The slopes of canalised brooks and ditches, which are the drainage water receivers, including open channels of other purposes are usually overgrown by grass. The slopes of naturalised channels are seldom if ever mowed. Therefore, when performing hydraulic calculations (for an average discharge at vegetation time, a maximum discharge during summer floods), it must be presumed that these channels are overgrown with luxuriant grass of 50-70 cm height. Thus, by estimating channel conveyance, i.e., possibility to allow the vegetation to exist, it is necessary to take the grass influence into account. For this purpose, it was necessary to work out a hydraulic calculation method for such channels. In the Lithuanian Institute for Water Management, a hydraulic calculation model was developed for naturalised channels overgrown by trees and grasses (Rimkus, 1996). To estimate the grass influence, the hydraulic research data published by Dąbkowski and Popek (1997) (Warsaw Agricultural University) were employed. To apply these data it was necessary to employ the method of equivalent hydraulic bottom.
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