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Miller, Frank Ltr Bee Branch Pr deffery A. Wickenkamp, P.E. Senior Project Manager Camp Dresser & McKee Inc. 125 South Wacker Drive, Suite 600 Chicago, IL 60606 Dear Mr. Wickerkamp, 602 E. 22nd St. Dubuque, IA 52~1 ~ 11-18-03 ~_.~. o~ Following the Sept. 25, 2003 meeting of the Dubuque BBCAC, I visited with you briefly about an engineering alternative for the storm water/storm sewer problems being experienced periodically here in Dubuque. For some time I have been working with Dr. Jos. Schaefer, P. E., formerly of Loras College's Department of Physics & Engineering, & now an Engineering Professor at Iowa State University, Ames, Iowa. Initially Dr. Schaefer did not have specific figures on such items as storm sewer dimensions, so I have updated his preliminary calculations to more closely reflect these figures as now known from information you provided at the Sept. 25, 2003 meeting of the Bee Branch Citizen Advisory Committee. Because I will be sharing this communication with members of the BBCAC, & others, I will be including some of my initial comments to Dr. Schaefer (as quotes from communications to him) so that you may understand the background of my proposal(s) on this matter. From my communication to Dr. Schaefer on 3-18-03: "The solution to the periodic flooding problem, proposed by the city's engineering consulting firm, is a large open ditch some 6 or more blocks in length, costing between 20 & 25 million dollars, to carry the excess runoff which the existing storm sewers cannot handle. It would displace a considerable number of home owners and, having experienced a similar runoff ditch system in Japan, I know it would initiate a separate set of problems of its own. Even if the displaced people got fair market value for their homes & property, it would be very difficult for them to locate replacement housing here in Dubuque because their homes are generally older & quite modest by today's standards. What follows is my alternate proposal, which I have made to two members of the city's Engineering Division: Deron Muehring, Civil Engineer, & Gus Psihoyos, PE, Assistant City Engineer. They indicated that they would take my idea to their engineering consulting firm but after several weeks 1 have heard nothing. If you are willing, I would ask your informal opinion of the physics of the plan, particularly from your background in hydraulic engineering. There may be some fundamental reason that it will not function, either theoretically or practically but as yet no one has raised such a concern. I know that it has never been done before - (at least these two engineers have no knowledge of such a system) - and would probably need some testing. First, the philosophy behind my proposal. The city engineers state "we have the flooding problem because the existing storm sewer essentially gets 'full' when we have very heavy rain in a short time span". The storm sewer empties into the retention basin (and ultimately the river) by means of gravity. Because it is not practical to change the gradient of the storm sewer to increase the rate of water transport, the conventional thinking is that we need something bigger, i.e. the large open ditch to carry the water. It seems to me that this is an example of a sort of one dimensional thinking. After all, "bigger is better" is often a legitimate solution to many problems but if that's the only avenue of thought, perhaps some other solution is not even considered, causing, in this case, considerable expense, social upheaval, & the perpetuation of what caused the runoff problem in the first place. Page 2 Proposal #1. My contention is that there is another way to transport the runoff. Certainly one can go 'bigger' but has anyone considered causing the water in the existinq storm sewer to flow faster? And I'm not talking about changing the gravitational gradient of the existing storm sewer to achieve this. This idea came to me while considering a plumbers' method (which was used in my basement) to quickly remove a pool of standing water which had formed when we had to dig up a branch sewer line because it had rusted through (over the past century) & plugged. The trench which resulted from us breaking up the concrete floor & removing the soil underneath to gain access to the sewer line soon filled with water because the line was blocked further down stream from where we dug. However, when we cleared the blockage the water in the trench slowly began to recede as it flowed, through gravity, into the unblocked main sewer line. At this point the plumber took a hose with a nozzle on the end, stuck it into the defective line directing the flow towards the main sewer line & the trench emptied in a matter of a few tens of seconds, far quicker than it had been, even through he was actually adding water to what was already there. I knew immediately that what has causing the increased flow rate was the use of Bernoulli's Principle. The increased speed of the water exiting the nozzle caused a reduced pressure because of the higher velocity & subsequently the effect of atmospheric pressure caused the water to flow at a much higher speed. Such Bernoulli devices are often employed to create vacuums in devices designed to 'pick