The Army Corps is proposing to build new levees and flood walls along West Avenue, Groveland and Park Place. While this may afford some level of protection to homeowners in the Groveland area flood plain (assuming the levees do not fail, in which case the results can be catastrophic), it may do so at the expense of other homeowners (like me) who live upstream of the levees and at the expense of properties, such as the high school, which are on the other side of the flood plain, directly across from the proposed levees.
In a 2012 paper “Levee Effects upon Flood Levels: An Empirical Assessment,” Southern Illinois University Prof. Nicholas Pinter looked at all river gauges in Illinois and Iowa that had at least 50 years of hydrological data.
During this period, levees were constructed near 15 of these gauges. Two gauges were downstream of the levee and 13 were at or upstream of the levee. These levees vary widely in stream size, alignment and degree of flood plain restriction.
Despite these differences, Pinter found that their effect on flood levels was remarkably consistent. He found levees did not affect flood levels downstream. But in all 13 cases, levee construction increased flood levels at and upstream of the levee. The before levee/after levee change in flood levels upstream was extremely clear in all cases (very high statistical significance) and the increase in flood level (for a given rate of discharge) was “generally large in magnitude.”
The smallest observed increase during a flood stage was 11 inches. Pinter’s empirical study confirmed theoretical expectations that backwater effects from levee constrictions reduce flow speed upriver and thereby increase flood levels.
Disturbingly, Pinter also found that the simple one-dimensional models used by the Army Corps (like HEC-RAS) are unable to fully capture this effect and systematically underestimate the increase in flood levels caused by building levees.
I think it would be far better for the community as a whole to find solutions to flooding which help everyone rather than help one group and harm others. Two such ideas are realigning the supports of the BNSF bridge (which would lower flood levels by one foot according the Corps) and replacing the very low, debris clogged Forest Avenue bridge, which currently acts like a dam during flood and raises water levels with a higher suspension-style bridge.
Jeffrey Miller
Riverside
