Walter McDonald, PhD Student, Civil and Environmental Engineering, Virginia Tech
The weir upstream of the LEWAS site in the culvert underneath West Campus Drive was constructed in 2003 and is the planned location for a level measurement device. The following document contains a breif background into the weir construction and calibration, recommendations for improvements to flow measurement at the weir, and preliminary plans to place a stage measurement device behind the weir.
Model Weir vs. Constructed Weir
The weir in the culvert is a contracted rectangular weir at low flow with 150 degree v-notch sidewalls extending above the rectangular crest to the walls of the culvert. A scaled model of the weir was constructed and calibrated in a flume to create a depth-flow relationship as shown in Fig. 1. The weir was calibrated in the lab with “free” or modular flow conditions where the height of the lowest point of the weir crest elevation is higher than the height of the water downstream. The weir in the model is placed 1.5 inches above the channel bed, thus creating a “free” flow condition. In this case, the water will pass through critical flow and discharge can be estimated based on the depth of water above the crest of the weir at a location upstream.
Fig. 1. Weir model
The weir upstream of the LEWAS site has a submerged flow or “non-modular” flow condition where the discharge is partially under water. Submerged flow occurs when the downstream water surface is near or above the crest. In this case the crest is the bottom of the channel so in all conditions the water downstream water surface is above the crest as illustrated in the diagram of the weir in Fig. 2. Because of this condition, changes in the downstream depth can affect flow rate and accuracy of measurement should not be expected (USBR 2001). To properly determine flow rates in submerged flow conditions the downstream depth as well as the upstream depth need to be measured, however this will only be an estimate.
Fig. 2. Diagram of weir below West Campus Drive
Two separate weirs were created as part of the project. Weir #1 was constructed underneath West Campus Drive just upstream of the upper Duck Pond and Weir #2 was constructed underneath a pedestrian bridge below the drill field. Fig. 3 illustrates base flow conditions at Weir #1 where submerged flow conditions are apparent; the elevation of the crest is equal to the elevation of the stream bed. Fig. 4 illustrates base flow conditions at Weir #2 where free flow conditions exist. It is clear that the elevation of the water downstream of the weir is below the lowest level of the crest as there is water exhibiting “free” flow beyond the weir.
Fig. 3. Photos of submerged weir
Fig. 4. Weir beneath pedestrian bridge below drill field
One recommendation to establish free flow conditions at Weir #1 would be to modify the weir structure so that the crest elevation is raised 3 inches above the bed elevation to the bottom of the wing walls. This would create a backup behind the weir and free flow conditions as long as backwater does not occur at least 3 inches above the bed elevation.
Placement of the stage measuring device
The stage measurement device must be placed upstream of the weir a sufficient distance in order to properly measure stage behind the weir. It is recommended that the gage be placed at least 4 times the maximum depth of water to flow over the weir as shown in Fig. 6 (FAO 1993). The gage was calibrated in the lab to the maximum height of the v-notch weir which corresponds to a height (H) of 52.9 cm. This would mean that the gage would need to be placed a minimum of 211.6 cm behind the weir. Because of this the pipe expansion as seen in Fig. 5 would be 1 m too close to the weir and would not be a suitable location to measure stage. Following the recommended minimum distance the stage measurement location would have to be placed at least 1.1 m behind the pipe expansion cross section.
Fig. 5. Pipe expansion 1.1 m upstream of weir
Fig. 6. Stage measurement location behind weir
Candidate Stage Measurement Devices
Possible stage measurement devices include a pressure transducer or an ultrasonic transducer. Due to sedimentation that has been documented at the site, a pressure transducer may not be practical given the sedimentation events that would cover or damage the sensor. An alternative is an ultrasonic transducer which could be mounted at the top of the culvert and measure stage from above. This device would be completely removed from the flow of water except for rare full flow conditions and its ability to function would not be affected by sediment. However, sedimentation would still affect the stage measurement as it would displace the water above the channel bed, thus giving a stage reading that is higher than what is actually occuring. Fig. 7 illustrates an ultrasonic configuration.
Fig. 7. Ilustration of downward looking ultrasonic depth sensor in pipe. (Hach 2013)
- FAO (Food and Agricultural Organization of the United Nations), (1993). “Structures for Water Control and Distribution (Irrigation Water Management Training Manual).” FAO, pp. 29-37
- USBR (United States Department of the Interior Bureau of Reclamation), (2001). “Water Measurement Manual.” Washington, DC.
- HACH Co., 2013. “US9001 and US9003 Ultrasonic Level Sensors”. Product Data Sheet, Loveland, Co.