A primary irrigation development challenge, during conditions of climate induced streamflow reduction, is to prevent filtration losses from the river Chu at a reach 48 km along the river entry into the Chu valley. An estimate of losses in this area (the so-called crevasse zone) is about 20 m3/s. One potential solution is to construct a second bypass canal that is lined by concrete.
It is necessary to point out that filtration losses are discharged into the river Chu downstream (below town Tokmak). Reduction of filtration losses will result in a reduction of discharge into downstream areas. One consideration to address prior to the construction of a bypass canal is the time interval within which the reduction of losses will influence groundwater discharge to the riverbed. A section of the area of interest along the riverbed is provided in Fig. 20.4.
Alteration of discharge downstream of Tokmak is characterized by the change of groundwater flow near Tokmak AQ(t). To estimate the required amount, we use the analytical equation of S. F. Averyanov [3] given by
where AQN is the amount of filtration losses change in the crevasse zone of the river Chu, m3/day; t is a period of time from the moment of the filtration losses change, days; a=T/|m is a ratio; T is the hydraulic transmissivity, m2/day; and m is the storage coefficient.
Hydrogeologic investigations of this area [4, 5] provided the following parameter values: T = 10,000 m2/day, |i=0.18, and L=48,000 m. Typical value of filtration
0 t 100 Years
Fig. 20.5 Alteration of the groundwater flow near Tokmak in response to filtration losses (Q1(t) - decrease of losses by 10 m3/s; Q2(t) -decrease of losses by 20 m3/s)
Fig. 20.5 Alteration of the groundwater flow near Tokmak in response to filtration losses (Q1(t) - decrease of losses by 10 m3/s; Q2(t) -decrease of losses by 20 m3/s)
losses in the crevasse zone were determined to be about 20 m3/s [5, 6]. Using these data we considered loss events (1) losses decreased by 10 m3/s, and (2) losses decreased to zero (reduction by 20 m3/s). The results of calculations in are presented in Fig. 20.5. The results indicate that a reduction of filtration losses will result in decreased discharge, however, it be delayed over many years. For example, within the period of 20 years, the outflow into the groundwater discharge area will decrease only 50% of the filtration losses reduction volume.
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