Qing-hua ZHANG*, Yan-fang DIAO,, Jie DONG

1. College of Water Conservancy and Civil Engineering, Shandong Agricultural University, Taian 271018, P. R. China

2. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

Impacts of water surface area of watershed on design flood

Qing-hua ZHANG*1, Yan-fang DIAO1,2, Jie DONG1

1. College of Water Conservancy and Civil Engineering, Shandong Agricultural University, Taian 271018, P. R. China

2. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing 210098, P. R. China

In order to analyze the impact of the water surface area of a watershed on the design flood, the watershed was classified into a land watershed and a water surface watershed for flood flow calculation at the same time interval. Then, the design flood of the whole watershed was obtained by adding the two flood flows together. Using this method, we calculated design floods with different water surface areas of three reservoirs and analyzed the impact of water surface area on the flood volume and peak flow. The results indicate that larger water surface areas lead to greater impacts on the flood volume and peak flow. For the same watershed area, the impact of water surface area on the flood volume and peak flow is positively proportional to the flood frequency, i.e., the higher the frequency, the greater the impact becomes.

watershed; water surface area; design flood; flood frequency; flood volume; peak flow

1 Introduction

In the design of water conservancy and hydropower projects, it is required that design floods at different flood frequencies, including peak flows, various flood volumes, and hydrographs, be calculated. Reasonable design floods play an important role in engineering design since their reliability and accuracy directly affect the engineering design scale and operation control schedule.

The design flood is usually estimated based on the flow or storm data. Under the condition that the actual flow data within a watershed are not available, storm data may be used to estimate the design flood. Currently, studies of the design flood focus primarily on its estimation at ungauged sites, calculation methods of design flood synthesis, and optimization methods for fitting the design flood frequency curve.

Zhou et al. (2004) proposed a regional regression method to estimate the design flood atungauged sites. Sui (2005) proposed the calibrated hydrologic engineering center-1 (HEC-1) model and applied it to rainfall-runoff simulation for an ungauged watershed. Castellarin (2007) also applied probabilistic envelope curves to design flood estimation at ungauged sites.

To investigate the calculation method of the design flood synthesis, Xie et al. (2006) used the first-order second-moment method to analyze the design flood synthesis influenced by upstream reservoirs, and then deduced the design flood hydrograph from the typical flood hydrograph. Liu (2005) adopted the stochastic simulation to calculate the design flood synthesis in the Cuijiaying navigation and hydropower project. Ji (2005) proposed the probabilistic combination method to calculate the design flood for cascade reservoirs.

For optimizing the fitting method of the design flood frequency curve, Song and Kang (2008) presented three fitting methods based on the simulated annealing, the genetic algorithm, and a coupling of the simulated annealing and genetic algorithm. The ant colony system (ACS) algorithm, inspired by the ant foraging principle, was also used to solve the problem of homogenous frequency enlargement of design floods, and the sketch of ant foraging for design flood computation was constructed with detailed processes (Xie et al. 2007). Four continuous probability distribution functions (PDFs), namely, the two-parameter Beta, Weibull, Gamma, and Lognormal distribution functions, were employed to fit the hydrographs of the Brahmani River in eastern India for a period of 22 years (Pramanik et al. 2010).

In estimation of the design flood from rainstorm data, the focus is on the precision of runoff generation and flow concentration forecasts. The Monte Carlo simulation technique is presented based on the joint probability approach, which incorporated the probabilistic characteristics of key input variables, such as the rainfall intensity, duration, temporal pattern, initial loss, and their correlations in flood estimation (Ataur et al. 2002). Jain et al. (2000) used the GIS-based geomorphological instantaneous unit hydrograph (GIUH) approach to estimate the design flood. A structure of a continuous semi-distributed rainfall-runoff model, named MISDc, was also presented by Brocca et al. (2011) for flood simulation in the upper Tiber River in central Italy.

