Study on Sediment Transport Mechanics in Shallow-Grade Storm Drain System

In the Texas coastal plain shallow-grade drain system, it may not be possible to maintain the required minimum self-cleaning velocity 3 ft/s at all points in time. It leads to sediment deposition and reduction of inflow capacity with time in culverts or storm drain pipe, and an associated high cost of cleaning and highly potential flooding risk. Recent studies shows that the sediment load from runoff and other sources into the drain system is the key parameter to determine the sediment transport mechanics in the drain system. To design a self-cleaning storm drain system, the sediment transported by runoff and the captured flow through the drain system structures needs take into account. The objective of the study is to better understanding the sediment transport mechanic in shallow-grade storm drain system, to evaluate the sediment concentration in the drain system under different hydrological, road, inlet and drain structure conditions, and to assess whether the minimum velocity is appropriate to prevent the sedimentation in shallow-grade drain system of the Texas coastal plain. An extensive literature review synthesizes a suite of empirical equations for calculating sediment concentration in the culvert/drain pipe developed by the experimental studies. The sediment concentration in drain system is determined by sediment characteristics culvert/pipe characteristics such as size, roughness and hydraulic parameters including water depth and flow velocity. The sedimentation in the drain system and runoff have been collected from Southeast Texas shallow grade area to conduct the sediment characteristics analysis. The sediment samples from pipe sump, culvert and ditch have the median grain size medians for of 0.225mm, 0.16mm and 0.335mm, and the specific gravity of 2.52, 2.62 and 2.55, respectively. The samples are considered to possess cohesive properties with more than 10% grain size less than 0.075mm. The runoff samples at the storm drain inlet for nine rain events have been collected and analyzed. The median of sediment is 0.032mm and 78.3% of the sample grain size is less than 0.063mm. The sediment concentration in the runoff samples varies from 52.2 ppm to 360.7 ppm. The hydraulic and sediment transport analysis indicates that the maximum required self-cleaning velocity to transport the deposited sediment is 10.04 ft/s for the culvert and 4.72 ft/s for the drain pipe. To quantify the sediment transport rate by the runoff, a diffusion wave model for predicting water depth and flow velocity is developed for the different road and hydrologic conditions. The model predicted the water depth is between 0.08mm and 3mm, and the velocity is between 0.02m/s and 0.23m/s. The maximum sediment transport capacity by the runoff is determined using the flow velocity distribution on the road and the grain size characteristics of the runoff by applying sediment transport equation in open channel flow. The maximum predicted sediment concentration in runoff varies from 173 ppm to 5209 ppm under different hydrological and road conditions. The regression analysis indicates the sediment concentration in runoff is a function of the rainfall intensity, the longitudinal and the cross slope. The required self-cleaning velocity is less than 2.85ft/s in the pipe to transport the sediment being carried in the runoff, which is much less than the required velocity to carry the deposited sediment in the drain system. It indicts the sediment in the drain system is not only from the runoff, but also from the other sources, which play the main role to determine the self-cleaning flow velocity. With the high cohesive sediment, the sediment transport mechanics in the coastal plain drain system includes sediment erosion, suspended load and bed load. The suspended load is the dominant sediment transport mechanics. Designing a self-cleaning culvert requires the velocity close to the maximum design velocity, which is not practical. The recommendation is to consider a culvert screen or gully pot at the entrance to filter or collect the grain size greater than 1mm. The hydraulic and sediment transport analysis on the culvert and the drain pipe indicates the self-cleaning velocity is a function of the sediment grain size, grain percentage, specific weight, deposited sediment width, water depth, and drain system size. Therefore, the single minimum flow velocity is not appropriate to prevent the sediment deposition in the drain system of the Texas coastal plain.