Most dams have a low-level outlet that consists of a closed conduit through the dam with a slide gate or valve to regulate flow. These outlets are used mainly for irrigational purposes but also for flushing the reservoir and controlling the reservoir elevation. When discharging through the low-level outlet works, negative pressures can develop on the downstream side of the gate creating a potential for cavitation damage and vibration. To minimize these effects, air vents (vented to the atmosphere) are installed on the downstream side of the gate to limit downstream pressure to something above vapor pressure (i.e., near atmospheric pressure). Previous air venting studies have been mostly limited to large dam outlet geometries, which typically feature a vertical gate in a flat-bottomed discharge tunnel. The large-dam air demand analysis has been based on the Froude number of the supercritical flow at the vena contract (located between the gate and the hydraulic jump) and the water flow rate. Small to medium-sized embankment dams typically utilize a slide gate installed on the sloped upstream face for flow control, followed by a vertical elbow connected to a sloping pipe. With this outlet geometry, there is no 1-D vena contracta flow, no classical hydraulic jump, and no representative Froude number. Additionally, no head-discharge characteristic data have been found for inclined slide gates (vented or non-vented) for small to medium-sized dams. Consequently, unless a flow measurement structure is installed in the discharge channel downstream of the dam, determining the water discharge rate based on gate opening and head on the gate, and consequently the air demand is problematic. This study focuses on quantifying air demand and air vent sizing for the small to medium-sized embankment dam low-level outlet geometries by providing: 1. Cd values as a function of gate openings and air demand; to better estimate flow rates from outlet works of similar geometries. 2. Flow conditions for varying operating conditions. 3. A new relationship for sizing air vents as a function of driving head and gate opening. 4. The magnitude of negative pressures for non-vented conduits. 5. A foundation for future studies and development of air demand research. This thesis presents the findings of this study.

A physical experimental setup of a circular, gated closed conduit was built at the Utah Water Research Laboratory (UWRL). Setup configurations were modified and data were measured to aid in the study of physical variables on air demand. It was determined that gate opening, gate and water surface roughness, and conduit length all were significant variables on the air demand measured through the conduit air vent. It was also determined that no noticeable air velocity profile existed above the air-water interface. A linear relationship was found between the air flow rate to water flow rate ratio (air-demand ratio) and head-to-gate height ratio when identical conduit geometry was used. Data obtained from this study illustrated that the use of the Froude number is an incomplete way to quantify air demand due to the effects of changing conduit geometry.

This diploma thesis deals with the reconstruction of small reservoir in collaboration with the company VZD Invest s. r. o. The first part aims at the basic description of the thesis subject – the small reservoir. Characteristics of small reservoir and calculations are processed in the second part. The third part is focused on the description of the current conditions. The main part consist of characteristics calculations of the small reservoir and proposal of the necessary reconstruction for future technical function. In conclusion, safety assessment of the proposed measures are evaluated.