Design and calculation of storm sewers. How a storm drain is designed
Storm sewer is a system of water collectors, pipes, channels and collectors necessary to collect rainwater flowing from roofs and areas with impervious coatings. As with the construction of any other communication system, at the first stage of work, you need to draw up a project taking into account the requirements of SNiP.
At the design stage, the type of drainage system will be determined, the layout of the channels will be drawn up, the required diameter of the pipes will be calculated, the place of water discharge and the location of water intake points will be determined. A storm sewer project drawn up taking into account the requirements of SNiP will help you navigate the following points:
- What materials will need to be purchased?
- What should be the diameter and footage of the pipes?
- How will the installation work go?
- What will be the duration of the work?
- Will special equipment be required?
Ultimately, on the basis of the project, an estimate of the costs that construction will require can be drawn up.
Types of storm sewers
Before you begin to perform calculations, you should decide on the type of storm sewer. Advice! Despite the fact that the deep drainage system is often built in parallel, according to the requirements of SNiP, they should not be combined. They are placed in parallel, one above the other, while the storm system should be above the drainage system.
Types of storm drains according to the method of water drainage
There are three types of drainage systems for rainfall water:
- Closed drains. This is the most difficult option, for its implementation you need to perform a serious hydraulic calculation in order to choose the right pipe diameter. Water in this case is collected in special collections - storm water inlets, trays. Then the collected moisture enters the pipe system, through which they move by gravity or with the help of pumping stations. Water enters the collectors, from where it is discharged outside the site, an example of a possible outlet direction is reservoirs, ravines or drainage installations.
Advice! Closed-type storm sewers using large-diameter pipes are most often installed on city streets or in industrial enterprises. But sometimes it is this type of drainage that is best suited for private areas. The most striking example of the expediency of such a choice is that the site has a large area.
- Open drains. This option, on the contrary, is the simplest. In this case, the moisture is collected and removed by means of a system of trays installed in ditches dug with an inclination towards the collector. Trays from above are covered with decorative removable lattices.
- Mixed sewers. This option for arranging the system provides for the installation of elements of both types mentioned above. Mixed conduits are built to reduce the cost of building closed systems.
If it is planned to build a closed or mixed type system, then pipes with a diameter of 100 to 150 mm are used for private buildings. You can more accurately determine the diameter of the pipes by calculating, taking into account correction factors. When performing calculations, not only the diameter is taken into account, but also the level of slope of the pipes to ensure the optimal flow rate.
Types of storm drains by type of drainage system
There are two types of system arrangement:
- Spot water collection. It is carried out by installing local storm water inlets connected by pipes into a single network. It is necessary to plan the installation of fence points in problem areas, for example, under drainpipes and in lowlands on the territory of home ownership.
- Linear collection of water. This option is suitable for collecting moisture from large areas, an example of such areas is paved areas, concrete paths, etc.
Storm sewer design
Residential development projects always include a part dedicated to the construction of a linear sewer. But the owners of private plots, as a rule, have to take care of the construction of drainage systems on their own.
When constructing communications such as storm sewers, it is better to entrust the calculation of the main indicators to specialists, since they are produced according to rather complex formulas and require knowledge of the requirements of SNiP.
Regulations
When drawing up a project, one should strictly adhere to building and sanitary rules. So, if the calculation is carried out 2.04.03-85 is the main document.
The procedure for drafting projects
As a rule, the development of project documentation is entrusted to specialized organizations that have permission to perform such work. To start drafting a project, specialists need to obtain the following information:
- Topographical plan of the area where the storm water will be built.
- Data on the nature of soils, that is, the results of a geological study of the site.
- Territory development plan.
- Specification for connection to centralized systems, if such connections are implied.
- Desired methods for organizing the collection and disposal of water, determined by customers.
Based on the collected information, a technical task is drawn up, according to which the project is then developed, taking into account local conditions and the requirements of SNiP.
What is included in the finished project?
