Surface transient storage in dead zones: Residence times from stream morphology, velocity and CFD modeling

Oregon State University, Corvallis OR

Investigators

Abstract

Transient storage in streams is the temporary retention of water and solutes in eddies and other "dead zones" (surface transient storage, or STS) and in the stream's subsurface (hyporheic transient storage, or HTS). Transient storage zones are where most of metabolism takes place in streams, and they are also very important to the fate and transport of heat and pollutants in streams. However, scientists have no easy, reliable method for quantitatively separating STS from HTS. Furthermore, in STS we do not know very precisely how the residence time distribution (RTD) and its mean relate to physical characteristics such as stream velocity, size of the dead zone, and number of eddies, even though such a relationship must exist and is intuitive. Preliminary field work indicates that the mean residence time in STS dead zones has greater variance (after accounting for differences in physical dimensions and velocity) than in dead zones in artificial channels or engineered dead zones in groynes. Preliminary work with a large-eddy simulation (LES) model suggests that it will be feasible to use this advanced computational fluid dynamics (CFD) method to simulate and understand dead zone residence times in natural streams. We will do detailed work on 5 dead zones at two discharges (for a total of 10) to characterize their physical dimensions, roughness, velocities, and residence times. Three of these will be simulated with LES. The LES, in turn, will be used to train less precise but cheaper Reynolds Averaged Navier-Stokes (RANS) models of the same dead zones. RANS models of all 10 dead zones will be developed, and the sensitivity of the RTD and mean residence time to each physical characteristic will be measured. We will use the LES and RANS models to develop a quantitative relationship between the RTD and the physical characteristics of the dead zone. This relationship will be tested on 20 dead zones where the physical characteristics and RTDs have been measured and that have no CFD model. The RTD relationship will be based on field-measureable parameters, and will not require a CFD model. To understand the limits of the RTD relationship, we will test it in the field against a number of non-ideal STS features, such as those with large wood or boulders and those in higher-velocity, larger streams. In addition to the 3 LES simulations on different STS sites, we will do a scaling analysis on the LES results to much higher Reynolds numbers to predict results (and the limits of the RTD relationship) for higher-velocity and larger streams. Lastly, we will develop a physics-based classification scheme for STS. This will allow us to state the limits of the RTD relationship in terms that are qualitatively and quantitatively easy to use in the field, and it will also help hydrologists and stream ecologists to communicate about STS. A quantitative RTD relationship for STS will allow hydrologists and stream ecologists to do a better job at predicting the movement of nutrients, pollutants, and heat in streams and rivers. This, in turn, will help with management of problems such as hypoxia in the Gulf of Mexico, nutrient loading in streams, excess stream temperature, and the cleanup of pollutant spills. The development of the RTD relationship will train several graduate and undergraduate students in both hydrology and CFD modeling, and will advance the field of CFD modeling. We will run a short course and conference on CFD modeling in hydrologic and allied sciences.