In the process of the hydraulic design of culvert, the computation of flood discharge is most important and is necessary to compute probable rainfall intensity. The probable rainfall diagram proposed by the KICT(Korea Institute of Construction Technology) consists of 49 pieces according to rainfall duration and return period. But the use of the probable rainfall diagram can generate considerable error due to designer's personal misjudgement and has an inconvenience for its use in the computation. Therefore in the present study, developed is the general equation of probable rainfall intensity which is a function of regional coefficient, return period and rainfall duration.
Usually the drainage pipe has the characteristics of open channel flow, but when the event rainfall occurs over the design rainfall, it normally has the characteristics of pressure pipe flow. Therefore in this study, the part of friction flow characteristics of drainage pipes is examined as an open channel flow, and throughout the experimentation we will develop new friction coefficient equation of drainage pipe.
In order to analyze the flow in drainage pipes, we have to compute some water levels, typically critical depth, normal depth, and water level at outlet of pipes.
Critical depth is defined as the depth when Froude number is 1 and specific energy is the minimum. The determination of critical depth is very important in the computation of open channel flow because it determines the direction of flow influence and the method of analysis. Critical depth can be directly computed for triangular or rectangular channel by using the traditional method, but for trapezoidal or semi-circular channel the solution has to be obtained by using the method of trial and error. Even though it can be solved, the process is very complex and requires long iteration. In the present study, proposed are several non-dimensional numbers related with the critical depth, and after obtaining the solutions of the equation by transforming autonomous variables and to subordination variables. And developed are the explicit equations; second order polynomials and third order polynomials for circular channel, and third order polynomial expression and power law for trapezoidal channel by checking the distribution tendency of the solutions.
Computation of normal depth is also very important in the design of the channel or the analysis of the water flow. The drainage pipe has generally the curvature like circular or U-type pipe. In this case, the derivation of solution equation is very difficult because the change of hydraulic radius and area versus depth is irregular. However if the ratio of the area and the diameter, or the hydraulic radius and the diameter of pipe is expressed by power law, the process of equation development becomes very simple, and the explicit equation can be obtained. Therefore, developed are the explicit normal depth equations for circular and U-type pipe, and the normal depth equations are also derived from Hagen(Manning) equation because of its wide usage in the business.
In order to analyze unsteady flow, we have to know at least one known water level at control point. In the case of supercritical flow, the control point is the inlet water level, and in the case of subcritical flow the control point, is the outlet level. But mostly the slope of drainage pipe is mild, and the flow of pipes is subcritical, but rarely supercritical. Therefore in order to analyze the unsteady flow in drainage pipe, we have to compute the outlet water level as accurate as possible.
In the method of culvert design some errors are found and corrected which are in the derivation of equation by the whole investigation. And proposed is the computation method of velocity and friction loss factor which are not clear enough in its concept. Besides developed are good efficacious logic when developing the culvert design program.