up' water from floors, etc. Joe, I don't need to explain the Bernoulli Equation to you but I will write it here in case you have someone else look at this. P + 1/2j~ v2 +p g h = a constant. P is pressure, j~ is mass density, v is velocity, g is the acceleration due to gravity, & h is the height. The three terms in the equation are related to the pressure, kinetic energy & gravitational potential energy, respectively. Since changes in the gravitational potential energy within the storm sewer are relatively small, they can be ignored. Therefore, if one can increase the velocity there is a reduction in the pressure,. The application of Bernoulli's Principle is what enables us to explain why air planes fly, curve balls curve, & the behavior of tsunamis (tidal waves), aspirators, etc. So, what sort of mechanism am I proposing for the storm sewer. First, I propose using the existinq storm sewer, but converting it into a two-stage operation. The existing main storm sewer, which causes the problems, is large enough so that a small end-loader (Bobcat) can enter it for cleaning & debris removal purposes. (The channel is approximately a rectangle with an arched top, some 10.5 feet in height and 16 ft in width. Figures corrected from original estimate.) The idea of a two- stage system is to place, against one wall to still allow space for cleaning equipment, a smaller diameter storm sewer pipe, e.g. say 9 feet in diameter, which would have strategically placed openings for water entry along its length. (These 'intakes' should probably be near the bottom of this pipe to reduce the intake of floating debris.) See diagram on separate page. Near the retention basin end of this pipe would be a large version of the hose nozzle described in the application above powered by a high velocity pump. Perhaps it only needs to be something like the pumps used by fire engines, or perhaps it needs to be something like the pump/nozzle arrangements used in placer mining operations. This pump/nozzle arrangement would be used only under the following conditions. When the rain is 'normal' & the storm sewer does not fill beyond the top of the 2nd interior pipe, the sewer operates just as it does now. However, with heavy short term rain the storm sewer fills beyond the top of the interior pipe. Then sensors would trigger the pump/nozzle arrangement, causing a higher flow rate in the inner pipe, thus lowering the pressure, allowing atmospheric pressure along the length of the entire system to move the water faster than is done by the gravitational gradient. Once the water level fell below the top of the interior pipe, the pump/nozzle arrangement can be automatically shut down. The source of the water for this pump/nozzle system could be the storm sewer system itself, city water mains, or as one of the city engineers suggested, it could come from the 32nd St. detention basin where they need to get rid of some water anyhow. Page 3 These are all details that could be worked out. I also believe the cost of converting the existing system to atwo stage system would be far cheaper than the proposed 20 to 25 million dollar price estimate for the open trench. However, I have no expertise in estimating such costs accurately. To summarize, the proposed two-stage storm sewer has the following advantages: It is based upon well known scientific principles; It utilizes existing facilities; The storm sewer would continue to operate as a gravitational drainage system as it does at present; The second stage would go into operation only when needed; The social disruption would be minimized; Presumably the costs would be significantly less than the proposed open ditch solution; Safety concerns of an open ditch of rapidly running water would be eliminated." End of my communication to Dr. Schaefer on 3-18-03 In an initial e-mail reply, on or about March 20, Dr. Schaefer commented as follows: "1 agree with your comments about the additional problems one could expect to occur with an open ditch, & I share your conclusion about observations made of such systems in Japan. There might be a better way to proceed, & at least the city should try to explore alternatives. Your comments about the socio-economic consequences for the current residents who would be displaced are very appropriate. I wonder where those who would be displaced could find comparable housing at affordable prices. Open ditches lead to stagnant water, no matter how well designed, & it is easy to think of dozens of problems associated with water standing in open ditches, e.g. mosquitoes, disease, & safety of children. in principle your idea works very well, and we know that it works on a small scale with both air & water .... A factor that you have not included in your analysis is the friction of the walls. Such losses are not insignificant .... So the upshot is that I know the system you describe works, but the question is whether it would be practical in terms of the size & cost of the equipment that it would require & in terms of the loss of flow rate due to friction. This could be a fertile new design area for retrofitting systems such as Dubuque where similar run off problems have developed .... If I get some time... I will make a quick calculation to estimate the frictional losses for