Ecological approach to management of open drains

Abstract

Most natural streams were made into drains when they were converted into recipients of subsurface tile systems. During the process the variety of biotic and abiotic conditions within and around the streams was totally destroyed, and the new watercourses (open drain recipients) were deprived of the greater part of the ecological importance that the former streams had possessed in the landscape. The data collected over the last decade in Lithuania about drain bed deformations, sediment accumulation, vegetation cover successions, intentional afforestation of slopes, beaver expansion and succeeding water quality changes, revealed some self-perpetuating natural alterations which raise the ecological significance of open drains as refuges and corridors in the landscape for vegetation (the succession of herbage and appearance of woody vegetation on slopes), and the barriers protecting surface runoff from diffuse agricultural pollution (beaver impoundments). On the other hand, these alterations are useful in terms of maintenance of the functioning of those drains as recipients because they result in steadier (roughly parabolashaped) cross-section profiles, less threat of bed silting up and no need for the mowing of slopes (because of the growth of woody vegetation). This paper presents and discusses the study results highlighting the above-mentioned bidirectional effect of self-renaturalization, and considers the preconditions RÉ SUMÉ La plupart des écoulements naturels ont été canalisés dans des drains lors de leur conversion en collecteurs des flux de drainage souterrain. A cette occasion, la variété des conditions biotiques et abiotiques internes et externes aux écoulements a été totalement détruite, et les nouvelles voies d'eau (collecteurs à ciel ouvert) ont été privées de l'essentiel du contenu écologique des ruissellements naturels. Les données rassemblées depuis dix ans en Lithuanie sur les déformations des collecteurs, l'accumulation des sédiments, les successions du couvert végétal, le reboisement intentionnel des pentes, la prolifération des castors et les changements de qualité de l'eau, ont révélé quelques unes des altérations autoentretenues qui posent la question du poids écologique des drains à ciel ouvert comme refuge et couloirs de végétaux dans le paysage (succession des herbes et apparence de végétation boisée sur les pentes) et barrières protégeant le ruissellement de surface de la pollution agricole diffuse (barrages de castors). D'un autre côté, ces altérations sont utiles en termes de maintenance des drains collecteurs parce qu'elles conduisent à des profils en travers plus stables (forme grossière de parabole), une moindre menace pour l'envasement du fonds et un moindre besoin de tondre les pentes (présence de végétation boisée). Cet article présente et discute les résultats d'étude qui mettent en évidence les impacts bidirectionnels de la re-naturalisation

Ecological impacts of early 21st century agricultural change in Europe – A review

“…Stoate et al (2006) report how the creation of 'buffer pools' which capture field drain water before it enters a watercourse, can halve phosphorus concentrations and, despite being eutrophic, provide a wetland habitat for numerous invertebrate and bird species. Ditch stretches below beaver dams had 30-40% lower concentrations of dissolved inorganic nitrogen and phosphorus than in stretches above the dams (Lamsodis et al, 2006).…”

Assessment and Research of River Restoration Processes in Regulated Streams of Southeastern Lithuania

“…For example, if the slopes of the streambed overgrew with woody or grassy vegetation and Manning's hydraulic roughness coeffi cient exceeded the value of 0.060-0.080, complying with requirements for the hydraulic conductivity of streambeds, a section free of trees should be formed at the bottom of the slope, or trees should be thinned out along the whole slope, or vegetation should be completely removed from one of the slopes, thus decreasing the roughness coeffi cient of the streambed within acceptable limits (0.060). Under cutting down trees or shrubs, leaving the southern slope overgrown is recommended [11,21] because, as mentioned above, wooden vegetation growing on the southern slope will produce stronger shadows into the streambed as well as inhibit the scale of an overgrowth of the northern slope and the bottom with grassy and water vegetation, thus decreasing streambed silting.…”

“…Works by Lamsodis et al [11] and Thomas et al [12] widely described the composition of the species of woody vegetation. Lamsodis [13] investigated the distribution of woody vegetation according to frequency and density in reclamation trenches.…”

An Assessment of Self-Purification in Streams

“…The formed ponds act as precipitators, resulting that significantly less nitrate concentrations flow out from the pond [43]. Research in the Nevėžis River Basin [22] shows that beaver ponds (comparing annual concentrations of inputs and outflows) retain relatively more phosphorus than nitrogen and more soluble mineral N and P compounds: on average 28% nitrate and ammoniacal nitrogen and 43% of orthophosphate phosphorus. But organic forms are much less retained.…”

“…The authors suggest an alternative way of maintaining regulated streams (trenches): to allow the overgrowing of slopes with woody vegetation, the crowns of which would shadow the channel and strengthen the slopes [8,21,22]. Depending on the tendencies of species distribution in landscape and in cross-section of the trench, the restoration of dendrology in a desirable direction can be promoted artificially by planting special species or correcting their varietal composition in slopes, also by forming protective zones.…”