Sand Cap Placement and Cap Thickness Monitoring: A Case Study at a Confined Disposal Facility

Thin sediment capping is a commonly used technique to prevent mobilization of contaminants from sediments into the environment. A 70‐m‐deep subaqueous confined disposal facility (CDF, 350,000 m²) at Malmøykalven, Oslofjord, which received dredged contaminated sediments from Oslo Harbor, was capped with 148,900 m³ of sand in 2009. This research serves as a case study regarding some of the key considerations involved with the cap placement and monitoring of the cap layer. Uncertainty is included in all the cap thickness monitoring methods and a combined use of them provided a better understanding of the cap coverage and structure at the site. An open water disposal model (STFATE) was used to simulate the behavior of the barge‐released cap material. The modeling results were consistent with field observations regarding the material spread, and the results provided insight into the relatively high material losses calculated. Better knowledge obtained of material settling resulted in cap properties and cap monitoring methods that are useful when planning similar operations. ©2015 Wiley Periodicals, Inc.     

Monitoring the volume and lateral spread of disposed sediments by acoustic methods, Oslo Harbor, Norway

Acoustic equipment, including interferometric sonar and parametric sub-bottom profiler, have been used to determine the volume and lateral spread of dredged sediments disposed in the natural submarine depression in the Bekkelag Basin, inner Oslofjord since the beginning of the Oslo Harbor remediation project in 2006. This natural depression is used as a subaqueous confined disposal facility (CDF). Calculation of the volume of disposed sediments in the CDF is based on elevation change, derived from two high-resolution bathymetric datasets obtained in 2004, i.e. before the onset of the remediation project, and in April 2008. Seismic profiles across the CDF have been used to estimate the settlement of the original seabed, caused by loading-induced dewatering and compaction of the seabed sediments under the disposed masses.Detailed bathymetry and backscatter data demonstrate the lateral spread of disposed sediments within a well-confined area covering ca. 195,000 m². The sea bottom within this area is distinctly softer than the surrounding seabed as shown by very low acoustic backscatter amplitude, signifying a very loose surface character of the disposed sediments. The thickness of the disposed sediments reaches 6 m the deepest part of the original depression. The volume calculation of the disposed sediments in the CDF, based solely on bathymetry data, gives a value of ca. 310,000–320,000 m³. Settlement of the original seabed as a result of loading has been estimated to be 30 cm at 5 m thickness of the disposed sediments. Under the condition that the settlement rate is linearly correlated to the thickness of disposed sediments, the settlement corrected volume of disposed sediments is ca. 330,000–340,000 m³. Presented results demonstrate high accuracy and good reproducibility of acoustic seafloor data, and indicate a great potential of such methods as monitoring tools in environmental projects that involve dredging and subaqueous disposal.