81 FIGURE 5.17: MONTHLY AVERAGE TURBIDITY TRENDS FOR THE TWO MANAGEMENT SCENARIOS. THE SOLID BLACK LINE IS FOR SCENARIO 1 (CURRENT SITUATION), AND THE BLACK DOTTED LINE IS FOR M22 Scenario. 87 FIGURE 5.20: MEDIAN 92 YEARS OF SIMULATED WATER DEPTH. THE SOLID BLACK LINE IS THE MEDIAN OF ALBANIA.
INTRODUCTION
- Background of St. Lucia
- Motivation
- Advantages and Disadvantages of Turbidity
- Research Question
- Aim
- Objectives
- Outline of Dissertation
From a management aspect, the control of the condition of the mouth can therefore have serious consequences in the lake basin 22km upstream of the mouth. The low water levels allow very turbid conditions to develop, affecting the biological functioning of the St.
I NTRODUCTION
W IND I NDUCED WAVES
Basically, their research showed that at some critical depth, the fetch no longer defines the wave growth, but rather it is defined by the water depth. Some researchers base the water depth at which the waves interact with the seabed to be approximately half the wavelength (Chao et al, 2008).
W IND -W AVE M ODELS
- Method 1 – empirical model
- Method 2 – process based approach
It is at this depth (for a given wind speed and range) that wave growth becomes depth limited. It is therefore possible to delineate areas within the lake where wave growth is limited in depth.
W AVE M EASUREMENT
- Accelerometers
- Zwarts Pole
- Pressure Transducers
- Capacitance & Resistive Wave Gauges
Accelerometers are used in wave buoys (Niclasen & Simonsen, 2008). The advantage of using accelerometers to measure wave height is that they measure actual surface change rather than inferring wave height from theoretical calculations (such as with pressure transducers). . Significant wave height is defined as the average of the highest one-third of the wave heights (Reeve et al, 2004).
W AVE BOUNDARY LAYER
E ROSION AND D EPOSITION
For argument's sake, this liquid mud has no critical shear stress because it is a liquid. The shear stress exerted by the waves is strong enough to erode the surface of the lake bottom.
L IGHT A TTENUATION THROUGH W ATER
This is because the flow is in one direction, while water waves cause water particles to oscillate near the lake bed. It is highly unlikely that the upper limit of measurable turbidity will be reached in South Lake; however the limit of the YSI 6920 (instrument used in the field) will need to be evaluated.
S UMMARY
- Field Study
- Data Collection
- TSS and Turbidity Relationship
- Wave Height
- Turbidity Build Up
- Model Calibration
- Sensitivity Analysis
- Depth–Fetch Domain
To determine whether the water column was well mixed, the turbidity value was checked at three different levels: (1) just below the water surface, (2) at a depth of half the water depth, (3) near the bottom of the Lake. The depth and position of the YSI was the same as that of the water sample, which varied for each sample point. Each filter was then placed in an oven at 110°C overnight and weighed; this was then the combined weight of the filter paper and sediment.
Then a graph of turbidity versus TSS was plotted and the relationship was found (see Chapter 5 for the graph). The wave height was taken as the height reading of the crest minus the headway height of the trough. The significant wave height was then calculated as the average of the highest third of the wave heights.
This was done to determine the response of the model to any change in parameters.
- Model Structure Overview
- Wind-Wave Growth
- Formulation
- Mathematical Model
- Wind Speed
- Wind Setup
- Water Depth
- Wave Height Prediction
- Wave Period Prediction
- Orbital Velocity
- Bed Shear Stress
- Erosion
- Deposition
- Spatial Model
Wave growth is a function of drag (x), wind speed (U), water depth, and duration. This distance is known as the equivalent drag and is representative of the time in which the wind transfers energy to the waves. Hs can be related to the energy of the wave spectrum, so it is not included in the dimensional analysis.
To investigate the diurnal pattern of the wind speeds, an E.M.D (empirical mode decomposition) was used. The haul used for the wind setup (i.e. L, according to figure 4.2) was the length of the Southern Lake. The velocity was then calculated by dividing this flow rate by the water depth and width of the lake (4714m).
The energy of the spectrum was needed to calculate the significant wave height.
Model Calibration
- TSS & Turbidity calibration
- Wave Height Prediction
- Turbidity Build up
- Model Calibration and Validation
- Sensitivity Analysis
- Depth-Fetch Domain
Since it is unrealistic to have a negative turbidity reading, the way this ratio was interpreted was that between concentrations of 0 – 0.1 g/L, the turbidity was 12.7 NTU (this was the estimated background light scattering, due to biology), for concentrations above 0.1 g/L, TSS was estimated from the previously mentioned ratio. This is the value measured in the field when the wind speed was zero and the water was clear. Note the relationship between the wind speed and the associated reaction time for the sediment resuspension.
