Fatal Currents – Low Head Dam Presentation

this is a video presentation of a rolling current in the Jordan River as presented at the 2012 Salt Lake watershed symposium presented by dr Roland Hotchkis and Edward Kern from Brigham Young University first we’re going to take a look at a specific incident that occurred on the Jordan River then we’re going to take a step back and look at a few basic principles of fluid mechanics namely energy dissipation and hydraulic jumps then we’re going to talk about the types of hydraulic jumps at overflow structures after that we will talk about the solutions to limit danger at overflow structures now let’s look at what happened on the Jordan River as reported by a local news agency and I know it looks small but it’s a pretty deadly little thing two people are dead after kayaking over a deceptive looking waterfall in the Jordan River the couple from Sandy and their friend were simply trying to enjoy a day on the river they put their kayaks in near 114 south and the Jordan River Parkway they ran into trouble when each went over a small waterfall near Murray it was this waterfall that proved to be deadly for Joseph Glasser and his wife Kelly their kayaks flipped and got sucked into the water by the undertow Kevin witness saw the glasses tried to make it over the waterfall I ran down and grabbed a branch to see if I get a branch out to him but they were right in the middle he saw the couple’s friend a third kayaker come through and make it across safely the two got 49 the blesser out of the water we drag him up on the bank and started CPR on him about then very fire department got on sight swift water rescue teams use the ropes to pull 51 year-old kelly Glasser out of the fast-moving water they then began CPR when they were found here neither one of more breathing or had a pulse Kelly Glazer and a husband were rushed to Intermountain Medical Center both for pronounced dead wizna says despite having floatation devices on the to simply underestimated the power of what looked to be a small simple waterfall the people don’t realize what a small little water falls maybe a foot and a half and tops but it just it has an undercurrent that is just deadly now there are signs posted in the area warning voters to get out and walk around the rapids because the current is too Swift now let’s talk about what happened on that tragic day as water goes over a drop structure like shown in this figure it picks up momentum when that water reaches the surface of the water at the base of the structure it entrains a lot of air which it then pulls to the bottom of the channel now the water has a lot of momentum so it carries this air downstream along the bottom of the channel when the air finally begins to rise it rises at what we call the boil point anything upstream of the boil point gets pulled back towards the face of the drop structure anything downstream goes on its merry way down the river the next question is is this an isolated incident or has this occurred elsewhere the following video is a body recovery attempt that happened in Binghamton New York Binghamton New York 1975 a television news team videotaped this attempt to recover the body of a fireman drowned during a rescue today before the boat is approaching the boil that the fire chief of the boat in Binghampton New York that day there was at least one survivor however not everyone in that boat survived at BYU a student named John diamond began a database to keep track of fatal incidents that have occurred across the United States he has recorded 65 locations across the country where fatalities have occurred and a total of over 300 deaths are recorded this database is woefully incomplete we know that there is at least one location in every state where a fatality has occurred based on the literature that we’ve read we also know being that I am a native of Southern California that there are many drainage canals in Southern California where this has been a problem now let’s talk about why low-head dams are built in the first place usually it is because we need a guaranteed water surface elevation if we have a field we want to irrigate we don’t want to have to pump the water out because that’s expensive instead we go a little bit upstream in the river we Dam it up so that the elevation of the water is just higher than our field and then we let the water drain to our field like shown here hello head dams and diversions come in all different sizes take for example this one at about 4,500 South on the

