Study Notes on Runoff Estimation and Flood Analysis in the Philippines

Fundamentals of Runoff Estimation and Flood Analysis

• Flooding Overview in the Philippines

  • Flooding is a persistent and escalating hazard, particularly in Metro Manila and other rapidly urbanizing regions.

  • Primary drivers include heavy rainfall, low-lying terrain, and inadequate water management systems.

  • Reliable runoff estimation and flood analysis are essential for mitigating these risks.

• Key Concepts in Runoff Estimation

  • Runoff Estimation: The process of calculating the volume, flow rate, and timing of excess rainfall that travels across land surfaces into drainage systems, streams, or rivers. This calculation accounts for losses such as infiltration, evaporation, and plant absorption (interception).

  • Surface Runoff: Water that flows over the ground surface instead of soaking into the soil.

  • Drainage Area: The entire land area that contributes water to a specific common outlet point.

  • Rainfall-Runoff Relationship: The correlation determining how much rainfall converts into runoff, influenced by land cover, soil types, and topography (slope).

• Flood Analysis and Causes

  • Flood analysis utilizes runoff results combined with terrain data, drainage specifications, and historical records to assess risk, design control structures, and forecast inundation.

  • Core Causes of Flooding:

    • Heavy precipitation and upstream water flow.

    • Urban development and concrete surfaces (imperviousness).

    • Inadequate or poorly maintained drainage systems.

    • Low-lying terrain and tidal effects.

    • Climate-related shifts in weather patterns.

• Flood-Prone Areas in the Philippines

  • Metro Manila: Marikina, Pasig, Taguig, Manila, Malabon, Navotas, and Valenzuela.

  • Provinces: Bulacan, Pampanga, Nueva Ecija, Pangasinan, Tarlac, Cagayan Valley, Maguindanao, North Cotabato, and Oriental Mindoro.

The Rational Method for Runoff Estimation

• Definition: The Rational Method estimates the peak floodwater flow (peak runoff) from a small watershed during heavy rainfall. It is fundamental for designing urban drainage systems.

• Formula:     $Q = 0.278CiA$

  • $Q$ = Peak runoff or design flood ($m^3/s$)

  • $C$ = Runoff coefficient (dimensionless; represents the fraction of rainfall becoming runoff)

  • $i$ = Rainfall intensity ($mm/hr$)

  • $A$ = Drainage area ($km^2$)

  • $0.278$ = Conversion factor

• Detailed Variable Definitions:

  • Runoff Coefficient ($C$): Indicates the proportion of rain that flows over land versus soaking in.

    • High $C$ ($0.7$–$0.9$): Industrial areas, roads, concrete surfaces, and bare rock generate more runoff.

    • Low $C$ ($0.2$–$0.4$): Grasslands, forests, and sandy soils generate less runoff due to high infiltration.

    • Specific Examples: Parking lots ($C = 0.90$), Residential areas ($C = 0.40$–$0.70$), Forests ($C = 0.20$–$0.30$).

  • Rainfall Intensity ($i$): The rate of precipitation measured in $mm/hr$. It is derived from Intensity-Duration-Frequency (IDF) curves based on:

    • Rainfall duration (for the Rational Method, duration should equal the Time of Concentration, $T_c$).

    • Return period (e.g., $2$-year, $10$-year, $25$-year, etc.).

  • Drainage Area ($A$): The total land area draining to one outlet, typically measured in $km^2$. Larger areas generally produce higher runoff volumes.

• Time of Concentration ($T_c$):

  • The time required for runoff to travel from the hydraulically most distant point in the watershed to the outlet. Peak runoff occurs when the entire drainage area is contributing to the flow.

  • Kirpich Formula components:

    • $T_c$ = Time of concentration (minutes)

    • $L$ = Length of the longest flow path (meters)

    • $H$ = Elevation difference between the highest point and the outlet (meters)

  • Impact of physical features: A longer flow path increases $T_c$; a steeper slope decreases $T_c$ by accelerating flow.

• Application Process:

  1. Determine the drainage area ($A$).

  2. Calculate the time of concentration ($T_c$) using the Kirpich formula.

  3. Use $T_c$ as the duration to find rainfall intensity ($i$) from the IDF curve for the required return period.

  4. Select the runoff coefficient ($C$) based on land use.

  5. Solve for peak runoff ($Q$).

• Calculation Example:

  • Given: Area ($A$) = $2.00 \times 10^0 \text{ km}^2$, $C$ = $0.50$, $i$ = $80.00 \text{ mm/hr}$.

  • $Q = 0.278 \times 0.50 \times 80.00 \times 2.00 = 22.24 \text{ m}^3/s$.

SCS Curve Number Method

• Definition: Unlike the Rational Method which finds peak flow, the Soil Conservation Service (SCS) Method estimates the total volume (depth) of runoff produced during a storm event.

• SCS Runoff Formula:     $Q = \frac{(P - 0.2S)^2}{P + 0.8S}$

  • $Q$ = Runoff depth ($mm$)

  • $P$ = Total rainfall ($mm$)

  • $S$ = Potential maximum water retention/absorption by the soil ($mm$)

• Potential Maximum Retention ($S$):     $S = \frac{25400}{CN} - 254$

• The Curve Number ($CN$):

  • Represents the runoff potential of an area based on soil type, land use, and land cover.

  • Low $CN$ ($30$–$60$): Forests, grasslands, and sandy soils (high infiltration, low runoff).

  • High $CN$ ($70$–$100$): Urban areas, paved roads, and clay soils (low infiltration, high runoff).

