The design task of flood drainage ditch is to open ditches for flood diversion, regulate ditches, and buildstructureSo as to intercept and eliminate mountain torrent runoff in an organized and timely manner.[1]
Chinese name
Flood discharge ditch
Foreign name
drainage ditch
Role
Drain the flood out of the reserve into the nearby water body
Classification
Open channel, closed channel, flood intercepting ditch, etc
Key points of design
Go deep into the site, comprehensively consider and reasonably arrange
Flood peak determination method
Flood survey method, reasoning formula method, etc
In order to minimize the damage caused by floods and protect the production of cities and factories and the safety of people's lives and property, it is necessary to carefully carry out the urban or factory'sFlood control planning。Flood control standards shall be reasonably selected according to the specific conditions of the city or factory.The mountain terrain has a large slope, a short catchment time, and a short flood duration, so the water flow is rapid and fierce, and the water flow also carries impurities such as sand and gravel, with a large scouring force, which is easy to damage the factories and towns under the hillside and cause serious losses.Therefore, ditches must be opened at the periphery of factories and towns threatened by mountain torrents to intercept mountain torrents, and the flood shall be led out of the protection zone and discharged into nearby water bodies through flood drainage ditches.
Determination method of peak discharge
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Flood drainage ditch is a drainage structure on a small catchment area.Generally, there is no measured flow data for the small catchment area, and the required design flood flow is often calculated indirectly using the measured storm data.It is assumed that the rainstorm has the same frequency as the flood discharge.At the same time, considering that the valley area of mountain rivers is generally only a few square kilometers to dozens of square kilometers, the water is small at ordinary times, even dry, the water volume increases rapidly in flood season, and the concentration is fast, and the protected area can be reached in tens of minutes.Therefore, the peak flow is mainly calculated, and the total flood volume and hydrograph are not studied.[1]
Flood survey method
Flood investigation method includes morphological investigation method and direct analogy method.The morphological survey method is mainly to go deep into the site, investigate the traces of flood level, deduce its occurrence frequency, select and measure the channel section, and follow the formula
Calculate the flow rate, and then follow the formula
Calculate the peak discharge of the survey.Where n is the roughness coefficient of the channel;R is the hydraulic radius;I is the water surface gradient, which can be replaced by the average river bottom gradient.Finally, the flow at a certain frequency obtained from the investigation is converted into the peak flow at the design frequency through the flow variation coefficient and modulus coefficient method.[1]
Inference formula method
There are three kinds of reasoning formulas, including those of the Hydrology Research Institute of the Chinese Academy of Hydraulic Sciences, each of which has its own assumptions and scope of application.For example, the formula of Institute of Hydrology, Chinese Academy of Water Sciences is:[1]
S - rainstorm rainfall, namely the maximum hourly rainfall corresponding to the design return period, mm/h;
τ - concentration time of the basin, h;
N-rainstorm intensity attenuation index;
F - drainage area, kmtwo；
When using this reasoning formula to calculate the design peak discharge, more basic data are required and the calculation process is tedious.When the drainage area is 40~50kmtwoThe above formula has good application effect.
Empirical formula method
Common empirical formula calculation methods include: general regional empirical formula;Simplified formula of Highway Research Institute;Contour method of the Second Railway Design Institute;Calculation method of the Third Railway Design Institute.
The most common application is the general regional empirical formula with drainage area F as the parameter:[1]
Where Q is the design peak discharge, mthree/s；
F - drainage area, kmtwo；
K. N - coefficient and index varying with region and flood frequency.
This method is convenient to use and simple to calculate, but has strong regional characteristics.When it is used in adjacent areas, attention must be paid to whether the specific conditions of each area are consistent, otherwise it is not appropriate to apply it.
For the above three methods, special attention should be paid to the flood survey method.On this basis, combined with other methods for design calculation.
Key points of design
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The design of flood discharge ditches involves a wide range of factors.Therefore, it is necessary to go deep into the site and reasonably arrange flood drainage ditches according to the overall planning of towns or factories, the division range of natural watersheds in mountainous areas, mountain slope topography and geomorphic conditions, the original natural flood drainage ditches, flood trend, flood scouring, local engineering geological and hydrogeological conditions, local meteorological conditions and other factors.Flood drainage ditches include open channels, closed channels, flood intercepting ditches, etc.[1]
(1) The layout of flood drainage ditch shall be closely coordinated with the overall planning of the plant area and considered uniformly.
In the design of concentrator and general layout, attention must be paid to flood discharge.Flood drainage ditches shall be reasonably arranged according to the planning of the general plan, so as to avoid setting plant buildings or residential buildings on mountain flood entrances and flood main channels.A distance of more than 3m shall be reserved between the flood drainage ditch and the building to prevent the building foundation from being scoured by water.[1]
(2) The flood drainage ditch should use the original mountain torrent ditch as much as possible, and can be properly renovated if necessary;
The original mountain torrent ditch is formed by the scouring of flood for several years, and its shape and bottom material are relatively stable. Therefore, the original natural ditch should be used as the flood drainage ditch as far as possible.When the original ditch cannot meet the design requirements and must be renovated, attention should be paid not to change greatly, try not to change the hydraulic conditions of the original ditch, and make the best use of the situation to make the flood discharge smoothly.[1]
(3) Flood drainage ditch shall make full use of natural terrain gradient
The flood drainage ditch should be along the vertical direction of most surface water flow, so the terrain slope should be fully utilized to enable the intercepted mountain flood to flow into the receiving water body with the shortest distance gravity flow.In general, there is no midway pump station in the flood drainage ditch. At the same time, when the flood drainage ditch intercepts the flow of several intercepting ditches, its intersection should be inclined to the downstream as far as possible, and connected in an arc, so that the flow can enter the flood drainage ditch smoothly.[1]
(4) The use of open channel or closed channel for flood drainage ditch shall be determined according to specific conditions
Generally, it is better to use open channel for flood drainage ditch, but when the flood drainage ditch passes through the urban area or the plant area, because of the high building density and large traffic volume, it should use closed channel.
(5) Determination of slope of flood discharge ditch
The slope of the flood drainage ditch shall be determined according to the terrain, geology, protective masonry, slope of the original flood drainage ditch, scouring and silting conditions, and generally not less than 1%.When designing the slope, the flow velocity in the ditch shall be increased evenly to prevent siltation in the ditch.When the longitudinal slope is very large, water drop or chute shall be considered, but it shall not be set at the turning.[1]
