blue carbon wetlands is fairly well known, but that of sand pits is not studied. In the long-term, the long cyclic sequestration of (organic) carbon is only possible if the long-term fate of pits and wetlands are known. Predicting their status over a longer period is quite challenging.
The third level includes all off site impacts on primary production and fine sediment sedimentation and its consequences for (organic) carbon sequestration. As indicated, these are largely unchartered waters, and more research into the whereabouts of fine sediments, phosphorus and (organic) carbon is needed.
As part of the Ecoshape program, a first tool is being developed whose aim it is to produce a carbon footprint based on all three levels. The presentation will briefly describe the architecture of this tool and some primary results from several marine engineering projects. The aim is to identify dredging strategies and marine engineering designs that have smaller carbon footprints.
Last update: 3 October 2016