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MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT THUY LOI UNIVERSITY LUU VAN QUAN THE RESEARCH ON DETERMINING OPTIMAL SIZE AND LAYOUT OF DETETION POND FOR THE MIXED URBAN AND AGRICULTURAL BASINS Major: Planning and management of water resources Code : 62-62-30-01 THE ABSTRACT OF DOCTORAL THESIS HANOI, 2015 The thesis was completed at the Thuy loi University Supervisors1: Assoc. Prof. Dr Tran Viet On Reviewer 01: Prof. Dr. Duong Thanh Luong Reviewer 02: Prof. Dr.Tran Huu Uyen Reviewer 03: Dr. Nguyen Van Tuan The thesis will be presented to the Doctoral Evaluation Council at Room 123-A1 buiding, Thuyloi University. ………………………………………………………………………………………………………………………….... ………………………………………………………………………………………………………………………….... Time of presentation: ……………………. If you would like to get more information on this thesis, please feel free to visit: - The national Library - Thuyloi University Library INTRODUCTION 1. The necessity of thesis In recent decades, due to the demands of the development, constructing the urban areas and industrial zones have shifted the soil structure of many agricultural regions to urban - agriculture mixed regions. This change has increased flooding in these areas. Using detention pond to regulate stormwater and reduce flooding is an effective solution. However, there are no researches on the effects of detention pond to the construction cost of the drainage system for the mixed areas. Thus, the theme: "The research on determining optimal size and layout of detention pond for the mixed urban and agricultural areas" very meaningful to reality and science. 2. Objectives Proposing a methodology to determine optimal size and layout of detention pond to reduce the construction cost of the drainage system for the mixed areas. 3. The scope of work and object of study The thesis focused on determining optimal size and layout of detention pond to reduce the construction cost of the drainage system for the mixed areas, applying to the Western Hanoi. Object of study: the drainage system for the mixed areas, applying to the Western Hanoi. The scope of work: - The optimal size and layout of detention pond to reduce the construction cost of the drainage system for the mixed areas in terms of ditermining topography, ground, slope of canal as well as location of constructions. - Regardless of sewage or water quality as well as other costs such as operating costs and maintenance of drainage system. - Applying to the western Hanoi. 4. Research methods and approaches 4.1. Approaches: Approaching the system, inheriting available researches. 4.2. Research methods and used techniques 1 Inherited methods, modeling and simulation methods, analysing system and optimization, statistical methods. 5. Scientific and practical significance Scientific Significance: The thesis provides scientific methodology for determining the optimal size and layout of detention pond to reduce the construction cost of the drainage system for the mixed areas. Practical Signigicance: The thesis proposed the percentage of area and form of rationl layout of detention pond in the Western Hanoi. This result can be applied in planning of detention pond for the Western Hanoi and other areas with similar conditions. 6. New contributions of the thesis 1. This thesis provided a scientific method to determine the capacity and layout form of detention ponds for the mixed urban and agricultural drainage to reduce the construction cost. The new method includes: constructing objective functions, identifying components of the objective function, solving optimization problems, suggesting a decision alternative (i.e. instead of proposing and calculating each of available alternatives in detail, these alternatives were compared, and then the best alternative was selected). The new method overcomes a limitation of the existing method which limits quantity of alternatives, hence the new method is considered more appropriate than the old one. 2. The thesis has applied the new method successfully for a case study- a basin in Western Hanoi. This area is currently being urbanized, and industrialized with the combination of agricultural land and urban land. The thesis used SWMM5.0 (Storm Water Managament Model of EPA, USA) to analyze and evaluate 500 options including