Dredging

About: Dredging is a research topic. Over the lifetime, 3300 publications have been published within this topic receiving 28325 citations.

Temporal Patterns in Seawater Quality from Dredging in Tropical Environments.

Abstract: Maintenance and capital dredging represents a potential risk to tropical environments, especially in turbidity-sensitive environments such as coral reefs. There is little detailed, published observational time-series data that quantifies how dredging affects seawater quality conditions temporally and spatially. This information is needed to test realistic exposure scenarios to better understand the seawater-quality implications of dredging and ultimately to better predict and manage impacts of future projects. Using data from three recent major capital dredging programs in North Western Australia, the extent and duration of natural (baseline) and dredging-related turbidity events are described over periods ranging from hours to weeks. Very close to dredging i.e. <500 m distance, a characteristic features of these particular case studies was high temporal variability. Over several hours suspended sediment concentrations (SSCs) can range from 100–500 mg L-1. Less turbid conditions (10–80 mg L-1) can persist over several days but over longer periods (weeks to months) averages were <10 mg L-1. During turbidity events all benthic light was sometimes extinguished, even in the shallow reefal environment, however a much more common feature was very low light ‘caliginous’ or daytime twilight periods. Compared to pre-dredging conditions, dredging increased the intensity, duration and frequency of the turbidity events by 10-, 5- and 3-fold respectively (at sites <500 m from dredging). However, when averaged across the entire dredging period of 80–180 weeks, turbidity values only increased by 2–3 fold above pre-dredging levels. Similarly, the upper percentile values (e.g., P99, P95) of seawater quality parameters can be highly elevated over short periods, but converge to values only marginally above baseline states over longer periods. Dredging in these studies altered the overall probability density distribution, increasing the frequency of extreme values. As such, attempts to understand the potential biological impacts must consider impacts across telescoping-time frames and changes to extreme conditions in addition to comparing central tendency (mean/median). An analysis technique to capture the entire range of likely conditions over time-frames from hours to weeks is described using a running means/percentile approach.

Recovery of the seabed following marine aggregate dredging on the Hastings Shingle Bank off the southeast coast of England

Abstract: The aim of this study was to investigate the effect of dredging intensity on the physical and biological recovery times of the seabed following marine aggregate dredging. Two areas of seabed, previously subject to, respectively, relatively high and lower levels of dredging intensity, were identified on the Hastings Shingle Bank. Two reference areas were also selected for comparative purposes. All four sites were monitored annually over the period 2001–2004, using a combination of acoustic, video and grab sampling techniques. Since the site was last dredged in 1996, this was intended to provide a sequence of data 5–8 years after cessation of dredging. However, an unexpected resumption of dredging within the high intensity site, during 2002 and 2003, allowed an additional assessment of the immediate effects and aftermath of renewed dredging at the seabed. The early stages of recovery could then be assessed after dredging ceased in 2003. Results from both dredged sites provide a useful insight into the early and latter stages of physical and biological recovery. A comparison of recent and historic dredge track features provided evidence of track erosion. However, tracks were still visible 8 years after the cessation of dredging. Within the high dredging intensity site, recolonisation was relatively rapid after the cessation of dredging in 2003. Rather than indicating a full recovery, we suggest that this initial ‘colonization community’ may enter a transition phase before eventually reaching equilibrium. This hypothesis is supported by results from the low intensity site, where biological recovery was judged to have taken 7 years. Further monitoring is needed in order to test this. An alternative explanation is that the rapid recovery may be explained by the settlement of large numbers of Sabellaria spinulosa. As the resumption of dredging within the high intensity site limited our assessment of longer-term recovery it is not yet possible to assume that a 7-year biological recovery period will be applicable to other, more intensively dredged areas at this or more distant locations.

A comparison of direct macrofaunal mortality using three types of clam dredges

Abstract: The white clam Spisula solida is harvested along the entire coast of Portugal using mechanical dredges. In this study, the total direct mortality of the macrobenthic community caused by three types of clam dredges (north dredge—ND, traditional dredge—TD, and the metallic grid dredge—GD) used in the S. solida fishery was determined and compared. The relationship between mortality and catching efficiency for each type of dredge was also assessed. Our results showed significant differences for total direct mortality between the ND and both the GD and TD dredges. This difference was largely attributed to the mortality of animals that died in the dredge track as a direct result of the physical damage inflicted by the dredge passing. It was also found that the damage to uncaught individuals is directly related to gear efficiency. The lower catching efficiency of the ND (64%) led to a higher proportion of damaged individuals being left in the dredge path, when compared with the more efficient GD (98%) and TD (90%) dredges. Short and long-term implications of the impact of dredging on the composition of benthic communities are discussed. From fisheries management and ecological points of view, there are obvious advantages to introduce into the bivalve dredge fisheries more efficient and selective dredges in order to reduce the number of damaged individuals and by-catch, and consequently decreasing the impact on the macrobenthic communities.