Experimental Study on Gully Vegetation in Soil Conservation

 

LI GUIFEN and SHI BING

 

China Institute of Water Resource and Hydropower Research(IWHR),

Beijing ,100044,China

 

 

ABSTRACT

To planting vegetation in gullies on the upstream of the Yellow River which is classified sandstone area plays a role in debris guarding. Through field survey, model test and prototype observation, we know that the 'flexible dam' made of seabuckthorn, framentizedly planted, can control soil and water losses, reduce the quantity of coarse sand which comes into the Yellow River annually meanwhile.

 

INTRODUCTION

The problem of the Yellow River is just fluvial sand, which mainly comes from the area of soil and water losses of 430 thousand square kilometers. There is a source area of coarse sand occupying 156 thousand square kilometers that leads to downstream sedimentation and bed aggravation of the Yellow River. Especially part of the sandstone area of 11 thousand square kilometers turns out and conveys a great deal of coarse sand on account of the landscape of bare hills, turbulent flow and countless gullies, which are entitled 'abominable soil and water losses in the world.' For the past years, people had tried to find an effective measure among a variety of technical alternatives, such as planting seabuckthorn and other plants on hillside. Unfortunately coarse sand by weathering still went into the Yellow River constantly skipping people's effort. In 1992, the Specialist, Bi Cifen put forward a proposal on the basis of field investigation that the flexible dams of vegetation should be constructed by means of planting seabuckthorn community in gullies. The objection of flexible dams is to fix coarse sand in gullies, to reduce large amount of investment for large-scale construction and to rehabilitate eco-environment rapidly. From 1995 to July 1998 several gullies have been developed and the effect of debris guarding is notably obtained. In order to make a theoretical explanation for 'flexible dam' physical model and analysis were successively done, and the proposal was proved proper for environmental quality improvement and coarse sand resisting. It was also proved the effective measure controlling soil and water losses in sandstone areas.

 

PLANNING PRINCIPLE OF THE FLEXIBLE DAM OF SEABUCKTHORN

The sandstone area has large gradient, strong shearing force of flow, lots of loose coarse particulate accumulation in alleys, but hasn't fine materiel for dam construction. Taking advantage of the well-developing rhizosphere, drought-enduring and strong reproductive capacity of seabuckthorn, several rows of seabuckthorn are planted as frame in alleys with vertical direction of the flow. The first flood in flood season brings lots of silt and dry plants branches, they stop water flow to form community backwater, and this has the function of debris guarding, drainage and overflowing. With the deposition of the riverbed after the flood season, the plants grow taller in transverse direction and form the 'flexible dam' community.

The amount of 'flexible dam' of seabuckthorn can be estimated with the following formula:

(I)

m: the amount of flexible dams of seabuckthorn; : the total drop of gullies; : the drop between steep dams and hydraulic drop.

According to the above principles, test sections of plants were planted in a short gully. Nine sections of vegetation were planted, measuring facilities were settled to make prototype observation after the flood season.

 

PHYSICAL MODEL TEST OF 'FLEXIBLE DAM' OF SEABUCKTHORN

In order to testing and verify the cause of formation and regularity of the 'flexible dam' of seabuckthorn in different flow conditions, model test was done in the National Key Laboratory of Sichuan Union University. The test was carried out in a glass tank with 16 meters long and 0.3 meters wide. According to actual investigation, the particulate with size of 2~20mm in the deposition is 15.5%, what of 0.05~2mm account for 78.5%, and what under 0.05mm account for 6%. The bottom of riverbed was simulated in the tank in accordance with the above proportion Feather was used to simulate the vegetation of seabuckthorn.

The forces that put on the gully are analyzed according to the vegetation planting situation, it is shown in Figure 1. Therefore, the relevant formula of vegetation factors and the relative shearing stress of the riverbed surface and the relevant formula of vegetation factors and the relative sediment delivery ratio are established.

