| Summary: | 博士 === 國立中央大學 === 土木工程研究所 === 89 === The urban storm sewer design model proposed in this dissertation analyzes conduits and detention ponds. The approach in analyzing conduits is developed from the Rational Formula incorporating design hyetograph. Given a rainfall event of design hyetograph falling in all subcatchment, the unit hydrograph of each subcatchment evaluated by the Rational Formula is applied to design the initial conduit. The downstream discharge hydrograph of conduit is synthesized by the mean of the lagging time to be used in designing the downstream conduit. For a case study, the peak flow in conduit are the largest, the middle and smallest, for the central peak type, the later peak type and the early peak type of rainfall pattern, respectively. It is found that the maximum peak flow in conduit occurs in the case that the duration of the design hyetograph being three times of the unit rainfall interval.
Due to the lack of actual discharge measurements, the result calculated by the SWMM model for several regions in Taiwan are employed to verify the confidence of the proposed model. The difference between the water depth, as well as discharge, calculated from the proposed model and that obtained from the SWMM model is insignificant.
This model, incorporating the Rational Formula and design hyetograph overcomes the defect of the Rational Formula alone. The Rational Formula provides only the peak flow, while this model provides the complete discharge hydrograph. Not only it can be applied to design the conduit of sewer, but also to design the detention pond. Furthermore, this model, requires the same traditional fundamental data, is convenient to be applied for a system of trunks and branches of sewer conduits.
The second part of this dissertation discusses the dimensionless approaches in the model of detention pond sign. Currently three methods, namely, the Simple method, the Reservoir method and the Progress method, are used to evaluate the volume of the detention pond. The Simple method is mandated by the authorities in Taiwan. The triangular hydrograph of inflow and outflow, are assumed in the Simple method. In the Simple method, the longer the base time of inflow hydrograph is, the larger the volume of detention pond is required. As for the Reservoir method, a dimensionless arbitrary shape inflow hydrograph can be applied. The Reservoir method is routed by a set of dimensionless equations based on hydrologic balance. In the case when the shape of inflow hydrograph is triangular, the volume of detention pond is subject to a characteristic value, α (α= tp / tb, tp being the peak reaching time of inflow hydrograph , and tb being the base time.). The larger value of α is, the greater the volume of detention pond is required.
Furthermore, the Simple method is analyzed by the dimensionless theorem. For a triangular shape of dimensionless inflow hydrograph, the result shows that the volume of detention pond evaluated by the Simple method is larger than that by the Reservoir method. It is found that when the characteristic value α is between 1/6 to 5/6, the volume of detention pond evaluated by the Simple Method is 1.32 to1.02 times that evaluated by the Reservoir method. Moreover, the Progress method is routed by a set of hydrologic balance equations, and the inflow hydrograph is derived from design hyetograph. So the Progress method cannot be analyzed by the dimensionless theorem. For a case study of the Progress method itself, the result shows that the later the peak rainfall time of design hyetograph is, the greater the volume of detention pond is required. In other words, the volume of detention pond is the largest, the middle and smallest, for the later peak type, the central peak type and the early peak type, respectively. As the duration of design hyetograph exceeds one hour, the increasing volume of detention pond becomes insignificant.
The final part of this dissertation proposes the concept and the design approach of a double detention pond. The structure of double detention pond is to add a separate pond within the traditional detention pond and install a set of one-way gates. The puny pond can release water quickly at the early stage during the inflow. The volume of the puny detention pond is evaluated by the equations of hydrologic balance, and the volume of huge detention pond is evaluated by computing the overflowing water from the puny pond. According to dimensionless analysis, for the triangular inflow hydrograph, the volume of double detention pond is subject to the characteristic value α and the peak reducing value Q*( Q*= qp/ip, qp being the peak of outflow hydrograph, ip being the peak of inflow hydrograph). The larger the value of α is, the more the saving volume of double detention pond is. Moreover, the larger the value of Q* is, the more the saving volume of double detention pond is. A case study shows under 1-hr duration of design hyetograph, the volume of traditional detention pond is the largest, the middle and the smallest, evaluated by the later peak type, the central peak type and the early peak type of rainfall pattern, respectively. Furthermore, the saving volume of double detention pond is 47.3%, 51.0% and 57.5%, for the early peak type, the central peak type and the later peak type of rainfall pattern, respectively. Similar result is also identified for the case of a triangular inflow hydrograph.
Key words: Rational Formula, storm sewer, double detention pond, design hyetograph
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