Synthesis of TxDOT Storm Drain Design
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2003-10-01
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Alternative Title:Synthesis of Storm Drain Design: Current Methodologies and Need for Alternatives to the Rational Method [Project Title from Cover]
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Abstract:The Texas Department of Transportation (TxDOT) currently uses the Rational method for development of design peak flow rates for its highway storm drainage design. Watersheds for which it is used have drainage areas less than 200 acres. The Rational method is an "instantaneous" peak discharge method that is popular due to its simplicity. The Rational method assumes a linear relation between rainfall rate for the time of concentration of the watershed and peak instantaneous discharge. The drawback of the Rational method, however, is that the time distribution and accumulation of flow cannot be precisely accounted for through each node (inlet) and run (conduit) of the system. Instead, the accumulated effects of all contributing sub-basins and branches are assumed to be “lumped” into a single equivalent basin when designing or analyzing each successive node or run. Use of peak discharge values also limits the hydraulic design and analysis of the system to the assumption of simple steady-state flow conditions. While this may result in a simple design process, the inability to consider unsteady flow and the inherent storage available in these systems may result in the missed opportunity to develop more cost-effective designs. Simple steady-state flow assumptions may also be inadequate to address the complex hydraulics that could be associated with the need to include non-traditional hydraulic features, such as in-line water quality basins. Therefore, the proposed study is intended to be the first phase in evaluating TxDOT procedures for storm drain design, not only in terms of the adequacy of current TxDOT practice relative to new directions in the field, but also in anticipation of the need to evaluate more complex features that might be required by changes in water quality regulations. The study is accomplished by completing two tasks: (1) a literature review to synthesize both the technical approach (Rational method versus other hydrological methods) and modeling efforts of drainage networks with various computer software packages, and (2) use of modeling tests on simple cases to examine storm drainage design.
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