The Effects of Flooding on Structures. Or What to Expect when the Drought Ends Violently

Transcription

1 The Effects of Flooding on Structures Or What to Expect when the Drought Ends Violently

2 Let s Define Flood Increase in discharge compared to normal level Direct runoff of rainfall (we re talking about Texas) Increase in discharge correlates with increase in depth of flow/ water surface elevation

3 Probability? Roughly 25,000 on-system stream crossing bridges Roughly 17,000 off-system stream crossing bridges Roughly 42,000 stream crossing BRIDGES (>20 open span) Unknown number of culverts too small to be in the database.

4 Probability? Assuming each stream crossing is located on a stream defining a unique watershed with unique characteristics, that s 42,000 opportunities for a damaging flood event at all times; Assuming a 1% annual exceedance event (simply for convenience), that s an expected value of 420 per year statewide

5 Probability? 420 per year- is that realistic? Probably not for a number of reasons, but you get the drift- the probability of seeing structures you have an interest in be subjected to major flooding is not insignificant. Should you consider behavior/ performance in the face of flooding? (I do)

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11 Notice Anything?

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13 To paraphrase Ron White- It ain t THAT the water is flowin!

14 Debris- Solid materials carried by floodwaters. Floating or relatively buoyant material originating in the watershed and transported by the stream. Might be houses, cars, boats, or other manmade materials, but most commonly is composed of trees, limbs, and other vegetative matter.

15 Debris- Solid materials carried by floodwaters. Structures may function as debris traps if they are not designed with debris in mind. Debris clogs the structure exacerbating flooding of adjacent land, as well as inducing large lateral loads due to flow impediment.

16 Debris Look at the watershed and stream. Try to visualize debris production potential. What kind of debris and how much might be produced? How might structure configuration accommodate or impede debris passage? There s no real standard answer.

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18 Scour Anecdotally, since such records have been kept, Texas has not seen the prevalence of problems due to scour as it is predicted by HEC 18 and is apparently prevalent in other states. Why is this? What is it about Texas that is different?

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20 Hypothesis: Much of Texas is subject to flash flooding that appears to be of extreme severity. Many bridges that were built on types of foundations that were most vulnerable (early 20 th century) were destroyed by floods before scour came to our attention. Texas was driven to the use of relatively scour-failure-resistant foundations decades ago by necessity.

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23 Scour- What we DO see Erosion damage to headers and approach roadway is very common when a structure (bridge or culvert) is overtopped by floodwaters. Undermining is often invisible from the roadway!

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27 Streambed Material Mobile bed material is present in many Texas streams. After the fact, it may present symptoms similar to the classical scour phenomenon about which so much is written. It actually involves much more extensive material transport upstream, downstream, and through a structure than we are used to thinking about.

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34 Summary and Conclusion We are conditioned by process and education to think only about flowing water when we do hydraulic analyses of bridges. Solids transported by the flowing water are very often the actual vehicles of bridge damage.

35 Summary and Conclusions Acknowledgement of the transport of solids by floodwaters is essential to planning survivable structures. There is little guidance- our procedures treat only water. Streams ALWAYS transport more than just water. Solids might be anywhere in the water column.

36 Summary and Conclusions Allowing the passage of debris might encourage a designer to open up a stream cross section at the structure to pass water and debris. Opening up the stream cross section invariably results in the inhibition of bed sediment movement at relatively small flows; this destabilizes the stream.

37 Summary and Conclusions There is a delicate balance between the natural rating curve (stage/discharge) of the stream, the rating curve of the structure, the ability of the structure to pass debris, and the ability of the stream to move bed material that arrives from upstream past the structure.

38 Summary and Conclusions Failure to account for debris passage in the configuration and maintenance of a structure increases the risk of damage to the structure during large events. Failure to account for bed material movement in the configuration and maintenance of a structure often destabilizes the stream and results in chronic, long-term problems that are difficult to resolve.

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41 Questions? George R. Herrmann, P.E., P.H., CFM