Utilization of stormwater runoff models for flood control in Tokyo

Transcription

1 Utilization of stormwater runoff models for flood control in Tokyo Masayuki Sugai, Jun Okamoto Tokyo metropolitan government(tmg), Nishishinjuku 2-8-1, Shinjuku-ku, Tokyo , Japan ( 1. ABSTRACT Tokyo Metropolitan Government (TMG) makes use of rational method and uniform flow calculation to get volume of runoff and sewer size and gradient in case of sewer design. However, in this method, it is difficult to evaluate hydraulics phenomenon in sewer precisely in consideration of topography of a basin and backwater influence. In order to improve the design method enabling the hydraulic gradient keep under ground level in consideration of the topography or backwater influence, TMG examined effective model technique for utilization of runoff analysis simulation and made design manuals for staffs. For the example of utilization of the manual, the most suitable storage method was studied. 2. KEYWORDS runoff analysis; effective model technique; suitable storage method 3. SIGNIFICANCE OF INTRODUCTION OF RUNOFF ANALYSIS SIMULATION The sewerage bureau of TMG has been implementing various project to solve many problems for sewerage system, such as reconstruction of deteriorated facilities etc. since achievement of sewered rate of 100% in Flood countermeasure is one of the most important problems. Particularly of late year, the inundation damages occur due to increase of a rainwater runoff caused by development of urbanization and frequent occurrence of local concentrated downpour. Therefore efficient and effective storm water control is demanded. As for the conventional design concept of the storm water control, rational method + assumption of uniform flow is used. Because this calculation method comparatively had little setting parameters, a calculation was simple and easy. However, there leave some inundation districts such as low ground or areas along rivers because this calculation method does not contain enough consideration of topography of a basin and backwater influence. TMG decided to introduce the design method utilizing runoff analysis simulation for the purpose of controlling the stormwater at any places in Tokyo. The runoff analysis simulation model TMG examined this time consists of abstraction model and runoff model, conduit hydraulics model, flooding analysis model. This simulation model can estimate gaps between peak hydraulic gradient line and ground level in consideration of the situation such as sewer depth, topography, backwater influence. In addition, this simulation can calculate the hydraulic gradient line from lowest downstream (river water level etc.) to the highest upstream (branch sewer water level), and can display a result visually. Therefore, effect comparison of anti-inundation measure institution plan becomes easy. Further, TMG examined a new concept of a sewer design and input method of simulation data to implement sewer design utilizing runoff analysis simulation.

2 Intensity[mm/hr] 4. APPROACH FOR INTRODUCTION OF RUNOFF ANALYSIS SIMULATION 4-1.Decision making of facility dimensions Design values for storm water control in Tokyo are given as below, - Design rainfall intensity :50mm/hr, I = 5,000/(t+40) 120 I: rainfall intensity, t: concentration time - Design runoff coefficient :0.8 (80%) mm/hr-Centralized hyetograph And rain waveform to use for simulation is centralized hyetograph as Figure 1. The decision making of present and designed 20 facilities dimensions was changed based on 0 from flow capacity of each branch sewers to Elapsed time[min] peak hydraulic gradient line of a whole branch sewer keeping 1.0m below ground Figure 1. Design rainfall hyetograph level. But, as for trunk sewers, we do not apply this ways of decision making due to followings reasons. - sewers have 20% margin of dimension based on Rational Method + Uniform Flow - Review is difficult because trunk sewer is large, and it takes a long term to plan. - When reviewing trunk sewers dimensions, there are too many related sewers to cope with Decision making of sewer dimensions is shown in Table 1, illustration of the design result of runoff analysis using simulation is shown in Figure 2. Table 1. Decision making of sewer dimensions Sewer Rational method + Uniform Flow Runoff analysis using simulation Subsidiary trunk Branches - Section determination (20% margin) - Initial sectional setting - An outline check of capacity - Initial sectional setting - An outline check of capacity - Only as for the evaluation (Is free surface secured?) - Section determination (Peak hydraulic gradient line do not exceed 1.0m below ground level) - Section determination (Peak hydraulic gradient line do not exceed 1.0m below ground level)

