View File - Development Services - City of Oxnard

WATER RESOURCES TECHNICAL REPORTfor the LAS, cannot be calculated. Instead, this value is simply reported as the averageheight above the goal across the coastal index cells. For example, if the average water levelelevation for a scenario is 10 feet in the index cells and the average goal is 4 feet, then theaverage water levels for the scenario are reported as “6 feet above the goals.”Lower Aquifer System. As described in Section 2.0, groundwater levels are significantlybelow sea level in the LAS, especially in the southern Oxnard Plain and Pleasant Valleyareas. For the LAS, there is a subset of both coastal cells and inland cells selected as "goal"cells as shown in Figure 2-5. There is an inland component for the goals in the LAS becauseof the pumping depression that may be causing water quality problems by inducing poorquality water from bedrock areas and from vertical flow through salt-rich clays separating(and within) the upper and lower aquifer systems. The goals in the selected cells vary from 5feet at the coastline to as much as 33 feet inland. Similar to the UAS described above, thecoastal “goal” is to maintain groundwater levels along the coast to provide a hydraulicbarrier to inland flow, or seawater intrusion. The inland goals are based on the averagewater levels in the LAS that would substantially reduce the existing head differencebetween the UAS and the LAS in that region (and thus the associated water qualityproblems), The percent overdraft reduction is calculated as follows:• First, calculate the difference in head between the goal in each of the selected "goal" cellsand the head modeled in the Base Case . This number becomes the "overdraft" fromwhich the percent overdraft reduction is calculated (denominator).• For each scenario, calculate how much the head has risen relative to the Base Case. Thisis the raw number that is used to calculate the reduction in overdraft (numerator).(Scenario elevation – Base Case elevation)Or, Percent Overdraft Reduction =(Goal elevation – Base Case elevation)For example, in cell R22/C35, the goal is 30 feet and the Base Case head from the model forthat cell is 10 feet. Therefore, a 20-foot rise in head is needed to eliminate the overdraft inthat cell. If Scenario 2a results in a head of 22 feet in that cell, then there has been animprovement in 12 feet from the Base Case value of 10 feet. Because there needs to be afull 20 feet of rise to eliminate the overdraft, this scenario provides 12 feet of the 20 feet, ora reduction in overdraft of 60 percent.Or, Percent Overdraft Reduction =22 feet – 10 feet)(30 feet – 10 feet)=12 feet20 feet=60%The reduction in overdraft reduction is the average overdraft reduction across all the "goal"cells. It is calculated by summing all the "goal" cell head rises above the Base Case for aparticular scenario (12 feet for this one example cell), and dividing that by the sum of all theheads needed to eliminate overdraft in all the "goal" cells (such as the 20 feet in thisexample).W112003002SCO LW1458.DOC/ 033390002 83