Efficiency of contour benches, filling-in and silting-up of a hillside reservoir in a semi-arid climate in Tunisia

Abstract

The El Gouazine catchment area (18.1 km 2 ), located in semi-arid central Tunisia (average annual rainfall 350 mm), was equipped with total retention contour benches on 43% of its surface area between June 1996 and July 1997. In order to analyze the efficiency of these benches on the catchment area, different thematic maps (slope, lithology, land use, hydrographic network) were crossed using GIS with a map that located the benches and their breaks. Specific topographical surveys were also carried out on a series of 14 benches' in order to characterize the development of their holding capacities. To evaluate the impact of the contour bench installation on the catchment area, the change of liquid and solid inflow was measured at the catchment area outlet two years before and eight years after the bench installation. On-site surveys showed that contour bench dysfunction in the El Gouazine catchment area could not have been caused by man. However, three main physical causes could explain the dysfunction: placement of benches on gypsum clay soils, location of breaks on the hydrographic network, and slope greater than 25%. Topographic surveys showed that the benches initial holding capacity varied between 1 and 3 m 3 per linear metre for a construction standard fixed at 2.28 m 3 . These surveys also showed that, nine years after their construction, the benches had lost 10 to 50% of their initial holding capacity. After installing benches in the El Gouazine catchment area, solid inflow was only reduced by 30%, whereas liquid inflow was reduced seven to eight times during a fouryear period, which greatly limited the possibility of downstream irrigation. Annual runoff coefficients, at 4.5% before bench installation, were again at 5.1% in 2002-2003, and the silting-up rate was evaluated, in June 2005, as identical to its value before installation. Bench breakage and silting up of upstream canals explain these changes. The results of this study should serve to improve contour bench installation plans of catchment areas in semi-arid climates by both increasing bench efficiency and, at the same time, conserving sufficient water flow to hillside reservoirs. To cite this article: N.

Parameterization of the Effect of Bench Terraces on Runoff and Sediment Yield by Swat Modeling in a Small Semi‐arid Watershed in Northern Tunisia

“…However, SWC structures underlie certain temporal evolution – such as the decreasing SWC efficiency over time through siltation or the appearance of sudden structure breaks (Baccari et al, ). Moreover, sediment accumulation may also lead to minor changes of the hill slope topography.…”

Combining Landsat observations with hydrological modelling for improved surface water monitoring of small lakes

“…Author-produced version of the article published in Journal of Hydrology, 2018, N°566, p. 109-121. The original publication is available at https://www.sciencedirect.com Doi: 10.1016/j.jhydrol.2018.08.076 These are inferior to errors generated from extrapolating capacity loss based on studies on 15 nearby surveyed reservoirs (figure 2), due to the strong disparities in silting rates and the difficulties in erosion modelling, especially over extended periods (Albergel and Rejeb, 1997;Baccari et al, 2008;Hentati et al, 2010;Lacombe, 2007;Ogilvie, 2015). The Gouazine reservoir benefited from the longest and most reliable time series (figure 3) due to regular maintenance, field observations and six updates to the stage-surface-volume rating curves but results on other reservoirs enabled to confront the method on lakes of different capacities ranging between 50 m 3…”

Assessing Floods and Droughts in Ungauged Small Reservoirs with Long-Term Landsat Imagery

“…and efforts to reduce soil erosion notably through contour benches. The influence of water and soil conservation works is also not constant, as these are shown to degrade with breaches potentially increasing erosion processes [46], while farmland abandon and forest expansion as seen in other Mediterranean areas can also in time modify erosion patterns and their amplitude [47]. Based on successive surveying of lakes, studies showed that specific erosion rates varied from 1.8 to 24.2 t/ha/year [30,31] on Gouazine and Fidh Ali respectively over the 1990-1999 period, as a result of lower crop development, clayey vs. marly soils, greater forest cover and greater contour bench development.…”