Due to the influence of natural and human activities, the flood control standards of many rivers and lakes (excluding seawalls here) are very low. Judging from the actual situation of resisting 1998 flood, many dikes are facing the real threat of flood, and the abnormal water retaining of 1 ~ 2m sub-dike appears, which is extremely serious. Under normal circumstances, in order to relieve the threat of overflow, the top of the dike must reach the design elevation specified in relevant codes. When the dam body is heightened, the dam slope and dam top should be reinforced accordingly.
It is necessary to design and build new dikes on a new basis, such as rebuilding dikes, returning dikes to the beach due to bank collapse, cutting and straightening dike lines, etc.
Section 1 Review of Levee Top Elevation
The crest elevation should be determined by the estimated design flood level h 1 plus a certain crest superelevation y. In principle, all embankments whose crest elevation does not reach the sum of the two should be raised and thickened to meet the standard. Therefore, in order to eliminate the danger of flood damage, we must first check whether the elevation of the top of the dike meets the requirements stipulated in the code.
First, the presumption of flood control standards for dike projects
The flood control standards for the objects to be protected in dike projects shall be determined according to the national standard "Flood Control Standards". The flood control standards of dike projects shall be determined according to the flood control standards of the protected objects with higher flood control standards in the protected areas. The dike engineering grade shall conform to the national standard Code for Design of Dike Engineering, as shown in Table 2- 1.
Table 2- 1 flood control standard grade of dike project [recurrence period (year)]
≥ 100
& lt 100 and ≥50
& lt50 years old, and ≥30 years old.
& lt30 years old and over 20 years old
& lt20, and ≥ 10
Levee engineering grade
1 2 3 4 5
For particularly important dikes, their flood control standards should be reported to the competent authorities for examination and approval after special argumentation. Flood control standards for dike projects in flood storage and detention areas and flood discharge areas shall be specially determined according to the requirements of river basin planning.
The flood control standard expressed in the flood return period corresponds to the peak flow value. Different river sections should calculate the design flood peak discharge value in the corresponding return period through flood frequency analysis, measure the vertical and horizontal sections of the river section at that time, analyze and select the roughness value, and push the water surface line to get the design flood level value along the river section. For the selected roughness, section, etc. The actual water surface line must be checked repeatedly under off-design flow.
The design flood level expressed by return period can generally remain stable for a long time. If the roughness or section of the river has changed greatly (such as siltation, bay cutting, etc.). ), the above steps must be adopted to recalculate the new design flood level along the river, so as not to threaten the safety of the dike.
When the water system is complex and there are many comprehensive factors such as diversion and jacking, it is difficult to estimate the design flood level with the design flood in a certain return period. In some river basin planning, the design flood level is often based on the actual flood level, and can also be used as the basis for rechecking the design flood level of the crest elevation after approval by the superior.
Second, the calculation of superelevation of dike top
Due to the influence of wind and waves and various uncertain factors, in order to ensure safety, a certain superelevation must be added to the design flood level.
Superelevation of dike top: y=R+e+A (2- 1)
1. wave climbing height r: under the action of wind and waves, wave climbing height often causes the danger of dike overflow. Wave climbing height can be calculated according to the method introduced in Code for Design of Dike Engineering.
The calculated wind speed of design waves of lake dikes and inland river dikes can be 1.5 times of the average wind speed in flood season over the years.
2. Height of windproof water surface E: When the wind blows along the water area, the water surface rises, that is, the height of windproof water surface exceeds the static water surface. In the case of limited wind area, it can be calculated as follows:
e=KV2F/(2gd).cosβ (2-2)
Where e is the height of the windproof water surface at the calculation point, m; K is the comprehensive friction coefficient, k = 3.6×10-6; V is the design wind speed, m/s, which is determined according to the calculated wave wind speed; F is the distance m from the headwind vector of the calculation point to the other side; D is the average depth m of water; β is the angle between the wind direction and the normal of the dam axis.
3. Safe heightening A: When designing the crest elevation, there should be a certain safe heightening value due to the limitation of observation data series in hydrological analysis, the change of river erosion and deposition, the change of mainstream position, the abrasion of crest, wind and rain erosion and other factors. The safety heightening value does not include the settlement heightening reserved for construction. This value should be based on the national standard Code for Design of Dike Engineering, and the engineering grade and wave prevention requirements should be analyzed and determined according to the provisions in Table 2-2.
Table 2-2 Safety Heightening Grade of Dike Project
1 2 3 4 5
Safety heightening (m)
Dike works that are not allowed to cross waves 1.0 0.8 0.7 0.6 0.5
Dike works that allow crossing waves.
0.5 0.4 0.4 0.3 0.3
4. Embankment superelevation value: In principle, the superelevation of river and lake embankments shall be calculated according to the above method. In the design of dike reinforcement, the calculated values of superelevation at the top of dike may be quite different, so it is difficult to use directly. Usually, according to the grade, material and river characteristics of the dam, the specified values are given in sections. For example, the superelevation of a particularly important dike in the middle reaches of the Yangtze River is 2.0m；; The superelevation of the secondary dike is1.5m; ; Generally, the superelevation of the tertiary dike is1.0m.