Existing Local Storm Drain Analysis. Consultant will perform a hydraulic analysis of the conveyance “capacity” of the existing City backbone storm drain system including subsurface storm drains under 36-inches in diameter, and open minor open channels. The larger backbone drainage systems will be evaluated in WSPG or XPSWMM. The hydraulic analysis will be based upon evaluation of the hydraulic grade line, rather than “normal depth” calculations. Hydraulic grade line (HGL) calculations will be performed utilizing the Los Angeles County Water Surface and Pressure Gradient (WSPG) program. The hydraulic model developed as part of this task will provide the baseline model to identify system deficiencies and to test the modification alternatives of the recommended storm drain improvements. The storm drain water surface profile models will only be generated for the main storm drain lines, using downstream tailwater controls in the major systems from the results of the XPSWMM analyses. Private or local facilities outside of public right-of-way (not including easements) intercepting the lot/building surface drainage will not be analyzed. All storm drains that outlet to channels will be evaluated. The water surface data and hydraulic information will be summarized in tabular format for each reach or element of the facility to allow easy use and review. The hydraulic models generated for the “existing” mainline storm drain systems will be analyzed in a sensitivity analysis to determine the maximum hydraulic “capacity” of the system. The maximum allowable hydraulic capacity will be defined as the highest elevation of the hydraulic grade line below the street that allows the surface inlets to function. A sensitivity analysis will be performed by factoring the “design” discharges to a lower amount until the storm drain operates without causing flooding of surface inlets.
