Monday, 11 June 2018

Early summer low flows

Stage = 0.313 m, Discharge = 0.48 m3/s

Early June, before the arrival of the seasonal rain front, is a period of low flows. Since the floods over 17-19 May there has been a period of almost continuous flow recession, only interrupted by a minor rainfall event on June 1st (see stage hydrograph plotted below).

Today's discharge measurement plots very close to the stage-discharge curves developed before the 19 May flood, indicating that the aggradation of the channel seems to have been temporary. The red symbols plotted below represent the three most recent measurements taken since the flood. Only the first measurement taken at stage value 0.65 m is significantly different from the established curves. Points plotting to the right of the curves indicate channel aggradation (sediment deposition), while points plotting to the left of the curves indicate channel degradation (erosion of the channel bed).

Thursday, 31 May 2018

Spring hydrograph

The winter to spring hydrograph above shows that there were two significant runoff events during January, followed by two smaller runoff events during February. The main snowmelt season starts at the beginning of March and continues until mid-May. An unusually heavy rainstorm occurs on May 19th causing stage to rise above 2m and estimated discharge to reach 100m3/s.

Today's discharge measurement plotted very close to the stage-rating curve prior to the May 19th flood, indicating that the aggradation of sediment in the reach may have been temporary. Future field work this month will confirm the status of the rating curve.

Cedar tree at the gauging section scarred by flood debris

Cedar tree roots stabilizing the bank at the gauged section

The following photos show the reaches of Takiya River below the gauging section over a distance of about 1 km to the small irrigation diversion close to the confluence with the Miomote River.

Tuesday, 22 May 2018

May 17-19 rainstorm event (>230mm)

Localized intense thunderstorms continued over the three days of 17-19 May, culminating in a major flood peak about 3am on the 19th with a stage of about 2.1 m (discharge >100 m3/s). Over the past 18 years of record, flood peaks have exceeded 2 m on only a few occasions during the June-August mid-summer season, but this is the first time to experience such a flood in May. The maximum precipitation intensities measured are new records for the month of May.

Precipitation (17-19 May):
Station       Total        Max intensity
Miomote   216 mm      23.5 mm/h
Takane      238 mm      38 mm/h
Takiya-2   235 mm      28.5 mm/h

The discharge measurement taken on 21 May showed that significant channel aggradation has taken place. We will have to quickly establish a new stage-rating curve and monitor for changes closely over this summer season.

Monday, 21 May 2018

Sunday, 20 May 2018

JICA Study Tour - Day 1

Tainai River irrigation headworks

Arakawa River from Maruyama Bridge, Sekikawa Village

Biotope pond for dragonflies alongside Arakawa River

Arakawa River irrigation headworks

Upstream view from Arakawa irrigation headworks

Thursday, 17 May 2018

Spring rains

Today over 70 mm of rainfall occurred with a maximum intensity of nearly 20 mm/h (Miomote Amedas Station). This is relatively heavy rainfall for May, which on average receives 150 mm of precipitation for the whole month (average for 1981-2010). This month has received 162 mm to date. When we arrived at the gauging station the river stage was steady at around 1 m, which is too high to take a discharge measurement by the wading method.

A visit to Miomote Dam showed that there was a large spillway discharge. The photo below also shows the hydro-electric generating plant on the left.

Saturday, 21 April 2018

Snowmelt season runoff

Stage = 57.25 cm, Q = 3.24 m3/s, Vmax = 0.94 m/s
Snowmelt season in full swing today with fine weather melt conditions. Stage starts to rise about 11am, and peaks about 6pm. Maximum velocities in the centre of the channel reaching nearly 1 m/s making it hard to hold the current meter in position. Unsteady flow and standing waves make it difficult to estimate the flow depth as it constantly fluctuates. However, despite the moderately high discharge level, the water remains extremely clear with no visible sediment load.

The hydrograph above shows a period of fine weather melt during late March where we can see the diurnal melt pattern rising day by day. In contrast, the larger and more abrupt peak flow on April 15 is a rain-on-snow event due to 54 mm of precipitation with a maximum intensity of 12 mm/h.