Modelling a Combined Sewage and Stormwater Flood Detention Basin

Transcription

1 Modelling a Combined Sewage and Stormwater Flood Detention Basin A. Pugh B.E. (Hons), Member A.W.A. Sales and Support Manager, Wallingford Software Pty Ltd, Australia S. Ratcliffe B.Sc(Hons), Grad Dip App Comp, ME, MIE Aust, CP Eng, CEng MICE Launceston City Council, Australia Abstract: The Margaret Street detention basin for combined sewage and stormwater flows is a unique solution to provide storm flooding protection within urban Launceston. Using a part covered and part open basin of 7ML and 23ML respectively, the basin configuration and operation provides first flush capture, containment and discharge to Council WWTP. The structure designed by GHD, is scheduled for completion by March The purpose of this paper is to discuss the creation of the InfoWorks CS model; how the catchment model was built, calibration issues, how complex control structures were considered within the model. The model was developed to allow testing and refinement of the operation under proposed and alternative system control scenarios for the Council. The results of this testing will also be discussed. Keywords: Hydraulic modelling, Combined, Sewer, Stormwater, Detention Basin, InfoWorks CS, WWTP. 1. Introduction Dating from the 1840 s Launceston City Council s drainage system is one of the oldest in Australia. The system was designed using the English system of collecting both stormwater and sewage discharges in the one same pipes. As Launceston expanded demands on the system increased and as a result the system became grossly overloaded with the result being regular flooding of the CBD and low lying residential areas. In the 1980 s Council spent approximately $20M on upgrade works in the system. These works were part of a strategy proposed by Gutteridge, Haskins & Davey (GHD) which had been developed by undertaking flow monitoring and modelling of the catchment. The strategy has so far been successful, although a major element of this strategy had yet to be constructed. The complete strategy included a 1.1 km long pressure gravity pipeline of 2.7m diameter to collect water from South Launceston and discharge directly into the river. It is estimated that this would cost $12M to complete and is considered to be beyond Council s current resources.

2 As a result alternative solutions were developed for managing the flows from the catchment. The Margaret Street Detention Basin was determined to be the best alternative and is an innovative solution which, when constructed, will: achieve the flood protection objectives of Council s strategy; maximise the use of existing stormwater infrastructure, be significantly less expensive and within Council s capacity to fund, deliver a significant improvement in amenity, and provide environmental benefits of the project including capture and discharge to the Council wastewater treatment plant of the first flush of stormwater, grit and floatables. In 2002 a more comprehensive hydraulic modelling study of the Margaret Street catchment was undertaken by Earth Tech Engineering with the following aims: To calibrate the proposed detention basin site in dry and wet weather flow conditions, To model the Margaret Street Detention basin and assess its performance for selected design rainfall events. Figure 1 Historical Flooding Event. 2. Building the model The raw data was provided in GIS format (ESRI SHP1) files and these were imported directly into InfoWorks CS2. Validation of the data was undertaken using the inbuilt routines within InfoWorks and where necessary data was input and/or inferred. Data flags were used during the model build to trace the data source and confidence, as this was seen to be an important Quality Assurance step. The model was simplified to 1 Developed by the Environmental Systems Research Institute (ESRI) a shapefile (SHP) stores nontopological geometry and attribute information for the spatial features in a data set. 2 InfoWorks CS is a dynamic hydraulic modelling tool supplied by Wallingford Software. It uses a four-point implicit finite difference scheme (the Preissmann scheme) with a non-linear Newton- Raphson iteration and an adaptive time-stepping algorithm to solve the St Venant equations.

3 approximately 1900 manholes and the ancillary structures and control regimes were created. Sub-catchment Delineation InfoWorks CS uses subcatchments as the mechanism for generating both dry weather sewer flows and wet weather runoff and infiltration for routing in the model. As this model contained three different systems, sewerage, stormwater and combined, it was necessary to ensure that there was no double counting of either population generated flows (sewage) or area based flow calculations (runoff and infiltration). It was decided to use only two types of catchments a storm catchment and a sewer catchment. This made the modelling simpler as you could view each type of catchment separately to ensure there was no overlap. For the combined network two separate sub-catchments were created. One, the foul catchment, represents the dry weather sewerage flows, and the slower inflow and infiltration effects. The storm subcatchment represents the faster wet weather rainfall runoff which will account for the majority of the wet weather inflow into the manhole. When delineating sub-catchments, the following aspects were considered: The location, grade, and system type of pipes not modelled. These pipes were considered to determine the modelled manhole into which any given discharges flows. The locations of properties relative to the pipework, via the cadastral base. Sub-catchments were delineated in such a way that their area did not exceed 10 hectares. 3. Calibrating the model The model was calibrated using data collected at the proposed site of the Margaret Street Detention Basin. There was fifteen months of data available, with flow gauges being located in both of the major pipelines.

