IMIS - Marine Research Groups

The second part of this work proposes a change in perspective regarding trash racks/screens in culverts and pump intakes: from trapping debris to divert/exclude towards a bypass, using screens. This proposal is made in an attempt to remedy the shortcomings in the methodologies needed to calculate the yearly amount of debris that may accumulate on the trash rack, the blockage percentage resulting from the dimensioning of the bar spacings, and the oversizing of the trash racks in case of clogging. Worked-out examples are also given there.

The third part of this work is dedicated to discuss the issue of fish migration/diversion/exclusion at intakes and channels in relation to the design of screens. The techniques presented for trash rack bar spacing dimensioning for culverts are usually favourable for fish migration (that is, the spacings are at least as big as the spacings required for fish migration) but, in general, this is not the only requirement for fish migration: hydrodynamics, and many other stressors, may deter fish from passing through the screen, regardless of the bar spacing. On the other hand, the dimensioning of screens for fish exclusion/diversion seldom agrees with the classical dimensioning techniques discussed in the first part, thus a brief discussion on behavioural fish exclusion facilities are made in order to put this contrasting ideas into context. Worked-out examples are also given here.

The last part presents a short discussion over the different approaches presented and gives recommendations for future studies related to the dimensioning of in-channel screens. It is important to mention that a dedicated manual for the dimensioning of screens lacks in Flanders. Hopefully, this text may serve as a sketch for a manual for the design of in-channel screens for the region of Flanders. For rapid access and use of the classical methods herein proposed, the author suggests to review section 2.1 and in particular Table 3 for a rapid dimensioning of the trash screen area given a yearly amount of debris that may travel with the flow, or just by using the intake’s area. The dimensioning of the trash rack bar spacings can be made using Figure 7; and in case an estimation of the flow velocities are needed then Figure 6 may come in handy. The bar spacing dimensions and flow velocities must be revised against the recommended values for fish migration/exclusion given in Table 5 and in Table 6. The latter tables can be used directly for the estimation of the bar spacings, in case information about the flow conditions is scarse. A workflow for the design of trash screens is presented in Figure 22, following the discussions held in the text.