Geotextiles are permeable fabrics used in various civil engineering and construction applications for the protection against erosion and scour. Geotextiles have predominantly been manufactured using synthetic materials due to their durability and precise design pore sizes. However, the environmental concerns associated with synthetic geotextiles have led to the exploration of sustainable alternatives, including natural fibers and biodegradable thermoplastic polymers. This study addresses the problem of designing stable geotextile filters. The problem statement resolves around the assumption that geotextiles of natural fibers are considered "open" filters. Existing design criteria for open filters do not directly account for load conditions or the use of fibers in the fabric. Instead, they rely on the relationship of the geometrical properties of the base and filter material. To address this gap, a new approach is proposed based on hydraulic pressure gradients. This approach considers the actual gradient and critical gradient at the geotextile interface. To achieve this, the geotextile will be evaluated within the context of an open channel with uniform flow conditions. The main research objective is to formulate a design method for geotextiles based on the principles of an open filter structures under a single granular filter layer. A comprehensive literature review has been conducted to investigate the hydrodynamic processes that define the actual gradient at the filter/base interface where the geotextile is positioned. The framework of linear wave theory proved to be a valuable tool for investigating and modeling these hydrodynamic processes. To define the actual gradient, the following hypothesis is proposed: The actual gradient can be defined by adding the average gradient to a turbulent component derived from measured turbulent wall pressure spectra at the top of the filter layer. Based on spectral approach and linear damping, it is hypothesized that these turbulent components undergo damping within the filter layer, resulting in a diminished turbulent component at the filter/base interface. From the literature review, it can be inferred that the dimensionless turbulent rough wall pressure spectra developed by Blake (1970) offer the most appropriate representation for the relevant hydraulic conditions (rough horizontal bed, uniform open channel flow). In a turbulent wall pressure spectrum, outer scaling variables and inner variables relate to different length scales within the flow, representing large-scale and small-scale fluctuations, respectively. In this hypothesis, it is assumed that the low-frequency region is scaled by the average velocity and water depth, while the scaling variables for the high-frequency region are presented in terms of the shear velocity and nominal diameter of the filter material. By combining both spectra, converting the pressure spectrum to the gradient spectrum, and transforming the spectral domain to the spatial record, the fluctuating gradient can be determined. The hypothesis was verified using measured data from experiments conducted by Van de Sande (2012) and Wolters & Van Gent (2012). By incorporating Van de Sande's measured dataset into the scaling parameters of the dimensionless pressure spectra and determining the stability parameter for open filters, it was concluded that qualitatively the results align with Van de Sande's findings, indicating stability or instability of the filter layer. However, in some test cases, the hypothesis suggested instability while the experiments indicated stability. This discrepancy may be due to the fact that while the stability parameter may indicate an unstable filter, it does not necessarily correlate with significant base material transport through the filter layer. In a quantitative analysis, the amount of dimensionless transport of base material was compared to the stability parameter. According to the data presented in figure 4.12, it can be observed that the stability parameter exhibits a value that is three times larger then the stability parameter defined by Schiereck (2012), while not inducing any significant transport of base material. This finding aligns with previous experimental investigations, such as the study conducted by Wolters & Van Gent (2012), which similarly demonstrated that the stability parameter could reach values that are two to three times larger, while still not resulting in notable base material transport. A direct quantitative comparison was conducted between the measured dataset of Wolters & Van Gent (2012) and the calculated gradients obtained from the hypothesis. The results revealed that the calculated gradients exhibited a comparable magnitude, ranging from 0.25 smaller to 1.50 larger in comparison to the measured gradients reported by Wolters & Van Gent (2012). This indicates a reasonably close agreement between the calculated and measured gradients. In conclusion, the research objective in this study was to formulate a design method for open geotextiles under a single granular filter layer. To achieve this, it was necessary to take some steps back and define the actual gradients, as well as identifying a suitable method for calculating these gradients. The actual gradient is calculated with the sum of the average gradient and the fluctuating gradient. The fluctuating gradient can be characterized by the turbulent wall pressures, which are derived from the pressure spectrum measured at the top of the filter layer, as described in Blake (1970). Assuming linear damping of the pressures within the filter layer, the actual gradient can be computed at the interface of the geotextile. Consequently, the assessment of stability entails a comparison between this calculated actual gradient and the critical gradient measured by Lemmens (1996). With a clearer understanding of how to calculate the actual gradients at the geotextile interface, the same approach employed in the hypothesis is applied to address the design of a stable open geotextile under a single granular filter layer. This has been demonstrated through a case study, where the thickness of the filter layer was determined to create a stable filter structure. Several recommendations can be made to improve the understanding and design methodologies in relation to the hypothesis. Firstly, there is a need for a better understanding of the spectral density of wall pressure fluctuations in open channel flow. This understanding will enable more accurate estimations of fluctuating gradients. Secondly, further research is necessary to incorporate the distribution of pressure at the filter/base interface for non-linear damping. Thirdly, it is advised to conduct tests using modern natural geotextile materials to refine the critical gradient determination and explore alternative approaches, such as considering critical filter velocities. Lastly, it is recommended to extend the design method to encompass wave conditions. Verifying the effectiveness of the design approach under wave conditions through comparison with measured data will ensure its applicability and reliability in practical scenarios. While further investigations are needed, it still offers a fresh perspective on evaluating the stability of open filter structures. If successfully refined, the design method could become a valuable tool in geotextile design and potentially find applications in other contexts involving "open" structures.