Abstract. One of the important factors in channel dimension is the Manning's coefficient (n).This coefficient is influenced not only by the channel roughness but also by the presence of plants in the channel. The aim of the study is to see the effect of the ratio between the height of the rigid plant and water depth on the Manning's coefficient (n) in open channel. The study was conducted in open channel with 15.5 m long, 0.5 m wide and 1.0 m high, in which at the center of the channel is planted with the rigid plants with a density of 42 plants/m 2 . The flow was run with a discharge of 0.013 m 3 /s at 6 ratios of H plants /H water , namely: 0; 0.2; 0.6; 0.8; 1,0 and 1,2, to obtain the velocity and water profiles. Then the value of n is analyzed using Manning's equation. The results showed that the mean velocity becomes decrease 17.81-34.01% as increase the ratio of H plants /H water . This results in increasing n value to become 1.22-1.52 times compared to the unplanted channel (n o =0.038). So, it can be concluded that the ratio between the rigid plant's height and water depth in the open channel can affect the value of Manning coefficient.
One of the eco-hydraulic methods used to reduce flooding and erosion in rivers is by vegetation along the riverbanks. The main problem in the Lawe Alas River is repeated flooding in the downstream areas that need to be addressed upstream. The purpose of this study is to design the proper vegetation diameter and the distance between plants that can reduce the velocity and flow of the river. The method of study in this research consists of conducting field surveys, measuring river discharge, and calculating vegetation diameter on riverbanks to reduce velocity and discharge based on the equation proposed by Merten. The planned diameter of vegetation with diameter is 0,1 m - 1,1 m, and the distance between vegetation 1 m - 11 m. The results showed that the maximum discharge capacity of the Lawe Alas River was 514,360 m3/s. For the return period of 50 years, 50 m from the riverbank design, vegetation with a diameter of 10 cm to 110 cm can reduce flooding in the resulting discharge 222,95 m3/s - 241,68 m3/s. Discharge is greater than 50 yearly flood discharge, which is 528,670 m3/s. In conditions without vegetation (diameter = 0) velocity = 1,725 m/s, the velocity produced by the design is 0,727 m/s - 0,789 m/s. Discharge and velocity are reduced by 54% - 58% so that the vegetation diameter of 10 cm is suitable for environmentally friendly designs on the riverbanks 50 m. In conclusion, all planned vegetation diameters can reduce discharge and velocity. Recommended vegetation is local vegetation, which is a tree that produces fruit that can be benefited to the community so that it can be planted and managed by local residents.
The presence of vegetation growing in the channel can cause increasing the Manning’s coefficient. The magnitude of Manning’s coefficient is depended on the characteristic and type of vegetation. The purpose of the research is to study and model the effect of density and height of vegetation on Manning’s coefficient in open channel. The study was conducted in open channel with 15.5 m length, 0.5 m width and 1.0 m height. At the centre part of the channel is planted with Elephant grass (Pennisetum Purpureum). The effect of different density and height of vegetation were run with fixed discharge to measure the velocity and water profiles in order to obtain Manning’s coefficients. The effect of those variables on the developed model was evaluated using statistical analysis of variance and paired t-test with significance level of 0.05. The results showed that Manning’s coefficient is increasing as increasing density and height of vegetation. If compared to un-vegetated channel, Manning’s coefficient increase in between 4.92 - 54.07%. Based on statistical analysis, it is found that Fcalculation = 367.776 > Fcritic = 19.452, tcalculation = 12.298 and 22.464 > tcritic = 2.571 and R2 = 0,967, showing that the density and height of vegetation significantly influence on the Manning’s coefficient with strong correlation variables in the model.
The problem of flooding that occurs around the Lawe Alas-Singkil, Aceh Province is the conservation function of the area from upstream to downstream needs serious attention. The area around the watershed is degraded due to several factors. This factor is closely related to illegal logging, exploitation of nature with mineral mining to the conversion of area functions. This study aims to identify the existing conditions of the riverbanks, how the relationship between community social activities and the banks of the Alas-Singkil River, and to study the physical conditions of the river from upstream to downstream. The study method in this research includes a literature study, hydrological conditions of the Alas-Singkil Watershed, field surveys, identification of existing conditions, and study of the ecology and physicality of the river. Survey the condition of flora and fauna in the study area and the economic and social activities of the community. The results of the study on the hydrological conditions of the Alas-Singkil watershed with an area of 10,090.13 km2 and a river length of 316.26 km, in the upstream form of mountains and natural forests where deforestation has occurred due to forest conversion to cocoa plantations, planting of corn on the hills and so on, The maximum annual daily rainfall is high (151-300 mm), so the potential for flood disaster is even greater. In the rainy season, large amounts of water cause mountains or hills in Southeast Aceh to melt and become a flash flood disaster. Many of the rivers in the middle and downstream have turned into plantations and rice fields and some are still swamps. The condition of the river in the Alas-Singkil watershed generally occurs silting or sedimentation in the river body, the existing condition of the Alas-Singkil riverbank, where there are still many settlements on the riverbanks. The downstream Krueng Singkil is experiencing more frequent flooding, either due to the overflow of Krueng Singkil in the upstream part and the blocked river flow and rising tides. The frequency of inundation and flood disasters in Singkil City reaches 3 to 4 times each year with an inundation height of 0.5-3.0 meters. It is necessary to repair the riverbanks by using vegetation components found on the banks of the Alas-Singkil River and overcoming floods in the upstream and downstream rivers to protect the ecosystem and people living on the riverbanks. The solution that can be done is to restore the conservation function of the area around the Lawe Alas watershed. and built channel diversion flood from the Lae Soraya river in the village of Gelombang directly to the sea.
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