By
Prof. Madhuri K. Rathi, Mr. Patil Dhananjay Rajiv
Amrutvahini College of Engineering, Sangamner
Abstract
The concentration of the inlet wastewater of urban sewage treatment plants is much lower than the expected level in the design stage, mainly because of the problems of construction, management and maintenance of the drainage systems. Through investigation of the urban drainage pipelines, primary problems of drainage network damage, local unreasonable elevation design, pipe blockage and drainage system confusion, etc. were found. Combining the local actual situation, some corresponding engineering and management measures and some feasible suggestions for drainage pipe construction, management and maintenance are put forward.
1. INTRODUCTION
1.1 Purpose
Providing adequate drainage in urban areas has been proven as a necessary component in maintaining the overall health, welfare, and economic well-being of a region. Drainage is a regional feature that affects multiple jurisdictions and all parcels of land. It is important to develop drainage policy that balances both public and private considerations. Certain underlying principles should be applied when planning drainage facilities. These principles apply to both water quantity and water quality management. Policy statements and technical criteria serve as the implementation tools for the underlying drainage principles.
1.2 Objectives
Drainage, flood control, and water quality protection. Drainage represents only one component of a larger urban system. The objectives are with respect to drainage, flood control, and water quality protection is to:
a) To protect the general health, safety, and welfare of the residents of the region.
b) To minimize property damage from flooding, including minimization of localized neighborhood flooding.
c) To ensure that new buildings and facilities are free of flood hazard from major and smaller storm runoff events.
d) To minimize water quality degradation by limiting the amount of sediment generation and erosion of channels.
e) To encourage the retention of open space, particularly along natural drainage ways.
f) To plan for large and small flooding events by providing both major and minor drainage systems.
g) To implement reasonable, cost effective best management practices (BMPs) for sediment control and water quality enhancement.
h) To manage stream and drainage channel corridors to maintain environmental diversity and to protect buildings and facilities from damage by channel erosion.
i) To stabilize channels to, among other things, minimize the disruption of existing infrastructure such as bridges and utility lines.
j) To comply with the applicable National Pollutant Discharge and Elimination System (NPDES) permit requirements.
k) To develop equitable methods to adequately fund construction, operation and maintenance, and administration of an up-to-date storm water management program.
l) To minimize future operating and maintenance expenses.
m) To educate the public on storm water policies and administrative procedures.
n) To build a regional storm water program based on understanding and cooperation with builders and developers, providing for effective administrative authority for the cities, counties and P-MRNRD.
1.3 Planning concepts
The following general principles apply when planning for and designing urban storm drainage systems (ASCE, 1992):
1.3.1 Drainage requires a regional solution
Drainage is a regional phenomenon that does not respect the boundaries between government jurisdictions or between public and private properties. Therefore, a successful plan must integrate regional jurisdictional cooperation, where applicable, to accomplish established goals.
1.3.2 Storm drainage is a sub-system of the total urban system
Drainage is a sub-system of all urbanization. The planning of drainage facilities must be included in the urbanization process. The first step is to include drainage planning with all regional and local urban master plans. Storm water management facilities, such as open channels and storm drains, serve both conveyance and storage functions. When a channel is planned as a conveyance feature, it requires an outlet as well as downstream space to safely convey and mitigate adverse impacts from the design flows. The space requirements for adequate drainage may become a competing use for space with other land uses. If adequate provision is not made in the land use plan for the drainage requirements, storm water runoff will conflict with other land uses, will result in water damages, and will impair or even disrupt the functioning of other urban systems (Tulsa, 1993).
1.3.3 Urban areas have two drainage systems
Urban areas are comprised of two drainage systems. The first is the minor or primary system, which is designed to provide public convenience and to accommodate relatively moderate frequent flows. The other is the major system, which carries more water and operates when the rate or volume of runoff exceeds the capacity of the minor system.
1.3.4 Runoff routing is a space allocation problem
Analysis and design of drainage systems generally should not be based on the premise that problems can be transferred from one location to another.
