Culvert formula (Water Overlay): Difference between revisions

Revision as of 10:09, 6 October 2020

Flow through culverts is based on an open channel flow calculation.

The actual height of the culvert is at least the height of the terrain on either end of the culvert and the provided threshold height:

B_{c}=\max(T_{c},B_{l},B_{r})

The radius of the culvert:

r={\frac {D}{2}}

The height of the water column at either end of the culvert, relative to the culvert, is calculated:

w_{l}=\max(0,w_{l}-B_{c})

w_{r}=\max(0,w_{r}-B_{c})

Flow depth d is:

d=\max(0,\min(D,\max(w_{l},w_{r})-B_{c})))

The loss coefficient for the culvert is calculated:

U={\sqrt {\frac {1}{1+{\frac {2g\cdot n^{2}\cdot length}{R_{h}^{\frac {4}{3}}}}}}}

The hydraulic radius R_h is calculated using the formula's in the image on the right.

The potential flow through the culvert is then calculated:

C=U\cdot K\cdot {\sqrt {2g\cdot \left|\Vert w_{l}-w_{r}\right|\Vert }}

Finally the actual amount of water flow is calculated:

\Delta f={\frac {\Delta t\cdot C}{\Delta x}}

Where:

$D$ = The CULVERT_DIAMETER attribute of the culvert.
$T_{c}$ = The CULVERT_THRESHOLD attribute of the culvert.
$B_{c}$ = The surface height of the base of the culvert.
$w_{l}$ = The water level on the entry of the culvert, relative to datum.
$w_{r}$ = The water level on the exit of the culvert, relative to datum.
$R_{h}$ = The hydraulic radius in the culvert^[1].
$K$ = Flow area, based on the height of the water in the (circular) culvert.
$g$ = Acceleration factor of GRAVITY, defined for the Water Overlay.
$L$ = The length of the culvert, calculated as the distance between the culvert's endpoints.
$U$ = Loss coefficient for culverts.
$n$ = The CULVERT_N attribute of the culvert.
$C$ = The potential rate of water flow through the culvert.
$\Delta f$ = The water flow which takes place.
$\Delta t$ = Computational timestep.
$\Delta x$ = Cell size.

↑ Hydraulic Radius Equations Formulas Calculator ∙ found at: https://www.ajdesigner.com/phphydraulicradius/hydraulic_radius_equation_pipe.php ∙ (last visited 2019-02-11)

@@ Line 20: / Line 20: @@
 The potential flow through the culvert is then calculated:
-: <math>C = U \cdot K \cdot \sqrt{ 2g \cdot \left |( w_{l} - w_{r} \right | }</math>
+: <math>C = U \cdot K \cdot \sqrt{ 2g \cdot \left | \Vert w_{l} - w_{r} \right | \Vert }</math>
 Finally the actual amount of water flow is calculated: