Philippine Tropical Hydrology and Design Storm Workflow - Examples & Applications

Example

A project-approved local IDF study gives a 25-year, 30-minute design intensity of 145 mm/hr145\text{ mm/hr} for a small urban catchment. The drainage area is 8 ha8\text{ ha} and the composite Rational runoff coefficient is C=0.72C=0.72. Estimate peak discharge using the SI form Q=0.00278CiAQ=0.00278CiA with AA in hectares.

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Example

A 12 km212\text{ km}^2 watershed is delineated automatically from a DEM, but a highway embankment crosses the mapped drainage path and field inspection finds two culverts. What should happen before the basin polygon is accepted?

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Example

A design storm has P=180 mmP=180\text{ mm}. A catchment model uses S=90 mmS=90\text{ mm} and Ia=18 mmI_a=18\text{ mm}. Estimate direct runoff depth using the Curve Number form.

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Example

A flood with return period T=50T=50 years is considered over a 30-year project life. Assuming independent annual exceedance probability p=1/Tp=1/T, estimate the probability of at least one exceedance.

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Example

An urban detention facility receives a modeled inflow hydrograph with a peak of 5.8 m3/s5.8\text{ m}^3/\text{s}. The downstream allowable release is 3.0 m3/s3.0\text{ m}^3/\text{s}. Why is subtracting the two peaks insufficient to size storage?

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Key Takeaways
  • Use locally accepted IDF information and preserve its return-period/duration basis.
  • Verify DEM drainage against roads, culverts, pipes, and field conditions.
  • Runoff models require explicit loss/antecedent assumptions.
  • Return period is an annual probability concept, not a deterministic recurrence schedule.
  • Storage design requires hydrograph routing, not peak subtraction.