Graduation of Inez van Tilburg

01 February 2018 10:30 - Location: Room F, faculty of Civil Engineering and Geosciences - By: Webmaster Hydraulic Engineering

"Risk-based vegetation maintenance in regional water systems" | Professor of graduation: prof. dr. ir. M. Kok, supervisors: dr. ir. B.G. van Vuren (TU Delft), S. Groot, MSc. (HKV Lijn in water), dr. ir. G.H.W. Schoups

Vegetation maintenance in regional water systems has a significant effect on flood risk, because the hydraulic roughness decreases after cutting of vegetation. In streams with floodplains the vegetation is relatively more important because of smaller water depth. A more intensive vegetation maintenance policy results in higher maintenance costs and lower ecological value. The aim of this research is to optimize vegetation maintenance policy in streams with floodplains by consideration of the aspects flood risk, ecological effects and maintenance costs. For the three aspects a dimensionless performance indicator is designed to assess the total performance of several examined vegetation maintenance policies. For the aspect `maintenance costs' data from the water board is retrieved and the aspect `ecological effects' is determined by a literature study. For the aspect `flood risk' several steps are executed. The vegetation maintenance policy is translated to probability on roughness coefficients by a roughness function, which is derived from developed growth curves and roughness coefficients of the stream. Stochastic modelling of the water level by the hydraulic model Sobek 1D is used to examine the influence of vegetation maintenance on water levels in the stream. A consequence model, Water damage estimator, translates the water levels into flood risk resulting in performance of the aspect `flood risk'. The three performances of the aspects are combined into the total performance of the maintenance policy. The results of the case study show that timing of cutting has the most influence on the flood risk, followed by cutting frequency (how often vegetation is cut) and cutting intensity (percentage of cross-section that is cut) has the lowest influence. For high performance of maintenance costs low cutting frequency is important and for high ecological value cutting of pattern and in August is important. In conclusion, for streams with floodplains the optimal vegetation maintenance policy is cutting of pattern in June. The total performance of the vegetation maintenance policy can be further optimized by developing a dynamic maintenance policy with `discharge' as performance indicator, where an extra cutting session is executed when a high discharge is forecast. A dynamic maintenance policy results in high total performance of the vegetation maintenance policy.