Physical Effects of Sea Sand Extraction / Fysische effecten van zeezandwinning

Abstract

FRAMEWORK OF EXISTING POLICY Several Dutch policy documents are relevant with respect to sea sand extraction on the Netherlands Continental Shelf (NCS), ie: - the government directive on shallow mineral reserves SOD (Structuurschema Oppervlakte Delfstoffen= Master Plan Surface Minerals), - the regional directive on extraction at the NCS RON/MER (Regionaal Ontgrondingsplan Noordzee = Regional Extraction Plan North Sea), linked to an Environmental Impact Assessment (MER = EIA), - the policy document 4e Nota Waterhuishouding (= 4th Note on Water Management), - and the government directive on rural areas SGR (Structuurschema Groene Ruimte = Master Plan Green Areas). Core principles are to use shallow mineral reserves as economically as possible, to achieve an optimal gearing to other user functions in the North Sea, and to maintain sustainable functioning of the North Sea water system. Several sea sand extraction guidelines have been adopted to realize these core principles. Since 1993, a number of developments have occurred which are not included in the RON/MER. A revised version (RON2) could give these developments their due. Partly for this reason it is desirable to examine the physical effects of temporary sand extraction in the nearshore coastal zone, of large-scale sand extraction by means of navigation channel overdimensioning, and of largescale sand extraction in designated extraction areas. TEMPORARY SAND EXTRACTION IN THE NEARSHORE COASTAL ZONE In the execution of beach nourishment operations, it may be desirable to construct a transhipment station in the nearshore coastal zone in the form of a temporary sand pit. A delivery pipe then extracts beach nourishment sand from the sand pit, after which dredgers refill the sand pit with sea sand. The maximum volume of such a sand pit is in the order of 1 million m3. Data from previous sand extraction operations in the nearshore coastal zone show that the characteristic time scale on which the pit is filled to about two-thirds of its capacity, is approximately six to twelve months. Model calculations indicate that an unfilled sand pit in the nearshore coastal zone causes the coastline to recede a maximum of several metres in one year. The direct consequences of a temporary sand pit for coastal safety are negligible if the sand pit is constructed at a depth of the coastal profile below NAP1 – 7m. Calculations with dune erosion model DUROSTA show that in that case the increase in dune erosion volume is less than 5%. LARGE-SCALE SAND EXTRACTION BY OVERDIMENSIONING OF NAVIGATION CHANNELS Overdimensioning, ie widening and deepening of the Euro-Maas Channel and the IJ Channel is one of the options for large-scale sand extraction. If the ‘Zero-Plus option’ in the present RON/MER is executed, the maximum mineable amount is estimated to be 320 million m3 for the Euro-Maas Channel and 200 million m3 for the IJ Channel. The consequences for shipping are generally positive: the width and depth increase and flow velocity crossing the channel decreases. However, depending on the design of the overdimensioning, around the jetty heads a complex flow pattern can occur which could hinder the shipping through the channel. Larger wave heights could furthermore occur, due to the fact that waves can penetrate further inshore through the navigation channels. Widening and deepening the navigation channels postpones the need for maintenance for an certain period of time. It increases the dredging margin, ie the difference between the actual depth and the target depth, as well as the distance between shipping traffic and slopes. Estuarine circulation, causing a shoreward flow of the salt seawater near the bottom, while the lighter, fresh river water near the surface flows offshore, may strengthen. This could result in an increase of silt transport towards the harbours. Erosion of the navigation channel banks could result in nearby existing cables and pipelines getting exposed, with risk of breach. Furthermore, erosion of the (deeper parts of the) coastal zone has a negative influence on the total sand budget of the Dutch coast. Over a period of 50 years, the erosion zone can expand over a distance of 6 to 20 kilometres along the deeper section of the coast. It’s influence on the nearshore coastal zone adjacent to the shipping channels is expected to be small, as a result of the dominating influence of the harbour jetties, resulting in sedimentation. The effect of navigation channel overdimensioning on coastal safety is of importance only with respect to the seawall around the jetties. This effect is not investigated in this report. LARGE-SCALE SAND EXTRACTION IN DESIGNATED EXTRACTION AREAS Large-scale sand extraction in designated extraction areas exceeding 100 million m3 could affect ecology, cables and pipelines, coastline maintenance and coastal safety. Sand extraction causes the entire bottom fauna of the extraction pit to disappear. The bottom fauna can recover only if the water residence time in the extraction pit is less than approximately 10 days, as a result of which the oxygen content remains at the same level. It is important, therefore, that vortices down the sand pit slope and stratification underneath the original seabed are avoided. By positioning the length of the sand designated extraction areas parallel to the flow direction, a process occurs which causes the designated extraction area to accelerate the flow, which in turn results in quicker water refreshment. The presence of a large-scale sand designated extraction area also has morphological consequences, the most important of which is orientation towards the tidal current and the length/width ratio. Model calculations show that continued deepening after construction is possible, depending