A climate proof water buffer for South Holland

Abstract

The Province of South Holland is conducting a heritage project to explore the historical significance of barge canals and their future role in a changing climate. This thesis aims to address the water challenges posed by increased floods and droughts caused by climate change through the implementation of a water buffersystem. The buffer should allow the region to serve as a drainage during storms and retain water during droughts, considering the projected climate conditions in 2100. The study focuses on a large case study area, the heart of South Holland, which is enveloped by barge canals and divided into three water boards. A water balance model, calibrated using existing pump time series, is utilized to simulate water flows at the polder level. The model demonstrates sufficient accuracy for predicting larger spatial and seasonal scales within the jurisdiction of the water boards. Future conditions are simulated in the model by incorporating KNMI scenarios. However, the scenarios underestimate the occurrence of droughts, resulting in an underestimation of polder inlet and an overestimation of outlet. 
An above-average extreme climate change scenario is used to compensate for these biases. Combining the model output with long term statistical storm and drought forecasts, this leads to a more realistic buffer design capacity. If the reservoirs can be operated as prescribed, the primary hydrological purposes of the natural water buffers can complement each other. The first criterion consists of the buffering of the increased inequality of net inflow distribution throughout an average expected year, for which 20 million m³ with an additional top layer of 115 mm would be necessary. The second criterion entails the draining of the increase of intense precipitation events. 7.5 million m³ would be necessary to drain the extra precipitation of a 1000 year return period storm event. Thirdly, the buffers should be able to provide water, compensating for the aggravation of drought conditions. 34 million m³ should suffice to compensate for the aggravation of droughts with a return period of 2 years and longer. As the criteria are conditionally compatible, 34 million m³ is the net minimum required buffering capacity for the center of South Holland. 
The final optimal buffering strategy entails the realization of a large buffer in the Noordplaspolder, connecting the Rotte and the Hoogeveensche Vaart, and of a smaller buffer in Schieveen. It also requires the expansion of the water reservoirs in the Eendragtspolder and Berkel. The larger buffer will feature a deeper canal, permanently submerged, linking the Rotte and the Hoogeveensche Vaart, which in turn will connect to the nearby boezem water network. The Noordplaspolder and Eendragtspolder will buffer the area of the two water boards Rijnland and Schieland and the Krimpenerwaard that are inside the case study area, Schieveen and Berkel will buffer Delfland. The Schie and the Vliet will play important roles as water carriers between Delfland and its neighboring water boards and the Nieuwe Maas. This strategy not only addresses water management but also offers recreational, historical, and ecological opportunities. It revives the functions of old barge canals, the Vliet and the Schie, and restores the connection between the Rotte and the Hoogeveensche Vaart, adding to the solution's historical value.