3.13.6Reducing Conveyance with Height
The conveyance of a cross-section may reduce with height where there is a sudden increase in the wetted perimeter compared with a relatively small increase in flow area, causing the hydraulic radius to reduce despite the water level increasing. If this occurs, a WARNING is issued and it is strongly recommended that the cross-section be reviewed and corrected.
The most common cause for the reduction in conveyance with height occurs when the extent of inundation across the cross-section increases markedly during the transition from in-bank to out-of-bank flow. The reducing conveyance with height problem is usually resolved by forcing a parallel channel analysis by specifying a change in resistance using the R, M or N flag discussed in the sections above.
Alternatively, as of Build 2005-05-AN, set Conveyance Calculation == ALL PARALLEL to force a parallel channel analysis based on splitting the cross-section into parallel channels at every distance (X) value. This approach ensures that there will be no conveyance reducing with height WARNINGS. Using this approach to calculate the hydraulic properties has a tendency to produce a slightly more efficient cross-section (higher conveyance), similar to the resistance radius formulation used by some schemes. Therefore, a slightly higher Manning’s n value (by ~10%) may be needed to achieve similar results.
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