May 10, 2016 - These results assist the analysis of the mixing process, and ... Keywords: CORMIX, CorTime, Time Series, Contour Maps, Dilution, Exceedance ...
Organized by:
IAHR-IWA Joint Committee on Marine Outfall Systems May 10th-13th, 2016, Ottawa, Canada
Method to classify and visualize time series simulations of submarine outfalls M.M. Ishikawa & T. Bleninger Programa de Pós Graduação em Engenharia de Recursos Hídricos e Ambiental, Universidade Federal do Paraná, Brazil
A.V. Falkenberg & R.C. Barletta CB&I Brazil, Florianópolis, Brazil
A.B. Trevisan & V. Dos Santos Casan, Florianópolis, Brazil ABSTRACT The increase of coastal population over the years carries the increase of waste and the necessity of the improvement of sanitation systems and its expansion. One option for these regions is the ocean disposal by submarine outfalls. To meet the growing demand with quality it is necessary improve the studies concerning the mixing processes and the characterization of near and far field. These two fields have very different characteristics, their divergences do not allow the modelling of the process with only one model. It is known that the major dilution occurs at the near field and consequently it is the most affected region. Therefore, the evaluation of the efficiency of submarine outfalls in this region is vital to guarantee the compliance of the regulations. There are basically three methods to solve the mixing process: the empirical, easy and fast to solve; the integral, which regards the velocity and density, but assumes steady state; and the numerical, that describes in detail the mixing but requires time and high computational effort. The integral method is commonly used due to the satisfactory results obtained without the necessity of sophisticated tools. However, it works in steady state while the ambient and discharge conditions are unsteady. CORMIX is a near field model based on integral methods. The use of time series can improve the results of the steady state model, showing the influence of the registered variations after running the programme several times. CORMIX has a post-processing tool that automatically run the simulations in time series, CorTime. It summarizes the dilution and plume characteristics of each time step simulated, and make a simple statistical analysis. This work will show post-processed results from CorTime, with the aim of get a better spatial visualization of the plume behavior, and have a more specific answer about the compliance of the system with the regulatory standards. The data used are from studies made in Florianópolis to the installation of a submarine outfall. The post-processing was made at MATLAB. It was obtained graphs of exceedance frequency (the frequency which the system efficiency is not in compliance), graphs of the average dilution and an video showing the plume variations over the time. These results assist the analysis of the mixing process, and at the decision making of projects. Keywords: CORMIX, CorTime, Time Series, Contour Maps, Dilution, Exceedance Frequency, Near Field
1 INTRODUCTION Many coastal waters suffered under pollution, due to the large population growth in coastal cities, and often on account of the poor management and control of the sanitation system. To be more specific, 60% of the world population lives within 100 km from the shoreline coast (UNEP, 2004). UNEP (2005) identifies the atmospheric deposition, the municipal, industrial and agricultural wastes and run-off as the activities that most affect productive areas of marine environment. However, the most damaging way in which cities pollute the coasts is the uncontrolled wastewater and sewage discharge. Consequently, the activities realized in the coasts are affected, as the offshore oil and gas extraction, trade and shipping, and the most profitable, the coastal tourism (UNEP, 2008). Furthermore, health issues can be related with the pollution of these regions. In January of 2016, CASAN (the water and sanitation agency of Santa Catarina State – Brazil) had to take emergency actions to control the outbreak of diseases registered by people who had swimming at Canasvieiras beach. The São Bras River (which has its river mouth at Canasvieiras) was blocked to mitigate the situation, since it is polluted with clandestine sources of sewage (MPF, 2016). To control the wastewater and sewage discharges of coastal cities, submarine outfalls systems may be very helpful. The concept of the system is the collection of the sewage to a treatment plant, after that, the effluent goes to the outfall, a pipe that discharges the liquid in the ocean by diffusers. Most of domestic sewage only goes through a preliminary treatment before its discharge, due to the high capacity of dilution and biodegradation of its characteristics substances. Other effluents (i.e. industrial that may contain toxic substances) require higher level of treatments depending on the sensitivity of the coastal ecosystem. The dilution is defined by the concentration decay of a solution by the increase of its solvent. It happens when an effluent is discharged by a submarine outfall. The dilution of a conservative effluent (no decay processes) in an environment with a background concentration is defined by:
S
c0 c
ca ca
(1)
Where the dilution S is dimensionless, c0 is the concentration of the solution that will be diluted (in this case the effluent concentration), ca is the background concentration (ambient concentration) and c is the final concentration (after the dilution). The mixing process can be divided in two regions: near and far field. The first one is the zone of initial dilution, where the effects of the discharge prevail. The second is the region where the dynamics o f the water body, as tides, govern the plume behavior. The different dynamics of near and far field implies the necessity of distinguished models. Coupling the near field model to the far field model is a manner to describe the entire waste field, but it is a complex method. 1.1 Objectives The major dilution of submarine outfalls occurs within the near field and consequently it is the most impacted area. This way, the paper works with a steady state near field model (CORMIX), to study the dispersion of plumes. Since the environment is unsteady, time series are used to obtain quasi-unsteady results (CorTime). The product of the model is taken to produce graphs to visualize the plume behavior along the time series, to identify the areas that exceed the limits and the distribution of dilution.
