Investigation on the Water Depth of Choked Flow due to Bottom Blockages in Circular Open Channels

Blockage detection in circular open channels (partially filled pipes, e.g., gravity sewer pipes) has gained increasing interest in the water industry. While there have been significant advancements in monitoring technologies for such systems (e.g., smart sewer systems), there is a lack of quantification of the hydraulic impact of partial blockages in circular pipes, which is a limiting factor for cost-effective detection. This study presents and validates a numerical framework to quantify the changes in water depth caused by extended bottom blockages in circular open channels. The analysis is based primarily on continuity and energy conservation with the assumption of a lossless transition at the blockage. The proposed approach can determine the critical blockage height that would induce choking, therefore a change in the upstream depth. It can also calculate the changed upstream water depth for various bottom blockage height values (above the critical height) for both subcritical and supercritical conditions. The proposed approach is numerically implemented and then validated through a series of laboratory experiments under the subcritical flow condition (which is the most common condition for gravity sewer systems). The numerically determined water depth results are consistent with the experimental results, which confirms that the proposed approach can accurately estimate the new upstream depth under the choking condition induced by a bottom blockage in circular open channels. The results contribute to advancing the hydraulic understanding in circular open channels with extended partial blockages, which is useful in the development of smart sewer technologies.

Spills and overflows out of the gravity sewer system damage the environment and bring risks to public health. The problem is typically caused by partial blockages, which can reduce the flow capacity of the sewer pipe and induce higher-than-designed flow depth on the upstream of the blockage (known as choked flow condition). This study helps water engineers and operators to understand how an extended bottom blockage affects the flow condition just upstream of the blockage in a circular gravity sewer pipe. Charts have been presented to enable the determination of (1) the critical height of an extended bottom blockage that would induce choking; (2) the expected upstream flow depth for blockages with various heights, with a given flow rate under choked flow condition; and (3) the expected upstream flow depth for various flow rates, with a given blockage height under choked flow condition. The results can be used to evaluate the risk of bottom blockage-induced spill for sewer pipes with various sizes, slopes and flow rates (e.g., some may be more tolerant to bottom blockages while some may be more sensitive). This information can then be used to prioritize preventative measures such as sewer depth monitoring or regular cleaning.