ISSN: 0852-0682.
E-ISSN: 2460-3945
Forum Geografi.
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July 2016: 58-68 Analysis Physical Characteristics.
(Miardini et al.
Analysis Physical Characteristics of Land for Estimated Runoff Coefficient as Flood Control Effort in Comal Watershed.
Central Java Arina Miardini1 and Pranatasari Dyah Susanti Institute of Technology Forestry Watershed Management Research Surakarta (Balai Penelitian Teknologi Kehutanan Pengelolaan DAS Surakart.
Jl.
YaniAePabelan.
Kartasuro PO BOX 295 Surakarta 57102 Telp.
Fax .
716959 Corresponding author .
mail: arinamiardini@gmail.
Abstract The vast area of critical land was evidenced to be a serious threat for watershed carrying capacity which eventually affected the hydrology imbalance in the watershed area.
The purpose of this study is to identify the physical characteristics of Comal watershed which have significant influence in determining the runoff and calculating the runoff coefficient by taking into account the parameters of watershedAos physical The method used in this analysis is Cook method, unit of analysis in this study is land mapping unit.
Peak discharge determination is performed by using the Rational method.
The analysis showed that runoff coefficient of Comal watershed is 61.
63%, which can be categorized as high.
The runoff coefficients and peak discharge calculations of each sub watershed, respectively.
Comal Hilir of 52.
with peak discharge 505.
68 m3/sec.
Genteng of 65.
04% with peak discharge 542.
44 m3/sec.
Lomeneng 00% with peak discharge 194.
23 m3/sec.
Srengseng of 64.
10% with peak discharge 270.
46 m3/sec, and Wakung Hulu of 62.
34% with peak discharge 686.
64 m3/ sec.
Comal watershed flood control priority on increasing the rate of infiltration, slope management, increasing vegetation cover and management of drainage density.
Keywords: watershed characteristic, land, runoff coefficient, flood.
Comal watershed Abstrak Luas lahan kritis yang makin meningkat mengancam keseimbangan hidrologi dalam DAS.
Salah satu DAS yang memiliki lahan kritis dan memerlukan prioritas penanganan adalah DAS Comal.
Tujuan penelitian ini adalah mengidentifikasi karakteristik fisik DAS Comal yang berpengaruh dalam penentuan koefisien aliran serta menghitung koefisien aliran dengan mempertimbangkan parameter karakteristik fisik DAS.
Metode penelitian yang digunakan untuk mengetahui karakteristik fisik DAS menggunakan metode Cook.
Unit analisis yang digunakan dalam penelitian ini adalah satuan lahan.
Penentuan debit banjir dilakukan menggunakan metode rasional.
Hasil analisis menunjukkan bahwa nilai koefisien DAS Comal yaitu 61,63% yang termasuk kategori tinggi.
Nilai koefisien aliran dan perhitungan debit pada masing masing sub DAS yaitu Comal hilir 52,65% dengan debit sebesar 505,68 m3/dtk.
Genteng 65,04% dengan debit sebesar 542,44 m3/dtk.
Lomeneng 64,00% dengan debit sebesar 194,23 m3/dtk.
Srengseng 64,10% dengan debit sebesar 270,46 m3/dtk, dan Wakung hulu 62,34% dengan debit sebesar 686,64 m3/dtk.
Pengendalian banjir di DAS Comal diprioritaskan pada peningkatan laju infiltrasi, pengelolaan kemiringan lereng, peningkatan tutupan vegetasi dan pengelolaan kerapatan aliran.
Kata kunci: karakteristik DAS.
DAS Comal, banjir limpasan, koefisien aliran.
metode cook, rasional Introduction Numerous watersheds in Indonesia are enduring the impact of degradation due to exploitative land use and environmental carrying capacity.
Based on Direktorat Jenderal Bina Pengelolaan Daerah Aliran Sungai and Perhutanan Sosial in Buku Statistik Kehutanan .
6 and 2.
, it is mentioned that critical land area was 77.
880,78 ha in 2006 and it increased by 104.
026,20 ha in It indicates the ineffective endeavors in managing watershed due to several obstacles.
The vast area of degraded land turns out to be a serious threat for watershed carrying capacity, which will eventually be a major cause for watershed hydrology imbalances.
One among Available online at http://Journals.
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(Miardini et al.
the impacts of hydrology imbalances in the watershed is flood.
Sinukaban .
also suggests that flood is an obvious indicator of watershed dysfunction caused by the decline of infiltration as the result of eliminated vegetation cover and inappropriate land use.
