Unit Hydrograph Analysis for Small Watershed in The Northern Part of Thailand, 1987

Tantitheravit, Thawat

Major Field of Water Resources Engineering, Faculty of Engineering, Kasetsart University, Thesis of Master Degree, Bangkok 10900, Thailand

Abstract

The analysis of unit hydrograph for small watershed from the streamflow gauging stations is for the purpose of developing the method or the model for being used in derivation of unit hydrograph which is necessary for the determination of flood hydrographs required in the design of the hydraulic structures and structures related to water resources development. Such analysis is necessary for being used in the design of flood hydrographs for small watershed of ungauged stations which are located nearby or are hydrologically similar in topography.

The data used in the study are streamflow records during the period of high floods of 34 streamflow gauging stations in the northern part of Thailand. Among the selected gauging stations for the analysis, 32 of them are located throughout the northern region, and the remaining are in the upper central region. The period of available records varies from 2 to 13 years and the catchment area varies from 6 to 229 square kilometers.

The determination of the physical characteristics of watershed of the analyzed stations were carried out from 1:50,000 topographic maps of Royal Thai Survey Department by measuring the catchment area A, the length along the main channel from the location of station to the remote point on the watershed divide L, the length along the main channel from the station to the point nearest to the centroid of the basin L_c, and the average channel slope S, as well as the culculation of the ratio LL_c/S^1/2

The unit hydrograph analysis for each gauging station were carried out by computing the unit hydrographs from various strom flood hydrographs and averaging for representative of the station. Two parameters of unit hydrograph, i.e., time to peak t_p and peak discharge q_p ,were then calculated and used in constructing the dimensionles unit hydrograph from the representative unit hydrograph. The dimensionless unit hydrograph is the unit hydrographs having time scale t/t_p and discharge scale q/q_p

The determination of relations of time to peak t_p vs physical catchment characteristic LL_c /S^1/2 and unit peak discharge q_p/A vs time to peak t_p were conducted by the method of regression analysis in the form of logarithmic relation.

The results of analysis give rise to the relationships classified into 3 principal river basins covering the whole area of the northern region :

Upper Chao Phraya basin t_p = 1.89533 (LL_c/ S^1/2) ^0.17263 and q_p /A =1.36404 (t_p) ^-0.62414,

Khong river basin t_p = 1.41377 (LL_c/s^1/2)^0.17833 and q_p/A = 1.07432 (t_p )^-0.57639, and

Salawin river basin t_p = 0.67296 (LL_c /S)^0.31874 and q_p/A = 0.91722 (t_p)^-0.41935 .

Similarly, the results of the calculated dimensionless unit hydrograph can be divided into 3 principal river basins: Upper ChaoPhraya ,Khong , and Salawin.

The obtained relationships between the parameters of the unit hydrograph and the physical catchment characteristics as well the dimensionless unit hydrographs can be applied in the derivation of the unit hydrograph of small ungauged watershed in the northern part of Thailand.

The analysis of unit hydrograph for small watershed from the streamflow gauging stations is for the purpose of developing the method or the model for being used in derivation of unit hydrograph which is necessary for the determination of flood hydrographs required in the design of the hydraulic structures and structures related to water resources development. Such analysis is necessary for being used in the design of flood hydrographs for small watershed of ungauged stations which are located nearby or are hydrologically similar in topography.

The data used in the study are streamflow records during the period of high floods of 34 streamflow gauging stations in the northern part of Thailand. Among the selected gauging stations for the analysis, 32 of them are located throughout the northern region, and the remaining are in the upper central region. The period of available records varies from 2 to 13 years and the catchment area varies from 6 to 229 square kilometers.

The determination of the physical characteristics of watershed of the analyzed stations were carried out from 1:50,000 topographic maps of Royal Thai Survey Department by measuring the catchment area A, the length along the main channel from the location of station to the remote point on the watershed divide L, the length along the main channel from the station to the point nearest to the centroid of the basin L_c, and the average channel slope S, as well as the culculation of the ratio LL_c/S^1/2

The unit hydrograph analysis for each gauging station were carried out by computing the unit hydrographs from various strom flood hydrographs and averaging for representative of the station. Two parameters of unit hydrograph, i.e., time to peak t_p and peak discharge q_p ,were then calculated and used in constructing the dimensionles unit hydrograph from the representative unit hydrograph. The dimensionless unit hydrograph is the unit hydrographs having time scale t/t_p and discharge scale q/q_p

The determination of relations of time to peak t_p vs physical catchment characteristic LL_c /S^1/2 and unit peak discharge q_p/A vs time to peak t_p were conducted by the method of regression analysis in the form of logarithmic relation.

The results of analysis give rise to the relationships classified into 3 principal river basins covering the whole area of the northern region :

Upper Chao Phraya basin t_p = 1.89533 (LL_c/ S^1/2) ^0.17263 and q_p /A =1.36404 (t_p) ^-0.62414,

Khong river basin t_p = 1.41377 (LL_c/s^1/2)^0.17833 and q_p/A = 1.07432 (t_p )^-0.57639, and

Salawin river basin t_p = 0.67296 (LL_c /S)^0.31874 and q_p/A = 0.91722 (t_p)^-0.41935 .

Similarly, the results of the calculated dimensionless unit hydrograph can be divided into 3 principal river basins: Upper ChaoPhraya ,Khong , and Salawin.

The obtained relationships between the parameters of the unit hydrograph and the physical catchment characteristics as well the dimensionless unit hydrographs can be applied in the derivation of the unit hydrograph of small ungauged watershed in the northern part of Thailand.