This is based on the fetch estimated for the sampling point at 2600m (refer to plate 5.1), the water depth measured was 0.7m, and the average wind speed during the sampling period was 6.9m/s from which a shear stress of 0.328. Dad was calculating. The model parameters that were changed were the wind speed (figure 5.14) and the water depth (figure 5.15) and each parameter was changed by 20%. The reason figure 5.14 shows changes of more than 100% is because the 20% change in wind speed was an increase in wind speed and therefore an increase in turbidity.
The wave heights are now therefore determined by the depth limit and since the water depth is held constant (see Figure 5.14), the change in wave height becomes proportional to the square of the change in wind speed (see equation (2). -4)).
Model Simulation Results
- Monthly Average Turbidity Values
- Turbidity for the Past Ten Years
- Daily Turbidity Trend
- EMD Results
- Spatial Model
However, regardless of the values of the estimated water depths, Figure 5.17 shows the importance of uMfolozi in stabilizing water levels in St. The wind speed used to determine the turbidity was the average of the 95% percentile (mentioned in chapter 4.3.). 1) over the past ten years. The effect of uMfolozi in recharging the water level in winter can also be seen in Figure 5.20; this effect is explained in section 1.1.
The wind speed used to determine the turbidity was the monthly average of the 95th percentile of the maximum value for each day. However, it can be seen that the sediment appears to be deposited relatively quickly from the water column. Taking the 95th percentile of wind speed as the wind speed causing sediment resuspension is consistent with the biological responses to daytime turbidity, ie.
It is interesting to note that for the SW wind more than 50% of the lake is in depth limited conditions for the two water level scenarios.
Development of Mathematical Model
Turbidity Trends
This was done to visualize the effects of the drought on the system. What it showed was that monthly cloudiness forecasts over the past decade were much higher than what the system would normally have experienced. It can therefore be concluded that the drought had a serious effect on the increase in monthly average turbidity.
This value was then halved for the summer month because it was assumed that more turbulent conditions in summer would result in a lower settling velocity, as it was assumed that greater turbulence would tend to break up any flakes. This effect is expected, since the inclusion of sediment means that the concentration of sediment in the water column at a given time would be the result of erosion and sediment flux. It can also be seen from Figure 5.23 that the daily turbidity trend follows the wind speed trend, i.e.
This can be explained by the fact that the sediment remains in suspension longer in more turbulent conditions.
EMD Summary
A time step of three hours was used when incorporating the settling velocity for the daily turbidity trend.
Spatial Model Development
This means that considering wave height as an indicator of high turbidity is short-sighted and it is important to consider both wave and water depth. The reason for this is that the effect of waves on the lake bed (i.e. shear stress) depends not only on the height of the waves, but also on the depth of the water. Using the depth retrieval domain derived from the model (Figure 5.16), the percentage of the lake that was in a depth-limited state was calculated.
This was due to the orientation of the lake and the location of the deep and shallow water. The reduction in wave height due to breaking and bottom friction was included via a rule used in the previously mentioned model. However, to improve the model it is recommended to discard this rule and investigate the actual relationships with wave heights and the effects of breaking.
The rule is used by this model, since; Research on the effects of end friction fell outside the scope of this research.
Biological Effect
This spatial model was then used to determine turbidity for a southwesterly and northwesterly wind blowing at 8m/s and water levels of 0 EMSL and -0.5 EMSL (1.14m and 0.79m water depth, respectively). What the model showed was that for a 0.5m difference in water level there was only a small change in wave height, however the change in turbidity was significant. This indicated that winds blowing from the southwest resulted in a higher proportion of limited depth conditions compared to the northwesterly wind direction.
Thus, in the last ten years, South Lake has far exceeded the value of 80 NTU, which means that the scale needs to be re-evaluated or the system has been in very poor condition in the last ten years. High turbidity values reduced phytoplankton productivity according to Perissinotto et al (2010b); this is in line with what Cloern (1987) suggests that turbidity limits phytoplankton productivity. High levels of turbidity would also have a negative effect on young organisms in the system.
Initially, the high turbidity would successfully mask the young organisms as suggested by Perissinotto et al (2010a), but during the winter months due to the high level of turbidity; zooplankton numbers would decrease (Carrasco et al, 2007), resulting in less food available for the young organisms to feed on.
Summation
Recommendations for Further Research
Effects of silt loading on the feeding and mortality of the mysid mesopodopsis africana in the St. Turbidity, Suspended Sediment and Water Clarity: A Review, Journal of the American Water Resources Association, vol 37, no. A new measure for direct measurement of the bed shear stress of wave boundary layer in wave trough, Journal of Hydrogynamics, vol 19, no.
Wind-driven estuarine turbidity maxima in the Mandovi estuary, central west coast of India, Journal of Earth System Science, vol. Anthropogenic influence on the water and salt budget of estuarine Lake St Lucia, South Africa. A review of the importance of freshwater inflows for the future conservation of Lake St Lucia.
Where: W(f+s) is the total dry weight (g) of sediment and filter and W(f) (g) is the dry weight of the filter.