 

1. Establishment of interrelation between vegetation factors and the relative shearing stress of the riverbed surface.

Suppose vegetation are planted in open channel regularly, and water flow can form even flow (or gradually varied flow similar to even). Make L as the length of forest zone, make the effective water-resisting area of I rows vegetation is A_i, the vegetation in the riverbed occupy the area of W_i, the total vegetation volume submerged in the water is V_f, A' and P' are the total area of sections and wet circumference, respectively (including vegetation). Make v' as the average flow velocity. Therefore, from the force balance (shown in Figure 3):

 

(1)

 

make , , , then

 

(2)

 

goes into both sides of the equation, and , then

(3)

 

C_Di is the average resistance coefficient, which relates with vegetation distribution and Reno Number of flow. According to the research of S.W.Shen, no matter paralleled or interlaced planting, the resistance coefficient C_D value of 1 to 4 rows attenuates the most rapidly; the C_D value of 4 to N row (N>4) keeps constant. For convenient analysis, undulation of the C_Di of inlet is neglected, and suppose C_di as a constant, i.e., C_di=C_D, then

 

(4)

 

make ,

 

(5)

When there is no vegetation in the open channel, the shearing stress of the riverbed bottom should be:

 

(6)

 

The equation (5) is divided by equation (6):

 

(7)

 

Table 1 lists parts of calculated results of the test in order to show the quantity of . It is shown in the Figure that is small, <. When F_v is small, approaches, otherwise, is relatively large. To make the calculation easier, make

m is a index which relates with f_v. The value of and m are shown in Table 1 and Table 2.

Figure 3 shows the results of this test and S.W.Shen's. It can be seen from the curves that will decrease with the increase of F_v,.

f_v ~ m relevant points are shown in Figure 4. From the relevant analysis, we have some equations:

forest alternate planted, m=2 x 10^-3 (8)

forest paralleled planted, m=3.4 x 10^-3 (9)

brush alternate planted, m=4 x 10^-3 (10)

m=k

(11)

 

Here k is a constant in the formula.

2. Establishment of interrelation between vegetation factors and the relative sediment delivery ratio.

From the Shields sediment delivery ratio equation:

 

(12)

 

Then the section sediment delivery ratio after planting can be express as:

 

(13)

 

Here Q: discharge, is critical shearing stress and d₆₅: representative particulate size.

From formula (10) and (12), formula (14) can be got:

 

(14)

 

_{In the formula,} = _{is the reduction of sediment delivery ratio. Formula (10) is divided by formula (8):}

 

(15)

 

Formula (15) is the relevant interrelation between F_v, f_v and sediment delivery ratio.

To check the reliability of the formula (15), some actual measured values of the writers and S.W.Shen are shown in Figure 4. It indicates that the calculated values well tally the actual measured values.

3. The model test indicates clearly that the upstream height of backwater and the amount of deposition in riverbed relate with the vegetation density directly. Relevant formula of the length of dam and the spacing between dams is put forward though tests, which can be the reference in prototype planting plan.

 

(II)

 

Here L_d is length of the dam; : the maximum height of backwater;J is water surface gradient of natural riverbed.

 

(III)

 

Here l_a is spacing between dams;l₁ ~ l_i is backwater length of each rows of vegetation;

x₁ ~ x_I: abscissa of vegetation.

The test results tally the theoretical analyzed values very well. It can be used as reference of prototype plan according to the preliminary test results.

 

THE ROLE THAT THE FLEXIBLE DAM OF SEABUCKTHORN PLAYS IN COARSE SAND RESISTANCE

Several real gullies are chosen to plant flexible dams of seabuckthorn according to the test results and theoretical analysis, which shows as the above Figure 1. This area is located at 1282.4~1449.1 meters above sea level, with a river basin area of 20.865Km². The main gully is 8.26Km long, 2.53Km wide (average in the river basin), with gradient of 1.63%, and the area of soil and water losses is 100%. The mean annual rainfall is 388mm, evaporation is 2265mm, which is 5.8 times of the mean annual rainfall. The sandstone reveals above the ground. Though there's weed and other kinds of arbuscle grow on the slope, the coverage of vegetation is less than 30%. Collapse always occurs at both banks when flood runs down the gully in flood season.

