The purpose of this study was to evaluate the method of estimating the areal precipitation reflecting the altitude of the mountainous terrain on Jeju Island by comparing and analyzing the areal precipitation using the Thiessen polygon method and the isohyetal method in mountainous streams. In terms of constructing the Thiessen polygon network, rainfall errors occurred in 94.5% and 45.8% of the Thiessen area ratio of the Jeju and Ara stations, respectively. This resulted in large areal precipitation and errors using the isohyetal method at altitudes below 600 m in the target watershed. In contrast, there were small errors in the highlands. Rainfall errors occurred in 18.91% of the Thiessen area ratio of Eorimok, 2.41% of Witseoreum, and 2.84% of Azalea Field because of the altitudinal influence of stations located in the highlands at altitudes above 600 m. Based on the areal precipitation estimation using the Thiessen polygon method, it was considered to be partially applicable to streams on Jeju Island depending on the altitude. However, the method is not suitable for mountainous streams such as the streams on Jeju Island because errors occur with altitude. Therefore, the isohyetal method is considered to be more suitable as it considers the locations of the rainfall stations and the orographic effect and because there are no errors with altitude.

In this study, we calculated the fixed-type Areal Reduction Factor (ARF) of the Hancheon River basin in Jeju Island, and compared the calculated ARF and the ARF of the four major river basins suggested by the Ministry of Land, Infrastructure and Transport. As a result, the maximum fluctuation ratios of ARF for the four major river basins calculated using area, frequency, and initial duration time were significant: 7.61% for the Hangang River basin; 12.69% for the Nakdonggang River basin; 8.09% for the Kumgang River basin; and 17.98% for the Yeongsangang River basin. In addition, the differences between the maximum and minimum value of ARF for the Hancheon River basin based on 48 hours was 2.13%, and it was smaller than the one for the four major river basins: 8.92% for the Hangang River basin; 11.41% for the Nakdonggang River basin; 8.87% for the Kumgang River basin; and 17.17% for the Yeongsangang River basin. The Yeongsangang River basin had the highest difference.

This study reviewed the applicability of the existing flood discharge calculation method on Jeju Island Han Stream and compared this method with observation results by improving the mediating variables for the Han Stream. The results were as follows. First, when the rain-discharge status of the Han Stream was analyzed using the flood discharge calculation method of the existing design (2012), the result was smaller than the observed flood discharge and the flood hydrograph differed. The result of the flood discharge calculation corrected for the curve number based on the terrain gradient showed an improvement of 1.47 - 6.47% from the existing flood discharge, and flood discharge was improved by 4.39 - 16.67% after applying the new reached time. In addition, the sub-basin was set separately to calculate the flood discharge, which yielded an improvement of 9.92 - 32.96% from the existing method. In particular, the steepness and rainfall-discharge characteristics of Han Stream were considered in the reaching time, and the sub-basin was separated to calculate the flood discharge, which resulted in an error rate of –8.77 to 8.71%, showing a large improvement of 7.31 - 28.79% from the existing method. The flood hydrograph also showed a similar tendency.

In this study, the Chiu-2D velocity-flow rate distribution based on theoretical background of the entropy probability method was applied to actual ADCP measurement data of Gangjung Stream in Jeju from July 2011 to June 2015 to predict the parameter that take part in velocity distribution of the stream. In addition, surface velocity measured by SIV (Surface Image Velocimeter) was applied to the predicted parameter to calculate discharge. Calculated discharge was compared with observed discharge of ADCP observed during the same time to analyze propriety and applicability of depth of water velocity average conversion factor. To check applicability of the predicted stream parameter, surface velocity and discharge were calculated using SIV and compared with velocity and flow based on ADCP. Discharge calculated by applying velocity factor of SIV to the Chiu-2D velocity-flow rate distribution and discharge based on depth of water velocity average conversion factor of 0.85 were 0.7171 m3/sec and 0.5758 m3/sec, respectively. Their error rates compared to average ADCP discharge of 0.6664 m3/sec were respectively 7.63% and 13.64%. Discharge based on the Chiu-2D velocity-flow distribution showed lower error rate compared to discharge based on depth of water velocity average conversion factor of 0.85.

This research aims at comparing the accuracy of flood discharge estimation. For this, we focused on the Oedo watershed of Jeju Island and compared flood discharge by analyzing the values as follows: (1) the concentration of the lumped model (HEC-HMS) and distributed model (Vflo), and (2) the in-situ data using Fixed Surface Image Velocimetry (FSIV). The flood discharge estimation from the HEC-HMS model is slightly larger than the Vflo model results. This result shows that the estimations of the HEC-HMS are larger than the flood discharge data by 4.43 to 36.24% and that of the Vflo are larger by 8.49 to 11%. In terms of the error analysis at the peak discharge occurrence time of each mapping, HEC-HMS is one hour later than the measured data, but Vflo is almost the same as the measured data.

As global warming has accelerated to weather in recent years, and The frequent floods are creating heavy rains and typhoons followed by considerable damage in Jeju. This study estimated design flood discharges and flood stage in Jeju, considering climate change in connection with RCP scenario, the 5th IPCC Report recently published. It also analyzed the period which might be subject to the risk of flooding in downstream of Oedo Stream. As a result, it has analyzed that there might be a risk of flooding when there were 80 years or more rainfall events in 35 years that rainfall would have increased by 10%, 69 years that 100 years or more heavy rain and rainfall would have increased by 20%, and 104 years that 100 years or more heavy rain and rainfall would have increased by 20%. It is expected that this study results of rainfall increasing trend caused by climate change will be helpful to minimize the damage of floods which will secure the future of Jeju.

This study Analyzed four of seven runoffs which had happened in 2012 in comparison with the runoffs shown in Kalesto data, using the fixed surface image velocimetry (FSIV) installed at Oedo stream, Jeju Island. As a result of identifying a runoff curve graph, it was analyzed that the flood runoffs calculated with two observation devices were almost equivalent. As the differences in peak flows were 10 m3/s, 0.7 m3/s and 3 m3/s, the very similar result values were calculated. Even though there were errors in RMSE(Root Mean Square Error) made by two observation devices according to the degree of the peak flow, the values of R² by flood event were 0.89, 0.87, 0.86 and 0.82, showing the result values almost close to 1. Therefore, there was a very high correlation in flood runoffs calculated with two observation devices. This research method was considered to be a very suitable method to measure unexpected flood runoffs which could happen in the island area such as Jeju island during bad weather.