Freie Universität Berlin Department of Earth Sciences Institute of Geographical Sciences
Master thesis
Impact Assessment of Floating Houses on Water Temperature and Dissolved Oxygen in Himpenser Wielen, Leeuwarden (Netherlands)
Janko Lenz Matriculation number: 4958994 Berlin, 29.01.2018
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Impact Assessment of Floating Houses on Water Temperature and Dissolved Oxygen in Himpenser Wielen, Leeuwarden (Netherlands)
by
Janko Lenz
in fulfillment of the requirements for the degree of
Master of Science
Geographical Sciences, area of concentration “Environmental Hydrology”
at the Freie Universität Berlin
First Supervisor: Prof. Dr. Achim Schulte, Freie Universität Berlin
Second Supervisor: Prof. Dr. Ir. Floris Boogaard, Hanze University of Applied Science Groningen
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4 Abstract
Large floating structures have the potential to overcome the challenge of land scarcity in urban areas. They offer opportunities for energy and food production or even habitation. On the other hand they influence the physical and chemical characteristics and hence living conditions in the water body they are floating in. A monitoring of these impacts is needed to enable the development of building legislation on future construction projects. For the present thesis floating houses at Himpenser Wielen in Leeuwarden (Netherlands) were selected, were a measurement campaign was carried out in autumn 2017. Two multi- parameter probes were utilized for monitoring of water temperature and dissolved oxygen content. Pioneering work was a 10-day measurement under a floating house. Vertical temperature profiles were recorded to detect differences in heat transport. Additionally an underwater drone was used for investigation of mussels and macrophytes underneath the floating house. Water temperature was lower in open water compared to the area under the house, by 0.15K on average. Checked against the shaded area it was lower by 0.14K at the water bottom and 0.1K near the water surface respectively. Dissolved oxygen content was higher in open water than in shaded area, by 0.8mg/l in shallower and 2.8mg/l in greater depths. Long-term measurement has a high potential for monitoring the environmental impact of floating houses. In the presented thesis it could show diurnal cycles of water temperature and dissolved oxygen, also in greater depths. Moreover a high dependency of these parameters on weather condition was determined. Further research, taking at least one year, would show effects of floating structures on the seasonal variations of water temperature and dissolved oxygen content. Other parameters, like nutrients, should also be investigated over longer periods. In addition the development of a more suitable method for measurement under floating houses remains as a challenge.
5 Acknowledgements
The author wrote his bachelor thesis about the impact of floating solar panels on the evaporation of lakes. After answering this question a new one came up: What is the impact of floating structures on water quality? During studies of Environmental Hydrology at the Freie Universität Berlin the concept for a master thesis project on this topic was developed. As solar companies only produce modules, but are not responsible for monitoring their environmental impact, the challenge arose to find a partner, who was interested in the concept and possessed the equipment for implementation. Fortunately INDYMO united both properties and accepted me for an internship. Special thanks are due to Mr. de Lima, who not only mentored me throughout this time, but was always available for exchanging knowledge and ideas. Conducting a measurement campaign at floating houses is inconceivable without the help of local residents. Of particular note are the families Luiks, de Roos and Renkema, who allowed me to fix devices on their property.
6 Content
Abstract ...... 5
Acknowledgements ...... 6
List of figures ...... 9
List of acronyms ...... 12
1 Introduction ...... 14
1.1 Problem setting ...... 14
1.2 Very Large Floating Structures ...... 15
1.3 Outline of the thesis ...... 17
2 Current state of research ...... 19
2.1 Theoretical background ...... 19
2.2 Potential impact of floating houses...... 22
2.3 Empirical Background ...... 23
2.3.1 Mega-Float ...... 23
2.3.2 Studies from the Netherlands ...... 25
3 Research approach ...... 30
4 Own preliminary works ...... 31
5 Study area ...... 33
6 Material and Methods ...... 38
6.1 Materials ...... 38
6.1.1 Devices ...... 38
6.1.2 Software ...... 42
6.2 Methods ...... 43
6.2.1 Period A ...... 43
6.2.2 Period B ...... 45
6.2.3 Period C ...... 46
6.2.4 Period D ...... 46
7 6.2.5 Weather conditions ...... 48
6.2.6 Underwater images ...... 49
6.2.7 Calibration ...... 49
6.2.8 Questionnaires ...... 50
7 Results ...... 51
7.1 Results of period A ...... 51
7.2 Results of period B ...... 58
7.3 Results of period C ...... 64
7.4 Results of period D ...... 71
7.5 Further results...... 76
7.6 Verification of hypotheses ...... 78
8 Discussion ...... 80
8.1 Discussion of results ...... 80
8.2 Discussion of methods ...... 81
8.3 Recommendations ...... 82
9 Conclusion ...... 83
10 Bibliography ...... 84
8 List of figures
Figure 1: Floating solar energy plant in Huainan, China ...... 15 Figure 2: Floating Farm (left) and Lilipad (right) ...... 16 Figure 3: Research concept of the present master thesis ...... 18 Figure 4: Energy budget of a lake ...... 20 Figure 5: Oxygen processes in open water ...... 21 Figure 6: Currents caused by wind ...... 21 Figure 7: Potential impact of floating houses ...... 22 Figure 8: Mooring site of Mega-Float models in Tokyo Bay...... 23 Figure 9: Vertical water temperature with and without Mega-Float I ...... 24 Figure 10: Mega-Float II airbase in Tokyo Bay, Japan ...... 24 Figure 11: Differences of DO caused by shade (left) and tunnel effect (right) ...... 26 Figure 12: Method scheme of DE LIMA and SAZONOV (2014) ...... 27 Figure 13: Minimum values for DO measured by DE LIMA and SAZONOV (2014) ...... 27 Figure 14: Dissolved oxygen content at IJburg 1 ...... 29 Figure 15: Locations of test measurements ...... 32 Figure 16: Location of the study area in the Netherlands ...... 33 Figure 17: Climate diagram of Leeuwarden ...... 34 Figure 18: Wind rose of Leeuwarden ...... 34 Figure 19: Location of the study area in Leeuwarden ...... 35 Figure 20: Location of the floating houses in the “Hoek” ...... 36 Figure 21: Floating houses in Leeuwarden ...... 36 Figure 22: Location of the measurement points in Himpenser Wielen ...... 37 Figure 23: TROLL9500 ...... 38 Figure 24: Van Essen CTD-Diver (left) and Mini-Diver (right) ...... 39 Figure 25: OpenROV and GoPro Hero 3+ (attached) ...... 40 Figure 26: Vantage Pro2 ...... 41 Figure 27: Measuring scheme at the FH1 and fixing in period A and C ...... 43 Figure 28: Attachment of the probes ...... 44 Figure 29: Measuring scheme at OW1 and connection between buoy and probes ...... 44 Figure 30: Measuring scheme at the FH2 in period B ...... 45 Figure 31: Canoe (left) and paddle with marks (right) ...... 46 Figure 32: Measuring scheme at the FH1 in period D and fixing of the Mini-Divers ...... 47 Figure 33: Measuring scheme at the OW3 in period D (left) and fixing (right) ...... 47
9 Figure 34: Localization of the weather station and the measuring points ...... 48 Figure 35: Fixing of the weather station and its operating ...... 49 Figure 36: Localization of the calibration site ...... 50 Figure 37: Calibration design for the Mini-Divers ...... 50 Figure 38: Dissolved oxygen content at FH1 during period A ...... 52 Figure 39: Water temperature at FH1 during period A ...... 53 Figure 40: Dissolved oxygen content at OW1 during period A ...... 54 Figure 41: Water temperature at OW1 during period A ...... 55 Figure 42: Autocorrelation of water temperature during period A ...... 55 Figure 43: Water temperature and DO at FH1 and OW1 on Day5 ...... 56 Figure 44: Wind conditions during LTA ...... 57 Figure 45: Dissolved oxygen content and water temperature during LTA ...... 57 Figure 46: Dissolved oxygen content at OW2 during period B ...... 59 Figure 47: Water temperature at FH2 and OW2 during Section1 ...... 60 Figure 48: Water temperature at FH2 and OW2 during Section2 ...... 61 Figure 49: Water temperature at FH2 and OW2 during Section3 ...... 61 Figure 50: Wind conditions during period B...... 62 Figure 51: Water temperatures and dissolved oxygen on 2nd of October ...... 63 Figure 52: Water temperature and dissolved oxygen at FH1 during period C ...... 65 Figure 53: Water temperature and dissolved oxygen at OW3 during period C ...... 65 Figure 54: Water temperature and dissolved oxygen (Part1, period C) ...... 66 Figure 55: Water temperature and dissolved oxygen at FH1 during LTC ...... 67 Figure 56: Water temperature and dissolved oxygen at OW3 during LTC ...... 68 Figure 57: Wind conditions during LTC ...... 69 Figure 58: Water temperature and dissolved oxygen content on 8th of October ...... 69 Figure 59: Dissolved oxygen content and high wind speeds at FH1 during LTC ...... 70 Figure 60: Vertical profile of water temperature at FH1 before sunrise ...... 72 Figure 61: Vertical profile of water temperature at OW3 in the morning ...... 72 Figure 62: Vertical profile of water temperature at FH1 around noon ...... 73 Figure 63: Vertical profile of water temperature at OW3 in the afternoon ...... 74 Figure 64: Vertical profile of water temperature at FH1 in the afternoon ...... 75 Figure 65: Mussel colony at the bottom side of the floating house ...... 76 Figure 66: Fish underneath the floating house ...... 76 Figure 67: Macrophyte on the bottom of the shaded area ...... 77 Figure 68: Algal bloom (left) and manure streaks (right) ...... 77
10 List of tables
Table 1: Space demand in the Netherlands in 2030 ...... 14 Table 2: Capacities of the measuring points ...... 37 Table 3: Specifications of TROLL 9500 ...... 38 Table 4: Specifications of van Essen CTD-Diver und Mini-Diver ...... 39 Table 5: Main specifications of GoPro Hero 3+ ...... 40 Table 6: Specifications of Open ROV 2.8 ...... 41 Table 7: Specifications ofVantage Pro2 weather station ...... 42 Table 8: Measuring times of the vertical profiles in period D ...... 48 Table 9: Measuring conditions during period A...... 51 Table 10: Frame conditions during period B ...... 58 Table 11: Wind speeds during period B ...... 62 Table 12: Frame conditions during period C ...... 64
11 List of acronyms
° Degrees ∆ Difference % per cent ACF Autocorrelation function C Celsius ca Heat capacity of air cm Centimeters cw Heat capacity of water
DO Dissolved oxygen E East ENE East-Northeast
ESE East-Southeast
Ebw Energy budget of a water body EU European Union F Fahrenheit FH Floating house FNU Formazine Nephelometric Units ft Feet g Gram / Acceleration of gravity h Hours K Kelvin kg Kilogram kJ Kilojoule km Kilometers km² Square kilometers l Liter LTA Long-term measurement in period A LTC Long term measurement in period C m Meters m² Square meters mg Milligram mm Millimeters MP Megapixels Mr. Mister N North NE Northeast 12 NNE North-Northeast NNW North-Northwest NTU Nephelometric Turbidity Unit NW Northwest OW Open water oz Ounces PAR Photosynthetic active radiation P Pressure
Qla Latent heat flux
Qlwa Long wave irradiance of the atmosphere
Qlww Long wave irradiance of the water
Qs Short wave irradiance of the sun
Qse sensible heat flux ρ Density ROV Remotely Operated Vehicle S South S Second SE Southeast sec Seconds SSE South-Southeast SSW South-Southwest SW Southwest T Temperature TR Temperature range USD US Dollar VLFS Very Large Floating Structures W Watts / West WD Water depth WNW West-Northwest WS Weather station WSW West-Southwest y Year
13 1 Introduction
"Water is a friendly element to whomever is familiar with it and knows how to handle it.” J.W. Goethe: Elective affinities, German original: Wahlverwandtschaften
Goethe’s quote reminds us that we have two opportunities: Either we could harm the water which in turn would harm ourselves. Or we treat it in a way that it promotes our well-being. If we modify a water body, even only at its surface, we have to be aware of the consequences for the whole environment.