When the design work is completed, the customer receives a document, which includes the following sections:
- Common data.
- Schematic diagrams of sewer networks.
- Site plan indicating the locations of the system elements.
- Detailed equipment specification.
- Estimate, that is, the calculation of the budget for the construction of the system.
In fact, a well-designed project is not just an example of calculating storm sewers, but a clear, detailed and step-by-step installation instruction. The design process requires special knowledge, experience and punctuality. Therefore, work should be entrusted only to competent specialists.
Design is the most important stage in the construction of communication systems, including the construction of storm sewers. When building capital houses, it is advisable to entrust this process to competent design companies, since without special knowledge and without taking into account the requirements of SNiP, it is impossible to complete a competent project.
Typical projects for individual residential construction or industrial sites imply the availability of project documentation for calculating storm sewers. The set of rules SP 32.13330.2012 contains all the formulas, tabular values and coefficients necessary for the calculation. Since it is impossible for a non-professional who holds a plan in his hands for the first time without help, here is the basic information on how to make calculations and not confuse yourself even more by reading the features of the hydraulic calculation of a storm drain.
The main goal pursued in the process of studying the arrangement of sewerage is the accurate calculation of the diameter and slope of the pipe, depending on the amount of precipitation falling in a particular area.
Important! Insufficient water supply capacity leads to a significant decrease in the efficiency of the sewer branch as a whole. And this threatens to flood the territory adjacent to the house during heavy rains.
The arrangement of storm sewers is strictly regulated and regulated by SNiP. Remember that the drainage system is an essential element, regardless of the purpose of the building.
It is not enough to observe the hydraulic calculation of storm sewers for the system to function properly, listen to some recommendations:
- For household wastewater and industrial waste, a separate drainage system is equipped.
- The place of discharge of effluents into natural reservoirs is coordinated with the sanitary and epidemiological service, bodies for the protection of water bodies.
- It is legally allowed to send from private households directly to the central sewer, without pre-cleaning them.
- For industrial enterprises, effluents must be passed through treatment facilities.
- The performance of centralized treatment facilities and its throughput determine the possibility of draining atmospheric precipitation from territories adjacent to private and industrial facilities.
- Whenever possible, try to organize a gravity mode for the descent of surface water.
- If it is required to provide a large production site or a whole settlement with a water supply system, then this, as a rule, is a closed type branch.
- Low-rise and suburban facilities are equipped with open sewer networks.
- Combinations of open and closed drainage systems in private individual residential construction have received practical application.
What formulas to use to calculate storm sewers
To determine the cross-section of drainage pipes, calculate the flow of rainfall in the region of residence. This factor depends on climatic and weather conditions.
Calculations are carried out according to the formula: QR= q20 YF, Where
- q20 denotes the estimated intensity of precipitation over 20 minutes;
- Y - moisture absorption coefficient of the coating (1.0 - for roofing, 0.95 - for soil, 0.85 - for concrete, 0.4 - for crushed stone).
How water is used in pressure mode
In the hydraulic calculation of storm sewers, an adjustment is made for the fill factor of the free drain in the event of a pressure regime (b):
Q= QRb, where b is taken from the table:
Important! Meaningndepends on the geographical location of the object.
Important! with a terrain slope of 1-3 cm per 1 m, the coefficientbincreases to 15%. If the slope is greater, then 1 is considered to be the indicator.
Take a look at the example of the calculation of storm sewers.
Practical calculation of the capacity of the water supply
Very often, the reason for the non-functionality of storm sewers is the neglect of the details of the calculation by the designers. Based on the general instructions of SNiP, repairmen and installers often make mistakes.
When calculating the diameter of a storm sewer, pipes with a cross section of 200-250 mm are often used. This is the optimal indicator for the unimpeded movement of wastewater through pipes. Even if the precipitation falls with greater intensity.
Important! Preliminary calculation and purchase of the necessary parts in accordance with the norms and requirements helps to reduce costs without compromising the functionality of the network.
