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Mortality of fish in the forebay and tailrace of a dam depends on predator density, water temperature, the amount of time in the zones, and the diurnal light distribution. An illustration of the zones is shown in Fig. 55. The full pool depth of the reservoir is H and h is the depth with a lowered pool elevation.
Predator Density / Volume Interaction
Predators may be concentrated in the forebay or tailrace when the depth of the regions is decreased by lowering the reservoirs. It is possible that concentrating predators increases the encounter rate between predators and prey and thus effectively increases the mortality rate in the forebay and tailrace.
This mortality increase can be included in CRiSP.1 runs by choosing the appropriate check box in the settings window opened from the RUN menu button. If the predator density/volume interaction is selected, predator density is a function of pool elevation for reservoir, forebay and tailrace regions. Predator density adjustments to the forebay and tailrace (Fig. 56) are given by1
(141)
where
- H = forebay (tailrace) depth at full pool
- h = forebay (tailrace) depth at a lowered pool
- P = predator density at full pool for the forebay (tailrace)
Forebay Mortality
Forebay mortality occurs while fish are delayed in passing the dam. The rate equation has the same form as used in the reservoir although predator densities and predation rate coefficients are different. The rate equation is
(142)
where
- X = smolt density in the forebay
- P(h)fb = predator density in the forebay as a function of forebay depth h (predators km-2) defined by eq (141)
= temperature in degree centigrade - a = predator activity parameter depending on prey species (this parameter has deterministic and stochastic parts)
- u = predator activity exponent independent of prey species
- b(t) = diurnal mortality rate factor 0 £ b £ 1 assumed to be constant over a dam time slice.
The fraction f of the forebay population surviving predators in each dam time slice 
t is
(143)
Tailrace Mortality
Tailrace mortality follows a process similar to those in the reservoir and forebay. The predation coefficient in the tailrace is larger than in the other regions, reflecting a temporary increase in fish susceptibility to predation after they exit the dam. The number of fish surviving the tailrace on each dam time slice is
(144)
where
- X0(t) = number of fish entering the tailrace at dam time step t
- X(t) = number of fish exiting the tailrace within dam time step t
t = residence time of fish in the tailrace (see eq (145)) - P(h)tr = predator density in the tailrace as a function of tailrace depth h (predators km-2). This is defined by eq (141)
- = temperature in degree centigrade
- a = predator activity parameter depending on prey species (this parameter has deterministic and stochastic parts)
- u = predator activity exponent independent of prey species
- b(t) = diurnal mortality rate factor where 0 £ b £ 1 assumed to be constant over a dam time slice.
Tailrace Residence Time
The tailrace residence time of fish, which is used in eq (144), depends on the level of flow and the volume of the tailrace. The equation is
(145)
where
- L = tailrace length as set by a slider at each dam (default is 1000 ft.)
- W = reservoir width
- Hu = depth of the tailrace, the upper depth of a reservoir at full pool
- F = flow at the project.
The tailrace time scale is not precisely defined and depends on the choice of the length scale L. In general, the length scale expresses the size of the region below a dam where predators are concentrated and the cross channel velocity distribution is influenced by turbine flows. The boat restricted zone of the tailrace is one possible measure of the length scale; BRZs are typically about 1000 feet long.
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1The limit h/H < 0.05 is arbitrary and required to prevent divide by zero errors. The limit equates to a river depth just over the head of most managers.
Columbia River Salmon Passage Model CRiSP.1.5 Theory, Calibration & Validation Manual
Copyright © 1996, Columbia Basin Research. All rights reserved.
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