1.1 Problem setting
Worldwide population growth and increasing urbanization created a serious problem: land scarcity, especially in metropolitan areas. In the future this challenge will become more and more relevant as rising sea levels and heavier floods are expected by the IPCC (2014). KOEKOEK (2010) calculated that in 2030 the Netherlands demand 3191km² more space than the country covers (see table 1).
Table 1: Space demand in the Netherlands in 2030
Source: KOEKOEK (2010, p. A5-8)
For a long time several countries with a high population density, such as Japan and the Netherlands, expanded their areas significantly through reclaiming land from the sea. As this causes negative impacts on the marine ecosystem, Very Large Floating Structures (VLFS for short) may offer an attractive alternative (WATANABE et al., 2004). In the following the concept of VLFS is described, with a focus on floating houses.
14 1.2 Very Large Floating Structures
Bridges are the most well-known infrastructures moored in water areas around the world. In contrast floating infrastructure became increasingly relevant only in recent years. Either they really float on the water (pontoon type) or they partly immerse (semi-submersible type). To prevent from drifting away they are fixed at the water bottom, by columns or chains. Notwithstanding these differences they are summarized under the name Very Large Floating Structures (ANDRIANOV, 2005).
Figure 1: Floating solar energy plant in Huainan, China
Source: DESIGNBOOM (online)
Very Large Floating Structures comprise several advantages: They are fast to construct and can be removed or expanded easily (WATANABE et al., 2004). DE GRAAF (2012) pointed out the adaptability to flood events and rising sea levels. VLFS can be used for energy production, for instance by solar power plants (see figure 1). Supplying food or human habitation (WANG and TAY, 2011) are other possible applications. Architect CALLEBAUT (2008) designed “Lilipad”, a floating ecopolis (see figure 2) and the SEASTEADING INSTITUTE (2014) further developed this concept. More examples, like oil storage bases, were complemented by TRIPATHY and PANI (2014) in their overview publication. There is a vital debate among architects, futurologists, engineers and economists about further developments in the field of VLFS. According to GRAND VIEW RESEARCH INC. (online) floating solar panel market size was 13.8 million USD in 2015. Until 2025 it is expected grow up to 2.7 billion USD. On International Floating Solar Symposium, held 24th until 26th of October 2017 in Singapore, opportunities and applications were discussed (ASIACLEANSUMMIT, online). The author joined the Floating Solutions Symposium "Let's Float", which took place on 27th of September in Delft. Among others the Floating Farm, which is located in Rotterdam, was presented as one opportunity for urban food production (see figure 2).
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Figure 2: Floating Farm (left) and Lilipad (right)
Sources: FLOATINGFARM (online), CALLEBAUT (online)
In contrast to futuristic cities floating in the sea, houses floating on inland waters are already reality. As there is neither a final definition nor a complete list of floating houses they are defined here as being not motorized, but moored and having a house number. As they have a draft of >1m they belong to the semi-submersible type of VLFS. An overview can be found on CLIMATESCAN.NL, which comprises 60 sites of floating urbanization in the Netherlands alone. Besides residential also industrial purposes are represented, e.g. the Limonade Fabriek in Streefkerk. VLFS may be one solution for human’s demand for space, but they may also create side effects, especially when implemented in big scale. WATANABE et al. (2004) stated, that VLFS are environmentally friendly as they do not damage the eco-system or disrupt the currents. The potential to increase local biodiversity and attract fish was complemented by INGER et al. (2009), who then again indicated the risk of habitat loss. Fresh water is not only a source of drinking water, it is also inhabited by fish which people consume. Moreover water areas provide opportunities for recreation and tourism. Any loss of these ecosystem services would harm human well-being (HÄRTWICH, 2016). Knowing about increasing threads on water bodies the EU Water Framework Directive was developed to prevent further deterioration of water quality. Its goal is to achieve good chemical and ecological status in European waters (WATER FRAMEWORK DIRECTIVE, 2000). The Netherlands adopted this goal in the National Water Plan. In its status report improving water quality, threatened by fertilizer was determined (NATIONAL WATER PLAN, 2014). After KITAZAWA et al. (2010) human intervention has resulted in local modification and destruction of the environments around the world. For this reason research of the environmental impact of floating houses is needed to enable the development of building legislation on future construction projects. Thus maximal sizes could be prescribed or simply decided, where installations should be sited (INGER et al., 2009).
16 1.3 Outline of the thesis
As set out above there is a knowledge gap about environmental impacts of floating structures. The main aim of the presented thesis is to quantify the impacts of floating houses on water temperature and dissolved oxygen content. Based on the current state of research, the author established his own research questions and hypotheses, which will be displayed in chapters 2 and 3 respectively. In order to find answers he decided to carry out an own measuring campaign. Prior to an internship at INDYMO several floating houses were visited to choose the most suitable location for the fieldwork, as explained in chapter 4. The study area is described in chapter 5. Based on previous studies and available equipment appropriate methods were selected for the conduction, which is explained in chapter 6. After carrying out the measurement campaign from 14th of September until 9th of October 2017 in Leeuwarden (Netherlands) the results were analyzed and will be presented in chapter 7. In chapter 8 the findings of this thesis will be compared to the state of the art. Moreover the used methods are evaluated to allow for giving recommendations on future studies. Conclusions are drawn in chapter 9. In figure 3 an overview of the research concept is displayed.
17 Theoretical and empirical background ‐ Considerations about possible impacts ‐ Methods used in preliminary studies ‐ Results of past measuring campaigns
Development of the fieldwork design ‐ Establishment of research questions ‐ Choice of a suitable location ‐ Selection of appropriate methods
Fieldwork ‐ Conduction of measurements
Analysis ‐ Presentation and explanation of results
Discussion ‐ Evaluation of used methods ‐ Comparison of results to previous works
Conclusion ‐ Summary of findings ‐ Recommendations for further reseach
Figure 3: Research concept of the present master thesis
18 2 Current state of research
Prior to assessing the impact of floating houses the factors which drive water temperature and dissolved oxygen content need to be explained. At first the main processes are described from a theoretical approach. Subsequently the major research studies in this field are presented.
2.1 Theoretical background
The specific heat capacity of fresh water (cw) is about four times the one of air (ca), as is apparent from equations:
cw= 4,182 kJ/kg·K ca= 1,005 kJ/kg·K each at T=20°C [2.1]
Air heats up faster than water, but also cools down more quickly. For this reason air and water hardly ever have exactly the same temperature. Hence at the air-water boundary heat is exchanged permanently, from warmer to cooler aggregate. To which extend the whole water body is heated or cooled depends on vertical energy transport (DYCK und PESCHKE, 1983).
The energy budget of a water body (Ebw) is essential for understanding the physical processes in a lake. It consists of five components (HENDERSON-SELLERS, 1984, see figure 4), which can be displayed as:
Ebw= Qs+Qlwa−Qlww−Qse−Qla whereby: [2.2]
Qs = short wave irradiance of the sun
Qlwa = long wave irradiance of the atmosphere
Qlww = long wave irradiance of the water
Qse = sensible heat flux
2 Qla = latent heat flux each in W/m
Qs depends on the solar radiation and cloudiness of the atmosphere, whereas Qlwa is proportional to the air temperature. Short wave radiation is partly reflected at the water surface, but the remaining share can penetrate to great depths, especially blue visible light. That’s why water in lakes “looks” blue (SUMICH and MORRISSEY, 2004).
19 Long wave radiation of the atmosphere is hardly reflected at the water surface, but almost totally absorbed in the upper layer (VIETINGHOFF, 2000). In the water radiation is converted to heat. Both are energy inputs for the lake, whereas the following represent energy losses.
Qlww is dependent on the water temperature, Qse and Qla are strongly forced by the wind. Furthermore it applies, that the bigger the difference between water and air temperature, the higher is the sensible heat flux. It is created by thermal conduction, in contrast to diffusion forming latent heat flux (CSANADY, 2001). We have to keep in mind that the main external driving forces of the water temperature are wind, solar radiation and ambient temperature.
Figure 4: Energy budget of a lake
Source: Own illustration, adapted from HENDERSON-SELLERS, 1984, p. 34
A physical property of cold water is that oxygen can dissolve easier than in warm water, for instance 10.92mg/l at 10°C compared to 7.53mg/l at 30°C (UNIVERSITÄT MÜNSTER, 2007). According to Henry’s law solubility is proportional to pressure. But there are more processes which influence the oxygen content of water, as can be seen in figure 5. In open water wind forces waves, which ensure material exchange between air and water body [a]. By reaeration oxygen is brought into the water, but partly oxygen is also taken out of the water. Currents and dispersion lead to the transport of the oxygen to other areas [b]. Living autotrophic organisms, like algae and macrophytes produce oxygen by photosynthesis [c]. Bacterial organic degradation of dead creatures consumes oxygen [d]. In the sediment oxygen is consumed, for instance by macrobenthos [e] (BOL and TOBÉ, 2015). Photosynthesis prevails in the upper, euphotic zone, whereas oxygen consumption predominates in the deeper, dysphotic zone (HENDERSON-SELLERS, 1984).
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Figure 5: Oxygen processes in open water
Source: BOL and TOBÉ (2015, p. 14)
A closer look is to take at the transport process [b]. Not only reaeration [a], but also horizontal and vertical currents are enforced by wind, as can be seen in figure 6. The stronger the wind, the further heat and oxygen is transported. Oxygen is 100 times less diffusive (HENDERSON-SELLERS, 1984).
Figure 6: Currents caused by wind
Source: Own drawing, adapted from DYCK und PESCHKE (1983, p. 213)
Wind also causes convection by pressing warm surface water into greater depths (forced convection). Natural convection occurs when water flows in from other areas. It arranges in order to density, if it has an unequal temperature and/or nutrient content (DYCK und PESCHKE, 1983). This is important for understanding the processes in the area shaded by a floating house, which is considered in the next section.
21 2.2 Potential impact of floating houses
Figure 7: Potential impact of floating houses
Source: BOL and TOBÉ (2015, p. 16)
Floating houses have an influence on all three main driving forces of the water temperature (see figure 7). They block the incident short wave solar radiation, depending on their size and the sun position (HÄRTWICH, 2016). This shade effect impedes the growth of phytoplankton and macrophytes below the platform (BURDICK and SHORT, 1999). In the shaded area besides the house long wave radiation, reflected for instance by clouds, still enters the water. However, plants produce less oxygen, because photosynthesis is reduced (BOL and TOBÉ, 2015). As a floating house is a barrier for wind and waves, the reaeration of the water body is weakened on the lee side. Between two houses a tunnel effect may occur at higher wind velocities causing better mixing of the water column (FOKA, 2014). BOL and TOBÉ (2015) assume that the barrier effect leads to increased accumulation of suspended particles, as well as debris, underneath the platform. Under a floating house the temperature should be more stable throughout the day and year, because cooling down during night and in winter is diminished by reduced exchange processes (FOKA, 2014). Moreover a heat exchange between the house and its surrounding occurs, which heats up the water (BOL and TOBÉ, 2015).
22 It has to be added that the surfaces of platforms get colonized by sessile organisms (COLE et al., 2005). By using oxygen for respiration they deplete the dissolved oxygen content of the surrounding. Excreted nutrients get dispersed increasing the nutrient concentration in the water as well as at the water bottom. Dead mussels also fall down and get decomposed, which increases the oxygen demand at the water-sediment interface (KITAZAWA et al., 2010 and HÄRTWICH, 2016).