1.3.5 Storm water runoff as a resource
Storm water runoff and the facilities to accommodate the runoff can be an urban resource when properly included in the urban system. Drainage ways can provide environments for various life forms such as aquatic life, mammals, birds, and vegetation. In many cases the drainage facilities can provide areas for active and passive recreation for citizens to enjoy. Although sometimes a liability to urbanization, storm water runoff can be beneficial as an urban resource. When storm water runoff is treated as a resource, water quality aspects become important. As such, it is important to implement best management practices (both structural and nonstructural) for water quality and effective erosion and sediment control.
1.3.6 Utilize the features and functions of the natural drainage system
Every site contains natural features that may contribute to the management of storm water under existing conditions. Each development plan should carefully map and identify the natural system. Natural engineering techniques can preserve and enhance the natural features and processes of a site and maximize post-development economic and environmental benefits. Good designs improve the effectiveness of natural systems, rather than negate, replace, or ignore them.
1.3.7 Post-development flow rates
In new developments, post-development flow rates shall be controlled to achieve the goals and objectives of the Watershed Master Plan.
1.3.8 Design the storm water management system from the point of outflow
The downstream conveyance system should be evaluated to ensure that it has sufficient capacity to accept the major and minor storm design discharges without adverse backwater impacts on the proposed conveyance system. Adverse downstream impacts such as flooding, stream bank erosion, and sediment deposition must also be prevented or mitigated.
1.3.9 Provide regular maintenance
Failure to provide proper maintenance reduces both the hydraulic capacity and pollutant removal efficiency of the system. Effective maintenance relies on clear assignment of tasks and a regular inspection schedule. Maintenance for local amenities (such as open drainage ways, BMPs detention/retention facilities, etc.) will be provided consistent with the policies established in the Watershed Master Plan, in compliance with the applicable local codes and regulations, and implemented through advance formal agreements between the entities with jurisdiction or responsibility.
1.3.10 Preventive and corrective actions
In existing urban settings, it may be necessary to develop a storm water management strategy based upon both preventive and corrective measures. For example, structural corrective measures such as inlets, storm drains, interceptor lines, channelized stream sections and reservoirs affect and control storm runoff and floodwaters directly. Nonstructural corrective measures, such as flood-proofing and land use adjustments, help limit activities in the path of neighborhood storm runoff or in river floodplains. Preventive actions available for reducing storm runoff and flood losses include: flood-prone land acquisition, floodplain regulations, and control of land uses within flood-prone areas.
1.4 Criteria summary
1.4.1 Drainage design and technical criteria
The design criteria are based on national engineering state-of-the-practice for storm water management, modified to suit the specific needs. The criteria are intended to establish guidelines, standards, and methods for effective planning and design. The criteria should be revised and updated as necessary to reflect advances in the field of urban drainage engineering and urban water resources management.
1.4.2 Minor and major drainage systems
Every urban area has two separate and distinct drainage systems, whether or not they are actually planned for and designed. One is the minor system and the other is the major system. To provide for orderly urban growth, reduce costs to taxpayers, and avoid loss of life and property damage, both systems must be planned and properly engineered and maintained.
1.4.2.1 Minor drainage system
The minor drainage system is typically thought of as storm drains and related appurtenances, such as inlets, curbs and gutters. For residential areas, downtown areas, and industrial/commercial areas, the minor drainage system design will provide capacity and management for the 10-year return frequency storm runoff, under assumed ultimate upstream development conditions. During design, the hydraulic grade line for all enclosed systems shall be determined to ensure that inlets act as inlets, not outlets. All easements for newly constructed storm drainpipe should be a minimum of 30 feet wide. In situations where the engineer can clearly demonstrate that an easement less than 30 feet is adequate, the City may consider such a request. Easements wider than 30 feet may be necessary for storm drainpipe and surface water flowage where a drainage way must be designed and maintained to carry storm water flow in excess of the storm drain pipe capacity.
