51
3.4. Result and Discussion
3.2.1. Basin Model Development
Developing a model basin is an important step in conducting analysis on hydrology system with HEC-HMS. A configuration needs to be developed in the
model basin to describe physical representation of a watershed based on its hydrology elements. There are seven hydrology elements available in HEC-HMS;
Sub-basin, Reach, Reservoir, Junction, Diversion, Source, and Sink. The research employs 19 sub-basins, 9 reach, 9 junctions, and 1 outlet.
Developing a model basin also includes calculation in 4 main sub-models, i.e.: loss model, transform model, base flow model, and routing model. Methods
and parameter needed for model basin input are listed in Table 13.
Table 13. Method and Input parameter for HEC-HMS
Model Method
Parameters
Loss SCS Loss Model
Initial Abstraction Curve Number
Percent Impervious Transform
SCS Unit Hydrograph SCS Lag Time
Baseflow Recession Baseflow
Initial Discharge Recession Constant
Threshold flow Ratio to Peak
Routing Lag
Lag Time
1 Loss Model
Rain fall in a certain watershed will go through infiltration, interception, evaporation and other form of loss processes before becoming run off. Loss model
calculates the amount of effective rainfall in total rainfall loss by employing precipitation loss. The research uses SCS method, since it is simple, measurable,
and stable USACE 2000. Bedient and Huber 1988 states that SCS approach has been applied well in several countries, due to its ability to consider land use, soil
hydrology characteristic and can be used in an un-measured area. SCS parameter that is needed as input in loss model is initial abstraction Ia,
curve number CN, and imperviousness percentage IMP. Initial abstraction is a function of land use and land cover i.e. interception, infiltration, depression storage
52 and previous soil humidity. In SCS method, Ia value is calculated based on potential
maximum retention and the curve number. Determination of curve number and width of impervious area is based on calculation in section 3.2.5.
Land use in upstream Cisadane watershed consists of 5 types of land uses, i.e.: water, grassland, estate, settlement, and forest area. Land hydrological
condition is determined by hydrology soil group HSG, which is based on soil type. Curve number at each sub-basin is calculated based on area weight of each land use
according to its HSG. Based on the calculation, average of curve number in Upstream Cisadane Watershed in 2010 is 87.757. Beside curve number, the weight
of impervious area also affect run off volume in each watershed. Table 14 shows curve number, imperviousness and initial abstraction in each sub-basin in upstream
Cisadane watershed.
Table 14. Curve Number Percent Impervious at each sub-basin in Upstream
Cisadane Watershed for Simulation Year 2030
Sub Basin Curve Number
CN Imperviousness
IMP Initial Abstraction
Ia
W280 0.445
0.444 0.440
W300 0.472
0.482 0.476
W310 0.629
0.610 0.593
W340 0.333
0.333 0.321
W350 0.525
0.531 0.523
W370 0.525
0.539 0.534
W380 0.420
0.421 0.421
W390 0.492
0.495 0.495
W400 0.452
0.448 0.443
W410 0.576
0.582 0.576
W420 0.494
0.501 0.501
W430 0.586
0.597 0.593
W450 0.398
0.409 0.409
W460 0.469
0.462 0.457
W470 0.583
0.571 0.566
W480 0.470
0.498 0.600
W490 0.629
0.618 0.615
W500 0.589
0.615 0.637
W510 0.596
0.602 0.631
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