This section describes the computational methodologies employed by Culvert Studio. It is highly recommended that you review the computational methods and equations used so that you will better understand the output and results. It is not the intention of this section to provide the basis of the theories used or to demonstrate how they were derived. But rather provide the actual equations and methods employed by the software.
The program uses only widely accepted methods within the industry. Procedures described in HEC-22, HDS-5, HEC-14 and the Rational method are the primary methodologies. This section will provide a summary of the concepts used but it is not intended to be all-encompassing. Below is a list of publications which provide details on the methods used.
FHA Hydraulic Engineering Circular No. 22 Third Edition, 2009
FHWA Hydraulic Design of Culverts (HDS5) Third Edition, 2012
Hydraulic Engineering Circular Number 14, Third Edition, 2006
Why Culvert Modeling Can Be Difficult
Culvert Studio follows calculation procedures described in HEC-22 and HDS-5. Culverts are complicated. Their analysis is difficult and often times confusing. Their flow regimes change with seemingly little reason. Their barrels may flow full or partly full but full flow throughout their length is rare. Generally at least part of their length is in partial flow. The upstream end may be totally under water while underneath, the barrel is in supercritical flow ending downstream in subcritical flow. Raise the tailwater a little and the entire flow regime changes to full.
Culvert Studio sorts out these hydraulic anomalies by using time-tested methods and some sophisticated algorithms.
Culvert Studio sorts out these hydraulic anomalies by using time-tested methods and some sophisticated algorithms. This chapter outlines those methods. It starts with an overview of a puzzling concept… Inlet and Outlet Control.
Inlet and Outlet Control
Culverts flow under two regimes; Inlet Control and Outlet Control. Inlet control implies that it is more difficult for water to get in the pipe than it is to get through it. During outlet control, it is more difficult for flow to get through the barrel than it is getting inside of the barrel.
Inlet control is a lot like traffic going from a four-lane highway into a two-lane tunnel.
Inlet control is a lot like traffic going from a four-lane highway into a two-lane tunnel. As the traffic nears the tunnel, it must squeeze together causing a traffic jam that affects the cars approaching the tunnel. Once in the tunnel, traveling is easier and traffic speeds up. You’ll find that culverts often flow in partial depth throughout its barrel while under inlet control. Traveling is easier because most of the cars are still trying to get in the tunnel.
If, on the other hand, there was an accident inside of the tunnel, traffic would slow down even more after entering. Traveling is more difficult. This is outlet control.
Inlet control is largely influenced by the entrance geometry of the pipe such as edge configuration, pipe area and shape. Outlet control is influenced most by n-value (barrel roughness), pipe area, shape, length and slope.
So how does one determine the flow regime of a culvert? The solution is to compute the hydraulic profile assuming both exist, and then selecting the one that produces the highest headwater, Hw.