Operational Water Quality Management for Marina Reservoir, Singapore

Transcription

1 Operational Water Quality Management for Marina Reservoir, Singapore Tjitte Nauta 1, Chang Chian Wui 2, Johannes Smits 1, Elizabeth Lee 2, JanJaap Brinkman 1 1 Deltares Delft Hydraulics, PO Box 177, 2600 MH, Delft, The Netherlands; 2 Public Utilities Board, 40 Scotts Road #22-01; Environment Building Singapore , Republic of Singapore; pub.gov.sg Abstract By damming the Marina Channel a new reservoir was recently created downtown Singapore. Through the flood protection structure, the Marina Barrage, Marina Bay and the Kallang Basin will be turned into a confined freshwater reservoir. In addition to flood control the new reservoir will provide another source of drinking water for Singapore, as well as a stable water level for a variety of water activities and events. By means of spillage plus a proposed recirculation & treatment scheme the reservoir will be connected to the Upper Peirce, Lower Peirce and MacRitchie Reservoirs. The Public Utilities Board (PUB) in Singapore commisioned Delft Hydraulics to map out the transition from a well-flushed estuarine water system to a freshwater system and to guide the future operational water management of the reservoir. Hereto, a fully integrated and comprehensive 1D-3D water quality modelling framework has been developed and applied to assess the future water quality and the effectiveness of mitigating measures. It was demonstrated that with maximal source control in place, continuous recirculation and simultaneous treatment and with ample artificial areation to enhance vertical mixing, water quality problems like eutrophication, oxygen depletion, bacterial pollution and high turbidity are strongly reduced though not completely eliminated. To address future operational water quality management problems an on-line Operational Management System (OMS) was developed that supports day-to-day decision-making based upon continuous rainfall forecasting, water quantity and quality monitoring and water quality model forecasting. The first version of the OMS addresses the optimal tidal flushing and water level control scheme for brackish conditions that has been put in place for the period , i.e. the period before the system will be converted to a freshwater reservoir and during which various mitigative measures are implemented. This paper provides a descriptive overview of all the steps leading to improved operational and strategic management support for Marina Reservoir. Keywords: Reservoir management, water quality modelling, mitigative measures, operational management system

2 Introduction The Marina Reservoir (see Figure 1), fed by the largest and most urbanized catchment in Singapore, has been created recently with a 350-meter long barrage across the mouth of the Marina Channel. The barrage includes 9 gates, 7 pumps (of 40 m 3 /s each) and 2 low level sluices. This conversion of Marina Bay into a fresh water reservoir will contribute in 2010 to the future water supply of Singapore by contributing some 10% of the current water demand. The Marina Reservoir will also serve to alleviate problems of flooding in the low-lying areas in the city and enhance the value of the Marina Bay as a recreational resource. Kallang Bay Marina Bay Marina Channel barrage Figure 1 Marina Reservoir catchment and Upper and Lower Peirce Reservoirs The completion of Marina Reservoir and scheduled other reservoir schemes such as Punggol and Serangoon will transform two-thirds of Singapore into water catchment areas. All of the five main waterways draining into Marina Reservoir (i.e. Singapore River, Stanford Canal, Rochor Canal, Kallang River and Geylang River) have served important functions to the economic development of the country since the mid 1800s, with the development of many financial institutions and various commercial activities concentrated along its banks. As such, the waterways supported the growth with the majority of the population living and working around the rivers. Industries such as boat repairs, goods processing plants flourished along these waterways (Hon, 1991). By the mid-1970s, the increased population and industries around these areas started causing pollution problems. The waterways had degenerated into an open sewer. Water in the river systems were black and odoriferous, and spread out into the surrounding seas from the mouth of the basin (Chou, 1998).

