Vulnerability Assessment of Cugman River Watershed

Vulnerability Assessment
of Cugman River
Watershed
Manolito M. Pasco
Senior Science Research Specialist

Geographical Location
The Cugman river
watershed stretches from the
northern portion of the
province of Bukidnon toward
the coastal area of Cagayan de
Oro City, Misamis Oriental with
its main drainage outlet
emptying down the coast of
Macajalar Bay. Geographically it
lies between 8°13’25’’ to
8°28’30’’ north latitude and
124°41’45” to 124°49’20” east
longitude.

Significance of the Vulnerability
Assessment of Watershed
1. It is used as planning tool and as guidance in the
preparation of Integrated Watershed Management and
Development Plan.
2. It provides guidance to LGU on landuse zoning and
disaster preparedness
3. It provides technical basis for policy
formulation/issuances
4. It recommends mitigating measures and strategies to
reduce the destructive impact of natural and manmade
hazards.

Objectives
1. To identify the potential hazards and the critical physical
and anthropogenic factors that influence the occurrences
of such hazards.
2. To facilitate zoning of watershed areas according to level
of vulnerability to natural and man-made hazards
3. To recommend preventive or mitigating measures as
guide in the formulation of intended interventions in the
watershed
4. To enhance disaster preparedness by LGU.
5. To provide valid technical information as basis in the
formulation of policies

Methodology
A. Landslide Hazard Vulnerability Assessment
Conceptual Framework
Watershed Characterization Report
Review and Validation of
Data/Maps
Hazard Identification
Collecting/ Gathering of
Additional Data
Soil Erosion Hazards
Physical Factors
Landslide Hazard
Identification of Landslide
Critical Factors
Mapping of Critical Factors
Flooding Hazards
Anthropogenic Factors
Physical Factors Category
Maps
Overlaying of Maps and
Analysis
Landslide Vulnerability Map
(Physical Factor)
Preparation of Critical Factors
Category Maps
Overlaying of Maps and
Analysis
Landslide Hazard
Vulnerability Map
Anthropogenic Factors
Category Maps
Overlaying of Maps and
Analysis
Landslide Vulnerability Map
(Anthropogenic Factor)
Field Validation/ Ground
truthing
Formulation of Mitigation
measure & policy
recommendation
Landslide Hazard Vulnerability
Assessment Report
Review, revision and
refinement of report

2. Identification of Critical Factors Influencing
Landslide Hazard
Physical critical factors- Slope, Rainfall, Landuse, Soil type,
and Geology
Anthropogenic critical factors- Farming system/Landuse,
and Road Network
3. Mapping of Critical Factors
Selected thematic maps from watershed
characterization report
Validation, rectification and/or refinement of thematic
maps of critical factors e.g. slope map, rainfall map,
landuse map, soil map, geology map and road network map

4. Preparation of Landslide Hazard Category Map
of the Critical Factors
• Basis for categorizing the level of vulnerability of physical
factors
Table 1. Rating of Critical Factors (Physical Factors)
Vulnerability
Class
Not
Vulnerable
Rating
Slope
(%)
Rainfall
(Annual –
mm)
Critical Physical Factors
Vegetative Cover Soil Type Geologic
Formation
1 0 – 8 50 >2,016
Domestic agri-crops
Bolinao
clay
Quarternary
alluvium/ Cagayan
terrace deposits

Basis for categorizing the level of vulnerability of
anthropogenic factors
Table 2. Landslide hazard values due to farming system
Rating
Vulnerability
class
1 Very low
vulnerability
2 Low vulnerability
Extend of disturbances by farming system
The farming system in the watershed do not favor landslides
The farming system in the watershed slightly favor minor landslide
de
3 Moderate
vulnerability
4 Highly vulnerable
The farming system in the watershed slightly favor major landslide
de
The farming system in the watershed highly favor minor landslide
5 Very highly
vulnerable
The farming system in the watershed highly favor major landslide

Table 3. Landslide hazard values based on relative distance from the road
Rating
Vulnerability class
Relative distance from the edge of road
1 Very low
vulnerability
>60 meters away from the edge of the road
2 Low vulnerability 45.1-60 meters away from the edge of the
road
3 Moderate
vulnerability
30.1-45 meters away from the edge of the
road
4 Highly vulnerable 15-30 meters away from the edge of the road
5 Very highly
vulnerable