2 MIXING PROCESS As mentioned previously, the discharge of effluents is divided in two regions, near field and far field (Figure 1). These two fields have very different characteristics, the first is an interplay between discharge and ambient, the scales are small, so it must be described in three dimensions, and the boundaries can be neglected. Its significant initial dilution occurs basically because the turbulence caused by buoyancy and momentum. With this in mind, it is possible increase the turbulence working with the discharge design (diffuser length, port spacing, volumetric flow, etc).
Figure 1: Laboratorial experiment for turbulent jet, with buoyancy and inclined in a reservoir with linear stratification. Source: Socolofsky et al. (2013)
Meanwhile the far field is dominated by the ambient flow conditions, it is necessary a large knowledge of the region, the scales are larger, then the boundaries need to be regarded, the process can be simplified and described in two dimensions. At the far field the oceanic turbulence becomes the responsible for the mixing, making more difficult to work with the system in order to improve its efficiency. The characteristics scales of time and length are shown at Figure 2:
Figure 2: Characteristic temporal and spatial scales in submarine outfalls. Source: Socolofsky et al. (2013)
Both of the fields have the same governing equations (continuity, momentum and transport equation), however, its differences leading them to different simplifications and consequently, different models. For the near field there are three methods of solution: Empirical methods: simple and fast to solve, but has many simplifications and does not have physical fundaments; Integral methods: transform the partial differential equations to ordinary differential equations, because the self-similarity assumption and the steady state simplification, turn them easier to solve; Numerical methods: used in complex cases where simplifications are not allowed, requiring time, sophisticated tools and high computational efforts; At this paper the integral method is utilized, in view of its convenience and for produce the main results (dilution and trajectory in an unbounded water body). A better description of the integral method can be consulted in Socolofsky (2013) and Jirka (2006). 3 CORMIX CORMIX is software to analyze, predict and design toxic or conventional discharges in different water bodies. Based on an integral method for near field, it contemplates situations of single port, multiport diffusers and surface discharges. It was developed in cooperation of several institutions, among them the U.S.EPA (Environmental Protection Agency), Cornell University, Oregon Graduate Institute and MixZon. It makes qualitative analysis with length scales (empirical method, where the boundaries are regarded) selecting a pre-classified flow that can identify a key aspect. The quantity analysis is made by the CorJet (Buoyant Jet Integral Model). Besides the plume geometry, CORMIX is also recommended to assess the regulatory mixing zone according to the water quality standards, based on the initial dilution (Doneker, 2007). As can be expected, the model works in steady state, although the environment is unsteady. In an effort to regard the parameters that are susceptible to the hydrodynamics variations in the water body, the time series are employed. 3.1 CorTime CorTime is post-processing tool of CORMIX, available at the version GTR (Research), which allows the automated calculation of time series data. In doing so, the steady state model is run several times turning the ambient or discharge varying notable. In other words, the time changes are visualized as a discrete data. The input file contains the parameters required to run CORMIX (hydrodynamics data from the environment and discharge characteristics). CorTime assists the analysis of the plume behavior under time varying conditions; the tool is also used to the far field coupling (Doneker, 2007). The main output of CorTime is the Status Report, where the characteristics of the modeled plumes, one per time step, are summarized. As the plume central line coordinates at the end of the near field and at the end of the regulatory mixing zone, its dimensions, dilutions, concentrations and the time travel until this point. It is also possible see graphs with the frequency distribution of the parameters, the parameters
vs. time and the distribution of the final coordinates around the source. To follow the objectives of this work, the Status Report is used. 4 STUDY SITE The case study chosen to present the method is based on studies made to the project of Santa Catarina Island submarine outfall (CB&I Brazil and CASAN). The Island is part of the Florianópolis municipality; it has an estimated population of 589,720 inhabitants and can reaches almost 1 million during the summer, due to its touristic beaches (IPUF, 2007). According to ABES (2008), only 12% of the Santa Catarina State population has sewage treatment. Therefore CASAN targets the improvement of the sanitation system, to cover up 100% of the population with the wastewater service until 2030. The Santa Catarina Island submarine outfall is one of the works to improve the wastewater treatment. There are three possible locations for the system: Rio Vermelho, Ingleses and Canasvieiras (Figure 3-a), each location had an ADCP deployed for one year set up for currents, waves, pressure, temperature and salinity measurements, which were used to calibrate and validate the hydrodynamic numerical model Delft3D-FLOW.
