Impact of urbanization on drainage modulus in tidal river networks

Abstract: Drainage modulus is the rate of water removal per unit of area used in drainage design. Reasonable drainage modulus is of great importance for waterlogging control. With the urbanization in tidal river networks，more and more impermeable areas are gradually replacing original permeable areas, thus changing the rainfall-runoff process of drainage areas. Different from rural areas, storm water in urban areas is collected by drainage pipe network，routing storm water runoff directly to main pipes or channels, causing less infiltration, but more overland runoff. Urbanization, with the loss of vegetation and replacement of soil with impervious surfaces, makes traditional computing methods of drainage modulus inadequate for urbanized areas. While inland river stage is affected by tidal backwater in tidal river networks, waterlogging risks increase. Based on the characteristics of urbanized drainage areas in tidal river networks, a two-layer computing model of drainage modulus was built. In the model, the catchment of first layer was divided into two sub-drainage areas, urban and rural, which adopted SWMM and Guangzhou comprehensive unit hydrograph method to establish rainfall-runoff models, respectively. On the second layer, based on their shared drainage channels (rivers), a river networks dynamic model was established by MIKE11 HD. Lateral inflows of rainfall runoff were derived from the simulated data of first layer, coupled with MIKE11 HD. Using the Dagang drainage area as a case study, the impact of urbanization on drainage modulus was studied through this two-layer computing model of drainage modulus. Drainage modulus under different designed rainstorm in 2003 and in 2012 was calculated. The relationship between drainage modulus and return periods under different underlying surface condition in 2003 and 2012 was analyzed. Results showed that due to the increasing ratio of urban area from 9.69% to 24.25% from year 2003 to 2012, drainage modulus under design standards with return period of 20 years increased from 1.766 m3/(s·km2) to 2.808 m3/(s·km2), drainage modulus under design standards with return period of 30 years increased from 2.909 m3/(s·km2) to 4.197 m3/(s·km2), which showed that the resistibility of waterlogging was weakening. When drainage modulus reached the 1.766 m3/(s·km2), the drainage area can meet design standards with return periods of 20 years in 2003, but it can only satisfy design standards with return periods of about 15 years in 2012. When drainage modulus reached the 2.909 m3/(s·km2), the drainage area can meet design standards with return periods of 30 years in 2003, but it was not able to satisfy design standards with return periods of about 20 years in 2012, which showed that waterlogging control standards of the study area was reduced in the process of urbanization. The result of this study may provide a reference for computing drainage modulus and help determine a reasonable value of drainage modulus for drainage engineering design in tidal river networks of urbanized areas.