Effects of a deep sand extraction pit

Abstract

DEEP SAND EXTRACTION ON THE NETHERLANDS CONTINENTAL SHELF (NCS) In the “Nota Ruimte” (=National Spatial Strategy) the Dutch Government has announced to allow for deep sand extraction on the NCS, where former legislation only allowed for extraction till a depth of 2 m below the initial seabed. This new legislation has been worked out in the “Regionaal Ontgrondingenplan Noordzee 2” (= Regional Extraction Plan North Sea 2). It prescribes that for sand extractions with an extraction volume exceeding 10 million m3 or an extraction area exceeding 500 hectares an environmental impact assessment is required, and that an ecological study is required when the intended extraction depth exceeds 2 m below the initial seabed. It appears that there is little experience with the effects of deep sand extraction pits on existing values and user functions, due to the legislation prior to 2004. Since it is the responsibility of the Dutch Government to develop legislation on sand extraction, and to judge the proposals for sand extraction before granting an extraction license, Rijkswaterstaat, North Sea Directorate has asked Rijkswaterstaat, The National Institute of Coast and Sea/RIKZ to study the hydraulic and morphological responses of a deep sand pit and the risks of oxygen depletion and deposition of fines inside such a pit. The general conclusion is that there are no indications that a deep sand extraction pit with a final water depth of 40 meter necessarily leads to unacceptable effects on existing values and user functions, and therefore it is expected that deep sand extraction will be an interesting alternative for shallow sand pits with a volume of more than 10 million m3. For the environmental impact assessment of a proposed deep sand extraction pit, numerical models are available for useful predictions of the hydraulic and morphological response of such a pit. THE PUTMOR MEASURING CAMPAIGN From autumn 1999 till summer 2000 there was an opportunity to carry out measurements at a temporary deep sand pit (referred to as the PUTMOR pit) of the Lowered Dump Site (LDS) near Hook of Holland [Figures 1.1 and 1.3]. The PUTMOR pit was located at an initial water depth of 23 m and was left open for a period of 10 months, after which it was refilled with dredged material from the Port of Rotterdam. The pit had a content of about 4.5 million m3, a length of 1300 m, a width of 500 m and an extraction depth varying between 5 and 12 m. Within a period of 10 months, Rijkswaterstaat gathered data about the hydraulic conditions, the water quality and the morphological changes. THE IMPACT OF A DEEP SAND EXTRACTION PIT ON VALUES AND USER FUNCTIONS There are two major concerns related to deep sand extraction. The first concern is that benthic communities cannot re-establish on the bottom of a deep sand pit due to oxygen depletion and deposition of fines. The second concern is that a deep sand pit will harm existing cables, pipelines and offshore constructions, and the coastal defence system. To address these concerns, we have formulated the following questions: • What effects has a deep sand pit on the flow conditions, stratification, oxygen depletion deposition of fines and bottom changes? • Can the original benthic communities recover on the sand pit bottom? • Is there a risk of damage to cables, pipelines and offshore structures? • What is the effect on the sand budget of the coastal system? • Is it possible to judge a sand pit design with an extraction depth of more than 2 metres below the initial seabed using hydraulic and morphodynamic models? With the help of the PUTMOR measurements, which was made up with other measuring data from the NCS, we came to the following conclusions: • The PUTMOR measurements showed an increase of the flow rate (discharge per meter width) inside the pit with one-third, but the flow velocity near the bottom of the pit has a decrease of one-third compared to the measured flow velocities outside the pit. • The PUTMOR measurements did not show stratification and oxygen depletion inside the pit, below the initial seabed. In the upper ten metres of the water column, the usual haline stratification was measured, resulting from the fresh water discharge from the river Rhine. On the NCS, there are no records of oxygen depletion due to this haline stratification, which is probably the result of the temporal presence of a halocline during the tidal period and the upwelling of oxygen rich water from the offshore. Thermal stratification is not expected since the water depth inside the pit is less than 40 m, which is the minimum water depth for thermal stratification found at the NCS. Besides, the water inside the PUTMOR pit was refreshed four times a day due to the tide. In general, there is no chance on long-term haline or thermal stratification within a deep sand pit on the NCS with a final water depth less than 40 m pit, and we expect that the risk of oxygen depletion in such a pit is negligible. • When there is a large deposition of fines, there may be a negative impact on the recovery of benthos communities. In the PUTMOR pit, we did not measure a large deposition if fines. However, the deposition of fines is very site-specific, depending on the local flow velocities, the suspended concentrations of fines and the characteristics of the sand extraction pit. • We expect that recovery of benthic communities on the new seabed within a deep sand extraction pit is possible. • The morphological changes of a large pit in deep water are very slow, although they depend on the local conditions. The backfilling of a deep sand pit at an initial water depth of more than 20 m is expected to take a period of centuries. The risks on offshore infrastructure and coast at a distance of more than half a kilometre away from the sand pit seem very small. • There are numerical models available to judge the hydraulic and morphological responses of a deep sand pit. Calculations with the numerical model DELFT3D showed that flow velocities were predicted at a satisfactory level. This model also gives a good qualitative prediction of the backfilling/flattening and migration of a pit, a trench or a dump site under various environmental conditions, although the modelling of the magnitude of migration and backfilling or flattening in time should be further improved.