Modeling Erosion Using ILWIS

ILWIS is an open source GIS software freely available to download  http://www.ilwis.org/ .
Download ILWIS 3.3 Academic for this practical.

Modeling erosion from pyroclastic flow deposits on Mount Pinatubo

1.       The given data set is to study the phyroclstic erosion of the given area, and  by examine the given data set understand them. The main phyroclastic deposit eroded by the rain water in different rainy seasons. 

The Digital Terrain Models given for after the main rainy sessions of 1991, 1992, 1993 as DTM91, DTM92 and DTM93

Dtmpre – Digital terrain model before eruption

Dtmerp – Digital terrain model after eruption

Catchpre – Catchment before eruption

Catchere – catchment after the eruption     

 Calculate the catchment area changes after the volcanic eruption in square kilometers and fill the following table

Catchment type	Area before eruption	Area after eruption	Changes of area
Abacan
Pasig
Sacobia
Sapangbato
Taug

 Step 1,

We have to calculate the Area before eruption and the Area after eruption and the calculate the different between them,

Open ILWIS - Right click on Catchpre - Statistics - Histogram

Step 2,

Histogram shows the details of the  Area before eruption, we required area in square Kilometers, therefore we have to add new colomn and calculate the area by using simple command in ILWIS.

We have to add new column to the attribute table,

Then we have to open the command line in ILWIS,

Then assign values to your new column by using following command (In this practical "Area_km2" is my new column name )

Area_km2 = Area/1 e 6

Just hit Enter,

Then do the same procedure for the "Catchere " data set also and calculate the difference with the Area before eruption.

After the main volcanic eruption the area filled with the phyroclastic material therefore Calculate the total volume increase after the main eruption 

Step 1,

Here we have to calculate the total volume increased,  for this step we have to use Dtmpre – Digital terrain model before eruption and Dtmerp – Digital terrain model after eruption and create new layer with the difference. for that use following command in ILWIS. (Here "Pfthick" is the name of my new layer)

Pfthick=iff(DTMERP - DTMPRE > 0, DTMERP - DTMPRE,0)

Hit Enter - Show - and press ok, 

Then Right click on the new layer and statistics -  Histogram 

we have to calculate the volume therefore we have to multiple Value by the Area, for that we have to add a new column and using command line we can assign the values to the new column,

Volume = Area*Value

After the first rain (1991) season the original situation has changed in to new situation due to the erosion

Step 1,

Total erosion map of the area 

We have to calculate the total erosion of the area, for that use following command,

TotalEro = iff(DTMERP - DTM91 >0, DTMERP - DTM91,0)

Step 2,

Erosion map of the phyroclastic deposit

 Here we have to calculate the total  phyroclastic deposit

TotPhyro=iff((TotalEro>0)AND(DTM91>DTMPRE),DTMERP-DTM91,0)

Step 3,

Erosion map of the other deposits in 1991

Here we have to calculate the other deposits of the catchment areas,

OtherDepo=iff((TotalEro>0)AND(DTM91<DTMPRE),DTMPRE-DTM91,0)

Step 4,

Sedimentation map of 1991

Sedimentation=iff(DTM91>DTMERP,DTM91-DTMERP,0)