A.2 Flooding and flood mitigation

The question asks for the name of the time interval between peak rainfall and peak discharge on a storm hydrograph. These are two distinct features plotted on the same graph.

Lag time is defined as the delay between the moment of maximum rainfall intensity and the moment of maximum river discharge. It reflects how long rainwater takes to travel from where it fell to the river channel via overland flow, throughflow, and groundwater pathways.

Peak rainfall occurs at 14:00 and peak discharge at 17:00, giving a 3-hour delay. This gap is precisely what lag time measures — it is not a limb, not a gradient, and not base flow.

The correct term is lag time. It is a fundamental diagnostic feature used to assess how quickly a drainage basin transfers rainfall into river flow, and is closely linked to flood risk assessment.

The question asks for the specific hydrograph term describing the time gap between peak rainfall and peak discharge. Understanding each component of a storm hydrograph is required.

The recession limb (also called the falling limb) is the portion of the hydrograph after peak discharge, where flow gradually returns to base flow. Its duration is not the gap between rainfall peak and discharge peak.

Base flow is the steady, background level of river discharge maintained by groundwater seepage. It is not a time interval between rainfall and discharge peaks.

The rising limb is the section of the hydrograph where discharge increases toward its peak. A gradient is a rate of change, not a time interval — this option does not describe what is asked.

Only lag time correctly describes the time delay between peak rainfall and peak discharge. It is the standard hydrological term for this interval on a storm hydrograph.

River X drains a recently urbanized catchment. Urbanization replaces permeable vegetation and soil with impermeable surfaces (roads, rooftops, pavements) and adds engineered stormwater drains. River Y drains a forested catchment on permeable sandstone, which promotes interception and infiltration.

In River X's urbanized catchment, rainfall cannot infiltrate impermeable surfaces, so it becomes rapid overland flow that is quickly routed into the channel via storm drains. In River Y's forested, permeable catchment, rainfall is intercepted by canopy, infiltrates sandstone, and reaches the channel slowly via throughflow and groundwater flow.

Fast delivery of water to River X's channel means short lag time and high, sharp peak discharge — a flashy hydrograph. Slow delivery to River Y's channel means long lag time and low, broad peak discharge, even with the same total rainfall input.

River X (urbanized) has a shorter lag time and higher peak discharge than River Y (forested, permeable). Lag time and peak discharge are inversely related — factors that shorten one simultaneously raise the other.

The question tests understanding of how land use and geology interact to shape storm hydrographs, specifically lag time and peak discharge.

Identical total rainfall does not produce identical hydrographs. The pathways water takes to the channel — determined by land use and geology — fundamentally alter the shape of the hydrograph. This option ignores all catchment characteristics.

River Y's forested, permeable catchment promotes interception and infiltration, routing water via slow subsurface pathways. This produces a longer lag time and lower peak discharge — the opposite of this option.

Urbanization produces impermeable surfaces and fast drainage networks, which shorten lag time and raise peak discharge, not lengthen or lower them. This option reverses the expected relationship.

River X has a shorter lag time and higher peak discharge than River Y. Impermeable urban surfaces and engineered drains accelerate runoff to the channel, producing the characteristic flashy hydrograph associated with high urban flood risk.