design documents, the parameters selected were: elevation, crown
width and slopes on both water and landside. The elevation of the
levee determines the maximum stage of the water in which the
levee will be not overtopped and is the main parameter traditionally
considered to assign a level of protection to a levee reach (DWR,
2010). It must be pointed out that most of the lands in the Delta are
below sea level therefore these levees act as dikes, continuously holding
back the in-channel water. It is also noteworthy that the last
major levee-failure flood took place in Upper Jones Track in 2004
and the specific reason, although unknown, was not flow overtopping.
The full geometric parameterization of the levee (elevation,
crown width and slope) can be used to identify stressed levees
which may collapse before overtopping occurs. A low slope together
with a narrow crown may be produced by internal or external erosion
which might be an indicator of the poor condition of the levee and evidence
a higher risk of failure, even if the narrow crown meets the
minimum elevation criteria. Fig. 3 shows areas where three of four
shape parameters do not meet the USACE standards in a highly populated
area. In the urban margin (E), results show that the mean value
of the crown width parameter is of 3.3 m (Table 1) and 99% of the levees
are narrower than the minimum required (Table 4). This condition
and the fact that 81% of the profiles in the landside with slopes
less than minimum required (41%) might imply an important degradation
of the levees, even if the minimum elevation of the levee criteria
is reached for the area. From the west margin, it should also be
noted that 98% of the profiles have a landside slope below the minimum,
though levee elevations are higher than those in the east
margin.