3　Model Descriptions

The drainage rainfall-runoff model is established by combining the hydrologic model,the pump and sluice model,the hydrodynamic model.In this study,a simple hydrologic model is used based on the dividing of sub-district.The pump and sluice model is employed to simulate the outflow of each sluice or pump,and the outflow is coupled as inflow to the hydrodynamic model.

3.1　District division and hydrologic model

It is effective to improve the accuracy of hydrologic simulation by dividing the study area into sub-districts.The four main factors for this dividing are the similar surface condition,bounded by rivers or canals,control area of second-level sluices or pump stations,and the accomplished district by limited drainage area stations.However,the joint control area of some second-level sluices and corresponding pump stations is varied according to the water level around,So it is not easy to accurately divide the joint control drainage area of each couple.After investigation,due to the electricity bills of drainage,the control area of the second-level pump station is relatively clear,it can be accurate to the administrative village,and considering the model is most applied for flood years when second-level sluice hardly has the chance to drainage,so the joint control area of the couple second-level sluice and pump station is the same as that of the second-level pump station.According to the above principles,the entire Diaocha lake area can be divided into 73 sub-districts.

According to the characteristic of rainfall-runoff of lower plain area:the total rainfall is the deciding factor to runoff depth and the process of runoff is flat,a simple hydrologic model is used:

Where,R is the runoff in sub-districts,P is daily precipitation,Ia is initial abstraction,La is later abstraction,and b0 is the reduction coefficient of initial abstraction.The value of Ia and b0 is analyzed through the test data collected from the Hanchuan runoff test field and La is equal to the evaporation by E601.

3.2　Pump model and Sluice model

3.2.1　Pump model

Second-level pump stations usually belong to township or village.There is no fixed operation and scheduling rules,in the principle of maximizing their own interests,township or village usually fully load the pump station no sooner than the rain starting,so there is nearly no surface water soon after the rain stops.Therefore,the outflow of the second-level pump station is a function of the drainage capacity and the remaining waterlogging.Drainage capacity can be approximated by the design flow of the pump station.Owing to the small control drainage area of each station and the rapid confluence velocity in crisscrossed ditch,the remaining waterlogging depends mainly on the runoff depth yielded.The outflow of the second-level pump station in the Spump model can be expressed by a bilinear function:

Where,Qpump is the outflow of second-level pump station(m3/s),V is the remaining waterlogging of sub-district drainage area(m3),Qmaxpump is Maximum drainage capacity(m3/s),V0 is the minimum remaining waterlogging required for the pump station to enter full load operation(m3),b1 is a parameter(The value is equal to the slope which can be analyzed from measured data)(1/s).The remaining waterlogging in the drainage area is simulated by a linear reservoir.

3.2.2　Sluice model

Most of the upstream part in the Diaocha Lake area is self-draining area,and the outflow of the self-draining area is controlled by the water level of the downstream main canal.It is mainly a function of the scale of the second-level sluice and the water level under sluice.Part of the sluice measured data show that its process line approximation shown in Fig.2 right,the sluice simulation model can be expressed as:

Where,Qsluice is the outflow of second-level sluice(m3/s),Qmaxsluice is the maximum drainage capacity(m3/s),Z is the water level under sluice(m),and Z0 is a parameter(critical water level above which the outflow will be limited),b2 is a parameter(Can be obtained from the measured data analysis).

3.3　Hydrodynamic model of Drainage Network

The Saint-Vernan equations[3]is used to simulate the water level and flow in the main canal,including the Xiatianmen River,the Nanzhi river,the Beizhi river,the south canal,the north canal,the east canal,the west canal,the Minle canal,the pump station river,the Fenshui canal and the sluice canal.

Treat the storage area and the spare storage area as the intersection with storage and the intersection of canals as the intersection without storage,continuity equation and momentum conservation equation is used to simulate the water level and flow of the intersection.

Treat the main control sluice,such as the Hukou sluice,the north floodgate,the south floodgete and the east floodgete,as inner-boundary node,the hydraulic formula of sluice is used to simulate the flow of those sluice.

The upper boundary condition is flow,the value is the outflow of the Tianmen flood control sluice.The lower boundary condition is the relationship between water level and flow of the first-level pump station or sluice.

3.4　Model Coupling

Using the hydrologic model to simulate the runoff that will be taken as the inflow of pump model or sluice model in the sub-district,and the outflow of pump model or sluice model is coupled to the hydrodynamic model by the form of external inflow.According to the drainage network and the status of second-level sluice or pump stations in sub-district area,the outflow is generalized into a point inflow or distributed inflow.Specific diagram of canal inflow is shown in the Fig.4.