COSTA BAJA RESORT AND MARINA

REVIEW OF MASTER DRAINAGE HYDROLOGY AND HYDRAULICS REPORT

080331

Dr. Victor M. Ponce

  1. Introduction

    This report contains specific comments regarding the report entitled "Costa Baja Resort and Marina: Master drainage hydrology and hydraulics report," prepared by VA Consulting, of Corona, California, March 2008, hereafter the "subject report."

    The subject report considers the hydrology and hydraulic design for the planned development of an 18-hole golf course in the Costa Baja Resort and Marina, La Paz, Baja California, Mexico. It calculates the size of drainage structures, including box culverts and drainage channels.

  2. Design return periods

    The subject report considers design return periods of 100 and 1000 years. There are seven subbasins, ranging in area from 1.52 to 163.6 ha. A federal highway (Mex 11) is within the limits of the planned development. The chosen return periods are judged to be too conservative.

    Established hydrologic practices consider 10-yr return periods for small urban drainage. Moreover, return periods up to 100 yr may be justified for larger areas (A). In the present context, it is recommended to use a 10-yr return period for very small basins (A less than or equal to 10 ha), 25-yr for small basins (A less than or equal to 25 ha), 50-yr for midsize basins (A less than or equal to 50 ha), and 100-yr for large basins (A greater than 50 ha) and federal highway crossings.

    There appears to be no justification for the use of 1000 yr in the subject report, particularly in the present setting (a golf course in close proximity to the ocean), even with a freeboard hydrograph.

  3. Intensity-Duration-Frequency

    Intensity-duration-frequency (IDF) curves should be obtained locally, based on 24-hr storm precipitation data available for La Paz.

  4. Peak discharge methodology

    The subject report uses the modified rational method for peak discharge calculations. In general, the rational and modified rational methods are recommended for urban drainage design in small basins, where hydrograph attenuation is small and can be disregarded. For example, the State of Virginia Drainage Manual limits the applicability of the rational method to basins smaller than 10 ha (20 acres).

    Basins greater than 50 to 100 ha are usually better modeled using hydrograph methods such as NRCS TR-55. The latter methodology does not require IDF curves, because the intensity-frequency relation is already included in the chosen NRCS type storm.

  5. Field visit

    On March 27-28, 2008, the Consultant performed a field visit to the site. He was hosted by Ignacio Bustos Guzman, of RUAL, and John Bowden, of Piza Design.

    On March 27, the Consultant inspected the two southern basins. The condition of the terrain and natural channels was noted. The existing drainage facilities, including culverts, were observed and measured. On March 28, the Consultant inspected the two northern basins, and the remaining northern area of the development. The condition of the terrain and natural channels was noted. The existing drainage facilities, including conveyance channels and culverts, were observed and measured. During the visit, the Consultant gained an understanding of the golf course facility, its location in relation to the existing natural drainage, and the drainage issues associated with the development.

    On March 28, the Consultant attended a meeting at Costa Baja Resort, with geologist Filiberto Martinez and representatives of the geotechnical consultants. The competence of the underlying parent rock was discussed at this meeting.

  6. Tasks

    The following tasks are identified.

    1. Obtain the complete 24-hr storm precipitation record for La Paz.

    2. Determine 24-hr storm depths for return periods of 10, 25, 50, and 100 yr.

    3. Establish an intensity-duration-frequency curve applicable to La Paz.

    4. Select appropriate return periods between 10-yr and 100-yr.

    5. Determine hydrologic properties for each location of interest (22 locations).

    6. Calculate peak discharges using both rational method and TR-55 method.

    7. Use rational method for the smaller basins (less than 10 ha).

    8. Use TR-55 method for the larger basins (greater than 10 ha).

    9. Calculate required size of structures.

    10. Select design cross sections.

    11. Recommend changes in existing design.

  7. Summary

    The subject report considers 100-yr and 1000-yr return periods. These are judged to be too conservative. The recommendation is to perform new hydrologic and hydraulic analysis, using 10-yr to 100-yr return periods. A reduction in size is expected, with resultant savings in construction costs.

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