Which basin shape tends to increase lag time and reduce peak discharge?

Which basin shape tends to increase lag time and reduce peak discharge?

• A. Elongated shapes
• B. Circular shapes
• C. Rectangular shapes
• D. Steep mountainous basins

Answer: (A)

Basin shape influences how far and how long runoff must travel to reach the outlet, plus how much storage and friction the water encounters along the way. Elongated basins create longer flow paths, so rainfall water takes more time to reach the outlet. This extra travel time, along with the increased opportunity for infiltration, absorption, and temporary storage along the route, spreads the runoff over a longer period, increasing the lag time and reducing the peak discharge. In contrast, basins with shorter, more direct paths—like circular ones—let water reach the outlet quickly, producing a faster concentration and a higher, sharper peak. Steep mountainous basins typically funnel runoff rapidly, again yielding a short lag time and a high peak. Rectangular shapes tend to be intermediate, but the main effect is that longer drainage paths associated with elongated shapes attenuate the flood wave.

Basin shape influences how far and how long runoff must travel to reach the outlet, plus how much storage and friction the water encounters along the way. Elongated basins create longer flow paths, so rainfall water takes more time to reach the outlet. This extra travel time, along with the increased opportunity for infiltration, absorption, and temporary storage along the route, spreads the runoff over a longer period, increasing the lag time and reducing the peak discharge. In contrast, basins with shorter, more direct paths—like circular ones—let water reach the outlet quickly, producing a faster concentration and a higher, sharper peak. Steep mountainous basins typically funnel runoff rapidly, again yielding a short lag time and a high peak. Rectangular shapes tend to be intermediate, but the main effect is that longer drainage paths associated with elongated shapes attenuate the flood wave.