The Pipingontspanner

Abstract

The Dutch flood defences are subjected to the new Dutch safety assessment program (WBI 2017) that incorporates the latest insights and safety regulations. As a consequence of the updated piping assessment model, studies are expected to identify more piping prone areas in the Netherlands, which makes it worthwhile to investigate alternative piping mitigation solutions that could be less expensive. A new potential solution is the Pipingontspanner, which consists of relief wells in combination with a water catchment area. The Pipingontspanner relieves water pressure fromthe aquifer and uses the water as counter pressure against the Uplift mechanism, thereby reducing the likelihood of piping.

The study investigates the technical and economic feasibility of the Pipingontspanner as a piping mitigation measure. A problem analysis was used to identify the components that form challenges in realising the concept and pinpoint the research to the functioning, effectiveness and applicability of the Pipingontspanner concept.

A hydraulic model and a design approach demonstrate the functioning of the concept. The hydraulic model describes the flow underneath the dyke, through the well and towards the basin above it. A critical point for the flow calculation appeared to be the time-dependent interaction between the river and the basin water level, which leaves only numerical calculation methods to describe the problem. The numerical program Modflow was chosen to predict the Pipingontspanner groundwater flow by simulating a flood wave scenario for a green dyke with piping problems and implementing relief wells and a basin. A parametric design approach was followed to create a Pipingontspanner model that could obtain the optimal configuration for a measure that can only be calculated with a groundwater flow model. For the verification of design configurations, relevant failure mechanisms have been included in this model. For the selection of the optimal configuration, a cost-benefit analysis has been used as an evaluation criterion. The Pipingontspanner model creates, calculates, verifies and evaluates the different design configurations.

The effectiveness of the Pipingontspannerwas illustratedwith a sensitivity and a cost analysis. For a variety of subsoil and hydraulic conditions, the sensitivity analysis showed that the safety factor for Uplift increases substantially for higher permeability of the aquifers and slowly growing hydraulic loads. The influence of the cover layer permeability and storage coefficient is negligible on the performance of the Pipingontspanner. On the other hand, the costs analysis showed that well maintenance, well monitoring and basin dyke construction costs are the main cost drivers of the design.
The total cost grows exponentially if the basin width behind the dyke is limited as the number of wells increases for smaller basins.

The applicability of the Pipingontspanner was demonstrated through a case study of a green dyke
in Tiel with piping problems. In addition, the design and costs of the Pipingontspanner in Tiel were compared against a traditional piping berm measure to illustrate the economic feasibility of
the concept. The results showed that the Pipingontspanner is not only able to mitigate the piping problem, but it does so with a smaller footprint and lower total cost than the piping berm. However, the case study also showed that limitations in the current groundwater flow model setup required an adaptation of the case geometry to prevent the drying up of the top layer cells, which would terminate the simulations.

The Pipingontspanner concept has proven to be technically feasible and economically competitive compared to the piping bermunder the conditions of:
1. a permeable aquifer with a minimum transmissivity of 25 m^2/d;
2. a minimum hinterland space of 10 m behind the dyke.