System Capacity Calculation Example
We take 100 m 2 for the area of \u200b\u200bthe adjacent territory, which is 0.01 of 1 hectare of land. Presumably, we will divert water from this area. Let's say that the object is in MO.
Based on the calculation table, it is determined that q20 for Moscow and microdistricts is 80 l / s. The moisture absorption coefficient for the roof is 1.
Based on certain indicators, the calculation of rainwater is as follows: QR= 80 x0.01 = 0.8 l/s.
In 90% of cases, the roof slope exceeds 0.03 (>3 cm per 1 m), then the filling factor of the free reservoir during the pressure mode is taken as 1. From this it emerges that: Q= QR= 0.8 l/s.
Important! After determining the calculation indicators, you will have the opportunity not only to calculate the diameter of the pipe for storm sewers, but also determine the required slope of the drain.
Good recommendations are given in the reference book by A. Ya. Dobromyslov “Tables for polymeric materials. Non-pressure pipelines. Here, the novice master will find the calculated data presented in the form of tables. It is definitely clear that for a flow rate of 0.8 l / s, a pipe with such a diameter and slope is suitable:
- 50/0,03;
- 63/0,02;
- 75/0,01.
Important! Remember that the slope of the pipes is inversely proportional to the diameter.
It is easy to guess that the design begins with the selection and purchase of material. And here the main thing is not to make a mistake in choosing, because otherwise all the work will go down the drain.
What material is suitable for the pipeline
According to SNiP, it is permissible to use asbestos-cement, steel and plastic (PVC) pipes.
Asbestos-cement pipes, although they are used, are very rare. This is an economical option, but the material is fragile and heavy (1 m of a pipe with a cross section of 100 mm weighs at least 25 kg).
Steel plumbing will be easier, but there is a but here too! The metal is prone to corrosion.
Therefore, products made of PVC plastic are preferable. Combining light weight, the ability to operate for a long time, ease of installation.
Features of the laying depth
When designing and calculating storm sewer treatment facilities, soil characteristics are also taken into account, including the level of soil freezing. The optimal location of the pipe is below the freezing line of the soil, but above groundwater. These conditions are not easy to comply with due to the uneven terrain, therefore it is determined that the pipe should be at least 70 cm from the surface of the earth.
The principle of installation of risers
It is impossible to imagine a storm sewer without risers with point / linear storm water inlets connected to them, the vertical fastening of which is carried out using clamps.
Important! Note that the calculation of the storm sewer riser fixing interval is based on the type of material used. If it is PVC, then the clamps are fixed every 2 meters, if steel - 1.35 m.
Secured territory
The SNiP provides for the organization of "security zones" at a close distance from the storm drain. Remember that there must be at least 3 m from the construction site, garden, park, landfill, parking to the sewer.
Design is the most important stage of the entire system for arranging and calculating pipes for storm sewers, regardless of the type of building.
Here are collected the basic formulas that will be useful when making calculations for both the craftsman and the beginner. But this method may turn out to be false if specific installation conditions are present (water pH, soil acidity, the number of bends and turns along the entire length of the sewer). The most correct decision is to entrust the work to specialists who, in the course of the hydraulic calculation of storm sewers, use an example to calculate every little thing. This option is efficient and economical.
Can you imagine a landscaped area without storm sewers? What would the yard then turn into due to heavy rain or a thunderstorm? There would be a lake in front of the house, and the exterior finish would soon deteriorate due to exposure to water, leaving no other options but to complete the house and the surrounding area.
Many people ask why the city drains cannot cope with water drainage, and the entire settlement turns into a real polynya. Neglecting the recommendations for calculating drainage systems in individual residential construction, a person compensates for one of the factors contributing to the malfunctioning of the entire central drainage system. Farther on the sites of the main nodes, unscrupulous craftsmen, trying to save money, neglect the same, exposing the risk of an immediate failure in the operation of the entire city network. Therefore, traffic jams, blockages, low throughput of the system, and flooding are formed.
