4.2.1. Formulation of River Regulation Schemes

According to the results of flooding analysis and survey results in the TieShan River basin, the main problems are as follows:


In order to solve these problems, the following three regulation schemes are proposed:

(1) Scheme 1: Widening the downstream width and heightening dikes on both sides of the TieShan River to improve flood control capacity to reach the standard of a 20-year return period.


**Figure 6.** Flooding situation in the study area.

4.2.2. Comparison and Selection of Three Schemes

For Scheme 1, considering the difficulty and high cost in land acquisition and demolition, the general practice is to build dikes on both sides of the SheGong River, maintaining the existing river width. However, according to the results of the water surface calculated by the simulation model (Figure 7), if we only build dikes to improve the flood control capacity to reach the standard of a 20-year return period, the elevation at the top of dikes must be 1.5 m above the ground at least (the safe height above the water surface is taken as 0.6 m), which is probably not allowed for the surrounding environment. Therefore, Scheme 1 that only adopts general engineering measures is not recommended here.

**Figure 7.** Comparison of current bank elevation and design flood level (20-year return period).

If analyzing from the whole region perspective, it can be found that the surplus of flood discharge capacity in TieShan River is the key of the solution. In Schemes 2 and 3, the construction of tributary-1 is to take advantage of this convenience, and it can play a role in alleviating the flood pressure of SheGong River and avoid the construction of dikes. At the same time, taking the construction cost of engineering projects into account, it is still more reasonable to open up the tributary than to build dikes. Therefore, the next content is mainly to compare Scheme 2 and Scheme 3 (Figure 8).

**Figure 8.** Schematic diagram of Scheme 2 and Scheme 3.

Through simulation, it can be determined that after the adoption of Scheme 2 or Scheme 3, there is no flooding with the 20-year return period. The difference between Scheme 2 and Scheme 3 is the treatment of the culvert. In Scheme 2, the culvert is to be widened; in Scheme 3, a part of water flows that should have passed through the culvert is transferred to the TieShan River through tributary-3. Considering the land acquisition and demolition, the costs of Schemes 2 and 3 are shown in Table 3.


**Table 3.** Cost of land acquisition and house demolition for Scheme 2 and Scheme 3.

The simulation results of water surface lines with Scheme 2 and Scheme 3 are shown in Figure 9. It can be seen from Figure 9 that when flooding with a 20-year return period occurs, the water level of the culvert is below safe altitude; the water level of the culvert in Scheme 2 is 0.4 m lower than the water level in Scheme 3 on average, but in the downstream of SheGong River, the water level is significantly higher than that in Scheme 3.

**Figure 9.** Simulation results of design water surface line.

In terms of the cost of land acquisition and house demolition, Scheme 3 is lower than Scheme 2. When looking into the water level reduction effect, Scheme 2 has a better reduction effect at the culvert, but Scheme 3 has a better reduction effect in the downstream area. Considering that the downstream area has a higher grade in the development planning, the final regulation scheme is to adopt Scheme 3, which is easier to implement in terms of economic cost and is more effective in reducing flood hazards in the downstream area.

Through the comparison of Schemes 1 and 2, it can be found that a sluice in the downstream of SheGong River can be set and a tributary in front of the sluice transferring floodwater from the upstream of SheGong River to the downstream of TieShan River can be opened, avoiding the widening of the downstream width and heightening dikes on both sides of the SheGong River. In Scheme 2, the downstream of SheGong River could remain the original width but the culvert would need to be widened, while in Scheme 3, the culvert needs to be slightly widened, or can even remain unchanged after opening up a tributary in front of the culvert. Considering the land acquisition and demolition, along with the simulation results of water surface lines, Scheme 3 is the optimal one.

#### **5. Discussion**

Due to high urbanization development, there is nearly no extra land around the downstream of the SheGong River that is surrounded by various kinds of buildings. The mid- and the downstream of the river are also the reaches with extremely serious flooding. Therefore, there is not enough space for flood control construction in the SheGong River. Large land occupation, high cost of land acquisition and demolition, difficult demolition work, local resident disputes have increased the difficulty of traditional river flood control measures. If one wants to reduce land acquisition and demolition and keep the original river width, then the embankment height should be at least 1.5 m above the ground, which is disharmonious to the surrounding environment. However, if one makes full use of the flood bearing capacity of the downstream of the Tieshan River and develops new tributaries, it would be an efficient and economical solution. The scope of land acquisition, which is mainly farmland, will be minimized, and the cost of compensation for land acquisition and demolition will be at its lowest. The reduction of land acquisition, demolition and compensation disputes, and the avoidance of raising river embankment make the flood control measures of the river simpler and reduce flooding risk. According to the flood simulation analysis, the flood bearing capacity of the TieShan River is still at a surplus. Flood water in the SheGong River can be transferred into the TieShan River so that the buildings around the SheGong River can remain and the work is greatly simplified, whilst the flood pressure in the mid- and downstream of the SheGong River is effectively reduced. This is from a whole region perspective, given that flood control of the SheGong River is not conducted independently in the SheGong River.