2.3 Empirical Background
In this chapter major studies of the impacts caused by floating structures will be presented. The focus lies on a campaign conducted in Japan and several works from the Netherlands. The Dutch research projects are extensively described to allow for comparison of used methods and gained results.
2.3.1 Mega-Float
In Japan, in the western Tokyo Bay south of Yokohama, a test aircraft runway was launched in 1995 (see figure 8).
Figure 8: Mooring site of Mega-Float models in Tokyo Bay
Source: KITAZAWA et al., 2010, p. 462
23 Technological Research Association of Mega-Float was established to conduct the "Research and Development of an Ultra Large Floating Structure" for a three-year program (SATO, 2003, p. 377). Besides technical research also the physical environment was investigated, such as water temperature and salinity (TABETA et al., 2003). Strong currents lead to fast water renewal under the platform, which could explain why almost no differences were detected. A supplemented model showed a slight decrease of the water temperature, in the surrounding as well as underneath of Mega-Float I in comparison to natural conditions (KYOZUKA et al., 1997). In a depth of 10m water temperature decreased by about 0.2K at the northern and southern edge of the platform and by about 0.3K at its center (see figure 9) – temperature differences are always given in K, as usual in physics.
Figure 9: Water temperature with and without Mega-Float I
Source: KYOZUKA et al., 1997, p. 158
By the end of 1997 it was decided to continue with a follow-up project called Mega-Float II from April 1998 to March 2001, after which it was removed. Whereas Mega-Float I had a size of 60 x 300m, Mega-Float II had a length of about 1,000m and a width of 60m (partially 121m). The draft was maintained at 1m (SATO, 2003, see figure 10).
Figure 10: Mega-Float II airbase in Tokyo Bay, Japan
Source: WANG and TAY, 2011, p. 64
24 The main focus of Mega-Float II was for take-off and landing of aircrafts, but the environmental studies were expanded. Chlorophyll α and dissolved oxygen were measured continuously for one year, additionally vertical profiles were taken (TABETA et al., 2003). The researchers measured the quality of bottom material, water organisms and benthos by taking water samples. Out of these observations an ecosystem model was developed (SATO, 2003). Neither a decrease in current velocity nor variations in temperature and salinity were observed under Mega-Float II. The concentration of dissolved oxygen was slightly lower in the deeper column below the platform, but did not reach hypoxic or anoxic levels, not even in summer (KITAZAWA et al., 2010). The concentrations of chlorophyll α were lower, but of nutrients, especially phosphate and ammonium, higher in the upper 5m under Mega-Float II (TABETA et al., 2003). These effects were mainly caused by the sessile organisms colonizing the floating structure, less by the impeded surface heat and salinity fluxes (KITAZAWA et al., 2001).
Note Mega-Float seems to be the best studied floating object to date. Due to major differences in framework conditions a 1:1 comparison to floating houses in fresh water systems is not practical. A runway has a small draft compared to its length. Tokyo Bay comprises saltwater and strong currents occur. The water column underneath the platform is much higher than in Dutch lakes.
2.3.2 Studies from the Netherlands
FOKA (2014) conducted a study at Harnaschpolder, Delft. 14 times, between July and September, the dissolved oxygen and water temperature were measured for 2 minutes at each depth, using a Hydrolab MS5 multi-probe. For continuous measurement over 20 days in September two CTD-Divers plus two Mini-Divers collected water temperature and pressure data. With a Baro Diver air temperature and air pressure was recorded. During that time the ambient conditions were monitored by a Vantage Pro2 weather station. She compared three locations: Between two floating houses, in a shaded area and in open water. For the former she detected a reduction of dissolved oxygen by 10% (1mg/l), compared to the latter. These differences occurred only in the upper layers (<1m depth) and mainly around noon, whereas in the morning and evening at both sites similar values were recorded. During stronger winds a tunnel effect between the houses was determined, causing better mixing of the water column, leading to lower gradients of dissolved oxygen.
25 For water temperature the difference was 0.5K, temperature variations in depth were very small. Differences in oxygen levels could not be explained by differences in water temperature, but were attributed to less photosynthesis. Unfortunately, results for the shaded area were not presented. Based on the data collected by the divers, FOKA developed a numerical model for the dissolved oxygen budget. It was calibrated with data from the multi-probe. Moreover it was calculated that the floating houses block 40% of the solar radiation, leading to differences in the amount of dissolved oxygen. As can be seen in figure 11, in the upper layer the DO at the floating house (DOf) was about 4mg/l lower than in open water (DOo) on some days. The effect decreases with depth. The model predicted that at the air-water-boundary the reaeration is negative – meaning that more oxygen is released to the atmosphere than taken up by the water – and stronger winds boost this process. As figure 11 shows, wind accounts for lower DO levels in the upper layers by about 1mg/l. It is to note that the correlation coefficients of the model were low, especially the DO values in the bottom layer deviated from the measurements. Hence the proportion of shade and tunnel effect could not be identified. Continuous measurements, also under floating houses, were recommended by the author.
Figure 11: Differences of DO caused by shade (left) and tunnel effect (right)
Source: FOKA (2014, pp. 71 and 73)
DE LIMA and SAZONOV (2014) carried out a study of water quality parameters at 16 locations of floating infrastructure in the Netherlands, between August and October 2014. A remotely operated underwater drone carried a TROLL9500 multi-probe, a CTD-, a Mini-Diver and dove underneath the floating objects (see figure 12).
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Figure 12: Method scheme of DE LIMA and SAZONOV (2014)
Source: DE LIMA and SAZONOV (2014, p. 8)
For some sites they additionally used a folding rule to place the sensors under the floating structure. Collected data was split into two fractions: Under or near structure and open water (see figure 13).
Figure 13: Minimum values for DO measured by DE LIMA and SAZONOV (2014)
Source: DE LIMA and SAZONOV (2014, p. 40)
27 In figure 13 the results of DE LIMA and SAZONOV’s measurements in 2014, separated by location, are shown. The minimum values of dissolved oxygen were significantly lower (∆>1mg/l) in five cases, in seven cases slightly lower (∆< 1mg/l) and in four cases slightly higher under or near the floating structure, compared to open water. At all sites except for Harnaschpolder (House 2) the required oxygen level for aquatic life (4.5mg/l) was met, in open water as well as under or near the floating structure. The exception was explained by the drone raising anoxic mud. Although no flow velocities were measured, they were held responsible for the different extent of change in the values. A relation to the object’s size or the water depth beneath draft could not be established. They concluded that the impact of floating structures on water quality is quite small, but new habitats are created. For further research they recommended a more continuous data collection and measurements in different water depths.
BOL and TOBÉ (2015) conducted a follow-up research at five sites with floating objects. The sensors measured mainly water temperature, oxygen ratio and salinity. For analysis the recordings were divided into three parts: Underneath the structure, near the structure and in open water. The results were classified according to Water Framework Directive. At the floating community IJburg water temperature and oxygen ratio showed only slight differences. For both indicators the results were classified as „very good water quality“. At the wetlands Boerenwetering in Amsterdam measurings on 26th of May 2015 showed a higher oxygen ratio in open water (54%), whereas underneath and near the floating structure 50% were recorded. Nevertheless the results for all sites were classified as „good“. On the same day at Entrepothaven underneath the houseboats higher temperatures and oxygen ratios by about 5% – compared to open water – were recorded. At the Water Villa in Middelburg it was the other way around. For oxygen ratio open water scored better („very good“, compared to „good“). On the previous measurements it scored worse. At the houses floating in Harnaschpolder (Delft) measurings on 21st of April 2015 showed big differences. The oxygen ratio under the house was 76.7%, nearby 92.6% and 106.5% in open water. Notwithstanding all three sites were classified as „very good“ for this indicator. They certify the drone a high potential for ecological monitorings, even so they recommend long term measurements in addition. The authors concluded that the floating structures they investigated have a positive environmental impact by forming new habitats and thus increasing biodiversity.
28 HÄRTWICH (2016) investigated the impacts of floating houses on the sediment, especially on benthic organisms. At four locations her measurements took place between 6th of April and 1st of June 2016. An OpenROV 2.7 drone was utilized, equipped with a CTD-Diver, a TROLL9500 and a GoPro camera. The last-mentioned was used to identify mussels, macrophytes and algae at the sediment surface. Additionally sediment samples were taken at one site. She detected mussels at all four platforms, but no macrophytes at the water bottom underneath the floating houses. At three of those no significant differences of dissolved oxygen content were recorded, compared to open water areas. She attributed this to high flow velocities. At IJburg 1 oxygen content decreased with greater depth, stronger than in open water (see figure 14). At this site also organic enrichment, higher nitrogen and organic carbon content was determined, in comparison to open water. Ecosystem monitoring before and after installation of floating houses was recommended by the author.
Figure 14: Dissolved oxygen content at IJburg 1
Source: HÄRTWICH, 2016, p. 20
Most of the Dutch studies went far beyond impact assessment. BOL and TOBÉ (2015) as well as HÄRTWICH (2016) suggested countermeasures to reduce negative effects. FOKA (2014) developed a numerical model to predict the changes over the year. From the thesis author’s point of view it is worth to take one step back. Better understanding of the effects of floating houses on physical, chemical and biological processes is a prerequisite for modeling as well as to deploy countermeasures in a targeted manner. The present thesis should make a small contribution on this path.
29 3 Research approach
Based on theoretical and empirical background of potential impacts of floating houses on water temperature and dissolved oxygen content the author’s research questions and hypotheses of the present thesis were defined. This thesis focused only on the effects on two parameters. The reasons for selecting dissolved oxygen and water temperature were their simple and reliable measurability and the availability of appropriate devices. Moreover dissolved oxygen is a prime indicator for biological processes (HENDERSON-SELLERS, 1984).
Research question 1: To what extend is the water temperature affected by floating houses?
Hypothesis 1: Water temperature in the shaded area is lower than in open water. This will affect the whole water column, because temperature gradients get balanced out by heat transport.
Hypothesis 2: Water temperature underneath the floating house is slightly higher than in the shaded area as well as in open water. As the measurement campaign was mainly conducted in autumn, water tends to cool down over time, but energy transmission to the atmosphere is limited by the floating house.
Research question 2: To what extend is the dissolved oxygen content affected by floating houses?
Hypothesis 3: Dissolved oxygen content in open water is higher than in the shaded area, where photosynthesis is reduced. This affects mainly the upper layer, as photosynthesis is prevailing there.
Hypothesis 4: Oxygen content underneath the floating house is lowest, because neither photosynthesis nor reaeration takes place there.
The objective is to quantify the changes and relate them to the different influence factors. A classification according to WATER FRAMEWORK DIRECTIVE (2000) is not expedient, because existing differences would get blurred (see the study of BOL and TOBÉ, 2015). Moreover several measurings throughout a year would have been required.
To answer the research questions the first task was to find a suitable site for carrying out the measurement campaign, which will be described in the following chapter. 30 4 Own preliminary works
In this short report it will be explained which measures were undertaken to find a suitable location for conducting the research campaign for the present master thesis. That site had to meet several framework conditions:
1. Floating objects side by side to a not influenced open water area 2. A water depth of >1.5m to allow for measurement underneath the floating object 3. Stagnant water to minimize external disturbances 4. Feasibility of measurements, by protection from theft of devices
In the Netherlands a significant number of floating objects were developed in recent years. In a first investigation period from 29th of May until 1st of June 2017 five locations were visited including test measurements (see figure 15). These measurings should answer the following questions:
1. Which site fulfills best the framework conditions? 2. Which method is appropriate to answer the research questions? 3. Which additional equipment is needed?
