Discontinuous Shallow Flow A Numerical Method for Discontinuous Shallow Flow Fritz R. Fiedler University of Idaho Department of Civil Engineering
Model Objectives Solve 2-D hydrodynamic flow equations surface flow, rainfall, infiltration stiff hyperbolic equations non-linear source term Applications rainfall-runoff process wetlands flood plains
Overland Flow
Microtopography
Equations
Numerical Challenges Non-linear hyperbolic system Strong source terms (equations stiff when h~0) Small depths / dry areas Large gradients Discontinuous flow regime
Vector Form
Vector Form
Approach Select basic numerical scheme Modify basic scheme to address problem-specific challenges Develop algorithm Develop code Test Iterate? (start simple)
MacCormack Scheme Predictor, Backward Difference Corrector, Forward Difference
Split MacCormack Scheme Lx1 Operator:
Friction Slope: stiff!
Friction Slope: Point-Implicit Treatment
Convective Acceleration Upwinding For the Lx1 operator: If flow is in the -x direction (j+1 to j)
Smoothing Function
Lateral Inflow and Infiltration
Algorithm Input Define grid Initialize Solve time loop compute lateral inflow (Newton’s Method) compute h, p, q output?
Computer Code do 102 j=1,Nx-1 delh1(j,k) = - dtohx*(pc(j,k)-pc(j-1,k))+dt*0.5*re(j,k) delp1(j,k) = - D(j,k)*dtohx* & (convacc(j,k) + g*hc(j,k)**2 - g*hc(j-1,k)**2) & - D(j,k)*dt * & ( g*(hc(j,k)+hc(j-1,k))*(z(j,k)-z(j-1,k))/hx & +Ko*0.01*pc(j,k)/8./hc(j,k)**2 & +pc(j,k)/hc(j,k)*re(j,k) ) & + D(j,k)*dt/hx**2*eps1*(pc(j-1,k)-2.*pc(j,k)+pc(j+1,k)) delq1(j,k) = - dtohx* & ( pc(j,k)*qc(j,k)/hc(j,k) & -pc(j-1,k)*qc(j-1,k)/hc(j-1,k) ) & + dt/hx**2*eps1*(qc(j-1,k)-2.*qc(j,k)+qc(j+1,k)) 102 continue
Comparative Numerical Examples Dam Break Problem Published results Physical Model Results
Dam Break Problem
Kinematic Wave Solution
Microtopographic Surface
Overland Flow Depths
Flow Depths and Velocity
Flow Channels
Overland Flow Depths
Cumulative Infiltration
Simulated vs. Experimental