In hydraulic engineering construction, large quantities of diversion works and water storage works have been built to fully utilize rain and flood resources. Since these water storage works have small watershed areas, the quantity of active water mainly depends on water diversion from other watersheds. These water storage works have large water surface areas, occupying a large part of the entire watershed. At present, there has been little research on the design flood calculation for such projects. In this study, taking three reservoirs as examples, we calculated design floods with different water surface areas, considering the land area and water surface area, and analyzed the impact of the water surface area on the design flood calculation. In this paper, we also put forward suggestions for design flood calculation with the water surface area taken into consideration.

2 Research method

In the process of flood formation, rainfall outside the boundary of the reservoir water surface (hereinafter referred to as the land watershed) forms a flood process after runoff generation and flow concentration processes, whereas rainfall inside the boundary of the reservoir water surface (hereinafter called water surface watershed) directly forms a flood process without causing runoff generation and flow concentration processes. Thus, a watershed was classified into a land watershed and a water surface watershed for flood flow calculation in this study. Consequently, the two flood flows calculated at the same time interval were added together to form the reservoir inflow design flood of the whole watershed. It should be pointed out that the design flood is defined in this paper as the reservoir inflow design flood.

2.1 Design flood of land watershed

The method of calculation of the design flood of a land watershed is the same as the current approach of design flood estimation based on storm data, in which the storm data within the watershed or the regional rainstorm isogram are used to estimate the point rainfall, area rainfall, net rainfall, and net rainfall interval distribution at different flood frequencies. Then, the flow hydrographs at different frequencies are estimated with the instantaneous unit hydrograph method.

2.2 Design flood of water surface watershed

The flood volume is the rainfall on the water surface watershed. According to the principle of the water balance for reservoir flood routing, the average flood flow can be approximately calculated as follows:

wheretQ is the average flood flow during the tth time interval (m3/s), tΔ is the time interval (h),WS is the area of the water surface watershed (km2), andPtR is the rainfall at the tth time interval and frequency P (mm).

3 Calculations and analysis

3.1 Study cases

The design flood is related to the design storm, watershed area, runoff generation, and flow concentration conditions. This study took three reservoirs as examples to calculate design floods of different frequencies, including the Shengli Reservoir in Taian City and the Wujiazhuang and Yangzhuang reservoirs in Linyi City. All the three reservoirs are situated in mountainous areas. It was assumed that the design rainy period was 24 h, with a time interval of 1 h selected for flood routing. Basic information on the three reservoirs is shown in Table 1.

Table 1 Basic information on three reservoirs

3.2 Calculation of design flood

Design floods of five flood frequencies, 0.1%, 1.0%, 2.0% , 3.3%, and 5.0%, with seven different percentages of water surface area, 0%, 5%, 10%, 20%, 30%, 40%, and 50%, were calculated with the method described above. The percentage of 0% means that the watershed consists of land areas without water surface. Flood volumes and peak flows of all reservoirs for the five flood frequencies with seven different percentages of water surface area are shown in Table 2 and Table 3. The flood hydrographs of the Shengli Reservoir for the flood frequency of 1.0% with seven different percentages of water surface area are shown in Fig. 1.

Table 2 Flood volumes of three reservoirs

It can be seen from Fig. 1 that flood flows at a given time increase with the percentage of water surface area. Meanwhile, the peak times of the seven flood hydrographs are the same, owing to the fact that rainfall in the water surface watershed directly forms a flood process without causing runoff generation and flow concentration processes.

Table 3 Peak flows of three reservoirs

Fig. 1 Flood hydrographs of Shengli Reservoir at flood frequency of 1% with seven different percentages of water surface area

3.3 Calculation of relative addition rate

To analyze the impact of the water surface area of watersheds on design floods, the relative addition rates of the flood volume and peak flow with six different percentages of water surface area (5%, 10%, 20%, 30%, 40%, and 50%), as compared with those without water surface area at each flood frequency, were calculated and are shown in Table 4 and Table 5.