centralized detention ponds at the headworks or decentralized ponds at major channels and at minor channels. The results show that: i) For this study area: The appropriate ratio of detention pond area to total drainage basin area fluctuates from 2% to 3.82% depending on two factors: (1) layout form of detention ponds (i.e. decentralization or centralization). The more dispersive the detention ponds are, the higher the ratio is; (2) land acquisition cost. The more the land 2 acquisition cost is, the smaller the ratio is and vice versa; ii) With the same ratio, the more dispersive the detention ponds are, the less the land acquisition is and vice versa. This result (first time) proved (through specific data) the effectiveness of the motto "spraying water, burying water" proposed long time ago by water resources experts of Viet Nam. The research results can be applied for other similar drainage areas. 7. The layout of thesis There are three main chapters: Chapter 1. Overview of detention pond, effect of detention pond to drainage system for the composite areas. Chapter 2. Methodology and research tools. Chương 3. Researching and determining the optimal size and layout of detention pond for the Western Hanoi. CHAPTER 1. OVERVIEW OF DETENTION POND, EFFECT OF DETENTION POND TO DRAINAGE SYSTEM FOR THE COMPOSITE AREAS 1.1. The function of detention pond 1.1.1. Terms The detention pond is a terminology standing for lowlands, valleys where have water storage capacity (temporary or permanent), formed in natural or artificial conditions. The system is responsible to drain simultaneously for either agriculture and urban area called the drainage system for the mixed areas. 1.1.2. The function of detention pond The detention ponds in drainage system simultaneously perform multiple functions such as regulating stormwater, reducing flood, storing water for irrigation, aquaculture, improving the microclimate, creating good ecological environment. In this research, detention pond (called HDH) focuses only on the function of regulating stormwater and reducing flood. 3 1.1.3. The connecting forms between detention pond and chanel. There are two connecting forms: detention pond is on channel and detention pond is beside of channel. (1) Pond on channel is construction connected from pond to channel by spillway, culvert or channel or combination between channel and spillway, channel and culvert. (2) Pond beside channel is part of channel extended with function of regulating water. 1.2. Overview of using detention pond 1.2.1. On the world Detention pond used to reduce flood in urban areas have built widely since 06’s of 20th century all around the world. However, the uneven contribution of pond area with total drainage area is diffirently divieded among cities as well as continents. 1.2.2. In the Vietnam The detention pond in urban drainage system have area ratio from 1% to 5%, most natural lakes, and scattered distribution. Some detention ponds use less efficient due to the location, area ratio, capacity, operation or connecting system between pond and channel. 1.3. Overview of researches on detention pond 1.3.1. On the world There have been many researches on the effect of detention pond to the flooding, environment and landscape ... Some studies which have figured out the approaching method by using detention pond to store the rainfall for other purposes is very effective either technique and economy. Some studies shown that the concentrating pond reduce peak discharge and flooding better than dispersing pond. 1.3.2. In the Vietnam Some studies on sustainable drainage have considered the detention pond under the aspect of dispersion, rainfall reducing at source (stored in households and increased permeability). The studies about the influence of the detention pond to the drainage headworks were also considered. In addition, studies of other fields (environment, landscape, ecology) were also announced. 4 In conclusion, either worldwide and Vietnam doesn’t have any research on relationship between size and layout form of detention pond to the construction cost of the drainage system for the composite areas. 1.4. Conclusions of chapter 1 The results of research show that: 1) Area ratio of detention pond in cities on the world and in the Vietnam are very different. 