Attendees at the 2017 Alden Forum on Hydropower and Fish Passage
Based on presentations given by the various speakers, the primary takeaways from the forum include the following:
Cake consumed during a break at the 2017 Alden Forum, showing companies and agencies in attendance
The format and content of the forum was highly rated by the attendees and led to many in-depth and productive discussions. The setting appeared to be more conducive to open dialogue among all of the participants compared to typical relicensing meetings and agency consultations.
Planning for additional forums addressing other relevant topic areas related to fish passage and other environmental issues is underway by Alden staff, and may include hosting events in other regions of the U.S.
HydroVision International is the largest gathering of hydro professionals worldwide. Over 3,000 hydro professionals and over 300 hydro related product and service providers were on the exhibit floor. The participants are from 51 countries. The event highlights perspectives on the role of hydropower, explores issues affecting the hydropower industry, covers issues and concerns affecting hydro resources, publicizes current market opportunities and challenges, and facilitates development of a vision to meet challenges and to ensure sustainable development.
Alden is active in supporting the hydro industry, with physical hydraulic modeling, 3D and 2D numeric modeling, fish passage design and testing. We attend Hydrovision every year, and participate in the exhibit, as well as technical conference and training sessions.
I was able to meet with most of our dams and hydro clients and teaming partners. The attendance from the private power producers, utilities, consulting companies and equipment manufacturers were adequate but there was clearly less participation from the federal government, especially from the U.S. Army Corps of Engineers (USACE). I observed increased energy and optimism in developing renewable and small hydro with a hope of relaxed regulation and a faster FERC approval process. That being said, folks seemed to think the price of natural gas will remain low in the foreseeable future and the development of hydro assets may remain relatively less competitive in general. Overall, the need for hydraulic, environmental, and fisheries work appears steady, nonetheless.
The spillway failure at the Oroville Dam impacted the dam owners and hydro industry, overall. Most dam owners are concerned and are taking steps to ensure their dams are safe, irrespective of regulatory requirements. Clearly, the need for support in the area of dam safety will remain strong.
Here a few highlights of the conference I can share:
Testing of Flood Control America’s One World Trade Center flood protection barrier at Alden
During the first week in May, I attended the Association of State Floodplain Managers (ASFPM) Conference in Kansas City. This conference is generally recognized as the key floodplain conference in the U.S. In addition to floodplain managers, associated consulting firms and product vendors regularly attend.
We thought it would be a good idea to go, given Alden’s recent activity in flooding related testing and modeling. Given the nature of my interest, I primarily attended sessions on floodproofing and modeling, and also spent time in the exhibit. The sessions were well attended and had good quality presentations.
For me, the key takeaways from the conference were:
We welcome your comments on flood proofing, flood protection, and modeling.
Mixing between fluids of different properties goes on all around us every day (e.g., think of stirring milk into your coffee or smoke billowing from a chimney). In many flows of engineering relevance, velocity differences between fluid bodies generate turbulent motions that, in turn, greatly enhance the mixing process; small-scale chaotic eddies that characterize the turbulence are much more effective than molecular diffusion at mixing fluid properties such as momentum, heat, salinity, sediment load, or pollution concentration.
When fluid bodies are of different densities the effects of gravity weigh heavily on the turbulent mixing process (pun intended). For example, when warm air escapes from a chimney it accelerates upward in a turbulent billow because it is lighter (less dense) than the cooler air around it, and gravity acts to drive turbulence through buoyant convection. On the other hand, in a stably-stratified lake, gravity acts to suppress turbulence at the thermocline where lighter, warm water overlies heavier cold water.
The signatures of turbulent mixing in stratified flows are perhaps most obvious in the sky above us. Clouds provide a convenient flow visualization method! An anvil-shaped thunder head reveals convectively-generated turbulence in an unstably-stratified environment, whereas rolling billows – think of the sky in van Gogh’s Starry Night – indicate shear-generated turbulence in a stably-stratified environment. A great example of the latter type of cloud was recently observed outside Alden's Fort Collins office (Figure 1).
Figure 1: Lenticular cloud bands forming at the crests of atmospheric gravity waves in the lee of the Rocky Mountain Front Range (flow is toward camera).
