Calculating Discharge from Culverts under Inlet Control Using Stage at the Inlet

Discharge through circular culverts under inlet control can be calculated using several empirical and theoretical methods; however, most methods require knowledge of site-specific characteristics, and some may even require that a rating curve be developed for each individual culvert. This process is onerous and often too time-consuming for land managers. The objective of this study was to identify a simple and accurate method to determine discharge at culverts. In this study, discharge at culverts was measured in forested watersheds using trapezoidal flumes and salt tracer tests and compared to discharge estimated using theoretical and empirically derived equations. The equation that best estimated culvert discharge was determined. This equation requires two variables: the diameter of the culvert and the height of the water at the culvert inlet. While this study used capacitance devices to measure and record stage at the culvert inlet, simple staff gauges or crest gauges could be used to measure water height adequately and subsequently calculate discharge from culverts.