Jordan River about three miles away from where that deadly current developed on this diversion you can see that even during low flow like right now you can still divert water out of the river because you have that guaranteed water surface elevation now let’s take a step back and start looking at some basic principles of fluid mechanics this here is the hydraulic jump this is the ultimate cause of the keeper or the roller or that dangerous current so what you have is you have very fast-moving water coming from the right it’s very fast it’s very shallow then you get a very very turbulent region where the water level increases dramatically after that you have a higher what we call tail water elevation so the water downstream is much higher than the water coming in and therefore it is also slower so a question that was posed during the Salt Lake watershed symposium was does the hydraulic jump occur naturally and the answer to that question is yes so let’s look at an example this is my kitchen sink turn on the water and you get very very fast-moving water where the water stream meets the sink then as the water goes up against the side of the sink its impounded and it tries to get back towards where the falling water is but it can’t that is a hydraulic jump that you see here where the water is very very shallow very very fast then you have turbulence and air bubbles then you have the water impounded against the side of the sink now let’s talk a little bit about what we can do to predict hydraulic jumps we are going to talk about the principles required to predict hydraulic jumps but we are going to do it very quickly let’s talk about a few ways that we can describe the flow the first is with something called specific energy it’s essentially the amount of energy that the fluid has according to Bernoulli’s equation the next way we have to describe the flow is the momentum factor the momentum factor takes into account the momentum of the fluid as well as the hydrostatic force of the fluid if we assume steady flow we can find what the specific energy and momentum factor are for different water surface elevations in the graph here the water surface elevation is shown on the horizontal axis an important fact is that momentum is conserved across a hydraulic jump the water going into the hydraulic jump has the same momentum as the water coming out of the hydraulic jump but there is energy loss as water passes through the hydraulic jump there is a lot more energy going into the jump than there is leaving the jump hydraulic jumps can form when there is a sudden change in the slope of a channel or where fast-moving water meets slow-moving water like in my sink when the water couldn’t go anywhere when it hit the edge of the sink now that that is out of the way let’s look at how we make a hydraulic jump in the lab this is water flowing in the flume and the hydraulics lab at BYU to make a jump we create hightail water or we force the water surface on the downstream end of the flume to rise we do this by raising a gate at the end of the float as we continue to raise the gate we fourside Relic jump to migrate up the channel in this case it is more important to look at what is happening on the downstream end of the the fast-moving water would have continued to flow quite quickly if we did not force the water to slow down at the base of the flow now take a moment to look at what’s happening at this hydraulic jump what’s coming in what’s happening at the jump and then what happens afterwards now let’s talk about hydraulic jumps at drop structures there are four cases of hydraulic jumps at drop structures the first case is case a where the falling water has enough momentum to push the hydraulic jump a short distance away from the face of the structure this is called a swept out jump the second is case B it is similar to case a because the momentum of the falling water is able to push the jump away from the face of the structure but this is also where the jump forms right at the base of the structure this is sometimes called the optimum jump because it minimizes the distance the jump is away from the structure but also forms a safe jump that means that the energy from that is easy to contain these two cases form what we call fully developed hydraulic jumps this is where the momentum factor of the falling water is greater than or equal to the momentum factor after the jump in other words when the falling water coming down the structure has enough force to push the tailwater away from the face of the structure we think that calling a case B jump an optimum jump is a bit of a misnomer from a public safety standpoint the reason is that a case B jump is sensitive to tail waterdepth if we lower the tail water the hydraulic jump will move away from the structure and will no longer have a case B jump but a case a jump if we raise the downstream water then it will form a

case C jump a case e jump is no longer a fully developed jump but a submerged jump since the hydraulic jump is very turbulent there is a lot of air entrained in the jump when the jump is forced against the face of the drop structure the air entrained water is forced to the bottom of the channel where it travels downstream the air rises to the surface and forces a large amount of water to follow some of this water that is forced to the surface doubles back towards the face of the drop structure the KC or submerged hydraulic jump is defined by the current doubling back at the surface it is as recirculating current toward the face of the drop structure that can make the submerge hydraulic jump dangerous a case d jump forms when the tailwater is nearly the same as the surface of the water passing over the structure air entrainment is minimal and the water simply starts to move a bit faster as it travels over the structure since there is no upstream surface velocity this jump does not pose a public safety hazard like a case see jump good so now let’s look at the different cases of hydraulic jumps in the lab we put a drop structure in the flume and we’re going to watch as barbies buddy ken goes over this drop structure now let’s watch that from a different angle ken is swimming having a good time he goes over the structure passes through the jump and safely continues his exciting journey now for the case B jump Ken goes over still passes the jump still has a good day notice how little the tail water changes from case B to KC so here’s Ken on a lovely summer day and his day is now ruined and just look at how fast the water on the surface is moving back towards the structure and this shows why this current is just so dangerous now we’re going to increase the tailwater and as you can see the KC jump still forms although in this case the current is weak enough that Ken might be able to swim out of it now for a case D jump here’s Ken passing the structure and able to go home from his vacation here he is again and having a great time now let’s look at these cases outside the lab this is a place called Howard’s hole it’s located at 7800 south on the Jordan River I would call this a case B since the jump is pretty much right at the base of the structure and when the kayaker capsizes he is ejected and still having a blast next is croydon wave this is on the Weber River on the head tag run between hennifer and Taggart you saw this is a case D jump or the drown out jump the standing waves in the downstream surface velocity are what define this type of jump and as you can see when the kayaker can no longer stay in the wave he is ejected out and is continuing to have fun with his buddies as you may have guessed this is the Jordan River near 6570 curve this is the boil point anything upstream of this point goes back towards the drop structure anything downstream continues down the river see if you can spot all the reasons that this current could be dangerous one reason this could be dangerous is because of the speed of the water moving back towards the structure another reason is that there’s so much air in this water if someone were to try to float in this they would not float very well it’s like trying to swim in foam it just doesn’t work so what can be done to decrease the risk associated with these currents first there is alteration of the channel if we can prevent the current from developing in the first place