5. Overlaying of category maps and
determination of landslide hazard
vulnerability
A. Physical factors
V L =0.30Sl + 0.25R + 0.20Lu + 0.15S + 0.10G
Where:
V L = Landslide Vulnerability Hazard Value
Sl = Slope (30%)
R = Isohytal Rainfall (25%)
Lu = Landuse (20%)
S = Soil Type (15%)
G = Geology (10%)

B. Anthropogenic factor
VL= Hazard Value of Farming System + Hazard Value
of Relative Distance from the Road ÷ 2
Basis for the Landslide Vulnerability Value (Physical and
Anthropogenic Factors)
Table 4. Rating ranges of vulnerability class
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

6. Overlaying of landslide hazard vulnerability maps
both the physical and anthropogenic factors
Where:
V L = 0.9Lp + 0.1La
V L
= Landslide Vulnerability value
Lp = Landslide Vulnerability value due to physical factors (90%)
La = Landslide Vulnerability value due to anthropogenic factors (10%)
Table 4. Rating ranges of vulnerability class
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

B. Soil Erosion Hazard Vulnerability
Assessment
1. Physical Factors
Conceptual Framework
Rainfall
Category Map
Slope Category
Map
Landuse
Category Map
Soil Type
Category Map
Geology
Category Map
Overlaying/Spatial
Analysis
Vulnerability
Class
Soil Erosion Hazard
Vulnerability Map

a. Two methods of Assessing Soil Hazard Vulnerability
The GIS-assisted mapping techniques and spatial analysis
Use of USLE- Universal Soil Loss Equation
b. Identification of Critical Factors that Influence the occurrence of
Soil Erosion: Rainfall, Slope, Soil type, Landuse, Geology
c. Preparation of Critical Factors Category Maps. (The categorized
maps of critical physical factor used in landslide
assessment are the same maps in assessing the soil erosion
vulnerability except the weight which are changed to the
following):
Vse = 0.40 R + .20 Lu + 0.20 Sl + 0.15 S + 0.05 G
Where:
R= Rainfall (40%)
Lu= Landuse (20%)
Sl= Slope (20%)
S= Soil type (15%)
G= Geologic age/ Formation (5%)

Overlaying of Soil Erosion Critical
Physical Factor Category Map
Using the above equation the soil erosion hazard value is
determined. The hazard value is referred to Table 4 to determine
the Hazard Vulnerability Class.
Table 4 Rating ranges of vulnerability class
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

2. Anthropogenic Factors
Conceptual Framework
Farming Practices/
Landuse
Farming System
Category Maps
Relative Distance
from the Road
Road Network
Category Map
Overlaying/Spatial Analysis
Sev = (FS + RD) /2
Vulnerability Class
Soil Erosion Hazard
Vulnerability Map
(Anthropogenic)

a. The landuse and road network category maps used in landslide
vulnerability assessment are the same maps used in soil
erosion vulnerability assessment (anthropogenic factors)
b. Overlaying of the landuse category map and road network
category maps. The same methodology used in landslide
vulnerability assessment was also employed where the following
equation was used
Vse= Hazard Value of Farming System + Hazard Value
of Relative Distance from the Road / 2
The value of Vse was also referred to Table 4

Overlaying of Soil Erosion Hazard
Vulnerability Maps (Physical and
Anthropogenic Factors)
V se = 0.9SEp + 0.1SEa
Where:
Vse= Soil Erosion Vulnerability value
SEp= Soil ErosionVulnerability value due to physical factors (90%)
SEa = Soil Erosion Vulnerability value due to anthropogenic factors (10%)

C. FLOOD HAZARD VULNERABILITY
ASSESSMENT
Identification and Verification of Flood Hazard
• It involves the analysis of the geomorphological attributes of the
watershed, the existing landuse, the rainfall pattern and actual verification
from the ground including interview from the local residents on the
occurrence of flood.
Mapping of Flood Prone Area
• The flood prone area mapping involved the calculation of areas with elevation
below the elevation of the highest flood watermark. The generated flood prone
area map was validated using the Global Positioning System readings of the actual
location of flooded areas within the watershed.

