Replacement floors (AKA “magic bottoms” or “false floors”) are a way of correcting flume hydraulics without the need for costly demolition and construction. 

The primary uses for replacement floors are correcting the finished elevations of flumes whose floors have been set too low or which are experiencing submergence due to downstream conditions.  Occasionally, replacement floors are used to correct a material failure in a flume's floor or an otherwise uncorrectable distortion caused by inadequate cribbing / poor technique during installation.

Securing

In using a replacement floor the intent is to raise the base line hydraulics to a desired height. Although configuration vary from flume to flume, replacement floor are usually secured either through wedging/screwing into place (for small differences in elevation) or lag bolting (for large differences in elevation).  The floors are permanently mounted into existing flumes; they are not designed for later removal.

Upstream Consideration

The height of a replacement floor can be as much as needed; the only limiation is the amount of head the upstream channel can comfortably accomodate.  In practice, though, it is rare to raise the floor of a flume more than 1/6th of its height.

Setting the Floor Elevation

For applications where a replacement floor is being used to reduce or eliminate the effects of downstream submergence, determining the correct height for the replacement floor is similar to that used in setting a flume in an existing channel for free-flow.

Applicability  

Only flumes with vertical sidewalls can be fitted with replacement floors.  These include the Parshall, Montana (short Parshall), and Cutthroat styles.

Flumes with sloped sidewalls (RBC and Trapezoidal) or varying exit geometry (HS / H / HL) cannot use replacement floors as a change in floor elevation results in a change in the cross-sectional area of the flume.  Use of a replacement floor in these flumes would require field re-rating of the flume.