Rough bottom ramp
Rough-bottom ramps are generally made up of blocks of relatively uniform dimensions arranged one against the other and forming a channel of varying slope.
Warning about the passability of the structure
In the absence of pools and protruding elements in the flow, these ramps do not provide rest areas for upstream migration fishes. Fish therefore have to swim over them in one go, generally using their maximum swimming speed. The crossability of a rough bottom ramp depends on the flow velocities, the water heights above the crest of the embankments, the ‘quality’ of the water surface (absence of jumps and slumps making it difficult, if not impossible, for fish to swim), and also the length of the ramp that the fish have to cover before becoming tired. This is why rough bottom ramps are not considered to be crossing devices as such (Larinier et al. 2006). This type of device can be useful (1) to make a sill directly passable by species with the best swimming ability (salmonids, even shad and lampreys) or (2) to form pre-dams combining short ramps with reduced falls and intermediate basins.
Calculation method
Inclined apron
The inclined apron is calculated by discretizing the pass width into several horizontal aprons whose number is determined by the parameter “Number of flow slices".
Relationship between upstream head on the crest and flow rate
The relationship between the upstream load and the flow is represented by a formula of free weir.
Relationship between water depth, flow velocity and uniform flow
From the flow supplied or calculated at the inlet, the head of water and the flow velocity in the channel are calculated using the Manning-Strickler formula for a uniform flow for a rectangular cross section equivalent to that of the ramp.
Strickler coefficient is estimated from the \(D_{65}\) size of riprap:
With \(a\) the Strickler correction coefficient function of how the riprap is placed and the level at which they are joined (Larinier et al., 2006):
- spilt rock, not grouted : 21
- riprap in a compact arrangement without jointing : 15.5
- 30% grouting : 16.7
- 50% grouting : 18
References
Larinier, M., J. Chorda, et O. Ferlin. 1995. « Le franchissement des seuils en enrochements par les poissons migrateurs : étude expérimentale ». GHAAPPE 95/05-HYDRE 161. irstea. https://hal.inrae.fr/hal-02575575
Larinier, Michel, Dominique Courret, et Peggy Gomes. 2006. « Guide technique pour la conception des passes à poissons “naturelles” ». Rapport GHAPPE RA. Compagnie Nationale du Rhône / Agence de l’Eau Adour Garonne. http://dx.doi.org/10.13140/RG.2.1.1834.8562