Extended paper MSD 2019 Adhy Yulistiyanto

Extended Abstract, 9th International Workshop on Multimodal Sediment Disasters
Prediction of Bed Degradation Due to Sand Mining Activities
Using An Analytical Approach
Adhy KURNIAWAN*1, Bambang YULISTIYANTO2,
*1 Department of Civil Engineering, Vocational School, Universitas Gadjah Mada, Yogyakarta, Indonesia
E-mail: [email protected]
2 Department of Civil and Environmental Engineering, University of Gadjah Mada, Indonesia
Sand mining in the river downstream of Merapi is mostly done by the community because the quality of
sediment material is very good for construction. In the upstream area, mining is carried out at sediment
deposition locations around Sabo dam using heavy equipment. In the downstream areas, mining is carried
out by the community using pumps that are accommodated on river banks. The sand mining activity needs
to be regulated and controlled to prevent environmental damage, failure of infrastructures and economic
and social problems.
Infrastructure failure can occur if mining activities result in changes in river morphology and changes in
river bed levels. To overcome this problem, it is necessary to investigate changes in river beds due to
removal of the sediment by sand mining activities. In this present study, an analytical approach to river bed
degradation, St Venant Exner derivation, is used to predict the bed degradation in a function of time and
distance.
Key words: bed degradation, sand mining, mining regulation
1. INTRODUCTION
In the business supply chain, mining activities
involve quite a lot of workers, miners, transport
workers, and material storage entrepreneurs. The
activity has been regulated through regulation with
the obligation to do licensing in the sand mining
business. Nevertheless illegal mining activities are
still often found, and in the implementation of mining
it has a negative impact on the environment.
In a recent years, community mining activities have
shifted from using manual equipment (senggrong) to
switching to using pumping equipment as shown in
Fig. 1 and Fig. 2. The pump is driven by using a
generator with gasoline and diesel fuel, so the result
of these activities is waste fuel pollutes the river
body. Sand mining activities using these pumps have
increased sharply. Based on the results of field
monitoring and based on aerial photographs (Fig. 3
and Fig. 4), it was seen that in early 2012 there was
no activity using pipes and pumps, then started from
2013 there were more than 80 vacuum pipes sand that
lies along the Progo River, Yogyakarta.
Fig. 1 Sand mining equipment uses pumps
Fig. 2 Sand mining activities by community
Extended Abstract, 9th International Workshop on Multimodal Sediment Disasters
equation.
2. BED DEGRADATIONS
Degradation can occur when the supply of solid
discharge is reduced (Williams and Wolman, 1984,
Wu and Wang, 2008), the water discharge is
increased, or a lowering of a fixed point on the
channel bed at the downstream, sediment removal
from river by mining activities (see Fig. 5).
.
Fig. 3. Instream sand mining using pumps (2018)
2.1 Analytical solutions
The analytical solutions will certainly help to
explain the evolution of the bed of the channel, when
the variations of water discharge can be readily
neglected. In the degrading channel the flow is being
considered as being steady and quasi uniform.
Computation of a degradation is only possible if the
conditions assumed for a parabolic model are well
fulfilled, namely: quasi steadiness of the flow, quasi
uniformity of the flow at Fr<0.6, validity for x>
3h/Se , where Fr is Froude number, x is longitudinal
distance, h is water depth and Se is energy slope,
respectively (Graf and Altinakar, 1998).
Fig.5. Scheme of degradation channel
Fig. 4. Mining spot downstream of Bantar bridge
Sand mining or sediment excavation at the bottom of
the river results in a process of hungry water, where
digging holes locally can increase the depth of the
flow, and excavation activities also allow the
widening of the flow. Both of these conditions result
in the speed of river flow taking more sediment below
or downstream of the mining pits. This continuous
process results in river bed degradation (Kondolf
1997, Gob, et al., 2005). Sand mining carried out at
inappropriate locations, can cause damage to
infrastructure, damage to the river environment,
disruption of flora and fauna life in the river,
pollution of river quality (Lee, et al., 1993, Rinaldi,
2005 and Rijn, 1996). The intensive turbulent flow
occurs in a scour hole contributes the active
movement of sediments (Kurniawan and Altinakar,
2002). The present study aims to investigate the bed
degradation by using analitycal approach of Exner
In the present study the analytical solution of
Exner equation is applied. The Exner equation
describes conservation of mass between sediment in
the bed of a channel and sediment that is being
transported. It states that bed elevation increases (the
bed aggrades) proportionally to the amount of
sediment that drops out of transport, and conversely
decreases (the bed degrades) proportionally to the
amount of sediment that becomes entrained by the
flow.
Exner equation can be expressed as follows:
𝜕𝑧
𝜕𝑡
1
+ (1−𝑝)
𝜕𝑞𝑠
𝜕𝑥
=0
(1)
The equation for the parabolic model was given as:
𝜕𝑧
𝜕2 𝑧
− 𝐾 𝜕𝑥2 = 0
𝜕𝑡
(2)
The coefficient K, in the parabolic model is
approximately given by:
𝐶 2ℎ
𝜕𝑞
𝐾 = 3𝑈(1−𝑝) 𝜕𝑈𝑠
(3)
Extended Abstract, 9th International Workshop on Multimodal Sediment Disasters
The solution to
transformations is:
Eq.
𝑧(𝑥, 𝑡 ) = ∆ℎ 𝑒𝑟𝑓𝑐 (
(2),
𝑥
2√𝐾𝑡
use
of
Laplace
)
(4)
The use of the model is limited to Fr<0.6, and
x>3Rh/Se.
Sand mining contributes to the construction of
buildings and development, however the negative
effects of sand mining include the permanent loss of
sand in areas. Permitted mining locations (safe
zonation area) need to be determined to reduce the
risk of environmental damage, and it is necessary to
ban miners in prohibited areas.
3. RESULTS AND DISCUSSIONS
REFERENCES
In the present study, the observation of mining site
was in the downstream area of Progo river, where the
mining pumps were very active. The field data of
hydraulic parameters were obtained, the channel
width is 106.3 (m), hydraulic radius, Rh, is 1.04 (m) ,
mean diameter grain size,d, is 0.7 (mm), the water
discharge, Q is 153.2 (m3/s), Froude number, Fr, is
0.3. From the data, then we can obtain the coefficient,
K, to calculate the dimensionless, z/h in the
temporal and longitudinal variations. Result of
evolution of bed degradation can be seen in the Fig.
6. The vertical dash line is the boundary of validity
of this model, which is valid only x>3Rh/Se. For
distances of x<3Rh/Se, the solution is only an
indicative one.
0
5000
x (m)
10000
15000
0
z/h
1 week
2 weeks
4 weeks
boundary
-1
Fig. 6. Bed degradation profiles
From these results it can be seen that the river bed
degradation that occurs will spread downstream.
Bridge Pajangan, Bantul where is located in the
downstream part of sand mining site need to be
alerted to the decline in the river bed.
The field measurement should be carried out to
compare the results of this model.
4. CONCLUSION
Prediction of river bed degradation was carried
out by using analytical solution of Exner equation.
Temporal evolution of bed degradation can be
described by this model.
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