Conduit Slot options

Overview

Stability in conduit units may be affected in two main areas - at low depths and flows and also at the transition from free surface flow to surcharged flow. As a tool to aid these issues, the user may optionally add an artificial slot at the base or top of a conduit.

A slot in the bottom of a conduit can help the model run at lower flows and depths by allowing the hydraulic properties to reduce at a slower rate and to exist at small negative depths. A slot in the top of a conduit enables the surface width to reduce also at a gradual rate and can aid the transition from free surfaced to surcharged flow.

The dimensions of the slot are chosen by default to minimise the differences in hydraulic properties between the conduit containing a slot and its true properties, i.e. without a slot, thus minimising the effect on output results.

The user may specify the existence of a slot globally, i.e. in every conduit throughout the model, or individually. The dimensions in the slot may be similarly specified, or left as default values.

Global option - run form

The 1D Scenario Data form (Low Flow options tab), which can be opened by going to Simulation Tab > Run Simulation button enables the modeller to specify the global existence of a bottom or top slot (Apply conduit bottom/top slot), and also to override the default dimensions of these (Bottom/Top slot depth/height/dh).

Local option- unit form

Additionally, each conduit unit form allows the modeller to override or reaffirm the global setting of the existence or the dimensions of either slot. If values are specified in the unit form, this will override the global setting.

NB A zero in any of the dimensions indicates that the default value will be used.

Note

The culvert slot option only applies to the geometric shaped conduits (circular, rectangular, full arch or sprung arch), and not to symmetric conduits (conduit section unit).

Methodology

Bottom slot

To achieve a smooth transition from flow within the slot to flow within the main culvert area, a smooth shape for the slot is chosen to effect this. For the bottom slot, this is achieved by elliptical quadrants see Murray (2003), as shown in the figure below.

In order to preserve the hydraulic properties of the conduit unit, the default area of the bottom slot is chosen to be equal to the area of culvert which it replaces. Thus, above the slot, the area is independent of whether a slot is present. Also, the default dimensions of the slot are chosen to minimise the difference in conveyance between the conduit without a slot and one with a slot. This necessitates that the top of the bottom slot is above the invert level, rather than coincident with it, which would add area and perimeter to the conduit, which could be significant for a small conduit.

The slot dimensions are specified by

• a 'dh' value, which is the distance between the top of the slot and the invert of the culvert.

• A depth of slot value, which is the total depth of the slot, including dh.

The user does retain the flexibility to alter these dimensions, which would typically be dependent on the shape and size of culvert, although typical values may be in the vicinity of 0.1m and 0.5m respectively.

Top Slot

For a top slot, any stability problems that may occur are often caused by a sudden change in free surface width. For a rectangular conduit, this is particularly pronounced as a culvert transitioning to surcharged flow has a sudden decrease in wetted width from that of the culvert barrel to zero.

To achieve this, a gradual decrease in the width, with no effect on the actual area or perimeter, is employed. For a rectangular conduit, the bottom of the top slot

may be at the same height as the conduit soffit, and reduce to zero width at the top of the top slot; for circular or arch conduits, it is recommended that the bottom of the top slot is below the soffit level.

As with the bottom slot, the slot dimensions are specified by

• a 'dh' value, which is the distance between the bottom of the top slot and the soffit level,

• a height of slot value, which is the total height of the top slot, including dh

Again, the user has the flexibility to alter these dimensions, which would typically be dependent on the shape and size of culvert. Typically, dh would be zero for a rectangular culvert, and 0.04*radius for circular/arch conduits. A typical slot height would be half the barrel height.

It should be noted that without a top slot (or above its top), the conduit units in Flood Modeller automatically apply a slot of zero effective width to model surcharging culverts. The top slot option adds additional functionality in that it smoothes the top-width transition to zero.