Riverbed Hydrologic Exchange Dynamics in a Large Regulated River Reach. Issue 4 (10th April 2018) - Zhou, Tian
Abstract: Hydrologic exchange flux (HEF) is an important hydrologic component in river corridors that includes both bidirectional (hyporheic) and unidirectional (gaining/losing) surface water‐groundwater exchanges. Quantifying HEF rates in a large regulated river is difficult due to the large spatial domains, complexity of geomorphologic features and subsurface properties, and the great stage variations created by dam operations at multiple time scales. In this study, we developed a method that combined numerical modeling and field measurements for estimating HEF rates across the riverbed in a 7 km long reach of the highly regulated Columbia River. A high‐resolution computational fluid dynamics (CFD) modeling framework was developed and validated by field measurements and other modeling results to characterize the HEF dynamics across the riverbed. We found that about 85% of the time from 2008 to 2014 the river was losing water with an annual average net HEF rates across the riverbed ( Qz ) of −2.3 m 3 s −1 (negative indicating downwelling). June was the only month that the river gained water, with monthly averaged Qz of 0.8 m 3 s −1 . We also found that the daily dam operations increased the hourly gross gaining and losing rate over an average year of 8% and 2%, respectively. By investigating the HEF feedbacks at various time scales, we suggest that the dam operations could reduce the HEF at seasonal time scale by decreasing the seasonal flow variations, while also enhance the HEF at subdaily time scale by generating high‐frequency discharge variations. These changes could generate significant impacts on biogeochemical processes in the hyporheic zone. Plain Language Summary: Dam operations regulate downstream water flow and modify the surface water and groundwater interactions. In this study, we simulated river and groundwater dynamics in a 7 km long reach of Columbia River using a high‐resolution numerical model. Due to the upstream dam regulations, the daily river stage variation of our study reach could be as high as 2 m. We found that about 85% of the time from 2008 to 2014 the river was losing water with an annual average net HEF rates across the riverbed of 2.3 m 3 s −1 . June was the only month that the river gained water, with gaining rate of 0.8 m 3 s −1 . We also found that the daily dam operations enhanced the losing and gaining activities, which may lead to great impact on riverbed biogeochemical processes. Key Points: A coupled CFD model was used to simulate hydrologic dynamics in a large regulated river reach (about 7 km long, 800 m wide) Daily dam operations increased the river gaining and losing rates by 8% and 2%, respectively This approach can be extended to basin scale by combining with geomorphological and hydrogeological characterizations