View source for Subsidence calculation

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==Overlays==
The following overlays are part of the Subsidence Collection:
* [[Subsidence (Overlay)|Subsidence]]
* [[Subsidence (Overlay)#Oxidation|Subsidence (Oxidation)]]
* [[Subsidence (Overlay)#Settlement|Subsidence (Settlement)]]
* [[Ground water (Overlay)|Ground Water Low (GLG)]]
* [[Ground water (Overlay)|Ground Water High (GHG)]]

==Calculations==
During a calculation step, the following aspects are calculated in order:
* The temperature at the start of the year is calculated
* Based on that, the "a" parameter of the oxidation formula is calculated
* The [[Subsidence (Overlay)#Oxidation|oxidation subsidence]] is calculated
* The [[Subsidence (Overlay)#Settlement|settlement subsidence]] is calculated

* The water level is lowered by the amount of subsidence times the indexation
* The ground water level is lowered based on the change in water level compared to the surface of the terrain
* The new ground water level serves as input for the next calculation step

===Oxidation Calculation===
The amount of subsidence due to oxidation is calculated by the following formula:
{{code|1= Subsidence = GLG * a - clay thickness * b - c}}

This formula was provided by experts, who have established this formula empirically.

===Settlement Calculation===
The amount of subsidence due to settlement is calculated by the following formula:
{{code|1= Subsidence = (Peat fraction * PEAT_A + Top layer * TOP_LAYER_A) * log10(days)
+ Peat fraction * PEAT_B
+ Top Layer * TOP_LAYER_B
+ Height Increase * HEIGHT
}}

* The peat fraction and thickness of the top layer are attributes in the project. The exact attributes which provide these can be configured as [[#Keys|keys]] in the overlay.
* The height increase is a result of the actions taken during a [[session]], such as the creation of [[dike]]s.
* The days are equal to the number of days in a year, times the current year being calculated.
* PEAT_A (-0.015853041) is a constant, for the effect of the peat fraction over time.
* PEAT_B (-0.02348519) is a constant, for the base effect of the peat fraction.
* TOP_LAYER_A (-0.006617643) is a constant, for the effect of the top layer thickness over time.
* TOP_LAYER_B (0.010061616) is a constant, for the base effect of the top layer thickness.
* HEIGHT (-0.200468677) is a constant, for the base effect of the height of added materials.

This formula is based on provided expert data in the form of a reference table, indicating the amount of subsidence based on the parameters used in the formula above. The formula's results conform to the reference table to within a fifth of the margin of error of the original table.

===Ground Water change Calculation===

==Configuring overlays==
The overlays part of the Subsidence collection of overlays all have a number of ways to configure them. Both values which serve as input for the overlays directly, as references to attributes of areas which provide input for the calculations.

===Keys===
Each overlay part of the Subsidence collection has a "Keys" tab in the [[right panel]] in the editor. Most keys are attributes of [[area]]s. When the overlay calculates, it will look per [[Grid overlay|grid cell]] for the existence of these attributes.