Understanding that the calculation of sewerage is an important factor, and you cannot do without replenishing it, will save you from a number of common problems. The main thing is to do everything right, and the sewer will work properly.
An accurate calculation of storm sewers is the key to optimizing the estimate for such construction. After all, overly deep bedding and overly wide channels hit the customer's wallet no less hard than errors in the project.
Therefore, any storm sewer project starts with approximate calculations based on averaged geodetic and meteorological data, and finishes with cost optimization based on “local realities”. And in this article we will go from start to finish, touching on the subject of calculations and the methodology for optimizing the results obtained.
A typical project begins with the preparation of a technical task, in which all technical nuances are discussed, the results are announced and the cost of the work done is specified. The preparation of terms of reference for storm water is regulated by GOST 3634-99 and the relevant SNiP.
The next stage of the project is the calculation of storm water from tabular and "local" geodetic and meteorological characteristics.
Moreover, at this stage, the following characteristics and parameters of the future conduit are calculated:
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- Type of storm drain design. At this stage, the design scheme of the storm sewer is determined (external or internal, deep or surface, and so on).
- Based on the scheme, the number and type of water collectors - storm wells or collectors (and the location of these elements relative to the foundation) are determined.
- According to the location of the water collectors, the footage of the drainage pipeline and / or storm trays is considered. Moreover, the footage is affected by the depth of the storm sewer, and the slope of the outlet channel, and the distance from the catchment point to the drainage well.
- In the final, the needs for shut-off and control and connecting fittings are determined, as well as the number and location of manholes.
Moreover, most of the parameters are affected by the calculation of water flow and the cross section of storm pipes, the magnitude of the slope and the depth of the laying, or rather the results obtained in the process of these calculations. Therefore, we will talk about such research separately.
Water consumption calculation
The calculation of the flow rate (volume) of water flowing through the storm pipes is the basis for all further research. After all, they are repelled from it when determining both the throughput and the diameter and the slope of the outlet pipeline. Moreover, water consumption most of all affects the estimated volumes of water collectors and collectors. After all, the entire volume of wastewater will be accumulated in these stormwater elements.
Well, the consumption itself is calculated by the formula:
V=q20 x S x D
Where V is the expense itself q20- this is a reference value indicating the volume of precipitation (in liters) falling on an area of \u200b\u200bone hectare (10 thousand square meters), S is the roof area converted from square meters to hectares (10,000:1), and D is the coefficient of moisture absorption by the foundation soil.
Moreover, both the volume of precipitation and the moisture absorption coefficient are indicated in a special collection of building codes (SNiP 2.04.03-85). Only q20 is displayed in graphical form, with reference to the map of the former USSR, and D - in tabular form, with reference to the type of soil.
Calculation of the diameter of the storm sewer pipe
An accurate calculation of the pipe diameter is associated with very complex calculations, which take into account the roughness coefficient of the inner surface of the drainage pipeline, the velocity of the liquid flow through the pipe, the slope of the drain, and other quantities.
Therefore, in most cases, it is customary not to go into details, but to operate with the minimum possible diameters specified in SNiP 2.04.03-85. And in this standard it is mentioned that the minimum diameters of free-flow drainage networks are 200-250 millimeters.
It is this diameter that guarantees the optimal flow rate of wastewater in a free-flow pipeline - 0.7 m / s, thanks to which it is possible to ensure rapid disposal of wastewater of an average daily volume.
Slope calculation
Knowing the diameter of the pipe and the flow rate of the liquid, it is also possible to determine the minimum slope of the storm sewer, which guarantees the flow of liquid under the influence of gravity. However, in SNiP 2.04.03-85, these values are linked, first of all, with the diameter of the pipeline.
