Two goals are followed in this research. The first one is develop a computer code to solve the navier- stokes equations in new approach. Other one is design a new model of manholes to dissipate flow energy with a stepped spillway core. To reach the first goal used finite difference method and try to solve flow equations fully implicit in an equation system. After analyze of bad results and some of tries, found that boundary conditions make problem if in the one node we have mixed boundary conditions. To solve this problem virtual nodes used and boundary condition applied with some difference. In this new method flow equations solved faster with more stability and convergence. Virtual compressibility is not required if this new method is used. Several coette flow samples are solved by new method. Results of solutions of this method compared with exact solutions to check this method. New method shows that has some benefits in compare of Drops are most important structure in urban runoff system. When water level decrease in lower elevation, mostly has dangerous velocity for down stream structures. In this research tried to design new models of manholes with stepped spillway approach that dissipate flow energy. With primary simulation of USBR suggestions, a new model of manhole with stepped spillway approach designed. It has include a pipe outlet structure as first part, then a stepped spillway and at last a compound of stilling basin and transform. All of design parameters effect must be distinguish to make manhole efficient and smaller. To find out effect of each parameter large number of manholes simulated. Models that satisfied conditions used for regression. At last efficiency of it can describe with exponential equation and can be design easily.