The Problems with Top-Hinged Traditional Flap Gates The graphics and text below are from the Golden Harvest web page describing their model GH-39 flap gate1. The text in green italics - which is intermixed with the excerpts from the Golden Harvest web page - are my comments regarding the information presented on this particular web site. Note that these comments apply to virtually all top-hinged flap gates produced by any manufacturer.
Backflow Prevention A trivial amount of backflow will actually be beneficial at many flap gate locations. With a Variable Backflow Flap Gate (VBFG) by Juel Tide Gates, backflow can be completely prevented when necessary, or some modest amount of backflow can be allowed at times. Backflow prevention occurs as long the flap gate does not catch debris. If sticks or other debris are present in the area that drains to the flap gate2, the partially cracked-open flap gate will not always pass the debris. If a piece of debris hangs up after passing part way through the opening at the flap gate, it will prevent the flap gate from closing completely when there is seating head acting on the flap gate. The amount of backflow resulting from trapped debris causing a leakage past a flap gate may be trivial ‌ or it could be catastrophic. The total volume of water that passes depends on the 1 2
http://www.goldenharvestinc.com/products/flap_gates/index.php It would be a very unusual location that did not have a large quantity of sticks and debris in the drainage basin.
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size of the culvert, the area of the opening at the flap gate, the driving head, and the duration of the high water event. Debris caught at a flap gate could potentially damage the flap gate. The racking forces on a fouled flap gate can be extreme – especially when the debris hangs up close to the hinge axis and a large seating head develops. Articulated hinges (arms with two pivot points) solve this problem. Unfortunately not all flap gates use articulated hinges. Note that the GH-39 does not use articulated hinges. Since the VBFG opens wide under a very minimal flow, debris simply cannot hang up in the flap gate. When the VBFG is configured to allow backflow, the tide gate opens wide even in standing water. If debris somehow hangs up on the VBFG during backflow, the VBFG will open wide on the following ebb tide and the debris will be released and swept downstream.3 Problem solved. Trash Racks Trash racks are commonly used to catch debris upstream from flap gates and prevent the debris from interfering with the flap gate. Trash racks are not 100% effective at capturing debris. They require frequent cleaning and the head loss at a partially-blocked trash rack could result in flooding.
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To my knowledge, this has never happened at any of my VBFG’s.
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The photo above shows the less than two year-old trash rack on the “Big Ditch” at the $7.7 million Fisher Slough project - “Creating New Wetlands for Salmon“. This Recovery Act-funded project is located in Skagit County in Washington State.4 The trash rack in the photo is upstream from the siphon and two normally-open manual sluice gates. There are traditional top-hinged flap gates on the headwall at the downstream ends of the siphon pipes. The photo was taken less than a week after the trash rack was cleaned / raked. (Note the debris piled atop the approach walls to the left and right of the trash rack.) The O&M burden associated with keeping this trash rack clear of debris is considerable.
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The Nature Conservancy was the client for this project. The engineering design work was initiated by Tetra Tech Inc. and completed by Shannon & Wilson Inc. Mr. David Cline, PE was the project manager for Tetra Tech - and later for Shannon & Wilson. I can only imagine the awards and accolades that D. Cline will receive for this important high-profile political wetland restoration project. (If D. Cline has a conscience or any ethical standards, he should decline any award that is offered for this abomination.)
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The layer of floating black muck5 which extends well upstream from the trash rack is disconcerting. The biological oxygen demand (BOD) associated with this rotting material - along with the fact that it functions as a barrier to oxygen transfer from the air into the water suggests that the dissolved oxygen (DO) of the water beneath this layer is very low.6 It is possible that the trash rack in the photo on the preceding page will never be 100% blocked with debris for the full depth.7 If the trash rack is allowed to become substantially or fully clogged with debris when a high runoff event occurs, there are only two possibilities: 1. Water backs up at the trash rack and flooding occurs. 2. The plugged trash rack collapses under the growing differential head. a. The wreckage (mangled galvanized steel) from the imploded trash rack then damages the manual sluice gates and prevents them from being operated. b. With luck, the flap gates will not be damaged in the process. c. Note that flooding may occur before the trash rack experiences a differential head sufficient to cause it to fail catastrophically.8 d. Flooding is inevitable Had my side-hinged VBFG’s been used on the headwall downstream from this siphon, a trash rack would not be necessary.9Problem solved.