4 Figure 2 Map showing the extents of the Margaret Street Catchment Site of Detention Basin

5 Dry Weather Calibration Dry weather flow data was available for the Low Level Gravity Pipeline (LLGP) and from this a representative hydrograph was developed for weekdays and weekends. This data also allowed a per capita flow rate to be established for the lower half of the catchment this flow rate and hydrograph profile was assumed to apply to the northern section also. In addition to calculating the generation rate, tidal levels at the outfall were obtained to improve the accuracy of the calibration. The levels were applied at the outfall to simulate boundary conditions at the outlet of the catchment. Wet Weather Calibration Rainfall entering the sewerage system is modelled in InfoWorks by using runoff surfaces. These surfaces have different parameters relating to both speed of response and quantity of response. Typically runoff surfaces are developed to represent flow from impervious areas as having a fast response whilst flows from pervious areas were classified as slow response. For the Margaret St catchment the four main areas were classified. The key runoff characteristics of these surfaces are: Roof and Road are both impervious surfaces, Open Ground assumed to be predominantly a pervious surface, and Long term response is generated from pervious surface with very slow inflow characteristics. Ideally fast response inflow should be all directed into the stormwater or combined network, however experience indicates that a lesser percentage will also enter into the sewerage system via leakage from sealed areas such as roofs, driveways and roads, damaged manhole lids and illegal stormwater connections. Slow responses from open ground area enter the stormwater system, again with anecdotal evidence suggesting that some flow will enter the sewerage system. As there was no way of separating the wet weather inflow into the sewerage system and the inflow into the storm water system a base level of inflow into the sewerage system was assumed. This arbitrary amount was set at approximately 4 % of the rainfall. The total quantity of flow and shape of the predicted outflow was calibrated by adjusting the percentages of flow from the storm catchments. The inflow from the storm catchments was found to be approximately 47% of the rainfall in the urban areas, and 31% in the undeveloped catchments. The calibration of the model was determined to be successful with the predicted wet weather flow volume for the verification event being 93% of that observed at the monitoring points. Peak flows predicted by the model were slightly lower than those observed by the field monitors. Figure 3 shows a sample calibration plot.

6 It was noted that the flows predicted by InfoWorks were substantially less than those predicted by other modelling exercises. While it was assumed that these earlier predictions were probably over estimations we wanted to check the realism of the model. As it was considered that some of this may be due to extra flow reaching the detention basin from surface routing, overland flow paths were added into the model. It was interesting to note that this had very little impact on the total flows received at the detention basin. 4. Modelling the Detention Basin The construction of the detention tank in the model was initially substantially simplified. It was noted however that this was not accurately modelling the flow situation in the tank and a more detailed model of the detention tank was developed. Figure 3 - Sample Calibration Plot. Rainfall (mm) Observed Flow Predicted Flows The flow regime (refer Figure 4) is such that in dry situations the flows pass through the primary detention, first flush storage, into the polluted chamber of the Outlet Control Structure and into the LLGP. The tank is designed to store both sewerage and stormwater flows in a covered flow control facility with the cleaner water from extreme rainfall events being detained in an above ground storage basin. The flows out of the tanks and the above ground basin are controlled by two large radial gates which protect the downstream system from excessive surcharge and flooding. There is also a valve that shuts the flow from the covered storage thus storing the first flush for subsequent discharge and treatment. There are flap valves located throughout the structure to prevent back flow of polluted water into cleaner areas.

7 Figure 4 Schematic showing Hydraulic Flow Paths in the Detention Basin The modelling of the hydraulic elements was relatively straightforward with most elements being modelled as they would physically exist. It was necessary to create a Real Time Control (RTC) routine in the model to simulate the operation of the outlet valves, pumps and radial gates as the operation of these would be governed by conditions in the downstream pipe network. It was important that this detail was included as a key part of the project was going to be validating the set points of the telemetry for the basin. Figure 5 shows how the model displays the different elements in the model.

8 Figure 5 InfoWorks CS Model Figure 6 shows the difference in the flow through the radial gates by adding a new rule that controlled the flow passing through the radial gates. Originally the control on these gates was based on water level in the gate manhole. A rule was added to throttle the flows allowed out of the detention tank to reduce any downstream flooding. The rule stated that if the flow was greater than 3m³/s the radial gate should start to be closed if the flow was below 3m³/s the gate should be opened. This allowed the gates to operate as the hydraulic simulation was proceeding optimising the flows downstream. Figure 6 Comparison of Flows Exiting the Tank with the Control Rule Original Operation Controlled Operation Controlled Operation Original Operation The results in the model showed that downstream flooding was reduced by 75% by adding this rule to the model.

9 5. Outcomes Launceston is one of the few cities in Australia with a combined sewerage and drainage system. By using a dynamic hydrologic and hydraulic model that was able to consider all three different system types, sewerage, drainage and combined; it has been possible to develop a model that has been calibrated to the catchment and can be used for testing the theory of the detention tank. While the model build and calibration has been completed and a preliminary investigation of the detention tank operation has occurred it is already envisaged that the model will be used to verify the following Fine tuning the set levels of the radial gates to reduce flooding in the downstream catchment, The effect of tidal position on the storage basin, Volume of stormwater detained in open storage for different storm durations and frequencies, and Water Quality analysis. As discussed in this paper the model has already demonstrated that improvements in operation may be able to optimise the detention tank operations and reduce downstream flooding while minimising detention times. Acknowledgements The authors would like to thank both the organisations of GHD and Earth Tech Engineering for their work on this project, and their assistance in preparing this paper. References Earth Tech Engineering (2003), Margaret Street Catchment Model Build and Calibration Report, Earth Tech Engineering, Melbourne. Tabart, K and Brayford (2001), G, Unique City Detention Basin Solution for Combined Sewage and Stormwater Flood Control, 6 th Conference on Hydraulics in Civil Engineering, pp GHD (2002), Margaret Street Detention Basin Schedule of Drawings, CD InfoWorks CS Online Help.