Simulation: Breach at Las Cruces Flood Control Dam could inundate about 1,200 acres, town told

At the Town Council workshop on Sept. 23, GIS analyst Tim Korner and Taos Soil and Water Conservation District manager Peter Vigil presented a simulation showing what a catastrophic breach of the Las Cruces Flood Control Dam could do to downstream neighborhoods.

The simulation modeled a 190-foot breach with the reservoir at crest and produced a short, high peak flow that would inundate roughly 1,200 acres. Korner said about 90% of that area would be covered within three hours; peak depths closest to the dam could reach about 12 feet and areas around El Monte Sagrado could see 6–8 feet of water. The model estimates roughly 1,000 structures within the inundation footprint, a nighttime population of about 1,595 and a daytime population of about 1,179.

Korner said the breach hydrograph in the model would peak near 80,000 cubic feet per second (cfs) for a brief period and that the simulation did not account for spillway discharge (about 8,000 cfs capacity) or deeper site-specific infiltration behavior, nor for the increased runoff that follows severe wildfire burn scars.

The presentation emphasized time to inundation: the areas in the red band on Korner’s maps would begin to see water within 10–15 minutes; many downstream areas would see significant flows within the first hour. Korner walked council members through FEMA hazard metrics (dVmax, depth × velocity) and maps that classify outdoor and indoor-person hazards, building collapse risk by construction type, and potentially lethal flood zones for children and adults.

Why it matters: the Office of the State Engineer Dam Safety Bureau inspects the dam every three years and has issued a recent report with corrective actions. Korner told the council that because the Las Cruces structure is classified as a high‑hazard dam, state statute requires the town to have an operations and maintenance manual and an emergency action plan and to complete hydrologic, hydraulic and geotechnical analyses to verify the dam’s capability to retain the design storm.

Town and district officials identified near‑term maintenance items the town can do without outside contractors — clearing woody vegetation, rodent abatement, repairing fencing and lubricating the slide gate so it can be closed in an emergency — and said they will work on an operations and maintenance manual this winter. More technical items — an emergency action plan and engineering analyses to verify as‑built conditions and capacity for a 500‑year event — will require outside assistance.

Vigil said the Taos Soil and Water Conservation District and its board are prepared to partner with the town on public outreach and planning. Korner and Vigil said the town has already contacted the Natural Resources Conservation Service (NRCS) about grant opportunities; NRCS has indicated it may fund an initial structural analysis at 100% and, if rehabilitation is required, could fund up to 65% of construction costs, though rehabilitation funding timelines are typically long (the presenters said 10–20 years is common).

Council members asked about the effects of wildfire and how much smaller storms could become catastrophic after burn scars. Korner and Vigil said wildfire can sharply increase runoff and debris flows by reducing infiltration and raising surface runoff velocities; they declined to give a single rainfall threshold because the response is site specific.

Korner summarized the presentation by urging action on the items the Dam Safety Bureau identified and said the intent of the original partnership (Town of Taos, Taos Pueblo, Soil Conservation Service/USDA and the local conservation district) was to protect downstream development. Vigil said the district will cosponsor a dam‑owners conference in Taos in May and will work with the town to brief residents and elected officials.

Next steps: town staff and partners said they will (1) complete basic maintenance with town crews and the conservation district; (2) finish the operations and maintenance manual over the coming months; (3) pursue NRCS funding for an initial structural analysis; and (4) seek funding and technical support for the required emergency action plan and geotechnical/hydrologic studies.

Korner and Vigil emphasized that the presentation was a worst‑case scenario intended to focus planning and to prompt compliance with state dam‑safety requirements.