Assessing Performance Characteristics of Sediment Basins Constructed in Franklin County, Alabama
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The objective of the research project was to monitor the performance of newly designed sediment basins that were constructed on the ALDOT 502 project in Franklin County. The project included four tasks: (1) assess performance characteristics of sediment basins on the 502 project, (2) collect cost data and perform a literature review, (3) perform a survey of the current state-of-the-practice, and (4) prepare project reports. All tasks proposed have been completed. Through completing the study, the following conclusions have been developed: • A field-scale data collection plan to monitor and evaluate sediment basin performance was developed and implemented using ISCO 6712 portable automatic stormwater samplers, flow modules, a rain gauge, and weirs. • Sediment basin 4 on the 502 project did effectively remove sediments at the early stage of the construction when the basin’s influent most likely contained relative large percent of large-size sediment particles. For example, sediment basin 4 removed 97.9% and 83.7% of sediments generated by rainfall events on 11/16/2011 and 12/5/2011. • A floating skimmer allowed for effluent to be discharged uniformly and slowly, providing longer detention time for sediments to settle in the basin. Data analyses on decay (reduction) coefficients for total suspended solids (TSS) and turbidity allowed us to quantify the sediment-settling rate of soils on the 502 project in Franklin County, AL. • Appropriate PAM (or floc log) added into inflow is crucial to aid sediment settling and reduce turbidity of effluent. For example, the performance of basin 4 was superior for the rainfall event on 11/16/2011 when correct PAM was used in the inflow channel than the performance for the rainfall event on 12/5/2011 when the wrong PAM was used. • Rainfall events with subsequent high rainfall intensity impulses generated high turbidity inflows from the construction site and suddenly increased in-basin turbidity that could be several times higher than turbidity of water already in the basin. • Resuspension of settled sediments significantly increased in-basin sediment concentration and turbidity when the basin has experienced a number of rainfall events with large amount of settled sediments inside basin. • An under-designed sediment basin (from a volumetric standpoint) more frequently allowed highly turbid sediment-laden runoff to directly flow over the emergency spillway to the downstream receiving water body. Based upon the results of the data collected and observed site conditions throughout the research period, the following recommendations are provided to ALDOT to improve sediment basin design and installation to maximize performance efficiency and cost effectiveness: • Use at least 3,600 cubic feet per acre draining to the basin from the contributing area to size the sediment basin. • Increase the number of PAM floc logs placed at the bottom of inflow channel to properly dose for the average flow rate of 2-yr 24-hr runoff. The number of floc logs should be based on the manufacturer recommended dosage and the expected inflow rate of stormwater runoff. • Consider increasing the number of floc logs placed on the sides of inflow channel to dose for the average flow rate of 2-yr or 10-yr 24-hr runoff. These storms will have higher water depths, resulting in a greater amount of inflow, therefore requiring a higher dosage of PAM. • The height of the baffles, once installed, should match the full depth of the sediment basin, including the additional freeboard depth used by the emergency spillway.