2) Most of the research focused on the function of environmental improvement and storing water for many different purposes. 3) Concentrating pond has good efficiency for reducing the peak discharge, flooding area and the flooding depth depends on the scale and layout form of detention pond. 4) Detention pond in agricultural drainage system have researched at the simple level: aquaculture pond, storage water in fields. 5) Effect of detention pond to headworks is very clear. There are no researches on relationship between size and layout form of detention pond to the construction cost of the drainage system for the composite areas. According to above reasons, the thesis: “The research on determining the size and form of rational layout for the composite area” is sponsored to research. CHAPTER 2. METHODOLOGY AND RESEARCH TOOLS. 2.1. The detention pond in the drainage system 2.1.1. The detention pond in the urban drainage system The detention pond is one of terms of urban drainage system. The detention pond originated from either nature or artificiality is distributed in residential areas or park to store rainfall, reduce flooding, improve the environment and provide water for irrigation. 2.1.2. The detention pond in agricultural drainage system The detention ponds in agricultural drainage systems are natural ponds or lakes existing before headworks were built. The natural ponds are sparsely distributed and the artificial pond has function on aquaculture or irrigation. These detention ponds have function to store rainfall, reduce the peak discharge for downstream constructions, and provide water for irrigation. 5 2.1.3. The detention pond for composite areas The drainage system for composite area includes simultaneously drainage construction for urban areas and for agricultural areas. Constructions in drainage system are characterized by incharge area is urban or agricultural. 2.2. Making the problem to determine the size and layout of detention pond Figure 2.7. Drainage system layout for the composite areas The problem is applied to the composite areas. The initial conditions of the problem stated that the drainage ground, land using and infrastructure need to be planned in the researching area (determing location of the drainage headworks, regulating culvert and canals). Therefore, the detention pond area may be reached to a certain rate. Requirement: Determing the size and layout form of detention pond in order to minimize the construction investment cost and ensure no waterlogging corresponding the design storm. 2.3. Method 2.3.1. Proposing the sequence to solve the problem a. Traditional method It is based on experience, by calculating and direct comparision, the designers give out the best appropriate method. The steps of solving will be shown in the figure 2.9. The advantage of this method is easy to implement and the disadvantage is that the calculation volume is too large and the result depends subjectively on the proposing person. 6 b. New method The author proposes a new method to determine the size and form of rational layout for detention pond (Figure 2.8) including building objective function, solving the optimization problem by regression method, proposing a reasonable solution. Studing basin characteristic: Studing basin characteristic: Topography, geology, land Topography, geology, land using… using… Proposing some solutions: (channel, detention pond) Proposing some different solutions: (canal, Determining the designing flow rate of each method Establishing methods (hydraulic, hydrological calculating – determining designing flow rate and method scale by modelling) Economy, technical calculating of each method Establishing regression function Comparing methods, determining the reasonable method Solving multi-objective optimization problem (using empirical planningmethods to solve) Proposing selected method Proposing selected method Figure 2.8. New method Figure 2.9 Traditional method (Diagram of determining steps of the reasonable layout of the detention pond) The basic advantage of this proposed method is to increase the considered cases (more than 500 cases), regardless of the experience of proposer. Thus, the rationality of the proposed method is very high. 2.3.2. Facility to determine the size and layout form of detention pond The construction area should take advantage of the natural ponds and farmland and are not coincided to the technical infrastructure already planned. 