3 Subsidiary Branches e.g Water level using simulation G.L-1.0m Hydraulic Grad(Current) W.L G.L Branch Increasing Capacity Hydraulic Grad (Increased capacity) Branch Increasing Capacity Subsidiary Increasing Capacity Runoff analysis using simulation Rational Method + Uniform Flow * sewer was already finished in condition of rainfall intensity 50mm/hr-coefficient of discharge 80%, and branches are constructed in condition of 50mm/hr-80% Figure 2. Illustration of the design result of runoff analysis using simulation 4-2. Effective model Easy models of facilities and basin are important to utilize runoff analysis simulation. TMG has sewerage ledger called SEMIS (SEwerage Mapping and Information System). A lot of data necessary for the model are prepared for SEMIS, and effective model can be made effectively by utilizing them. On the other hand, the following problems are given. 1) SEMIS database has a lot of data item and data files are classified in detail. The data extraction for model is complicated. 2) Hydraulic structure (weir, orifice, gate, pump) data are not available in SEMIS database, it is necessary to supplement them with other data. We are trying to solve these problems as follows. 1) Software to extract necessary data from SEMIS will be developed. 2) Database of hydraulic structure will be developed. 4-3.Making of a design manual The bureau has formulated design manuals for software (InfoWorks CS, MOUSE, XP-SWMM) used mainly in Japan now. The manuals make TMG staff utilize runoff analysis simulation for design. The manuals contain a series of way of model using SEMIS data, method of calibration and simulation, evaluation method of calculation result, examination of measures for facilities, and they exhibit result product. By utilizing these manuals enlightenment of a way of thinking of runoff analysis simulation is enabled for TMG staff, and unevenness of result examined by a designer decreases.

4 5. EXAMINATION OF EFFECTIVE DESIGN FOR STORAGE FACILITY 5-1. Examination method Evaluation of facility such as a network effect of sewer network or storage effect at plural spots is enabled by using runoff analysis simulation. It spreads a choice of design methods and promotes more effective design of facility. Therefore, with the manual that was made this time, we examined the most suitable storage technique for model districts. The model district is intended for three topographies (incline, swale, flat land). We compare 2 storage types and 2 establishment models (the collection, dispersion) of storage facility. Here, the storage types are offline type and in-line type (Table 2). At first the measure point was set at the spot where hydraulic grade line suddenly rose to by a calculation result of an existing facility. The case of setting a storage facility close to measure point is called collective installation and also the case of installing three or four small storage facilities is called dispersed installation (Table 3). facility scale was set so that the water at measures point has free surface. Table 2. type type Offline storage In-line storage Existing sewer Manhole Basic concept Weir sewer Existing sewer Weir wier Existing sewer Invert Regulator Water intake structure structure Weir diverted water from an existing system Installed deeper than an existing system, and not effective after filled up Direct influent from an existing system Installed at the same depth to an existing system 5-2. Examination result Examination result is shown in Table 4. In case of flatland, every kind of storage type and establishment model turned out to be effective. However, effective technique was restrictive in the swale topography, and there was no effect in some cases. In addition, in the case of the incline topography, it became clear that the in-line storage that has not been adopted very much was effective due to the characteristic that hydraulic gradient level did not continue in the presence of drop connection.

5 Table 3. establishment policy Type Collective installation Dispersed installation Basic concept Scale (Close to remedial point) Remedial point Remedial point (Several point in upstream of remedial point) The storage capacity is designed so that peak water level of remedial point does not become a pressured state Table 4. Adaptability of a storage facility for the topography situation Topography Incline Swale Flatland In-line Offline Collective Good Good Dispersed Excellent Poor Collective Good Excellence Dispersed Poor Poor Excellent 6. EXAMINATION OF DATA MANAGEMENT Keeping the input data and sewer network data after basic design was performed by runoff analysis simulation, the data can be utilized again at the time of revising design and a detail design. It can evaluate the influence of revision of design on sewer network. TMG is going to plan practical use of accumulated model data at the time of design for reconstruction and repair. The image of utilizing model data is shown in Figure CONCLUSIONS From now on, TMG is going to plan positive introduction of runoff analysis simulation based on the manuals made in this study. In addition, the bureau will utilize more existing sewerage ledger data (SEMIS data) for effective model and is going to develop software to simplify complicated data extraction from them and a database of hydraulic structure such as weir in future. And the bureau will develop unification management of model data and the input data, and make use of it for detail design and a change design in future restructuring and repair.

6 Basic Design Document collection / investigation Modeling Calibration Present situation evaluation Drafting of a construction plan Development of a construction schedule / Calculation of a construction expense Summary Sewerage bureau management SEMIS data Model data (Present situation) Model data (After construction) Modeling data extraction Inflection of basic design data Inflection of detail design reflection data Reflection of detail design revision contents Reflection of completion data Reflection of rivised construction data Ability inspection of a maintenance route Revision of a model data When a design revision occurs Drawings of completion Execution Detail design Document collection / investigation Method of construction examination Design of a construction route Revision of a model Capacity inspection of a maintenance route Determination of design When a design revision occurs Document collection / investigation Estimation Figure 3. Illustration of utilizing model data Construction 8. REFERENCE JIWET: Japan Institute of Wastewater Engineering Technology (2006) The distributed hydraulic model utilizing manual