RESULTS AND DISCUSSION
1. Landslide Hazard Vulnerability Assessment
a. Critical Factors Identification
Slope, Rainfall, Landuse, Soil type, Geologic
Age/Formation, Farming System and Road Network
b. Mapping of Critical Factors
•The thematic maps of critical factors validated and rectified
are the following:
•Physical Factors
Slope, Rainfall, Landuse, Soil type, Geologic
Age/Formation

Mapping of Critical Factors
Digital Road Landuse Geologic Rainfall Slope Elevation Network Map
Map

Categorization of Critical Physical Factor Maps
Basis for the categorization
Table 1. Rating of Critical Factors (Physical Factors)
Rating
Critical Physical Factors
Vulnerability
Class
Slope (%)
Rainfall (Annual
– mm)
Vegetative
Cover
Soil Type
Geologic
Formation
Not
Vulnerable
1 0 – 8 50 >2,016 Domestic
agri-crops
Bolinao
clay
Bolinao
clay
Himalyan formation
Quarternary
alluvium/ Cagayan
terrace deposits

Critical Physical Factor Category Maps
Road Geologic Landuse Rainfall Slope Soil Network Hazard Hazard Map
Map
Map

Overlaying of Critical Physical Factor Category Maps
Slope Hazard Map
Soil Hazard Map
Landuse Hazard Map
Rainfall Hazard Map
Geologic Hazard Map
Road Network Hazard Map

VL = 0.40 Sl + 0.30 R + 0.15 S + 0.10 VC + 0.05 G
Table 4. Rating ranges of vulnerability class:
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

Landslide Hazard Vulnerability Map
(Physical Factors)

Categorization of Critical Anthropogenic Factor Maps
Table 3. Landslide Value Based on Relative Distance from the Road
Rating
Vulnerability class
Relative distance from the edge of road
1 Very low
vulnerability
>60 meters away from the edge of the road
2 Low vulnerability 45.1-60 meters away from the edge of the
road
3 Moderate
vulnerability
30.1-45 meters away from the edge of the
road
4 Highly vulnerable 15-30 meters away from the edge of the road
5 Very highly
vulnerable

Table 2. Landslide hazard values due to farming system
Rating
Vulnerability
class
1 Very low
vulnerability
2 Low vulnerability
Extend of disturbances by farming system
The farming system in the watershed do not favor landslides
The farming system in the watershed slightly favor minor landslide
de
3 Moderate
vulnerability
4 Highly vulnerable
The farming system in the watershed slightly favor major landslide
de
The farming system in the watershed highly favor minor landslide
5 Very highly
vulnerable
The farming system in the watershed highly favor major landslide

Overlaying of Categorized Critical Anthropogenic Maps
Landuse Category Map Road Network Category Map

V A = FShv + RDhv / 2
Table 4. Rating ranges of vulnerability class
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

Landslide Hazard Vulnerability Map
(Anthropogenic Factor)

Overlaying of Landslide Hazard Vulnerability Maps
(Physical and Anthropogenic Factors)

V L = 0.9Lp + 0.1La
Table 4. Rating ranges of vulnerability class
Landslide vulnerability class
Rating Ranges
Very low Vulnerability 4.2

Final Landslide Hazard Vulnerability Map of
Cugman Watershed

Table 5. Landslide Vulnerability Assessment (Physical & Anthropogenic Factors)
Barangay
Area (ha)
%
cover
Highly Vulnerable
Area (ha)
%
cover
Moderately
Vulnerable
Area (ha)
%
cover
Low Vulnerability
Area (ha)
%
cover
Very Low
Vulnerability
Area (ha)
1 Tablon 1175.1874 11.47 35.1795 2.99 364.0163 30.98 407.3272 34.66 368.6644 31.37
2 Cugman 378.3973 3.69 - 22.295 5.89 30.9509 8.18 325.1514 85.93
3
F.S.
Catanico
670.4717
6.54
%
cover
3.0413 0.45 79.239 11.82 99.5334 14.85 488.848 72.91
4 Gango 554.7846 5.41 3.546 0.64 43.9292 7.92 359.0725 64.72 148.257 26.72
5 Palabucan 650.8001 6.35 4.0677 0.63 154.1056 23.68 433.9694 66.68 58.6574 9.01
6 Kiliog 239.2789 2.33 - - 85.05 35.54 45.6663 19.08 108.6139 45.39
7
Poblacion 3953.4092 38.57 58.7288 1.49 852.1337 21.55
2091.753
4
52.91 950.7936 24.05
8 Sta Fe 467.6932 4.56 15.5595 3.33 39.4485 8.43 412.6852 88.24 - -
9 Crossing 198.1756 1.93 4.7597 2.40 50.6814 25.57 142.7363 72.03 - -
10 Laturan 956.5129 9.33 4.9828 0.52 66.2785 6.82 308.7386 32.28 - -
11
San Jose 379.9999 3.71 38.8986 10.26 100.5297 26.52 817.1846
215.0
5
- -
12 Maambong 460.4115 4.49 28.4016 6.17 305.9271 66.44 126.0828 27.38 - -
13 Sil-ipon 164.4627 1.60 2.3851 1.45 162.6776 0.06 - - - -
Total 10,249.59
100
199.542
6
1.95
2326.311
6
22.70
5275.700
6
51.47
2448.985
7
23.89