1.4.2.2 Major drainage system
The major drainage system is designed to convey runoff from, and to regulate encroachments for, large, infrequently occurring events. When development planning and design do not properly account for the major storm flow path, floodwaters will seek the path of least resistance, often through individual properties, thus causing damage. An assured route of passage for major storm floodwaters should always be provided such that public and private improvements are not damaged. For subdivisions in Omaha, this need is to be provided for both in watershed headwaters settings and along major drainage ways. The 100-year return frequency storm under assumed ultimate upstream development conditions shall be the major drainage system design storm for all new developments. Runoff from major storms should pass through a development without flooding buildings or homes. Overland flow routes can be provided using streets, swales, and open space. Open channels for transportation of major storm runoff are desirable in urban areas and use of such channels is encouraged. Open channel planning and design objectives are best met by using natural, or natural-type channels, which characteristically have slow velocities, and a large width to depth ratio. Optimum benefits from open channels can best be obtained by incorporating parks and greenbelts with the channel layout. To the extent practicable, open channels should follow the natural channels and should not be filled or straightened significantly. Effort must be made to reduce flood peaks and control erosion so that the natural channel regime is maintained. Channel improvement or stabilization projects are encouraged which minimize use of visible concrete, riprap, or other hard stabilization materials to maintain the riparian characteristics.
1.4.3 Storm runoff computation
The calculation of the storm runoff peaks and volumes is important to the proper planning and design of drainage facilities. Potential methods for calculation of runoff shall require advance approval from the City Department of Public Works.
1.4.4 Detention
Detention facilities shall have release rates that do not increase the potential for downstream flooding and are consistent with the policies of the Watershed Master Plan. Submittal of hydraulic design calculations is required to document that major and minor design storm peak flows are adequately attenuated.
1.4.5 Streets
The primary drainage functions of streets are to convey nuisance flows quickly and efficiently to the storm drain or open channel drainage with minimal interference to traffic movement and to provide an emergency passageway for the major flood flows with minimal damage to adjoining properties, while allowing for safe movement of emergency vehicles. The allowable use of streets for new land development in metropolitan Omaha for minor and major storms runoff in terms of pavement encroachment .
1.4.6 Flood corridor management
In all watersheds where Flood Insurance Study (FIS) floodway has not been delineated, development shall preserve a corridor with a minimum width consistent with the policy of the Watershed Master Plan.
1.4.7 Water quality
Both structural and nonstructural best management practices are recommended that address long-term Storm water quality enhancement. Effective, reasonable, and cost-effective should be selected for implementation on a site-specific basis and in a manner that is consistent with the Watershed Master Plan.
The following is a list of voluntary that should be considered:
Create temporary ponding areas on parking lots and in landscaped or turfed open areas of building sites;
a) Use porous or turf pavement for remote parking areas.
b) Reduce the amount of impervious area directly connected to the storm drain system.
c) Intentionally create longer vegetated drainage paths for minor storm events.
d) Encourage use of constructed wetlands.
e) Develop multipurpose extended detention facilities.
f) Use retention facilities (wet ponds) where feasible.
The following is a list of voluntary non-structural BMPs that is encouraged:
a) Use of appropriate vegetation to reduce the need for fertilizer and pesticides.
b) Preservation of environmentally sensitive areas to protect them from development or other disruption.
c) Set aside more open space.
d) Preserve or re-establish riparian vegetation.
e) Implement staged grading of developments to minimize the amount of land disturbed at one time.
1.5 Limitations
The interpretation and application of the provisions shall be the minimum requirements for promotion of the health, safety, convenience, order and general welfare of the community. The standards, however, should not be construed as rigid criteria. Rather, the criteria are intended to establish guidelines, standards and methods for sound planning and design. The City may set aside these criteria in the interest of the health, safety, convenience, order and general welfare of the community.