The two parallel cloud bands seen in the photograph are occurring at the crests of atmospheric waves that are occurring in the lee of Colorado’s Front Range Mountains (wind is coming toward you in the picture). Flow over the mountains disturbs the stably-stratified atmosphere and generates a train of gravity waves similar to surface waves in a ship's wake. Low pressure at the wave crests causes water vapor to condense and form the "lenticular" cloud bands you see in the picture. Air is actually moving through the clouds rather than the clouds moving with the air! A good cross-section schematic of mountain lee waves is show in Figure 2 from Durran (2013).
Figure 2: Schematic of mountain lee waves and lenticular clouds (from Durran, 2013). Flow is from left to right.
While the lee waves themselves are not breaking into turbulence, it appears that a crosswind acting perpendicular to the main flow is doing something interesting to the cloud bands. See those curling billows in the photograph (close up picture in Figure 3)? Those are caused by shear between the lighter air above the cloud and the heavier air below. The fancy name for these shear-driven billows is Kelvin Helmholtz instabilities. "K-H" instabilities can be observed in many natural flows with density stratification and are common in thermally-stratified flows of oceans, lakes, and rivers. As the billows roll up, they lift heavy fluid up and push light fluid down – working against gravity. Eventually, the coherent billows collapse into smaller-scale, chaotic, turbulent motions that mix the two fluid bodies.
Figure 3: Zoomed photograph of Kelvin Helmholtz instabilities due to shear generated by a cross wind.
The computational fluid dynamics (CFD) models used at Alden can capture K-H instabilities as demonstrated in Figure 4 which shows a snapshot from a simple two-dimensional simulation of warm, lighter water (red) moving across cold heavy water (blue). In many cases of engineering relevance, however, domain size and complexity often preclude the resolution needed to explicitly model the flow structures such as K-H billows that ultimately drive mixing. Instead, numerical models rely on assumptions about what’s going on at the unresolved scales of the turbulence. These assumptions form the basis for turbulence models that approximate the mixing process.
Figure 4: Kelvin Helmholtz instabilities occurring on a density and shear interface between warm water (red) and cold water (blue) as captured in a highly-resolved (Δx = 0.50 cm) two-dimensional simulation. Each billow is approximately 10 cm tall.
Because no model is perfect, keys to a successful modeling effort include calibration and validation. A promising approach to calibration and validation using field data comes in the form of airborne thermal imaging. Alden is currently developing a drone-mounted infrared camera system that will provide overhead snapshots of mixing of water bodies of different temperatures – an application of focus being thermal plumes discharged from power plants. IR images taken recently from a power plant discharging cooling water into a major river are shown in Figure 5. IR imagery provides a valuable check on the lateral mixing predicted by a given CFD model and serves as yet another tool in Alden’s arsenal for understanding and solving mixing-related problems.
Figure 5: Aerial infrared image taken by Alden of a thermal plume discharged from a power plant on a major river.
Flow is from top to bottom of the image. Note the complex structure of the mixing/shear line and dilution of the plume in the downstream direction.
Durran, D.R. (2013, Jan. 29) Trapped lee waves over the western U.S. http://www.youtube.com/watch?v=P84WoxbDXCg.
Searching for eels is an activity reserved for those who like to stay up late. Only under the cover of darkness is one able to have the best chance to find these nocturnal fish. Surveying for American eels, Anguilla rostrata, with lanterns at night or “shining” is a common method to document their presence in rivers across eastern North America. Juvenile American eels often congregate downstream of obstacles that block their upstream movement (eels are catadromous fish, meaning adults spawn in saltwater and the young move into freshwater to rear and mature before returning to the marine environment to complete the reproductive cycle). Hydroelectric dams are common impediments to upstream migrants along river courses. Finding optimal areas to establish passage routes for eels to move upstream is a primary reason to shine for eels.
Eels, being a fish, become more active as the water temperature rises. When the temperature is above 10 degrees C, which generally occurs from May to October in New England, eels of all ages and sizes become more active. The recently born glass eels, (so called because of their transparent appearance), are carried northward by Atlantic Ocean currents, floating along like crystalline feathers until tides pull them ashore where they begin a process of metamorphosis into the more recognizable elongated fish. These eels, now called elvers, grow and mature to the point where they are capable of swimming against the river current to seek inland freshwater and a chance to grow to appreciable sizes, (mature female eels in the St. Lawrence River can reach 1 meter in length and weigh up to 8 kilograms). Eels are long-lived fishes, some individuals are almost 30 years old before returning to the central Atlantic Ocean to complete their lifecycle and give life to the next generation.
