General @ Thursday July 31, 2008 10:20 am by WunderKraut

What I’m Doing Right Now

Addressing comments on my first drainage report for a bridge replacement in Texas:



Funniest comment is comment #2:

In the first paragraph under “Hydrology”, suggest re-wording the last sentence; one does not “determine the hydrology”. Rather, one uses hydrology to estimate peak flow rates.

Smoke and mirrors. That’s all it is. Smoke and mirrors.

That’s why it’s been tough learning the hydrology and hydraulics side of engineering. There are no real hard rules. Merely a loose matrix of standard practices, assumptions and magic.

Especially magic.

3 Responses to “What I’m Doing Right Now”

  1. on 31 Jul 2008 at 4:50 pm   Tracy

    hummmm, can we say railroad… *yawn* :-) Just teasing my friend… you are so smart and so beyond me, so glad there are geeks, I mean smart people like you and Vince in this world…. LOVE ya…. Mean it!

  2. on 31 Jul 2008 at 9:30 pm   Dave

    I’m curious as to how you treat the lakes being within the drainage area. I’ll have to email you about that.

  3. on 01 Aug 2008 at 7:37 am   WunderKraut

    Dave,

    You have to look at the function of the lake. In this case, there is a pretty large lake in the basin. After looking at aerial photos, it became clear that the lake was for an upscale subdivision and was being used for aesthetics/recreation. As a result, it is assumed that the lake was not designed to function as a detention pond.

    That means flow in = flow out. So, in my calculations and routing methods, I assumed the lake wasn’t there. Now, I’m not sure because I don’t have the HEC-HMS model right now, but I think I did consider the surface area of the lake in my composite Curve Number. In other words, in regards to routing, the lake isn’t there, but in terms of the runoff, it is there.

    Also, this basin is HUGE, 43 sq miles.

    If this large lake had been designed and was functioning as a detention/flood control facility, then you would ignore the contributing drainage area. It would be like the lake makes all the area draining to it disappear from the overall basin. But you do have to be careful when making that assumption.

    Our two owners had to go in and fix another engineers mistake where that engineer ignored the contributing area of a large lake near the bridge in question. So, it’s not a hard rule.

    Also, I think the rules of your Metro North Georgia thingy, say something about ignoring the lake if it impounds less than 10% of the basin. Something like that.

    And finally, if it you can develop an outfall hydrograph for the lake if you know enough about the outlet control structure, spillway, etc. Then you can route that hydrograph with the rest of the basin.