If a fish must leap to enter the culvert outlet, it will require a plunge pool below the culvert outlet that provides sufficient “take-off” conditions.  To determine the characteristics that define a suitable plunge pool for leaping, a number of researchers have made detailed observations of leaping fish in both natural and laboratory environments.

Stuart (1962) studied leaping fish (“salmon parr and minnows”) at natural waterfalls and observed that successful leaping occurred when the pool depth below the fall was 1.5 times the drop height of the falls; and that this condition resulted in a good standing wave location. He also observed that most successful leaps originated at the standing wave. At a later date, Aastrude and Orsborn (1984) did experiments in the laboratory and also found that adult pacific salmon often leapt with greater success from the standing wave.  However, they also found that Stuart's observation about the D/H ratio was not generally applicable; that, in fact, the formation of the standing wave was a function of the entrained air and shape of the water jet; not the height of the falling water. These authors suggested two conditions for optimal leaping: “1) depth of penetration of the falling water (dp) should be less than the depth in the plunge pool (dpp); and 2) depth of the plunge pool must be on the order of, or greater than the length of the fish (LF) attempting to pass” (Powers and Orsborn 1985, page 42-43).  Their rationale is that these two conditions will assure that the plunge pool will be stable and the depth will avoid turbulence that disorients fish and reduces the propulsive power of the fish’s tail.  

Lauritzen (2002) has done the most direct study on how depth of the outlet pool and height of the falls affects leaping success.  He tested Kokanee salmon (Oncorhynchus nerka) with an average total length of 29 cm at seven pool depths (8, 15, 23, 30, 38, 46, and 66 cm) at four fall heights (0, 12, 25, and 36 cm).  Fish leaping was not significantly (P>0.05) affected at the pool depth and drop heights tested.  However, there was significant relationship between fish leaping and the ratio of pool depth to fall height.  The ratio that best supported leaping varied between 0.6 and 1.2 depending on fall height, with higher falls having smaller ratios.

When the pool depth drop to less than 8 cm or was greater than 36 cm leaping ceased.  A pool depth of 8 cm was too shallow to generate adequate take-off speed that were directed at the crest of the fall.  At 36 cm, the pool depth was considerably lower than the bottom of the standing wave and fish would continue past the bottom of the wave, leave the current generated by the falls, and aborted the leap.

It appears that one of the conditions suggested by Powers and Orsborn’s (1985) was not supported by Lauritzen’s (2002) studies. Apparently, the plunge pool depth does not have to be greater then the total length of the fish attempting to pass, because the average total length of the kokanee tested was 29 cm and leaping did not stop until the pool depth was 8 cm. 

Lauritzen (2002) states that the preferred fall height is dependent on the pool depth and the mean ratio of depth to height is 1.0.  However he also notes that other factors besides the D/H ratio affects jumping such as the flow rate and gradient of the falls. 

While it may be preferable to base the depth of the plunge pool on penetration depth of the falling water or depth of the standing wave, this information is generally not known.  Instead, to determine if the pool depth is sufficient for leaping, FishXing relies on the flow dependent ratio of D/H – (maximum depth within the pool)/(total drop height measured from water surface within the culvert outlet to the pool surface). 

See: Minimum Plunge Pool Depth Calculations, Entering the Culvert: Leap or Swim