Deh-Asodah Check Dam Concept Review

The proposed dam is intended to provide alluvial aquifer recharge to a karez network in Maiwand District, Afghanistan. The karez network is currently reliant on diesel pumps to withdraw water, which necessitates the cultivation of opium poppy due to the high operating costs. The proposed dam is designed to reduce the need for pumping and thus the need for opium cultivation. However, Tetra Tech has several concerns with the proposed design. The company notes that no study has been provided to indicate that the dam will provide the intended recharge and alleviate the need for pumping. Additionally, no analysis has been provided of the safety and integrity of the proposed dam under various loading conditions, which could have dire consequences downstream. Tetra Tech also expresses concerns regarding the numerous assumptions made regarding key design features such as soil properties, topography of the dam site, and the impoundment area. The company notes that the Technical Specifications contain vague references to materials and construction, which can vary greatly from one person to another. Clear direction for materials and construction is critical to the durability of the completed structure. The review also highlights several inconsistencies in the Technical Specifications, including the use of both Fahrenheit and Celsius temperature references, apparent reference to an outdated Indian Standard Code for concrete strength, and reference to material conformance to unspecified requirements. Furthermore, the Final Scope of Work and the Technical Specifications contradict one another regarding the strength of concrete to be used for the cement and sand mortar and for the reinforced concrete slab. Based on the review findings, Tetra Tech cannot confirm that the proposed Deh-Asodah Check Dam will perform its intended function of aquifer recharge and cannot determine if the structure will be structurally sound. The company does not recommend that USAID proceed with the advertising and construction of the Deh-Asodah Check Dam until additional studies have been performed and a more comprehensive technical specification is prepared. A geotechnical review was conducted by Tetra Tech's geotechnical and structural staff. The review notes that the data available to Tetra Tech did not include a hydrogeologic investigation of the area of the check dam and karez groundwater regime. Tetra Tech has concerns that the construction of the check dam will not provide an adequate source of groundwater recharge to the karez system. The review also highlights the need for a hydrogeologic study to evaluate the feasibility of the check dam recharge scheme. Specifically, a hydrogeologic study should include the elevation of the water table, information on the elevation of the check dam and the karez system, distance of the karez from the check dam impoundment, and hydraulic conductivity of the rock and soil below and adjacent to the check dam and the impoundment. In addition, the review notes that no dam safety performance requirements were specified for the proposed check dam. Dam structural performance criteria include, but are not limited to, stability under certain loads, seismic design, and hydrologic design requirements. These requirements should be defined to help ensure the long-term stability, operability, and safety of the dam. The review also highlights the need for a topographic survey of the site and the impoundment area, as well as a geotechnical investigation to evaluate the dam foundation and availability of suitable materials. An investigation should include drilling along the dam axis, the abutments, and under the energy dissipation structure, as well as testing for engineering properties and soil permeability. Geologic mapping should also be conducted in the vicinity of the dam and the impoundment, including the karez. Finally, the review notes that a flood hydrologic investigation should be conducted to determine the design flood for the structure. The design flood is a critical project element that impacts dam safety. Aerial photography shows that the watershed for the stream may be substantial, which implies that runoff volumes may also be substantial. The study should estimate the design flood for the spillway, as required by local regulations.