The first test was conducted at the “Floating garden” in Rotterdam [1]. But due to bad accessibility of the object and precarious environment the location was not shortlisted. In Lelystad [2] the inhabitants were better off. A measurement series was carried out from one’s balcony and another in open water. The results showed a high variation, probably caused by strong currents that were visible. Moreover one inhabitant told that the houses sometimes hit the water bottom. In Leeuwarden [3] the water body was deeper, this site also showed the most unequivocal results. As this was caused by the drone, which hit the ground several times, this test showed that using (only) a drone would not be the best method to answer the research questions. Nevertheless it can be used as an additional tool, for instance to detect colonization of mussels at the floating houses – which worked during the test. The next test site, in Utrecht [4], showed similar problems as Lelystad and at the fifth location, in IJburg [5], the water flow was even more intensive. The results were heavily dependent on local conditions and hence a comparison between different sites not very reasonable. For the present thesis only one site was chosen. The decision was made for Leeuwarden, because the challenge of hitting the ground easily could be overcome by taking a measurement chain instead of a drone.
31 The resulting requirements for the research campaign were:
1. Measurements at different sites (open water, under and near a floating house) in the same depth for same duration 2. Duration of each sample should be at least 24 hours to get diurnal cycles 3. A fixing tool is needed for measurements underneath the floating house 4. Additionally colonization by mussels should be monitored using the drone
Figure 15: Locations of test measurements
As it always took a while until the sensors adapted to new conditions after each movement, e.g. to a greater depth, the author opted for long term measurements where the adaptation time is negligible. Dynamic measurements, like with a drone, have to be considered as random samples. In contrast static measurements provide continuous data. As no metallic chains were available, ropes were a cheap solution to fulfill the third requirement.
32 5 Study area
The city of Leeuwarden is the capital of the Dutch province Fryslân. It is located about 110km NNE of Amsterdam (see figure 16). As it is about 20km far from the North Sea, part Waddenzee, Leeuwarden’s height is just 5m above sea level.
Study area
Figure 16: Location of the study area in the Netherlands
Like the whole country Leeuwarden experiences a moderate maritime climate, Cfb after Köppen and Geiger (CLIMATE-DATA.ORG, online). From December until February frost occurs on 12 days per month on average. In summer the city faces five heat days, in winter five days of snow (METEOBLUE, online). November boasts the highest amount of precipitation (84mm, see figure 17), while August is the warmest month with 16.1°C. Annual rainfall is 798mm and mean temperature at 8.7°C (CLIMATE-DATA.ORG, online). In figure 18 the occurrences of wind velocities, separated into nine classes and based on hourly means, are displayed. As can be seen the main wind direction is SW (982h/y), followed by WSW (839h/y) and SSW (800h/y). On 51 hours winds with a speed of >61km/h occur (METEOBLUE, online).
33
Figure 17: Climate diagram of Leeuwarden
Source: CLIMATE-DATA.ORG, online
Figure 18: Wind rose of Leeuwarden
Source: METEOBLUE, online
34 The floating houses where the measuring campaign took place are located in the quarter “Hoek”, 5km southeast of the city center (see figure 19).
Study area
Figure 19: Location of the study area in Leeuwarden
Figure 20 shows the “Hoek” neighborhood. Seven houses of the “Skûtesân” street float in the Himpenser Wielen. This water body has a north-south extension of 1.2km and a west- eastern sweep of 600m. For prevailing south-westerly winds the fetch – the distance it blows over water - is just 35m. Besides no data was available from the local water authority Wetterskip, Himpenser Wielen can be categorized as shallow and temperate freshwater lake. As the measurement campaign took place in autumn it can not be said, whether the lake is dimictic (stratified in summer) or pleomictic (continuous summer mixing, after DYCK und PESCHKE, 1983). Himpenser Wielen drains to the east into the Van Harinxmakanaal, which connects Leeuwarden to Harlingen at the North Sea. The floating houses are located in the flow shadow.
35
Figure 20: Location of the floating houses in the “Hoek”
In a project named “Het Blauwe Hart” the seven floating houses were installed in Himpenser Wielen in 2005 (see figure 21). According to the architect, Johan Sijtsma, they were the first floating houses with a mooring system in the Netherlands (SIJTSMA, online). The houses have a diameter of 7.69m and a draft of about 1.30m (OOMS, see appendix).
Figure 21: Floating houses in Leeuwarden 36 Measurements were carried out at sites under (FH2), near floating houses (FH1) and in open water area (OW1-3), because one aim of this thesis was to compare locations with these different characteristics (see table 2). As the campaign was conducted in autumn and FH1 was located at the northern edge of the house, it can be assumed that it faced no direct sunlight throughout the day. Hence it can be defined as “shaded area”. The distance of FH1 and FH2 to OW1-3 was about 83m to ensure same frame conditions (see figure 22).
Figure 22: Location of the measurement points in Himpenser Wielen
Table 2: Capacities of the measuring points
Measuring point Characteristic Easting* Northing* FH1 Near floating house 5°51'03.4"E 53°10'35.8"N FH2 Under floating house 5°51'03.5"E 53°10'35.6"N OW1 In open water 5°51'07.8"E 53°10'35.8"N OW2 In open water 5°51'07.9"E 53°10'35.6"N OW3 In open water 5°51'07.9"E 53°10'35.5"N *Source: GOOGLE MAPS, online
Of the seven floating houses of Skûtesân the one with house number 22 is located halfway. Therefore it would have been the first choice site for the campaign. During the test measurements in May, as well as in September, it was tried to get in contact with the owner of the house – without success. Fortunately, his neighbor Mr. Luiks, owner of number 20, was willing to help.
37 6 Material and Methods
6.1 Materials
6.1.1 Devices
Various equipment was used to measure water temperature and dissolved oxygen content. First of all two TROLL9500 probes were utilized to record the before mentioned parameters with corresponding sensors. DE LIMA et al. (2015) had previously applied the same sensors for water quality and ecology monitoring under floating structures. Its specifications are displayed in table 3.
Table 3: Specifications of TROLL9500
Operating temperature -5 to 50°C Storage temperature -40 to 65°C Dimensions 4.7 x 55.25 cm Weight 1.9 kg Memory 222,000 records Standard Sensors Accuracy Range Response time Dissolved oxygen ±0.1 mg/l 0 to 50 mg/l < 60 sec Temperature ±0.1°C -5 to 50°C < 30 sec Turbidity ±5% or 2 NTU 0 to 2,000 NTU 5 sec Source: IN-SITU INC., 2012
With a weight of 1.9kg TROLL9500 is relatively heavy. It contains a high storage capacity and has a good accuracy. On the other hand the response times for water temperature and especially for dissolved oxygen are rather long. TROLL9500 consists of a twist-lock connector to enable reading out the data. The sensors for the measured parameters (see figure 23) are usually covered by a nosecone to protect from mechanical stress. Measurement interval was set to 10 seconds to get high resolution data.
Figure 23: TROLL9500 Source: DIRECTINDUSTRY, online
38 To verify the intended depth a van Essen CTD-Diver (see figure 24) was applied, which also provides information about conductivity and water temperature. Its suitability for the purpose was proven by MARTIN et al. (2006), who investigated a Karst aquifer. The relevant specifications of the CTD-Diver are presented in table 4. Measurements every minute were assumed to be sufficient for depth monitoring. Schlumberger Mini-Divers were used to create vertical profiles of the water body (see chapter 6.2.4). MITZ et al. (2014) used them for temperature control in a hatchery for aquaculture, DESSIE et al. (2014) applied their pressure measuring to monitor river discharge. Furthermore, a Mini-Diver (see figure 24) was put on the shore to monitor the atmospheric pressure. Its specifications are also displayed in table 4. Sample interval was set to 5 seconds to get information about convective processes.
Table 4: Specifications of van Essen CTD-Diver and Schlumberger Mini-Diver
CTD-Diver Mini-Diver Length 135 mm 90 mm Diameter 22 mm 22 mm Weight 95 g 55 g Memory 48,000 measurements 2x24,000 measurements Sample interval 1 second to 99 hours 0.5 seconds to 99 hours Pressure (both) Temperature (both) Range Water depth of 10 m -20 to 80°C Accuracy ± 0.5 cm ± 0.1°C Resolution 0.2 cm 0.01°C Sources: VAN ESSEN (2017) and SCHLUMBERGER (2014)
Their low weight made the application easy, resolution and accuracy are sufficient for the purpose. High frequency measurements would fill the memory very fast, e.g. in 13 hours when recording every second. This is a challenge for long term experiments. The response time, as being 3 minutes for both devices (SCHLUMBERGER, 2014), is very long, particularly for short term measurements like the vertical profile.
Figure 24: Van Essen CTD-Diver (left) and Mini-Diver (right) Sources: VAN ESSEN (2017) and SCHLUMBERGER (2014)
39 Underwater images helped to detect colonization by mussels. A GoPro Hero 3+ (see figure 25) was used to take these images. Recently ORICCHIO et al. (2016) conducted a monitoring of benthos dynamics guided by this device. Main specifications are shown in table 5.
Table 5: Main specifications of GoPro Hero 3+
Weight 2.61 oz Underwater Depth Up to 131.2 ft Camcorder Sensor Resolution 12 pixels Effective Photo Resolution 12.0 MP Source: CNET, online
With a weight of 74g the GoPro could easily be attached to the drone. The diving depth of 40m and the resolution were satisfactory for the approach.
Figure 25: OpenROV and GoPro Hero 3+ (attached)
An OpenROV 2.8 drone (see figure 25) was used to carry the GoPro and provide light for taking images. The specifications of the drone are presented in table 6. Its weight force had to be balanced out with polysterene.
40 Table 6: Specifications of Open ROV 2.8
Physical specifications Weight 2.6kg Dimensions 30cm long x 20cm wide x 15cm high Nominal battery life 2-3 hours (depending on use) Performance specifications Maximum depth 100m Maximum tether length 300m Maximum forward speed 2 knots Temperature capability -10°C to 50°C Source: OPENROV, online
Wind is the main driving factor for mixing of the water column. Moreover rainfall and air temperature have an influence on the water temperature. To measure these three parameters the weather station Vantage Pro2 (see figure 26) was utilized. Its specifications are shown in table 7. The Integrated Sensor Suite (ISS), collecting the weather data, is powered by a solar panel and sends the data to a console.
Figure 26: Vantage Pro2
Source: WEATHERSHACK, online
As can be seen in table 7 the resolution for rainfall is 0.2mm. This means that rainfall is displayed as 0 (zero) until the rain bucket is filled with 0.2mm of rain and then tips. Also the resolution for the velocity of wind, as being 1km/h, is to be considered as rather low, especially during calm winds. In contrast wind direction was set to “compass rose”, as assumed to be sufficient for the present thesis.
41 Table 7: Specifications of Vantage Pro2 weather station
Integrated Sensor Suite (ISS) Operating Temperature -40° to +65°C Weight (with batteries) 0.85 kg Rainfall Resolution 0.2 mm Accuracy ±4% of total or ± 0.2mm, whichever is greater Outside Temperature Resolution 0.1°C or 1°C (user-selectable) Range -40° to +65°C Sensor Accuracy ±0.3°C Radiation Induced Error 2°C at solar noon Wind Direction Range 0 - 360° Display Resolution 22.5° on compass rose, 1° in numeric display Accuracy ±3° Wind Speed Resolution 1 km/h Range 0 to 322 km/h Accuracy 1.5 km/h or ±5%, whichever is greater Source: DAVIS, 2017
6.1.2 Software
Data collected by the weather station was stored in a remotely placed console. Using the program WeatherLink data was read out, followed by a first visualization. It allows for plotting several measured parameters, thus for instance rain events could be detected. As statistical analyses or comparisons were not possible, the data was exported as txt-file. CTD- and Mini-Divers stored their records internally. The data was read out utilizing the program Diver-Office. To derive the depth from measured pressure values it had to be converted by equation 6.2. For further analysis data was converted from dat- to mon-file.