{| class="wikitable"
! Attribute
! Default
! Description
! Example
! Remark
|-
| Water level
| WATER_LEVEL
| The surface water level, measured from Amsterdam Ordnance Datum (NAP).
| -2.90
| When absent, "0" is assumed.
|-
| Output Level
| WATER_LEVEL_OUTPUT
| The attribute to write the final water level value to.
| -3.20
| If the water level is indexed, subsidence will cause the water level to lower. By writing it into an attribute, the end value can be used. This option can be disabled by unchecking the related checkbox.
|-
| Ground Water Level
| GLG
| The Ground Water Level, measured from the surface of the terrain.
| 0.5
| 
|-
| Indexation
| INDEXATION
| The amount of indexation the water is subject to, from 0 (0%) to 1 (100%)
| 1
| The surface water level is lowered each year by an amount equal to the subsidence times the indexation. From the perspective of the surface of the terrain, the water level in a location with 0% indexation will appear to increase as subsidence takes place.
|-
| Clay Thickness
| CLAY_THICKNESS
| The thickness of the clay layer on the peat, for the calculation of the oxidation component of subsidence.
| 0.2
| When absent, the attribute DEFAULT_CLAY_THICKNESS of the overlay is used.
|-
| Toplayer Thickness
| TOPLAYER_THICKNESS
| The thickness of the layer covering the peat, for the calculation of the settlement component of subsidence.
| 5
| When absent, the attribute DEFAULT_TOP_LAYER_THICKNESS of the overlay is used.
|-
| Peat Fraction
| PEAT_FRACTION
| The fraction of the soil composed of peat, for the calculation of the settlement component of subsidence.
| 0.4
| When absent, the attribute DEFAULT_PEAT_FRACTION of the overlay is used.
|-
| Subsidence
| SUBSIDENCE
| Whether or not subsidence should be calculated in a given area. Subsidence is calculated when the value is greater than 0.
| 1
| When absent, "1" is assumed (and thus subsidence is calculated).
|-
|}
Besides these attributes, 2 more keys can be configured.

'''Years'''<br>
The amount of years to simulate during the calculation, in 1-year steps. It's possible to set this value anywhere between 1 to 1000. This key is linked to one of the attributes of the overlay. Changing this value changes the attribute as well.

'''Ground Water Tiff'''<br>
If the option to use a Ground Water Tiff is checked, a GeoTiff can be selected to use for the ground water levels. By default, a few GeoTiffs are available with open data concerning ground water levels.

===Attributes===
Each overlay part of the Subsidence collection has attributes. All attributes have a default value, but can be changed to configure the subsidence calculation.
{| class="wikitable"
! Attribute
! Description
! Default value
! Remark
|-
| A
| The A parameter in the [[Subsidence (Overlay)#Oxidation|Oxidation formula]].
| 0,023537
| This value is recalculated between 1-year time steps of the calculation.
|-
| B
| The B parameter in the [[Subsidence (Overlay)#Oxidation|Oxidation formula]].
| 0,01263
| 
|-
| C
| The C parameter in the [[Subsidence (Overlay)#Oxidation|Oxidation formula]].
| 0,00668
| 
|-
| CLIMATE_FINAL_TEMP
| The final temperature in the final year of the total simulation time.
| 0,67
| This is the temperature at the end of the year. The last 1-year calculation step uses the temperature from the beginning of that year.
|-
| CLIMATE_OXIDATION
| The oxidation factor used to recalculate the climate values between 1-year calculation steps.
| 0,67
| 
|-
| CLIMATE_SOIL_TEMP_FACTOR
| The soil temperature factor used to recalculate the climate values between 1-year calculation steps.
| 0,5
| 
|-
| DEFAULT_CLAY_THICKNESS
| If no clay thickness value can be found in a particular grid cell, this value is used instead.
| 0,2
| 
|-
| DEFAULT_PEAT_FRACTION
| If no peat fraction value can be found in a particular grid cell, this value is used instead.
| 0,4
| 
|-
| DEFAULT_TOP_LAYER_THICKNESS
| If no top layer value can be found in a particular grid cell, this value is used instead.
| 5
| 
|-
| HI_PASSIVE_DRAINAGE
| When passive drainage is applied, the GHG (highest ground water level) is increased by this amount.
| -0.1
| Due to definitions "Increase" means that the distance between the surface and this ground water level increases. The default value causes the ground water level to come closer to the surface.
|-
| LOW_PASSIVE_DRAINAGE
| When passive drainage is applied, the GLG (lowest ground water level) is increased by this amount.
| 0
| Due to definitions "Increase" means that the distance between the surface and this ground water level increases. The default value does not affect the ground water level.
|-
| YEARS
| The amount of years to simulate during the calculation, in 1-year steps. It's possible to set this value anywhere between 1 to 1000.
| 0
| This attribute is linked to one of the keys of the overlay. When this attribute is changed the key is changed as well.
|-
|}