density of riverine flood water

[asabender]

I am trying to design a hydrostatic retention system for a riverine flood. I know the elevations and flow volumes during the design flood event, taken at the location, which is a real event from 2010 exceeding the FEMA 100 year flood. The project area is in a still water zone. I am trying to find an adequate value for the unit density of such water. I know that the CT river had a maximum sediment load of 780 mg/l. FEMA was no help. Any suggestions for a unit weight of riverine flood water?

 

[cvg]

62.4

 

[asabender]

cvg: 62.4 is airless distilled water. The entire Connecticut River ran 780 mg/l (62.5 pcf) in a less severe flood event, measured in its estuary. I will clearly have more sediment than that. For consideration, if I presume that my my flood event runs 2000 mg/l my unit density will be 62.61 pcf. 4000, which I would think to be, perhaps, moderately conservative, gets me 62.82 pcf. Those fractions of a pound add right up with my considerations, and those values are not overly conservative. 62.4 is not acceptable in this instance.

I'm in a much smaller river than the CT River with a much higher velocity. I'm on an interior re-curved bank within a mile of an outfall, all of which got chewed up (trees, concrete and all) during the flood event I'm designing for. My intake vents protrude less than 18" from soil which could suspend and intake.

All other considerations aside, does anyone have a reasonable value of unit weight or sediment load for a 150 year flood in a bendy little river which usually passes 2k CF passing 30k CF during the flood?

 

[cvg]

flood studies generally assume density at 62.4 regardless of the amount of debris. debris is usually handled in other ways such as by adding freeboard or large trash racks. suspended sediment also generally increases your mannings n value. Unless you are dealing with salt water where the actual water density is greater, not sure why 0.2 pcf makes any difference? what sort of calculation are you making where it is even considered? and if it does, add a safety factor. what the heck is an intake vent?

without giving any data, how could anybody possibly give you a good value for the sediment load of an unnamed river?

 

[asabender]

Good point on the n value, but my situation is hydrostatic. Have you ever heard of a sediment load getting it higher than 63 for a riverine flood?

It's a strange job. The vent is a "flood vent" but with a 1 way valve on it (backflow preventer), because we don't want the hazardous material coming back out of the building. I need the vents because I don't have a reasonable factor of safety over floatation. The vents have to protrude through the adjacent embankment to get the right elevation and invert clearance. I'm putting an ACE design-basis scour apron in the area. With the backflow prevention, this means I will be pumping the building up like a balloon and dealing with the hydrostatic forces. I know the "sediment" load I'm going to add within the building from the fraction which could be hazardous material. I know that that bank could be mostly gone after the flood. Right now I'm just conservatively using 64.0 for the inbound water.

 

[asabender]

The flood event:

http://files.engineering.com/getfil...561.00065_-_worst_case_rise_of_quinebaug..pdf

 

[cvg]

64 seems conservative for your loading calcs. suggest that you obtain the FEMA detailed study for the river to verify the actual water surface elevation for location instead of USGS graph river stage graph. It was updated in 2013 and is available online. It gives the actual water elevations.

 

[asabender]

That was the starting point. As mentioned, it's not the 100 year flood. Apparently it was just short of what they consider the 500 year flood. My data and my standards are based on a single real event, not a statistical analysis. FEMA says this area will not flood. It flooded. They probably have the topography wrong. My guess is that they are off by 2' based on an error made in the area several years ago.

 

[beej67]

I've peer reviewed many recent flood map revisions who incorporated very detailed LIDAR for topography and sections, and then paid no real attention to manning's roughness coefficients, leading to significant flood elevation errors. That said, I am increasingly of the opinion that the NOAA NWS isohyetal maps are all terribly wrong to begin with, so we may all be designing off bad data.

I think it's pretty clear after Harvey that the ones in Texas are very wrong, and the ones in Louisiana are likewise very likely to be deeply flawed.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[dik]

62.4 is normally used, if you want to be conservative, use 62.5... The suspended stuff weighs whatever, displaced by it's own volume in water. It contributes very little to the density of water.

Dik

 

[cvg]

the rainfall data in texas may be wrong, but not the only problem. the storm surge backed everything up by at least 15 feet and subsidence has resulted in some areas that have dropped 8 - 10 feet. The same problem as NOLA which is below sea level and still sinking. You cant expect posative drainage when you are at 2 feet above sea level and the sea walls overtop. Personally, I think that low lying areas susceptible to hurricanes should probably be designed for 200 year or higher. and rebuilding (or refurbishing) houses in the floodplain should absolutely not be allowed.

 

[asabender]

I think that building in flood plains should be allowed. I think that you should have to design for a reasonable probability... like 50% in a 50 year design life. If you were in an ICF house of piers 2' above the design flood, your house would probably still be standing when it got slammed with 8 extra feet of water... wet and in need of finishes, but standing.

The problem is that everyone intentionally skirts those codes. People ignore scour, people ignore subsidence/sealevel rise/severity increase/etc. In New England, where I am, people don't remember a big storm. If you find some centenarian, they may remember the Long Island Express of '38, but people just don't realize that THAT isn't as big as our design storm. We haven't been hit hard since the 1600's. I constantly work on mega mansions that are dipping their toes in the atlantic, skirting all the design codes by jumping from the unclear and crappy IRC to municipal codes.

 

[cvg]

building in flood plains is already allowed in most areas assuming your finished floors are above the 100-year water surface elevation. my point is that there are existing homes that are in floodplains that do not meet this criteria. they get flooded on a periodic basis and then get rebuilt in the same location. that is just crazy.