Mananga Watershed

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               The Mananga Watershed is located in the lower portion of Cebu. This covers 2 municipalities and 16 barangays, with a total area of 7,900 ha. The main river of the watershed is the Mananga River, while its tributaries include Manipis River, Sinsin River, Wapairan River, Managuksok River and Morga Creeks (University of San Carlos & DENR Region VII, 2000).

 

  On May 7, 2014, Saturday the USC Phil LiDAR 1 Team (together with the Phil LiDAR 2 Team) conducted an ocular inspection in the Mananga watershed to familiarize the place. During the ocular inspection rough measurements of boundaries using GPS and calibrated tape were made, pictures were taken, and some markers in the field were indicated in maps provided by USC-WRC. The digitized map is based on NAMRIA maps having scale of 1:50,000. The contour interval of the map is 20 m which is not the level of refinement required to develop reliable rainfall-runoff relation and flood forecasting models. Since Mananga is one of the protected watersheds in the province of Cebu as declared in Presidential Proclamation 1074 and 581 (University of San Carlos & DENR Region VII, 2000) the area is expected to have minimum human incursion. This is to some degree true in the upper part of the catchment but not in the lower part where human activities have made significant changes to the landforms. The available map may therefore be reliable in the upper part of the catchment but not in the lower part.

 

          A map of Mananga Watershed was also prepared from available data in the USC Department of Civil Engineering GIS Laboratory (see Figure 3.1). One can see roads and trails that would permit access to the watershed.

 

Staff Gages

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        There are staff gauges (markers that indicate the depth of water) in the river. These were installed by USC-WRC. The staff gauge shown in Figure 3.2 is marked on the column of the bridge at Camp 4, Talisay City. The highest mark is 3 m.

 

      One concern that can be observed from Figure 3.2 is the lowering of the river bed which is manifested by the emergence of a portion of the pile cap. The lowering of the river bed poses a threat to the structural integrity of the connection between the piles and the pile cap because they will be exposed to the impact of strong water current during flood events. In addition, the lowering of the river bed has implication on the water storage capacity of the watershed.

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  About 300 m downstream of the staff gages described in Figure 3.2, one can find another set of staff gages installed at different levels (see Figure 3.3) by USC-WRC. These staff gages were put in place to capture both low and high flows. They were securely fastened to withstand the impact of the water current. During high flows both staff gages can be read simultaneously to get an idea of the water surface slope, which is an important parameter in estimating the flow velocity. These staff gages were installed by USC-WRC.

 

Rain Gauges

 

         A number of rain gauges were also installed in the Mananga Watershed. Two prototypes of these rain gauges are shown in Figure 3.4. The white one is a manual rain guage, wherein, the rainfall collected by the gage is stored in the lower cylinder and this will be measured by an observer using a graduated cylinder. The measurement is done twice a day one at 8:00 AM and the other at 5:00 PM. This type of raingage can only provide average daily intensity. Actual rainfall duration and intensities at shorter time interval cannot be extracted. The black one is a recording tipping bucket type rain gauge. The red box fastened at the post supporting the gage is a data logger that stores the rainfall collected by the rain gauge. The data can be downloaded to a laptop computer. The recording raingage can provide rainfall intensities at shorter intervals; say 5, 10, or 15 minutes and it can also provide the duration of the rainfall event. The two rain gauges are placed side by side for checking purposes or as backup if in case the recording rain gauge fails to record the rain. The two gages are installed in Jaclupan, Talisay City particularly at the site where the siltation and infiltration basins for the water impounded by the 7-m high Jaclupan weir. The facilities are maintained by the Metro Cebu Water District (MCWD). There are a number of rain gauges installed by USC-WRC all over Metro Cebu for MCWD projects and of these about 7 or 8 rain gauges can be made as reference for watershed study of Mananga.

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Stevenson Screen

 

A Stevenson screen is a louvered enclosure which is made of wood and painted with white color. It is used to house meteorological instruments such as thermometers and hygrometers. The design of such enclosure is to allow circulation of air while protecting the instruments from direct rays of the sun and direct exposure to rain. The Stevenson screen shown in Figure 3.5 was installed by USC-WRC when the center undertook a project for MCWD in 2011. The enclosure houses a thermometer and a hygrometer that monitor temperature and humidity, respectively, continuously. The data collection started on July 10, 2011 and it is continuing up to the present. The location of this meteorology station is at the infiltration basin of the Jaclupan weir.

