IB Geography A.2.2 Human Impacts on Flood Risk Notes

Urbanization Produces Faster Runoff and Higher Local Flood Peaks

1. Roofs, roads and paved surfaces replace permeable soil and vegetation, reducing infiltration and interception. A larger share of rainfall therefore becomes rapid surface runoff.
2. Storm drains and culverts connect impermeable surfaces directly to channels. This shortens transfer time, steepens the rising limb and can raise peak discharge.
3. Construction compacts soil and removes vegetation before buildings are completed, so flood hazard may rise during the development phase as well as after it.
4. Urban rivers may receive runoff from several paved tributary areas at once. If channel capacity, bridges or drains form bottlenecks, water can back up into particular neighbourhoods.
5. The distribution of damage is uneven. Low-income households may occupy lower-cost flood-prone land, have less insurance and recover more slowly even when the physical depth of flooding is similar.

Deforestation Removes Interception and Weakens Soil Storage

1. Removing the canopy reduces interception and evapotranspiration, so more rainfall reaches the ground and remains available for runoff.
2. Loss of roots and soil organic matter can reduce infiltration and water storage. Logging machinery or grazing may compact exposed soil and intensify this change.
3. Erosion from bare slopes increases sediment delivery. Deposition in channels, drains or reservoirs can reduce their effective capacity and increase local flooding.
4. The effect varies with storm intensity, soil, slope, regrowth and the scale of clearance. Deforestation can increase runoff, but it should not be presented as the sole cause of every major flood.
5. Upstream land-cover change can therefore alter flood frequency and magnitude downstream, separating the place where risk is produced from the place where damage occurs.

Channel Modification Redistributes Rather than Eliminates Flood Risk

1. Straightening shortens the channel and raises its gradient, so water is transferred through the modified reach more quickly. This may reduce local water levels while increasing the speed and timing of flow downstream.
2. Deepening, widening or dredging increases local channel capacity, but sediment can return and maintenance is costly. Faster flow may also increase erosion farther downstream.
3. Levees confine water within the channel and protect land behind them up to a design level. They can raise water levels within the embanked reach and cause severe losses if overtopped or breached.
4. Bridges, culverts and undersized drains can obstruct flow, especially when debris blocks the opening. Floodwater then spreads into areas not protected by the modified channel.

Human Change Alters Flood Distribution, Frequency and Magnitude

1. Distribution changes when runoff is routed toward new locations, development occupies floodplains, or defences transfer water elsewhere.
2. Frequency can increase when smaller storms generate enough runoff to exceed local channel or drainage capacity more often.
3. Magnitude can increase when synchronized runoff produces a higher peak, or decrease locally when added storage and channel capacity contain the event.
4. Several small changes across a basin can combine. Paving, deforestation and channel engineering may each alter different parts of the same hydrograph.
5. Flood risk results from hazard, exposure and vulnerability. Managing runoff alone is therefore incomplete if settlement continues on flood-prone land or warnings do not reach exposed groups.