About the effectiveness of sand-bags
There have been a few studies related to the effectiveness of sandbags in flood events, these are usually based on observations performed after flood events.  Overall, studies have shown that sandbags only have modest protection in its use as a domestic (1).

A major factor the research shows is that the maximum effectiveness occurs to about 1 metre depth (1), they can, on occasion, hold out in higher flood levels, but as Reeve and Badr (2) determined, leakage between the bags increases rapidly with increasing flood water levels.  However, due to the unique nature of each flood conditions, there is still a large degree of uncertainty as to what effectiveness actually means in terms of sandbags (1).  However, the major degree of effectiveness is highly dependent on the number of rows of sandbags and the duration of the flood (2).

Asides from floodwater height and duration, and height of sandbags, other factors that affect their effectiveness include:


Waves on the flood waters increase instability of sandbag walls/dikes, especially as floodwater depth increases (5).
Ambient conditions - temperature mainly, if low enough - ice could form between the bags decreasing their effectiveness (5).
Speed of flood waters - having eyewitnessing (and being caught in) a flash flood (3), fast moving water itself (and the debris it invariably contains) can severely degrade the effectiveness of sandbags.
The time taken to build the sandbag wall, as well as build up the height of the wall should the flood levels rise.


In reference to the comment about alternatives, one major alternative is preventative action, even though it is impossible to fully protect against floods, particularly large-scale and flash-flooding. After the 2002 floods of the Elbe River in Germany, it was found that household preventative measures lessen the damage from a large scale flood (4).  Related, flood minimalisation techniques are explored including soil bioengineering (6).

References

(1) Botzen et al. 2008 Willingness of Homeowners to Mitigate Climate Risk through
Insurance

(2) Reeve and Badr, 2003 Performance of sandbags for domestic flood defence, Proceedings of the ICE - Water and Maritime Engineering, Volume 156, Issue 4, 01

(3) Worst Queensland Floods In 35 Years 11/01/11 YouTube news clip

(4) Kreibich et al. 2005 Flood loss reduction of private households due to building precautionary measures -- lessons learned from the Elbe flood in August 2002 Natural Hazards and Earth System Science

(5) Raymond, 2009, Wave action and the effects of the environment on sandbag dike performance Thesis - University of Manitoba

(6) Dhital and Tang, 2015 Soil bioengineering application for flood hazard minimization in the foothills of Siwaliks, Nepal Ecological Engineering