3 In 1977, a 10 year cleaning programme of the Singapore River and the basin was initiated by the Singapore government. This included 1) cleaning and dredging of the waterways, 2) phasing out of polluting activities, 3) removal and /or relocation of farms, improper workshops and 4) development of suitable infrastructure, factories, housing and food centres for those affected by the relocation (Chua and Loo 1987; Hon 1991). Beaches were created at Kallang Basin, with the dredged river bed and sections of the banks covered with sand, to transform Kallang Basin into a riverside park (Hon 1991). For many years this rehabilitated urban catchment in combination with an efficient tidal flushing of the Marina Bay did not reveal any significant water quality problems. Now with the conversion towards a freshwater reservoir and the intensification of the shoreline use it may not go unnoticed as during the process water quality problems and hence impacts on different user functions may arise: eutrophication, possibly leading to nuisance algae blooms (including toxic bluegreens), oxygen depletion and smell problems; silt, solids and organic matter runoff, leading to high turbidity (brownish water); bacterial contamination and related disease vectors; accumulation of micro-pollutants (heavy metals and organic substances) and accidental spills of chemicals; and accumulation of diesel range and gasoline range organics Litter accumulation Initial mosquito breeding (when the fish stock is not yet developed). From early 2008 till end of 2009 (Table 1), when different mitigation programs are still under construction, the reservoir will be operated as a well-flushed brackish water system with sufficiently high salinity to avoid likely problems with bluegreens and mosquito breeding. During this period the reservoir will already be used for flood prevention, creating storage whenever needed, and for all sorts of recreational activities, requiring water level control and well-controlled water quality. Despite the reduced flushing compared with the previous open bay situation it was believed that the implementation of aerator systems could overcome problems with oxygen depletion. Hereafter (i.e. at the end of 2009), the system will be gradually converted to a freshwater reservoir. The duration of this conversion will depend on the rainfall and therefore on the season. Although the salinity will quickly drop to low levels, it may take a longer time (up to a few months) to make the water potable. Due to the conversion to a freshwater system this period may demonstrate some adverse water quality due to the expected degradation of living organisms Table 1 From bay to freshwater reservoir. Marina Bay Brackish reservoir Transition to freshwater reservoir Till early2008 Early 2008 end of End of 2009 medio well-flushed estuary with no water quality problems 2009 well-flushed brackish reservoir with no significant water quality problems 2010 controlled conversion to freshwater system with possible water quality problems Freshwater reservoir Medio 2010 onwards controlled freshwater reservoir with all measures implemented and with minimal problems

4 Programme implementation PUB has put in place various action programmes to ensure that the water quality in Marina Reservoir will meet the aesthetic, recreational and drinking water needs in the new downtown of Singapore. With the exception of securing low salinity levels, water quality for drinking does not seem to be an issue nowadays as advanced membrane filtration will effectively remove most of the pollutants and pathogens. The key programmes are: i. To control pollution at source by sewer rehabilitation, silt control at construction sites, catchment surveillance and litter control so as to reduce occurrences of algae blooms (especially bluegreens that may be toxic and produce scums on the water surface), brownish water (mainly caused by rain events bringing in silt) and bacterial contamination (faecal coliform (FC)) which could affect the aesthetic and recreational water quality of the Marina Reservoir; ii. To implement mitigation measures such as a recirculation system which will circulate up to 5 m 3 /s of water from the Marina Reservoir to a treatment plant to remove pollutants, then into Upper Peirce, and finally to the five main tributaries flowing into the Marina Reservoir; iii. Educate the public in keeping the catchments clean through the Active, Beautiful and Clean (ABC) Waters Programme which is set-up to systematically identify opportunities to transform the utilitarian waterways and reservoirs to vibrant, beautiful and clean streams, rivers and lakes; iv. Engage available knowledge and technology to set-up of comprehensive field surveillance programme and to develop a state-of-the-art water quality modeling framework and an operational management system that will monitor and predict the water quality in the Marina Reservoir, enabling reservoir operators to make decisions to operate the barrage and pumps or other measures to manage water quality in the Marina Reservoir. Water quality modelling framework To better understand the systems, guide the implementation of mitigative measures and prepare for the future operational management at PUB, a modelling framework (see Figure 4) has been set up to describe: Rainfall run-off to the various sub-catchments of Upper and Lower Peirce Reservoirs and Marina Reservoir (hydrological model HYMOS, HYMOS is the Deltares - Delft Hydraulics information system for water resources management in general. It covers all data storage and processing requirements for analysis, planning, design and operation of water management systems. The Sacramentomodule simulates the rainfall-runoff process in part of the catchment, where the attention is on the land-phase of the rainfall-runoff process. It is assumed that the open water system in the segments contributes little to the shaping of the hydrograph. The conveyance of water through the drains, canals and rivers (1 D hydraulic model SOBEK, Sobek is a Deltares - Delft Hydraulics 1D and 2D instrument for flood forecasting, drainage systems, irrigation systems, sewer overflow, ground-water level control, river morphology, salt intrusion and water quality. The model is used to simulate the hydraulics of the canal and river system within the catchment (see Figure 2). It relates to a combined application of the Channel Flow and Sewer Flow modules.

