3.3 Water Resources
Local currents in nearshore waters are complex and include longshore currents, which flow parallel to the
shore, and cross-shore and rip currents, which move in an onshore-offshore direction. The combination
of these currents makes up the littoral transport process. Longshore currents in the coastal zone and driven
primarily by waves striking the shoreline at oblique angles. Overall, longshore currents produce drift and
sediment transport (turbidity) from north to south. Wave exposure affects the receiver beaches from the
south and west. For further discussion of littoral transport processes affecting the proposed receiver sites,
refer to Section 3.1.
Seasonal fluctuations in wave patterns and currents also cause substantial changes in water quality,
especially turbidity. Warming by the sun is the primary factor that affects surface water temperatures in
southern California from June to October.
Seasonal upwelling and downwelling also affect water quality within the area. Upwelling occurs when
northern winds displace surface waters offshore, resulting in replacement by colder, deeper waters. These
colder waters have lower dissolved oxygen, but they have higher salinity and, most importantly, are richer
in nutrients. Upwelling is generally present from late March though July in the San Diego County area.
Downwelling occurs when southern winds push offshore waters towards the shore, thus, pushing nearshore
surface waters down and causing warmer waters and lower salinity than is typical for deeper waters.
Chemical Properties
3.3.2
The similarities and differences in chemical properties expected at the borrow sites and offshore the
receiver sites are described below. The initial focus is factors associated with the water, specifically
temperature, salinity, dissolved oxygen and pH. This is followed by sediment characteristics.
Temperature
In areas near the borrow sites, seasonal thermoclines stratify the water column. Waters typically are
stratified during the summer and early fall, unstratified during the winter, and transitional (e.g., stratification
weakening or increasing) in late fall and spring. Thermoclines represent barriers to mixing between surface
and bottom waters. Surface water temperatures generally are highest from June through September and
lowest from November through February. Temperatures near the bottom generally are higher from
October through January and lower from April through June. Historical temperatures in the study area
range from 52 to 74 F near the surface and from 49 to 61 F near the bottom.
Page 3.3-2
Regional Beach Sand Project EIR/EA
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