By “Operates Automatically” they mean that the GH-39 cracks open “with small differential head pressure”. This precludes fish passage unless there is sufficient differential head pressure for the gate to open wide enough for the fish to squeeze through. If the flow is sufficient for the GH-39 to open wide enough for fish passage, the flow velocity may exceed the fish’s burst speed. Even if the flow is less that the fish’s burst speed, the fish simply may not be willing to attempt to squeeze through the opening to get past the flap gate.
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The consistency of the film is like filthy heavy crude oil and the thickness is on the order of an inch or more. The photo does not do this film justice. The smell is also… interesting. 6 My money says: “ZERO” - otherwise known as “anoxic”. 7 I’m sure that the diking district won’t mind paying a diver to check that the lower portion of this trash rack is not clogged periodically – say every week. What would you have to pay someone to go swimming in that mess? 8 The suspense is killing me!!! Tick, tick, tick, tick ... 9 Amateurs, biologists, and even the average otherwise seemingly competent engineer should not attempt to do this sort of design work.
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If sharp edges are present at the flap gate opening, potentially lethal descaling injuries to the passing fish are possible. My side-hinged VBFG opens very wide with a small differential head pressure. If the VBFG is set to allow backflow, it actually opens wide in standing water. Passage is possible for the even the most sloth-like species of fish, amphibian, insect, waterfowl and mammal.10 Problem Solved.
Flood control and salinity control require that the flap gate (and the trash rack) not be fouled with debris. If the flap gate never allows any backflow, “salinity control” effectively amounts to salinity elimination since the water upstream from the flap gate will be fresh water with no salinity. With the VBFG, the salinity of the water upstream from the flap gate can actually be controlled. The salinity can be increased or decreased by simply increasing or decreasing the amount of backflow that is allowed to pass through the VBFG during the flood tides. Note that water having some amount of salinity is desirable for many important organisms that utilize the water upstream from flap gates in estuaries. Some types of invasive non-native plants – including Reed Canary Grass - die when exposed to brackish water. Reed Canary Grass is virtually impossible to eradicate using other weed-control methods. With a VBFG and backflow of brackish water – problem solved. With a VBFG and backflow of brackish water, the salt-tolerant flora that will ultimately replace the non-native invasive plants is spectacular.
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I have watched baby ducklings playfully pass back and forth through the open VBFG at Edison Slough. Imagine the fate that awaits a duckling that attempts to pass through a top-hinged flap gate under any flow condition. I once saw a river otter transit the culvert, the vault containing a VBFG, and the outfall pipe at Port Stanley Had he attempted this at the wrong time with a heavy top-hinged flap gate, he would likely get caught and drown!
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The photo above was taken upstream from the VBFG11 at Edison Slough in June of 2012.Three years of tidal flushing resulted in an incredible transformation of the vegetation along the Slough.
Tide gates are a remarkably challenging realm within the field of engineering.12 A number of critical design issues must be considered and addressed for a successful tide gate / flap gate project/design: •
Hydrology &Hydraulics (including the possibility of water hammer) with non-steady state variable tidal water levels
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I was paid $7,500 by Skagit County to design, fabricate and install the VBFG mechanism on the tide gate at th Edison Slough. This amounts to 1/1,000 the cost of the $7.7 million Fisher Slough debacle. 12 A typical run-of-the-mill engineer will face insurmountable challenges when doing this type of design work. (Solution: Hire an engineer who really knows what they are doing and has a passion for this line of work.)