WD = 9806.65 * (PW-PA) / (ρ*g) whereby: [6.1]
WD = Water depth
PW = pressure of the water in cmH2O
PA = pressure of the air in cmH2O ρ = density of the water ≈ 1000kg/m³ g = acceleration of gravity ≈ 9.81m/s²
To get WD in m, it applies in good approximation: WD = (PW-PA) / 100 [6.2]
42 WinSitu retrieved the data of the different sensors of TROLL9500. The data files were converted from bin- to csv-format. As its sensors recorded temperature in °F (TF), values had to be converted in °C (TC) after equation 6.3.
TC = (TF - 32) * 5/9 [6.3]
Further elaboration was done in Excel, statistical analyses were coded in R. ArcGIS 10.4.1 was used for drawing maps. With WRPLOT wind roses were plotted in some cases. As the program only allows to plot complete days it was not applicable for all measuring segments.
6.2 Methods
The field campaign can be subdivided into four periods (A, B, C and D). In this chapter it will be described, which methods were applied and how.
6.2.1 Period A
One of the intended investigations was a comparison of the water column shaded by the floating house with an environment in open water. For this purpose a TROLL9500 and a CTD-Diver were fixed to one another (hereinafter probes, see figure 28) and to a rope. This rope was released from the northern edge of the floating house (FH1). From 14th to 19th of September 2017 the probes remained in five different depths for about 24 hours in each case. An anchor was used to prevent the probes from drifting (see figure 27).
Figure 27: Measuring scheme at FH1 and fixing in period A and C 43 For measurement in open water a location (OW1) was selected which shared the same frame conditions as FH1, but was not in permanent shade by a floating house. It had the same distance from the shore, so it was expected to have the same water depth. Similar to FH1 it was partly shaded from the East and West (see figure 21 in chapter 5).
Figure 28: Attachment of the probes
For setting up the position of the probes at OW1 they were attached to each other and to a rope, as at site FH1 (see figure 28). In order to keep the position an anchor and a buoy were used (see figure 29). To balance out the buoyancy of the buoy two additional weights were fixed at the TROLL9500. After dropping the anchor the devices were released from the shore, where afterwards the rope was fixed. By pulling the rope from the shore it was possible to move the probes to another depth.
Figure 29: Measuring scheme at OW1 and connection between buoy and probes
Due to logistic reasons the probes at OW1 were removed on 19th of September, whereas the ones at FH1 remained on the water floor until 22ndof September.
44 6.2.2 Period B
As a second aim of the thesis a comparison between the area under a floating house and the same section of the water column in open water was intended. Therefore two ropes were fixed at TROLL9500, which was still attached to a CTD-Diver. The other ends of the ropes were fixed at the eastern and western side of the floating house, so that the probes were in a central position under the house (FH2, see figure 30). Starting at the water floor the probes remained in three different water depths, each for about three days, from 22nd of September until 2nd of October. Moving the probes was done by pulling or releasing both ropes by equal length.
Figure 30: Measuring scheme at FH2 in period B
Water depth at FH2 was measured using a paddle and marked with red tape (see figure 31). For measuring in the open water again a location east of the floating houses was chosen. By means of a canoe (see figure 31) a position with about the same water depth in open water (OW2) was found, where the equipment was released– similar to the principle in period A. The measuring procedure was the same as at site FH2.
45
Figure 31: Canoe (left) and paddle with marks (right)
6.2.3 Period C
As the results of period A were not satisfying (see chapter 7.1), its measuring scheme was conducted once more. The procedure was reversed and took from 2nd until 6th of October 2017, this time starting at the water bottom. In four steps the probes were moved to the water surface. Also in this case the water depth at FH1 was measured with the paddle (blue mark, see figure 31), positioning of the equipment was done with the help of the canoe. As the water depth at FH1 was greater than at FH2 by about 20cm, a site OW3 was selected, which was further away from the shore than OW2 (see figure 21 in chapter 5).
6.2.4 Period D
Because the measurements at different depths in periods A, B and C were conducted at different days the results could not be compared to each other (see chapter 7.1). Hence it was decided to create two vertical profiles, one at the floating house and one in open water, in one day. Therefore, four Mini-Divers were fixed at a rope with knots and burdened with a weight (see figure 32).
46
Figure 32: Measuring scheme at FH1 in period D and fixing of the Mini-Divers
A buoy should secure vertical position of the rope whereas a knot above the buoy should allow adaptation to different water depths. This was necessary because the canoe was not available and hence the divers were released from the edge of the most eastern floating house, as close as possible to OW3 (see figure 33).
Figure 33: Measuring scheme at OW3 in period D (left) and fixing (right)
47 On 4th of October five short term measurements were carried out (see table 8). As only four Mini-Divers were available the measurements at FH1 and OW3 could not be conducted simultaneously, but as soon as possible after each other.
Table 8: Measuring times of the vertical profiles in period D
Segment FH1 OW3 Sunrise 7:43:00 – 7:53:00 7:57:00 – 8:17:00 Morning 10:47:00 – 11:07:00 10:34:00 – 10:44:00 Noon 13:20:00 – 13:30:00 13:37:00 – 13:57:00 Afternoon 16:31:00 – 16:51:00 16:14:00 – 16:24:00 Evening 19:04:00 – 19:14:00 19:18:35 – 19:38:35
Due to long response times of the sensors after changing of places measuring time at the second site was set to 20 minutes in each case.
6.2.5 Weather conditions
As external factors like wind and rainfall have an impact on water temperature and dissolved oxygen content, these parameters were monitored using the Vantage Pro2 weather station. Usually it has to be mounted two meters above ground and 30m away from impervious surfaces, ideally on grass (DAVIS, 2017). As these frame conditions were not fulfilled in the Hoek surrounding the station was mounted on 22nd of September on the top of a 5m flagpole at Skûtesân 17 (WS, see figure 34).
WS
FH1 FH2 OW1‐3
Figure 34: Localization of the weather station and the measuring points
48 Fixing and deployment was done in a way that the solar panel heads south, rain collector and anemometer were not impeded by another or the pole (see figure 35).
Figure 35: Fixing of the weather station and its operating
6.2.6 Underwater images
Additionally to the measuring of water temperature and dissolved oxygen living conditions at the floating house were investigated with the usage of underwater vehicle OpenROV. On 28th of September several dives were done, from the northern as well as from the western edge of the floating house. A flashlight and the GoPro were attached to the OpenROV, which was remote controlled (see figure 25 in chapter 6.1.1). The GoPro took videos during the dives, screenshots can be seen in chapter 7.5.
6.2.7 Calibration
As several TROLL9500, CTD- and Mini-Divers were deployed it was necessary to check, whether they correspond to each other. No reference, like a test bed, was given, that’s why the calibration was carried out 7km northwest of the Hoek at the river Potmarge near the INDYMO office in Leeuwarden (see figure 36). Calibration in that sense does not mean that it was possible to tune the sensors. But if major differences in the calibration measuring occurred, a correction factor should be used for elaborating the results of the measurements in periods A-D. To achieve same results under same conditions the four Mini-Divers used in period D were fixed next to each other (see figure 37). The same applies for the two probes.
49
Figure 36: Localization of the calibration site
Figure 37: Calibration design for the Mini-Divers
6.2.8 Questionnaires
To gather more information about the study area and its development over time questionnaires were designed and handed to house owners, sent to companies participating in the project “Het Blauwe Hart” and to responsible authorities at the city of Leeuwarden. In the appendix one can find questionnaires filled by house owners. In these questionnaires the author also informed about his master thesis and the background of the research.
50 7 Results
This chapter presents the results of the measurement campaign. That was subdivided into the four periods A, B, C and D. The main findings will be explained and – whenever data is available – related to the recorded weather conditions.
7.1 Results of period A
In the first part of period A measurements in five depth steps were conducted at sites FH1 and OW1. At both sites the probes hang for approximately 24 hours at each stage. However, the measuring depths differed between both sites every day, as can be seen in table 9.
Table 9: Measuring conditions during period A
Measuring Start Sunset* Sunrise* Average depth Average depth segment at FH1 at OW1 Day1 14.09. 19:53 7:11 0.35m 0.70m Day2 15.09. 19:51 7:13 0.76m 1.09m Day3 16.09. 19:48 7:15 1.37m 1.17m Day4 17.09. 19:46 7:16 1.56m 1.20m Day5 18.09. 19:43 7:18 1.78m 1.24m *Source: SUNRISE-AND-SUNSET.COM, online
The major reason for the differences is that at OW1 water was shallower although it had the same distance to the shore as FH1. Therefore the results from FH1 and OW1 will be shown separately in the following. To allow comparison all presented figures of Days1-5 of period A run from 14:09 to 13:54 on the next day. On the first two days of period A dissolved oxygen content (DO for short) at FH1 was above 4.5mg/l, which is adequate for healthy aquatic life (see aquatic life threshold in DE LIMA and SAZONOV, 2014). On Day3 it decreased from 5.62mg/l at 14:51 to 4mg/l at 3:38 in the night and 3.71mg/l at 9:41. At the deepest two stages dissolved oxygen remained under the aquatic life threshold and decreased further to a minimum of 3.07mg/l at 2:40, before it stabilized until the end of this measuring part (see figure 38).
51
Figure 38: Dissolved oxygen content at FH1 during period A
At the first two stages (0.35m and 0.76m depth) water temperature followed a diurnal cycle. After a maximum in the afternoon temperature decreased during night and rose from 9:30 in the morning (see figure 39). On the third day water temperature decreased already in the afternoon and hardly recovered in the morning. At 1:56 in the forth night a sudden drop by 0.2K occurred, whereas water temperature rose before and remained almost constant afterwards. Interestingly, at the greatest depth water temperature also followed a diurnal cycle, solely the increase in the morning was lower. The maximum temperature on Day5 was 14.5°C and the minimum at 13.9°C, on average water temperature was lower by 0.6K than on Day2.
52
Figure 39: Water temperature at FH1 during period A
As no weather data was available for this period only a cautious interpretation is given: The increase of water temperature and dissolved oxygen in the mornings of Day1 and Day2 can be explained by diffuse sunlight entering the water body, heating it up and allowing for photosynthesis. Nevertheless some questions remain: Why did the dissolved oxygen content decrease on Day3? What led to the drop of water temperature on Day4? Why did it follow a diurnal cycle on Day5?
In open water direct sunlight is available for phytoplankton, depending on cloudiness, and oxygen can pass the air-water boundary. However on Day1 the dissolved oxygen content at site OW1 remained relatively stable (see figure 40). On Day2 it decreased from a maximum of 6.5mg/l at sunset to a minimum of 5.5mg/l in the morning. On Day3 dissolved oxygen content decreased further and on Day4 it remained just above the aquatic life threshold for most of the time. On Day5 oxygen levels reached 3.5mg/l, the minimum value during this period.
53
Figure 40: Dissolved oxygen content at OW1 during period A
At OW1 the water temperature followed a diurnal cycle on all measured days (see figure 41). But there were slight differences: In the morning of Day1 the biggest increase occurred, which can be explained by short and long wave radiation entering the water. Day2 showed the highest temperatures in total while measured in deeper position than Day1. As this is also true for site FH1 it can be assumed that Day2 was the warmest and brightest day of the period. On Day5 the measured values were lower by 0.5K compared with Day2.
All in all water temperatures and dissolved oxygen at FH1 showed less fluctuation than at OW1. Increases and decreases of water temperature at OW1 did not lead to opposite effects on dissolved oxygen levels. Other factors prevailed, but photosynthesis was not predominant as no diurnal cycle of DO at OW1 was visible. Horizontal and vertical currents lead to mixing of the water column.
54
Figure 41: Water temperature at OW1 during period A
Due to unequal measuring depths a direct comparison of FH1 to OW1 is not scientifically justifiable. Hence autocorrelation was tested to decide, whether measurings on different days, for instance Day2 at FH1 to Day1 at OW1, could be compared. Autocorrelation function (ACF) was used to calculate, for how many lags (times of measurement, in this case 1 lag = 10 sec.) water temperature values at OW1 have a relation to each other. For dissolved oxygen it was done in the same way (see appendix).
