Assignment
1. Overview
This assessment is designed to test your achievement of selected learning objectives of Modules 6 to 9. In particular, the assignment involves estimation of urban design discharges, reservoir routing and water balance. It is intended to reinforce and extend your knowledge on urban hydrology, using a low density residential subdivision as a case study.
The assignment is divided into three parts:
• Part A - Estimation of Minor and Major design flows for the urban subdivision.
• Part B - Preliminary analysis of stormwater detention
• Part C - Preliminary water balance analysis of rainwater harvesting
2. Part A - Minor and Major Design Discharges
2.1 Proposed Broadmeadows residential subdivision
A drainage scheme is to be designed for a 28 lot residential located at Broadmeadows, near the city of Rockhampton, Queensland. A copy of the subdivision layout plan can be downloaded from Studydesk (Subdivision Lots Layout.pdf). The subdivision includes a network of proposed internal roads (Roads 02, 03 and 04).
Your task is to prepare a Minor system design check of the stormwater system upstream and including Pit 04/01 based on the preliminary layout and then a Major system check. The Minor system is to cater for the 2 year ARI storm and the Major system for the 100 year ARI storm. The system includes stormwater line 1 (pits 01/01 to 04/01) and line 2 (pits 01/02 and 02/02).
Pits will be based on Brisbane City Council standard drawings. Initial surface levels at pit locations are given in Table 1.
Table 1: Initial design details at pit locations
|
Pit/Location Finished
|
Surface Level (m LD)
|
|
01/01
|
304.9
|
|
02/01
|
304.9
|
|
03/01
|
304.87
|
|
04/01
|
304.57
|
|
01/02
|
304.47
|
|
02/02
|
304.47
|
|
LD = Local datum
|
Use the following information:
1. The location of the subdivision is 23.40° S, 150.62° E
2. The road kerbs are BCC ‘Type D' kerb and channel. Roads 03 and 04 will have a 6.5m wide road pavement (kerb to kerb). The width of Road 02 is 8m. All roads have a dual crossfall of 1:30 (i.e. roadway has a central crown).
3. Assume all road pits will be 2400 lintel ‘lip in line'. The pits along Road 02 are on grade and all other pits are in sag. Sag pits have different hydraulic capacities compared to on grade pits due to the effect of water ponding.
4. The proposed lots will be developed to ‘low density residential' with an expected percentage impervious within the lot area of 60%.
5. Soil permeability can be classed as ‘Medium'. Pervious parts of the subdivision can be classed as ‘Medium density bush' or "Good grass cover'.
6. Assume no flow of stormwater runoff into lots from neighbouring lots (i.e no flow across shared lot boundaries). Once flow meets an internal shared lot boundary, assume concentrated overland flow' conditions. Assume flows across lot boundaries to the street can occur. Use the contours shown on the layout plan to identify the subcatchment boundaries draining to each pit. This may mean that the full area of some lots may not drain to the nominated pits.
7. To simplify the analysis, assume that there is no stormwater entry to Roads 02, 03 and 04 from external areas outside of the proposed subdivision. It is assumed that there is existing drainage to service the existing main road on the eastern entry to the subdivision. The catchment boundary shown as the red line on the subdivision layout plan can be used on this basis.
8. The minimum travel time to a pit is 5 minutes and the maximum is 20 minutes. Reminder: Partial Area check is based on the most remote, directly connected impervious area to the pit. The travel time may be longer than 5 minutes.
9. In completing the Hydrologic Design Sheets, it is acceptable to combine together lot subcatchments that have the same fraction impervious, rather than report CA values for individual lots.
10. It is acceptable to scale up the CA values based on the frequency factor ratio F100/F2- this will simplify the calculations for the Major Storm analysis
11. Do not use standard inlet times
12. For pipe flow travel times for Sheet 2, use a minimum pipe gradient of 0.3% otherwise the road slope.
13. Gutter flows in the street during the Minor storm should be contained within the full roadway width with zero depth at the crown. The maximum flow depth at the road kerb during the Major storm should contain flooding to within the road reserve. This corresponds to a maximum water depth of 0.25m. dV product should not exceed 0.6 m2/s. At the sag pits, check both the ponded water width at the pit and the roadway flow hazard approaching the pit during the Minor Storm.
