EDGEWOOD CORE

Citizens for an Open and Responsible Edgewood

This letter is shared with permission of the author, Tom O. McGill. Mr. McGill has lived in Edgewood since 1978. As a concerned citizen with extensive experience in engineering, electronics, and high-level technology, Mr. McGill is uniquely qualified to analyze complex matters. He has worked at National laboratories, high-tech firms, and taught in academia.

Mr. McGill sent this to all Edgewood Town Councilors and town senior staff by email on August 13, 2021.

Dear Councllors:
     Thanks to Carla [deputy town clerk}, for providing the information today concerning the Section 16 bridge in a courteous and timely manner.
     The information has only deepened concerns which I expressed in a video I sent to you January 15, 2021, and followed up with detailed letters on April 4, and July 9, 2021. I expect nothing. I hope, if nothing else, that these documents may be historically useful in any future court action.
      Video is available at https://youtu.be/vPNSqarY8B4
     I did a quick review and these are the issues I see right away.
     It is regrettable that the installation of the bridge has progressed so far without addressing these concerns with the public. I will try again, because I think reexamination of the project is still relevant. I doubt you will be issued a development permit. I included a request for a permit application in my IPRA request. Nothing was provided, therefore I assume an application has not been prepared. I see no requirement in the bid documents for the bidders be responsible for a permit.
      I notice that part of a P&Z review often includes flood zone requirements as part of the NFIP National Flood Insurance Program which suggests to me that Edgewood is a NFIP community (Ordinance 1999q). This bridge site is rated A (flood zone) by FEMA, so would require a development plan, 44 CFR 60.3(b)  I would think. I do not believe this installation would qualify. There is no chance whatsoever that this bridge is within the hundred year flood requirement, not the fifty year, not the ten year, and not the five year flood requirement (barely). Please see illustration. Building a bridge you know will be overtopped is irresponsible.
     The bridge will be installed adjacent to a school in an area designed to attract recreational and leisure users. The worst-case consequences include risk to life and health of citizens including unsupervised children. For a flash flood to occur at the bridge site, a heavy rain needs not be present downstream of the 35 square mile floodplain. It does not necessarily occur in the middle of the night. It is frighteningly fast.
     I am not doubting the engineering work on the bridge design. To the contrary, it is exactly the engineering I am citing to warn you that your decision to build this bridge in this location is unwise. I do not think the engineering is adequate, however.
     The primary causes of a typical bridge failure are contraction scouring and local abutment scouring. The most common causes after scouring include collision, overloading, and overtopping. This is misleading, however, because very few people in modern times will build a bridge that is highly susceptible to overtopping. There are guidelines prohibiting it.
    The engineering confines itself to scouring, both contraction and abutment. It confines itself to a five-year flood calculation with a ten-year check. It concludes that the bridge will be stable after a five year flood. I have huge problems with this. They acknowledge the probability of having to repair earthworks after a main event. Most of the engineering calculations concern encroachment. FEMA requires assurance that this structure will not significantly impact the culverts on US 344, 600 yards away.
     I went to the proposed bridge site after a rain in the last few weeks. I marked out the water level rise. Without a permanent survey marker, I had to estimate height. I calculate that the flood level qualified at or near a two year flood. This is not a surprise. There is a fifty percent chance of a two year flood every year. It did intuitively validate the engineering assessment in the sixty percent drawing package provided to me, signed early March 2021. On the illustration from the engineering set reproduced below, flood levels are labeled WSEL, water surface elevation level.
     There are several kinds of failure. The bridge can go away completely, but it can also sustain costly damage. It can stand, but you will realize that you have to install additional measures in the future, like rip-rap which you almost certainly need now, but is very problematic and expensive. The bridge road surface can be damaged and have to be replaced. It can be a Venus Fly Trap to entice, trap and wash away children during an extremely exciting flood event.
     It is woefully inadequate to design for a five year flood. There is a 20 percent chance of a five year flood every year. There are no engineering calculations for failure from overtopping. A ten year flood will introduce the possibility of a failure of the road surface through lifting and buckling. There are no predictions for that. The only prediction in the original documentation is that the bridge will survive a two year flood.
     The abutments are substantial, but the bridge itself is balanced on four 3/4 inch bolts and sits on elastomeric pads. This, as you know, is to allow for expansion and contraction. Without calculations, I don’t think tensile or shear failure is out of the question. The geometry of the bridge in water will cause turbulence and a water level rise of eight tenths of a foot. The results of this can be hard to calculate.
     The caissons are in the stream bed, not on the embankment. This exaggerates the forces of the flow in the narrow stream, if they were closer to ground level the water would be distributed more widely and be less turbulent. The bridge is not quite parallel with the flow of the stream, nor is it level from one side of the arroyo to the other. These are small conditions that are not accounted for in the calculations. Scouring will be accessible from all sides of the Caissons. This bridge is a very common Short bridge over a narrow channel with a vertical wall abutment. The configuration looks to me more like calculations for pier scour might be relevant. Maybe it doesn’t matter.
     Both the Froelich and Hire scour calculations are based on soil particle size. There appears to be no discrimination on the cohesiveness of the soil. Again, consider this is more of a question. I don’t see variables for this in the equations. It is my observation that the soil, when disturbed, will fall away easily. At least for the thinnest layers. I agree the caissons are very deep and unlikely to be totally exposed, or moved. I don’t know. Prudence requires an ongoing inspection protocol against excessive damage.
     All of the calculations are based on clear water scouring. There is no accounting for debris or waterborne material. This is not a reasonable assumption.  Again, consider this a question.
     The bridge is 14 feet wide. The steel bollards were apparently replaced with wooden bollards for cost reasons. It looks like an automotive bridge. I think we can assume it will be used that way, eventually. For that reason, I would be careful of taking advantage of exemptions not meeting requirements for heavier loads.
    Thank you for your consideration.
     Thomas O. McGill

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