The ignorance of this damage will just add the long list of critical watersheds in Indonesia.
Flooding will be discussed in this paper is the flood derived from land or known as the overland flow.
Type flood that comes from surface runoff before entering the river an inquiry on land physical characteristics which affect the amount of runoff is required so as to formulate recommendations for flood control in a given watershed in the context of upstream to downstream watershed management efforts.
The magnitude of the flood is strongly influenced by the intensity of the rainfall, watershed area and runoff Static factors such as rainfall and watershed area is a natural factor, whereas a more dynamic runoff coefficient so that the flood control can be done by managing the runoff coefficient.
Rencana Pembangunan Jangka Menengah/ RPJM) 2010-2014 explicitly states a number of 108 watershed in Indonesia are included in critical condition and watershed priority (The Ministry of Forestry has issued Decree No.
SK.
328/Menhut-II/2.
Comal watershed is one among Priority I watersheds.
Comal watershed has an essential function especially for its raw water supply for numbers of daily needs such as irrigation, households, and industry for 5 districts.
The categorization of Comal watershed as Priority I is mainly caused by its flood intensity.
For instance, based on the data from Badan Nasional Penanggulangan Bencana/BNPB, flood took place in Bodeh.
Comal.
Ulujami.
Ampel.
Gading.
Taman.
Petarukan, and Pemalang on 4 March 2014.
The condition is worsened by the increasing population density and intensive natural resources exploitation, resulting in the massive watershed conditions such as landslides, erosion, sedimentation, flood, drought (Suharyono et al.
, 2.
The aim of the study is to identify the physical characteristics of Comal watershed which influence the runoff determination and to calculate the runoff coefficient by examining the parameters of watershed physical The yield of the inquiry will be the basis of flood control recommendation in Comal watershed, particularly in sub watersheds with flood vulnerability as the efforts to uphold upstream to downstream watershed management.
WatershedAos response to the waters system is strongly affected by the physical characteristics of watershed.
The characteristics can be defined as specific illustration of watershed adduced by the parameters related to morphometry, topography, soils, geology, vegetation, land use system, hydrology, and human activity (Seyhan, 1.
According to Supangat .
, the watershedAos condition relied upon the nature as well as on land use system as human involvement.
In flood control efforts.
Method Study was carried out in Comal watershed (DSA) in September to November 2015.
Total area of Comal watershed is 82140.
30 ha with 99 km in perimeter.
The main river of Comal watershed is Kali Comal of 104.
km in length.
Materials used in this study consist of: Map of Comal watershed, data of daily precipation data of 10-year series .
from 12 rain stations (Randudongkal.
Sipedang.
Sumubkidul.
Banjardawa.
Warungpring.
Bongas.
Moga.
Brondong.
Ponolawen.
Kaliwadas.
Sokowati, and Klayera.
, digital data of Landsat 8 Imagery of 2014 Path/row 120/66 spatial resolution of 30 x 30 m.
RBI Digital Map scale 1:25000.
Soil Type Map scale 1:250000, and Forest area map.
The tools used in this study include Notebook ASUS Core i3 with capacity of 6 GB in RAM and harddisk 500 GB.
Software Arc GIS 10.
Software Ms.
Word, and Ms.
Excel.
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The physical characteristics of watershed was analyzed by using CookAos method in accordance to Chow .
and Meijerink .
In this method, the estimation of runoff was carried out through runoff coefficient by examining the wathershedAos characteristics of slope, inflitration, vegetation cover, and drainage density.
The analysis unit in the study was land mapping unit.
Meijerink .
in Gunawan .
affirms that land mapping unit is a preferred interpretation unit and landscape visual mapping unit in association with hydrology process.
The step of the study is illustrated in Figure 1.
Figure 1.
Research Flowchart The value of runoff coefficient, subsequently, was classified based on CookAos method which was divided into 4 .
classes with detail categorization as follows:
Table 2 Runoff Coefficient Classification Class Criteria Low Normal i High Extreme Source: Meijerink .
Value (%) < 25
>25-<50
>50-<75
>75-100
Results and Discussion The Physical Characteristics of Study Area The analysis indicates there is a total of 247 land mapping units in Comal which consists of 2353 poligons.
The amount of runoff in each land mapping unit is calculated by considering the characteristics factors of watershed in CookAos method, which slope, drainage density, vegetation cover, and infiltration.
These Forum Geografi.
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parameters, furthermore, are classified and seep into the ground will be a runoff that proportionally scored based on their influence will gather in the tributaries so that the peak on runoff coefficient (Gunawan, 1.