your geometry." Dr. Schaefer's detailed numerical analysis appears below. This calculation addresses the problem of frictional effects caused by the secondary pipe. From Dr. Schaefer's March 22, 2003 communication (with updated numeric values & appropriate supporting editorial changes): This is a calculation comparing the flow in a channd with a secondary pipe in it to the flow with no such pipe present. For simplicity I treated it as open channel flow, even though the storm sewer is enclosed. The situation shown in the diagrams would approximate open charmel flow if the storm sewer is not completely full, e.g. the water level was within, say, an inch of the top so that the water did not make contact with the top. The same assumption is made for the pipe, i.e. there is a small area at the top of the pipe that does not have contact with the water. Obviously, that is not the case because for the given conditions the secondary pipe would, indeed be full. A more complete calculation could be made for closed channel flow, but I do not want to go through all of that today. Page 4 I assume that the original storm sewer and the Secondary pipe are made of the same material so that they have the same frictional effect. Perhaps the original storm sewer is rough concrete or even brick so that in reality it would have a different friction coefficient than new, smooth concrete that would be presumably used for the secondary pipe, but my intent is not to get an exact answer. More about that later. Using the dimensions from the diagram, assuming the depth of the charred to be 10.$ ft., the width to be 16 ft., the diameter of the secondary pipe to be 9 ft., and assuming the channel to be rectangular in shape (the July 6, 1899 diagrams from city records show the shape of a good portion of the Bee Branch line to approximate a rectangle with an arched top), one can calculate certain mathematical properties of the storm sewer charmel & secondary pipe which will be needed in further calculations, e.g. areas, perimeters, etc. Because these calculations do not need to be extremely precise, small variations in dimensions, certain estimates & assumptions, etc., do not significantly effect the final result which may be off by a few percent due to these variations. If you wish to skip the mathematical portion of what follows, proceed to the results at the asterisk, *, on page 6. With the assumptions regarding open channel flow, the flowrate is given by the Mamning Equation, which is: Q= (k/n) A (Rh)2/3 So1/2 where OAs the flowrate in either m3/s or ft3/s, k is a conversion factor that is equal to 1.00 for S Iunits and is equal to 1.49 for British units (Manning was a Civil Engineer, sohe did not express the equation in dimensionless parameters the way most such equations in fluid mechanics are expressed). I will compare the two cases, so if both calculations are conducted in the same units, the factor k will cancel out anyway. n isthe Manning coefficient that represents the friction effect of the fluid on a particular surface. For finished concrete n = 0.012, for unfinished concrete n = 0.013, and for brickwork n = 0.015 so the assumption that the surfaces have the same Manning coefficient is justified. If you prefer you could make a calculation with one value for the original channel and another for the pipe, but as you will see that difference will produce a small effect compared to the change in the wetted perimeter. A is the cross sectional area of the channel, in either square feet or square meters, Rh is the hydraulic radius of the channel. It is calculated by dividing the cross sectional area of the channel by the wetted perimeter of the channel carrying the fluid. The hydratd~c radius has the dimension of length. You have to be careful to not confuse the hydraulic radius used in the Manning equation with the hydraulic diameter used in calculations for the closed pipe. It does not necessarily follow that hydraulic radius is one half the hydraulic diameter. So is the slope of the channel, which I will assume is the same for both cases. Page 5 Case 1 The original storm sewer in the present condition Area: A = (10.5 ft) (16 ft) = 168 ft2 (Due to the irregular & changing shape of the Bee Branch channel, I believe an area of 140 ft2 is a more representative value of the actual average area, and this value, i.e. A = 140 ft2, will be used in the calculations which follow.) Wetted Perimeter, "P": Under the assumptions listed above water is in contact with both sides of the channel a_nd with the bottom of the channel so that P = 10.5 ft + 10.5 ft+ 16 ft = 37 ft Hydraulic Radius: Rh= A / P = 140 ft2 / 37 ft= 3.78 ft Thus we have the flowrate for the original condition, Qo as: Cb = (k/n) (A) (Rh)2/3 So1/2 = (k/n) (3140) (6.3.78)2/3 So1/2 = 340 (k/n) So1/2 Case 2. Pipe with circular cross section ofdiameter 9 ft placed inside the original storln sewer. Area: I assume that there, s no reduction in the area for flow produced by the insertion of the pipe, i.e. the wall thickness for the pipe is negligible. This is an assumption in your favor, the effect of which can be calculated in a more precise mathematical analysis. Wetted Perimeter, "P": The water touches one 10.5 ft wall of the original channel. The pipe makes contact with