Approaches to Water Monitoring and Modelling [Mar 2008]
Assessing water quality pressures on fluvial systems
The population in certain areas of England has grown rapidly in the last few decades. As a result, additional water will be needed to meet predicted levels of customer demand in the next 10 years in several parts of the region. This expanding population is also creating a larger energy demand. It is inevitable that conflicts will arise between the pressure exerted on fluvial systems to meet these water and energy needs and more stringent environmental protection afforded by the Water Framework Directive (WFD) requirements (WFD UK TAG, 2007).
But how can we identify these potential environmental impacts and minimise/manage them in a targeted fashion to achieve compliance against environmental quality standards? This article details the monitoring/modelling approaches that can be used to identify potential impacts and benefits associated with water resource investigations and power station discharges.
Water resource schemes
Water companies are currently undertaking planning for new strategic water resources to address this shortfall. Where water resources development is to take place, water companies need to demonstrate that the requirements of the EC Directive of Environmental Impact Assessment have been met (EC Directive 97/11/EC). Appraisal of the baseline water quality conditions in the catchment of interest forms part of this assessment. The generated baseline assessment is ultimately used in water quality models, which aim to simulate impacts arising from different strategic water resource options in the catchment.
Existing data?
Work should aim to complement existing data to enhance our knowledge of the current and historic water quality environment that will be influenced by the various water resource options. The Environment Agency’s routine data collection programme and water company discharge/abstraction data provide valuable information to assess long term and spatial water quality variation in catchments. Baseline reviews of data and site specific appraisals/ ranking based on environmental quality standards can be completed.
Missing information?
Existing data certainly performs an important role and should not be overlooked. However, gap analysis invariably reveals a number of issues for specific water resource schemes that can not be addressed by this type of nationwide monitoring. For example:
- Lack of determinands / frequency of monitoring in analysis suite at key sites to address forthcoming WFD guidelines/legislation
- Short term fluctuations in water quality, particularly in highly dynamic systems
- Influence of low frequency, high magnitude storm events on nutrient loads
- Sedimentation of fish spawning beds
To facilitate the Environmental Impact Assessment process, a programme of monitoring has been initiated to infill these apparent gaps. The following section presents preliminary results.
Data scarcity?
Additional spot sampling sites have to be added in data-poor catchments to provide improved spatial and determinand set coverage (Figure 1). The choice of determinads should be governed by the objectives of the project but new standards for the WFD-R1, WFD-RS and WFD-PS suites under the Water Framework Directive should be borne in mind during this selection process (WFD UK TAG, 2007).
Crash, bang, wallop?
Routine Environment Agency data provide a long-term record of historical changes in water quality in water courses. However, this provides no indication of short-term, high magnitude changes (e.g. algal blooms, sediment flushing, pollution incidents, and tidal variation). High temporal resolution monitoring is necessary to improve our understanding of the dynamics and drivers of short-term changes in water quality in these catchments (Figure 2).
Big storm, big effect?
![Figure 8: Cooling Towers at Cottam [2000MW] Power Station
AWE International Issue 14 Mar 2008
© AWE International 2008](images/articleimages/233.jpg "Figure 8: Cooling Towers at Cottam [2000MW] Power Station
AWE International Issue 14 Mar 2008
© AWE International 2008")
Routine Environment Agency sediment and nutrient data collected at monthly intervals provide a long-term record of historical changes in water quality. However, this gives no indication of short-term, high magnitude changes resulting from rainfall events which generate overland flow/wetland flushing and discharges from wastewater treatment works during peak periods of demand. High temporal resolution data is necessary to fully understand the dynamics and drivers of short-term changes in sediment and nutrient concentrations and compute annual nutrient loads delivered in the catchments.
The transport of nutrients through a catchment during these periods can dominate the annual budget. ISCO 3700 series autosamplers with depth-trigger switches were deployed in catchments with a non uniform collection time programmes biased towards capturing ‘first flush’ events (Figure 3).
All clogged up?
