# Friction in saint-venant : Torrential test case with Manning

In this example, we reproduce the MacDonald benchmark[1]. We test the friction source term in a torrential flow. The leading equations can be written as :

\displaystyle \begin{array}{cc} \partial_t h + \partial_x q = 0 \\ \partial_t q + \partial_x \left(\frac{q^2}{h}+ \frac{1}{2} g h^2 \right) = -gh (S_0 + S_f) \end{array}

S_0 is the slope and S_f is the friction term in its kinematic form.

## Declarations

We call the saint venant solver which takes into account the rain source term.

#include "grid/cartesian1D.h"
#include "saint-venant.h"
#include "saintvenant/manning.h"

int LEVEL;

scalar e[];
double e1 = 0., e2 = 0., emax = 0.;
int ne = 0;
double pause, tmax, q0 = 2.5, z0, h0, tb =400 ;

// Analytical solution for h and dh/dx
double hex(double x){
return pow(4/G,1/3.)*(1 - 0.2*exp(-36*pow(-0.5 + x/1000.,2)));
}
double dhex(double x){
return pow(4/G,1/3.)*0.0144*exp(-36*sq(-0.5 + x/1000.))*(-0.5 + x/1000.);
}

// Manning friction term in kinematic formulation
double sf(double x){
return  -n*n*q0*q0/pow(hex(x),10/3.);
}

// Z and dz
double dzex(double x) {
return (q0*q0/(G*pow(hex(x),3))-1)*dhex(x)+sf(x);
}
double zex(double x, double z) {
double dx = L0/N;  return z + dx/3.*(dzex(x-dx)+dzex(x-0.5*dx)+dzex(x));
}

## Parameters

Definition of parameters and calling of the saint venant subroutine run().

int main()
{
n = 0.04;
pause = 0.2;
L0 = 1000.;
X0 = 0;
G = 9.81;
tmax = 500;
for( LEVEL = 7; LEVEL <= 10; LEVEL++){
e1 = e2 = emax = 0.;
ne = 0;
N = 1 << LEVEL;
run();
fprintf (stderr, "%d %g %g %g\n", N, e1/ne, sqrt(e2)/ne, emax);
}
}

## Boundary condition

h[left] = dirichlet(max(hex(0),0));
eta[left] =  dirichlet(max(hex(0)+zb[],zb[]));
u.n[left] = dirichlet(max(q0/hex(0),0));

h[right] = dirichlet(max(hex(1000),0));
eta[right] =  dirichlet(max(hex(1000)+zb[],zb[]));
u.n[right] = dirichlet(max(q0/hex(1000),0));

## Initial conditions

event init (i = 0)
{
DT = 1e-2;
z0=0;
foreach(){
zb[] = zex(x,z0);
z0=zb[];
u.x[] = 0;
h[]=0;
}
boundary(all);
}

## Computing error

event error (i++; t<=tmax) {
foreach()
e[] = (h[] - hex(x));
norm no = normf (e);
if(t > tb){
e1 += no.avg;
e2 += no.rms*no.rms;
ne++;
if (no.max > emax)
emax = no.max;
}
if( N == 1024){
static FILE * fp1 = fopen("ErrorN1024.dat","w");
fprintf(fp1,"%g \t %g \t %g \t %g \n",t,no.avg,no.rms,no.max);
}
}

## Gnuplot output

We can use gnuplot to produce an animation of the water surface.

/*
event plot ( t <= tmax; t += 15 ) {
if( N == 64 || N == 1024 ){
printf("set title 'Manning friction fluvial ----- t= %.3g , N = %i '\n"
"p[%g:%g][-60:1]  '-' u 1:($2+$4) t'free surface' w p pt 1,"
"'' u 1:4 t'topo' w l lt 4,"
"'' u 1:5 t'Analytical' w l lt -1 \n",t,N,X0,X0+L0);

foreach()
printf (" %g %g %g %g %g %g\n", x, h[], u.x[], zb[], hex(x)+zb[], t);

printf ("e\n"
"pause %.4g \n\n",pause);
}
}
*/

Print the water profile along the channel at final time

event printprofile ( t = 399){
char name[100];
FILE * fp;
sprintf(name,"profil-%i.dat",N);
fp=fopen(name,"w");
foreach()
fprintf(fp,"%g  \t %g  \t %g \t %g \t %g \t %g  \t %g \n"
,x,h[],zb[],hex(x),u.x[],u.x[]*h[], h[] > dry ? u.x[]/(sqrt(G*h[])) : 0);
fclose(fp);
}

## References

[1] I. MacDonald, M. Baines, N. Nichols, and P. G. Samuels, “Analytic Benchmark Solutions for Open-Channel Flows,” no. November, pp. 1041–1045, 1997.