Consultants present HUC‑10 regional drainage study; consultants and city map $1.5B program of projects to reduce flood risk

Consultants from Civil Systems Engineering (CSC) gave Harlingen commissioners a detailed briefing on a regional HUC‑10 watershed flood‑planning study that covers roughly 287–297 square miles across parts of Willacy, Cameron and Hidalgo counties. The study uses HEC‑RAS/2D hydrologic modeling, recent LIDAR, NOAA rainfall records and observed storms (including major events in 2018, 2019 and March 2025) to map flood risk and prioritize projects.

CSC said the study divides the watershed into 39 subbasins to identify viable, bankable projects and to develop the benefit‑cost analyses required by the Texas Water Development Board (TWDB). The consultants described a “toolbox” of measures — channel deepening and widening, regional and neighborhood detention basins, larger storm‑sewer trunk lines, additional inlets, backwater restrictors and selective reclamation of resacas for storage — and recommended updating regional drainage criteria and enforcing higher detention standards for future development.

Consultants warned that some major conveyances no longer meet original design capacity because of sedimentation and operational constraints. CSC estimated that IBWC‑managed facilities (notably an arroyo/floodway with an original design capacity near 21,000 CFS) currently operate at a fraction of that capacity in some reaches. The presentation noted five newly installed stream gauges and recommended integrating gauge data into early‑warning systems and future project validation.

The consultant team said some projects show strong benefit‑cost ratios in urbanized areas (citing several individual basins above 5:1) but that many rural or future‑condition projects have lower ratios until they are pared down or targeted. They estimated a program‑level scope in the hundreds of millions to roughly $1.5 billion for the full master drainage plan, a figure that includes both existing flood relief and future‑condition drainage improvements across the Arroyos/Colorado basin.

City staff and commissioners pressed consultants on prioritization, TWDB thresholds and how to improve benefit‑cost results by breaking large basins into smaller, more targeted projects. Consultants and the city engineer said many improvements will require coordination with irrigation districts, the International Boundary and Water Commission and Hidalgo County; they also highlighted that some actions (dredging, operational changes) depend on external partners and federal funding.

CSC recommended phasing projects so local investments can leverage state and federal grants, and proposed that some near‑term detention and channel fixes be completed before new home construction in key subareas. City leaders asked that any development proposals include conditions to ensure necessary drainage and sewer upgrades are completed as proposed.