SAGA Study : Ondoy Philippines

SAGA STANDARD TERRAIN ANALYSIS (25-Apr-2013)

References: Referencing Terms links all point to Wikipedia Online

Base Map: South Luzon (National Capital Region Manila, Philippines)

Comments: Due to the severity of flooding which occurred in the National Capital Region of Manila in the Philippines during Tropical Typhoon Ondoy (2009) I thought this would be a good area to base a standard Terrain Study on.  This study only involves the standard outputs from SAGA and does not consider made-made effects on terrain due to deforestation, unplanned developments, Storm water drainage outflow or many of the other parameters which would have directly impacted on the flooding devastation that occurred.

SAGA provides some good tools for performing Terrain Analysis and Simulations. In this post i will demonstrate the Standard Terrain Analysis Module, and have opened a 90 Meter DEM (extracted from DEM Explorer) and then applied SAGA Module Tool "Terrain Analysis - Compound Analyses" to the original DEM. After selecting the loaded DEM, move to the Module TAB and select Terrain Analysis - Compound Analysis > Standard Terrain Analysis. User will then see the following...

I then select the 90M DEM which had just been loaded as the grid system and then the 90 M DEM Grid Elevation. At the bottom of the Dialog there are options which the user can modify prior to running the model against the DEM. I will not go through these options...maybe at a later post. Once the Grid and Elevation has been selected from the loaded DEM click OK, this will start the module execution. You will need to wait for some time until the module completes (the steps in execution are shown at the bottom of the SAGA window as it executes).

Once the processing has finished all 17 analysis components will be displayed in the Data Tab as shown  The first layer i add to the map is Analytical Hill-shading (Shaded Relief)  as shown below. To add new layers to the map from the data TAB just double click on the data layer and a popup will be displayed asking where should this layer be added, as we are going to show all data analysis in one map we will choose Map 1. The first layer to add in is the Analytical Hill-shading as shown below.

Hill-shading

The next layer we will overlay is the 90 M DEM Elevation Contour.

DEM Elevation

If the Analytical Hill-shading was not loaded first the DEM Elevation would have shown as flat with elevation body as shown below.

90 M DEM Elevation (Flat)

NO CELL DATA

Please note that for all data files generated an associated legend is created which represents the upper and lower boundaries of the data analyzed in the original DEM. Sometimes DEM's contain cells which contain no data, these cells are normally treated by most GIS applications as NULL and assign a default value to each cell when reading in. Normally its -9999 or similar, and the user can define this in properties for the data. SAGA has a number of tools to convert NULL cell values into a value by using the Module Function Grid Tools > Close Gaps.

Module: Close Gaps

(c) 2002 by O.Conrad
Menu: Grid > Construction
Description
Close gaps of a grid data set (i.e. eliminate no data values). If the target is not set, the changes will be stored to the original grid. 
I won't explain the process for running this module function on a DEM Grid in this post.

Next we overlay the Watershed/Drainage Basins determined from the SAGA compound analysis on the DEM which is shown as follows...

Watershed/Drainage Basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea, or ocean.

Other terms that are used to describe a drainage basin are catchment, catchment area, catchment basin, drainage area, river basin and water basin.^[1] In North America, the term watershed is commonly used to mean a drainage basin (though in other English-speaking countries, it is used only in its original sense, to mean a drainage divide^[2]). Drainage basins drain into other drainage basins in ahierarchical pattern, with smaller sub-drainage basins combining into larger drainage basins.^[3]

Sometimes it is necessary to analyze water channeling/movement from a lower level within a Watershed/Drainage Basin, SAGA outputs Watershed Sub Basins as one of the 17 outputs in the Standard Terrain Analysis and for our example it is shown below here...

Sub Basins

Multiple sub basins may have an effect of water flow, sink fill effects, drainage etc...depending on the geological makeup of the land within each sub basin. Soil Types, Vegetation, Rainfall pocketing can all collectively change what happens within a watershed area.

To obtain an understanding of water movement through basins/sub basins we need to look at the Channel Network

Module: Channel Network
(c) 2001 by O.Conrad
Menu: Terrain Analysis > Channels
Description
This module derives a channel network based on gridded digital elevation data.
Use the initiation options to determine under which conditions channels shall start.