2. METHODOLOGY
2.1 Data collection methodology
a) Collecting information, including pipe network built drawings, meteorological data, hydrological data, etc., and conduct research and analysis.
b) Making a further research on the map of pipe network system, combined with in-situ field investigation to determine the location of all sewage wells and the sewage flow direction, understanding the scope of the pipe network collection. Numbering the sewage wells is well included.
c) Sampling for the main pipes and branch pipes to determine sewage pipes that are low in Chemical Oxygen Demand (COD) concentration and recording weather condition, as well as the water level of the adjacent rivers.
d) Making a detailed investigation on the main pipes and branch pipes of sewage where COD concentration is low or appear mutations to address the network problems. Corrective advices should be proposed for further improvement.
e) Areas that are of the most concern include: pipes, flap valve, inverted siphon pipes, culvert, and pump station, sewage interception wells which are close to rivers or ponds.
f) Employ robots, sight glasses, closed-circuit television (CCTV) in assisting the investigation.
2.2 The problems of drainage network
The following problems were recognized through four months deep investigation into the drainage network.
a) Sewer damage Sewer was damaged by transformation of roads and bridges, construction of water pipes and gas pipes, crossover operation of sewer pipes. Sewage pipes experienced deforming, disjoint, sink, water leakage and some other relevant issues, hence finally result in collapse of surrounding roads.
b) Unreasonable local elevation design Sewage only piled up in the pipes and even refluxed because the elevation design is unreasonable in the sewage pipe junctions.
c) Severe blockage of pipe networks the blockage in the sewer pipes which are not conducive to sewage collection was generally made of construction waste (such as stone, cement), mud, plastic, foam, etc. The scour capability of low-speed flow is not high enough to rush the blockages into downstream pipe network thus garbage was easily stored in the slow flow pipe which forms a vicious cycle and results in more serious siltation .The silting of the inverted siphon pipe across the river is the most serious part. To illustrate, a section of DN600 inverted siphon which was jammed by a large number of sand bags and garbage led to cross section reduction around 20cm2 and cut down the transmission capacity of the sewage.
d) Improper flap valve installation
e) The installation of flap valve is rigorous. It must be tilted installed at a bias angle between 8°–15° to make it function well under stress. The flap valve does not open freely for its aging. And it is opened by the bricks in order to drain the flood during the monsoon, so that the river water flows into the pipe network through the flap valve.
f) Quality problems of inverted siphon pipes In south China, there exists high density river network; it is commonly seen that inverted siphon pipes are used in the sewage network engineering. The old inverted siphon pipes are made of steel reinforced concrete. In spite of its low cost, its impermeability, subsidence-resistance and seismic-resistance are quite limited. The inverted siphon pipes are embedded under the river bed, bearing pressure for a long time, which makes the wall of pipes cracked easily.
g) Manholes and manhole covers Manholes are always covered by construction site. As to the manhole covers, some problems do exist: firstly, the manhole covers are easily breaking for their using low-quality material manufacturing; secondly, some manhole covers experienced difficulty in opening after a long time enclose; thirdly, it is hard to distinguish sewage wells from rainwater wells and water supply wells due to irregular management.
h) Wasted rivers the functions of a river are mainly described as landscape in town. However, they are usually formed to receive the domestic sewage and waste water due to a series of difficulties such as lack of capital, planning and poor construction conditions. It attributes to the damage of the original functions and the ecological environment of river. Meanwhile, rainwater and river water are likely to enter drainage networks through wasted rivers.
i) Disorder of drainage system some non-professionals (such as real estate developers, construction workers) who do not understand the role of rain water pipes privately discharge sewers into rain water pipes without the application license, registration and approval, resulting in rain water mixed with sewers. Some communities which should have had separate drainage systems have appeared the case that rain and sewage mixed under inadequate community supervision and regulatory measures and, lax supervision of final acceptance.
j) Incomplete drainage network construction the dirt holding rate is approximately 40% at present, which is way to reach the standard of 70%, indicating that the drainage network construction is incomplete. It is still commonly seen that sewage was discharged directly to the rivers in some places, especially in the country side, further enhancing the pollution of rivers.
2.3 Corrective measures
In view of the above issues proposed and by combining with the local actual situation, effective, rational, economic and feasible corrective measures are to be developed to tackle the problems.
