Evaluation of Drop-Check Structures for Farm Irrigation Systems

Abstract

The slope of irrigated lands varies greatly, requiring gradient control and energy-dissipating structures in most irrigation systems. Water-level control structures are also required for good water management on irrigated farms. Combination check and drop structures are often used where both types of control are needed in the same ditch. The headwall extension length and cutoff wall depth requirements for these structures vary for different soils and site conditions. Most commercial structures of a given hydraulic capacity have fixed dimensions and do not provide sufficient headwall length or cutoff wall depth for many field conditions. They also have stilling basins that are generally too narrow or too short or both. Washed-out structures and eroded ditches are a problem on many farms. A common standard design for a stable structure at a given site is a formed, cast-in-place, concrete structure. However, because of economy and convenience of installation, prefabricated structures are frequently used. Additional design and site limitation information is needed to reduce failures of prefabricated structures. Structures are needed that provide adequate erosion and water control, economy, ease of installation, and ready acceptance by the farmer. This study was made to obtain information to improve the design of drop and check structures for farm irrigation systems, and for use by the USDA Soil Conservation Service and other agencies in evaluating small structures. The study involved the evaluation and comparison of the field performance of various conventional and standard structures as they are commonly used. Several experimental structures were also included. The hydraulic and structural characteristics, together with installation and maintenance requirements, were observed. The amount of scour occurring upstream and downstream from each structure, together with bank erosion and undercutting, were observed through four seasons of operation.