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• • • • • • • •
Extreme corrosion (wet/dry cycles with salt water or brackish water in some locations) Potentially catastrophic scour downstream from the flap gate during high flow conditions Foundations on thick layers of poorly-consolidated fine-grained soils (differential settlement) Groundwater hydrology and salt water intrusion concerns Sedimentation, shoaling, and channel geomorphology effecting the ability of the downstream ebb channel to convey drainage efficiency Constructability Operation and maintenance requirements Biological impacts (marine growth, vegetation, effects on wetland animals)
Sedimentation / Shoaling The potential for sediment depositing and burying the flap gate is particularly important. This must be considered at each project. It should be obvious that digging out a shoaled-in flap gate on a regular basis would be a major inconvenience. The digging out a shoaled-in flap gate must be scheduled to take advantage of low tides or low river levels. Equipment will have to be brought on site and the vegetation at the access point will be severely disturbed. The excavated material should be hauled and disposed of off-site. Handdigging in the soft muck may be possible in some instances. If a large amount of material must be removed, hand-digging is impractical. In many jurisdictions, acquiring a permit to do this sort of work will be challenging - if not impossible. In contrast to the situation with a top-hinged flap gate, the daily ebbing and flooding flows through a VBFG make shoaling-in of a VBFG impossible. Problem Solved.
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The flap gate shown in the photo below is at Edison Slough in the town of Edison, located a few miles north of Burlington Washington. This flap gate is situated in a stagnation zone of the flow field where sediment gradually accumulated. Not long after the installation was completed, the culvert and flap gate became half-buried in sediment.
The shoaled-in flap gate in the photo above is the very flap gate that Golden Harvest used for their web page promoting the GH39 (see the photo to the right).13 The photo to the right was taken immediately after the flap gate was installed. It is safe to assume that “Flexible Design Customized to Meet Most Project Applications” did not meet this particular project application. The “flexible designer” should have realized that the pile of mud in the corner behind the flap gate was a subtle hint that sedimentation was going to be a problem here. If the shoaled-in flap gate was not enough, the culvert and tide gate to the right of the GH-39 in these photos is Golden Harvest’s first attempt at a copy of my Aberdeen tide gate design. They refer to this as their knock-off as the “GH-850”.
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It’s a bit ironic given their name, but Golden Harvest has the opposite of the Midas Touch.
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After the GH-850 at Edison Slough closed on the first flood tide, it never opened again. “It just sat there, stuck shut”.14 In January of 2009, I retrofitted the GH-850 with my VBFG control mechanism.15 It has been working flawlessly for over three years (as of June 2012) and it has not shoaled-in at all. It’s not as if Edison Slough was half way around the world from Golden Harvest’s Headquarters (located in Burlington Washington). It would have been a minor inconvenience for their “flexible designer” to go to the site and assure that the GH39 that they provided met “project applications”.
Note that Edison Slough is only 6.4 miles away from Golden Harvest’s main office as shown in the image above:16
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See my YouTube video about the GH-850 and other failed tide gates at the Edison Slough project http://www.youtube.com/watch?v=b7obU2-Wgv0 15 The cabinet with Golden Harvest’s nameplate on it containing thousands of dollars’ worth of hydraulic controls is entirely superfluous. 16 Image compliments of Google Earth.
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With a VBFG by Juel Tide Gates, actual engineering is applied by an award-winning, intelligent engineer who has more than 25 years of engineering experience and an unmatched knowledge and passion for environmentally-sound tide gates.
Golden Harvest, Inc. supplies Flap Gates in stainless steel, aluminum, and spun aluminum and supplied with neoprene seals. Flexible design available in square, rectangular and circular shapes. If you order a GH-39 from Golden Harvest, be sure to specify the appropriate stainless steel alloy. 316 stainless should be specified for salt and brackish water and less-expensive 304 stainless may be used for fresh water. When your GH-39 is eventually replaced with a VBFG, be sure to recycle the metal.
Automatic, circular opening for gravity flow and pump discharge can be aluminum or stainless steel. Stainless steel construction is recommended for pump discharge applications where the flap gate is located such that pressures are violent at the discharge pipe. Pressures are what they are. What makes a pressure “violent”? If by “violent” they are referring to “water hammer”, a vent in the roof of the culvert located behind the flap gate will significantly reduce extreme negative pressure associated with a water hammer event.
Flap gates have a built-in taper. With a small differential pressure on the back of the gate, it opens automatically to allow discharge. When the head is greater on the face side of the gate, the flap closes automatically to prevent back flow. The above paragraph is meaningless. They are attempting to describe the concept of “unseating cracking pressure” – or the differential pressure required for the closed flap gate to crack open. The third sentence is totally wrong. All flap gates are closed when there is no differential head acting on the flap gate. They do not require the head to be greater on the face side of the gate for the flap gate to close automatically.
