Environmental Engineering Reference
In-Depth Information
Fig. 3.2 Surveying a wetland staff gage to a local datum. Rod is held on a screw projecting from
the plank on which the staff plate is mounted. Water-level is at 26.38
timing and frequency of observer visits to the site. Staff gages are subject to
movement because wetland sediments tend to have a relatively large content of
organic material and are, therefore, often poorly competent, meaning they are
loosely compacted and may readily deform. Pipes or rods to which staff gages
are attached should be driven deeper to provide a stable anchor if sediments are soft.
If the wetland surface freezes during winter, any change in the elevation of the ice
surface over the course of the winter, such as a rising ice surface during snowmelt,
can also move the staff gage, either horizontally or vertically. Therefore, the height
of the staff gage relative to the local datum needs to be determined at least annually
to provide inter-annual continuity of stage data. Staff plates can be stacked verti-
cally if wetland stage varies over a distance greater than the length of a single staff
plate. Because this method is so simple and relatively robust, staff-gage values
commonly are used as the reference value when automated sensors are used
to collect more frequent stage data. Staff plates need to be cleaned regularly to
remove chemical or biological accumulations at or near the water surface.
3.3.1.2 Float-Based Gage
A float and counterweight connected to opposite ends of a tape or wire draped over
a rotating pulley is another wetland-stage measurement method that has been in use
for many decades. The float moves up and down with the wetland water level,