14. Unless otherwise defined, the drainage design check will be in accordance to the 2007 edition Queensland Urban Drainage Manual (QUDM). (The updated 2013 edition remains provisional). Use 1987 AR&R IFD data to generate design rainfall intensities.
15. Undertake the Major design flow check for Road 02 at just downstream of pit 04/01.
2.2 Scope of Part A
The scope of Part A is to undertake a Minor and Major analysis of the proposed drainage system by completion of Hydrologic Design Sheets 1, 2 and 3 (as per Module 7 of the Study Book).
After completion of the Design Sheets, review the results and make suggestions on revisions to the proposed road drainage system (e.g. requirement for additional pits, if any). It is not required to reanalyse and update the Design Sheets.
The design sheets shall be supported by:
1. A plan showing the boundaries of each pit subcatchment and the flowpath to each pit used in the time of concentration calculation.
2. Design assumptions (refer to report template)
3. A worked example of how an (arbitrarily) selected row of each design sheet was determined, by providing the calculations in more detail.
4. An output summary - giving the pit size at each pit location, the Minor discharge in each pipe reach and the Major flow characteristics (discharge, flow width, dV product) at the selected road location.
2.3 Useful materials for Part A
The following materials have been provided and can be downloaded from StudyDesk or from external websites:
1. Subdivision layout base plan
2. Gully pit designs and hydraulic capacity charts can be downloaded from https://www.brisbane.qld.gov.au/planning-building/planning-guidelines-andtools/guidelines/standard-drawings/index.htm
3. 2007-08 and 2013 Provisional QUDM can be downloaded from https://www.dews.qld.gov.au/water-supply-regulations/urban-drainage
4. Refer to Bureau of Meteorology website for online IFD tool for design rainfalls.
3. Part B - Rainwater Detention Analysis
3.1 Proposed Use of Rainwater Tanks for Detention
As part of whether to make the installation of rainwater tanks for new dwellings a mandatory requirement, the local council decided to use the subdivision as a test case. The purpose of the test case was to identify the stormwater detention benefits that may arise due to specially designed rainwater tanks.
It was decided to first undertake a desktop analysis and this is the scope of Part B of this assignment.
The concept design of the rainwater tank for the dual purpose of onsite detention and household water supply is shown as Figure 1. You have been asked to provide an initial estimate of the reduction in Minor storm peak discharge from a typical house roof area when the airspace in the tank is used to detain roof runoff. (Part C of the assignment will involve an analysis of the ability of the rainwater tank as a supplementary water supply).
3.2 Scope of Part B
The scope of Part B is to undertake a reservoir routing analysis of the proposed rainwater tank. For the purpose of the assignment, the following simplifications can be used in the analysis:
1. Assume a typical house roof area equal to 150 m2 with a 5 minute time of concentration.
2. It was decided to do the detention analysis based on the 2 year ARI 20-minute duration storm and the 2013 AR&R IFD rainfall intensities
3. Apply the principles of the Time-Area Method to generate the inflow hydrograph from the roof into the tank. Assume that the time-area diagram is linear. Assume 1mm initial loss and a 20% proportional loss to allow for gutter overflows during the storm.
4. Compute the storage routing based on the above inflow hydrograph. Setup reservoir routing computations in Excel. Use a 30 second timestep to achieve stable outflow discharge estimates. Interpolation of data from the lookup table is preferred to using the simpler LOOKUP function in Excel to ensure a more accurate analysis. Use a depth increment of 0.01m to setup the lookup table.
5. Assume that the water level at the start of the storm matches the invert of the orifice plate (at 100% water supply capacity).
6. To define the tank outflow, assume that ‘free flowing' discharge through the square orifice plate occurs and the tank is not subjected to any backwater affect. For consistency adopt an orifice coefficient Cd=0.61 and weir coefficient C= 1.8. Hint: The outflow discharge will be in three stages as water depth increases: no flow, weir flow and orifice flow.
4. Part C - Rainwater Harvesting Analysis
4.1 Proposed Use of Rainwater Tanks for Water Supply
The concept rainwater tank given in Part B is also intended to provide a non-potable water supply to the house and also for other uses such as garden irrigation. As part of the desktop analysis, you have been also asked to quantify the water supply benefits of the rainwater tank system.