on the geometry of the sand designated extraction area. Within a period of 50 years, banks caving in and the extraction pit being moved could cause problems with respect to cables and pipelines within a zone of several kilometres around the extraction pit. If the extraction pit is located near the seaward boundary of the coastal foundation zone, this could cause sand loss in the coastal zone. (At the time of publication of this report, the seaward boundary of the coastal foundation zone was not formally established. This will probably be the established NAP –20m depth contour.) The effects of large-scale sand extraction on coastline maintenance and coastal safety seem to be limited. Based on a limited number of numerical model calculations to predict the effect of a future Second Maasvlakte, the conclusion is drawn that there is no significant increase in hydraulic boundary conditions (water levels, wave heights and wave periods), and that structural receding of the coastline position is not to be expected within a period of 5 years. However, the effect on the coastline position over a longer period, for example 50 years, has not been clearly mapped out as yet. RECOMMENDATIONS For temporary sand extraction in the nearshore coastal zone along the straight Dutch coastline and the central coastal sections of the islands, the following recommendations are given: • It is recommended in any case that after the sand nourishment the temporary sand pit be filled to the original sea bed elevation. • It is furthermore recommended to demand compensation for the loss of sand transported to the pit from the surrounding area during the use of the temporary sand pit. This compensation can be achieved by overdimensioning the sand nourishment or by creating an extra sand buffer at the temporary sand pit. The volume of this compensation can be estimated by means of a formula presented in this report. • To prevent any adverse consequences of the temporary sand pit to coastal safety, it is recommended to carry out the extraction operations seaward of the NAP –7m line with a surface area under 10 ha. For the Zeeland and South Holland islands and for the heads of the Wadden Islands, further conditions need to be determined for temporary sand pits in the nearshore coastal zone. For large-scale sand extraction by means of overdimensioning of the navigation channels, the following recommendations are given: • To prevent wave reflection on the navigation channel banks it is recommended that the slopes be at least as gentle as 1:7. • With respect to cables and pipelines, it is recommended that an influence zone of 6 to 20 kilometres, running parallel to the navigation channels, be taken into account over a period of 50 years. Possible measures are the creation of a 1,000 metre buffer zone, deeper digging in of cables and pipelines, and periodical checks for possibly exposed cables and pipelines. • As a result of the complex flow pattern around the jetty heads, it is recommended that navigation channel entrance design be studied using a numerical model. • It is recommended to investigate beforehand if navigation channel overdimensioning results in estuarine circulation changes, and if this is the case, to indicate whether an increase in sedimentation of the harbours may be expected. • It is recommended to chart the positive and negative effects on the sand budget of adjoining coastal sections. It is furthermore recommended to define the boundaries of this coastal zone parallel to the navigation channels where an influence of the navigation channel is accepted, taking into account a maximum overdimensioning and expected expansion of the morphological influence area in the longer term. • It is recommended to carry out further studies into the effect of navigation channels on wave conditions in the nearshore coastal zone and to examine how overdimensioning affects wave conditions during extreme conditions. • It is recommended to examine the effects of large-scale sand extraction by navigation channel overdimensioning in connection with the effects of other large-scale interventions in the North Sea, eg during future Environmental Impact Assessment (EIA) studies. For large-scale sand extraction in designated extraction areas the following recommendations are given: • With respect to cables and pipelines, it is recommended that influence zones of several kilometres around the large-scale sand designated extraction areas be taken into account over a period of 50 years. Possible measures are the creation a 1,000 metre buffer zone, deeper digging in of cables and pipelines, and periodical checks for possibly exposed cables and pipelines. • It is recommended to create a buffer zone of several kilometres between large-scale extraction pits and the coastal zone. • Despite the fact that large-scale sand extraction in designated extraction areas does not seem to have any significant adverse effect on coastal safety, it is recommended to include this aspect when considering large-scale sand extraction in designated extraction areas. • If considering large-scale sand extraction in a sand designated extraction area, it is recommended to carry out a study of the oxygen concentration at the bottom by means of a water quality model. • With respect to the geometry of the sand designated extraction area, it is recommended not to draw up quantitative guidelines. Instead it is recommended to examine the effects on the relevant user functions beforehand for every large-scale sand extraction operation, for example by means of a Milieu Effect Rapportage. • It is recommended to examine the effects of large-scale sand extraction in designated extraction areas in connection with the effects of other large-scale interventions in the North Sea, eg during EIA studies. NOTE ALSO A DUTCH VERSION IS ADDED TO THE FILES