Numerical modelling of morphological stability of proposed restoration measures along the Havel River

Transcription

1 Numerical modelling of morphological stability of proposed restoration measures along the Havel River Small scale morphological evolution of coastal, estuarine and rivers systems 6 7 October 2014, Nantes, France Petr Jiřinec 1, Rocco Buchta 4, Bertram Monninkhoff 2, Christian Tomsu 3 1 DHI, a.s. Na Vrsich 5, Prague, CZ, 2 DHI-WASY GmbH, Volmerstraße 8, Berlin, D 3 DHI-WASY GmbH, Büro München, Breslauer Weg 74, Geretsried, D 4 NABU Projektbüro Untere Havelniederung, Ferdinand-Lassalle-Str. 10, Rathenow, D Contact: Petr Jiřinec, p.jirinec@dhi.cz

2 Agenda Project overview Morphological processes Methodology Restoration measures Conclusions

3 01. Project overview

4 Project overview: Purpose Characteristics of the LOWER RIVER HAVEL area: outstanding importance as a natural river and floodplain one of the largest contiguous wetland areas acts as an important link in the waterway connections at least between east and west of Germany Directive: Richtlinie zur Förderung der Errichtung und Sicherung schutzwürdiger Teile von Natur und Landschaft mit gesamtstaatlich repräsentativer Bedeutung einschließlich der Förderung von Gewässerrandstreifenprojekten Carrier: Naturschutzbund Deutschland e.v. (NABU) Project: Investigation of the riparian zones in terms of satisfy safe navigation conditions and morphological stability of restored river reaches in the area Untere Havelniederung zwischen Pritzerbe und Gnevsdorf

5 Project overview: Location of project area Project area ca. 80 km northwest of Berlin

6 Project overview: Havelberg Stadt MK15 Model area and measure areas MK7 ca. 14 km of the Havel River km to MK1 abandoned river branch Vehlgast Dorfhavel up to 2 km wide flood plain river longitudinal slope < 1 MK4 MK2 MK5 Strodehne bridge

7 02. Morphological processes

8 Morphological changes - observed in period Morphological changes (m) global sedimentation up to 0.2 m

9 Morphological changes - observed in period Morphological changes (m) erosion up to 0.2 m dominated, affected by dredging

10 02. Methodology

11 Methodology: Two step approach flow conditions hydrodynamic model MIKE 21FM water levels, discharges, flow velocities, Froude numbers and bottom shear stresses morphological changes (erosion x sedimentation processes) morphodynamic model MIKE 21C all morphodynamic processes, helical flow, updated bathymetry during simulation Software package: MIKE by DHI

12 Methodology: Methodological workflow

13 Methodology: Finite Elements mesh of MIKE 21FM nodes and elements element sizes range from 7 m² in the river and trenches to 1950 m² outside the dikes

14 Methodology: Bathymetry of whole model of MIKE 21C software MIKE 21C x 559 computational cells cell size 0.5 to 15 m)

15 Methodology: computational grid of MIKE 21C Detailed schematization in main channel and old river branches

16 Model MIKE21C grain size parameters and erodible bed definition 0,6 mm Bed load + suspended load Engelund-Hansen formula 0,48 mm grain size (mm) 0,2 mm 0,7 mm i = S u h R S 0,8 mm erodierbar

17 Methodology: Model calibration and validation observed simulated MIKE 21C Longitudinal profile simulated MIKE 21FM

18 ST verification, period simulation steady flow observed data Mittlere Monatsabflüsse, 1 Jahr Berechnung - Morphological changes pattern comparable - Absolute values of erosion and deposit are comparable - Issue the width of simulated changes area is underestimated at steady flow approach simulation mean monthly values

19 DHI-WASY ST verification, , 3 years period

20 ST verification, , 3 years period observed data simulation steady flow

21 03. Restoration measures

22 Measures include typical restoring interventions like removal of bank protections, initialization of floodplain vegetation, restoration of abandoned flood reaches or historical river branches satisfy safe navigation conditions morphological stability of restored river reaches Example: restoration and reconnection of the abandoned historical river branch Vehlgast - Dorfhavel

23 Measures: Vehlgast Dorfhavel; river bottom optimization dredging the abandoned old river branch Groyne to stabilize and redirect actual state Opt projected 2 Opt state 3 Opt 4

24 Measures: Long term evolution after 10 years current stage Vehlgast Dorfhavel; river bed evolution Long term evolution After 10 years projected stage Q = 3.4 m 3 s -1 MQ = 88.9 m 3 s -1 Additional simulations using a cohesive sediment approach showed that there is no risk of mud remobilization out of the old river branch

25 Measures: Vehlgast Dorfhavel; navigation channel

26 Measures: Vehlgast Dorfhavel; navigation channel

27 Measures: Vehlgast Dorfhavel; navigation channel

28 04. Conclusions

29 Conclusions The overall simulation results show that the applied two-step approach is suitable to investigate the hydrodynamic and morphodynamic effects of measures along a river. The results of the applied approach can be used as reliable basis for approval procedures but also as a professional basis for conflict resolution strategies for different interests of river development The study verified validity of applied approaches for sand transport and fine-grained sediment (mud) remobilization assessment and identified limits for their application.

30 Thank you for your kind attention Petr Jiřinec