Flood Routing Analysis in Sitio Crossing Gabi, Poblacion, Compostela, and Davao De Oro Using HEC-RAS and GIS Software: Basis for Flood Detention Pond Design
Awing, Dan Joshua C. | Niog, Yvan Gueen M. | Prollo, Kristele Jan T. | Bengil, Jerico S. | Reyes, Britt Joseph G. (2021)
ABSTRACT
This quantitative-descriptive study aimed to conduct a hydraulic analysis of the stream, particularly in Sitio Crossing Gabi, Brgy. Poblacion, Compostela Davao De Oro. The study focused on providing significant data, which served as the basis for flood detention pond design. The researchers utilize the Geographic Information System (GIS) and Hydrologic Engineering Center River Analysis System (HEC-RAS) software in getting the hydraulic analysis of the stream. Engineering surveys were conducted to gather the primary data of the study. Through the integration of the HEC-RAS software and GIS, the proponents were able to make thematic maps and parameters necessary to the study. According to the data in the pond report, the detention pond, trapezoidal in design, should have a bottom elevation of five meters (5m), a bottom length of fifty meters (50m), a bottom width of twenty meters (20m), a slide slope of 1.5 (H:1), a total depth of ten meters (10m), and a contour area of 1,107 m² with a storage capacity of 527 m3. This study has provided valuable information that would aid in constructing a flood detention pond and lower the risk of flood damage and accidents to the residents of Sitio Crossing Gabi, Poblacion, Compostela, Davao de Oro.
Keywords: flood detention pond design, topographic map, geographic information system (GIS), hydrologic engineering center river analysis system (HEC-RAS), quantitative & descriptive research, Compostela, Davao De Oro, Philippines
INTRODUCTION
Poor drainage system can cause flood. Flood is a surge of water that overflows the usually dry land and an area of study in the hydrological discipline. In order to avoid lethal floods, it is important to have very effective drainage systems and strive towards ensuring free flow of water during and after heavy downpours. As most people are well aware, the immediate consequences of flooding include loss of human life, property damage, crop destruction, livestock loss, and degradation of health conditions due to waterborne diseases.
Drainage problem has been an issue in some countries. A study from Bangladesh proved that a not well-structured and low-quality drainage facilities can result water logging and flash-flood that caused damage to the roads and conflict to human activities. A poor drainage system can be an acute problem for some areas and can be easily dealt with the correct disposal of waste and proper maintenance of water channels (Mark & Chusit, 2015).
Neglecting drainage problem can cause additional issues in addressing flood hazards. In the Philippines, a poor drainage system affected numerous barangays and cities when the water surges due to heavy downpour. Integrating and improving canals cannot keep up with the large amount of water particularly during rainy season (Rulona, 2018).
The lack of a Master Drainage Plan is one aspect that contributes to flooding. It has been a problem in the Municipality of Sto. Tomas. According to the Provincial Disaster Risk Reduction and Management Council (PDRRMC), the municipality was under a state of calamity after torrential rains flooded several towns, causing destruction to properties and displacements of thousand people. The primary aspect has been a lack of adequate drainage outlets and unorganized culvert sizes. A master drainage plan will aid in the prevention of flood-related activities in the immediate future.
In order to reduce the chances of major flooding, this study provided a solution by creating a road map for potential drainage-related activities within new or current projects in the selected barangays of the Municipality of Sto. Tomas using HEC-RAS and GIS software.
OBJECTIVES OF THE STUDY
This study aimed to create a flood detention pond design with the help of conducting a geographic mapping of flood-prone areas at the locality in Sitio Crossing Gabi, Brgy. Poblacion, Compostela, Davao de Oro.
To accomplish the goal of this study, the following objectives were established:
1. To obtain a topographic map in Poblacion, Compostela, Davao de Oro using Geographic Information System (GIS).
2. To conduct a hydraulic analysis of the main canals and creeks that are located in Sitio Crossing Gabi, Poblacion, Compostela using Hydrologic Engineering Center – River Analysis System (HEC-RAS) software to determine:
2.1. Cross-sectional Areas;
2.2. Normal Flow;
2.3. Peak Discharge;
2.4. Drainage Area;
2.5. Rainfall Intensity
3. To map out the flood-prone areas in the Poblacion, Compostela using Geographic Information System (GIS) indicating a predicted time frame of 25 years that is based on;
4. To determine the possible location of Flood Detention Pond in Sitio Crossing Gabi, Poblacion, Compostela.
5. To propose a Flood Detention Pond Design specifications based on the determined results of the hydraulic analysis in terms of:
5.1. Detention Pond Area;
5.2. Detention Depth;
5.3. Detention Pond Storage or Volume
MATERIALS AND METHODS
This study followed a quantitative study that employed a descriptive design specifically utilizing the Geographic Information System (GIS) and Hydrologic Engineering Centers River Analysis System (HEC-RAS) software. A good descriptive analysis shows what we know about a population's capabilities, demands, techniques, behaviors, and laws in a way that is pertinent to certain research (Loeb et al., 2017). In addition, descriptive research is often used as a prelude to quantitative research designs because it gives a comprehensive summary and some useful links as to which variables should be tested quantitatively (Shirima, 2020). The researchers gathered data and information to map out flood-prone areas, employed a descriptive research design, and created a basis for flood detention pond design.
