9.1.2 Fixed Installations

Open Channel Installations

Open channel flow is flow defined as gravity flow in any stream, canal, ditch, flume, or partially full pipe or tunnel not under pressure. It is different from closed channel flow in which the closed channel is full and under pressure. This section provides installation and monitoring advice in such open channels as natural streams and engineered conduits such as concrete channels as well as installation recommendations in monitoring water level in flow control structures such as weirs, flumes, pipes and orifice discharge devices.

To ensure the integrity of monitoring data, it is vital to choose the monitoring location with care. In natural channels, choose a location on the stream where the flow is least turbulent without surcharging, yet representative of the immediately upstream reach of the channel in slope and bottom surface roughness. In streams or channels with deep or rapid flow, or of uneven or slippery bottom materials, take extra care for your personal safety in installing equipment. Do not install equipment by attaching to structures that may interfere with debris flow, threaten the stability of flow at that location or the security of the equipment. When possible, install the Levelogger in a stilling well device. The stilling well will protect the logger from floating debris and rock saltation as well as dampen the effects of surface waves or turbulence. If a stilling well cannot be used and the logger must be affixed to an anchor structure instream such as a concrete slab or large stable boulder, consider protecting the logger from floating debris and rock saltation by placing it inside a short perforated section of protective 1” dia. pipe. Avoid monitoring in the vicinity of flow obstacles that can cause hydraulic jump. In pipes, the logger should be located at least 2 – 4 pipe diameters down- and upstream of pipe turns, junctions, inlets, outlets, turbulent joints or valves. Place the logger upstream of zones potentially subject to surcharging. Always bear in mind that the logger does not have to be at the deepest point in the stream or at the bottom of the pipe, it need only be located in the safest location within the stream/pipe profile just below the lowest anticipated water line. In this case use an offset value or the Reference Datum adjustment to compensate the data for the level offset.

This Section discusses flow in open channels determined by one of the following methods: Area-Velocity, Slope-Hydraulic radius and Hydraulic structure(s).

Using the Area-Velocity method the Levelogger provides readings of the head of water above the pressure transducer. The practitioner will use the water depth to determine the cross-sectional area of water, then the water velocity to ultimately derive flow:

The cross-sectional area of water in natural channels is most commonly determined by surveying the shape of the channel bottom at the monitoring station. In circular pipes the wetted area can be determined by using a section of a circle equation and in other engineered channels, by determining the channel geometry. Either water head will be related to area by using and equation, where head is the only variable, or the Head: Area information is compiled in a Look-up table. The water velocity can be determined in a number of ways, by estimation taking into account the channel geometry, area and surface roughness, by spot measurement in which the stream is velocity profiled both vertically and across its cross-sectional area to develop a rating curve, or by deployment of a logging velocity meter that can accurately characterize the velocity over the cross-sectional area. Together the area multiplied by average velocity to derive flow.

In the Slope-Hydraulic radius method, techniques like the Manning formula or variations such as Lanfear and Coll or Chezcy’s equations are used to estimate flow based on changes to the cross sectional area and wetted perimeter. The cross sectional area and wetted perimeter must be predetermined and are proportional to head. The Manning formula:

Hydraulic structures are among the most common and reliable means of measuring flow. Four types of hydraulic structures and the methods used to measure them are discussed here: weirs, flumes, pipes and orifice discharge devices.