Low flows and peak flows

This year has seen below average precipitation during June (125mm compared to 180mm average). You can see in the hydrograph above that the period 17-26 June was completely dry, and we can even see small downward dips in the water level during day-time as evapotranspiration rises. This phenomena is only seen during mid-summer when river and ground water levels are relatively low and soil conditions are dry. A minimum stage of 0.261m is observed on 26 June.

During 27-28 June a rainstorm drops 66mm (max. 9mm/h) causing a moderate peak flow. However, soil and ground water storage is barely replenished, and flow levels quickly decrease to near 0.3m after a week or so.

Ta = 22C, Tw = 17C, Stage = 0.32m, Q = 0.357m3/s

Today we also carried out a survey of the channel long-profile, measuring by level the elevation of the channel thalweg and the water surface slope. Over the 90m reach shown in the photo above, the channel slope was 1.14%, while the channel slope over the 100m upstream of the gauge was 0.78%. The overall water surface slope for the 190m reach was 0.948%, with a drop in elevation of 1.801m.

Considering the reach between 50m upstream and 50m downstream of the gauging point, the slope was 0.84% compared to 0.90% surveyed in 2004. This suggests the slope at the gauging point has decreased slightly since 2004. We are currently undertaking an analysis of the hydraulic geometry and stage-rating curves for the gauging point, for which estimates of channel slope are essential. Measurements of channel slope during flood discharge will also be required.