The flexible dams of seabuckthorn planted in the test area since 1992 have taken obvious effect of sand resistance in gullies. Deposition appears in the whole gully riverbed in which dam of seabuckthorn is planted. The sediment delivery character changes from balance sediment delivery to unbalance sediment delivery. The gully deposition decrease progressively from bottom to surface, the maximum annual mean aggression is 0.5 ~ 0.6m. In one gully, coarse sand deposit takes place in the first distant upstream of the flexible dam, triangle deposition is formed there. The sentiment reduces to some extent in the second upstream dam. To the third and forth dams, even to the checkdam of the main gully, the deposition formed by fine particulate is like the alluvial fan. When the water in riverbed goes down, obvious rift appears in these places. Figure 8 shows the gully deposition in the two years. It clearly indicates that besides the deposition of the dam body, the deposition pattern of upstream dam shows that the upstream backwater has the same deposition as that of reservoir.

According to the on-the-spot investigation in July of 1998, the seabuckthorn communities planted in this two years grew very well, what is more, new seabuckthorn grew traversal and lengthwise of the riverbed. This is the unique character of seabuckthorn. It is preliminary estimated that several thousand Yuan RMB are enough to set up a series of flexible dams, while an earth checkdam needs more than 100 thousand Yuan RMB. After the checkdam is full filled, flood will overflow and scour the dam, it will be a vicious circle of scouring and forming gully. The flexible dam of seabuckthorn has the effect of flexible energy-reduction, and it will grow with the deposition of riverbed, then a good circle of eco-environment rehabilitation will be formed. It is really a new way to bring the sandstone area under control.

 

REFERENCES

1.      Bi Cifen, Yu Zhuode and Li Guifen, A New Proposal for the Control of Soil-Erosion in Sandstone Area, Paper Collection of the 3rd Seminar on Hydraulic Science & Technology, 1996.

2.      Bi Cifen and Shen Mei, Ecological Construction is the Key Technology for the Radical Watershed Comprehensive Control, Upstream & Middle Stream Administration Bureau under the Yellow River Commission, Mar. 1998.

3.      Shi Bing, Cao Shuyou, Bi Cifen and Li Guifen, The Calculation of Backwater of Gully Flexible Dam in Arid Area and the Rational Dam Distance, Sichuan Union University, 1997.

 

Table 1 Relevant Factors Check

Type of Vegetation	(M)	S0 ?	fv	Fv	hb	fv-hb
high forest	0.2000 0.1190 0.0600	2 2 2	0.00393 0.00196 0.00982	0.0302 0.0153 0.0077	0.003930 0.001960 0.009820	0 0 0	0.686 0.409 0.205	2.402 1.800 0.331
brush	0.0240 0.0667	2 2	0.00077 0.00231	0.0350 0.0940	0.000875 0.002900	-0.00021 -0.00059	0.500 0.024	3.780 12.40

 

Table 2 m Coherency

Type of Vegetation	Mode		1-	Fv	fv	m	Remark
high forest	alternate	0.286 0.442 0.629 0.308 0.265 0.268 0.283	0.714 0.558 0.371 0.692 0.735 0.732 0.717	0.0302 0.0153 0.0077 0.0272 0.0258 0.0273 0.0274	0.003928 0.001964 0.009820 0.003630 0.003625 0.003626 0.003626	0.096 0.140 0.204 0.102 0.084 0.093 0.087	Q=4.786m3/s S0=2 ? rectangular section
high forest	parallel	0.188 0.331 0.305 0.301 0.335 0.228	0.812 0.669 0.694 0.699 0.665 0.772	0.0594 0.0606 0.0303 0.0302 0.0302 0.0435	0.007860 0.003928 0.003928 0.003928 0.003928 0.005892	0.074 0.115 0.104 0.102 0.117 0.083	Q=5.89m3/s S0=2 ? rectangular section
brush	alternate	0.777 0.735 0.452 0.448 0.394	0.223 0.265 0.548 0.552 0.626	0.0307 0.0350 0.0940 0.0760 0.0870	0.007670 0.007670 0.002310 0.002310 0.002310	0.431 0.396 0.254 0.230 0.205	Q=6.82 l/s S0=2 ? trapezoidal section

 

 

 

 

Figure 5: Gully desposition in the Test