Landslide Vulnerable Area By Barangay

Soil Erosion Hazard Vulnerability Assessment
The critical physical and anthropogenic factors influencing
landslide are the same factors that influenced soil erosion. Hence,
the categorized physical and anthropogenic factors maps are the
same. It varies only with the assignment of weight.
Vse = 0.40 R + 0.20 Sl + 0.20 Lu + 0.15 S + 0.05 G

Overlaying of Soil Erosion Hazard Vulnerability Maps
(Physical and Anthropogenic Factors) is shown below:

Final Soil Erosion Hazard Vulnerability Map
of Cugman Watershed

Table 10. Soil Erosion Hazard Vulnerability Area by Barangay
Barangay
Area (ha)
%
Cover
Highly Vulnerable
Area
(ha)
%
cover
Moderately
Vulnerable
Area (ha)
%
cover
Low Vulnerabiliy
Area (ha)
%
cover
Very Low
Vulnerability
Area (ha)
1 Tablon 1175.1874 11.47 78.841 6.7 490.2849 41.77 286.1173 24.35 320.0849 27.24
2 Cugman 378.3973 3.69 44.9304 11.87 93.1601 24.62 240.3068 63.51
3 F.S. Catanico 670.4717 6.54 8.5558 1.28 91.5289 13.65 25.847 3.85 544.536 81.22
4 Gango 554.7846 5.41 75.2816 13.57 396.7263 71.51 82.7767 14.92
5 Palabucan 650.8001 6.35 151.516 23.28 294.744 45.29 204.5401 31.43
6 Kiliog 239.2789 2.33 11.1068 4.64 59.7576 24.97 168.4144 70.38
%
cover
7
Poblacion 3953.4092 38.57 10.2388 0.26 875.1763 22.14 1692.6614 42.81 1875.3327 34.79
8 Sta Fe 467.6932 4.56 6.8796 1.47 103.6911 22.17 357.1225 76.36 - -
9 Crossing 198.1756 1.93 8.7358 4.41 189.4398 95.59 - - - -
10 Laturan 956.5129 9.33 23.1728 2.42 640.8617 67 292.4784 30.58 - -
11 San Jose 379.9999 3.71 4.7102 1.24 302.6385 79.64 72.6512 19.12 - -
12 Maambong 460.4115 4.49 78.371 17.02 283.1241 61.49 98.9744 21.48 - -
13 Sil-ipon 164.4627 1.6 30.7532 18.7 133.7095 81.3 - - - -
Total 10,249.59
99.98
250.2582 2.44 3393.2796 33.12 3670.1802 35.88 2935.9916 28.64

Soil Erosion Vulnerable Area By Barangay

C. Flood Hazard Vulnerability Assessment
Result on the delineation of the flood prone areas based from the common observation
that areas below the elevation of the highest water landmark are prone to flooding is
shown in map below

RECOMMENDED MITIGATING
MEASURES/INTERVENTION ON HIGHLY
VULNERABLE AREAS
Table 8 and 9 show the recommended mitigating measures/
intervention by barangay on highly vulnerable areas to soil
erosion and landslide respectively.
Table 8. Landslide Hazard Vulnerable Areas by Barangay and
Recommended Mitigating Measures
Table 9. Soil Erosion Hazard Vulnerability Area by Barangay and the
Corresponding Recommended Mitigating Measures.

Limitation of the Vulnerability
Assessment Study
1. Absence of comprehensive soil map.
2. Insufficient geological data.
3. Absence of rainfall data within or adjacent to watershed area.
4. Assigning of weight to the model is subject to change
depending on the result of ground truthing.
5. Inadequate watershed characterization terminal report.