the location will be inherently less dangerous another option is to train swift water rescue teams public education is another option lastly public access to these locations may be restricted all of these options have the potential to make a location less dangerous the route we selected for our research was alteration of the channel in order to create a scale model of the structure and channel at 6500 South the site was surveyed during low flow using survey grade GPS Lindsey Esplin can be seen here standing chest deep in the water trying to obtain points near the base of the drop the resulting model was studied by John Guymon the water flows from the upper right towards the bottom left the model extends downstream far enough for us to replicate the flow conditions at the base of the hydraulic structure it is a 1 to 23 scale model seen with 1 to 23 scale cars for I reference what John found was that there was something interesting on the channel bottom at the base of the drop there was an apparent scour hole followed by a zone of deposition the deposition zone increased the depth of the tailwater while the scour hole was long enough and deep enough to allow the recirculating current to form this video was taken during low flow two years after the survey was performed and it is still very clear that boulders have been deposited a little ways downstream of the drop and that they impound water near the face of the structure when looking at solutions for retrofitting low-head dams there are a few considerations that must be weighed the reason we will be researching retrofits is for the safe passage of people however other considerations include energy dissipation the whole reason hydraulic jumps form is to dissipate energy if there is not enough energy dissipation then we are likely to see channel alteration by way of scour and deposition we also need to be able to pass large debris such as boulders and trees cost is another important consideration from a public safety standpoint if a retrofit cost too much then the structure is unlikely to get retrofitted until problems with public safety have become evident fish passage may be a consideration in areas with fish species that migrate variants of tail water could be another issue if the tail water is subject to change then the retrofit must be able to operate under a broad range of tail waterdepth without developing a recirculating current lastly ice passage could be an important factor in cold regions now let’s review some of the retrofit options that are available the step spillway has been effective if the step height and length are designed properly under certain circumstances a six step configuration with a one two six slope has been effective so for each foot of the drop it would have to go downstream six feet however in large channels the one to six slope also means that the volume of concrete needed to construct the spillway is very large which makes the step spillway expensive also there are configurations of step spillways that have proven ineffective and have still developed a recirculating current caution must be exercised when designing a step spillway another option is a 1 to 10 slope in Boulder spillway the 1 to 10 slope is steep enough that the entrained air does not get carried along the bottom of the channel which prevents the recirculating current from developing the boulder face helps to dissipate energy as the water travels down the spillway although the 1 to 10 slope in Boulder spillway has been effective it requires even more concrete than the step spillway and is therefore even more expensive in some circumstances right-of-way could also be problematic if the drop is more than just a few feet in cases where the submerged hydraulic jump forms in a scour hole the raised grouted stilling Basin may be an economical option sometimes a structure is configured in such a way that there is no recirculating current when it is first constructed but then scour occurs at the base of the structure and the dangerous current then begins to form in these cases it may be possible to stop the recirculating current by filling in the scour hole there are cases when the scour hole has been filled with large rocks called riprap however there have been numerous cases when this riprap has been transported out of the scour hole it was meant to fill and the dangerous current returned if boulders are used to fill the scour hole they don’t move as easily but then other public safety hazards are introduced such as foot entrapment to prevent the transport of the riprap a layer of grout can be used to keep it in place the race grouted stilling Basin comes

with a few risks if the hydraulic jump occurs beyond the grouted region then scour could become a problem downstream also deposition may occur after the hydraulic jump and cause the tailwater to rise this could cause the dangerous current to return in the correct circumstances this option may be a viable solution it is relatively inexpensive and if designed properly may provide a long-term solution when John Guymon was studying the current at 6500 South he investigated the raised grouted stilling Basin in this image the water is coming from the right side the dark tan region is where the scour hole has been filled he also found it was necessary to remove the deposition that had occurred the borders of the region where the material was removed is outlined in black with the scour hole filled and the deposition removed a case a jump developed John found that if the deposition was removed but the scour hole was left in place that the submerged jumps still formed but was smaller than with the deposition in place if the scour hole was filled and the deposition was left in place a case B jump developed since a case B jump can easily become a submerged jumped with minor tail water changes this solution was not recommended the solution that John recommended was to fill the scour hole and remove the deposition so that the jump would be less sensitive to tail waterdepth our plan for further research is to design a generalized retrofit that is economic easy to scale for various channel geometries and insensitive to tail waterdepth the US Army Corps of Engineers tested a spillway with a flip lip this lip would stop the downward velocity of the falling water and would prevent the air and drain water from traveling to the bottom of the channel this configuration also effectively forces the surface velocity to be directed downstream you may notice that there is still a recirculation zone but since the surface flow is directed downstream the current will pass a person or other floating debris the problem with this configuration is that it is sensitive to tail waterdepth if the tail water were to rise it is possible to form a submerged jump that develops a dangerous current the research we plan to perform over the next few months will be to model a modified flip lip spillway with staggered lip heights we believe this will provide enough downstream momentum to force a downstream surface velocity while also providing energy dissipation and insensitivity to varying tailwater depths since some structures have straight drops some are sloped and some have a curved face called an OG crest our goal is to allow for the flip lips to be installed on both sloped and OG crested spillways on structures that have a sheer face a sloped spillway with flipped lips could still provide an economic solution since the slope of the spillway could be relatively steep it is now time to conclude the presentation thank you for watching if you’d like to follow our research please visit KRC project group CT wiU net you will be able to see our progress and results as well as explore our interactive database of fatal locations across the United States you