Hydraulic Structures HYDRAULIC STRUCTURES

Hydraulic Structures Hydraulic structures are devices which are used to regulate or measure flow. Hydraulic structures form part of most major water engineering schemes, for irrigation, water supply, drainage, sewage treatment, hydropower, etc. Hydraulic structures grouped under three headings: Flow measuring structures, Ex: Weirs and Flumes Regulation structures, Ex: Gates and Valves Discharge structures, Ex: Spillways

Flow Measuring Structures Open channel flow occurs with a free surface open to the atmosphere, thus meters using change in pressure cannot be used as in pipe flow measurement. For open channel flow measurement, a change in depth of flow at some point is typically measured and correlated with water flow rate. The most common methods of measuring open channel flow rate are with a weir (sharp crested weir (v notch weir and rectangular weir), broad crested weir) or a flume (Venturi flume and Parshall flume).

Weir Basics A weir is basically an obstruction in the flow path in an open channel. The weir will cause an increase in the water depth as the water flows over the weir. In general, the greater the flow rate, the greater will be the increase in depth of flow, The height of water above the top of the weir is the measurement usually used to correlate with flow rate.

Types of Weirs The two major types of weir are sharp crested weir and broad crested weir. The crest is the term used for the top of the weir, where the water flows over it. The two diagrams here show a sharp crested weir and a broad crested weir. The sharp crested weir has a sharp surface at the crest or top, where the water flows over it, and the broad crested weir has a broad flat surface at the top.

The height of the water above the crest of the weir is called the head over the weir and is shown as H in both of the diagrams. It is the parameter that is measured and used to determine the flow rate.

Types of Sharp Crested Weir Two common shapes for a sharp crested weir are the v notch weir and the rectangular weir. A rectangular weir (horizontal across the top) may go all the way across the channel (called a suppressed rectangular weir). If it is inset at the sides as shown in the diagram, then it is called a contracted rectangular weir.