Control Surface Approach for Inlet Control Performance of Pipe Culverts: Comparison of Experimental Data and Design Frameworks

Culverts are widely used hydraulic structures, and commonly are designed using the Federal Highway Administration (FHWA) or USGS frameworks. This paper presents the control surface method (CSM), a generalized method for estimating inlet control performance based on the geometry of the inlet and slope of the culvert. CSM design values were compared with available experimental data for circular pipe culverts, and agreement was within 5% for the discharge coefficient (CD) and 8% for the pressure term (kD). CSM performance was compared with FHWA and USGS performance for 14 different pipe culvert inlets. Within the span of valid inlet geometries, FHWA performance was estimated within 4% and 3% for unsubmerged and submerged inlet control flow, and USGS performance was estimated within 7% and 5% for unsubmerged and submerged inlet control flow, respectively. Overall, the CSM estimated both FHWA and USGS performance and experimental results with an average absolute error of 1%–2% across all inlets and flow types considered. Differences between CSM and FHWA performance remained largely constant with increasing slope, but differences between CSM and USGS performance increased. It is concluded that the CSM can be used as a complementary method to existing design frameworks, allowing for estimation of inlet control performance for a wider range of inlet geometries, and for development of hydraulically efficient inlets. The CSM also can be used to reduce the uncertainty associated with differences between assumed and as-built culvert inlet geometries in hydraulic culvert design. Based on the results, a span of valid inlet geometries and maximum and minimum performance estimates were determined for the CSM. The CSM is limited to ungated, unblocked, unembedded, and nonmitered pipe culverts of constant slope, and the included span of valid inlet geometries. Further work is required to extend the method to other inlet configurations, cross section shapes and outlet control flow types.