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WATER SYSTEM ANALYSIS SALLANDSE WETERINGEN AND CANALS OF

The Sallandse Weteringen drain the catchment area of , an area of approximately 50.000 ha in the west of the province of , the Netherlands. Flowing through the canals of Zwolle, water is discharged on the Zwarte Water River and ultimately into lake IJssel. The dikes along the streams and canals are classified as secondary flood defences and must be able to hold back water levels with a return period of 200 years (frequency 1/200 years).

A test conducted by the regional water board in 2008 and a model study, dating from the same year, showed that at several locations the dikes do not meet the required safety standards.

Using a 1-dimensional hydraulic model (SOBEK) of the Weteringen, city canals and a part of the Zwarte Water River, characteristics of the water system and the effectiveness of different measures, aimed explicitly to reduce the design water levels, are analysed. The model results indicate that some 9.3 kilometers of dikes are not at the required height.

Time series of water levels at different points along the Weteringen and canals shows that the water levels in the city of Zwolle are predominately influenced by the downstream water levels (at the mouth of the Vecht River). Water levels in Zwolle are only slightly influenced by the discharge coming from the upstream Weteringen. Further upstream of Zwolle, the influence of the downstream boundary gradually decreases while the discharge of the Weteringen becomes more dominant. This system characteristic proved to be very important in the effectiveness of possible measures.

In order to comply with the safety norm it proved to be necessary to reduce the water levels at the mouth of the Vecht River with as much as 50 cm or implement a combination of upstream and downstream measures. This mainly because the water levels in Zwolle are only slightly influenced by the discharge from the Weteringen.

Above pictures show the threathening situation during the fall of 1998. An extreme precipitation event (approximately 1/80 – 1/100 year frequency) caused near-floodings in the city of Zwolle (left picture) and along the Weteringen (right picture). Gert-Ruben van Goor

Graduation Date: 26 August 2010

Graduation committee: University of Twente (UT) Dr.ir. D.C.M. Augustijn Dr.ir. P.R. van Oel

HydroLogic Ir. J. de Graaf