Factors affecting flood hydrograph, baseflow separation

Factors affecting the flood hydrograph

Climatic factors

• Rainfall intensity and duration: high intensity gives a higher, sharper peak; long duration gives a larger volume and a broader hydrograph.
• Areal distribution and storm movement: a storm moving downstream gives a higher peak than one moving upstream.
• Antecedent moisture: wet soil increases runoff.
• Evaporation, temperature and snow-melt.

Catchment factors

• Size: a larger area gives a longer time base and a later, lower specific peak.
• Shape: a fan or circular shape gives a high, sharp peak; an elongated shape gives a flatter one.
• Slope: steep slopes shorten the time to peak and increase the peak.
• Land use, vegetation and soil: forest and permeable soil reduce the peak; urban and bare areas raise it.
• Drainage density: a higher density gives a quicker response.
• Storage: lakes, ponds, depressions and flood plains flatten the peak.

Channel factors: cross-section, roughness, slope and channel storage.

Methods of baseflow separation

1. Straight-line method: join the start of the rising limb to the end of direct runoff by a straight line.
2. Fixed-base length method: the end of direct runoff is taken at $N=0.83A^{0.2}$ days after the peak ($A$ in km$^2$). Extend the pre-storm recession forward to the time of the peak and join it to this end point.
3. Variable-slope method: extend the pre-storm recession forward to the peak time and the post-storm recession backward to an inflection point; join the two by a straight line.
4. Semi-log recession (master recession curve) method.