Drop Inlet Failures
Brian Dillard, Rachel Oller, Ryan Stricklin, Mary Womack
The Natural Resources Conservation Service (NRCS) is a federal agency that works hand-in-hand with the people of the United States to improve and protect their soil, water, and other natural resources. For decades, private landowners have voluntarily worked with NRCS to prevent erosion, improve water quality, and promote sustainable agriculture. NRCS has implemented many methods to address these concerns in a variety of situations on private and public land (NRCS website, 2005). During a storm event, a large amount of runoff results from developed land or land used for agriculture. Haan, et al. (1994) states that in situations where there are few abstractions to rainfall, such as soil infiltration or plant cover, runoff volumes will be much higher than on land that is undeveloped such as pasture. The peak discharge rate will also increase and the need to control these high volumes and peak flows is important to decrease soil erosion. NRCS uses drop inlet grade stabilization structures (GSSs) as one method to control large runoff volumes. These structures primarily prevent gully erosion and involve placing corrugated metal pipe (CMP) with diameters as great as 60” to route water from higher to lower elevations. Per discussion with Chris Stoner, Agricultural Engineer with NRCS, implementation of these structures increases the stability of channels by preventing gully erosion and consequently reducing sediment deposition downstream. Although no laws require the use of these structures, farmers, landowners, and even county commissioners install these for the protection of land and roads.In the 1950’s, various entrances for drop inlet GSSs were tested. It was found that canopy and sliced entrance structures were more effective in producing full pipe flow at lower heads than the conventional blunt entrance (Stoner, 2000). (Figure 2 provides an example of a sliced hood inlet, and Figure 3 depicts the canopy hood inlet in the field.) A pipe experiencing full pipe flow moves a greater volume of water in a shorter period of time than pipes flowing partially full. This is important because it reduces initial storage volume, exposure of water to soil, and is more cost effective. It was also found that vortex formation around the sliced and canopy entrances was less of a concern. (Stoner, 2000) Since the 1950’s studies, these structures have been widely implemented.
Since the 1980’s, Oklahoma has implemented many canopy and sliced inlet GSSs to control high runoff volumes over rural land. Though proven to be very useful over the years, an increasing number of failures of the inlets have occurred. In a NRCS report, Chris Stoner outlined the first noticed collapse on a sliced inlet. The entrance of a 42” corrugated metal pipe had failed the first time it flowed. The left side had folded inward, creating a 40% blockage of flow. Since that time, other failures have been noticed and reported. These occurrences were typical of 48” diameter or greater pipes with a 16 gauge thickness. In 1995, the NRCS recommended the use of canopy inlets instead of sliced hoods, because the canopy added extra strength to the structure. In 1997, the inlet thickness was increased to 14 gauge for pipes with diameters greater than 42”. However, a failure was reported in November 2000 of 14 gauge pipe. The report by Stoner also details characteristics of the failures, which interestingly enough are all similar. Always occurring on the left side looking downstream, the pipe folded inward, consequently blocking the flow and limiting the capacity for which it was designed. Because the time of failure is difficult to determine, the magnitude of head causing the collapse is also difficult to determine.
Scope of Work
NRCS is seeking an analysis of canopy and sliced inlets to establish criteria for providing increased strength for corrugated metal canopy inlets, including: Determining design parameters that govern the need for increased strength; Identifying pipe sizes, corrugations, and gauges that need increased strength;Proposing changes to the Oklahoma NRCS Conservation Practice Standards to reflect the analysis. The NRCS also requests alternative methods for strengthening and a cost comparison of options.