RAINFALL INTENSITY DURATION (FREQUENCY) RIDF DAVAO CITY A NALYSIS OF RAINFALL IN D AVAO C ITY
Rainfall intensity, duration and frequency analysis of the available rainfall data in Davao City . Part of B.BE Third year internship research water management at Rotterdam University of Applied Science. Project Document name Version Organizations Website Author Date, location
Hydrologic model of Davao City A.01 4.2 HELP Davao, TropICS, Ateneo de Davao, DOST XI http://davao.vincentoskam.nl Vincent Oskam 4 January 2013, Davao, Philippines
Region XI Davao
Rainfall Intensity Duration (Frequency) RIDF Davao City Analysis of rainfall in Davao City
I. INTRODUCTION
METHODS
Davao City is the biggest city on the Island Mindanao in the Philippines, with 244 000 hectares of land one of the largest cities in the world on land area (Davao City), located in the Davao bay it is a delta city. As third year student water management, this research has been part of the internship requirements of the Rotterdam University of Applied Science.
PROBLEM/ISSUE According to Dutch PUM expert M.M. Vierhout, who was asked to advise the city of Davao, many parts of the city are subject to waterlogging and flooding during heavy rainstorms. The flooding is said to be mostly of short duration (a few hours) and only shallow inundation (10 – 40 cm) (Vierhout, 2012). Best practice to design storm water drainage systems is the rational method (Vierhout, 2012) (World Health Organization (WHO)). To compute design capacity using the rational method, the Rainfall Intensity Duration Frequency (RIDF) has to be known. RIDF curves show the ratio between intensity and duration of rainfall events with a certain probability.
The data from scarcely available rain gauges in Davao City has been used to analyze rainfall intensity and duration frequency (RIDF). To make an accurate assessment of RIDF, long term high frequency measurements are necessary. Unfortunately high frequency measurements, e.g. every half hour, has started less than three years ago. Therefor the reliability and representability of this assessment is limited and should be interpreted as such. However, this is the maximum possible at this point.
Illustration 1.1 Rain gauges in and around Davao City (Oskam, 2012)
To calculate the expected return time of certain events the following method has been used. ∑ ∑ = Probability of rainfall in a year where p is greater than x ∑ =Total number of recorded events where p is greater than x ∑ = total number of recorded events Years = years of registered data
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Rainfall Intensity Duration (Frequency) RIDF Davao City Analysis of rainfall in Davao City
KEY FINDINGS Long term rainfall data for Davao City is only available from Pag-Asa Davao and only of low frequency (every six hours). Representative high frequency measurements are available from PREDICT Panabo and Manila Observatory Shrine Hills Davao.
(TROPICS) at Ateneo de Davao University. Pag-Asa Davao who provided long term rainfall data. Professor Heikoop from the Rotterdam University of Applied Science who made this internship opportunity possible.
Analysis of Rainfall Intensity Duration Frequency (RIDF), give an indicative design rainfall event of 80 mm/hour or 120 mm/2 hours. This event has an approximated return period of two years (between one and three years). Best practice for designing storm water drainage capacity is one or two years.
ORGANIZATION OF THE PAPER Chapter one shows the rainfall events greater than 48 mm in six hours for the most representative rain gauge stations. Chapter two covers the RIDF analysis over the Manila Observatory rain gauge in Shrine Hills Davao. Chapter three contains the conclusions of the analysis. I.
Introduction ....................................................... 2
1.
Intense precipitation events........................ 4
2.
RIDF Manila observatory .............................. 5
3.
Conclusions......................................................... 5
II.
