Source: County of Santa Clara

Land use is one of the factors that affect flood frequency and magnitude. One of the major goals of Phase 1 of the study was to understand the potential flooding affects of land use changes over time. Four different land use conditions were chosen to span the extent of the reasonable land use changes and associated flooding affects. Modeling the watershed in different conditions gives insight into potential future flooding problems and allows the impacts of development trends to be identified.

 

Each of the four conditions was chosen based on both individual characteristics and patterns that can be established between them. First, the model was developed and calibrated using existing conditions. Then, the four conditions were selected and modeled. The following four conditions allow the model to explore watershed response to changes that might affect downstream flooding.

• Back in Time to 1947:

  The historical perspective provides a glimpse of how flooding has changed due to known shifts in land use. The year 1947 is significant because it was just before the Corps’ levees were built and had conditions similar to when the 1955 flood occurred. In addition, three of the four existing reservoirs and some additional levees were not yet in place in 1947.

• General Plan Build Out:

  This scenario allows the model to predict the watershed flood potential using the urban and agricultural land uses for each planning departments. This is the best estimate available for future conditions within the watershed. While the horizons of the individual general plans vary greatly, this scenario is intended to approximately represent the years between 2015 and 2020.

• Ultimate Build Out in 2050:

  This scenario represents a worst-case scenario, in terms of flooding, due to urbanization. The model predicts how the watershed would respond to significantly increased growth in the cities beyond what the general plans currently allow. The year 2050 is the approximate end of the economic life of a project started at the time of this report.

• Changes in Agriculture:

  Agriculture can play a large role in the amount of runoff and therefore flooding in an area. This scenario does not represent any particular time period but parallels the Ultimate Build Out scenario in that it represents a worst-case agricultural hydrologic conditions.

Hydrology Model Results of Four Watershed Conditions

• Back in Time to 1947:

  Peak and average design discharges were higher in 1947 than they are today. Reservoirs existing today in the upper reaches of the watershed provide some incidental flood protection in the lower Pajaro River area.

• General Plan Build Out and Ultimate Build Out in 2050:

  These two watershed scenarios have been grouped together due to similarities in both their goals and results. Both conditions were chosen to see the effects of urbanization on runoff but at different times in the future; consequently, results show similar trends.
• The model results indicate that urbanization affects small storm discharge more than it affects large storm discharge. For the General Plan Build Out scenario, all changes in storms larger than the 50-year event are less than 3% for both peak and 3-day average discharges. For the Ultimate Build out Scenario, the largest change is approximately a 5% increase in maximum annual peak discharge and 3-day average flow. The lack of significant changes is probably due to the small amount of urbanization upstream or the San Benito River modeling point.
  
  Urbanization has a significant effect on the peak discharge of the smaller storms (2-year to 25-year). The impervious surfaces added by the development of urban areas generate more runoff and discharge in smaller events. The discharge frequency of a given storm will decrease with the additional urbanization. In other words, what was previously considered a 25-year storm would be expected to occur every 23 years.


• Changes in Agriculture: Model results indicate that even if all current agricultural uses in the watershed were converted to row crops under poor hydrologic conditions, the changes in peak discharge and 3-day discharge for the 50-year to 200-year return periods are well under a 2.5% increase from existing conditions. However, the 2-year to 25-year return periods show a much larger impact, increasing flows up to almost 9.5% in some locations. The major impact comes from the Lower Soap Lake watershed that includes agricultural uses in the South Santa Clara Valley, the Hollister Valley, and the Bolsa. Changes in the San Benito River watershed were very small, as only a small percentage of that watershed is currently used for agriculture.