A lot of the county and FEMA and other "official" flood plain maps are pretty much just point to point connected straight line, quick draw sorta maybe somewhere near to where it oughta be generalities that don't really follow contour lines or realistic topographic profiles. They are based (call it guessed at) on 20 foot contour maps (or worse) and therefore are not precise by any means. That is probably intentional since surveying all the miriad of streams to a foot or so--which is really what's required--would be an impossible task and are understood to not be cast in stone. As lots are bought and sold and developed flood plain data is surveyed in and incorporated into the database and the base maps further refined.
It's important to remember that flood plains cannot be demarcated with any real precision anyway. All streams have a variable gradient which does not follow the contour lines exactly or for long and is affected by many factors including the gradient itslef at any particular point or section of the stream, the width and depth of the actual entrenched streamway at any particular point, and the profile (cross section) of the flood plain itself--the broadness or flatness of the plain or the steepness and depth of canyon walls which might limit a stream's spreading--which are always changing relative to the water moving downstream. All of the workings of a stream are quite dynamic and are affected by the gradient and imperviousness of the tributary upon which rain falls, rain pulses which cause pulses in the stream which are differentially affected by water already impounded by chokes or bends or other factors that were different a few minutes before, other pulses coming into the main c hannel from other tributaries either before, while, or after it does, etc, etc. Flood plain maps are at best half-assed guesses by engineers working from imprecise maps and who've probably never set foot on the site. Even when a particular stream may have been surveyed and the local gradient and approximate watershed known, there are still too many variables--especially as the watershed gets larger--to make anything that resembles a definitive mark on the wall or to put a stick in the ground and say, "Here it is." On the other hand, the sea level change maps can be figured fairly closely by simply following and/or interpolating the marginally accurate contour lines that are readily available on all topo sheets. None of the variables mentioned above regarding flood plains will play any part in predicting within a few feet or centimeters pretty nearly exactly where sea level will reach at any given worldwide rise. That's another type of flooding altogether. --Ediger -------------- Original message ---------------------- From: "Scott" <back2scool...@hotmail.com> > > Re: [Texascavers] RE: Sea Level Rise--the MapIf you live in Travis county you > can go to the City of Austin website, under Development, GIS viewer and it > has > plat maps that you can add property lines, utilities, zoning, fema flood > plains, > and lots more on. ITs actually a really useful tool. I have not checked to > see > if other counties have as nice a viewer....or even a viewer. > > > ----- Original Message ----- > From: John P. Brooks > To: ronra...@austin.rr.com ; 'Allan Cobb' ; texascavers@texascavers.com > Sent: Saturday, September 29, 2007 7:32 PM > Subject: Re: [Texascavers] RE: Sea Level Rise--the Map > > > You can typically find flood plain information with regards to a specific > piece of property on Property Appraisal District maps. Dallas County > Appraisal > District has online maps that are quite useful for checking these things. > Seems > like San Antonio has a similar site....as does the Austin area. The appraisal > district of course get their information from other sources....but I have > found > that appraisal districts are very good at pointing out unbuild-able land...as > it > is in their best interest to know where and what is build-able. > In addition, many regions or cities maintain zoning and planning maps that > are > on line. These maps will usually have topography and flood limits. I know the > North Texas Council of Governments keeps that information for much of the > Metroplex. I have found that some cities keep fairly detailed GIS type maps > in a > pdf format on line...and this is also a good source for flood plain and other > encroachment ( ie easements, ROWs, etc.. ) information on property. > > On 9/26/07 10:50 AM, "Ron Ralph" <ronra...@austin.rr.com> wrote: > > > I plugged in my house on the top of a fluvial divide that has not flooded > since the Pleistocene and got back a Low to Moderate risk rating with a > button > to push if I wanted to buy insurance. I think this is a scam or at best a > scare > tactic. Where are the FEMA flood zone maps with 25 year and 100 year averages? > > Ron > > > > > ---------------------------------------------------------------------------- > > From: Allan Cobb [mailto:a...@oztotl.com] > Sent: Wednesday, September 26, 2007 10:36 AM > To: texascavers@texascavers.com > Subject: Re: [Texascavers] RE: Sea Level Rise--the Map > > > I saw that Ediger didn't include the link either so I did a google search > for sea level rise map and it was the first hit. > > > > http://flood.firetree.net/ > > > > I would like to add the sea level rise WILL NOT affect this year's TCR > site. > Mother Nature is not invited so we are not expecting floods! > > > > Allan > > > > ----- Original Message ----- > > From: Minton, Mark <mailto:mmin...@nmhu.edu> > > To: texascavers@texascavers.com > > Sent: Wednesday, September 26, 2007 9:44 AM > > Subject: [Texascavers] RE: Sea Level Rise--the Map > > > > Ediger said: > > > >Here is the map of sea level rise I've been looking for. > > > > Where? There was no URL given. Remember - no attachments on > Texascavers. :-) > > > > Mark Minton > > > > > --------------------------------------------------------------------- Visit our website: http://texascavers.com To unsubscribe, e-mail: texascavers-unsubscr...@texascavers.com For additional commands, e-mail: texascavers-h...@texascavers.com
