Ground bottom flow formula (Water Overlay)

Hoe is kwel geïmplementeerd in het Tygron Geodesign Platform? (Dutch only)

Underground seepage from outside the project area is calculated per grid cell. The formula is based on the principles used in the formula of Pankow for estimating seepage^[1].

The water level change due to seepage is calculated using the following formula:

\Delta wl_{x,y}={\frac {h_{d}(t)-h_{x,y}}{c_{x,y,tf}}}\cdot {\frac {\Delta t}{24\cdot 60\cdot 60}}

The external bottom head pressure is variable over time by the introduction of $h_{v}(t)$ :

h_{d}(t)=p_{x,y,tf}+h_{v}(t)

Finally, the actual amount of water seeping into the ground from the bottom is calculated as followed:

\Delta s_{x,y}={\Delta wl_{x,y}}\cdot {ws}

where:

\Delta wl_{x,y}

= The water level change due to seepage, in meters.

\Delta t

= Computational timestep in seconds.

h_{x,y}

= Ground water head in the grid cell.

h_{d}(t)

= Additional ground water level (or head) of the external zone causing the seepage at time t, in meters

p_{x,y,tf}

= Ground water level (or head) of the external zone in meters at timeframe

tf

.

h_{v}(t)

= Additional variation of bottom head pressure over time

c_{x,y,tf}

= Resistance of the separating layer in days at timeframe

tf

ws

= Water storage fraction of the soil

\Delta s_{x,y}

= Seeped in water in m

Notes

The calculated $\Delta wl_{x,y}$ is essentially the change in groundwater level, inherently taking into account the porosity of the soil. The final $\Delta s_{x,y}$ is representative of the actual amount of water, i.e. the height of the water, if the relevant quantity of water wasn't in the underground but sat on the surface instead.
$p_{x,y,tf}$ and $c_{x,y,tf}$ can both be provided as spatially variable values using prequel overlays. These prequels can have multiple timeframes, as indicated by the formula, allowing changing head and resistances over time.
$h_{v}(t)$ can be provided as a set of values, variable over time.
A seepage head lower than the water level in the project is also allowed, resulting in water seeping out to a region outside the project area.