Surrounded by complete darkness after nightfall during the warm spring or summer months, throw on a pair of waders, and the search for the secretive eel begins. A careful inspection of the area of interest such as a dam tailwater using flashlights or lanterns is best achieved from the shore or among the rocks and boulders of the riverbed. If the river is inaccessible for wading, a pair of binoculars can help view from a distance by aiming a strong spotlight at the suspected congregation area. Eels do not need a lot of flowing water to stimulate them to try and ascend. They can seek to move upstream through even the smallest trickle of water if that is all that is available, so be sure to pay attention in areas where these conditions exist. Look for rivulets of water flowing among boulder fields that offer a constant and uninterrupted path. They may try and lift themselves up with their tails through a plunging waterfall but they need substrate to support their long muscular bodies and push against it to get up, over and through the point of passage they seek. Eels even have the extraordinary ability to travel overland if the desire is strong enough to move past an obstruction. During warm humid nights, eels can be observed doing just this.
Figure 1: Migrating eels in a New England River
Just because its night-time and it’s the month of June doesn’t mean that eels will be on the move. Environmental conditions such as air and water temperature, precipitation, percent cloud cover, and lunar phase influence eel behavior. Variability in these conditions during the spring and summer months can cause eel activity levels to increase or decrease. Keeping track over a season the exact locations and environmental conditions that cause the greatest number of eels to congregate will provide the best information to decide where to establish an upstream passage route. There are many ways to provide upstream passage for eels (ladders and traps are common), but it is critical to know the optimal location to place these facilities. Eel shining is a proven way to establish these locations by observing the conditions that eels prefer at a site of interest. Alden advocates using night-time surveys as a low-tech yet effective method to shine the light on upstream eel behavior.
During spill season at hydroelectric dams, more water flows into the upstream reservoir than can be used to generate electricity in the powerhouses. This excess flow must pass through a number of different flow release structures in order to bypass the dam and powerhouse. Spillways, diversion tunnels, and low-level sluice gates are commonly used to route flow past dams. Open channel spillways are one of the most common flow release structures at high head dams, and create a highly aerated, turbulent jet of water that exits the spillway up to 150 feet above the river downstream of the dam. This waterfall of aerated flow can plunge to the bottom of the tailwater pool, where the bubbles of atmospheric gases are slowly dissolved into solution with the water. The deeper the jet plunges, the more pressure is exerted by the water on the bubbles, dissolving them faster and preventing them from rising to the surface. This is why we see a frothy white plume of flow that can stretch up to half a mile downstream of a dam when flow is being released, as shown in the photo of Boundary Dam spillway below.
Once the water has dissolved all the gases it can hold in equilibrium (saturation level), the river can exceed its saturation level and becomes supersaturated with dissolved gasses. This is a function of the pressure exerted on the gas bubbles at depth, and the travel time of the bubbles to reach the surface. The sum of all the gasses dissolved into solution is called the Total Dissolved Gas (TDG) concentration. High TDG is a hazard for aquatic life, especially migratory fish such as salmon and steelhead. When fish come into contact with high TDG concentrations at depth, their tissues absorb the gases. When they later swim to the surface, the gasses come out of solution and form bubbles, which can cause trauma around the gills and fins. This trauma is known as Gas Bubble Trauma and has become a major problem for fish populations that must use fish passage systems to bypass dams. Some good photos showing Gas Bubble Trauma in fish can be found in this linked article in the Billings Gazette:
Click on image for full article
The Environmental Protection Agency (EPA) has introduced regulations that limit TDG production to 110% of saturation. This limit is enforced regardless of the TDG level of the water coming into the reservoir, which may be at or near saturation due to upstream dams, waterfalls, or other conditions. High head dams and hydroelectric projects are required to be relicensed with the Federal Energy Regulatory Commission (FERC), and must prove that they meet the new EPA regulations before being granted a license. The owners of a number of affected projects have reached out to Alden to help them find cost-effective solutions to TDG problems, which we have explored extensively using computational fluid dynamics and physical modeling, as well as structural and operational changes to the projects.
In Part II we’ll explore the use of energy-dissipating devices to reduce spillway plunge depth and bubble transit time – stay tuned!