The discharge will increase.
result of analysis is presented in Table 2.
Drainage density is an important characteristic Slope is included in the factor affecting the of watershed as it is used to evaluate the amount of rainfall will eventually surge as potential of runoff.
It is the comparison Goro .
and Suhardiman .
between total area of watershed and total that the slope will affect the runoff and the length of all the streams and rivers in a water possibility to seep into the ground will watershed (Raharjo, 2.
A watershed with be smaller so that it will increase the potential one watercourse and numbers of tributariesAo for flooding in a watershed.
branch has a higher drainage density compared to a watershed with a watercourse and a few The percentage of slope in each sub watershed of tributaries (Indarto, 2.
The highest can be figured out in Table 2.
In this table, drainage density of (>5 mill/mill.
in Comal Genteng and Srengseng sub watershed watershed (Table .
occurred in 4 .
sub have the highest percentage of slope in watersheds, while Comal Hilir sub watershed class >30%, respectively, of 43.
76% and has the highest drainage density in the class Lomeneng and Wakung hulu sub of 0-1 mil/mill2.
High drainage density allows watershed are evidenced to have the highest runoff from the area above the tributaries to percentage in class 10-30%, respectively, of be efficiently drained, since the rainfall will 7% and 49.
59%, meanwhile Comal Hilir flow into the tributaries quickly, rainfall will sub watershed has the highest percentage of be divided and will reduce the peak discharge.
89% in slope of 0-5%.
The dominant slope in Comal watershed turns out to be susceptible.
Infiltration is important process of runoff.
since the potential of land degradation will It determines the amount of rainfall which result on higher amount of runoff coefficient.
can be absorbed into the soil.
The higher In addition, land with hilly to steep conditions is the value of infiltration, the smaller is the causing rainfall pours down in relatively difference between rainfall intensity and high speed, hence, it have inadequate time to infiltration capacity, since the runoff is smaller It is similar to Suhardiman .
and so is the peak discharge.
According to who suggests slope is an elevation of land Widianto et al.
, .
, in the case of rainfall surface that will affect the rate of infiltration.
is directly poured down to the ground, the soil aggregates will be destroyed and it causes As illustrated in Table 2, it can be observed the space of macro pore is reduced so that that the dominant vegetation cover in the class the infiltration rate is declined.
In addition to of >80%, which are Genteng sub watershed the vegetation as the resistor of the amount .
6%).
Lomeneng sub watershed .
65%), of rainfall into the ground, soil infiltration is Srengseng sub watershed .
28%), and also affected by the nature of soil physical Wakung Hulu sub watershed .
3%), while properties.
It is stated by Rohmat and Sukarno Comal Hilir sub watershed is on class of .
that the infiltration rate is a function 10-80% with the percentage of 45.
of permeability parameters and soil moisture The information of vegetation cover is very which is closely related to the soil physical significant due to its highly correlation properties.
Based on the result of the analysis with the runoff.
The higher is the amount of (Table .
, on infiltration parameter, there are vegetation in a given land, the lower will the three sub watersheds are included in the slow runoff (Saribun, 2.
It is similar with the class, namely.
Genteng of 68.
Lomeneng study conducted by Rajarjo .
, in which of 77.
68%, and Srengseng of 94.
Comal water unretained by the vegetation and cannot Hilir of 61.
97%, and Wakung Hulu of 49.
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are included in the category of medium.
indicates that soil texture is evidenced to have influence on infiltration rate.
According to Arsyad .
, soil properties which determine and limit infiltration capacity is soil structure and it is powerfully affected by water texture and content.
In addition.
Fonth and Adisoemarto .
in their study imply that the relative size of soil particle is expressed in terms of texture refers to the smoothness or roughness of the soil.
Soils with smooth texture, such as clay, have small pore spaces, hence, the infiltration is slow as it requires long time for rainfall to fill the soil pores.
Table 2 Parameter of Runoff Coefficient Estimation Values in Each Sub watershed Parameter of Runoff Coefficient Estimation Sub Comal Hilir Genteng Lomeneng Srengseng Wakung Hulu Slope Vegetation Cover Class Area Percentage (%) >30% 0-5% 10-30% Drainage Density .
ill/mill.
