Assessing Hyporheic Exchange: A Review of Methods With Emphasis On Flows and Sediment

Surface water-groundwater interaction in the shallow subsurface along streams can provide various stream functions, including temperature regulation, nutrient cycling, pollutant attenuation, and habitat creation. For these benefits, the hyporheic zone is increasingly considered in river assessments, restoration planning, and engineering design. There is strong interest from the scientific community and practitioners in identifying and quantifying functions in the hyporheic zone. There are two main complications in hyporheic zone assessment. First, hyporheic zone assessment requires detailed knowledge and experience from a range of scientific disciplines. Second, hyporheic zone functions vary widely in space and time, presenting challenges in scaling results and reaching conclusions. These two issues make it difficult to perform hyporheic zone assessments efficiently and effectively.

Hyporheic zone assessments often focus on characterization of hyporheic exchange flows. The hyporheic flow paths, flow rates, and residence times are critical in shaping the presence and magnitude of hyporheic zone functions. The flow rates and residence times are heavily dependent on hydraulic gradients and sediment hydraulic conductivity which can vary greatly in river systems. Sediment hydraulic conductivity is particularly important because it can vary over 7 orders of magnitude in space and time. Also, the type of hyporheic flow informs the available assessment methods, models, and tools. For example, assessment of (1) vertical hyporheic exchange induced by in-stream structures, (2) lateral hyporheic exchange across meander bends, and (3) bank storage during storms could each have different assessment approaches. As a result, a hyporheic zone assessment strategy for a site can be complex with multiple approaches.

The purpose of this technical paper is to provide a review of methods and techniques to perform a hyporheic zone assessment, with particular focus on hydraulics. The paper also includes recommendations for assessing hyporheic zone effects on water quality and ecology. Methods and techniques will be organized by type of hyporheic exchange, such as in-stream structure induced exchange, exchange across meanders bends, and bank storage during storms. A goal of this paper is to provide a reference and starting point to develop a hyporheic zone assessment plan that can be applied to a particular river system. This paper also identifies common problems in implementation and knowledge gaps that can be addressed in future efforts and research.