Figure 42: Autocorrelation of water temperature at OW1 during period A
55 As figure 42 shows, after one day (8640 lags) there is no relation between the water temperature values anymore, as ACF≈0.1. Good autocorrelation is valid for just about four hours, as ACF≥0.7 for these times, which means that at least 49% of the values can be determined (0.7²=0.49). For DO good autocorrelation applies for less than 3 hours. For this reason no comparison of measurements of different days during period A is drawn. But despite unequal depths, measurements on Day5 were conducted at the bottom of FH1 and OW1 respectively.
Figure 43: Water temperature and DO at FH1 and OW1 on Day5
Water temperature at FH1 was higher than at OW1 throughout most of Day5 (see figure 43), by 0.11K on average. Neither short nor long wave radiation can transmit to this depth at FH1. Horizontal and vertical currents should be comparable at FH1 and OW1. Hence the author assumes that the floating house itself may serve as a source of heat. DO levels at FH1 were lower than at OW1, by 0.69mg/l on average. Less oxygen was produced by water plants and more was used by soil-dwelling organisms (benthos) and by decomposition of dead creatures. On 19th of September the probe at OW1 was removed for logistic reasons. At FH1 a long- term measurement was conducted, called LTA. Measurements during LTA went from 19.09. 3 p.m. to 22.09. 3 p.m., 72 hours in total. As the weather station was installed on the same day, its records will also be presented here.
56
Figure 44: Wind conditions during LTA
In figure 44 wind conditions during LTA are presented. Prevailing wind directions were classified by recorded high speeds. Beam thickness represents the frequency of occurrences. During LTA winds from WSW were dominating as occurring for roughly 20% of the time and having highest velocities.
Figure 45: Dissolved oxygen content and water temperature during LTA
57 Dissolved oxygen levels did not reach aquatic life threshold during LTA (see figure 45). Nevertheless DO shows a certain pattern, namely a steep increase in the afternoon, followed by a slower decrease during the night. Despite being recorded in a depth of 1.88m, just above water bottom, no anoxic conditions (<2mg/l) occurred. Water temperature (WT) followed a diurnal cycle during LTA. Until about one hour after sunset it rose, followed by a decrease until about 9 a.m. On the third day the maximum was reached: 15.16°C at 18:07. Interestingly, water temperature and dissolved oxygen values showed a positive correlation (0.18 for the first, 0.75 for the second and 0.78 for the third part). As photosynthetic activities of phytoplankton should be reduced to a minimum they should have a negative correlation, because when water temperature rises it can take up less oxygen (see chapter 2.1). The sharp increase of WT and DO on 21st of September around 3 p.m. catches the eye. Even though during LTA moderate wind conditions dominated, from 11:30 until 16:30 on that day higher velocities (from southern directions) occurred, with a maximum of 7.6m/s at 13:00. Moreover, ambient temperatures showed highest values of this period, around 18°C. Hence it can be assumed that between the floating houses waves were caused mixing up the water column. Despite no measurements of wind speed between the houses were carried out one should take into account that there the wind may be accelerated by the tunnel effect. As a result energy flowed from air to water and then was transported even to greater depths. The mixing can also explain the increase in DO. The oxygen produced by photosynthesis employing plants in shallower depths was transported to deeper regions.
7.2 Results of period B
On 22nd of September the second probe was brought back to the study site and another measurement series was carried out. The first probe was attached under the floating house at position FH2. The other probe was put into open water at site OW2. In three sections at different depths records were taken (see table 10).
Table 10: Measurement conditions during period B
Measuring Start End Average depth Average depth segment at FH2 at OW2 Section1 22.09. 19:33 25.09. 15:48 1.80m 1.51m Section2 25.09. 16:08 28.09. 14:00 1.63m 1.21m Section3 28.09. 14:10 02.10. 12:17 1.85m 1.38m
58 Unfortunately, the measured dissolved oxygen content at FH2 was very close to 0mg/l during all three sections. That’s why the corresponding figures are not presented here. In figure 46 the dissolved oxygen content measured at OW2 is given. To ensure same durations results are shown from 19:33 on the first day until 14:00 on the third, almost 66.5h in total for each section.
Figure 46: Dissolved oxygen content at OW2 during period B
During period B dissolved oxygen content at OW2 remained at values <4mg/l. Nevertheless in all three sections DO showed diurnal cycles (see figure 46). Usually the increases can be explained by photosynthesis, but as measuring took place in depths >1.2m questions arise: Is there enough sunlight available in such depths? Are there enough plants performing photosynthesis? Because light intensity was not measured and neither sites OW1 nor OW2 or OW3 were investigated with the drone, these questions remain unanswered. As DO values measured during Section2, in shallowest depth, were the lowest, it can only be assumed that there are macrophytes living on the water bottom (producing oxygen), but less phytoplankton in 1.21m depth. Some particularities occurred during period B: After sunset of 26th of September DO rose twice (around 20:30 and 6:00, Section2), despite no photosynthesis could have taken place at those times. It can be stated that both occurred when after a phase of relative still air gusts of >4m/s blew over Himpenser Wielen. But this is not a final explanation, because oxygen level dropped after 6:30 even though wind speed further increased.
59 Although measurement depths differed slightly between FH2 and OW2 in each section, comparisons of water temperatures can be drawn. Measurements during Section1 and 3 took place near the water bottom and during Section2 20-30cm above it. During most of Section1 water temperature was higher at FH2 than at OW2, 0.25K on average (see figure 47). But it’s clearly visible that increases and decreases were softened and less fluctuation occurred at FH2, compared to OW2. Moreover temperature range (TR) is TR1FH2≈0.35K in total, whereas TR1OW2≈1.35K.
Figure 47: Water temperature at FH2 and OW2 during Section1
Hence it seems that the floating house serves as a temperature buffer for the water underneath. Air-water energy transmission is diminished by the house. As indicated by Day5 of period A, the house itself could serve as a source of heat.
60
Figure 48: Water temperature at FH2 and OW2 during Section2
During Sections2 and 3 water temperatures at FH2 and OW2 correlated with each other (0.957 and 0.924 respectively, see figures 48 and 49). Temperatures at FH2 were higher, 0.13K for section2 and 0.09K for section3 respectively – whereby the latter is within the accuracy of the sensors. The lower volatility at FH2 lasted for all sections.
Figure 49: Water temperature at FH2 and OW2 during Section3
61 It has to be investigated, how far weather and especially wind conditions provide explanations for the differences in temperature development during period B.
Figure 50: Wind conditions during period B
In figure 50 for each direction the frequency of occurrences of wind (averaged high speed) records is displayed1. During Sections1 and 2 winds from eastern directions were dominating, whereas during Section3 southern directions prevailed. Despite missing data for 28th to 30th of September it can be stated that during Section3 winds were stronger than on the days before (see table 11).
Table 11: Wind speeds during period B
Measurement name Average speed Average high speed Highest speed Section1 0.9m/s 2.3m/s 6.7m/s Section2 1.5m/s 3.5m/s 6.3m/s Section3 2.7m/s 4.6m/s 13.4m/s
During the weak winds of Section1 the floating house served as a buffer. Oppositely during Sections2 and 3 water temperatures at FH2 and OW2 correlated with each other. The mixing fostered by the wind was sufficient to compensate the missing air-water interaction at FH2.
1 For sections1 and 2 it is based on half hourly measurements, during section3 values were taken every minute, the number of occurrences was then divided by 30. 62 On 1st of October wind force slowly increased from noon on and in the first eight hours of 2nd of October 2.4mm rain fell. In figure 51 it can be seen that dissolved oxygen at OW2 reacted to these external forces. Three major increases occur, which correspond to gusts, 9.8m/s from S at 1:02 and 2:15, plus 12.1m/s at 5:01 from S respectively. After 6:15 south-westerly winds dominated, with a maximum of 13.4m/s at 7:34, causing a strong decrease of dissolved oxygen content at OW2. This clearly indicates that wind mixes up the whole water column. For dissolved oxygen it also shows a dependency on the wind direction. Southerly winds bring water from water areas with higher oxygen concentrations, whereas winds from SW bring water from areas that are more influenced by the floating house and contain less oxygen.
Figure 51: Water temperatures and dissolved oxygen on 2nd of October
One remark is to make about the rain. In the whole campaign it played a minor role, as only 4mm were recorded in the 12 days where weather data was available. Most of it fell on 2nd of October and hence it was tried to establish a relation to the development displayed in figure 51. The two slight increases of water temperature at FH2 (0:38 and 3:07) can not be related to rain events, even considering a delay of 1.5 hours. Only four of the 12 events took place in temporal proximity to significant changes of dissolved oxygen content at OW2.
63 First of them (at 4:38) occurred short before a decrease, whereas the next three (4:54, 5:06 and 5:09) within the following increase. No major difference of wind speed or direction was detected between the events. Hence no effect of rain events on water temperature or dissolved oxygen, at least in greater depths and for such small rain amounts, was observed. Thus also no difference in the effect on open water compared to the area under the floating house could be determined.
7.3 Results of period C
Starting from 2nd of October another stepwise measurement at FH1 was carried out. Measurements at OW3 were taken as a reference, because it was further away from the shore than OW1 and OW2. This time measuring started at the water bottom (see table 12). At OW3 rope was released at the beginning of each measuring segment, but for some reason the buoy didn’t bring up the probes. So they remained in a depth of ≈1.70m.
Table 12: Measurement conditions during period C
Measuring Start Sunset* Sunrise* Average depth Average depth segment at FH1 at OW3 Part1 02.10. 19:10 7:42 1.77m 1.71m Part2 03.10. 19:07 7:44 1.45m 1.68m Part3 04.10. 19:05 7:46 1.14m 1.71m Part4 05.10. 19:02 7:48 0.25m 1.71m *Source: SUNRISE-AND-SUNSET.COM, online
At the beginning of period C dissolved oxygen content decreased from 4.38 to 1.31mg/l at FH1 (see figure 52). During Part2 it slightly increased to 2.3mg/l and further to roughly 4mg/l at the end of Part3. In shallowest measuring depth oxygen level reached 6mg/l. These highest records correspond with lowest temperatures during Part4 (<14°C), but are also caused by frequent reaeration. DO was higher during days than in the nights of Part2 to 4, suggesting photosynthetic activity to depths of at least 1.45m.
64
Figure 52: Water temperature and dissolved oxygen at FH1 during period C
Figure 53: Water temperature and dissolved oxygen at OW3 during period C
65 In the first two Parts at OW3 dissolved oxygen values were lowest (≈5.2mg/l), increased to 6.6mg/l in Part3 and 7.13mg/l at the end of Part4 (see figure 53). In total dissolved oxygen at OW3 showed higher values than at FH1 and more fluctuation, despite being recorded at the water bottom. This indicates a large number of macrophytes at OW3. Moreover water temperatures at OW3 showed similar differences as at FH1 (≈1.5K between Part1 and Part4 in both cases). This indicates that water temperature is less dependent on water depth, but driven by weather conditions.
One objective was to investigate, whether there is a difference in the effects of floating houses in greater depths, compared with shallower regions. That’s why only for Part1 a direct comparison to FH1 is drawn here. Water temperature at the bottom of FH1 was higher than at OW3 most of the time of Part1 (see figure 54), on average by 0.14K. Dissolved oxygen was lower at FH1 throughout Part1, on average by 2.8mg/l, even under the aquatic life threshold. Whereas dissolved oxygen at OW3 fluctuated between about 10 a.m. and 6 p.m. it remained almost stable at FH1. This may suggest sudden shifts from sunny to cloudy and back on 3rd of October, governing the photosynthesis at OW3.