Class
Area
Percentage (%) >80% 10-80% Infiltration Rate
Class
Area
Percentage (%) Class
Area
Percentage (%) Moderate >2-5 Slow
0-10%
>1-2
Ignored 5-10% >30% >80% Moderate 0-5% 10-80% >2-5 Slow
10-30%
0-10%
>1-2
Igored 5-10% >30% >80% Moderate 0-5% 10-80% >2-5 Slow
10-30%
0-10%
>1-2
Ignored 5-10% >30% >80% Moderate 0-5% 10-80% >2-5 Slow
10-30%
0-10%
>1-2
Ignored 5-10% >30% >80% Moderate 0-5% 10-80% >2-5 Slow
10-30%
0-10%
>1-2
Ignored 5-10% Estimation of Runoff Coefficient The value of runoff coefficient in each sub watershed in Comal Watershed (C) ranges 65 to 65.
04 with an average value of 63 (Table .
From the analysis, the highest coefficient value was obtained by Genteng sub watershed and the lowest was Comal Hilir sub watershed.
C value indicates that Comal watershed can be categorized as high, it means that high amount rainfall becomes runoff and potential to overland flow.
(Figure .
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Table 3 Runoff Coefficients of Sub watershed in Comal Watershed Sub watershed Score S Score V Comal Hilir Genteng Lomeneng Srengseng Wakung Hulu Rata-rata Source: Data Processing and Analysis, 2015 Score I Score D Figure 2.
Map of Runoff Coefficient of Comal Watershed Score C Forum Geografi.
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Assessment of peak discharge used in this study is done through Rational Method which is affected by the factors of runoff coefficient, rainfall intensity, and watershedAos area.
Runoff is the most dynamic response in a watershed that is related to rainfall (Liang et al.
and Mu et , 2.
Data of maximum rainfall intensity is determined through the daily rainfall data of rain gauge station.
Assessment of the maximum daily rainfall is done by calculating the Thiessen coefficient of each rain station.
The result of rainfall calculation indicates the maximum daily precipitation in Comal watershed was 124.
26 mm which occurred on January 29.
rainfall intensity also affects the amount and velocity of runoff.
Based on the calculation, rainfall intensity of Comal watershed was 91 mm with time of concentration of 7 hours 54 minutes.
The value of peak discharge in each sub watershed can be calculated as follows in Table 9.
Estimation on the peak discharge in Comal watershed which was calculated in each sub watershed adduced that the highest discharge was in Wakung Hulu sub watershed of 686.
m3/sec and the lowest was in Srengseng sub watershed of 270.
46 m3/sec.
The largest contributor of overland flow in Comal watershed was Wakung Hulu sub watershed.
Rainfall and runoff are interconnected, and Wakung Hulu sub watershed is located in the further, they constitute a major problem in upper area of Comal watershed.
It is dominated the field of hydrology (Liang et al.
, 2.
by hilly to steep slopes, high drainage density.
Monde .
suggests that in addition to slow-to-moderate soil infiltration, and low the extensive catchment area, the amount of density vegetation cover.
Table 9 Calculation of Peak Discharge (Q) of Sub watersheds in Comal Watershed Sub watershed Comal Hilir Genteng Lomeneng Srengseng Wakung hulu I .
m/h.
A .
Q .
3/se.
Source: Data Processing and Analysis, 2015 In managing the attempts of flood because from the four analyzed parameters should be of land .
verland flo.
control in Comal determined.
In accordance to the assessment watershed, the most significant parameter of correlation on C value (Table .
Table 5 Correlation Value of Runoff Coefficient Estimation Factors Sub watershed Slope Comal Hilir Genteng Lomeneng Srengseng Wakung hulu Source: Data Processing and Analysis, 2015 Vegetation cover Implication toward Flood Control Efforts The main target of flood control is the restoration, reclamation and conservation of land use in accordance with the function Infiltration Drainage density and the region or spatial plan that can be controlled runoff because the functioning of vegetation cover on any land use and functioning of water infiltration into the soil Forum Geografi.
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Flood management in each watershed will be different according to the contribution of each parameter in the assessment of runoff Based on the correlation analysis, the priority control for Comal watershed are:
increasing the rate of infiltration, .
slope management, .
increasing vegetation cover and .
management of drainage density, with the following explanations:
Increasing the rate of infiltration Increasing infiltration capacity can be increased through the making biopori b.
especially in residential areas and adding soil improvement materials.
According to Subagyo .
, the runoff, soil, and infiltration capacity controls are vital in the runoff management.
Biopori is a small hole in the ground which is naturally formed by the activity of soil organisms.
The existence of the organic material can be as a unifying or binding soil grains .
, a source of nutrients, enhancing the ability of soil holds water .
olding capacit.
, an addition to the cation exchange capacity .
ation exchange capacit.
as well as the energy source for microbes and makroba soil (Gardiner and Miller 2.
