Lessons Learned from Sediment Transport Model Predictions and Long-Term Postremoval Monitoring: Marmot Dam Removal Project on the Sandy River in Oregon

The 14-m-tall Marmot Dam was removed during the summer of 2007, and the cofferdam protecting the working area was breached during a storm on October 19, 2007, allowing approximately 750,000 m(3) of reservoir deposit to be eroded freely and released downstream to the Sandy River. Prior to the Marmot Dam removal, sediment transport models were developed to predict the transport dynamics of both gravel and sand, providing key pieces of information for stakeholders and regulatory agencies to select the most appropriate dam removal alternative. A monitoring program was implemented following dam removal that was designed to examine model predictions and assess when potential fish passage issues related to dam removal were no longer of concern. Comparisons of model predictions with field observations indicate that the model successfully predicted the erosion of the impoundment deposit, the deposition of sediment in a short reach downstream of the dam, and the lack of deposition in the majority of the Sandy River. The model overpredicted sediment deposition in a reach 7 to 12 km downstream of the dam. Further examinations indicated that the overpredicted sediment deposition may be attributed to underestimating the gravel particle abrasion coefficient during initial modeling. Comparisons also indicated that the model significantly underpredicted the suspended sediment concentration during the first 10 h immediately following cofferdam breaching but correctly predicted the minimal increase in suspended sediment concentration thereafter. (C) 2014 American Society of Civil Engineers.