Figure 54: Water temperature and dissolved oxygen (Part1, period C)
66 After the four measurement parts of period C, 24h each, the two probes remained at FH1 and OW3 respectively. From 6th to 9th of October a long-term measurement was conducted, called LTC, almost 72h in total. By doing so a comparison of near surface measurements was intended. The probes at OW3 remained at the water bottom also throughout LTC, as no data was read out and hence the error not realized. For this reason the results of the measurements at OW3 and FH1 during LTC are presented separately in the following.
Figure 55: Water temperature and dissolved oxygen at FH1 during LTC
At FH1 measurements during LTC were taken in 0.29m depth. Water temperature showed a diurnal cycle and a cooling trend (see figure 55). Thus it could be expected that dissolved oxygen levels show an increasing trend. But this is not the case. Dissolved oxygen also followed a diurnal cycle and mostly rose when water temperature increased. All in all DO showed a decreasing trend, but remained above the aquatic life threshold. Both can be explained by photosynthesis. During the days long wave radiation entered the water and plants started producing oxygen, during the night this process was stopped. In cooler water activity of plants is reduced, hence less photosynthesis is conducted, which explains the DO decrease trend.
67 At OW3 one would expect that the relation between photosynthesis and DO levels is even more pronounced, as more sunlight enters the open water and results from preceding measurements of period C indicate a large number of macrophytes. In total OW3 contained more oxygen (>6mg/l), but the pattern was totally different (see figure 56). The first strong rise of DO during LTC occurred on 7th of October after sunset. On the following two days water temperature and oxygen negatively correlated during the days (-0.588 and -0.15) and positively correlated during the night (0.806).
Figure 56: Water temperature and dissolved oxygen at OW3 during LTC
Measurements at OW3 during LTC took place in 1.75m depth, at the water bottom, hence photosynthesis should not have been the dominating process. The better oxygen solubility in cooler water partly explains the records. Nevertheless they stand in contrast to preceding measurements at OW3. To resolve this contradiction external factors have to be investigated.
Due to malfunction of the weather station data was lost for the period 2nd until 8th of October (04:05). For the remaining part of LTC, until 9th of October (18:42), wind conditions are presented in figure 57. Rain didn’t play a major role, as just three events, each of 0.2mm, were recorded. Winds from WSW occurred most frequently, whereas winds and gusts from W and WNW were strongest, up to 17m/s each.
68
Figure 57: Wind conditions during LTC
As can be seen in figure 58 at 13:35 on 8th of October water temperature at OW3 increased from 12.65 to 12.83°C, whereas DO decreased from 7.64 to 7.17mg/l within 12 minutes. During this time a strong gust of 16m/s from W occurred. This gust caused waves and mixed up the water, bringing warm low-oxygen water from the floating houses to OW3.
Figure 58: Water temperature and dissolved oxygen content on 8th of October
69 Surprisingly, this wind event hardly lead to any reaction at FH1, which is not shaded to the west and where measurements were carried out much closer to the water surface. It can be assumed that westerly winds bring water to FH1 from areas that are influenced by the neighboring floating house and hence contain similar amounts of oxygen.
Records at FH1 during LTC were taken near surface. Hence they were appropriate to verify a hypothesis of FOKA (2014) and thus help understanding the processes influenced by floating houses. From the results of her modeling she stated that the near water surface area releases more oxygen to the air than it binds, boosted by the wind. In order to verify this, measurements at FH1 during LTC need to be related to anemometer recordings.
Figure 59: Dissolved oxygen content and high wind speeds at FH1 during LTC
Figure 59 shows DO content at FH1 and wind speed (highest records) from 4:05 on 8th of October until 18:42 on 9th of October. Usually wind speed is a continuous variable, but in this case the measurement interval was 5 minutes, in contrast to 10 seconds for DO, that’s why wind speed is displayed in discrete values here. The first three increases of dissolved oxygen content (at 4:20, 4:55 and 6:10) occurred during relatively strong winds from WNW (13, 14 and 13m/s respectively). The sharp decreases (from 5:01, 6:36 and 9:28) correlate with a slowdown of the wind. Increasing DO values from 10 o’clock onwards can be explained by photosynthetic activity.
70 Nevertheless the disproportionate increase at 13:22 can be related to strong winds, up to 17m/s from WSW, while the decrease from 14:17 on occurred during slowdown after 17m/s from W. After sunset at 18:55 photosynthesis stopped and DO levels decreased. The two interim increases (at 22:30 and 0:33) can not be related to wind patterns. On 9th of October no gust stronger than 9m/s blew over Himpenser Wielen. Also dissolved oxygen did not reach same levels as on the day before. Besides probably higher cloudiness another explanation are the weaker winds on that day. Hence the author concludes that the area near surface takes up more oxygen than it releases to the air. This process is fostered by strong winds. As the measuring period was just under 40 hours this is no final refutation of FOKA’s hypothesis. Longer-term measurements of wind and near-surface dissolved oxygen content are needed.
7.4 Results of period D
On 4th of October vertical profiles of water temperatures were taken in Himpenser Wielen. Measurements started at 7 a.m., before sunrise, and took until 19:38, after sunset. Four Mini- Divers were released at FH1 and OW3 alternately (see table 8 in chapter 6.2.4). This was done, inter alia, to detect differences in the adaptation to the increase of ambient temperature in the morning, and decrease after sunset between OW3 and FH1. Because the temperatures at both sites remained constant for the 10 minutes of the initiated periods, a comparison is not presented here. Instead the results of measurements are displayed, where the divers remained at one site, 53 minutes in the first and ≈2.5 hours in the subsequent cases. The first vertical profile was taken at FH1 before sunrise. As can be seen in figure 60 water temperature decreased in all measured depths. At the water surface heat was emitted to the atmosphere, which’s temperature, measured by a fifth Mini-Diver, was lower, 12.21°C at 7 o’clock and also decreased to 12.05°C. The diver at the water bottom recorded highest temperature values. As warmer water is less dense than cooler, convective rise occurred. But as the temperature differences between surface and bottom layer were very low (≈0.2K) the extent of convection was quite small. Because no weather data was available it can not be said, whether natural or forced convection occurred.
71
Figure 60: Vertical profile of water temperature at FH1 before sunrise
Unfortunately, adjustment of the rope at OW3 did not work as it should have. That’s why three divers measured near the water bottom. The fourth diver, measuring in a depth of 1.57m, recorded lowest temperature values (see figure 61). As ambient temperature was still lower, unless increased from 12.05°C to 13.62°C in this time, heat was transmitted upwards.
Figure 61: Vertical profile of water temperature at OW3 in the morning
72 At this point the question arises, whether the three divers, measuring at 2.32, 2.33 and 2.38m respectively, were located at the water bottom or even in the sediment. The vertical profiles were taken parallel to measurements of Part2 during period C. There the probes recorded in a depth of 1.68m, which was assumed to be close to the water bottom. This can be confirmed by a measured average turbidity of 10 FNU, which means that the water is not recommended for drinking. But for sediment substantially higher values are to expect. The Mini-Diver used for the vertical profile did not measure the turbidity. But as records were taken 64-70cm deeper they can be assumed as in the sediment. This also applies for following measurements in depths >1.90m at OW3. For FH1 it is more complicated to define the water-sediment boundary. Measurements during Part1 of period C showed a turbidity of 3.3 FNU in a depth of 1.77m, but during LTA turbidity was only 1.56 FNU in a depth of 1.88m. Hence the author suggests sediment as in depths of >2.10m.
Figure 62: Vertical profile of water temperature at FH1 around noon
Despite measuring at a similar depth as diver3 (green) diver4 (purple) measured significantly higher water temperatures at FH1 around noon (see figure 62). During calibration this diver was unremarkable, hence the author did not use a correction factor for the records of diver4. Nevertheless this characteristic should be taken into account, especially because it applied for all vertical profiles. One could also argue that the green diver showed too low values (see especially figures 60 and 63). Conclusions need to be drawn with caution.
73 Notwithstanding this it can be seen in figure 62 that at the shallowest depth of FH1 again lowest temperatures were measured. Ambient temperature increased from 13.86°C, reached a maximum of 14.67°C at 12:36 and then decreased to 14.32°C during this part. As heat exchange with the atmosphere was very limited, water temperatures at all depths remained constant at FH1 around noon.
The vertical profile of water temperature at OW3 in the afternoon shows similar characteristics (see figure 63). At least for diver1, measuring at 0.36m, it is a bit surprising. Ambient temperature ranged from a maximum of 14.91°C to a minimum of 13.84°C, in total >1K during that time. Hence water temperatures at OW3 should also show fluctuation.
Figure 63: Vertical profile of water temperature at OW3 in the afternoon
At FH1 water temperature decreased in the afternoon in shallower depths, whereas it remained constant at the water floor (see figure 64). Ambient temperature decreased from 14.08°C to 12.81°C during this time. Heat was emitted from shallower water depths to the atmosphere.
74
Figure 64: Vertical profile of water temperature at FH1 in the afternoon
All in all the vertical profiles show only small differences of water temperatures between the measuring depths, at FH1 and OW3 respectively, partly within the accuracy of the Mini-Diver. This shows the complete circulation of the water column, which is mainly driven by the wind. A stagnation with formation of a thermocline was not observed. Due to time-delayed measurements in unequal depths it is not possible to determine differences between shaded area and open water in adaptation to heating and cooling of the atmosphere.
75 7.5 Further results
Besides the measurement campaign the surrounding of the floating house was investigated with an underwater drone. On 28th of September it was launched from the northern and western ceiling to dive under the house. On both sides of the house colonies of mussels were detected, but also on its bottom side and on the water ground (see figure 65).
Figure 65: Mussel colony at the bottom side of the floating house
Moreover three times during the dives fish swam through the area under the floating house (see figure 66 in the red ellipse). This does not mean that fish stay there for longer time or even breed there, but together with the detected mussels it shows that an appropriate amount of dissolved oxygen must exist underneath the house. In turn it clarifies that recorded DO values (≈0mg/l) were measurement errors. Due to malfunctional device DO levels can’t be related to colonization or decomposition of mussels. The same applies for turbidity, which recorded negative values throughout period B.
Figure 66: Fish underneath the floating house
76 At the bottom of the shaded area, near FH1, macrophytes were detected (see figure 67), as well as west of the house, but not underneath the house. The occurrence of plants demonstrates that light intensity is high enough for photosynthesis, also in the shaded area. As vegetation in the open water was not investigated, it can not be compared to the floating house’s surrounding.
Figure 67: Macrophyte on the bottom of the shaded area
Algae were visible on both sides, drifting in the water and sometimes also at the surface (see figure 68 on the left).
Figure 68: Algal bloom (left) and manure streaks (right)
77 Questionnaires were another source of information, filled by the owners of floating houses at Skûtesân 20 (Mr. Luiks) and 28 (Mr. de Roos). According to Mr. Luiks, who lives there since 2005, water quality improved for that time. Today the water is clearer than in earlier times. Birds and anglers catch fish, up to 60cm big. Nevertheless both reported, that algal bloom occurs for some weeks in summer. Moreover Mr. de Roos stated that sometimes streaks are visible at the water surface, as from manure (see figure 68 on the right). Neither water authority nor municipality answered the questionnaires.
7.6 Verification of hypotheses
In order to be able to answer the research questions, the results of measurements under the floating house (FH2), in shaded area (FH1) and in open water (OW1-3) have to be compared. Averaged values are most suitable for this purpose. Measurements at FH2 and OW2 were conducted simultaneously and in similar depth during period B. The comparison shows that TFH2 > TOW2 by 0.15K. Due to malfunction of the sensor at FH2 dissolved oxygen levels can’t be compared here. To compare open water to the shaded area measurements during Part1 of period C fit best, carried out at the water bottom. It results in: TFH1 > TOW3 by 0.14K and DOFH1 < DOOW3 by 2.8mg/l. For determination of the impacts on the water’s surface layer Day1 of period A is most suitable. It appears: TFH1 > TOW1 by 0.1K and DOFH1 < DOOW1 by 0.8mg/l. Results from the vertical profiles are not used for comparison, because many records were taken in the sediment instead of in the water column. Without forgetting the sensor’s accuracies and the high dependency of water temperature and dissolved oxygen content on weather conditions, it can be stated:
Hypothesis 1: Water temperature in the shaded area is lower than in open water. This will affect the whole water column, because temperature gradients get balanced out by heat transport.
