From Hannah Ritchie, PhD student Cranfield University, Water WISER CDT
My PhD project is based on sand dams: small, concrete structures constructed across ephemeral streams, behind which sand accumulates, in which water is stored from the rainy season for use in the dry season. I am exploring the causes and impacts of lateral, longitudinal, and vertical losses of the water stored in sand dams to the sub-surface environment.
In 2022, I spent two months in Southeast Kenya in the short dry season (January – March), working on three dams being constructed that season and three mature dams. A lot of my time was spent collecting geophysical data, which we used to image the subsurface up to 25-30m deep, and data to constrain the geophysics, such as topography, water levels, rock depth, and soil profiles.
The geophysical method chosen was Electrical Resistivity Tomography (ERT), as we wanted to image the sub-surface saturation and any fracturing in the bedrock. We conducted four profiles at each site, three parallel to the dam wall and one perpendicular, using dipole-dipole and gradient arrays.
I also collected empirical data on loss pathways at the mature dams. I assessed 12 different criteria, including vegetation health, number of trees, ponding downstream of the wall, scouring of the wall, and erosion. This information will be used to score each dam on its level of potential loss and will be compared to the geophysical data, and later to remotely sensed data to assess the causes and impact of loss from the dams.