Part 1 of this series outlined how high concentrations of total dissolved gas (TDG) can occur downstream from high head dams when their spillways are open, and how this TDG can be harmful or even fatal to fish. Alden has been involved in several recent projects for which the objective was to reduce TDG downstream of high head dams. Alden performed the hydraulic and structural design of roughness elements that break up the high velocity jet of flow discharged from the spillway. TDG production is reduced by these roughness elements because they cause the jet to spread out and thereby reduce the plunge depth in the receiving water, which reduces TDG. The roughness elements work very well at reducing plunge depth, but they can cause cavitation, which can damage the spillway surface and the blocks themselves. The design and implementation of the roughness elements will be topic of another article. The present article focuses on reducing the potential for cavitation on the roughness elements.
Alden designed roughness elements have been installed on spillways at Cabinet Gorge and Boundary Dams. Cabinet Gorge Dam is shown in Figure 1. The first set of roughness elements installed at Cabinet Gorge Dam performed well at reducing TDG, but suffered cavitation damage (Figure 2). Cavitation can occur in high velocity flows on steep spillways, especially when roughness on the spillway surface causes flow separation. Air supply ramps are often used on spillways to lift the nappe from the spillway surface and supply air to the void underneath the nappe. Cavitation potential is reduced by introducing the air.
Figure 1. Cabinet Gorge Dam and 3D model for CFD (Dunlop, et al., 2016)
Figure 2. Cavitation Damage on Roughness Element (Paul, 2015)
Air ramps were installed upstream of the first row of roughness elements for one bay each at Cabinet Gorge and Boundary Dams to supply air and to lift the horseshoe vortices that form around the base of the blocks off of the spillway surface. The Cabinet Gorge air supply ramps are shown in Figure 3. The hydraulic design of the air ramp and the air supply ducts at Boundary and Cabinet Gorge Dams was performed by Alden and based on the Aerator Design Chapter (Chapter 5) of Dr. Hank Falvey’s “Cavitation in Chutes and Spillways – Engineering Monograph No. 42.” A conceptualization of the air ramp is shown in Figure 4. The flow over the ramp follows a trajectory influenced by: the velocity of the flow at the location of the ramp, the angle of the ramp, the angle of the spillway, and the air pressure in the air pocket underneath the nappe. The underside of the nappe entrains air which generates negative pressure in the air pocket underneath the nappe. Air is supplied to the air pocket by the air ramp, and the energy losses through the air ramp can be significant. The volume of air drawn through the ramp, the shape of the trajectory, the pressure in the air pocket underneath the nappe, and the energy losses of the air flow through the ramp are functions of each other, so there is no analytical solution to solve for the air flow rate. Alden developed an Excel VBA program to iteratively solve for the air flow rate for a given ramp geometry and spillway flow rate. The air ramp and the ducts supplying air to the ramp were designed by Alden to ensure that the high velocity air through the ramps would not cause sonic shocks and so that the unit flow rate of air on the spillway was approximately 10% of the unit flow rate of water, which has been shown to reduce the potential for cavitation (Falvey, 1990).
The air ramps installed at Cabinet Gorge and Boundary Dams successfully supplied air, and cavitation damage has not been observed since their installation.
Figure 3. Roughness Elements and Air Supply Ramp at Cabinet Gorge Dam (Dunlop, et al., 2016)
Figure 4. Conceptualization of Air Ramp (Based on Falvey, 1990)
No mention of what may be the 600 lb gorilla in the hydropower/fisheries mitigation room: the conversion what may well be 100 km of waterway immediately upstream of the dam from it's previous "lotic", i.e., flowing or riverine hydrology, to a radically different "lentic", i.e., essentially lacustrine or lake-like hydroecology. So even if upstream migrant somehow make it through the fish ladders, once past them they may well have to cope with a limnological situation for which they have no evolutionary or adaptive history. In most cases, say by comparison to the five or six salmonids and maybe a few non-salmonids of commercial aesthetic, and cultural significance that need to be mitigated in Northern/central California, Pacific NW, Alaska, and BC— I mostly work on the Mekong, and there are seldom fewer than 100 spp. from a dozen different taxonomic families, either resident or migrant, at any particular site on the mainstream. Of which very little is known of their detailed ecology.