Soil organic matter is a source of food for soil organisms, thus stimulating the formation of soil structure over crumbs and ultimately can improve soil infiltration capacity.
Biologically, to promote the improvement of soil infiltration capacity, the are Simple and inexpensive method is the treatment of compost or organic The addition of organic matter in the soil is capable in slowing down the rate of runoff, increasing the infiltration, and stabilizing soil aggregate (Nuraeni c.
et al.
, 2.
In addition to organic matter, utilization of soil fauna such as earthworms, can improve soil infiltration.
According to Subowo .
, endogeic earthworm can construct soil pores and unite the soil layers.
To support the earthwormAos breeding and populations, organic fertilizer is required.
Organic matter as feed either vertically or to the inner layer of soil, can stimulate the worm to crawl up to the inner layer, so as to strengthen the absorption of water and to improve the soil pores (Subowo, 2.
It is similar to the analysis delivered by Iqbal .
that the addition of compost can increase the activity and population of soil organisms, improve soil structure, increase the ability to bind water and soil aggregates, and increase infiltration.
Slope management Land management in areas with steep slopes to steep and increasing vegetation cover by applying the principles of conservation of soil and water.
combination of conservation techniques encompassing of vegetative, mechanics, and chemistry is ideal for flood control.
Mechanically, the factor of long and steep slopes can be overcome by cutting through the construction of terraces and contour planting.
According to Arsyad .
, bench terracing is able to reduce the slope length, as well as to decrease the runoff velocity and quantity.
Mechanical technique is accountable for the management of slope and river.
River rehabilitation is also significant in flood control effort.
Setyowati .
suggests the establishment of well built embankment, particularly on the bank of the river, which is expected to withstand the high discharge, the establishment of constructed channels flowing into the main river is an attempt to reduce the pool due to the poor infiltration, in addition to crops planting with solid roots on the riparian areas which can strengthen the natural levee.
Increasing vegetation cover The increasing vegetation cover can through several techniques such as agroforestry and the utilization of grasses.
Agroforestry systems and grasses can be applied as a flood control.
According to Atmojo .
, agroforestry has an important role in watershed management and flood control as it is accountable in Forum Geografi.
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covering the ground completely so as to reduce the runoff and to improve the soil infiltration capacity.
In addition, agroforestry systems with a variety of crops will construct solid roots, it will maintain the slope stability.
Soan et al.
, .
affirms the crops and their residue can be used as protective ground against raindrops and runoff carrying capacity and subsequently, it will improve soil infiltration capacity.
In addition to agroforestry, grasses can be used as terrace reinforcement.
Nuraeni et al.
confirms that mechanical technique must be balanced with vegetative conservation by planting terrace crops such as grasses which serves as compost, fodder, and soil binder to prevent landslides.
The combination between grass and agroforestry system can covered the ground surface, so it can reduce the runoff.
Management of drainage density According to Pallard et al.
, drainage density is a catchment morphology and may influence significantly the frequency regime of flood flows and control the formation of river flows.
Because of that, the management of drainage density is very important to reduce runoff.
water, .
dredging river sediment, .
creation of water storage as a water transit , .
Preparation diversion if necessary for the flow solver, .
Protection of river banks and reservoirs, .
construction Rehabilitation riverbanks and levee.
Conclusion Several physical characteristics of Comal watershed were analyzed, consisting of slope, infiltration, vegetation, and drainage density.
As previously elaborated, conclusion can be drawn that from 5 .
sub watersheds in Comal watershed, 2 sub watersheds are in slope of > 30%, 2 sub watersheds are in slope of 0-30%, and 2 sub watershed is in slope of 0-5%.
The dominant vegetation cover in Comal watershed is >80%, dominant drainage density is categorized in class >5mill/mill2, and dominant infiltration rate can be considered as Runoff coefficient of Comal watershed is 63% or high category.
The values of runoff coefficient and discharge calculation of each sub watershed are, respectively.
Comal Hilir 65% with peak discharge 505.
68 m3/ sec.
Genteng of 65.
04% with peak discharge 44 m3/sec.
Lomeneng of 64.
00% with peak discharge 194.
23 m3/sec.
Srengseng 10% with peak discharge 270.
46 m3/ sec, and Wakung Hulu of 62.
34% with peak 64 m3/ sec.
Comal watershed Some management drainage density can flood control priority on increasing the rate be through .
the normalization stream of infiltration, slope management, increasing with the manufacture of plaster riverbanks vegetation cover and management of drainage .
in order to prevent erosion, density.
landslides and increase the capacity of References