(a) (b) fdsaffd(b)sdflsdlf Figure 3: (a) The three possible localization of the Santa Catarina Island submarine outfall (b) Canasvieiras distribution of currents during summer (CB&I Brazil, 2015)
It was used the data of velocity (magnitude and direction), density and depth from one month during summer with Δt of 30 minutes, for Canavieiras at 3 km and 6 km. Figure 3 (b) presents the currents distribution also during the summer for Canasvieiras, resulted of the hydrodynamic modeling (CB&I Brazil, 2015).
Both locations have a predominant axis in NE and SW and average ambient density of 1024 kg.m-3 without stratification. At 3 km the average velocity is 0.11 m.s-1 and depth of 8.78 m, while at 6 km the average velocity is 0.13 m.s-1 and depth of 10.48 m. 5 METHODOLOGY The results from status report are used to sketch a triangular (for single port diffuser) or trapezoidal plume (for multiport diffusers), as the scheme at Figure 4 (as shown by Morelissen et al, 2014). The NFRX is the distance from the source to the end of the near field region towards to the ambient velocity (indicated by the angle phi, counted clockwise from the geographic north), and the NFRBH is the halfwidth of the plume. With these two lengths a right triangle is defined, and the coordinates of the corners are settled. The sources are matched as zero dilution, the corners and the NFRX point are matched with the final solution simulated by CORMIX. After that the dilution of the whole plume is settle by a simple interpolation. The process shown above is repeated for each time step. As long as the dilutions are defined, the concentration of the diverse substances founded in sewages can be estimated by the equation (1).
Figure 4: Scheme of the adopted procedure to interpolate the dilution of plumes
In order to visualize the distribution of the dilution, the average dilution is calculated with all the time steps and then is plotted with isolines. To identify the areas of risk it is necessary define a substance and settle the required dilution. For instance the total nitrogen, it has an emission standard of 45 mg.L -1 (Sperling, 2014), following the CONAMA 357/2005 resolution (Brazilian discharges standards) the stricter case for saltwater require a maximum concentration of 0.40 mg.L-1. Therefore, the required dilution is 112.5; if the dilution of the region is lower than this the area of risk is defined. However, the regulatory mixing zones (RMZ) define a limited area around the source where the initial dilution is allowed to occur until the water quality is in compliance with the standards (Doneker, 2007).
The regulators agencies also use to settle a value of exceedance frequency; it means that the maximum concentrations can be exceeded in a certain proportion, usually given in percentage. With this in mind, a graph indicating the exceedance frequency nearby the discharge was developed. Each point of the area had its dilutions compared with the required dilution, in the whole time series. If it was lower than the required it was considered an exceed. At the end the percentage of the exceedance frequency from the simulated time was scored and plotted also with isolines. In an effort to visualize the plume behavior, the concentrations of a plume to any substance can be obtained with the dilutions and equation (1). They are plotted for all the time steps and each time step is converted to a frame of a video. This way is possible to see the discharge dispersion along time. 6 RESULTS It was simulated a period of one month, with a Δt of 30 minutes, for two regions of Canasvieiras: 3 km, and 6 km from the coast, both have the same configurations (the multiport diffuser has 300 m of length with 30 diffusers of 15 cm of diameter, and the volumetric flow varying from 1.523 to 2.452 m³.s -1). The regulatory mixing zone was settled as an area within 200 m of radius from the middle of the multiport diffusers (point (0,0)). The diffuser line was settled as perpendicular as possible with the currents, to improve the spreading of the effluent, the angle alpha (see at Figure 4) is 120º at 3 km and 135º at 6 km. At Figure 5 the dilution contour graphs are shown, (a) for 3 km and (b) for 6 km.