Concern regarding the impact of certain water resource options on fish spawning beds in catchments had been raised. An increase in fine sediment deposition can be deleterious to spawning success (due to lower oxygen diffusion and impeded alevin emergence). Sediment data collection protocols (Figure 4) were devised to allow the following to be investigated:
- Suitability of river bed sediments as salmonid spawning grounds (Figure 5)
- Investigation of temporal changes in composition of bed sediments
- Relationship between flow and sediment character
Modelling
These monitoring data can be used to inform later modelling studies and can potentially flag up issues and processes that might not have been apparent from routine data. As part of one such water resources development scheme, Atkins was commissioned to develop a water quality model for a river in the South East of England.
The DHI modelling software Mike-11, with its Ecolab water quality module, was selected as the most appropriate tool for this work. The model extends from the coast to approximately 38 km inland, hence including the transition from a tidal to fluvial system. Water temperature, salinity, biochemical oxygen demand (BOD) and dissolved oxygen (DO) were simulated at a 5-minute time-step and over a six-week period. These four parameters represent important water quality indicators:
- Water temperature has a critical impact on aquatic life; biochemical reactions commonly experience a doubling in reaction rate with a rise of 10°C
- Similarly, changes in salinity can have a big impact on the ecosystem, as several aquatic species have narrow salinity tolerance
- BOD is a measure of the potential oxygen demand exerted within waters, generally arising from decaying organic matter
- DO concentrations in the water are a good indicator of the general water quality of the system. Fish kills are often due to asphyxia. Particular attention was paid to DO in this system, as there have been concerns with regard to low levels in the summer months
To infill data gaps, automatic water quality monitors were deployed and spot samples were collected at several locations along the river, specifically for this project. This data set, in combination with Environment Agency and Met Office data, was used to create boundary time-series, but also to calibrate and test the model. Typically, each water quality parameter was calibrated and tested against two to three points along the river.
Several modifications to the standard model were made to achieve a good fit with the observed data. For example:
- Cloud cover reduces the amount of light available for photosynthesis and may hence affect DO concentrations. Hourly cloud cover data were obtained and the DO equation was modified to include the effect of changes in cloud cover (based on Chapra et al., 2003)
- Turbidity varies significantly and dynamically along the river; it is expected to be at its maximum at the river mouth, gradually decreasing away from the tidal interface, and is strongly affected by the tidal cycle. It was considered that this may represent an important control on water quality by limiting the amount of sunlight available to algae. A turbidity factor, which varied spatially and dynamically in accordance with depth (and by proxy, tidal state), was introduced in the photosynthesis equation (based on Chapra et al., 2003)
- Field observations indicated the presence of dense marginal vegetation in the upper river reaches (see Figure 6). This reduces the amount of sunlight entering the water and the effect of wind on surface water cooling. A shading parameter was, therefore, included both in the calculation of DO and temperature
The result of the model development work was a significantly improved fit with observed data, as shown in Figure 7. The simulations replicate well the temporal and spatial detail present in observed data from this complex system, and have greatly improved our overall understanding of water quality processes in this tidal river. The water quality model has since been used as a tool to assess the potential environmental impact from a number of options to meet the forecast supply-demand deficit in the region.
Meeting power needs
The effects of power generation on the environment have very much come into the spotlight recently. Although there is a direct link between large scale power generation and the need to disperse waste heat in adjacent bodies of water, ironically the pressure to increase the use of renewable resources has also led to smaller scale but significant efforts use local water bodies as heat sinks. A good example is a district cooling scheme where large refrigeration plants cool water circulated throughout an entire district, with the waste heat dispersed at sea.
Indirect systems
The familiar sight of condensation clouds rising from the cooling towers of a power station indicates that indirect cooling. Cottam Power Station, located along the River Trent, is such an example (Figure 8) where latent heat of vaporation is used to reject a large amount of waste heat into the atmosphere.
Even when using cooling towers, there is still a need for water to be drawn from a nearby water body to make up evaporative losses and to assist with other processes such as flue gas desulphurisation.
