In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed dissolved oxygen concentrations drop well below the standard limit of 5 mg l -1 at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality parameters.

The dynamic behavior of bed-load sediment transport under unsteady flow conditions is experimentally and numerically investigated. A series of experiments are conducted in a rectangular flume (18 m in length, 0.80 m in width) with various triangular and trapezoidal shaped hydrographs. The flume bed of 8 cm in height consists of scraped uniform small gravel of D 50 ¼ 4:8 mm. Analysis of the experimental results showed that bed-load transport rates followed the temporal variation of the triangular and trapezoidal hydrographs with a time lag on the average of 11 and 30 s, respectively. The experimental data were also qualitatively investigated employing the unsteady-flow parameter and total flow work index. The analysis results revealed that total yield increased exponentially with the total flow work. An original expression which is based on the net acceleration concept was proposed for the unsteadiness parameter. Analysis of the results then revealed that the total yield increased exponentially with the increase in the value of the proposed unsteadiness parameter. Further analysis of the experimental results revealed that total flow work has an inverse exponential variation relation with the lag time. A onedimensional numerical model that employs the governing equations for the conservation of mass for water and sediment and the momentum was also developed to simulate the experimental results. The momentum equation was approximated by the diffusion wave approach, and the kinematic wave theory approach was employed to relate the bed sediment flux to the sediment concentration. The model successfully simulated measured sedimentographs. It predicted sediment yield, on the average, with errors of 7% and 15% of peak loads for the triangular and trapezoidal hydrograph experiments, respectively.

In water supply reservoirs, selective withdrawal is commonly implemented to control released water temperature for quality purposes. This study investigated the effects of selective withdrawal on hydrodynamics of a stratified reservoir through numerical modeling and analytical analysis. A 3-D hydrodynamic model was applied where observations of water temperature time series recorded every 30 min at the thermocline and measured temperature profiles along the water column were used to validate the numerical model. The effect of selective withdrawal from four outlets located along the water intake structure of Tahtali Reservoir in Turkey on water temperatures was investigated and the effects on thermal stratification structure were discussed. Withdrawal of the water at the bottom outlet was found to be the most effective choice encouraging the mixing of the water column and thus reducing anoxia. The results of this study can be used to guide the further investigations in stratified lakes for better management practices.

A distorted physical model, based on Ürkmez Dam in Izmir, Turkey, was built to study sudden partial dam break flows. The distorted model had a horizontal scale of 1=150 and a vertical scale of 1=30, containing dam reservoir, dam body, and downstream area-from dam body to Ürkmez urban area until the sea coast. In the model, the reservoir is approximately 12 m 3 , the dam body has a width of 2.84 m and a height of 1.07 m, and the downstream area is nearly 200 m 2 . The Ürkmez Dam was chosen because Ürkmez Town is located right at its downstream area, allowing the study of dam break flows in an urban area. Furthermore, the dimensions were suitable such that it allowed the construction of a physical model (dam reservoir, dam body, and downstream area) having a horizontal scale of 1=150 in the available space of 300 m 2 . The features creating roughness such as buildings, bridge, and roads were also reflected in the physical model. The dam break flow was investigated for sudden partial collapse, which was simulated by a trapezoidal breach on the dam body. The water depths at downstream area were measured at eight different locations by using eþ WATER L (level) sensors. The velocities were measured at four different locations by ultrasonic velocity profiler (UVP) transducers. The propagation of the flood was recorded by a high-defnition camera. The experimental results show that the Ürkmez area can be flooded in a matter of minutes, at depths reaching up to 3 m in residential areas in 4 min. The flood wave front can reach the residential areas in 2 min and to the sea coast in 4 min. Flow velocities can reach 70.9 km=h in sparse residential areas, close to dam body. Away from the dam body in the sparse buildings part of the town, the velocities can reach 27.7 km=h. In dense residential areas of the town, the velocities are too low (2.8 km=h) but flow depths can reach 3 m. Velocity profiles show similar behavior like unsteady and nonuniform open channel flow in nonresidential areas close to the dam body. In residential areas away from the dam body, the velocity profiles are more uniform, having lower velocity values. Vertical variations of velocities show markedly different behavior during rising and recession stages. The profiles are smooth during the rising stage in sparse residential area, yet it shows fluctuating behavior during the recession stage.

This study investigated flood inundation in an urban area due to a possible failure of Ü rkmez Dam in İzmir, Turkey. The estimation of flood hydrograph upon partial failure of the dam and routing of the flood hydrograph along the narrow valley downstream were first performed by the one-dimensional hydraulic routing model HEC-RAS. The twodimensional hydraulic routing model FLO-2D is then used to simulate the spreading of the dam-break flood after the flood wave exits the valley. Land use and land cover digital maps were utilized to find the spatially varying roughness coefficient for the floodplain. The influence of the buildings on the flood propagation was represented in the numerical model by the area reduction factor as well as the width reduction factor. The peak flow depth, peak flow velocity and time moment of the peak flow depth maps were shown in the GIS environment. The results reveal that flow depths can reach about 3 m in the residential area. In about 40 min after the dam-break, houses in the large section of the town would be under the maximum flow depths. The two-dimensional hydrodynamic model results were tested against experimental dam-break flow data of the distorted physical model of Ü rkmez Dam, which is consisted of the reservoir, dam body and downstream area including Ü rkmez Town. The model successfully simulated experimental flow depth data measured at different measurement locations.