7 The calculating scenarios are created from the combination of size and layout form of the detention ponds (concentrated or dispersed) in the determined region under the plane containing thescale and layout form axis (Figure 2.10). Main  cannal KC  VT1 Branch _ 01 Branch _ 02 . . . Branch _ i . . . Branch _ m N 01  VT1 N 02  VT1 . . . Ni  VT1 . . . Nm  VT1 KB1 KC  VT2 . N 01  VT2 . N 02  VT2 . . . . . . . Ni  VT2 . . . . . . . Nm  VT2 KB 2 .  . . . . . . . . . . . . . . . . . . . . . . . . . KC  VTn N 01  VTn N 02  VTn . . . Ni  VTn . . . Nm  VTn KB n Figure 2.10. The determining method of layout form scenario 2.3.3. Determining the size of the drainage system in scenarios After proposing the scenarios on size and layout of detention ponds, it is period to determine the construction scale in system by following steps: 1st Step: Determining the preliminary dimensions of channels, pumping stations according to the current regulations, namely follow to the formula in TCVN 7957: 2008, TCVN 4118-85. 2nd Step: Using Storm Water Management Model 5.0 (SWMM 5.0, EPA USA), to simulate and to check the system determined from the 1st Step. This problem suggests for the urban drainage system and appling “on-farm detention pond” method for the agricultural area. The calculating result of on-farm detention pond method is outflows and they are entered into the SWMM 5.0 model as inflows at Nodes. 2.3.4. Establishing the objective function and the constraint conditions a. General objective function General objective function for reasonable design problem is only minimum the construction investment cost. The general objective function form: m T  n  j j j C htj  C đm  C htk  C hdh    C đmi   Cthkj   C hdh   Min 1 1  1 j 8 (2.25) With: Cjht: The total construction investment cost of drainage system for the scenario j, Cjđm: The total construction investment cost for drainage headworks for the scenario j, Cjhtk: The total construction investment cost for conveying line (conveying cannal) for the scenario j, Cjhdh: The total construction investment cost for the detention ponds for the scenario j, J: Number of scenario J = 1, 2, 3, ..., n: Number of cannals in system, T: Number of detention pond in system. b. Constraint condition The constraint conditions in the simulation: 1) Fngập = 0; no flooding in the case of calculating rain. 2) Fhồ < (a%* Flưu vực); detention pond area is smaller than a% total drainage area. 3) Construction the detention ponds are workable. 4) Assuming the studing field has defined boundaries, planned ground of canal and pipepline, identified the slope of each channel segment, and fixed the structure form and the detention pond work in the gravity form. 5) The scenarios are siminar in work structure, unit price and calculating period. c. Establishing the objective function for studying area The identification of the objective function for the studying area is done on the basis of determining the regression function’s components. Following as 02 methods below: Method 1: Calculating directly the construction cost, establishing the regression function between construction cost and discharge. Method 2: Construction costs are taken from the project in the region. Establishing of regression function between the construction costs and discharge or area is to determine the objective function’s components. In this study, both of two ways has chosen to build the specific objective fundtion to studying areas. The order of elaboration of the regression function for each item is implemented as follows: * Drainage headwork is pumping station. The headwork of system is pumping station. The regression function corresponding to each pumps forming: 9 Cđm = f1(Qđm); (2.30) * Drainage channel system. Drainage channel system including multi-channel grade, each channel considers a representative length segment L (m). Chtk = ΣCkênh cấp 1 + ΣCkênh cấp 2 + ΣCkênh cấp 3… (2.31) * Grade 1 and 2 channels: Assuming the grade 1 and 2 channels have trapezoidal cross-section with earth channel and earth channel with embankment. The regression functions of grade 1 and 2 channels: Ck1,2 = f2(Qk1,2); (2.32) * Grade 3 channel: Most grade 3 channels’s common structure is prefabricated reinforced