Rainfall-runoff models for all the river basins have already been set-up in HEC-GeoHMS. However only one HMS model has been calibrated due to the inavailability of Automatic Rain Gage and Water Level Monitoring Sensors installed by DOST.
Kotkot River is located in Cebu Province in Visayas. It covers the municipalities of Compostela, Liloan, Consolacion and the cities of Cebu and Danao. From the entire stretch of the river an outlet point was chosen to analyze the rainfall-runoff relations and the flood inundation. The outlet point analyzed is located in the Cabadiangan Bridge with geographic coordinates of 10°27’19.13″N and 123°57’40.89″E.
The watershed delineated has drainage area of 66.36 sq km. The delineated watershed covers Danao City, Cebu City and the Municipalities of Compostela, Liloan and Consolacion. The river drains through the mouth at Compostela.
The model of the watershed of Kotkot River was generated using HEC-HMS. The generated model consists of 19 subbasins, 8 reaches and 9 junctions. The outlet of the watershed was designated as Outlet 6 in the model. These hydrologic elements are presented in Figure 4-2.
Soil and Land Cover
The soil shapefile was obtained from the Mines and Geosciences Bureau (MGB) of the Department of Environment and Natural Resources (DENR). The land cover shapefile was obtained from the National Mapping and Resource information Authority (NAMRIA). The soil map is presented in Figure 4-3 and the land cover map is presented in Figure 4-4. These data are used to estimate the losses in the watershed by using the SCS Curve Number Loss Model.
Precipitation data was taken from two automatic rain gauges (ARGs) installed by the Department of Science and Technology – Advanced Science and Technology Institute (DOST-ASTI). The nearby rain gauge station of Kotkot River basin is located in Brgy. Panas Consolacion with geographic coordinates of 10.446306N and 123.930639E.
Figure 4 4. The location map of rain gauges used for the calibration of the Kotkot HEC-HMS model.
The rainfall data used for calibration was the rainfall event last November 26-27, 2014. The rain has a duration of 21 hours. The total rain from this station is 59 mm with a peak of 6.5mm at 00:30 of November 27, 2015.
Rating Curve and River Outflow
A rating curve was developed from the data obtained in Cabadiangan Bridge in Liloan with the location described in the first section of this paper. From the various depth and discharge data acquired in the field, the rating curve was established. This is presented in Figure 4-6.
The obtained relationship of the discharge and depth is described as where:
The gathered rainfall and discharge data last November 26, 2015 was used for the calibration of the model. This data is presented in Figure 4-6.
Results and Discussion
From the calibrated Kotkot HEC-HMS Model, the observed outflow was compared with the HEC-HMS simulated outflow. The two discharge data are presented in Figure 4-7.
Model Outflow using Rainfall Intensity-Duration-Frequency (RIDF) Analysis Data
The Philippines Atmospheric Geophysical and Astronomical Services Administration (PAGASA) computed Rainfall Intensity Duration Frequency (RIDF) values for the Mactan station. This station chosen based on its proximity to the Kotkot watershed. The extreme values for this watershed were computed based on a 37-year record.Five return periods were used, namely, 5-, 10-, 25-, 50-, and 100-year RIDFs. Rainfall data with duration of 12 hours were used in the simulation of the models.
Results using Rainfall Intensity-Duration-Frequency (RIDF)
The summary graph in Figure 4-8 shows the Kotkot outflow using the Mactan Rainfail Intensity-Duration-Frequency curves (RIDF) in 5 different return periods (5-year, 10-year, 25-year, 50-year, and 100-year rainfall time series) based on the Philippine Atmospheric Geophysical and Astronomical Services Administration (PAG-ASA) data. In the 5-year return period graph (Figure 4-9), the peak outflow is 306.8cms.
This occurs after 4 hours and 20 minutes, and a precipitation of 1.52mm. In the 10-year return period graph (Figure 4-10), the peak outflow is 417.1cms. This occurs after 4 hours and 20 minutes, and a precipitation of 1.78mm. In the 25-year return period graph (Figure 4-11), the peak outflow is 559.5cms. This occurs after 4 hours and 10 minutes, and a precipitation of 2.12mm.
A summary of the total precipitation, peak rainfall, peak outflow and time to peak of the Kotkot River discharge using the Mactan Rainfall Intensity-Duration-Frequency curves (RIDF) in five different return periods is shown in Table 4.1.