tag:blogger.com,1999:blog-3501444543696395573.post3509166831460655194..comments2023-08-22T04:52:21.984-05:00Comments on An Arrow Through The Air: Little Osage CreekDavid A. Toddhttp://www.blogger.com/profile/16825539283421597579noreply@blogger.comBlogger4125tag:blogger.com,1999:blog-3501444543696395573.post-14705269659040662262008-11-16T17:19:00.000-06:002008-11-16T17:19:00.000-06:00Gary:The two computer programs I'm using are from ...Gary:<BR/><BR/>The two computer programs I'm using are from the US Army Corps of Engineers. I believe a lot of what they have done on riverine studies, especially at the Hydraulic Engineer Center (HEC) at Vicksburg, Mississippi. The two programs are HEC-HMS and HEC-RAS. At Vicksburg, they have large buildings with models to run water through and to calculations. But they have also done modeling based on the Mississippi River and other rivers with gauging stations. Of course, what these physical models do is give us a better handle on the parameters that have to be entered into the computer.<BR/><BR/>For example, for a given rainfall, how much water runs off rather than soaks in? Based on much empirical data, we have published parameters based on type of soil, type of surface features, and even somewhat based on the nature of the storm. For a given waterway, how much friction does the surface features generate? Some of this has come from the Corps, but a lot more from university studies over the last 100 years.<BR/><BR/>So I have good confidence in the computer models and published parameters. Now, in myself to properly apply those models and select the right parameters, I have less considence.<BR/><BR/>DaveDavid A. Toddhttps://www.blogger.com/profile/16825539283421597579noreply@blogger.comtag:blogger.com,1999:blog-3501444543696395573.post-83475108452973393492008-11-16T12:33:00.000-06:002008-11-16T12:33:00.000-06:00Certainly, the model software must be tested when ...Certainly, the model software must be tested when originally designed. Or is it like the general circulation models in climate science where they fudge poorly understood components with best guesses? In other words, do they actually run water through a section of river or a physical model (like a wave tank in ocean engineering) to measure the accuracy of the computer model?<BR/>-GaryAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-3501444543696395573.post-80545849213305002742008-11-15T20:20:00.000-06:002008-11-15T20:20:00.000-06:00Gary:I never did see the final report on the Minne...Gary:<BR/><BR/>I never did see the final report on the Minnesota bridge, though I thought a large part of the failure was maintenance.<BR/><BR/>We are not able to verify the model through calibration. This is much too small a project for that (only 3,800 feet of urban stream, though mody of the western tributary area is undeveloped. Verifying that an accurate model has been put in the computer is a question of checking upon checking upon checking. Making sure I have no typos. Making sure I have accurately stated the length of the creek channel, left overbank, and right overbank. Making sure I have used parameters that accurately define the drag of the flood plain against free flow. Making sure that each culvert is accurately defined by survey (or occasionally by approximate measures). It is then a matter of trusting the developers of the computer program to have accurately put together mathamatical routines that will give an accurate flood elevation. We really have no other way of verifying it, short of field calibration; which, when you are modeling the 100-year storm, it's kind of hard to get field calibration.<BR/><BR/>I will prepare a submittal to FEMA, asking them to change the flood map according to my analysis. In addition, I will prepare an engineering report of my findings and making recommendations to the City. One huge uncertainty for the future is how that western basin will develop. The City needs to make sure it develops in a way such that downstream flooding will not get worse. I'm going to recommend some ordinances they could enact to help the situation.David A. Toddhttps://www.blogger.com/profile/16825539283421597579noreply@blogger.comtag:blogger.com,1999:blog-3501444543696395573.post-22913588988025259232008-11-15T12:30:00.000-06:002008-11-15T12:30:00.000-06:00Dave, How do you verify the models? Or more spec...Dave, How do you verify the models? Or more specifically, calculate the margin of error? People tend to look for a final number and ignore the uncertainties. The report on the bridge in MN that collapsed a couple of years ago cited undersized gusset plates and overloading with new construction materials as leading to the disaster. Surely this bridge was modeled, but nobody seems to have paid attention to the results. There appears to have been no effort to quantify or at least publicize the structural limitations. <BR/><BR/>So, final question: how do you communicate the risks and probabilities to people who only want to know will the countryside flood while I'm still in office?<BR/><BR/>-GaryAnonymousnoreply@blogger.com