  • Specific Examples: Forest ($CN \text{ approx. } 55$), Agricultural land ($CN \text{ approx. } 75$), Urban pavement ($CN \text{ approx. } 98$).

• Calculation Example:

  • Given: Rainfall ($P$) = $100 \text{ mm}$, $CN$ = $80$.

  • $S = \frac{25400}{80} - 254 = 317.5 - 254 = 63.5 \text{ mm}$.

  • $Q = \frac{(100 - 0.2 \times 63.5)^2}{100 + 0.8 \times 63.5} = \frac{(100 - 12.7)^2}{100 + 50.8} = \frac{87.3^2}{150.8} = \frac{7621.29}{150.8} \text{ mm}$.

Engineering Applications of Runoff Estimation

• Urban Drainage Design: Used to size storm drainage networks, roadside canals, culverts, and sewers. Metro Manila applications include sizing canals in Quezon City, Manila, Makati, and Pasig.

• Flood Control Infrastructure: Planning and sizing detention ponds, retention basins, floodwalls, levees, diversion channels, and pumping stations. The SCS method specifically helps determine required storage capacities.

• Watershed Management: Evaluating how urbanization increases impervious surfaces and flood peaks. Used for planning green infrastructure like rain gardens and permeable pavements.

• Flood Hazard Mapping: Runoff data serves as input for hydraulic models to determine flood depth and extent, supporting disaster preparedness for areas near the Marikina and Pasig Rivers.

• Bridge and Culvert Design: Calculating peak runoff to ensure bridge openings can safely convey flood flows without overtopping or structural failure.

• Stormwater Management: Designing systems to improve water quality, recharge groundwater, and reduce erosion via Low-Impact Development (LID).

• Risk Assessment: Estimating the frequency and magnitude of floods to prioritize mitigation investments for schools, hospitals, and residential areas.

• Climate Change Adaptation: Redesigning structures to handle increased rainfall intensity and changing runoff conditions.

Philippine Case Studies

• Typhoon Ondoy (September 2009) - Metro Manila & Marikina River:

  • Rainfall: $450.00 \text{ mm}$ in $24$ hours (normal monthly average in one day).

  • Impact: $80\%$ of rainfall became runoff due to high impervious cover. The Marikina River reached a level of $20.6 \text{ m}$, exceeding dikes by $3.0 \text{ m}$.

• Tropical Storm Sendong (December 2011) - Cagayan de Oro & Iligan City:

  • Rainfall: $180.00 \text{ mm}$ in $12$ hours.

  • Impact: Steep slopes and deforestation caused runoff levels $40\%$ higher than forested conditions. Flash floods resulted in rivers rising $3.0 \text{ m}$ in under an hour.

• Typhoon Ulysses (November 2020) - Metro Manila, Pampanga, Cagayan Valley:

  • Impact: Saturated soil from preceding rains led to a higher $CN$, meaning the ground could not absorb more water. Runoff from mountains overwhelmed river systems, causing floods to persist for weeks.

Relevant Philippine Laws and Agencies

• Regulatory Framework:

  • Republic Act No. 10121 (DRRM Act of 2010): Mandates hydrologic analysis for infrastructure projects and shifts focus from response to prevention.

  • Presidential Decree No. 1096 (National Building Code): Rule VII requires drainage systems sized for maximum probable rainfall. Sets return periods at $10$–$25$ years for small areas and $50+$ years for major facilities.

  • Republic Act No. 9729 (Climate Change Act of 2009): Requires flood studies to use climate-adjusted rainfall depths and future-oriented IDF curves.

  • DPWH Department Order No. 173 (Series of 2012): Adopted the "Hydrology and Hydraulics Design Standards." Specifies the Rational Method for areas $\leq 80 \text{ ha}$ and the SCS Method for larger watersheds.

  • Republic Act No. 7160 (Local Government Code of 1991): Empowers Local Government Units (LGUs) to maintain local drainage and enforce stricter local return-period requirements.

• Key Agencies:

  • DPWH: Lead agency for flood control; publishes the mandatory "Design Guidelines Volume 3: Hydrology and Hydraulics."

  • PAGASA: Provides official rainfall data, IDF curves, and classifies storm return periods.

  • NWRB (National Water Resources Board): Coordinates national water policies and integrated river basin studies.

  • MMDA: Manages Metro Manila drainage and pumping stations; requires developers to submit runoff computations.

  • DENR: Manages land use and reforestation, which directly influences watershed $CN$ values.

Problems and Challenges in the Philippines

• Hydro-Meteorological Stress: High frequency of typhoons, southwest monsoons, and intense thunderstorms.

• Rapid Urbanization: Proliferation of concrete roads and buildings reduces infiltration, increasing runoff peaks.

• Environmental Degradation: Deforestation reduces the natural ability of the land to absorb rainwater, increasing erosion and river sedimentation.

• Infrastructure Deficiencies: Inadequate or clogged drainage systems that cannot accommodate modern runoff volumes.

• Data Scarcity: Lack of hydrological data in certain regions makes accurate prediction difficult.

• Climate Change: Increased frequency of extreme rainfall events renders historical data less reliable for future forecasting.

Recommendations for Flood Mitigation

• Methodological Integration: Use both Rational and SCS methods for a balanced assessment; calibrate parameters with local soil data.

• Technological Adoption: Utilize Geographic Information Systems (GIS) for precise flood mapping.

• Infrastructure & Policy:

  • Enforce strict land-use regulations and protect watersheds.

  • Upgrade drainage systems to account for climate-adjusted rainfall.

  • Regularly maintain and clear waterways of solid waste.

  • Establish early warning systems and increase public awareness of flood preparedness.