EVEREST June, 2013 - California Department of Boating and ...

Orange County Coastal Regional Sediment Management Plan
San Gabriel River Watershed
Second only to the Los Angeles River, the San Gabriel River is the most heavily controlled river
in southern California. Within its drainage there are 38 smaller check dams, 16 debris basins, 8
flood control and storage reservoirs, two larger flood control basins, several artificial percolation
basins, and near-complete channelization below the mountain catchments (Brownlie et. al.,
1981). The potential sand delivery of the San Gabriel River under natural conditions, assuming
no controls existed upstream, was estimated at 784,000 yd 3 /year. From Section 2.3.1 of this
report, the current sand delivery for this river was estimated at 20,000 yd 3 /year. The
764,000 yd 3 /year difference between these rates is provided as an estimate of the volume of
sand that is trapped in the upland sediment sources along this river. These three sand volumes
as well as those for other watersheds are summarized in Table 3.3.
Huntington Harbour Watershed
The watershed that drains into Huntington Harbour is large and heavily urbanized with much of
the material originating from drainage outlets, such as the Wintersburg, Sunset (i.e., Heil), and
Edinger (i.e., Bolsa Chica) Flood Control Channels. There are insufficient diversion systems,
sediment traps, silt traps, or catch basins to remove all the sediment, trash and debris from
these channels before it reaches Huntington Harbour. Results from a bathymetric survey of
Huntington Harbour in the year 2000 indicate that approximately 34,000 yd 3 of sediment needs
to be dredged (Tetra Tech, Inc., 2000). It has been estimated that the future sediment dredging
rate is approximately 11,000 yd 3 /year (Moffatt & Nichol, 2009b), with the sand portion being
much less. In addition, it can be assumed that a large and unknown quantity of potential RSM
beneficial sand is trapped in upland catchments that feed into Huntington Harbour.
Santa Ana River Watershed
The Santa Ana River is highly controlled and channelized and is the largest and most heavily
used river in southern California (Brownlie et. al., 1981). Under natural conditions it was
estimated that 430,000 to 650,000 yd 3 /year of sand could be delivered to the coast from the
Santa Ana River. USACE estimated a sand yield of 33,000 yd 3 /year (USACE, 2002b) and
Brownlie et. al. (1981) estimated that 140,000 yd 3 /year are currently being delivered to the
coast. From this, it is estimated that from 250,000 to 617,000 yd 3 /year are trapped in upland
areas. Roughly 226,000 yd 3 /year of sand is estimated to be trapped behind the Prado Dam
(CDBW et. al., 2002), with the remaining assumed to be distributed amongst other upland
sediment sources. A separate study by Slagel and Griggs (2006) estimated that dams and
flood control devices along the Santa Ana River reduce the rate of sand and gravel flow to the
coast by 243,000 yd 3 /year.
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