Evaluation of Best Management Practices for Highway Runoff Control (2006) / Chapter Skim
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Pages 84-105
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From page 84... ...
84 10.1 Design Guidance Apart from the LID Design Manual, specific BMP design guidance is not an objective of this project since good help is available, as listed below. However, BMP/LID selection inherently involves hydrologic and hydraulic considerations, discussed in this section.
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85 If a storage basin is configured to detain large storms, with large outlet capacity, small storms will essentially pass through unmodified. However, control of small storms is one of the cornerstones of water-quality control.
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86 Note: WQCV = water quality control volume. See source documentation for details and tables referenced in the figure.
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87 Peak-flow reduction may also be accomplished upstream in the watershed through "hydrologic source control" mechanisms (including components of LID) that reduce the volume of the runoff hydrograph and delay the peak through distributed storage.
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88 3. Designing a storage-release system that discharges postproject runoff that matches the preproject flow-duration characteristics.
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89 common today, flow-duration design criteria are becoming more popular with regulatory agencies. For example, King County, Washington, has a flow-control criterion requiring postdevelopment flow durations to match predevelopment flow durations for 50% of the 2-yr through 50-yr peaks (King County 1998)
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90 (Bedient and Huber 2002; King County 1998)
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91 shows event depths and Figure 10-8 shows event average intensities. Depths and intensities corresponding to specified frequencies may be obtained from such graphs and used in the standard hydrologic techniques previously described.
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92 hydrographs. For example, the 90% runoff depth and normalized maximum flow for the actual 33% imperviousness of the site is about 0.4 in./hr and 0.3 in./hr, respectively.
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93 0 1 2 3 4 5 6 7 8 9 10 0 10 20 30 40 50 60 70 80 90 100 Percent (less than or equal to)
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94 more robust method. Runoff volume or peak is made dimensionless by dividing by the mean storm event magnitude.
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95 Source: Heaney et al.
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96 site in Figure 10-7.
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97 0 2 4 6 8 10 12 14 16 18 20 80 85 90 95 100 Percent Annual Runoff Volume Captured Pe ak F lo w (in /hr ) Lisbon, FL The Plains, VA Block Island, RI Lost Hills, CA Note: Top curve is Lisbon, FL, 2nd curve is The Plains, VA, 3rd curve is Block Island, RI, and bottom curve is Lost Hills, CA.
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98 Source: Driscoll et al.
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99 Continuous simulations provide more than just a depth or flow rate that corresponds to a certain frequency. The output (hydrographs and pollutographs)
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100 results)
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101 includes federally supported models such as (Singh 1995; Singh and Frevert 2006) : • EPA Storm Water Management Model (www.epa.gov/ ednnrmrl/models/swmm/index.htm)
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102 Continuous hydrologic model input data typically include the following: Rainfall input. NCDC long-term precipitation data are available in four formats: • 15-min, 0.01-in.
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103 less accurate because the trade-off between maximizing volume captured and maximizing load captured is not explicitly included. The volume captured can be maximized by shortening the detention time, but waterquality control will be improved if detention time is increased.
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104 the final cost-effectiveness curve that shows total costs as a function of the percent pollutant removal (see Figure 10-25)
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105 • Allowing for an increase or decrease in the quantity of vegetation in the system as well as varying vegetation management strategies, such as altering vegetation type or species to achieve water-quality benefits. (Different plant species have varying pollutant-removal capacities and tolerances.
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