the other 10.5 ft wall and with the 16 ft base of the original channel. In an ideal world the pipe would be tangent to those surfaces with an infinites~ region of contact, but in reality there will be a greater length of the original channel not in contact with the water. While this gives a wetted perimeter of 37 ft, for this calculation I reduced the wetted perimeter ofthe original channel to 35 ft, again an assumption probably still in your favor. The pipe is wetted on both sides (inside & outside) so its wetted perimeter is 2 (,r) (D) = 2 (,r) (9 ft) = 56.5 ft. The portion of the outside wall of the pipe touching the two wails will not be wetted, but I believe that the 2 ft reduction in P for the channel covers that. It also covers the fact that for open channel flow the pipe will not be completely full. This gives a total wetted perimeter of P = 35 + 56.5 = 91.5 ft. Compare this to the 37 ft calculated for the original condition. You could reduce this number if you believe that the assumptions regarding contact between the pipe and the wails is not generous enough. Hydraulic Radius: Rh = A / P = 140 ft2 / 91.5ft = 1.53 ft Note the significant reduction in the hydraulic radius because the water is in contact with more surface. The real culprit here is the fact that the secondary pipe makes contact with the water on both sides, inside & out. Page 6 The flowrate for the condition with the secondary pipe, Qp is: Qp = (k/n) (A) (Rh)2/35ol/2 = (k/n)(36140) (2.1.53)2/3 So1/2 = 186(k/n) So1/2 When we calculate the ratio ofthe two flowrates with the assumption that both materials have the same Manning coefficient and both channels have the same slope, the (k/n) So1/2 te~ms cancel out and the ratio of the flowrates is: Qp / Qo = 186 / 340 = 0.547 *This means the flowrate with the secondary pipe in place is only 54.7% of the original flowrate because of the greater loss to friction. I suspect that you will have difficulty accepting that the frictional effect is so great, because I know that I had some difficulty in accepting it when I D_rst encountered it in teaching Fluid Mechanics from an engineering approach. You see, we both are accustomed to a Physics approach that assumes ideal, inviscid fluids. Stating this in another way, the reduction in flow that occurs because of the added friction due to inserting the pipe in the original storm sewer is 100% - 54.7% = 45.3%. That means that your system using the Bernoulli effect would have to bring about at least a 45.3% increase in flowrate inside the secondary pipe to break even without considering the cost of purchasing, installing, and operating the system. One might think that substitution a smooth steel pipe would help reduce the losses, but it turns out that the Manning coefficient for smooth steel is the same 0.012 as for finished concrete. Interestingly enough, for painted steel n = 0.014, although there may be some special epoxy paint that is slicker and might give a smaller value. I believe that all of the assumptions that I made in these calculations favor your system in any roundoffs, the neglecting of contact length, etc. This calculation leads me to believe that it would be difficult to implement your suggestion in a practical way because of all the losses incurred. End of Dr. Schaefer's communication of March 22, 2003 From Dr. Schaefer's analysis above, one would need to increase the flow rate in the secondary pipe at least a factor of 2 to have any meaningful gain with the two-pipe system. While I believe it is possible to achieve that type of gain on a small scale, I am not qualified to judge whether or not it can be achieved on the larger scale needed. Now having gone through ali of this it struck me that a simpler modification of this idea might be much more promising. Page 7 Proposal #2. books make the point that pipes are not really necessary for the Bernoulli effect to work. Therefore, what would happen if one discarded the idea of the second pipe & simply put the water injection nozzles near the end of the Bee Branch channel near 16th Street? This works on the smaller scale plumbing example mentioned above. Costs would be far cheaper, ff one could get a flow rate twice as fast due to the Bernoulli effect, it would be like having two identical Bee Branch chmmels for far less than building a large open ditch. ~ From another communication from Prof. Schaefer: "Perhaps you could convince the city staff to have the Iowa Institute of Hydraulic Research (rlHR) conduct some experiments with models to test the idea(s). The costs of such a study would depend upon how much of a complete analysis would be required, but I would think that $10,000 to $20,000 would finance an investigation that would give some reasonable answers. If the city wants to pursue that (i.e. this secondary pipe idea, Proposal #1), or if you simply want to ask some questions, I suggest contacting Rob Ettema. He is Chairman of Civil Engineering at the State University of Iowa, Iowa City. (Dr. Schaefer worked with him for 7 summers at l/HR. He can be reached at: 319-384-0596 or e-mail at robert- ettema@uiowa.edu" hoankyou for your time in examining these proposals. It is personal belief that posal #2 should be considered first, my Sincerely, Frank Miller, Member of BBCAC