concrete culverts or box reinforced concrete culverts and buried underground. The regression functions of grade 3 channel: Ck3 = f3 (Qk3); (2.33a) * Detention pond: In this studying, the detention pond has function of regulating rainfall and reducing flooding. The regession function of detention pond: Chdh = f4(Fh); (2.33b) * The specific objective function: Regarding to (2.25) and (2.30, 2.31, 2.32, 2.33a, 2.33b) established, the specific objective function has following formula: C = f1(Qđm) + f2(Qk1,2) + f3(Qkc3) + f4(Fh); (2.34) 2.3.5. Applying the objective function to choose an optimal scenario a. The relationship between discharge and area of the detention pond The relationship between total peak discharge on the drainage headwork, total peak discharge on channel system and pond area is the inverse relationship. b. The relationship between total cost and pond area 10 According to the above relationships, the figure 2.26 has summersized the relationship between construction investment cost and pond area or pond’s area ratio. In where: Cđm Construction investment cost in headwork, Chtk Construction investment cost for channel system, Chdh Construction investment cost for detention pond, Cht Construction investment cost for whole system. Figure 2.26. The relationship form between construction investment cost and pond area 2.4. Conclusions of chapter 2 Using experimental planning method to solve the problem to get the results of high accuracy. The extreme points of construction investment cost that is minimum (reasonable method) depends on the construction unit prices and unit costs for site clearance compensation. When compensation unit price is higher, the extreme points will close to the origin and when construction cost is higher, the extreme points tend to back away from the origin (Figure 2.26). The methodology also presents a problem description and resolution procedures. With the method described, this methodology can be applied to all the basins where have the composite area. The Storm Water Management Model 5.0 is selected to simulate the hydrology, hydraulics for urban area, on the other hand, the on-farm pond is selected to 11 resolve to the agricultural area. Methods and tools described above are applied to specific research areas in the west of Hanoi. CHAPTER 3. RESEARCHING AND DETERMINING THE OPTIMAL SIZE AND LAYOUT OF DETENTION POND FOR THE WESTERN HANOI BASIN 3.1. Selecting and describing the research area 3.1.1. Selecting the research area The west of Hanoi is selected to be a case study for the problem of determining the size and form of detention pond. The spindle is Nhue river from the Lien Mac culvert to the Ha Dong culvert. The Dam river, Cau Nga river and La Khe river also are studied in the research. The incharging area of drainage is 17,965 ha including 13,917 ha of urban planning, 2.090ha of greenbelts planning along Nhue river, 1,958 ha of agricultural land [68]. The drainage system satisfied the data and boundary conditions, so it satisfy the requirements of the proposed problem in Chapter 2. Picture 3.1. The study area in the West of Hanoi 12 3.1.2. Features of ponds and lakes in the study area The study area has flat topography; natural ponds are few in number and small in size. The natural pond is not involved in regulating rainfall. 3.2. Validation of the Storm Water Management Model 5.0 3.2.1. Purpose To ensure the reliability of the model, consistent with the research problem, using this parameter is to simulate the scenario of the research problem. 3.2.2. Validation results Table 3.1: The evaluation table of the error of the calculating and measuring process Measuring station Daily rain Hà Đông 24/8 - Đồng Quan 28/8/2010 Max discharge (m3/s) Total discharge (m3) Measur Calcu Error Calculati Error Measuring ing lating (%) ng (%) Error S/ 81,58 82,78 1,48 8.052,9 8.076,72 0,70 0,15 105,66 107,16 1,42 10.365,7 10.739,8 3,61 0,18 From table 3.1, the indicators on the maximum discharge, the total discharge and the line form are achieved. So it is possible to apply these simulations to test the model. Table 3.2: The evaluation table on the error of calculating and measuring process (02 rains) Max discharge (m3/s) Measuring Station Hà Đông Đồng Quan Hà Đông Đồng Quan Daily rain Total discharge (m3) Error Measu ring Calcul ating Error Calculatin Measuring g (%) Error (%) S/ 22/5 - 89,60 93,92 4,82 8.819,67 8.773,56 0,52 0,08 26/5/2012 101,19 101,36 0,17 11.671,77 11.549,44 1,05 0,32 17/8 - 65,11 66,73 2,68 7.192,45 7.340,79 2,06 0,10 19/8/2012 100,61 99,08 -0,25 11.091,29 11.022,57 0,62 0,08 From table 3.2, the error of line form is from 0.08 to 0.32, the error of maximum discharge is from 0.17% to 4.82%, the error of total discharge is from 0.52% to 2.06%. Thurs, the errors were smaller than allowance. 