Introduction
Determining legal riverbed boundaries in low-gradient floodplain environments is a persistent challenge, particularly in regions where traditional delineation relies on administrative or geomorphological approximations rather than hydraulic criteria. The Shahrstan Stream, a natural distributary of the Jajrood River in Tehran Province, exemplifies such a case. The Eastern Regional Water Authority designated a 600-meter-wide legal bed for the Shahrstan Stream near Abasabad village, an estimate that has sparked controversy due to its excessive breadth and apparent disconnect with hydraulic realities. To address this discrepancy, the current study investigates the hydraulic validity of various bed width scenarios for the Shahrstan Stream using a simple but robust analytical method. Instead of relying on conventional width estimation, the study adopts a performance-based approach focused on hydraulic behavior, specifically the ability of each width to sustain flood flows with sufficient flow energy. The Australian flood energy criterion (velocity × depth ≥ 0.3 m²/s) is used as the threshold for identifying active flood-conveying sections. The objective is to develop a scientifically grounded, technically defensible estimate for the legal riverbed width of a disturbed floodplain stream, an approach that promotes more accurate and replicable outcomes in future river management decisions.
Methodology
This study applies Manning’s hydraulic equation to evaluate the flood-carrying capacity of the Natural Shahrstan Stream under five proposed riverbed widths: 100, 200, 300, 400, and 600 meters. Given the extensive morphological changes and insufficient elevation precision in the study area, two-dimensional modeling was deemed impractical. Instead, a simplified analytical approach was adopted to assess stream hydraulics. The 25-year design flood discharge of the Jajrood River was estimated at 389 m³/s. Based on satellite imagery and relative inundation areas, this value was proportionally reduced to approximately 39 m³/s for the Shahrstan Stream. SPOT satellite data from 1986 and legal documentation from the Iran Water Resources Management Company and Sazeh Pardazi Iran Consulting Engineers supported the spatial and hydrological assessment. For each width scenario, Manning’s formula was used to calculate average flow velocity, depth, and discharge. The hydraulic validity of each scenario was evaluated against the Australian standard flood energy threshold (velocity × depth ≥ 0.3 m²/s), a widely accepted criterion for determining active flood channels. The approach emphasizes practicality and replicability in similar floodplain contexts lacking high-resolution data. It integrates both hydrological theory and regulatory frameworks to yield a rational estimate for the legal riverbed boundary, as stipulated by Bulletin No. 307.
Results and Discussion
The hydraulic analysis demonstrated that the 600-meter bed width, as designated by the water authority, is a substantial overestimate. At a flow depth of 1 meter, this width would produce a discharge of approximately 1330 m³/s—more than 34 times the estimated 25-year design discharge of 39 m³/s. Adjusting the flow depth to align with the target discharge requires a depth of just 12 cm, which yields a velocity–depth product of 0.065 m²/s. This value falls far below the minimum energy criterion, indicating insufficient hydraulic function. The intermediate widths—400, 300, and 200 meters—likewise failed to meet both the design discharge and the minimum energy threshold. Although deeper flows improved energy levels, they also produced excessive discharges inconsistent with the natural capacity of the stream, and shallower flows remained under-energized. In contrast, the 100-meter width demonstrated ideal hydraulic behavior. At a depth of 0.3 meters, the stream reached a velocity of 0.99 m/s and a discharge of 29.76 m³/s. This configuration achieved the target velocity–depth threshold of 0.3 m²/s while approaching the design discharge, offering a balanced and defensible estimate of the stream's legal riverbed. Higher flow depths further confirmed the flexibility of the 100-meter configuration, yielding discharges up to 219 m³/s, without violating hydraulic constraints. These findings align with international practices in floodplain delineation and emphasize the importance of integrating flow energy criteria into legal and technical assessments. Unlike the traditional approach based solely on width extrapolation, this method offers a rational and replicable solution for disturbed floodplain systems. It also validates the use of simplified hydraulic models in settings where complex simulation is not feasible but regulatory compliance is essential.
Conclusion
This study confirms that the 100-meter riverbed width is the only configuration that satisfies both the hydraulic energy threshold (velocity × depth ≥ 0.3 m²/s) and the 25-year design discharge (~39 m³/s). Wider widths (200 to 600 meters) either generate unrealistically high discharges or insufficient flow energy, making them incompatible with the hydraulic behavior of the Shahrstan Stream. Therefore, the 100-meter width stands out as the most technically and legally appropriate boundary, offering a model approach for future delineation efforts under Bulletin No. 307 in similar disturbed floodplain environments.