Figure 12. — Interchange induced by creating adjacent 

 regions of high and low permeability. 



AREA OF 

 DOWNWELLING 



I ■ I nil) Ji* iniLflVUljliu 



IMPERMEABLE STRATUM 



Figure 13. — Interchange induced by sheet pilings. 



Figure 14. — Interchange estabhshed in streambeds by 

 shaping the gravel bed into (a) a rounded profile and 

 (b) an angular profile. 



CONCLUSIONS 



A constantly changing supply of well-oxygen- 

 ated water is required in salmon spawning beds 

 for proper survival, growth, and development of 

 salmon embryos and alevins. Interchange between 

 stream and intragravel water is essential for the 

 delivery of adequate dissolved oxygen to pre- 

 emergent fish. Knowledge of the factors that con- 

 trol interchange aids in understanding mortality 

 factors and in developing means of increasing 

 production of salmon fry from spawning beds. 



Table 1. — Summary of efecls of permeability, depth, and 

 surface profile of streambed on interchange 



Streambed 

 characteristics 



Causes 

 upwelling 



Causes 

 dowuwelling 



Illustrated 

 in figure (s) 



Bed depth, 6(1) Decreasing in the Increasing in the 11. 



direction of flow. direction of flow. 



Permeability, tW do.... do 12,13. 



Bed-surface profile, Concave Convex 6,7,8. 



yi(.z) (see fig. 2). 



Waterflow within a spawning bed can be 

 described by Darcy's law and an equation of 

 conservation of mass for an incompressible 

 fluid. The potential causing intragravel flow, 



<t>=y+gcP/pg, 



is the solution of the equation residting from 

 combination of Darcy's la\\- and the mass con- 

 servation equation. 



The potential along the stream-streambed 

 boundary is described approximately by 



(t>{x)=yi (a;)-t- constant, 



where i/i (x) describes the profile of the streambed 

 surface. 



Interchange is controlled by the shape of the 

 longitudinal profile of the streambed and longi- 

 tudinal gradients of permeability and depth of 

 the gravel bed according to the approximation 



" n dx\ ax/ 



A concave streambed siurface induces upwelling, 

 and a convex siu-face induces downwelling. In- 

 creasing permeability of the streambed (measured 

 in the direction of waterflow) induces down- 

 welling, and decreasing permeability induces 

 upwelling. Increasing bed depth (measured in 



INTRAGRAVEL FLOW OP "WATER IN A STREAMBED 



487 



319-171 O . 68 - 4 



