2. No additional development has taken place in the watershed for drainage line N.
Still, every rain brings more erosion to the hillside and additional sediment transport to the
pond.
The pictures below show the effects of a moderate rain on March 21, 2011. According to
NOAA data, between midnight and noon, 0.48 inches of rain fell in the area. These pictures
show the sediment plume that was delivered to the pond only from the entry point of drainage
line „N‟ which later dispersed to the entire pond. This is typical of all rain events.
Photos are available showing upstream erosion during 2010-2011 rains.
Looking to the SW corner of the pond from hill Looking SW. Pond entry point of line “N”
across Attebury Road. Note outline of sediment watercourse is to the right. Note intensity of
plume. sediment plume as it disperses in pond.
3. Finally, after almost 2 years, a report was received from the City of San Marcos. This is
a Technical Memorandum from Lyle Engineering to the San Marcos City Attorney and
City Engineer, dated February 16, 2012.
This report confirms an increase of drainage discharge from 4.7cfs pre-project to
52.4cfs post-project. It also shows an increase in velocity from 8.8fps to16.1fps and
calls this “a significant increase in the 100-year peak discharge and velocity
along the “tributary” channel due to the construction of San Elijo Road and
adjacent development.”
Included in the ‘Conclusions’ portion of the report; “Based on these results, future
erosion along the tributary channel will be minimal going forward since a
majority of the naturally occurring surface soils that were present before the
San Elijo development have been washed away.”
This is only true if no further development is planned in the watershed feeding the
channel and a 100 year storm has taken place in the last 4 years – neither of which is
the case. Ignoring that, when was it approved that any surface soils present could be
“washed away”? And, where do they want to claim the surface soils went? The fact is
they were deposited into the pond and surrounding wetland. See previous and next
two slides……..
4. Picture taken approx. 8:30AM April 11, 2012.
See mud plume entering pond from wetland area fed by San Elijo Hills Drainage Line N.
Total rainfall in San Marcos on April 11, 2012 was 0.25 inches.
5. Picture taken approx. 4:30PM April 13, 2012.
See mud plume entering pond from wetland area fed by San Elijo Hills Drainage Line N.
Total rainfall in San Marcos on April 13, 2012 was 0.78 inches.