Direct systems
Direct cooling systems draw water from a water body and after passing it through the heat exchanger discharge it to same water body. Large quantities of water are required, as there is now no opportunity to make use of latent heat of vaporisation. Flows are typically about 50 times that for an indirectly cooled power station of the same overall power output (Figure 9).
The heat load from a large power station can be very significant, for example a 2GW direct cooled power station requires a flow of 30 tonnes per second of water heated 15°C above the inlet temperature to provide the necessary cooling for the condensers.
The environmental impacts can be large but, not only must new designs comply with appropriate regulations, existing consents to discharge are continually being revised as new regulations come into force.
All of these factors have led to an increased need for more accurate assessment of the thermal plumes generated by power stations of a wide variety of sizes and types. Apart from the increased pressure from regulations, greater use of renewable energy sources is also putting pressure on developers to use radical approaches and slightly more unusual bodies of water for waste heat rejection. The feasibility of such schemes needs to be carefully assessed, from the point of view of engineering viability and environmental impact.
Modelling
Up till a few years ago the accurate modelling of thermal plumes was expensive and laborious. The density gradients associated with large temperature differences between the receiving water and the thermal plume often leads to stratification, which requires a fully three dimensional approach. Attempts to bypass the need for such work often led to end of pipe conditions being imposed as part of discharge consents which were inaccurate and not clearly related to the regulations. The connection between the two is best specified by defining temperatures at the edge of the mixing zone, but then the edge of the mixing zone must also be established.
Some older discharge consents are based on a fixed temperature specified at the discharge point. Not surprisingly this says little about the real extent of the plume as although such consents can mention the edge of the mixing zone there is no reasonable definition of its location. Using a calibrated 3D numerical model of the receiving waters, the plume behaviour can be simulated for a variety of environmental conditions. In reality the plume is constantly changing as the tide washes back and forth and in estuarine locations varying fluvial flows from upstream strongly influence the plume behaviour. The weather can also have an influence on surface heat exchange.
Limits on temperature rise above ambient conditions, particularly in rivers and estuaries, are related to the sensitivity of cyprinid and salmonid fish to both temperature elevations and absolute temperatures. By defining the plume edge as the 98%ile temperature contour of 1.5°C or 3°C above ambient, a clearer picture of the plume extent can be derived from simulations. Such a technique can be applied to outfalls in all fluvial, estuarine and coastal environments to show compliance with regulations or to provide a basis for outfall location optimisation or to examine the feasibility of a proposed project.
Figure 10 shows the extent of the mixing zone associated with a large power station thermal plume. The location is well upstream from the sea but where tidal influence is still significant. It shows clearly how much of the river is uninfluenced although the plume is bank attached.
Conclusions
This article has shown a range of environmental investigations that can be applied to quantify impacts placed on fluvial systems from increased water and energy demand. Although these studies are site specific, the issues they seek to address are largely generic and can be applied to most urbanised European catchments. Indeed, with the inception of the WFD and catchment wide view on resource management, these studies will become more prevalent and important.
References
Acornley RM, Sear DA (1999) Sediment transport and siltation of brown trout (Salmo trutta L.) spawning gravels in chalk streams. Hydrological Processes, 13, 447-458.
Chapra et al 2003, S., Pelletier, G., Tao, H., 2003. QUAL 2K, A modelling framework for simulating river and stream water quality, User Manual.
EC Directive 97/11/EC (1997) Amending DIRECTIVE 85/337/EEC of 27 June 1985 on the assessment of the effects of certain public and private projects on the environment. 3 March 1997.
O’Connor WC, Andrew TE (1998) The effects of siltation on Atlantic salmon Salmo salar L., embryos in the River Bush. Fisheries Management and Ecology, 5, 393-401.
WFD UK TAG (2007) UK Environmental Standards and Conditions (Phase 2). SR1-2007. UK Technical Advisory Group on the Water Framework Directive. June 2007.
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