2.3.1. Engineering measures
a) Making a scientific and reasonable plan as soon as possible and integrate the pipe network construction, in order to improve the dirt holding rate. Transform both sides of the creek, and built sewage pipe network on the shores to collect domestic sewage from the direct discharge port to prevent the pollution. Speed up the construction of primary pipes as well as branch pipes construction. Collect all sewage coming from institutions, enterprises, factories and residents to the greatest extent to accomplish zero discharge of sewage.
b) Improve drainage systems towards better separation between rainwater and sewage collection to avoid illicit connections in some residential districts. The rainwater pipes must be separated from the sewage pipes. When the pipes are connected by the inhabitants, it should be done under the guidance that is right to the separate system. The existing illicit connections should be notified for correction under improved policy.
c) Maintain the destructed pipes. Such problems subsidence, collapsing and breakdown of the pipes led by the alternate construction should be worked out as soon as possible to avoid the infiltration of groundwater.
d) Opening channels should be reformed and wasted rivers should be conducted to intercept the sewage from both sides of them by laying sewage pipes.
e) Reforming flap valves and inverted siphons. Replace the old or malfunctioned flap valves with energy–saving flap valves which are strongly recommended. What is more, replace inverted siphons made of reinforced concrete with seamless steel pipes, which can work under high pressure and hold better properties such as seismic resistance, relatively light dead weight and long pipe section.
f) Inverted siphons, sewage pipes and inspection chambers should be desilting regularly. Combined high pressure-flushing and sludge suction vehicle is strongly recommended. Cleaning by high pressure water jet is a state-to-art technology that has been spread employed by developed countries for the reason that it not only shortens the construction time period, but also save the capital cost and performs well on silt cleaning.
3. CONCLUSIONS
a) A detailed survey of sewage collected area must be done before drainage network construction the consideration of the original pipeline flow trend and polluted inflows of the sewage collected area has usually been neglected before setting the sewage designed quantity of the sewage treatment plant. In addition, the hardhats builds the drainage pipes only according to drawing paper mechanically, problems whether sufficient sewage quantity can be collected by the building sewage pipes and even whether river water can flow backward the sewage pipe network have not been taken into account in construction. Hence, the outfalls in the built drainage network leading to water flowing backward or discharging directly into the river.
b) Separate system must be set in the future Despite such disadvantages as illicit connections, high investment and the pollution of initial rainwater, separate system has the advantages of easy pollution control, rain water collecting and recycling. The sewage treatment plant under such a system is able to treat the sewage effectively and being managed easily.
c) Integrate checking system of drainage network construction Pipe leakage, closed water test fail, location deviation of the pipes; deformation and subsidence of inspection well, great foundation deformation of the pipes are problems commonly to occur in the drainage network, whose main courses are quality of construction and materials. Discharge characteristics and watertight test must be done seriously when checking, and subsidiary facilities of the drainage network as materials of flap valves and inspection well, construction quality should also be double checked.
d) Enhance archives management of the drainage network Archives management of the drainage network must be carried out under the filing-up system of completion drawing. Outlets, temporary plugs, survey coordinate and diameter of the future pipes connection points must be marked on the completion drawing clearly for checking. The misunderstandings of only focusing on original record maintenance and putting such modification of filing works into subordinate position must be replaced, and set up the archives management system of the drainage network for modification and supplementation, which can file in denomination of every street and function as an index for information and data inquiry.
e) Establish GIS system for drainage network management Geographic Information System (GIS) system for drainage network management has four functions: information access and input, data storage and management, data conversion and analysis, results generation and output. GIS management system is widely used in the drainage network. The system, which is made of computer graphics and database, is a high technology for data processing and storage. Correlation attributes and geographical location are organically combined in such a system, and can be displayed to the users with picture-illustrated composing and accuracy style. Users can make decisions by its spatial analysis function and visualization. The needs for design, management and running of the drainage network inquiry can also be satisfied from it. The relevance municipal departments should carry out investigation of drainage network as soon as possible and set up the GIS system for drainage network management.
f) Strengthen the environmental protection education and encourage public participation Questionnaires must be done before the construction or reform of drainage network and encourage public participation to supervise the construction and maintenance of the drainage network, and to enhance the environmental protection.
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