When the Corps is tasked with designing and building a structure, how do the designers know if it will perform as intended? Since trial and error is a costly and uncertain method to determine future structure performance, engineers use models to reduce uncertainty and help ensure a structure’s performance will be up to par.
Before a weir was constructed on the Jemez River upstream of Jemez Canyon Dam in 2003, numerical modeling was used, although it was “limited to the extent of data supplied,” said Stephen Scissons, civil engineer in hydrology and hydraulics.
A weir is a low dam built across a river to raise the water level, divert the water, or control its flow. The Jemez weir was designed to stop erosion and the lowering of the local water table along the Jemez River, upstream of the former reservoir, behind Jemez Canyon Dam.
The dam, constructed in the 1950s, helps provide flood control for the Rio Grande near Albuquerque. In the 1980s, the Corps and the New Mexico Interstate Stream Commission (NMISC) established a pool of water behind the dam to increase sediment retention. However, in December of 2000, the NMISC discontinued providing the water for this pool, and all waters were released and the reservoir was emptied on Nov. 1, 2001. The riparian habitat that grew up around the pool was threatened with erosion and a lowering of the local water table. Thus, the weir was installed to help mitigate these issues.
The river’s behavior upstream of the weir has been a success, but downstream there has been excessive channel degradation and incision that could result in failure of the structure unless the channel reaches a stable condition or protection is added to the weir.
While building a physical model is unusual for the District, Supervisory Hydrologist Tamara Massong said the circumstances warranted having the District work with the Hydrology Lab at the University of New Mexico to gather data for modeling the structure to better understand and stabilize the weir and prevent further environmental changes on the Jemez.
“This particular model is assisting us with determining the deformation characteristics of the large rock apron just downstream of the weir, so we know how to address it,” Scissons said. “UNM has been used in the past; however, this is the first time that a mobile-bed, physical model has been made. It is just one example of how the District effectively leverages our relationships in the professional community to solve complex problems for the nation.”