Baseflow measurements were done on 8 rivers in the second year, having a total number of 10 rivers with baseflow measurements from the first year to the second year. The baseflow of the river was obtained by gathering velocity and depth measurements. These are incorporated in the surveyed cross sections of the rivers. The velocity is obtained by using a mechanical flowmeter while the depth is obtained by the use of depth gauge or by manual measurements. The cross sectional area were obtained either by using the total station or a dual frequency GPS. The acquisition of the cross section of the study areas are activities to be done by the Data Validation and Bathymetric Component of UPD. However, on instances when the data was still not acquired, the USC PhilLiDAR 1 team acquires the data to avoid delays in the acquisition of data.
Flow data acquisition for Mananga River was conducted in Camp 4, Talisay City. Figure 3-14 presents a photograph of the study area. Mananga River is one of the primary sources of potable water for the city of Cebu. The purpose of the activity were to record the normal base flow of the river, to document flooding, and to account the past flooding that affected the area (flow rate, flood level, stages of the flood, etc.).
Since Water level guides (see Error! Reference source not found. 3-15) are painted on the bridge piers so that nearby residents of the river can monitor the level of the river when flooding occurs helping residents to decide whether to evacuate or not. According to the locals, previous floods reached up to the “WARNING” sign and usually approximately 1.3 meters above the river bed.
Velocity was measured using a mechanical flow meter and is presented in Figure 3-16. We requested from the Camp 4 barangay captain two (2) local residents who are qualified to be trained in acquiring flow data using the flow meter and to conduct flow data measurements during extreme events (flooding) using floats.
Data was acquired for 30 days and the average discharge was obtained to represent the discharge of the river. The computed baseflow of Mananga River is 0.20
The table below summarizes the data acquired in Mananga River. The data in black are collected during fine weather while the data in red and dark red were obtained during rainfall events.
Kotkot River, Cebu
Base flow measurements at Kotkot River were done from November 13, 2014 to December 31, 2014. These measurements were conducted at a steel foot bridge in Barangay Cabadiangan, Liloan, Cebu which is presented in Figure 3-17.
For normal flow velocity we used the flow meter but for extreme events floats were used. The flow meter was secured on a wading stick and was submerged at a level where the rotor was almost right under the water surface as can be seen in Figure 3-18.
In using floats, a fixed distance of twenty nine meters (29m) was set up along the river channel as shown in Figure 3-19. The time it takes for the floats to traverse the 29-m distance was taken. The velocity can be computed from the distance and time.
On November 27, 2014 Typhoon Queenie hit Cebu, where storm signal Number One was raised by PAGASA. At around 7:00 am the water at Cotcot River started to rise and became murky. At around 9:00 am the water level rose to a depth of 0.85m having an average flow velocity of 1.55m/s with a cross sectional area of 9.32m2 . The flooding event had a maximum depth of 1.58m with an average flow velocity of 2.96m/s and having a cross sectional area of 18.1m2. These acquired event discharge data are then used in the calibration of the flood model of Kotkot River.
On December 7, 2014 Typhoon Rubie also hit Cebu under Signal Number Two. It gave a maximum depth of 1.66m with an average flow velocity of 2.65m/s and cross sectional area of 19.13m2. The flooding event lasted for two days.
On December 30, 2014 another typhoon called Seniang made landfall in Cebu around midnight and by 2:00 am the water level rose to a depth of 3.55m having a cross sectional area of 51.5m2, the flow velocity was not taken because it was too dangerous to perform the measurement. The water level reached its maximum depth of 3.95m around 4:00 am having a cross sectional flow area of 61.39m2. At 9:00 am the water level dropped to 1.69m having an average flow velocity of 2.54m/s with a cross sectional flow area of 19.52m2. The flooding event also lasted for two days. From the series of values obtained, an average discharge of _____ was computed to represent the baseflow of Kotkot River.
The final study area chosen for flow measurements is located before the spillway constructed by the Municipality of La Libertad. The cross section was obtained by the USC Phil LiDAR 1 team using a total station. The acquired cross section however does not display elevations in mean sea level but displays the difference in elevation with respect to a reference point. The cross sectional area survey is presented in Figure 3-20.
Baseflow of the river was obtained by conducting 30 days of flow measurements. These measurements were conducted from April 22, 2015 to May 22, 2015 under fair and sunny weather conditions. Velocity measurements can either be measured by the use of flow meter or by the use of floats. Flow data were taken for three straight hours in the morning and three straight hours in the afternoon with a ten-minute interval. Depth of the water was also measured corresponding to the velocity measurements obtained. During the start of the acquisition of the velocity and depth measurements, two research associates trained and guided the local hires. As mentioned earlier, the early stages of the acquisition of such data were conducted simultaneously with the acquisition of the cross sectional area. During the later part of the acquisition of data, revisits were done by two research associates to monitor, answer inquiries of the hired personnel, to check on the data, and to hand over the compensation of the hired personnel.
Since during the entire 30 days of the acquisition of flow data the weather was fair and sunny, the mechanical flow meter was used. The flow meter was secured with a wading rod and was submerged at a level that the rotor was almost right under the water surface. Figure 3-21 presents the acquisition of data conducted by the locally hired personnel.
For the gathering of data in La Libertad River, two personnel were hired. They are Mr. Arthur Gabunales and Mr. Samuel Garson. Aside from the acquisition of flow data for 30 days, they will also be conducting flow measurements during extreme events. Flow measurements during extreme events will be done by the use of floats.
