Live Oak Master Drainage Study
City of Live Oak / Reclamation District 777, California
West Yost jointly prepared a Master Drainage Study (MDS) for the City and the District. The goal for preparing a joint document was to ensure that the two agencies did not develop conflicting drainage plans. The purpose of the study was to allow the City to grow without causing upstream or downstream impacts to the agricultural areas or causing increased flooding within the City. West Yost developed design criteria for the City, developed an XP-SWMM hydraulic model, evaluated the adequacy of existing City drainage infrastructure and the District channel system, and analyzed four alternative drainage plans for buildout associated with the City’s new General Plan.
West Yost reviewed the design criteria for several communities in the Central Valley of California and recommended appropriate design criteria for the use in the MDS, including the 10-year, 4-day storm for storm drains, and the 100-year, 4-day storm for detention basin and open channels (with 1 foot of freeboard). An XP-SWMM model of the City’s storm drain system and the District’s open channel/culvert system was developed. The model data was based mostly on GPS surveys of open channel cross sections, culverts, and storm drains, but record drawings of facilities within the City were also used. The model was calibrated by running the 2-year and 10-year storms, and comparing the results with known flooding problems. Subsequently, the model was recalibrated for the older area of the City that had larger yards and lower impervious percentages than the rest of the town. Scenarios for the 100-year storm were also run.
The adequacies of the existing City drainage infrastructure and the District’s channel system were evaluated with the model. At the worst locations, flooding depths of under 3 feet were predicted for the 10-year storm and up to 4 feet in the 100-year storm. The District’s channel system was originally sized for a runoff rate of 15 cfs per square mile, which is less than even a 2-year storm event. Improvements to solve six existing City flooding problems were developed. The total capital cost of these improvements was estimated at $5.4 million.
West Yost also evaluated four alternative drainage plans for the buildout land use of the new City General Plan. West Yost prepared and gave a presentation describing and comparing the alternatives at joint City Council and Planning Commission workshop. After receiving their (informal) input, Alternative 4 was selected as the preferred project. A development impact fee study was prepared based on Alternative 4. Impacts fees were based on land use types and their associated impervious coverage. Commercial, industrial and high density residential impact fees were developed using a per acre of development basis. Residential impact fees were developed on a per dwelling unit basis.
West Yost provided engineering evaluation of four alternatives to meet the drainage requirements of the City’s new General Plan. The cost of the alternatives ranged from $36 million to $58 million. For each alternative, a life cycle cost evaluation of was prepared, and the most effective and cost-efficient alternative was recommended, resulting in major savings in future capital costs. This type of thorough evaluation process helps keep stormwater impact fees low, and illustrates to the local development community that the lowest cost drainage solution has been identified, which contributes to acceptance of the drainage plan.