Monday, 11 September 2017

Late summer low flows

A series of storm events during late August led to multiple peaks in the stage hydrograph shown below, the highest peak reaching almost 1.4 metres. September is normally a season of late summer low flows when the annual minimum daily flows are often recorded. Aggradation and accumulation of sediment in the gauging reach occurred in early summer floods, but these recent floods have led to net removal of sediment and deepening of the channel in the gauging reach. Changes in the stage-rating curve need to be closely monitored to maintain accuracy in discharge estimation.

Thursday, 10 August 2017

Change in stage-rating curve?

The 587 mm of rainfall for July was the second highest monthly precipitation since records began in 1981. The highest recorded was 639 mm for July 2013. Although the maximum flood stage and peak discharge was not particularly high last month (1.26 m, 30 m3/s at 11:30 on 24 July), the hydrograph shows multiple peaks around 1.0-1.2 m stage.

Comparing the measured discharge with the estimated discharge from the stage-rating curve, we find a discrepancy of about 25%. The discharge is 25% higher than we expect from using the current stage-rating curve, indicating degradation of the channel during the floods of 24 July. The removal of the sediment deposited from the tributary channel earlier in July is also evidence of likely channel degradation. Further discharge measurements are needed in the near future to confirm this change in the stage-rating curve.

Young hydrologist - first discharge measurement (Grade 4)

Friday, 21 July 2017

587 mm of rainfall in July

This July has had more than twice the usual rainfall, with 587 mm at the Miomote Amedas Station compared to an average of 263 mm (1981-2010). Maximum daily precipitation was 119 mm on July 24, and maximum hourly precipitation was 48 mm on July 18th. While the above hydrograph only shows data until July 21, it shows that the rainfall did not produce any particularly large floods. Instead there were multiple floods and peaks with a moderate stage of 0.8 m to 1.1 m.

The above two photographs show the water to be running very clear. However, the intense and localized storm of July 18 (max. 48 mm/h) seems to have been centered in the tributary basin which joins the main stem of Takiya River by the bridge abutment shown in the photographs below. There is a large accumulation of bedload sediment at the confluence below the bridge as the main stem did not appear to have the flow required to transport the sediment downstream.

Wednesday, 28 June 2017

Waiting for the rainy season

Stage = 31.3cm, Discharge = 0.38m3/s
No significant rainfall has occurred since the rainy season was officially declared, and discharge levels are close to annual minimum values. Early June saw a moderate rainfall event over 2-3 June with 56mm of precipitation. Visiting the upper part of the neighbouring Miomote River basin we could see that the high peaks (>1500m elevation) still had some snow cover. However, lack of rainy season precipitation could lead to low flow conditions in the coming weeks.

Asahi Mountains from near Okumiomote Dam

Monday, 29 May 2017

End of snowmelt season

The hydrograph above shows the recession of snowmelt runoff during the second half of May. The diurnal melt pattern has mostly gone by 24 May, and the subsequent peaks are due to rainfall (51mm rainfall over 26-27 May at Miomote Amedas station). A discharge measurement confirmed no change in the stage-rating curve, which has remained stable for nearly three years now.

Stage = 0.45m, Discharge = 1.18m3/s, Specific discharge = 0.0607 m3/s/km2

Monday, 22 May 2017

JICA field training

JICA (Japan International Cooperation Agency) participants on the course "Irrigation and Drainage Technology Based on Integrated Water Management" joined me at Takiya River for basic hydrological field survey. We measured discharge in three teams using different equipment and compared the results. Sampling of the channel bed materials using the Wolman Pebble Count was also undertaken. Participants came from various countries in Africa and Asia (Cambodia, Malawi, Mozambique, Myanmar, Philippines, Rwanda, South Sudan, Tanzania and Uganda) to study for six months in Japan.

Arakawa from Maruyama Bridge (Sekikawa Village)

Wednesday, 10 May 2017

Snowmelt season

Today I was joined by participants from my graduate class in Snow Hydrology. We observed the snowmelt runoff condition, and measured the discharge, confirming no changes have occurred in the stage-rating curve.

Above you can see the typical snowmelt runoff pattern in the stage hydrograph. The larger peaks above 0.8m are all connected to rainfall events, while the regular sine-wave fluctuations are due to fine weather snowmelt (e.g. early May). These fluctuations are closely related to the energy balance of the snowpack and the diurnal change in air temperature.