4.2 Scope of Part C
The scope of Part C is to undertake a water balance analysis and document the water supply benefits.
Assumptions can be made to simplify the analysis:
1. Use daily rainfalls observed at Broadmeadow for calendar years 2008 to 2010. This data can be downloaded from the BOM website. Check if there any missing data or accumulated totals.
2. The rainwater tank will be fully enclosed (no open top). Assume an empty tank at the start of the water balance simulation.
3. Adopt a volumetric runoff coefficient C =0.8 for roof surfaces (allowing for gutter and system overflows)
4. Use a constant daily demand of 0.3 kL/day to account for toilet flushing and other household uses for this preliminary assessment.
It was decided to use the average annual volume of water supplied by the tank as the key measure of tank performance. Express this volume in kL/year and as a percentage of the total annual household demand. Due to the direct supply to the house, the water balance should allow for the extraction of water from the tank if it is available (even if there is less than 0.3 kL stored). Also report the reliability of the tank supply, but this will be based on counting the days when the full daily demand is met. What was the longest time that the tank was empty?
It is normal practice to undertake a water balance analysis using significantly more than three years of historical rainfall data. Compare the 2008 to 2010 annual rainfalls with long term statistics and evaluate the likely effect on the tank performance if a longer rainfall input is used.
5. Submission
Your submission for Assignment 2 should include:
• A report that documents the hydrological analyses that have completed. A marking scheme is provided as Table 2. Use this marking scheme to check that you have addressed the full scope of the work. If an element of the assignment has not been documented in the file report than no marks will be given for that element. For reasons of consistency, a report template (ENV3105Report2Template.docx) must be downloaded from Study Desk and used to report your work. Complete each section of the report.
• An EXCEL spreadsheet containing your hydrological computations. Multiple spreadsheets will not be accepted - instead put your workings in separate sheets within the one EXCEL file. Include a list indicating the contents of each sheet to aid marking in the above report.
Part of the available marks has been allocated to reward reporting that is well set out and easy to follow. Submissions that are untidy and/or poorly structured and thus difficult to assess will attract less marks for this element.
Electronic submission of this assignment is required. One ZIP file will be accepted containing:
1. A single pdf document based on the template provided (rename the file based on the convention below)
2. A single EXCEL spreadsheet
The following filename convention shall be used: *Ass2.zip, *Ass2.pdf and *Ass2.xlsx, where * is your student number.
6. Marking Scheme
Table 2: Assignment Scheme
Assignment element Total
Part A Minor and Major Design Discharges (Technical work)
Design assumptions and output summary
Catchment plan showing subcatchment boundaries and flowpaths
Table of pit subcatchment details
Table of 2 year and 100 year ARI rainfalls
Table of runoff coefficients
Izzard table and hydraulic capacity charts
Output summary table
Suggestions for revisions to proposed drainage
Sheet 1 and worked calculation
Appropriate time of concentration estimates (Col 1 to 8)
Full area CA values and Rational Q estimates (Col 9 to 14)
Partial area CA values and Rational Q estimates (Col 9 to 14)
Pit inlet analysis and road hazard (Col 15 to 25)
Sheet 1 worked calculation
Sheet 2 and worked calculation
Full area calculations (Col 1 to 5)
Partial area calculations (Col 6 to 11)
Sheet 2 worked calculation
Sheet 3 and worked calculation
Appropriate time of concentration estimates (Col 1 to 8)
Road Q and capacity check (Col 9 to 18)
Sheet 3 worked calculation
Part B Rainwater Detention Analysis
Q2 storm hydrograph
Rainfall excess hyetograph and discharge hydrograph table
Assumptions
Detention Analysis
Tank outflow and storage plots
Discharge equations and assumptions
Lookup table ( Storage, Discharge Q and 2S/T +Q)*
Puls Method analysis*
Tank hydrograph plots and discharge results
Part C Rainwater Harvesting
Estimation of runoff R using volumetric runoff coefficient*
Correct use of input data rainfall P *
Correct daily storage, demand, overflow and reliability computations*
Daily Rainfall timeseries plot
Storage behaviour plot
Analysis results ( reliability, average annual volumes etc)
Effect on using a longer rainfall record
Reporting
Assignment report
Assignment EXCEL spreadsheet