5 Discharges Singapore River Flow (m3/s) Computed Measured Time Figure 2 Example of hydraulic model results for the Singapore River Emissions from point and non-point sources (emission model) were derived using latest land use information (Master Plan, 2003), storm water sampling data and routine monitoring data for the canal and river systems, other available measurements for specific source contributions (atmospheric deposition, sewers, industry, golf courses, construction sites, green zones, backyards, foodstalls, etc.) and literature data (characteristic emission data). The emission modeling, applying a SOBEK sub-module, was based on four distinct steps: (1) estimation of annual loads using land use information; (2) First check of annual load estimates with regard to reproducing the measured water quality in Marina Bay; (3) Calibration of the detailed spatial and temporal variable load estimates on the water quality measurements in the tributaries; and (4) Reverse modelling based on comparison of water quality measurements in Marina Bay with the results of the coupled 1D-3D water quality model fed with the results of the detailed load estimates. Transport, water and sediment quality for the three reservoir systems (3D water and sediment quality model Delft3D, Delft3D is Deltares - Delft Hydraulics 3D instrument that was set-up for the former open bay situation as well as for all three reservoirs (see Figure 3). Delft3D describes the hydrodynamics and water and sediment quality (FLOW, WAQ and ECO modules). Altogether, the number of active grid points in the Marina Reservoir model grid is about 2500 in the horizontal plane. The areas of the Marina Bay basin, Kallang basin and Marina Channel have a typical grid size of 25 m by 25 m. In the a maximum of 12 computational vertical layers (in the deepest parts) is used.

6 Figure 3 Example of calculated transition to freshwater reservoir computed with meteorological forcing and run-off representative of the year This year is considered to be and average year in terms of run-off. Conversion to freshwater reservoir can be done within six months depending on the season. Real-time control (SOBEK RTC module) operation of the barrage (as integral part of the coupled 1D-3D model). The SOBEK RTC module is applied to deal with the complex operational rules for the crest gates, pumps and pipes given specific conditions obtained from the SOBEK and Delft3d models (as of now water levels inside and outside of the barrage and salinity and dissolved oxygen at bottom layers inside the reservoir). data on meteorology, land use, bathymetry / topography, loads, WQ, etc. Hydrology Marina catchment / Hydraulics drainage canals Boundary conditions from hydrodynamic model of Singapore strait HYMOS / SOBEK Hydrodynamics Delft3D-FLOW Emissions from catchment Sobek-Emission design Mitigative measures 2007 Water quality Delft3D-WAQ / ECO Water Quality Management Plan 2008 Operational Management System 2009 post-barrage updates (2010) Implementation plan live system Figure 4 Schematized stepwise modeling approach indicating the phasing and timing of activities and the various models used. The whole batch of models will be re-calibrated after the transition period. All these modules were coupled to allow for an integrated analysis of all water quality management options.

7 The models have provided increased insight in the functioning of the interlinked systems through describing the complex hydrodynamics (water balances, stratification and effects of aeration) transport path, fate and effects of different water quality parameters (resulting in among others detailed nutrient balances, algal species composition and the occurrence of oxygen depletion, bacterial pollution, high turbidity and nuisance algal blooms). Figure 5: Calculated future water balance for the fresh Marina Reservoir (based on 2005 hydrological data) Total-P Atmospheric deposition 0.18 Total-N Atmospheric deposition Denitrification Marina tributaries 2.55 Kallang tributaries Storage Outflow SS Other runoff 2.21 Internal load Water 5.30 Sediment Settling Storage 5.48 Marina tributaries Kallang tributaries Storage Outflow SS Other runoff 9.57 Internal load Water Sediment Denitrification Settling Storage Burial Marina Reservoir Figure 6: Calculated nitrogen and phosphorous balance for the fresh Marina Reservoir (based on 2005 hydrological data) The model simulations so far showed that even after implementation of selected mitigative measures (aeration, recirculation, source control) some water quality problems in the future reservoir may still arise. Simulations also show that the various source control and mitigation programmes (as a combination of source control, aeration and recirculation), in particular 0.38 Burial Marina Reservoir