13 Therefore, the calculating line and measuring line is well reasionable with each other. The SWMM can be appied to calculate for this thesis. 3.2.3. The model parameters were chosen after testing a. The parameters of padded surface - Coefficient of surface roughness with hard coating: n = 0.025 – 0.15 - The parameters on the characteristics of soil permeability: Maximum permeable coefficient kmax = 8 mm/h; kmin =0.5 mm/h, the saturated seapage time is 5 days, average permeability is 25mm. b. The bed roughness coefficient: River and earth channel: n = 0.025 – 0.03; river and earth channel with embankment stone roof: n = 0.017. c. Calculating time: t = 60 s 3.3. Establishing the specific objective function for the West basin of Hanoi 3.3.1. The general objective function The Chapter 2 have determined the general objective function as following formula: n C = Cđm + Chtk + Chdh = C 1 m T đmi   Cthkj   C hdh => Min 1 1 3.3.2.Determining the components of objective function The construction costs and the clearance price at the fourth quarter period in 2013 of the west of Hanoi to determine the components of the objective function. By considering the impact of costs for site clearance to the selection of appropriate layout scripts, author has selected 03 cases of calculations, including: TH1 (100% of agricultural land), TH2 (85% of agricultural land and 15% residential land), TH3 (70% and 30% of agricultural land for residential land). a. The drainage system headwork is pumping stations To determine the parameters in the objective function of Cđm ~ Qđm, this studying used the data of 21 pumping station in the west of Hanoi and vicinity area and updated to the cost estimates with unit price at the fourth quarter period in 2013. 14 The regression function: TH1: Cdm = 1,6578*(Qdm)1,4532; (3.4) TH2: Cdm = 1,7064*(Qdm)1,4491; (3.5) TH3: Cdm = 1,7566*(Qdm)1,4451; (3.6) With, Cdm is construction investment cost in billion Dong, Qdm is the upstream discharge in m3/s. Figure 3.18. Diagram showing the relationship of Cđm ~ Qđm b. Drainage channel system * The regression relationship between discharge and construction investment cost of the channel grade 3: The channel grade 3 usually structured as reinforce concrete round sewer or box sewer buried underground. The regression function has formula as the following table: TH The regression function at the channel grade 3 with two structural types Rectangular sewer: Box sewer: TH1 Ckc3 = 0,6417*ln(Qkc3) + 0,6246; (3.11) Ckc3 = 0,4731*ln(Qkc3) + 0,487; (3.8) TH2 Ckc3 = 0,6764*ln(Qkc3) + 0,7114; (3.12) Ckc3 = 0,5179*ln(Qkc3) + 0,5736; (3.9) TH3 Ckc3 = 0,7010*ln(Qkc3) + 0,7983; (3.13) Ckc3 = 0,5628*ln(Qkc3) + 0,6603; (3.10) Where, Ckc3 is the construction investment cost in billion Dong per 100m, Qkc3 is discharge of the channel grade 3 in m3/s. * The regression relationship between discharge and construction investment cost of channel grade 1, 2: The channel grade 1 and 2 in researching system is the trapezoidal channel. The author used the data of Nhue River to calculate for channel grade 1 and 2. The regression function for the dredged and reinforced roof channel: TH1: Cc1-2 = 0,000003*(Qc1-2)2 + 0,0032*Qc1-2 + 2,393; (3.14) TH2: 2 (3.15) Cc1-2 = 0,000003*(Qc1-2) + 0,0041*Qc1-2 + 2,3192; 15 TH3: Cc1-2 = 0,000002*(Qc1-2)2 + 0,0051*Qc1-2 + 2,2455; (3.16) The regression equation for the dredged channel: TH1: TH2: TH3: Cc1-2 = 0,000003*(Qc1-2)2 + 0,0032*Qc1-2 – 0,0558; (3.17) 2 (3.18) 2 (3.19) Cc1-2 = 0,000003*(Qc1-2) + 0,0041*Qc1-2 - 0,1296; Cc1-2 = 0,000002*(Qc1-2) + 0,0051*Qc1-2 - 0,2034; Where: Cc1-2 Investment cost for channel grade 1 and 2 (billion/100m); Qc1-2 Designing discharge for channel grade 1 and 2 (m3/s); c. The detention pond Assuming the detention pond was built and reinforced roof by stone The regression function: TH1: Chdh = 4,8997*(Fho) + 9,7424; (3.20) TH2: Chdh = 6,2804*(Fho) + 9,7424; (3.21) TH3: Chdh = 7,6611*(Fho) + 9,7424; (3.22) Where, Chdh is construction investion and site clearance cost in billion, Fho is detention pond area in ha. The Software Eview 6.0 is applied to test the regression function on the probability of error and stability. 