Water storage with dams began early in human history. Dams are one of the oldest man-made constructions. Today, dams are used to store water for a variety of purposes, including: human consumption, food production, electricity production, industrial use, and flood protection. Dams are vital elements in modern society and represent large economic values. They also represent a potential risk.
With an estimated 40,000 large dams (those over 15 m [~50 feet] high) (and countless smaller dams) worldwide, the risk to life and property is large. A study by the International Commission on Large Dams (ICOLD) on the failures of large dams found that 35% of failures were attributed to erosion and seepage problems. The majority of failed dams studied either did not have a monitoring system or had a system that was out of order.
All dams have some seepage as the retained water seeks the path of least resistance through the dam and its foundation. Seepage becomes a concern if it is carrying material with it. By capturing and analyzing soil particles suspended in or washed along with seepage flows, engineers can monitor a dam’s health over time. Changes in seepage flows or composition can be good leading indicators of issues internal to the dam structure. While changes in a dam’s health over time are important, the greatest risk of dam failure is during the first filling of the dam. During this time the leakage rate must be very closely for abnormal leakage rate or composition.
To assist dam safety engineers and owners of small to medium sized dams and reservoirs, the Federal Emergency Management Agency (FEMA) – National Dam Safety Program has developed the Dam Seepage Monitoring System. The Dam Seepage Monitoring System is a single data system for the management of small to medium sized dams and reservoirs. One aspect of the system is the ability to configure seepage devices including weirs and parshall flumes.
Weirs and Parshall flumes are the most common primary devices for measuring seepage flows. In evaluating between these two options there are several factors that must be considered:
- Capture and retention of sediments / debris
- Sedimentation is usually a considered a drawback of weirs – here it is of critical importance. The combination of an inlet baffle plate and the weir plate proper provide a quiescent area in which suspended particles can fall through the flow stream to collect at the bottom of the weir pool. Larger debris will also be essentially screened from the flow stream by either the baffle or weir plates. Flumes, on the other hand, are quite good at passing solids and debris. The fall through the throat of the Parshall flume in particular makes the retention of sediments or debris unlikely. Here, secondary containment upstream or downstream of the flume will be required to capture the requisite solids. An alternative is the use of a HS / H / HL flume with a riser box in the approach section. The riser box acts in a manner similar to that of a weir pool – it provides a quiescent area through which flows must rise to enter the primary device.
- Flow rates and accuracy
- While weirs are commonly though of as being able to measure flows more accurately and at lower flow rates than flumes, this in not actually the case. The minimum head and flow rate for even the smallest V-notch weir is 0.2-feet [6.096 cm] at a rate of 3.990 gpm [0.2518 l/s]. Accuracy is +/- 2%. A 1-inch [2.54 cm] Parshall flume goes lower in both head and flow rate, 1.460 gpm [0.0921 l/s] and 0.60-inches [1.52 cm]; with an accuracy of +/- 3-5%. HS flumes go even lower in flow rate (0.0718 gpm [0.0045 l/s]), head (0.12-inches [0.30 cm]). All at an accuracy of +/- 3%. Additionally, the maximum head recommended for a weir is 2-feet [60.96 cm]. Depending upon the maximum anticipated flow rate, this may not be high enough.
- Installation Footprint
- For a weir to function accurately it is vitally important that the upstream weir pool is sized correctly. In applications where a weir is intended to read both low and high flow rates, the required size of the weir pool may not fit in the area in which seepage flows need to be monitored. In general, the installation footprint for a flume is smaller than that of a weir.