Storm Sewer Design [2]

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Benjamin Jonah Russell
5 years ago
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1 Class 5 [1]

2 Storm Sewer Design 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. (BS EN 752 suggests that for pipes less than 900mm, specifying a minimum velocity is adequate. The CIRA Method is used for larger pipes.) 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. [2]

3 CIRIA Approach [3]

4 Sewer Sediments - Characteristics Sewer sediments may be defined as any settleable particle that is found in stormwater or wastewater which can form a deposit under appropriate hydraulic conditions. Normally solids can be classified as one of 4 types: Type Size (m) SG (-) Gross > Grit > Suspended > Dissolved < [4]

5 Sediment Types [5]

6 Dundee [6]

7 Biofilm [7]

8 Sediment [8]

9 Sediment [9]

10 Physical & Chemical Characteristics [10]

11 Transport Zones Sewage Flow Near Bed Solids or 'Bed-Load' Material Class A Sediment Bed [11]

12 First Foul Flush? FLOW CLASS A SEDIMENT BED EROSION ' Bed-Load' or Near Bed Solids Material DWF ONSET OF STORM [12]

13 Bed-Load [13]

14 Bed-Load [14]

15 Precipitation Sediment Pathways Winter Grit, Litter, Surface Erosion, Road Surface and Constrution Materials, etc. Paved & Impermeable Areas Street Sweeping Gully Pots Gully Cleaning Industrial & Domestic Waste Water Infiltration Combined Sewer Sewer Cleaning Pollution Source CSO Structure (During Storms) CSO Structure (During Storms) Sewage Treatment Grit Screening & Sludge Removal (Where Undertaken) Receiving Waters (Pollution Impact) Liquid Solids [15]

16 Stormwater Pollution Suspended Solids C.O.D. SURFACE 35% SURFACE 22% DWF 20% DWF 6% EROSION 39% EROSION 35% SLIME 20% SLIME 23% [16]

17 Sewer Sediments Sources: Sanitary Sources of sewer sediments can be diverse, but can be placed in one of three categories: sanitary, surface & sewer. Sanitary Large faecal and organic matter with SG close to 1.0. Fine faecal and other organic particles. Sanitary refuse. Vegetable matter and soil particles from food processing. Materials from industrial and commercial sources. [17]

18 Sewer Sediments Sources: Surface & Sewer Surface Atmospheric fall-out (dry and wet). Particles from erosion of roofing material. Grit from road surfaces or re-surfacing works. Grit from de-icing operations on roads. Particulates from motor vehicles. Materials from construction works (e.g. building aggregates, concrete slurries, exposed soil, etc.) and other illegally-dumped materials. Litter from roads and paved areas. Vegetation, litter, silts, sands and gravels washed/blown from unpaved areas. Sewer Soil particles infiltrating due to leaks or pipe/manhole/gully failures. Material from infrastructure fabric decay. [18]

19 Road Runoff [19]

20 Gully Pot [20]

21 Sewer Sediments Problems Sewer sediments are associated with three principle problems: Effect Blockage Loss of capacity Pollutant storage Consequence Surcharging Flooding Surcharging Flooding Premature / DWF CSO operation Washout to receiving waters Shock loading at treatment plants Gas production [21]

22 Sewer Sediments Management Due to the problems which sewer sediments generate it is normal to attempt to minimises the occurrence of sediments via good design. This is essential as the removal of sediments from sewers can be both costly and disruptive. The need for sewers to be designed to carry sediment has been recognised for many years. Conventionally, this has been done by specifying a minimum 'self-cleansing' flow velocity. Although this approach has apparently been successful in many cases, a single value of minimum velocity, unrelated to the characteristics and concentration of the sediment or to other aspects of the hydraulic behaviour of the sewer, does not properly represent the ability of sewer flows to transport sediment. [22]

23 Sewer Sediments Management In particular, it is known that a higher flow velocity is needed to transport a given concentration of sediment in large sewers than in small sewers. It is also important to appreciate that conditions in gravity sewers are extremely variable. Flow rates and the sediment entering a system can vary considerably with time and location. Therefore, a sewer designed to be self-cleansing in normal conditions is still likely to suffer sediment deposition during periods of low flow and/or high sediment load. [23]

24 Sewer Sediments Management - Transport Transport : As wastewater flows over a sediment bed in a sewer, hydrodynamic lift and drag forces are exerted on the bed particles. If these 2 combined forces exceed the restoring force, then entrainment occurs, resulting in movement of the particles at the flow/sediment boundary. Not all the particles of a given size at this boundary are dislodged and moved at the same time, as the flow is turbulent and contains short term fluctuations in velocity. The limiting condition, below which sediment movement is negligible, may be set in terms of a critical boundary shear stress ( o ) or critical erosion velocity (v). These variables are linked using: o v 2 8 w [24]

25 Sewer Sediments Management - Entrainment Entrainment : Once sediment has been entrained into the flow, it travels, in suspension or as bed-load. Finer, lighter material tends to travel in suspension and is primarily influenced by turbulent fluctuations in the flow, which in turn are influenced by bed shear. Suspended transport is at at mean flow velocity. Heavier material travels by rolling, sliding or saltating along the pipe invert (or deposited bed) as bed-load. This type of movement is affected by the local velocity distribution, and advection velocities in this mode are considerably lower than the flow mean velocity. [25]