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By a “built-in taperâ€? I presume that they mean that the geometry of the flap gate and the seating surface result in the flap gate having a bias towards being closed. If the bias is very small, under a very slight differential pressure, the flap gate just barely cracks open. This is a problem with all top-hinged flap gates. Because of the bias, the head on the face side of the flap gate will actually be equal to or slightly less than the head on back side of the gate when the flap gate automatically closes. Determining if a flap gate will crack open and allow flow under a given scenario is surprisingly complicated. The analysis involves the geometry of the flap gate and the hinge, the density of the gate material, the density of the water17 upstream and downstream of the flap gate, and the water levels upstream and downstream of the gate. Below is a figure I used in a spreadsheet for determining if a round flap gate will crack open under any scenario of upstream and downstream water levels. Moments, M (where M = ÎŁ dM ; and dM = dF x LTH) are compiled for the hydrostatic pressure acting inside(upstream) and outside (downstream) of the gate.
If the net moment due to the hydrostatic pressures is sufficient to overcome the moment associated with the weight of the flap gate times the horizontal distance from the hinge axis to the center of gravity of the flap gate, the flap gate will just barely crack open.
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The density of fresh water is 62.5 lb/ft , the density of 100% salt water is 65 lb/ft , and the density of brackish water obviously falls somewhere in between.
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The analysis described above is academic. A heavy top-hinged flap gate may crack open under a small differential pressure, but it will not open very wide.18 A side-hinged VBFG opens wide with an infinitesimally small differential pressure. Problem Solved.
Flap gates act as natural skimmers of debris such as branches, logs and trash. It is recommended that frequent inspection and cleaning be scheduled to keep the flap gate working effectively. Installation should incorporate a vertical elevation drop below the flap gate of 300 to 600mm, to allow debris to work its way free of the gate. Protection of a flap gate should be considered when the installation projects into a stream flow where heavy debris or ice may be present. This sounds relatively innocuous; however frequent inspections and cleaning will be a major burden on the entity responsible for the operation of the flap gates. The consequence of a flap gate failure can be catastrophic - so this cannot be ignored. A “vertical drop of 300 to 600mm” will only solve the debris problem if, prior to installation, the GH-39 is physically dropped from this height onto a very hard surface – assuming that this drop is sufficient to irreparably damage the GH-39. A greater vertical height and/or multiple vertical drops would be prudent. Be responsible and recycle the scrap metal, then procure a VBFG from Juel Tide Gates. Heavy debris or ice under most conditions will not damage a VBFG. The VBFG is very heavy-duty and since it opens wide under minimal flow, fairly large debris and ice will pass through the VBFG easily. If the material in question manages to fit into the upstream end of the culvert, it will easily pass through the VBFG on the other end of the culvert.
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If you purchase a GH-39 from Golden Harvest, I suggest that you request that their flexible design include this analysis.
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Special variations include side-hinged flap, top hinged flap with buoyancy chambers and secondary buoyant door designs. The “secondary buoyant door designs” are actually top and bottom-hinged “pet-doors”. These designs are particularly susceptible to damage due to racking forces caused by snagged floating debris combined with a large seating head acting on the “pet door”. Note that the “pet door” design has been tried and has failed at a number of locations. With the VBFG, being side-hinged is not a “special variation”. Hinging the flap gate on the side rather than at the top solves a number of problems associated with traditional flap gates.
Mounting Options: Surface / 125# Flange / Spigot or Collar / Thimble
A better option for the GH-39 is a vertical drop of at least 600mm - into a dumpster.
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A Variable Backflow Flap Gate
Please see my website at www.jueltide.com and see my YouTube Channel: JuelTide Contact me with any questions at jeff.juel@jueltide.com or by phone at: (206) 765-6936
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The following page is the web page in its entirety as published on Golden Harvest’s web site on June 22 2012.
The use of Golden Harvest’s copyrighted Information is permitted under the “fair use” doctirne. The "fair use" doctrine is a defense to copyright infringement in the United States that allows the reproduction of a work for certain limited purposes such as criticism, commentary, parody, news reporting, research and teaching.
It is my sincere hope that the reader understands my criticism, appreciates my commentary, and laughs at my parody.
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