Severe flooding puts New Mexico dam and downstream communities at risk

Severe flash flooding in southern New Mexico forced the evacuation of several communities in Dona Ana County after intense rainfall caused rivers and drainage channels to rise rapidly and water to overtop the McLeod Dam near Garfield.

Residents of Hatch, Rincon, Salem and Garfield were ordered to leave their homes on September 29 as authorities warned that the 25 ft high earthen dam could fail. Water was seen flowing over and around the structure as runoff carrying debris from the surrounding mountains entered the drainage system. At the height of the emergency, the National Weather Service issued a flash flood emergency, its highest level of flood warning, because of the potential threat to life and property.

The McLeod Dam was constructed as a flood mitigation structure and has a storage capacity of around 750 acre-ft. Although the dam remained standing during the initial emergency period, overtopping raised concerns that continued flow could erode the embankment or its abutments and eventually create a breach. Homes, farmland and employee accommodation are located close to the structure, increasing the consequences of any failure.

The flooding developed quickly. Residents described water levels rising to conditions they had never previously experienced, while agricultural land and crops around Garfield were submerged. Debris transported from higher ground accumulated around channels and roads, adding to the difficulty of moving water safely through the area. Police went door to door in Hatch as evacuation orders expanded, while Garfield Elementary School was also evacuated.

Several state highways were closed because of flooding, including sections of NM 185, NM 140 and NM 187. State police, federal resources and swift-water rescue teams were deployed, while temporary evacuation centres were established around Las Cruces and Hatch. No fatalities or injuries had been reported during the initial response.

The event also brought back memories of the major Hatch flooding of 2006, but the latest emergency placed particular attention on the behaviour of local flood-control infrastructure. Earthen dams and levees are designed to manage defined flood events, but extreme inflows can create a different set of risks once water approaches or passes the crest. Overtopping can rapidly erode unprotected downstream slopes, while concentrated flow around the ends of an embankment can undermine the structure even if the main body initially remains intact.

For engineers assessing the aftermath, the condition of the embankment, evidence of erosion or internal seepage, sediment accumulation and the performance of spillways and downstream channels will be particularly important. The volume and speed of runoff entering the system will also help determine whether the event exceeded the conditions for which the local flood-control infrastructure was originally designed.

The flooding showed how quickly conditions can change in normally dry landscapes. After months of limited rainfall, a large volume of water arriving within a short period overwhelmed channels, inundated farmland and placed communities downstream of flood-control structures under immediate evacuation orders.