低湿地における堤防下浸透量の水文学的考察 (I)

-埼玉県新方領の堤防下浸透量について-

抄録

Niigataryo district is adjacent to Tokyo and consists of 4, 233 ha of low swampy land. It is located in the lower reaches of Nakagawa River in Saitama Prefecture. The district is surrounded by rivers, and the river beds are comparatively high. Therefore, a fair quantity of seepage water under levees is observed in non-irrigation period, and the quantity increases in irrigation period owing to artificial heightening of the water level of surrounding rivers. The water balance is shown by the following formula : P= (O-I) +E+ (G₂-G₁) +ΔS where P : precipitation, O : runoff, I : inflow of surface water, E : evaporation from the district, G₂ : outflow of ground water (G₂ is negligible because of the low level in the district), G1 : inflow of groundwater, ΔS : amount of change in water volume in the district. The results of the investigation are as follows.
(1) Non-irrigation period in 1952 and.1953.
i) I=0, Q_b =G₁-ΔS, E= P-O+ Q_b, where Q_b : base flow separated from runoff curve.
E is the only unknown factor, as Q_b is known. Therefore, G₁ can be calculated.
ii) The mean value of E obtained by the above formula is 1.7mm/day, and the mean value of G₁ (mean depth of 4, 233 ha) is 0.3-0.4mm/day.
(2) Irrigation period in 1954, 1955, 1959 and 1960.
i) The values of E and G₁ are unknown, but E can be approximately presumed from the value of E_p as fo llows : E= E_p× α where E_p : evaporation from evaporimeter, α : a constant (0.9-1.2) characterizing each 10 days of irrigation period.
The value of E obtained for the district is 3.6-4.0 mm/day.
ii) The caluculated value of G₁ is 2.0-2.5 mm/day, and 40-50l/sec per kilometer of levee length.
(3) The value of G₁ correspond to the gradient of ground water level under levees.
(4) The results of caluculation in the district proved to be applicable in other districts of similar conditions.