Weirs are engineered structures designed to raise and protect the stream bed elevation, forcing water over the weir crest and into an operating diversion. They can be permanent or adjustable and full span (crossing the entire channel width) or partial span. Permanent full-span weirs constitute a point along the stream channel where streambed elevation is nonadjustable. This can create problems in a dynamic and frequently adjusting stream system, as most channels are. Most adjustable weirs also affect channel dynamics in this way, because they have a permanent sill supporting adjustable panels, stoplogs, or gates. The effect is usually less than for permanent weirs, though, because during high flows, the weir can be lowered to the permanent sill or floor, which can be much lower than the crest of a permanent weir (Planning & Layout of Small Stream Diversions, USBR). As discussed in the consolidation section, the location described in Figure 1 below has three diversions close to one another and there is a chance of consolidating these diversions. In the meantime, we also have to consider the acequias that are connected as described in Figure 1 below. There is a possibility of consolidating diversions 7 and 8, as labelled on Figure 1, and replacing them with a concrete weir structure at the location of diversion 8.
Figure 1: Focus Area
Figure 2: Santa Margarita River O'Neil diversion weir at Camp Pendleton, CA
To clarify the applicability of the Weir in the location, further evaluation was conducted based on the factors below:
Flows: The purple lines in Figure 1 represent the flowpath of the acequias fed by the existing rock diversions. It is a must to understand the path of these diversion flows as they have a crucial contribution in supplying water to the connecting acequias. Careful study of peak and average flow rate and flood plain mapping without the weir should be conducted before deciding the prospective consolidation of existing diversions.
Cost Benefit: The costs of a weir can only be well estimated if there is a design and in this preliminary stage the costs can only be a rough estimation. In general, concrete weir structures are the most expensive of the alternatives considered here. At the end of the day, which alternative works the best is the ultimate question and based on the cost criteria, a concrete weir structure is at the top of the list of the most expensive alternatives.
Sediment Flow: A diversion dam backwaters streamflow and can cause sediment deposition, especially if the dam is not removed or if sediment is not sluiced during high flows. Upstream of the dam is the depositional area and the stream may be locally shallower and more prone to flood the adjacent lands (Planning & Layout of Small Stream Diversions, USBR). For example, the Santa Margarita River O’Neil diversion weir at Camp Pendleton, CA as shown in Figure 2. This steel pile diversion dam is in a river with very high sediment load. Heavily vegetated sediment accumulations upstream and downstream are visible in Figure 2, which is a photo taken shortly after a moderate flood had disturbed the channel (photo by Kathleen Frizel, Bureau of Reclamation).
Durability: For most structures intended to be stable over a long period of time, designs typically need to consider two critical pieces of information in the hydrologic analysis. First, these structures must be designed using the discharge that is responsible for the maximum scour immediately downstream from the structure. Second, most structures are required, through regulatory means, to be designed so that the added feature does not impact the water surface elevations at a specific discharge (e.g. 50-year or 100-year floodplain) (Weir Design Guidance, U.S. Department of the Interior Bureau of Reclamation). In terms of durability, concrete weirs can be considered the most durable alternative among the four different alternatives considered. However, poor design or construction can leave the structure vulnerable to erosion or decomposition of the weir material. Rock riprap may erode from the structure and wood may rot or erode overtime. Even high quality concrete may be eroded by bed material abrasion (Planning & Layout of Small Stream Diversions, USBR). Therefore, the design and construction phase play an important role in the durability of the weir structure.
Maintenance: Depending on the type of weir used (permanent non-adjustable weirs or adjustable weirs) the maintenance required may vary. Permanent non-adjustable weirs may be preferable in terms of maintenance since no power or operator manipulation is required and generally require less maintenance. Only annual inspections are necessary but inspection and maintenance activities might be needed after a high flow. For adjustable weirs, some systems require power for air compressors, hydraulic pumps, and other machinery to adjust the height of the panels. In addition to annual and high flow inspections and maintenance for the adjustable weirs, an operator must adjust the weir elevation as flows change, unless the system is automated. (Planning & Layout of Small Stream Diversions, USBR)
Fish Passage: In aquatic systems, structures like weirs act as a barriers to fish movement. There is a growing array of technological fish passage solutions for the movement of fish across such structures. A vertical slot fish way can be considered an effective fish passage option that can be incorporated into the weir. Fish are attracted by the water flowing through the fish way and enter through the vertical slots as shown in Figure 3 and 4. These slots lead into a series of small pools to create gentle flowing currents for smaller fish. The entire fish way is constructed to create a gradual ramp of water. This allows fish to travel between the lower and upper levels of the weir. These fish way work in a wide range of upstream and downstream water surface elevations so water flow does not need to be carefully controlled for fish passage purposes.
Figure 3: Vertical Slot Fish-Way showing fish passage.
Figure 4: Vertical-slot fish ladder, Santiam Water Control District diversion canal, OR.
References
Planning and Layout of Small-Stream Diversions [PDF]. (2013, March). United States Department of Agriculture Forest Service.
River Weirs – Good Practice Guide [PDF]. (October, 2003). R&D Publication W5B-023/HQP
Rock Weir Design Guidance [PDF]. (March, 2016). U.S. Department of the Interior Bureau of Reclamation