Impact of shrubby willows on seepage lines of flood protection dykes

Vegetation on flood protection dykes (FPD) is an ambivalent topic. It can provide multiple ecosystem services (e.g. erosion stability). Mostly herbaceous vegetation is used. Many rules, standards and technical codes ban woody vegetation from FPDs. While herbaceous vegetation leads to higher efforts regarding continuous maintenance work, it could be assumed that woody vegetation can provide additional benefits.  The spectrum reaches from less maintenance, impacts on biodiversity, shadowing effects that might counteract the drought stress of herbaceous vegetation under climate change aspects etc.. Nether the less effects of woody vegetation on FPDs reach from positive to negative influence, covering the spectrum of soil stabilization to tilling of the dyke matrix. Failure mechanisms of dyke stability with positive or negative influence of woody vegetation are surface erosion, subsurface erosion leading to ground failure, slope failure with deep sliding horizon and slope failure with shallow sliding horizon. A first step towards consideration of woody vegetation on FPDs is the prove of stability of dykes covered by woody vegetation. Up to now these issues were faced with individual expert opinions conducted for unique dyke situations and single tree individuals.
Focusing on the mechanism of ground failure, which is depending on the course of the seepage line through the dyke. The influence of the vegetation type covering the dyke surface on the seepage line has been analyzed over 15 years at an experimental dyke in Austria. Typical dyke related herbaceous vegetation communities were compared to woody vegetation implemented via soil bio-engineering techniques. Flooding experiments were carried out and the height of the seepage line in the dyke was measured cross-sectional at two to four points. The results represent newly built dykes considering the current state-of-the-art. The tested woody vegetation covers shrubby willows, implemented via cuttings and brush mattresses.  
Seven flooding experiments were conducted, covering durations from 5 to 49 days. Comparing the seepage lines of day 7 no clear patterns could be detected. For 42 % of measuring points the seepage line in the area of herbaceous vegetation is higher than in related areas with woody vegetation. For 37 % of measuring points the seepage line in the section covered by willows exceeds the height values related to herbaceous vegetation. 21 % of measuring points show almost the same heights for both vegetation types. Based on the conducted measurements no seepage section on the dyke surface could be assumed, dam stability appears not to be threatened by ground failure in both vegetation scenarios.
As first results it can be stated that continuously growing shrubby willows shows no negative effects on the seepage line of FPDs compared to herbaceous vegetation cover. Tough, this study shows some limitations: The results are only valid for dykes constructed under state-of-the-art standards and for shrubby species.