(a) fdsaffd(b)sdflsdlf
(b)
Figure 5: Dilutions contours for Canasvieiras (a) 3 km and (b) 6 km from the coast
With a distance of 3 km from the coast, the average dilution for one month is about 160 and it is met, in most part, before the regulatory mixing zone. It is a satisfactory value for the nitrogen. At 6 km from the coast the average dilution increases considerably, the average dilution before the mixing zone is 220 and can reach the value of 260 in some locations.
Through the two graphs in Figure 5 is possible see that one region has a better efficiency. The same configuration of diffusers increased 60% of the dilution in a different location. The exceedance frequency (Figure 6) was based on the required dilution of 112.5 (as the nitrogen example), also for 3 km and for 6 km in a same configuration. It was estimated the exceedances of 1%, 10%, 20%, 30%, 40% and 50%, the Brazilian standards usually establish 20%. The regulatory mixing zone has an area of 125,664 m²; the areas of each estimated exceedance are shown at Table 1: Table 1: Areas for each estimated exceedance frequency, for 3 km and 6 km 1%
10 %
20%
30%
40%
50%
3 km
227,600
96,400
67,200
56,000
51,600
8,800
6 km
193,200
78,800
60,000
52,400
44,000
18,000
Difference
- 15%
- 18%
- 10%
- 6%
- 15%
+ 104%
In general the areas decrease, except for the 50% that has its double at 6 km when compared with the 3 km. However it is the initial region, which has the major impact, and it is within the RMZ. Beyond this region, the main exceed is of 1%. Likewise the dilution contour, this second analysis reaffirms that the region at 6 km from the coast has a better efficiency. It can be visualized at Figure 6.
(a) fdsaffd(b)sdflsdlf
(b)
Figure 6: Exceedances frequencies for Canasvieiras (a) 3 km and (b) 6 km from the coast
The better mixing at the farther location can be related to the depth. Deep waters have better mixing and shoreline discharges have low dilution and very slow mixing (Bleninger et al., 2011). Further, deeply submerged plumes have lowest dilutions. It happens when the ambient is stratified and the plume is trapped in a layer of same density. In this case the plume has a small length of depth to cover, similarly
as a shoreline discharge. Meanwhile, in general, the surfacing plumes have much higher dilutions, since the effluent has a longer way to cross, resulting in a better mixing (Roberts, 1999).
Ambient velocity direction
Diffuser Line
At last, the variations of the plume along time can be seen in a video made from all the time steps. It helps to understand the two first analysis that are based on all of its frames. The Figure 7 presents in a simple way the idea of its concept.
Figure 7: Frames from the video to visualize the plume variations according to concentration
7 CONCLUSION As the principal dilution of submarine outfalls occurs at the near field region, it is important assess the region of discharge to guarantee its compliance with the standards, before the construction. Therefore the modeling is a powerful tool to guide the taking decision of projects. The integral methods need to be handle correctly, concerning the time variations since they are based on steady state. So, even with its limitations, the method is fast to solve and can obtain useful results, as in this paper. The employment of time series supported the variations of ambient and discharge. The same analysis for two different scenarios demonstrates the importance of the ambient conditions at the mixing process. The efficiency can be improved only by changing the location. It is possible make these analysis modifying the diffuser characteristics. All things considered, it can be concluded that work with a steady state model can generate satisfactory results along time.
ACKNOWLEDGEMENT The authors acknowledge the funding by the Brazilian Research Council, CNPq via project no. 132123/2014-2. MixZon Inc. is acknowledged for provide the academic license of CORMIX .And finally CB&I Brazil and CASAN for its contributions.
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