Table 4 Relative addition rates of flood volume of three reservoirs with different percentages of water surface area

Table 5 Relative addition rates of peak flow of three reservoirs with different percentages of water surface area

3.4 Impact analysis

It can be seen from Table 2 and Table 3 that the water surface area has a certain impact on the flood volume and peak flow. As a general rule, the flood volume and peak flow increase with the percentage of water surface area. In terms of the impact degree, the water surface area has a greater impact on the peak flow than on the flood volume. Below, we provide detailed analyses of the impact of the water surface area:

3.4.1 Impact of water surface area on flood volume

(1) It can be seen from Table 4 that, with the same percentage of water surface area, the relative addition rate of flood volume increases with the flood frequency. Taking the Shengli Reservoir as an example, for a water surface area of 10%, the flood volume increases by 1.8%, 2.5%, 2.7%, 2.8%, and 3.1% at the frequencies of 0.1%, 1.0%, 2.0%, 3.3%, and 5.0%, respectively, compared with the flood volume for a watershed without water surface area.

(2) Also, the greater the percentage of water surface area is, the greater the relativeaddition rate of flood volume at the same frequency. The relative addition rate of flood volume is positively proportional to the percentage of water surface area at the same frequency.

(3) Table 4 also shows that the relative addition rate of flood volume decreases with the increase of watershed area at the same frequency and percentage of water surface area. For instance, with a frequency of 0.1% and the percentage of water surface area of 10%, the watershed area of the Yangzhuang Reservoir (36 km2) is greater than that of the Shengli Reservoir (13.8 km2), while the relative addition rate of flood volume of the Yangzhuang Reservoir is 1.1% smaller than that of the Shengli Reservoir.

3.4.2 Impact of water surface area on peak flow

(1) Table 5 indicates that, with the same percentage of water surface area, the relative addition rate of peak flow increases with the flood frequency. Taking the Shengli Reservoir as an example, peak flow with a water surface area of 10% increases by 5.2%, 7.4%, 8.0%, 8.1%, and 8.9% at the frequencies of 0.1%, 1.0%, 2.0%, 3.3%, and 5.0%, respectively, as compared to a watershed without water surface area.

(2) At the same frequency, the relative addition rate of peak flow increases with the percentage of water surface area. The relative addition rate of peak flow at the same frequency is positively proportional to the percentage of water surface area.

(3) Table 5 also indicates that the relative addition rate of peak flow increases with the watershed area at the same frequency and percentage of water surface area. For instance, with the frequency of 0.1% and the percentage of water surface area of 10%, the relative addition rate of peak flow of the Yangzhuang Reservoir is 2.0% greater than that of the Shengli Reservoir.

In addition, it needs to be pointed out that the relationships between the relative addition rates of flood volume and peak flow and the percentage of water surface area should be nonlinear in theory, but the relationships are approximately linear in this study. That is because runoff generation and flow concentration were not considered in the water surface watershed calculation.

4 Conclusions and suggestions

From the analyses described above, the conclusions are the following: (1) For the same frequency, the greater the percentage of water surface area is, the greater its impacts on the flood volume and peak flow. In addition, the impact of the percentage of water surface area on the peak flow is greater than that on the flood volume. (2) The impacts of the percentage of water surface area on the flood volume and peak flow are positively proportional to the frequency. (3) For the same frequency and percentage of water surface area, the greater the watershed area is, the higher the relative addition rate of peak flow and the smaller the relative addition rate of flood volume.

Based on these conclusions, the following suggestions are made for design flood calculation: (1) When the watershed area is less than 30 km2with a water surface area of over10% or the watershed area exceeds 30 km2with a water surface area of over 5%, the impact of water surface area should be taken into consideration in the design flood calculation. (2) When the impact of water surface area is taken into account in the design flood calculation, the watershed should be classified into a land watershed and a water surface watershed for flood flow calculation at the same time interval, and the design flood of the whole watershed can be obtained by adding the two results of the flood flow calculation together.

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(Edited by Ye SHI)

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This work was supported by the Major Water Conservancy Scientific Research and Technology Promotion Project of Shandong Province, the Special Fund for the Public Welfare Industry of the Ministry of Water Resources of China (Grant No. 201201022), and the Open Foundation of the State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering of Hohai University (Grant No. 2011490111).

*Corresponding author (e-mail: zqh@sdau.edu.cn)

Received Oct. 30, 2012; accepted Jun. 24, 2013