Vulnerability assessment of Sta.Rosa-Silang subwatershed using SWAT

Transcription

1 Vulnerability assessment of Sta.Rosa-Silang subwatershed using SWAT Damasa B. Magcale-Macandog, Charles Gunay and Milben A. Bragais Institute of Biological Sciences, University of the Philippines Los Baños, College, Laguna, Philippines Paper presented during the 2017 ESRI User Conference, July 2017, Convention Center, San Diego, California

2 INTRODUCTION Sta. Rosa Silang subwatershed: rapid land conversion since 1990 s extreme disturbances weather altered water balance

3 Flooding from typhoon events flooding related Google search

4 Major flood disasters and related damages and losses as brought by typhoons in 2006 and 2009 in Laguna region (Source: RDRRMC, 2011)

5 Source: Presentation made by LLDA Secretary Acosta on Dec 2, 2014 in Santa Rosa, Philippines FLOOD HAZARD in Laguna Lake Greater Watershed

6 Study site Silang Binan Cabuyao Santa Rosa Silang-Santa Rosa Subwatershed, Philippines Area shaded in red (above), topography (right) Multiple cities in 120 km2 Rapid economic growth Population growth and migration (570,000 people) Massive land use changes in past 2 decades Flooding, environmental degradation, pollution, waste

7 LAND COVER CHANGES Source: Magcale-Macandog et al., 2015

8 INTRODUCTION Soil and Water Assessment Tool A physically-based, river basin-scale, continuous event model developed to quantify the water, sediment, and agricultural nutrient yields of large watersheds. The model bears its simulations on the area s soil types, slope, and land management conditions.

9 INTRODUCTION Soil and Water Assessment Tool Enables its users to study long-term impacts from the readily available inputs of spatial and climatic data Accurately simulate the surface runoff using SCS Curve Number method and the sediment accumulation using the Modified Universal Soil Loss Equation (MUSLE)

10 INTRODUCTION In this study, SWAT, inputted with LiDAR- and RS-derived maps and historical weather data, was used to quantify the ff.: potential flood volume potential erosion and sediment accumulation

11 OBJECTIVES This study generally aimed to assess the landslide and flooding high risk areas along the Santa Rosa-Silang Subwatershed using the SWAT hydrological model. Specifically, it intended to: a) Predict runoff and sediment yield in the site using SWAT model from the spatial and climatic inputs b) Identify high risk areas along the site vulnerable to landslides and flooding

12 MATERIALS AND METHODS Preparation of SWAT Model Inputs: Geographical data of the Santa Rosa-Silang Subwatershed a) Digital terrain model (DTM) b) 2014 land use map c) Soil map Historical climatic data from 1980 to 2015 a) Daily rainfall b) Daily maximum and minimum temperatures

13 Reconditioned digital terrain map (DTM) of Santa Rosa- Silang Subwatershed

14 SWAT Input - SWAT-defined Code Area (hectares) Percent Subwat. Area (%) LAND USES*: Residential (URBN) Orchard (ORCD) Pasture (PAST) Forest, Mixed (FRST) Agricultural, Generic (AGRL) land cover map of Santa Rosa-Silang Subwatershed

15 SWAT Input - SWAT-defined Code SOIL TYPES*: Clay (VERGENNES) Sandy loam (POOTATUCK) Sandy clay loam (RIPPOWAM) Loam (MIDDLEBURY) Clay loam (WALLKILL) Area (hectares) Percent Subwat. Area (%) Soil map of Santa Rosa-Silang Subwatershed

16 MATERIALS AND METHODS Preparation of SWAT Model Inputs: The 36-year (1980 to 2015) climatic data was converted and compiled to a particular format (.txt) required by the SWAT model.

17 MATERIALS AND METHODS Development of SWAT Model: The DTM map was entered in the model s platform. Streams and outlets were created based on the changes in elevation along the area.

18 SWAT Model Output

19 Table 2. Simulated surface flow and sediment yield volumes of the dominant 49 HRUs. HRU # Flow (cms) Sediment (tons) HRU # Flow (cms) Sediment (tons) Santa Rosa-Silang Subwatershed divided into 49 dominant HRUs

20 RESULTS AND DISCUSSION HRU #8, #13, #15, #21, #22, and #26 are those vulnerable to flashfloods based on their physical characteristics & exposure to extreme rainfall. Identified areas vulnerable to flooding along the Santa Rosa- Silang Subwatershed

21 RESULTS AND DISCUSSION The flooded areas are dominated by residential subdivisions and industrial facilities which makes the permeability of the land and soils to be extremely low.

22 RESULTS AND DISCUSSION HRU Nos. 3, 4, and 19 vulnerable to sedimentation HRU Nos. 33, 37, 40, 42, 44, and 47 are vulnerable to erosion.

23 RESULTS AND DISCUSSION Brgy. Tartaria (Upstream) Brgy. Sto Domingo (Mid stream) The portions vulnerable to erosion are mostly in sloping upstream areas in Silang and midstream areas in Santa Rosa.

24 RESULTS AND DISCUSSION Brgy. Aplaya (Shoreline) The downstream areas along the riverine system and lakeshore are most vulnerable to sedimentation

25 CONCLUSIONS A GIS-based hydrological modeling study was performed at the Santa Rosa-Silang Subwatershed to assess its physical vulnerability towards landslides and flashfloods. SWAT Input - SWAT-defined Code Area (hectares) Percent Subwat. Area (%) The SWAT model was used to simulate the potential surface flow and sediment yield volume based on the spatial and climatic data inputted in its platform. LAND USES*: Residential (URBN) Orchard (ORCD) Pasture (PAST) Forest, Mixed (FRST) Agricultural Land, Generic (AGRL) SOIL TYPES*: Clay (VERGENNES) Sandy loam (POOTATUCK) Sandy clay loam (RIPPOWAM) Loam (MIDDLEBURY) Clay loam (WALLKILL) SLOPE RANGES: TOTAL

26 CONCLUSIONS Results of the simulation of the model showed: high volumes of surface flow in the downstream and midstream areas of Santa Rosa along the riverine system; erosion occuring in sloping upstream areas sedimentation in the coastal and along the riverine system in the downstream area

27 Thank you!