Ponce Elec4 Ulo3

Miguel Jesse Rex T.
Ponce ELEC4 ULO3
Efficiency of HEC-HMS Rainfall-Runoff Models

calibrated based on field survey and gauges data
Figure 28. Outflow Hydrograph produced by the HEC-HMS model compared with
observed outflow
After calibrating the Davao HEC-HMS river basin model, its accuracy was measured
against the observed values. The comparison between the two discharge data are shown
in Figure 28.
The Root Mean Square Error (RMSE) method aggregates the individual differences of
these two measurements. It was identified at 15.11924.
The Nash-Sutcliffe (E) method was also used to assess the predictive power of the
model. Here the optimal value is 1. The model attained an efficiency coefficient of
0.604049.
A positive Percent Bias (PBIAS) indicates a model’s propensity towards under-prediction.

Negative values indicate bias towards over-prediction. Again, the optimal value is 0. In
the model, the PBIAS is -25.30.
The Observation Standard Deviation Ratio, RSR, is an error index. A perfect model
attains a value of 0. The model has an RSR value of 0.63.
The calibrated models of the other discharge points are used in flood forecasting.
DREAM Program offers the LGUs and other disaster mitigation agencies a water level
forecast tool, which can be found on the DREAM website.
Figure 29. Sample DREAM Water Level Forecast
Given the predicted and real-time actual water level on specific AWLS, possible river
flooding can be monitored and information can be disseminated to LGUs. This will help
in the early evacuation of the probable affected communities. The calibrated models can
also be used for flood inundation mapping.
Calculated Outflow hydrographs and Discharge

Values for different Rainfall Return Periods
4.2.1 Hydrograph using the Rainfall-Runoff Model
The outflow of Davao using the Davao station Rainfall 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 PAGASA data are shown in Figures 30-34. The
simulation results reveal significant increase in outflow magnitude as the rainfall intensity
increases for a range of durations and return periods.
In the 5-year return period graph, the peak outflow is 357.5 cms. This occurs 7 hours and
30 minutes after the peak precipitation of 25.1 mm, as shown on Figure 30.
Figure 30. Outflow hydrograph generated using the Davao 5-Year RIDF in HEC-HMS
In the 10-year return period graph, the peak outflow is 357cms. This occurs 7 hours after
the peak precipitation of 28.8 mm, as shown on Figure 31.
In the 25-year return period graph, the peak outflow is 527.5 cms. This occurs 6 hours
and 30 minutes after the peak precipitation of 33.5 mm, as shown on Figure 32.
In the 50-year return period graph, the peak outflow is 673 cms. This occurs 6 hours and
20 minutes after the peak precipitation of 37 mm, as shown on Figure 33.
In the 100-year return period graph, the peak outflow is 850.7 cms. This occurs 6 hours
and 10 minutes after the peak precipitation of 40.5 mm, as shown on Figure 34.
Figure 34. Outflow hydrograph generated using the Davao 100-Year RIDF in HEC-
HMS
A summary of the total precipitation, peak rainfall, peak outflow and time to peak of Davao
discharge using the Davao Rainfall Intensity-Duration-Frequency curves (RIDF) in five
different return periods is shown in Table 2.
Table 2. Summary of Davao discharge using Davao Station Rainfall Intensity Duration
Frequency (RIDF)
Total Peak rainfall Peak outflow
RIDF Period Precipitation (mm) (cms) Time to Peak
(mm)
7 hours,
5-Year 121.26 25.1 247.2 30
minutes
10-Year 140.49 28.8 357.5 7 hours
6 hours,
25-Year 165.65 33.5 527 30
minutes
6 hours,
50-Year 183.55 37 673 20
minutes
6 hours,
100-Year 202.15 40.5 850.7
10 minutes
4.2.2 Discharge Data using Dr. Horritt’s Recommended
Hydrological
Method
The river discharge values using Dr. Horritt’s recommended hydrological method are
shown in Figure 35 and the peak discharge values are summarized in Table 3.
Figure 35. Outflow hydrograph generated using the Davao 5-, 25-, 100-Year RIDF in
HEC-HMS
Table 3. Summary of Davao river discharge using the recommended hydrological

method by Dr. Horritt
RIDF Period Peak discharge (cms) Time-to-peak
5-Year 1758.6 23 hours, 20 minutes
The comparison of discharge values obtained from HEC-HMS, QMED, and from the
bankful discharge method, Qbankful, are shown in Table 4. Using values from the DTM
of Davao, the bankful discharge for the river was computed.
Table 4. Validation of river discharge estimate using the bankful method

Discharge Point Qbankful, cms QMED, cms Validation
Davao (1) 1501.82 1758.6 Pass
The value from the HEC-HMS discharge estimate was able to satisfy the condition for
validating the computed discharge using the bankful method. Since the computed value
is based on theory, the actual discharge values were still used for flood modeling but will
need further investigation for the purpose of validation. It is recommended, therefore, to
use the actual value of the river discharge for higher-accuracy modeling.

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Miguel Jesse Rex T.

Ponce ELEC4 ULO3

Efficiency of HEC-HMS Rainfall-Runoff Models

A positive Percent Bias (PBIAS) indicates a model’s propensity towards under-prediction.

Calculated Outflow hydrographs and Discharge

Table 3. Summary of Davao river discharge using the recommended hydrological

Table 4. Validation of river discharge estimate using the bankful method

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