Random Walk

With all the extreme weather in the midwest of late, the media are dutifully reporting the work of volunteers to fill sandbags and place them on the various levees and riverbanks to hold the river back, and the obvious question finally dawned on me: what happens to the sandbags after the danger is past? Put another way, why don't they just leave the sandbags there for the next time? Aesthetics?

Update (6/18/2008): I continued to do some digging after asking what may have seemed like a rhetorical question, and ran across a PowerPoint presentation from Dr. James Blatz of the University of Manitoba, a professor in their civil engineering department. The presentation mentioned the importance of timing around sandbag placement AND removal, but without any printed elaboration. Dr. Blatz was kind enough to respond to my query, as follows:

...The point of the importance of removing them is not obvious in my presentation as you have noted since there is some discussion I add around that point. The comment is actually specific to the area I discuss in the presentation (in around the Winnipeg region) where the dike corridors are right at the top of marginally stable riverbanks. The Red River cuts through a weak lacustrine clay layer and placing the large surcharges of sandbags on the tops of the banks can actually cause them to fail and take the bags with them thereby destroying the flood protection level. So part of the issue with timing is waiting for the river to rise enough such that the flood water acts as a lateral hydrostatic force on the riverbank allowing the added surcharge to be placed safely (the sandbag dike) and then when the water recedes below the sandbag level after the flood you need to remove the surcharge quickly or if the water drops too far, reducing the lateral hydrostatic force, the sandbags can cause the banks to fail. This is particularly a problem along the Red River since the dike corridor is relatively narrow since people build their homes close to the river. If you had the dike corridor quite a way from the natural riverbank edge this would not be a concern and you could leave them up as long as you like. However, the sandbag materials are usually very cheap (end run materials) and are not protected against UV degradation (with carbon black or another admixture that would increase the cost significantly) since they are generally only to be used for short term applications and as such after a few years exposure to UV the dike would be highly impacted and the integrity would likely not be safe for resisting flood waters. You would see many ruptured bags along the surface and sand running out and so on....

Thanks to Dr. Blatz for taking his valuable time to deal with such a trivial subject for a layperson. We now return you to your regular randomness.