Channels

Further it possible to run the D8 flow Analysis

Module: D8 Flow Analysis
(c) 2003 by O.Conrad
Menu: Terrain Analysis > Channels
Description
Deterministic 8 based flow network analysis
which provides Flow Direction, Flow Connectivity and Flow Network outputs from the selected DEM Grid Input.

Flow Direction

Assigns a flow direction based on the steepest slope of a triangular facet (Tarboton, 1997). Flow direction is defined as steepest downward slope on planar triangular facets on a block centered grid. Flow direction is encoded as an angle in radians counter-clockwise from east as a continuous (floating point) quantity between 0 and 2 pi. The flow direction angle is determined as the direction of the steepest downward slope on the eight triangular facets formed in a 3 x 3 grid cell window centered on the grid cell of interest.

In the following D8 Analysis i have zoomed in to show the effect of the analysis over the DEM. The Marikina Sub-Basin was extremely effected during Ondoy and the reason being that all upstream channels direct from up line sub basins which covers a large catchment area.

Flow Connectivity

D8 Flow Network

 D8 Flow network generated by the Terrain Analysis is output as a standard Shape as defined above in green overlaying the hill-shading layer. The following two schematics shows the Marikina watershed and channel network that has a direct effect on why during heavy rains flood waters occur.

Marikina Watershed (Google Exaggerated Elevation)

Marikina, well and truly lies on the discharge point of a large catchment area, which comprises of multiple sub-basins. All channel network flows push out into this Marikina basin/flood plain.

Overland Flow Distance to Channel Networks

The "Overland Flow Distance" measures the flow distance not only horizontally but also includes the slope of each grid cell - so the values represent something like a "true" distance measured on the surface of your DEM: in completely flat areas, the overland flow distance equals the horizontal distance, in steep areas, the overland flow distance is significantly larger than the horizontal one.

Overland Flow Distance to Channel Networks

Module: Overland Flow Distance to Channel Network
(c) 2001 by O.Conrad
Menu: Terrain Analysis > Channels
Description
This module calculates overland flow distances to a channel network based on gridded digital elevation data and channel network information.
The flow algorithm may be either Deterministic 8 (O'Callaghan & Mark 1984) or Multiple Flow Direction (Freeman 1991)
References:
- Freeman, G.T., 1991: 'Calculating catchment area with divergent flow based on a regular grid', Computers and Geosciences, 17:413-22
- O'Callaghan, J.F., Mark, D.M., 1984: 'The extraction of drainage networks from digital elevation data', Computer Vision, Graphics and Image Processing, 28:323-344

This function can be used to simulate potential environmental impacts on downstream watersheds for upstream soil leaching from landfills, made-made chemical spills etc. Spill Characterization is an essential component of environmental management these days, which also, when coupled with population demographics provides decision making data in disaster planning due to large environmental spills.

Catchment Area Flow Tracing
Provides clear representation of major channels and their resultant flow path.

Catchment Area Flow Tracing

Valley lines and drainage networks shows clearly the main drainage network delineation. In National Capital Region the network coming from the mountain range is clearly visible. Another strong network paths out from the mountain range down through Bulacan province.

Valley Lines & Drainage Networks

Final Reference Comments

On September 26, 2009, at about 6:00 pm PST, the 50-mph "Tropical Storm Ketsana" (called "Ondoy" in the Philippines) hit Metro Manila and dumped one month's rainfall in less than 24 hours, causing the Marikina River system, including the Manggahan Floodway, to burst its banks very rapidly. It is thought that blocked pipes and a poorly maintained sewer system, along with uncollected domestic waste, were major contributory factors in the speed with which the flood waters were able to engulf the surrounding area. The illegal settlers especially were blamed for flooding since their houses reduce the effective width and blocked the flow of the floodway. During the height of the storm, the Marikina River had a flow of about 3000 m³/s (106,000 ft³/s), and the head of the UP National Hydraulic Research Center stated that the floodway could have handled this flow without overflowing if there were no settlers on its banks.^[6]

Consequently, in February 2010, President Gloria Macapagal-Arroyo revoked Proclamation 160 that reserved 20 parcels of land along the floodway for 6700 urban poor families, and ordered the forcible relocation of the illegal settlers whose houses were blocking the waterway to Laguna de Bay.^[7]

There are many different social aspects and impacts that contribute to this type of flooding, hence the need to always be searching for improvement in the areas of management in Environmental, Urban Planning, GIS mapping and Analysis, and most important Public Awareness.