The specific location where measurements were taken along the Tanjay River was at Barangay Novallas Tanjay City with geographic coordinates of 9°29’59.82”N and 123°7’30”E. Measurements were done simultaneously with La Libertad River. The entire cross section of the river was obtained and the local hires were trained to obtain velocity and depth measurements. Only one cross sectional measurement was done and it was obtained last April 21, 2015. Figure 3-22 presents the photograph during the determination of cross sectional area and the local hire training for velocity and depth measurements. The cross section was obtained using a total station. This data was used for the determination of the cross sectional areas of the water flow corresponding to the obtained velocity and depth. The obtained cross section is presented in Figure 3-23.
Baseflow of the river was obtained by conducting 31 days of flow measurements from April 21, 2015 to May 21, 2015. The data were gathered under fair weather conditions. Velocity measurements can either be done using floats or using a flow meter. Since the acquisition of data was taken under the stated conditions, the mechanical flow meter was used. Two research associates were assigned for the training of local hires during the early stages of the data acquisition. Revisits were done during the later part of the data acquisition. Log of the revisits in the area is attached in this document. Figure 3-24 shows the gathering of velocity and depth measurements by the local hires.
The hired personnel for Tanjay River were Mr. Francisco Buenavista Jr. and Mrs. Brenda Aguilar. Aside from the acquisition of flow data for 31 days, they will also be conducting flow measurements during extreme events. Flow measurements during extreme events will be done by the use of floats. From the obtained data, the arithmetic mean was obtained to be 0.29 m^3/s and this value is used to represent the baseflow of Tanjay River.
The USC PhilLidar 1 team started the 30 days flow data measurement on Inabanga River. Inabanga River is one of the longest and widest rivers in the Province of Bohol with its headwater location reaching up to Danao, Dagohoy and Pilar, Bohol.
The chosen study area for flow measurements acquisition is in Brgy. Baguhan Bridge, Inabanga. The local hires were oriented and instructed how to use the mechanical flow meter. They were taught how to read and tally the data needed as seen in Figure 3-25.
Actual deployment of the flow meter was carefully observed to minimize possible error in flow data measurements. Proper positioning of the flow meter was inspected for it to be parallel with the river flow. Manual measurement for depth was performed by using steel tape. Figure 3-26 presents a photograph taken during the actual acquisition of flow data by the locally hired personnel as supervised by Research Associate Engr. Arthur Sabandal.
The flow data acquisition of Inabanga River commenced on June 6, 2015 and ended on July 6, 2015. The data gathering was inspected weekly by revisiting the local hires for one day to ensure proper data collection. From the obtained data, the average discharge calculated was 8.440m^3/s and this represents the baseflow of Inabanga River.
The location of the base flow measurement area is at Dorol Bridge, Balilihan for the Abatan River. The site is 17.4 kms from the mouth of the Abatan River. The local hires were trained to collect data for the base flow measurement for Abatan River. They used a bamboo raft to deploy the mechanical flow meter. Figure 3-28 presents the acquisition of data of the locally hired personnel while on a bamboo raft. It was ensured that the raft is in the same location during the entire course of the acquisition of the data. The data was acquired from July 15 to August 12, 2015. From the series of data obtained, an average discharge of 0.738m^3/s was computed.
The location of the base flow measurement is on Tugas Bridge, Trinidad Bohol for Ipil River. The site is located 10.6 km from the mouth of the Ipil River. Since the cross section of this area was not yet acquired by UPD-DVBC, USC PhilLIDAR 1 Team acquired the cross section by the use of dual frequency GPS. Figure 3-29 presents the set up at Brgy Tugas of the instruments used for the survey.
Training of local hires was performed under the Tugas Bridge because it was the only part that a person can stand in the river. The river banks were densely covered by mangroves and trees that thrive near river systems. The training of the locally hired personnel and the mechanical flow meter set-up. The acquisition of flow data was done from October 16, to November 11, 2015. From the series of data obtained, the baseflow was calculated to be 4.065m^3/s.
Guinabasan river is located in the municipality of Asturias. A steel bridge located at Barangay New Bago was selected as the study area. It is approximately 2.8 kilometers from the mouth of the river. The team was endorsed by the municipality MENRO to the Barangay Captain. The barangay captain was generous enough in assisting the team in hiring local hires for the 30 days base flow acquisition. The data acquisition started last September 2, 2015 and ended at October 2, 2015. The team managed to perform cross sectional survey of the said study site. The NAMRIA point which serve as the reference for the cross sectioning was located at Guinabasan Bridge. The cross section was acquired by USC PhilLiDAR1 team using a dual frequency GPS. The cross section acquired is presented in Figure 3-31.
According to the locals, the last strong typhoon to cause major flooding in the Guinabasan River was typhoon Basyang last January 2014 since then other typhoon causes flooding but not as much the damage done by Basyang. Two locally hired personnel acquired flow data for 30 days in Guinabasan. Figure 3-32 presents a photograph taken during the training. An average discharge of
Combado-Lusaran River (Balamban River)
Balamban river or what is locally known as Combado-Lusaran is located in the City of Balamban. The chosen study area is located at Barangay Biasong, Balamban. It is located near a spring. The team had difficulties in choosing a study area due to the rampant quarrying activities within the Combado-Lusaran River. According to the municipal MENRO, each contractor has permit for the said activity. The profile of the river changes from time to time due to these quarrying activities. Base flow acquisition started September 7 and ended on October 7, 2015.
According to the locals, the water in the river only becomes clear during dawn since no dump trucks are crossing the river. Once the working hours starts the water tends to become murky. Cross sectional area of the study area were obtained using the Dual Frequency GPS. The NAMRIA point, reference point, was located at Asturias Bridge which is approximately 5km from the study area.
A closer NAMRIA point was located along the Balamban Highway but the team does not have the coordinates of the said point. The last typhoon that causes major flooding was typhoon Yolanda. Typhoon Ruping occurred November 1990 caused the biggest damage in Balamban. Due to the quarrying activities recent typhoon causes only minor flooding.