8 sewer rehabilitation, silt control, re-circulation and aeration will help to significantly improve water quality in the Marina Reservoir. i. Without the programmes: The model predicts that recreational water quality standards will not be met 20% of the time in Marina Bay, 38% of the time in Kallang Basin and 7% of the time in the Marina Channel. Marina Reservoir will be prone to algal blooms. Silty water is also expected about times a year in Marina Bay and Marina Channel, and about 30 times a year in Kallang Basin. ii. With the programmes: The model predicts that the programmes will reduce the exceedance of the recreational water quality standards, to 10% of the time in Marina Basin, 30% of the time in Kallang Basin, and 3% of the time in the Marina Channel. They will also reduce the potential for algal scum formation, and improve the situation of silty water by %. Given the complexity of the catchment, the water quality modeling framework predicts that there would still be times when water quality in the Marina Reservoir may exceed target levels especially after heavy storms. Nevertheless, it is predicted that with all the programmes in place, water quality will be to that of Lower Seletar Reservoir, which is currently a popular spot for recreational activities. At the same time it is expected that the system will remain protected against eutrophication problems that occurred in other Singaporean Reservoirs. The Operational Water Quality Management Plan in the making, aims to address and prioritise the most practical and effective strategic and operational options for the management of the water quality of Marina Reservoir. This means that in addition to the various measures to prevent problems to occur curative measures are considered to mitigate the problems. Although many alternative measures have been discussed in literature, in practice most of them do not work well in systems with a constant pollution load or combine well with the existing boundary conditions (drinking water use, controlled water level, etc.). Operational Management System To address the future operational water quality management problems an operational management system (OMS) is under development to support day-to-day decision-making based upon continuous rainfall forecasting (using numerical weather prediction modeling and radar data), water quantity and quality monitoring information streams and the 1D3D water quality model forecasting (see Figure 7). The first version of the OMS addresses the optimal tidal flushing and water level control scheme for brackish conditions that has been put in place for the period , i.e. the period before the system will be converted to a freshwater reservoir. Monitoring Prediction Response Forecasting Figure 7 Set-up of OMS

9 The general role of the OMS is to provide up-to-date information about present and expected water quality conditions in the coupled Marina, Upper and Lower Peirce systems. Hereto the OMS will be: A control room application that monitors the system to support the day-to-day operational tasks of the managers a live system that runs 24-hours a day The OMS monitors and applies the rules as set by the overall Water Quality Management Plan in the making. The OMS assists to predict when day-to-day water quality problems appear and assists the PUB management to assess the size of the problem, reduce potential damage, take short-term mitigating measures and restore the preferred Marina Reservoir situation. The OMS supports operational management of the coupled Marina system by: Providing online information of current water quality state through related water quality variables, indicators and warning levels Predicting the expected water quality state using scenario predictions Providing warnings based on current and predicted states when preset targets can or will not be met Providing information regarding the expected effect of pre-selected available management measures Hereto, the OMS will manage and contain: All available data streams from different (online) sources of information The current and historic data required for proper evaluation of events (performance checks) The 1D3D modelling framework A tailored set of visual tools for operation and communication The on-line OMS output is already available on individual Personal Computers of preselected users. The future life OMS will be implemented in the control room of the Marina Barrage. Organisational set-up and capacity building The 1D3D modeling framework and Operational Management System have been transferred to strategic and operational treams within PUB. Both teams work close together to best manage the Marina Reservoir (see Figure 8).

10 Strategic team What if modelling framework & routine monitoring PUB management operational rules & advise reconstruction of events Operational team What to do when OMS & early warning monitoring Figure 8 Organisational set-up of Operational and Strategic teams illustrating the specific activities and available means (tools and monitoring). Conclusions The stepwise approach presented in this paper has already led to a unique coupled 1D3D modelling framework describing the water quality processes in the coupled Marina catchment multiple reservoir system. Current work involves the implementation of this framework in the Operational Management System to support the day-to-day operational management by making forecasts and recommendations on the mitigative or regulatory actions to take. As a The various programmes to manage water quality in Marina Reservoir are progressing well. The modelling study has shown that these programmes, especially sewer rehabilitation, silt control, recirculation and aeration, will significantly improve water quality in the Marina Reservoir. Nevertheless, the model study has revealed that there will still be periods when water quality is unlikely to meet target levels, especially after heavy storms. PUB will continue to engage the public and private sectors to reduce sources of pollution in the Marina Reservoir catchment, and explore new and innovative measures through R&D and the ABC Waters Programme to mitigate the effects. Acknowledgement The authors are grateful to PUB for allowing the presentation of the current state-of-the-art development of the modeling framework and operational management system considering the political sensitivity of the intermediate results. References Chou, L.M., The cleaning of Singapore River and the Kallang basin: approaches, methods, investments and benefits. Ocean & Coastal Management 38: Hon.J, Tidal fortunes A story of change: The Singapore River and Kallang River, Singapore landmark Books. ++++