And for pipes with DN200 (nominal diameter 200 millimeters), the minimum slope is determined by a factor of 0.007. Moreover, pipes mounted to storm water inlets (drainage trays, storm wells, and so on) are laid with a slope of 0.02 (up to 2 centimeters per running meter of runoff).
Open drainage trenches filled with gravel (trapezoidal, 30 cm wide at the bottom and 40 cm deep) have a slope of 0.003. And the slope of the drainage trays ranges from 0.003 to 0.005.
Determination of bookmark depth
The minimum depth of laying the stormwater pipeline is determined based on several factors:
- Groundwater level.
- Soil type.
- Depths of soil freezing.
Ideally, the minimum depth of laying should be less than the groundwater level and more than the soil freezing mark. That is, the storm drain will have to be deepened by 1.2-1.5 meters, if the groundwater level allows it. And this is only the “upper” edge of the drain, since the depth of immersion in the soil of the lower end is determined taking into account the height difference, according to the slope of the pipe.
Estimates for storm sewers: ways to optimize costs
A typical storm sewer consists of the following elements - water collectors, drainage pipes, sand collectors, intermediate wells (inspection and drainage) and a wastewater collection tank.
1. Moreover, the best results are shown only by a fully equipped sewer, in the design of which there are all of the above elements. Therefore, saving estimates by excluding any components is far from the best solution.
However, no one prevents us from combining some elements "in one bottle". For example, a manhole with a difference in nozzles can be turned into the same sand collector. And instead of trays - quite expensive products - use a perforated drainage pipe laid in a ditch filled with rubble.
2. In a word, there are a lot of options for combining functions. And each promises all tangible savings. In addition, it is possible to reduce the estimate by optimizing the size of sewage conduits or drainage wells. After all, the dimensions recommended in SNiP (200-250 millimeters) are suitable for industrial buildings and for entire blocks of one-story buildings.
3. But for a separate house equipped with a non-pumpable storm well, a 100 mm pipe is enough (a 150 mm pipe can be used to protect against flooding). The result is one and a half, and even double savings only on pipes.
4. Another way to save money is the already mentioned non-pumping storm well, buried to the horizon with high permeability. From such a well it is not necessary to pump out water or connect it to the central sewer. The drains will go away on their own, dissolving in sand that conducts moisture well.
As you can see, a creative approach to design gives a real chance for savings.
However, some arbitrariness is permissible only when arranging a household storm sewer, laid near a small summer cottage or a modest country house.
Really large buildings with a large roof or industrial facilities are best equipped with storm sewers, equipped according to the recommendations from GOST and SNiP. Otherwise, the owner of such facilities may pay a double price for their own indiscretion (and this is without taking into account the cost of dismantling an unsuitable storm drain).
This type of sewage is a complex engineering device, so the exact calculation of storm sewers during its design is of no small importance. In general, the installation of such a system requires an integrated approach; certain technical requirements must be observed in the process. The project must be approved by the Sanitary and Epidemiological Service, by the bodies for the protection of fish stocks and the protection and regulation of waters.
Stormwater design
Stages of drafting a project
The design process consists of the following steps:
- Storm sewer project: its development in relation to a specific site.
- Coordination of this project in all necessary instances.
- Direct implementation on site.
The first stage of the design is the development of project documentation. The basis for such work is the terms of reference, diagrams, drawings. When developing, it is necessary to rely on regulatory documents: SNiPs, SanPiNs, GOST 3634-99 and some others.
The main document at this stage is the terms of reference. It is compiled based on the wishes and requirements of the customer, filled in in accordance with GOST 19.201-78. The terms of reference reflect the terms of the work, the purpose of the device, technical and economic requirements, the control procedure. Applications are allowed if necessary.
Calculation of parameters included in the project
The methodology for calculating storm sewers includes the following key points:
- determining what type of sewerage should be used: internal or external;
- finding out the location of storm sewer wells, what will be their number;
- calculation of the required pipe length;
- selection of signaling devices, sensors and materials.