It must be acknowledged that this hypothesis was wrong. For the measurements, conducted in autumn, the opposite applies. The floating house serves as a source of heat, which mainly affects the bottom layer. At the surface layer the difference is a bit smaller, because the heat is (at least partly) transmitted to the atmosphere.
78 Hypothesis 2: Water temperature underneath the floating house is slightly higher than in the shaded area as well as in open water. As the measurement campaign was mainly conducted in autumn water tends to cool down over time, but energy transmission to the atmosphere is limited by the floating house.
It is true that the measurements during period B showed higher temperatures under the floating house, compared to open water. No cooling trend was observed for that time. Hence it has to be concluded again that the floating house acts as source of heat. The limited air- water interaction ensures that energy is stored under the floating house in autumn.
Hypothesis 3: Dissolved oxygen content in open water is higher than in the shaded area, where photosynthesis is reduced. This affects mainly the upper layer, as photosynthesis is prevailing there.
Open water contained more oxygen than the shaded area throughout the measuring campaign. However in the bottom layer the difference was much bigger than in the surface layer. This may be caused by the higher oxygen demand of the mussels colonizing the water floor, as well as for their decomposition by bacteria. Moreover from the measurements it can be assumed that at the bottom of the shaded area fewer macrophytes exist than in open water, and thus less oxygen is produced. As the open water was not investigated with the drone, this assumption can not be confirmed here. Furthermore it’s worth noticing that measurements at the surface and bottom layer were conducted on different days. Therefore the values are not directly comparable.
Hypothesis 4: Oxygen content underneath the floating house is lowest, because neither photosynthesis nor reaeration takes place there.
Due to malfunction of the sensor under the floating house, this hypothesis can be neither confirmed nor refuted here.
79 8 Discussion
In this chapter the used methods and obtained results will be discussed. Own results get juxtaposed to findings of previous studies. To improve impact assessment of floating houses on water quality recommendations will be given, how future research should be designed.
8.1 Discussion of results
FOKA (2014) found higher water temperatures in open water, compared to an area between floating houses, by 0.5K. For dissolved oxygen she found differences only near surface. In the here presented thesis water temperature under as well as near a floating house was slightly higher than in open water. The presented thesis proved lower DO contents in the shaded area, even to a greater extend in deeper regions. The results of DE LIMA and SAZONOV (2014) showed a wide range. In the original dataset, made available by Mr. de Lima, the area near a floating house had highest DO values (7.26mg/l on average), followed by open water (7.08mg/l) and under the structure (6.81mg/l). For water temperature they recorded 19.71°C under, 19.69°C near the floating house and 19.62°C in open water. As measurements were conducted with just one device, accuracy of the sensor has not to be taken into account. In accordance with the presented thesis their results indicate that the floating houses serve as a source of heat. For DO the results seem to contradict each other. FOKA made photosynthesis responsible for differences in dissolved oxygen contents, while DE LIMA and SAZONOV assumed a dependency on flow velocity of the water. In the presented thesis it could be shown that photosynthesis is an important factor in shallower depths. Moreover a dependency on wind speed and direction – and hence caused waves and currents – could be established. These both preliminary studies were carried out in a similar time of the year. In contrast BOL and TOBÉ (2015) as well as HÄRTWICH (2016) conducted their measurements in spring. The results can’t be directly compared to the ones of the present thesis, because plant growth is just starting then. Nevertheless their findings are in accordance with the here presented results. BOL and TOBÉ (2015) measured lower oxygen contents near the houses floating in the Harnaschpolder than in open water. HÄRTWICH (2016) found macrophytes in the shaded area, but not under the floating houses. The lower DO content under the house, which was measured in both studies, could not be verified in the present thesis. In the Mega-Float project slightly lower water temperatures and DO values were modelled for the area underneath the airport, compared to open water. For water temperatures, at least during the campaign (autumn), this could not be confirmed by the present thesis.
80 The statements of Skûtesân’s inhabitants concur with the findings of the National Water Plan, concerning improvements as well as challenges (see chapter 1.2). Since not all measurements worked as planned, the methods used for the presented thesis have to be discussed as well, in the following section.
8.2 Discussion of methods
For the measurement campaign just the existing devices could be used, supplemented by additional tools like ropes. Measurements were designed at three sites, open water as well as under and near a floating house, in a couple of depth stages. Having only two probes implicated the necessity to move the sensors from one site to another. Buoys and ropes should help to fix the probes to secure that both probes measure at the same depth during the same time. This did not work effectively enough. For the open water measurements the challenge was to remotely control the probes by pulling or releasing rope. Moreover the water depth at OW1 was not measured in advance.
It was the first time that long-term recordings were taken under a floating house. For this purpose two ropes were fixed at the probe, which was released from the ceiling. As low values indicate, the DO sensor probably was contaminated with mud before the ropes were tightened. Finding a better working method, which is applicable at other locations of floating infrastructure, is a task for future studies.
For the vertical profiles it was intended to measure in four different depths from the water surface to the bottom at the same time. Because tightening of the rope from the edge of the house did not function, some records were taken in the sediment and sometimes only one in the water. This could be overcome by tying a knot above the buoy. This knot should have the same distance from the diver measuring in greatest depth as the expected height of the water column, in this case ≈1.90m. Nevertheless the challenge remains that only simultaneous measurements in open water as well as in shaded area allow for comparison.
Due to malfunction and low storage capacity lots of data from the weather station was lost. This could be overcome by permanently connecting the console to a computer to ensure reading out frequently. On the basis of presented findings and limitations recommendations will be given for follow- up studies, in the next section.
81 8.3 Recommendations
TROLL9500 are expensive devices, heavy in weight. By fixing them to ropes and buoys one will hardly achieve same measuring depths. To overcome this, a metal measuring chain with data loggers is more suitable. As data loggers are relatively cheap a number of them could be used to measure simultaneously in small distances, for instance vertical or horizontal profiles. In this way measurements should last for longer terms, at least for one year, to gather information about developments in different seasons. Doing so may allow for separation between floating houses as source of heat (and probably cold in summer) and diminished air-water interaction (energy transmission at the water surface). A year-round monitoring would improve the understanding of the impact of floating houses on water bodies in many ways. Growth cycles of mussels and phytoplankton could be compared. Macrophytes and macrozoobenthos have to be investigated, water samples should be taken several times a year. As water quality consists of more indicators, such as nutrients, further research should include such measurements. Long-term measurements would also make differences of the impact of extreme events visible, like storms (with velocities >21m/s) or freezing of the water surface. Under these conditions it could be observed, whether the area underneath a floating house acts as a refuge area for fish. To separate more impact factors also currents should be metered, ideally at four sites: In open water, near and under a floating house, as well as between two houses. The latter enables to detect a possible tunnel effect, and could be accompanied by measurements with an anemometer. As photosynthesis was identified as important, light intensity should be measured, especially near and under the house, also in greater depths. Most suitable is a PAR probe, abbreviated for photosynthetically active radiation. These measurements should be accompanied by recordings of the solar radiation by a weather station. For all technical measurements it is recommended to set same sample intervals for all devices. This facilitates comparability, and hence allows for revealing relations between the parameters. Only after completion of these measurements a model can be set up to determine the impact of floating houses on water temperature and dissolved oxygen content. It would then be possible to vary the proportion of the floating house, to predict the effect of an implementation in big scale.
82 9 Conclusion
In Himpenser Wielen in Leeuwarden (Netherlands) a measurement campaign was carried out to assess the impact of floating houses on water temperature and dissolved oxygen content. Several 24-72 hour measurements in different depth stages were carried out in an area shade by a floating house, as well as in open water. Pioneering work was a 10-day- measurement under a floating house. Two TROLL9500 multi-parameter probes and two CTD-Divers were utilized for these monitorings. Vertical temperature profiles were recorded with CTD-Divers. Additionally an underwater drone was used for detecting mussels and macrophytes underneath the floating house. As the long-term measurements confirm, water temperature and dissolved oxygen content strongly depend on weather conditions. During weak winds the floating house serves as a buffer for water temperature, stronger winds mix up the whole water column and reach down to the area underneath the house. In the shaded area a tunnel effect could be detected. Water temperature was lower in open water compared to the area under the house and to the shaded area respectively. In autumn, when the campaign took place, the floating house serves as a source of heat for its surrounding. Dissolved oxygen content was higher in open water than in shaded area, whereby the differences were larger at the water bottom than at its surface. This could be explained by less oxygen production through photosynthesis in the shaded area.
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88 Declaration of Authorship
I confirm that this Master's thesis is my own work and I have documented all sources and material used. This thesis was not previously presented to another examination board and has not been published.
Berlin, 29th of January 2018 ______Place and date Signature
89 Appendix
Layout of a floating house at “Het Blauwe Hart”, Leeuwarden Supplied by Peter Lont, Ooms BV
90 Autocorrelation function Dissolved oxygen Explanations ccf(LTA, type = "correlation") x_t0 <‐ LTA$RDO..mg.L.[1:25920] Define x_t0 as x[‐1] x_t1 <‐ LTA$RDO..mg.L.[2:25921] Define x_t1 as x[‐n] head(cbind(x_t0, x_t1)) Confirm that x_t0 and x_t1 are (x[t], x[t‐1]) pairs cor(x_t1, x_t0) View the correlation between x_t0 and x_t1 acf(x_t1, lag.max = 1, plot = F) Use acf with x cor(x_t1, x_t0) * (25920)/25921 Confirm that difference factor is (n‐1)/n plot(acf)
Code and autocorrelation function of dissolved oxygen during period A
91 Our questions:
For how long do you live in this house? Since march 1, 2017
Since you live there, did you realize any changes in water quality or color? Yes; sometimes. There is a kind of foam or foam-stripes on the water; other times it seems dirty, brownish, like there is manure in the water. Like someone discharged something in the water.
Can you tell us about the presence of fish? Do people come fishing in this area? Yes, people come fishing. We see birds catching small fish.
Are there sometimes unpleasant smells? Can you explain and justify this? Not very mentionable
Is algae bloom a regular phenomenon? How often does it occur? In summer, a few times a month.
Did you realize a colonization of the house wall by mussels and did you ever eliminate them? We see mussels on objects we found in the water as old chairs, lamps and on the concrete box the house is floating on. Never removed them.
Did your house ever hit the ground? Is there any difference during low water levels? No, never noticed until now. There is approximately 50 cm of free space to the mud beneath the house.
Questionnaire filled by Mr. de Roos (excerpt)
92 Our questions:
For how long do you live in this house? Since 2005
Do you have any information about the mooring system? It is a concrete hull filled with polystyrene.
Since you live there, did you realize any changes in water quality or color? The water quality has improved, the water is more clear.
Can you tell us about the presence of fish? Do people come fishing in this area? Yes they do, also big fish until 60 cm
Are there sometimes unpleasant smells? Can you explain and justify this? I never smell something.
Is algae bloom a regular phenomenon? How often does it occur? Once or twice a year it will occur for some weeks and then it disappears
Did you realize a colonization of the house wall by mussels and did you ever eliminate them? I did not realize the mussels on the wall, they came by themselves and I do nothing about it.
Did your house ever hit the ground? Is there any difference during low water levels? No and no.
Does the water surface freeze during winter? Is there a difference to surrounding areas? We have been skating for 5 years in a row, but not the last two years. No difference between other areas.
Questionnaire filled by Mr. Luiks
93