Storm Sewer Design - Introduction

Transcription

1 Class 4 [1]

2 Storm Sewer Design - Introduction As urban drainage can not be expected to accommodate all rainfall events, the first step in the design procedure is to select an appropriate design storm. Normally the design storm selected will reflect both the costs associated with failure and different levels of protection. EN makes the following recommendations: Design to prevent surcharging, then check the sewer gives adequate flood protection when surcharged. [2]

3 Storm Sewer Design The next step is to collect catchment data. The area contributing should be determined using field surveys and/or contour maps. The basic pipe network should then be produced. This should, as much as is possible, make use of gravity to move flow through the network. The next stage is then to determine PIMP via surveys, maps or aerial photographs. This can then be used to determine the runoff coefficient. [3]

4 Storm Sewer Design Time of Concentration Flow times must now be considered. When a rainfall event begins rainfall does not immediately enter the sewer. The overland flow time is known as time of entry t e. This depends on: Surface Characteristics. System Characteristics Length of flow path. Rainfall Characteristics. Times normally range between 3-6 minutes (Section 4 WP V4) The time of flow (t f ) is the time taken for flow at the point of entry to reach a point X (the design point) in a sewer. The time of concentration t c is the time taken for runoff from the most distant part (in terms of travel time) of the catchment to reach a point X. t c = t e + t f [4]

5 Storm Sewer Design Rational Method Rational Method The rational method forms the basis of sewer design [ EN 752 Part 4]. This method assumes that the catchment is a fully impervious rectangular area (A) subjected to a depth of rain (D) which falls over a given time (t) at a constant intensity (i). Assuming no losses, the volume of rain accumulating on the surface is determined using DxA. Assuming the rainfall is steady and is being drained at the same rate as it is falling, the rate of flow entering the sewer (Q) can be determined using : Q DA t ia [5]

6 Storm Sewer Design Rational Method - Losses However, we know catchments are not 100% impervious & that losses do occur. We can use the runoff coefficient C [see Class 3 notes] to represent this: Q p CiA If we use: Q p Max flow rate (l/s) i Rainfall intensity (mm/h) A Catchment area (ha) This becomes (to correct for mixing units): Q 78CiA p 2. [6]

7 Storm Sewer Design Modified Rational Method The Modified Rational Method Further research with the aim of better understanding the Rainfall-Runoff process led to a modification of the Rational Method. The modification is to divide the runoff coefficient (C) into two parts C = C v C r Where: C v Volumetric runoff coefficient (-) C R Dimensionless routing coefficient (-) [7]

8 Storm Sewer Design MRM Cv & Cr The Modified Rational Method Volumetric runoff coefficient (-) The proportion of rainfall falling on a catchment which appears as runoff. Typical values are For impervious catchments this may be determined using: C v PR PIMP Dimensionless routing coefficient (-) This reflects the rainfall characteristics, catchment shape and the magnitude of the peak runoff. For design purposes, a value of 1.3 is recommended. Q CiA C C ia C p v R V ia [8]

9 Intensity (mm/h) Storm Sewer Design Storm Duration (min) Return Period 5 yr 2 yr 1 yr Remember Shorter Duration Storms Have Higher Intensities [9]

10 Flow Rate Storm Sewer Design Time of Concentration Storm 1 : t < t c Storm 2 : t = t c Storm 3 : t > t c Time Storm 2 : t = t c Storm 3 : t > t c Storm 1 : t < t c t c Time [10]

11 Storm Sewer Design Time of Concentration Due to the IDF relationship, each individual pipe in the network is designed with a different storm duration. The design storm duration should be set to equal the time of concentration. Why? Storm 1 : t < t c A point is not reached where all the catchment is contributing to the flow in the sewer. The event ends before flow from remote parts of the catchment enter the sewer. Storm 3 : t > t c With Storm 3, all the catchment does contribute. However, as the storm duration is greater than t c, the intensity (and therefore the runoff) is reduced. [11]

12 Storm Sewer Design Steps 1 & 2 We now have enough information to formalise the design procedure: 1. Assign a design rainfall return period (T), pipe roughness (k s normally 0.6mm), time of entry (t e ) and volumetric runoff coefficient (C v ). 2. Produce a preliminary layout of sewers, including tentative inlet locations. 1.0 [12]

13 Storm Sewer Design Step 3 3. Mark pipe numbers on the plan. To do this the longest route to the point of discharge is determined. The most distant pipe is numbered 1.000, the second etc. Branches are numbered in a similar way [13]

14 Storm Sewer Design Steps 4 to 6 4. Estimate impervious areas contributing to each pipe. 5. Make a first attempt [i.e. guess] at setting gradients and diameters of each pipe. 6. Calculate pipe-full velocity (V f ) and flow-rate (Q f ). This can be done using the Colebrook-White Equation: v 2 2gS f log10 k s 3.7D 2gS Alternatively hydraulic tables/charts may be used. D 2.51 f D [14]

15 k s = 0.6mm How is this chart used? Say we want to lay a pipe at a gradient of 1:500 which will convey a flow of 600l/s. What diameter will be required and what will the flow velocity be? (~750mm & ~1.25m/s) Hydraulic Chart Pipe Diameter (m) [15]

16 Storm Sewer Design Steps 7 to Calculate the time of concentration. For downstream pipes, compare alternative contributing branches and select the branch resulting in the maximum t c. 8. Obtain a rainfall intensity from IDF curves for t = t c (for design T). 9. Estimate the cumulative contributing impervious area. 10.Calculate Q using the MRM [Slide 8]. [16]

17 Storm Sewer Design Steps 9 & Check Q < Qf and Vmax > vf > Vmin. Vmin is normally specified to avoid sedimentation. This will normally be 1.0 m/s at pipe full condition. With some sewer materials, higher velocities can be a problem (especially old brick type). Further, high velocities can also lead to noise, hydraulic jumps, cavitation and safety problems. All of these issues should be considered carefully when the velocity exceeds ~3.0m/s. 10.Adjust pipe diameter and gradient as necessary (given hydraulic and physical constraints) and return to step 5 for each successive pipe. [17]

18 Example An engineer is required to design pipework which will convey runoff from part of a catchment to a pond. Based on the pipe and rainfall details below, select suitable pipe diameters using the modified rational method. Pipe Network Pipe 1 1ha 6ha Catchment Details Total Catchment Area = 25ha Estimated Impervious Area = 10ha Soil Classification = 0.30 UCWI = 85 t e = 6 minutes Pipe 3 Pipe 2 Main Road Pipe 4 River Floodplain Pipe Slope Length Area (-) (-) (m) (ha) ? ? [18]

19 Depth of Rainfall (mm) Depth of Rainfall (mm) Duration M1 Depth M5 Depth M10 Depth M20 Depth M30 Depth M50 Depth M100 Depth M200 Depth minutes (mm) (mm) (mm) (mm) (mm) (mm) (mm) (mm) M1 M5 M10 M20 M30 M50 M100 M Event Duration (minutes) M1 M5 M10 M20 M30 M50 M100 M Event Duration (minutes) Rainfall Data For Example [19]