3.3.3. The specific regression function for the West basin of Hanoi From the general regression function and established regression function, the specific regression function has formula as following: C = f1(Qđm) + f2(Qc1-2) + f3(Qkc3) + f4(Fh) => min 3.4. Establishing the scenarios of layout of the detention ponds. Based on the literature on drainage plan for the west of Hanoi, the drainage basin is divided into 03 drainage headwork. The detention pond area selected by percentage of drainage area ranged from 0% to 6%. About detention pond layout, this study considered 03 scenario groups: (1) pond concentrated in the drainage headwork, (2) pond distributed along the main channel (PT1), (3) pond distributed along the main channel and branch channels (PT2). a. The scenarios of the detention pond concentrated at the drainage headwork (TT) 16 The author proposes 12 scenarios of the detention pond that concentrated at the drainage headwork and had the same area ratio in the 03 drainage headworks. The area ratio of detention pond ranged from 0% (no pond) to 6%. Otherwise, the authorlso proposes 24 scenarios of the different detention ponds in every drainage headworks. Totally, 36 scenarios were considered. b. The scenarios of the distributed detention pond * The scenario of detention pond distributed along the main channel (PT1) including 11 scenarios, each scenario has 11 locations of detention pond.. * The scenario of detention pond dstributed along the main channel and branch channels (PT2) including 11 scenarios, each scenario has 38 locations of detention pond. 3.5. The result 3.5.1. The results of simulation of the flow corresponding to each scenario The discharge in calculating or modeling is the average value in hour. According to designing standard, this value will be used in designation. 3.5.1.1. The scenarios of the detention pond concentrated at the drainage headwork (TT) Figure 3.10. The relationship between upstream discharge and the detention pond area Figure 3.11. At Lien Mac inflow when the detention pond area changing At the upstream, discharge decrease when the pond area increases. The reduction is smaller and smaller (Figure 3.10 and 3.11). The peak discharge summary of the headworks reaches to minimum at TT666 (pond ratio 6%). When the pond area ratio is different at the headworks, the headwork discharge 17 summary reaches to minimum corresponding to pond ratio 5.3% (as scenario TT664). 3.5.1.2. The scenarios of the distributed detention pond The calculation results for the scenario group (PT1) and (PT2) present the total peak discharge at upstream and total peak discharge at channels are minimum when the pond ratio is 4.35% (as scenario PT1-6) and 4.58% (as scenario PT2-6) corresponding to the scenario groups PT1 and PT2. 3.5.1.3. Comparision between scenarios of concentrated and distributed detention pond The comparision is shown in the following diagram: Figure 3.15. The diagram of relationship Figure 3.16. The diagram of relationship between the total of peak discharge and the detention pond area between upstream discharge and the detention pond area The headwork peak discharge reduces in all 3 scenario groups (TT, PT1 và PT2). In which, the scenario of distributed detention pond along the main chaneel and branch channel (PT2) gave out the minimum value (figure 3.15). The diagram 3.16 has shown that the peak discharge of the channel system corresponding to scenario (TT) unchanged (horizontal line). On the other hand, both of the scenarios (PT1) and (PT2) decreased strongly. Especially, the scenario of pond distributed along the main channel and branch channels have strongest reduction (the pink line) Summary: Regulating effect of the pond depends on not only the pond scale, but also the form layout (concentrated or distributed). Considering overview of the system, if the distribution of detention pond is more and more large, the effect of peak discharge is more and more increase. 18
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