 

  The Field Validation Component (FVC) from UP-Diliman visited the University of San Carlos on September 18-19, 2014 for mentoring. An ocular survey was conducted at Mananga Bridge in Talisay City (see Figure 3.6), where an automatic water level sensor (AWLS) was installed. On September 30, 2014, the USC Phil LiDAR 1 Team conducted preliminary cross section and velocity measurements of Mananga in preparation for flood modeling mentoring. At the time of the measurements, the weather was fine and normal condition of the river was observed (see Figure 3.7).

 

Preliminary Cross-Section and Velocity Measurements

 

    The Field Validation Component (FVC) from UP-Diliman visited the University of San Carlos on September 18-19, 2014 for mentoring. An ocular survey was conducted at Mananga Bridge in Talisay City (see Figure 3.6), where an automatic water level sensor (AWLS) was installed. On September 30, 2014, the USC Phil LiDAR 1 Team conducted preliminary cross section and velocity measurements of Mananga in preparation for flood modeling mentoring. At the time of the measurements, the weather was fine and normal condition of the river was observed (see Figure 3.7).

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For cross-section measurements the following steps were adopted:

 

    1. Place one range pole/stick on each side of the river bank.

 

    2. Tie a rope connecting the two poles/sticks.

 

    3. Place markers along the rope indicating the desired distance interval. The distance interval may vary depending on the profile of the river. The lower the interval the more accurate are the data.

 

    4. Place the rod with the reflector in every marker and record every reading.

The data obtained from the total station are the vertical and horizontal angles and the horizontal and sloping distances.

    5. Calculate the cross-sectional area of the river (see Figure 3.8 and Figure 3.9).

 

    In this study the data obtained were plotted and the cross section was determined. All measurements are referred from the NAMRIA benchmarks at the edges of the bridge. The instruments used can be seen in Figure 3.10.

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    At the time of the field survey, the flow meters have not arrived yet. So we devise a methodology based on the Study Leader’s (Engr. Ric Fornis) experience in flow measurements using floats. Oranges have been traditionally used as floats for flow measurements because they have the right density to be half-submerged. This condition is necessary such that the movement of the oranges represents the water movement while their exposed part can be used for tracking their position.

 

    The following steps were then followed for velocity measurements using floats (see Figure 3.12):

 

    1. Establish a starting line and a finish line with a minimum distance of 20 meters in between.

 

    2. Release an orange before the starting line. As soon as the orange passes through the starting line start the timer. Record the time when it reaches the finish line. The velocity can be determined by dividing the distance (in this case 20 m) by the time.

 

    3. Perform at least five trials in order to lessen the error. Calculate the average velocity over the number of trials.

 

    4. In the event of a rainfall, perform a set of trials every 10 minutes. However, if there is no rainfall during the fieldwork perform the set of trials every 30 minutes since the change of water-level and velocity is very small.

 

    It has to be noted that quarrying is rampant in Mananga River. In fact, quarrying is visible right under the Mananga Bridge as shown in Figure 3.13. The location is right below the AWLS. This activity can change the cross section of the river and hence it can affect the flow of the water and the reading of the AWLS.

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    The flow measurements in Mananga River are presented in Table 3 1. Fiver trials were done to calculate the velocity in every 30-minute interval

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Kotkot River Basin

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  Kotkot is one of the major river systems in Cebu. It is part of the Kotkot-Lusaran Watershed and is a protected area (together with Mananga) under Presidential Decree 1074 and 581. Kotkot practically straddles the municipalities of Consolacion and Liloan. A possible site for hydrologic measurements is the Cansaga Bridge in the municipality of Consolacion, where an AWLS is installed (see Figure 3.14). This bridge is near SM-Consolacion.

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  Cross section of the area under the AWLS was obtained using the same methodology used in Mananga. Velocity of the flow was also measured using float method. A drainage adding to the discharge of the river can be observed in Figure 3.15. Velocity was measured at distances before the drainage and after the drainage to verify the assumption that the drainage can add up to the discharge and therefore affect the velocity. The measured velocity ranges from 0.18 to 0.31 meter per second for a distance of 6 meters. At a distance of 20 meters which is then affected by the drainage described earlier, the velocity ranges from 0.20 to 0.32 meter per second. Table 3 2 shows the flow measurement results for a distance of 6 m while Table 3 3 shows the measurement results for a distance of 20 m. This distance encompassed the drainage.

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