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Staff outlines stormwater management and arroyo protections; drainage design tied to 100-year standard
Summary
Capital Improvement staff presented on stormwater management, master drainage plans, arroyos, and floodplain responsibilities, reiterating that drainage structures are designed to a 100-year storm and that developments must either connect to regional systems or provide on-site detention.
City staff from Planning and Capital Improvement summarized stormwater-management requirements, arroyo protections, and floodplain responsibilities for the commission on March 13.
Oscar Villalobos of Capital Improvement (presented via Planning/Inspections) described the purpose of stormwater management as protecting public health, safety and welfare by controlling increased runoff from new development. He explained master drainage plans and drainage studies must show where runoff will go, including verifying that any regional pond cited by an applicant has sufficient capacity. Villalobos said the city’s drainage design criteria generally require drainage structures to be sized for a 100-year storm and that the applicable table of intensities differs by area of the city.
The presentation defined an arroyo as a dry riverbed or seasonally active creek that can carry stormwater during rainy periods; staff said arroyos are common in arid regions and often preserved as open space but may be channelized to support development when necessary. Villalobos explained that developments in a designated drainage study area may discharge historic flow to regional systems but must retain the additional runoff generated by development on-site. He emphasized the city participates in the National Flood Insurance Program and that FEMA-designated floodways must remain free of obstructions.
Commissioners asked how older drainage studies and changing design standards affect current review. Villalobos replied that older infrastructure may have been designed to lower return-frequency standards (for example, a 25- or 50-year design) and that current projects account for those differences: engineers must show the difference between historical design assumptions and current proposals and retain the incremental runoff where required. He also noted local soil and percolation coefficients vary by area and affect whether on-site retention is feasible, particularly in mountain development.
The presenter and commissioners discussed areas of El Paso that historically retain water (notably parts of central and south-central El Paso) and how some regional features (pumping stations, modified arroyos) function during extreme events. Staff reiterated that project reviews require drainage reports and calculations consistent with the Drainage Design Manual and city and federal floodplain regulations.
