114
8 Sediment-Storage Effects of Check-Dam …
Designing check dam system requires an estimate of (1) preferred dam sites;
(2) number of dams required and their heights for sediment interception and flood
detention; (3) the optimal sequence and interval for dam construction (Wu 1994).
Mathematical models have recently been utilized for designing check dam systems
(e.g., Wu and Huang 1995; Wan et al. 1995; Lin et al. 1995). However, several limitations exist in these models. For example, the Linear Programming Method (Wu 1994)
is a static model that produced results biased for the established condition whereas
the small watershed is an open and dynamic system. Although it is dynamic and
multi-objective, the Nonlinear Programming Method, another mathematical model,
is not credible enough to be applied in practical engineering (Li et al. 1995). To minimize the number of decision-making variables, only skeleton dams can be analyzed
using current mathematical models (Wu 1994).
Relatively stability of the check dam system plays an important role in the dam
design. The fact that the check-dam systems on the Loess Plateau of China always
exhibit a good relative stability is a unique geomorphological phenomenon. It has
been found that the Loess Mesa Ravine Region and the Loess Hill Ravine Region,
which cover 200,000 km
2 of the Loess Plateau in China, are among the areas having
the highest erosion rates on earth, and the most effective way to conserve soil and
water in these areas is to build check-dam systems in the gullies. At present, more
than 100,000 check-dams have been built, and the amount of sediments retained
by the check-dams is the largest among all measures (Xu et al. 2004a). Moreover,
official documents from the Chinese Ministry of Water Resources have stated that
163,300 check-dams will be built in this area before 2020 (MWRC 2003). Farmers
would prefer to plant crops in the deposited areas behind the dams, which is called
the dam-land, owing to their more abundant water content and more fertile soil as
compared with the slope land. According to field investigations on the check-dam
systems of the Shanxi Province (Fang 1996), 6000–7500 kg grains could be harvested
per hectare of the dam-land, which was 8 to 10 times higher than those on the slope
land. At present, the dam-land occupies only 9% of the whole farmland areas in the
Loess Hill Ravine Region, but their grain yields comprise 23.5% or higher of the
over-all grain production of this region (Xu and Wang 2000). However, when tens or
hundreds of check-dams are built in a small watershed area not larger than 100 km
2 ,
it will present quite complicated hydro-sedimentologic problems. Elements such as
the layout of the dam-sites, the heights of the dams and the areas controlled by the
small watersheds should all meet the requirements of relative stability during the
design and construction of the check-dam systems. The concept of “relative stability
of the check-dam system” first emerged in the 1960s, which was stemming from the
recognition of the key factors that determined the natural balance of the check-dams.
Inspired by the natural Juqiu, a kind of dam-land resulting from land-slips which
never overflows when impounding the floodwater and soil for centuries, people came
to recognize the important role of dam height and the ratio of dam-land area to that of
the controlled watershed. It was observed that if the above parameters have reached
certain values, the soil and water in the small watershed could be internally absorbed,
without the need of raising the height of the dam. Fang (1995) has suggested that
check-dam systems exhibiting good relative stability have to fulfill the following
8 Sediment-Storage Effects of Check-Dam …
Designing check dam system requires an estimate of (1) preferred dam sites;
(2) number of dams required and their heights for sediment interception and flood
detention; (3) the optimal sequence and interval for dam construction (Wu 1994).
Mathematical models have recently been utilized for designing check dam systems
(e.g., Wu and Huang 1995; Wan et al. 1995; Lin et al. 1995). However, several limitations exist in these models. For example, the Linear Programming Method (Wu 1994)
is a static model that produced results biased for the established condition whereas
the small watershed is an open and dynamic system. Although it is dynamic and
multi-objective, the Nonlinear Programming Method, another mathematical model,
is not credible enough to be applied in practical engineering (Li et al. 1995). To minimize the number of decision-making variables, only skeleton dams can be analyzed
using current mathematical models (Wu 1994).
Relatively stability of the check dam system plays an important role in the dam
design. The fact that the check-dam systems on the Loess Plateau of China always
exhibit a good relative stability is a unique geomorphological phenomenon. It has
been found that the Loess Mesa Ravine Region and the Loess Hill Ravine Region,
which cover 200,000 km
2 of the Loess Plateau in China, are among the areas having
the highest erosion rates on earth, and the most effective way to conserve soil and
water in these areas is to build check-dam systems in the gullies. At present, more
than 100,000 check-dams have been built, and the amount of sediments retained
by the check-dams is the largest among all measures (Xu et al. 2004a). Moreover,
official documents from the Chinese Ministry of Water Resources have stated that
163,300 check-dams will be built in this area before 2020 (MWRC 2003). Farmers
would prefer to plant crops in the deposited areas behind the dams, which is called
the dam-land, owing to their more abundant water content and more fertile soil as
compared with the slope land. According to field investigations on the check-dam
systems of the Shanxi Province (Fang 1996), 6000–7500 kg grains could be harvested
per hectare of the dam-land, which was 8 to 10 times higher than those on the slope
land. At present, the dam-land occupies only 9% of the whole farmland areas in the
Loess Hill Ravine Region, but their grain yields comprise 23.5% or higher of the
over-all grain production of this region (Xu and Wang 2000). However, when tens or
hundreds of check-dams are built in a small watershed area not larger than 100 km
2 ,
it will present quite complicated hydro-sedimentologic problems. Elements such as
the layout of the dam-sites, the heights of the dams and the areas controlled by the
small watersheds should all meet the requirements of relative stability during the
design and construction of the check-dam systems. The concept of “relative stability
of the check-dam system” first emerged in the 1960s, which was stemming from the
recognition of the key factors that determined the natural balance of the check-dams.
Inspired by the natural Juqiu, a kind of dam-land resulting from land-slips which
never overflows when impounding the floodwater and soil for centuries, people came
to recognize the important role of dam height and the ratio of dam-land area to that of
the controlled watershed. It was observed that if the above parameters have reached
certain values, the soil and water in the small watershed could be internally absorbed,
without the need of raising the height of the dam. Fang (1995) has suggested that
check-dam systems exhibiting good relative stability have to fulfill the following