26 Sewer Sediments Management - Transport Mode The mode of transport depends on the relative magnitude of the lifting effects due to turbulence, as measured by the shear velocity (U * ), and the settling velocity (W s ). Shear velocity is given by: U * o W s /U * Mode >0.6 Suspension Saltation 2-6 Bed-load In an urban drainage network, with graded materials of differing specific gravity, a combination of these modes will exist. [26]

27 Sewer Sediments Management Transport Mode 2 Suspension : Where a sewer has no deposited bed, the presence of sediment in the flow or moving along the bed will cause increased energy losses up to 1% of capacity may be lost. Geometry : The deposited bed reduces the cross-sectional area to convey a flow, to maintain a given discharge flow velocity (and energy loss) must increase. This becomes important once sediment depths reach ~10%. Bed Roughness : Usually the greatest influence sediments have on sewer performance is by increasing bed roughness. Sediment depths of 5% with dunes can reduce capacity by 10-20%! [27]

28 CIRIA method for sewer design - Intro The CIRIA method was developed to enable a self-cleansing velocity to be specified which represented a number of factors: pipe size, roughness, flow depth, sediment size, sediment type, concentration and the presence of a deposited bed. Fundamental to the approach are two factors: 1. Each pipe should be individually designed with its own self cleansing velocity. 2. Some deposition is acceptable. Self cleansing is defined as: An efficient self-cleansing sewer is one having a sedimenttransporting capacity that is sufficient to maintain a balance between the amounts of deposition and erosion, with a timeaveraged depth of sediment deposit that minimises the combined costs of construction, operation and maintenance. [28]

29 CIRIA method for sewer design - Basis It may seem that allowing some deposition would cause a reduction in the sediment transporting capacity of the flow, leading to possible blockage. However, laboratory evidence has shown that the presence of the deposited bed actually allows the flow to acquire a greater capacity for transporting sediment as bed-load. This is because sediment transport is capacity is related to the width of the deposited bed. This effect more than compensates for the reduction in velocity caused by the roughness of the bed. Thus, in principle, a small amount of deposition may be advantageous in terms of sediment mobility. [29]

30 CIRIA method for sewer design 3 Criteria The method specifies that designs should meet three criteria: 1. Transport a minimum suspended solids concentration. 2. Transport a course granular material as bed-load. 3. Erode cohesive particles from a deposited bed. Suspended and bed-load transport criteria. The assessment of designs is undertaken using laboratory derived criteria. For criteria 1 & 2 (suspended and bed-load transport) separate relationships are available for transport with & without a deposited bed. [30]

31 CIRIA method for sewer design Bed Erosion Bed Erosion Based on field and laboratory studies, it was recommended that the minimum shear ( o ) condition set should be 2 N/m 2 this is based on a bed roughness (k b ) of 1.2mm. b v f 4log 8 10 o 1 b k b 3.7D 2 [31]

32 CIRIA method for sewer design - Application 1.0 Design Criteria What depth of sediment deposition is allowable? Application 2.1 Sediment Data Data should be collected regarding the nature of the sediments found in the system. 3.0 Data Collation All relevant data should be collated, and then used to test each of the 3 Criteria 2.2 Hydraulic Data Data should be collected regarding the in sewer hydraulics 4.1 Criteria I Suspended Sediment Transport 4.2 Criteria II Bed-Load Transport 4.3 Criteria III Cohesive Sediment Erosion 5.0 Design velocity may now be selected OUTPUT Pipe diameters and gradients may now be selected [32]

33 [33] Application of the method is complex. Design tables must be produced to represent the application based on catchment specific data which is difficult to collect. Many of the relationships require detailed data collection, e.g. bed-load transport without deposition: ' ' , 1 1 ' d d d S g v Where D S g v v v D d A D C G t G L L t v CIRIA method for sewer design - Complex

34 CIRIA method for sewer design Simplified Design As an alternative, a simplified method is available which relies on typical (rather than actual) values for design. This data may then be used with a simplified design table. [34]

35 CIRIA method for sewer design - Plot [35]

36 CIRIA method for sewer design - Example Example A 1.0 km sewer is proposed to carry stormwater from a 5.0 ha area earmarked for a residential development. The suggested gradient in 1:175 and the design storm has a 30- year return period. Using the CIRIA method, select a pipe size which will ensure sediment deposition is limited to 2%. Assume k s = 0.6mm, C v = 0.8 & t e = 2.0 minutes. For rainfall intensities, use the IDF plot from the Class 4 notes. [link] [36]

37 CIRIA method for sewer design - Synopsis Synopsis Although the CIRIA method is more complex than using a simple minimum velocity criteria and is based largely on laboratory data, it does attempt to represent the complexity of sediment transport. Further, by allowing engineers to allow some deposition and tailor solutions to meet specific pipes it provides a framework whereby more cost effective solutions may be obtained. [37]

Continuing Education Associated with Maintaining CPESC and CESSWI Certification

Continuing Education Associated with Maintaining CPESC and CESSWI Certification Module 2: Stormwater Management Principles for Earth Disturbing Activities Sponsors: ODOTs Local Technical Assistance Program