Assessing performance characteristics of sediment basins constructed in Franklin County.
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Assessing performance characteristics of sediment basins constructed in Franklin County.

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  • Abstract:
    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 the 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 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

    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 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


     The height of the baffles, once installed, should match the full depth of the sediment basin

    and not be installed below the minimum elevation of the emergency spillway.

     Include a sediment storage volume (e.g., 500 ft3/acre disturbed) into the design

    specifications of sediment basins and a requirement to remove the sediment when it reaches

    one third of the height of the sediment storage volume.

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