CHAPTER 5 - INSPECTION OF WATER MEASUREMENT SYSTEMS

5. Rough Water Surface

Reducing turbulence or improving approach flow distribution can eliminate rough water surfaces that are not caused by wind. A rough water surface can cause errors in discharge measurements when a staff gage must be read to determine head or cross­sectional area of the flow. A stilling well will help reduce errors in head measurement, but every attempt should be made to reduce the water­surface disturbances as much as possible before relying on the well. Errors of 10 to 20 percent are common where a choppy water surface impedes accurate head determination. The area of piping to a stilling well should be about one-hundredth of the well area to dampen water surface oscillations. A larger area of piping may be needed to eliminate debris plugging or increase well response to changes in measuring head. A smaller area may be needed to dampen overly rough flow.

Specially constructed wave­damping devices (Schuster, 1970) are often required to obtain a smooth water surface. Figure 5-3 shows a schematic of an underpass type of wave suppressor successfully used in both large and small channels.

eqn
Figure 5-3 -- Underpass wave suppressor.

The channel may be either rectangular or trapezoidal in cross section. Constructing the suppressor four times as long as the flow is deep can reduce waves as much as 93 percent. The suppressor produces a slight backwater effect for the most effective vertical placement. The suppressor may be supported on piers, can be constructed of wood or concrete, and need not be watertight. The design of several other suppressor types, along with example cases, is covered in Peterka (1983). Figure 5-4 shows turbulence and waves in a Parshall flume produced by an outlet works stilling basin, which makes accurate discharge determination impossible. The log raft in the foreground was used in an attempt to quiet the flow; however, the raft was later lifted out of the water because of ineffectiveness.

photo
Figure 5-4 -- Turbulence and waves in a Parshall flume produced by an outlet works stilling basin. The log raft failed to quite the flow.

Figure 5-5 shows the water surface after removal of the log raft and installation of an underpass-type wave suppressor. This modification significantly reduced the turbulence and waves, making accurate discharge determination a routine matter.

photo
Figure 5-5 -- Underpass-type wave suppressor reduces turbulence and waves in Parshall flumes.