San Elijo Hills Drainage Diversion Update – May , 2012
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 picturesshow 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.
Finally, after almost 2 years, a report was received from the City of San Marcos. This isa Technical Memorandum from Lyle Engineering to the San Marcos City Attorney andCity Engineer, dated February 16, 2012.This report confirms an increase of drainage discharge from 4.7cfs pre-project to52.4cfs post-project. It also shows an increase in velocity from 8.8fps to16.1fps andcalls this “a significant increase in the 100-year peak discharge and velocityalong the “tributary” channel due to the construction of San Elijo Road andadjacent development.”Included in the ‘Conclusions’ portion of the report; “Based on these results, futureerosion along the tributary channel will be minimal going forward since amajority of the naturally occurring surface soils that were present before theSan Elijo development have been washed away.”This is only true if no further development is planned in the watershed feeding thechannel and a 100 year storm has taken place in the last 4 years – neither of which isthe 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 isthey were deposited into the pond and surrounding wetland. See previous and nexttwo slides……..
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.
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.