Two properties on the same street, with the same mapped flood susceptibility, can behave very differently over a decade if the catchment above them changes.

Land use upstream is one of the strongest influences on flooding, and it is one of the things a hazard screening cannot see.

What changes when a slope is cleared

Water arrives faster. Vegetation and forest litter intercept rain, slow runoff, and let water soak in. Bare ground sheds it. The same rainfall produces a higher, sharper peak downstream.

More material comes with it. Exposed soil erodes. That sediment travels down the channel and settles where the water slows, raising the bed and reducing capacity. Over time the same flow overtops the same channel more easily.

Slopes become less stable. Roots bind soil. Removing them reduces that binding, particularly on steep ground, which raises landslide susceptibility in a way the published map may not yet reflect.

What quarrying adds

Quarrying and sand and gravel extraction do three specific things:

1. They create cut faces and spoil. Loose material sitting on a gradient.
2. They change the channel. Extraction from a river bed alters its shape and its behaviour, sometimes destabilising banks and structures.
3. They generate sediment that moves downstream.

Mining, including small-scale mining, has the same effects and adds tailings. See buying in Davao de Oro for how that looks in practice.

What urbanisation upstream adds

A subdivision, a road, or an industrial estate converts absorbing ground into shedding ground. The runoff has to go somewhere, and it goes into the same channel as before, faster.

This is why an area that never flooded can start flooding without any change in rainfall. It is also why why flood maps change over time is worth reading alongside this.

Why it is not on the report

A hazard screening reads published national datasets at a point. Those datasets were produced from data captured at a time. Land use change since then is not in them, and there is no national point-level dataset of "what happened upstream last year" that we could read honestly.

So we say so, and tell you how to check it yourself.

How to check it yourself

1. Look at the catchment on satellite imagery. Publicly available imagery with a history slider lets you compare the same slopes across years. Bare patches, new roads, quarry benches, and new development are all visible.

2. Drive or walk upstream. Follow the channel or the road up the valley. Look for active quarries, cleared slopes, new subdivisions, and sediment in the channel.

3. Look at the channel itself. Is the bed sandy and wide, with fresh deposits? Are the banks raw and eroding? Is there timber caught on structures?

4. Ask the neighbours. "Has flooding got worse here over the years, and do you know why?" Long-term residents often have a very clear answer, and they usually name the cause.

5. Ask the LGU and the MGB. The municipal engineering office and the MGB regional office will know about permitted quarrying and about local geohazard assessments.

What to screen, for context

• Mapped flood susceptibility and elevation at the exact coordinates.
• Distance to the nearest mapped waterway.
• Slope and mapped landslide susceptibility for the property and, by walking, for the ground above it.
• The debris flow and alluvial fan flags, since more sediment supply is exactly what feeds them.

Checklist

1. Screen the exact coordinates.
2. Identify the catchment above the property on a map.
3. Compare satellite imagery of that catchment across several years.
4. Drive or walk upstream and look at the channel and the slopes.
5. Ask neighbours whether flooding has changed over time, and why.
6. Ask the LGU and the MGB about quarrying, mining, and new development upstream.
7. Weight recent local experience above an older map where they disagree.

The honest limit

A hazard screening reports published mapped conditions at a point. It cannot see land use change since the data was captured, and it does not model catchment response. The LGU, the MGB, satellite imagery, and the people who live there cover the gap between the published map and this year.

CheckHazard does not replace a professional geotechnical or engineering survey.