Dams are important structures having many functions such as water supply, flood control, hydroelectric power and recreation. Although dam break failures are very rare events, dams can fail with little warning and the damage at the downstream of the dam due to the flood wave can be catastrophic. During a dam failure, immense volume of water is mobilized at very high speed in a very short time. The momentum of the flood wave can turn to a very destructive impact force in residential areas. Therefore, from risk point of view, understanding the consequences of a possible dam failure is critically important. This study deals with the methodology utilized for predicting the flood wave occurring after the dam break and analyses the propagation of the flood wave downstream of the dam. The methodology used in this study includes creation of bathymetric, DEM and land use maps; routing of the flood wave along the valley using a 1D model; and two dimensional numerical modeling of the propagation and spreading of flood wave for various dam breaching scenarios in two different urban areas. Such a methodology is a vital tool for decision-making process since it takes into account the spatial heterogeneity of the basin parameters to predict flood wave propagation downstream of the dam. Proposed methodology is applied to two dams; Porsuk Dam located in Eskişehir and Alibey Dam located in Istanbul, Turkey. Both dams are selected based on the fact that they have dense residential areas downstream and such a failure would be disastrous in both cases. Model simulations based on three different dam breaching scenarios showed that maximum flow depth can reach to 5 m at the border of the residential areas both in Eskişehir and in Istanbul with a maximum flow velocity of 5 m/s and flood waves having 0.3 m height reach to the boundary of the residential area within 1 to 2 h. Flooded area in Eskişehir was estimated as 127 km 2 , whereas in Istanbul this area was 8.4 km 2 in total.

Sedimentation in the main pool of a deep ͑maximum depth: 50 m͒, 227 km 2 hydropower reservoir was modeled using a three-dimensional numerical model of hydrodynamics and sedimentation for different wind, inflow, and outflow conditions. Short-term velocity measurements made in the reservoir were used to validate some aspects of the hydrodynamic model. The effects of thermal stratification on sedimentation patterns were investigated, since the reservoir is periodically strongly stratified. Stratification alters velocity profiles and thus affects sedimentation in the reservoir. Sedimentation of reservoirs is often modeled considering only the deposition of sediments delivered by tributaries. However, the sediments eroding from the shorelines can contribute significantly to sedimentation if the shorelines of the reservoir erode at sufficiently high rates or if sediment delivery via tributary inflow is small. Thus, shoreline erosion rates for a reservoir were quantified based on measured fetch, parameterized beach profile shape, and measured wind vectors, and the eroded sediments treated as a source within the sedimentation modeling scheme. The methodology for the prediction of shoreline erosion was calibrated and validated using digital aerial photos of the reservoir taken in different years and indicated approximately 1 m / year of shoreline retreat for several locations. This study revealed likely zones of sediment deposition in a thermally stratified reservoir and presented a methodology for integration of shoreline erosion into sedimentation studies that can be used in any reservoir.

Acoustic Doppler current meters (ADV, ADCP, and ADP) are widely used in water systems to measure flow velocities and velocity profiles. Although these meters are designed for flow velocity measurements, they can also provide information defining the quantity of particulate matter in the water, after appropriate calibration. When an acoustic instrument is calibrated for a water system, no additional sensor is needed to measure suspended sediment concentration (SSC). This provides the simultaneous measurements of velocity and concentration required for most sediment transport studies. The performance of acoustic Doppler current meters for measuring SSC was investigated in different studies where signal-to-noise ratio (SNR) and suspended sediment concentration were related using different formulations. However, these studies were each limited to a single study sitȩ where neither the effect of particle size nor the effect of temperature was investigated. In this study, different parameters that affect the performance of an ADV for the prediction of SSC are investigated. In order to investigate the reliability of an ADV for SSC measurements in different environments, flow and SSC measurements were made in different streams located in the Aegean region of Turkey having different soil types. Soil samples were collected from all measuring stations and particle size analysis was conducted by mechanical means. Multivariate analysis was utilized to investigate the effect of soil type and water temperature on the measurements. Statistical analysis indicates that SNR readings obtained from the ADV are affected by water temperature and particle size distribution of the soil, as expected, and a prediction model is presented relating SNR readings to SSC measurements where both water temperature and sediment characteristics type are incorporated into the model. The coefficients of the suggested model were obtained using the multivariate analysis. Effect of high turbidity conditions on ADV performance was also investigated during and after rain events. Keywords