132
8 Sediment-Storage Effects of Check-Dam …
slope is homogeneous, and the deposition and inundation on a single dam is also
basically identical (Zhu et al. 1997). Therefore, the relative stability of the dam
system only represents the relative balance of the soil and water in some limited
small and ideal catchment (Shi 2005). Up to the present, the dam system and single
check-dam are not clearly separated in the literatures on the balance of the soil and
water. The concept of relative stability mainly replies to the relative balance of soil
and water retained by a single check-dam.
In combination with the model experiment of the dam optimization programming,
we performed the rainfall simulation experiments with several scenarios of dam
construction sequence and found out that the small dams in the reservoir were quickly
filled up.
The most contribution of the relative stability of the dam system is that the concept
of the critical coefficient for relative stability provides a quantitative criterion for the
dam system programming in the small watershed, with which the number of dams
could be figured out according to the corresponding reservoir capability.
8.4.3 Comparison of Sediment Loss and Dam-Land Area
The main purposes of the check-dam system design are to (1) retain sediment and
(2) form dam-lands. The first objective, which is the most important for the total
area of dam-lands, is determined by the retained sediment (BMUM 2004). In fact,
the effects of sediment-storage are determined by the order in which check-dams are
constructed. It is preferable to build the larger reservoirs with greater flood-control
capabilities in the lower reaches of the main gully (e.g., Scheme A here), rather than
building dams in the upper reaches first (e.g., Scheme B here).
The scheme with less cumulative soil loss is better, as the antecedent land cover
and the amount of rainfall is the same in both schemes. The amount of soil loss after
each rainfall event is shown in Table 8.3. The total amounts of sediment retained by
the dams in both schemes were obviously different. The total amount of soil loss in
Scheme B was 13.5 × 10
6 kg after 10 rainfall events, and only 10.6 × 10
6 kg in
Scheme A, 27% smaller than that in Scheme B. Especially in the first rainfall event,
sediment was impounded more effectively and much less soil was lost in Scheme
A than in Scheme B (Fig. 8.10). Thus, the order of dam construction in Scheme A
is preferable, for less sediment may escape to the Yellow River than in Scheme B.
Moreover, the dam system attains relative stability earlier than in Scheme B, thereby
increasing the benefit of the check dam System (Xu et al. 2004a).
The dam-land area is generally determined by the gully shape and local social and
economic conditions. After the dam has been built, the area of the deposited dam
land will rise and the gully erosion will decrease as time goes on. As a result, a better
benefit of the check dam system may be received. Since the third rainfall event, the
cumulative area of the dam-land in Scheme A was larger than that in Scheme B, as
shown in Fig. 8.11. After 10 rainfall events, the cumulative area of the dam-land was
23 hm
2 as the check-dams were built according to Scheme A, whereas the cumulative
8 Sediment-Storage Effects of Check-Dam …
slope is homogeneous, and the deposition and inundation on a single dam is also
basically identical (Zhu et al. 1997). Therefore, the relative stability of the dam
system only represents the relative balance of the soil and water in some limited
small and ideal catchment (Shi 2005). Up to the present, the dam system and single
check-dam are not clearly separated in the literatures on the balance of the soil and
water. The concept of relative stability mainly replies to the relative balance of soil
and water retained by a single check-dam.
In combination with the model experiment of the dam optimization programming,
we performed the rainfall simulation experiments with several scenarios of dam
construction sequence and found out that the small dams in the reservoir were quickly
filled up.
The most contribution of the relative stability of the dam system is that the concept
of the critical coefficient for relative stability provides a quantitative criterion for the
dam system programming in the small watershed, with which the number of dams
could be figured out according to the corresponding reservoir capability.
8.4.3 Comparison of Sediment Loss and Dam-Land Area
The main purposes of the check-dam system design are to (1) retain sediment and
(2) form dam-lands. The first objective, which is the most important for the total
area of dam-lands, is determined by the retained sediment (BMUM 2004). In fact,
the effects of sediment-storage are determined by the order in which check-dams are
constructed. It is preferable to build the larger reservoirs with greater flood-control
capabilities in the lower reaches of the main gully (e.g., Scheme A here), rather than
building dams in the upper reaches first (e.g., Scheme B here).
The scheme with less cumulative soil loss is better, as the antecedent land cover
and the amount of rainfall is the same in both schemes. The amount of soil loss after
each rainfall event is shown in Table 8.3. The total amounts of sediment retained by
the dams in both schemes were obviously different. The total amount of soil loss in
Scheme B was 13.5 × 10
6 kg after 10 rainfall events, and only 10.6 × 10
6 kg in
Scheme A, 27% smaller than that in Scheme B. Especially in the first rainfall event,
sediment was impounded more effectively and much less soil was lost in Scheme
A than in Scheme B (Fig. 8.10). Thus, the order of dam construction in Scheme A
is preferable, for less sediment may escape to the Yellow River than in Scheme B.
Moreover, the dam system attains relative stability earlier than in Scheme B, thereby
increasing the benefit of the check dam System (Xu et al. 2004a).
The dam-land area is generally determined by the gully shape and local social and
economic conditions. After the dam has been built, the area of the deposited dam
land will rise and the gully erosion will decrease as time goes on. As a result, a better
benefit of the check dam system may be received. Since the third rainfall event, the
cumulative area of the dam-land in Scheme A was larger than that in Scheme B, as
shown in Fig. 8.11. After 10 rainfall events, the cumulative area of the dam-land was
23 hm
2 as the check-dams were built according to Scheme A, whereas the cumulative