Flow in “boils” (pipe break-through seepage) in Dutch polders and elsewhere groundwater MSc project: Information: Prof.dr.ir.T.N.Olsthoorn, t.n.olsthoorn@tudelft.nl, 06-20440256 Drs. Perry de Louw, perry.delouw@tno.nl, 030-256 4806 Global description/context/background/problem The groundwater below the Holocene cover layer in the deep polders in the western Netherlands is under permanent pressure. This not only causes upwards seepage, it also causes local break-throughs of the covering layer after which a more or less permanent pipe flow of groundwater develops, thus short-circuiting the ditches with the underlying sandy aquifer. This phenomenon is quite common in the deeper polders where the less pervious cover layer is weak. Even though the existence of boils has been known for many years, research has recently taken off more intensively by Deltares. Recent research showed that boil seepage is the dominant mechanism of surface water salinisation in deep polders in the Netherlands and contributes 60% of the total salt load of the polder; the concentrated large fluxes in boils lead to strong upconing of deeper and therefore more saline groundwater in comparison with diffuse seepage. Since surface-water salinisation has become an important issue for water boards, especially with respect to future changes of climate, sea level and land subsidence, more knowledge of the mechanism behind these boils is needed. Hence, investigation is important for the long-term sustainability of these polders. The current study will focus on the actual flow inside a boil, i.e. a pipe-flow system, under different (geohydrological) conditions. We distinguish pin boils, which are small springs or upwellings of water, and sand boils, which are usually larger and discharge water containing fluidized sediment. However, the walls of the pipe are likely maintained dynamically by the combination of upward flow and the density of the fluidized sand in the pipe. The morphology of boils strongly depends on the composition of the Holocene cover layer as it may have different conduits or just one pipe through which water with or without sediment is discharged. Because it is not known how the boil system exactly works, this MSc aims at modeling the pipe flow under different geohydrological conditions, i.e. different type of boils, for which different conceptual models may be investigated against an actual data set obtained by Deltares over the last couple of years. Some simple field experiments will be carried out to collect data about flow velocities, pipe-diameters, boil morphology and the relation between head difference and flow velocities. The Msc will be done in close cooperation with and partly at TNO/Deltares in Utrecht under the supervision of Perry de Louw (Deltares, preparing his PhD on boils: in Dutch: “wellen”) and Theo Olsthoorn (TU-Delft/Waternet) (The pictures were borrowed from http://www.rijnland.net/wat_doet_rijnland/onderzoek_aan_water/onderzoek_aan_water/p ilot_weldichting. See this site for some background information. Aim/objectives To analyze conceptually and from the literature the hydraulics of pipe flow of break-though zones in deep polders. To implement one or more of the most likely conceptual models of this pipe flow under different geohydrological conditions (different types of boils). To benchmark and possibly calibrate these models on actual site data.. Working methods Analysis and modeling with advanced software allowing implementation of concepts at a high level (Matlab, Comsol, Maple). Expected outcome MSc report, and/or scientific paper Drs Perry de Louw DELTARES, Unit Bodem en Grondwater systemen Postbus 85467, 3508 AL Utrecht Princetonlaan 6, 3584 CB Utrecht Tel: +31 030 256 4806 Olsthoorn, Prof. dr. ir. Theo N. Professor Groundwater Hydrology / Sr. Hydrologist TU Delft / Factulty of Civil Engineering and Geosciences (CITG) Section Water Mgmt, Chair Hydrology Stevinweg 1, room 4.96 2628 CN Delft, NL; T +31(0)15 27 87346; M +31(0)6 204 402 56