Works cited......................................................... 5
ACKNOWLEDGEMENTS This paper would not have been possible without the help of several individuals and organizations. I would like to extend my gratitude to all whom made this research possible. All members of the HELP Davao network, the Department of Science and Technology, The TROPical Institute for Climate Studies Vincent Oskam, 2012
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Rainfall Intensity Duration (Frequency) RIDF Davao City Analysis of rainfall in Davao City
1. INTENSE PRECIPITATION EVENTS The rain gauges with the most representable rainfall data are PREDICT Panabo and Manila observatory in Shrine Hills (Oskam, 2012). Pag-asa Davao has registered more than 50 years of rainfall data, but only with a frequency of six hours. Rainfall events of more than 48 mm in six hours have been analyzed. Almost eight months of rainfall data from Panabo, with 15 minute frequency, shows one event of 80 mm in one hour. The intensity seems almost linear for the first hour and almost stops after that.
Analysis of almost three years of 30 minute frequency measurements from Manila observatory in Shrine Hills Davao show more intense rainfall with similar patterns. The intensity is almost linear until the rainfall nearly stops. Regular events of 60 mm in two hours are shown. The most intense is 80 mm in one hour and 120 mm in two hours. This would indicate a return period of approximately two years.
120
120
100
100
Rainfall in mm
Rainfall in mm
140
80 60 40 20
80 60 40 20
0
30 60 90 120 150 180 210 240 270 300 330 360
15 30 45 60 75 90 105 120 135 150 165
0
Duration in minutes
Duration in minutes
Station Station Source Location Recorded period Measurement frequency Consistency
PREDICT Panabo (PREDICT, 2010) X 125.68, Y 7.3 1-Feb-12 – 25-Sep12 Every 15 minutes Shows some missing entries
Illustration 1.1 PREDICT Panabo - accumulative rainfall > 48 mm/6hour
Vincent Oskam, 2012
Source Location Recorded period Measurement frequency Consistency
Manila Observatory Shrine Hills (WeatherLink, 2011) X 125.58, Y 7.07 29-Dec-09-18-Aug11 & 21-Aug-11-13Nov-12 Resp. Every 30 minutes & Every 5 minutes Consistent data
Illustration 1.2 Manila Observatory - accumulative rainfall > 60 mm/6hour
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Rainfall Intensity Duration (Frequency) RIDF Davao City Analysis of rainfall in Davao City
2. RIDF MANILA
II. Works cited
OBSERVATORY
4 hour
3 hour
2,5 hour
2 hour
1.5 hour
1 hour
160 140 120 100 80 60 40 20 0 30 mins
Rainfall in mm
Calculating the probability of a certain rainfall intensity over the total period of measurements gives the average statistical return period of rainfall intensity duration. The results with logarithmic trend line are shown in illustration 2.1. It must again be emphasized that assessment over this short duration can never be accurate, however they are expected to approximate the actual values and can be used as indicative values.
Duration Illustration 2.1
Davao
City. (n.d.). City planning and development office. Retrieved 09 19, 2012, from Office of the city planning and development coordinator: http://www.davaocity.gov.ph/cpdo/ Oskam, V. (2012). Hydrologic model of Davao City. Hydrology for Environment, Life and Policy (HELP) Davao. Davao: Ateneo de Davao university. PREDICT. (2010). Rain Gauge Panabo. Retrieved 10 2012, from PREDICT Philippine Real-Tim eenvironment Data Acquisition and Interpretation for Climate-Related Tragedy prevention: http://202.90.128.12/predict/station s.php Vierhout, M. M. (2012). PUM expert's Final Report on Mission Drainage of Davao. Davao City, Philippines. WeatherLink. (2011). DavaoMO NOAA 2011 Yearly Summary. Davao City, Philippines. Retrieved 11 15, 2012 World Health Organization (WHO). (n.d.). Annexe 6: Designing a simple drainage system.
3. CONCLUSIONS As long term high frequency measurements are scarce, Rainfall Intensity Duration Frequency (RIDF) can only be approximated. Using almost eight months of rainfall data from PREDICT Panabo and almost three years of rainfall data from Manila Observatory Shrine Hills Davao a design rainfall event of 80 mm/hour and 120 mm/2 hours is expected to be a good indicative approximation of a rainfall event with a return time between one and two years.
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