The decision is influenced by such parameters as the catchment area, the presence of water bodies (natural or artificial) in the immediate vicinity, the maximum amount of precipitation and the average rate of their precipitation, the speed with which runoff water moves and other factors that can characterize this object.
The calculation of water consumption and the cross section of storm pipes are described in the article "".
In order to have an idea of the monetary investment invested in the organization of this system, an estimate is made for storm sewers. It includes the cost of all components, namely:
- Point type water collectors.
- Water pipe systems.
- Various purposes of storm sewer wells.
- Effluent filters.
If necessary, the estimate can be supplemented with other elements at the request of the customer.
One of the important parts that the storm sewer scheme includes is a well. Depending on its type, it can collect wastewater, distribute it for treatment, water sampling, sediment erosion, flow measurement, inspection and maintenance of underground utilities.
The process of designing a rainwater sewer system has its own characteristics, which include:
- trace;
- calculation of hydraulic parameters;
- designing system elements, their linking.
Tracing is necessary in order to collect rain and melt water from a specific area and divert it to the place of release or treatment.
At what depth should storm sewers be placed
Despite the fact that this issue is being vigorously discussed at construction forums, SNiP 2.04.03-85 gives a clear answer to it, which states that the depth based on the experience of operating networks in this region is taken as the smallest depth.
Pro tip:
If there is not enough data, then the minimum depth of laying the stormwater pipeline for pipes up to half a meter in diameter is taken by 30 centimeters, and pipes with a diameter of more than half a meter are not less than 70 centimeters from the surface.
To determine the minimum depth, thermotechnical and statistical calculations are carried out.
For practical purposes, the depth of the storm sewer is such that it is possible to minimize the amount of work performed when digging a trench, as well as to ensure the pipes are safe from external damage and prevent freezing of sewage in the pipeline.
Storm slope
Pro tip:
To determine the minimum slope of a storm sewer, the type of drainage, cross-sectional diameter and surface coverage must be considered.
With an inner diameter of pipes of 0.2 meters, the slope should be 0.007, and if the inner diameter is 0.15 meters, the slope should be at least 0.008. If for objective reasons it is impossible to make such a slope, the standard is reduced to 0.007 for 150 mm pipes and to 0.005 for 200 mm pipes.
For open-type drainage systems, the minimum slope values for storm pipes are:
- for a drainage ditch - 0.003;
- tray of the roadway, the surface of which is asphalt concrete - 0.003;
- tray of the carriageway, which is covered with crushed stone or paving stones - 0.004;
- when the cobblestone tray of the roadway is covered - 0.005;
- separately located tray - 0.005.
Based on these numbers, you can see that the slope depends on the roughness: the larger it is, the greater the slope required. The diameter of the pipes also matters: the larger it is, the less slope is required to ensure the required flow rate.
Pro tip:
When constructing a drainage system on your own, it is best to use a "surface slope". The other two methods are applicable only if ready-made factory drainage channels are used.
The numbers given in the regulatory documents are the result of practical research in the construction of a large number of buildings, however, they are not a dogma for the developer. They provide the information needed to create a reliable system. If you adhere to these standards, then with the right one, it will serve uninterruptedly for many years.
Storm sewage is one of the most important systems for equipping a residential area, which, unfortunately, many owners simply forget about or treat it too lightly. And it is completely in vain - the hopes that rain or melt water will go away by itself often lead to the gradual swamping of the territory, to the destruction or failure of the laid paths and platforms, to the erosion and erosion of the structures of the foundations of the erected buildings, waterlogging of their walls and other negative consequences.
Storm sewage includes many different elements that are responsible for a specific area of water collection, for several such areas, or for the entire system as a whole - these are storm water inlets, pipes, wells, collectors. In order for them to be able to cope with their task, their parameters must correspond to the expected volumes of water. And when planning a system, the calculator for calculating the volume of stormwater, offered to the reader's attention, may be useful.
Below, under the calculator, a brief explanation of how it works will be given.