In this study, GIS was used to create a map that depicted the location of the detention pond design and assisted the researchers in creating maps to identify the detention pond, which served as a design basis for the researchers. Furthermore, HEC-RAS was used as an instrument in analyzing the canals and creeks to determine water rise levels in projected years. This software helped the researchers attain the results that they needed within their parameters and the objective of the study. The results obtained was used to locate the possible location of flood detention pond design and provided specifications for a flood detention pond design in Sitio Crossing Gabi, Poblacion, Compostela, Davao de Oro.
RESULTS AND DISCUSSIONS
The contour lines that represent the elevation or height from the mean sea level of the Brgy Poblacion, Compostela, Davao de Oro, which is essential to better understanding the terrain of the study area. This map has an elevation range of 73–85 meters, it has graduated colors and the darkest color denotes the highest elevation, whereas the lightest denotes the lowest elevation. Moreover, Figure 6 shows the topology of Brgy Poblacion, Compostela, Davao de Oro in a 3D map view.
Simulated peak discharge of the water flow. A computed peak flow of 19.17 m3/s in 5 years, 23.74 m3/s in 10 years and 29.77 m3/s in 25 years return period. Water surface elevation and the top width of the Crossing Gabi stream in a simulation using the HEC-RAS software within the given period of years.
Watersheds that cover the area. The maps reveal the drainage pattern of the area, which is the outcome of the topography, slope, the length of water flowing, the tributaries, and the area's waterways. Compostela has been identified as a low-lying area as it has a lower elevation since the province it belongs to, is a valley.
Flood-prone areas in Brgy. Poblacion, Compostela, Davao De Oro in the 25-year return period that is based on along with the guide of previously gathered parameters. The darkest color represents the area with high flood susceptibility; the lighter color represents the area where the occurrence of the flood is moderate and the lightest areas are those with low flood occurrence.
Possible location of the detention pond in Sitio Crossing Gabi, Brgy. Poblacion, Compostela, Davao de Oro. The highlighted blue shade is an area 100 feet, more or less, away from the stream, which is the suitable distance to build a detention pond according to DPWH Design Guidelines, Criteria, and Standards: Volume 3 – Water Engineering Projects Section 6.8 Detention Basins
The flood detention pond design specification summary is shown in Table 16. The data was determined through a series of computations in Hydrology Studio Software based on the results of the determined hydraulic analysis. The software utilizes the Rational Method in getting the discharge of the detention pond. The specifications then are calculated by the software that is shown in the table.
FINDINGS
The following were the summary of the findings enumerated and discussed in the preceding chapter
1. The river geometry of the Crossing Gabi stream estimated from Section 105 to Section 0 has a 21 meters’ interval and a total of 6 Sections. Having a width of 6.08, 6.08, 6.08, 6.08, 6.08 and 3.87 meters and a length of 2.11, 2.07, 2.03, 1.99, 1.95, and 1.91 square meters in descending order. The area of the cross-sections is 12.83, 12.58, 12.34, 12.10, 11.86, and, 11.61 square meters. The flow area in every cross-sectional area gathered is 0.96, 0.82, 0.66, 0.44, 0.64 and 0.42 in-squared meters from upstream to downstream, respectively. The stream has a drainage area of 347.3 square kilometers; the recorded maximum rainfall intensity was 234.4 mm. A rainfall duration of 2.85 hours and an average slope of the river of 0.059. The interpolated rainfall intensity duration frequency using the data gathered over 5 years, 10 years, and 25 years is 65.72, 81.36, and 102.03 millimeters per hour. A computed peak flow of 19.17 m3/s in 5 years, 23.74 m3/s in 10 years and 29.77 m3/s in 25 years return period.
2. The possible location of the flood detention pond in Sitio Crossing Gabi, Poblacion, Compostela, Davao De Oro was determined through the use of Quantum Geographic Information System (QGIS) software and Google Earth application. The selected location with a 30.48-meter radius distance from the main stream was based on DPWH DCGS Volume 3 Section 6.8 Detention Basins.
3. According to the hydraulic analysis' determined pond report data, the detention pond should have a bottom elevation of five meters (5m), a bottom length of fifty meters (50m), a bottom width of twenty meters (20m), a slide slope of 1.5 (H:1), and a total depth of ten meters (10m). According to the storage table, the pond has an area of 1,107 m² which can hold 527 m3 of water.
CONCLUSIONS
Based on the finding of this quantitative study, the following conclusions were drawn:
The researchers discovered that the QGIS generated map of the Barangay Población, Compostela, Davao de Oro has an elevation range of 73–85 meters above sea level which resides in the center of the tallest ridges of mountains where it is considered a high flood risk area considering the entire results that have been taken.
It is essential to understand the potential damage that a flood could cause to nearby residential and agricultural areas wherein locals rely on a living.
The researchers also ascertained through the flood-prone map that Sitio Crossing Gabi was one of the areas with high flood risk in the Barangay Poblacion where most areas that experience high flood occurrence are those near streams and riverbanks.
The suggested possible location of the flood detention pond was located 100 feet or 30.48 meters away from both the upstream and downstream parts of the stream wherein the area was a plain land and had enough distance away from the residential region and public roads making it a very suitable area for the pond.
The hydraulic analysis' pond report data determined that the pond should have a bottom elevation of five meters (5m), a length of fifty meters (50m), a width of twenty meters (20m), has an area of 1,107 m², and a capacity of 527 m3.
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