In this study, three possible flood control schemes are proposed. Scheme 1 is to simulate the design flood condition of the SheGong River without changing the existing river width. However, the bank elevation of the whole river in the scheme is lower than the design flood level; consequently the river channel will seriously flood. Due to the lack of flood bearing capacity in the middle and lower reaches of the SheGong River, the downstream of the SheGong River will suffer from flooding. A tributary can be opened in the downstream. There is a culvert in the midstream, which can be uncovered and widened or a tributary can be opened in front of the culvert. In Scheme 2, a flood gate will be built at the downstream of the SheGong River, and a tributary will be built in front of the gate to transfer floodwater to the downstream of the TieShan River. Meanwhile, the culvert at the midstream will be uncovered and widened. Scheme 3 is to open the tributary at the downstream of the SheGong River with the culvert at the midstream unchanged and build the second tributary in front of the culvert. Compared with Scheme 1, Scheme 2 effectively reduces flooding in the downstream of the SheGong River. Comparing Scheme 2 with Scheme 3, the design flood level of the original culvert in Scheme 2 is 0.4 m lower than that in Scheme 3 on average, and the design flood level of the downstream section is higher due to large flows, while tributary 2 in Scheme 3 can effectively reduce flooding pressure of the downstream of the SheGong River. Scheme 3 needs to build a new tributary because water flows in the bottom of the river in Scheme 2 are relatively large and the river needs to be widened. In terms of the cost of land acquisition and house demolition, Scheme 3 is lower than Scheme 2; for a water level reduction effect, Scheme 2 has a better reduction effect at the culvert, but Scheme 3 has a better reduction effect in the downstream. Considering that the downstream area has a higher grade in the development planning and that the cost of land acquisition and demolition cannot be neglected, Scheme 3 is preferable. This scheme makes full use of the surplus flood discharge capacity of the TieShan River from a whole basin perspective. Transferring river floodwater not only avoids a lot of land acquisition and demolition but also solves flooding in the SheGong River economically and effectively. With the rapid development of high urbanization in developing countries around the world, the problem of flood control and drainage in small and medium-scale river basins has become increasingly prominent. Therefore, this study can provide a feasible flood control measure for small and medium-scale rivers in other developing countries.

While this study provides a new idea of flood control, there are some caveats when the idea is applied in other regions. For one, MIKE 21 has high requirements for data; for tributaries and rivers, when simulating small-scale areas using the model, it needs a precise spatial resolution, whilst it is unable to predict elevation differences in floodplain modeling, which has its own significance in accurate results [54,55]. Second, there are some differences between the data obtained in the research area and the requirements of the model, and considering errors such as the generalization of the

river network and grids as well as the changes in working conditions in the model, the verification of the one-dimensional model and the two-dimensional model is seemingly simple. Third, we only tested three schemes in the study which are seemingly inadequate given that flood conditions across different regions are rather complicated, and therefore a variety of schemes should be tested, if allowed, for other regions. Last but not least, the development of tributaries will somewhat impact the ecological environment, which needs further study.

### **6. Conclusions**

To explore the most effective flood control strategy for small and medium-scale rivers in highly urbanized areas, this study took the SheGong River (the tributary of the TieShan River) basin as the case to demonstrate how flooding across small and medium-scale river basins can be controlled from a whole region perspective. The MIKE FLOOD model was coupled with MIKE 11 and MIKE 21 to simulate flooding in the basin for different return periods, and the results showed that the mid- and downstream areas of the basin suffer from serious flooding with a 20-year return period, yet the downstream of the mainstream, i.e., TieShan River, can still sufficiently bear more flooding. Therefore, it is possible to deal with flooding over the SheGong River basin by consideration of the flood bearing capacity of the mainstream TieShan River from a whole region perspective. Further, three possible flood control schemes were compared to determine the most cost-effective one. It showed that the construction of two new tributaries transferring floodwater in the mid- and downstream of the SheGong River into the downstream of the TieShan River is the best way to reduce flooding pressure in the SheGong River basin. The scheme of opening up new tributaries to replace the reconstruction of the original river channel can reduce a large number of land acquisition and demolition projects and in turn, reduces the compensation cost of land acquisition and demolition. In highly urbanized areas, the rivers with abundant flood bearing capacity can be fully utilized to control floods. When the flood bearing capacity of small and medium-scale rivers is inadequate, floods in the middle and lower reaches of the river can be transferred to the river with abundant flood-carrying capacity. The new idea of dealing with flooding from a whole region perspective can consider and make full use of the flood bearing capacities of both the mainstream and tributaries. This can not only avoid a large number of land acquisitions, demolitions and compensation fees in highly urbanized areas but also reduce the risk of river flood control, which is particularly useful for highly urbanized areas in developing countries around the world.

**Author Contributions:** Conceptualization, Z.L. and Y.C.; methodology, Z.L.; software, S.W. and F.L.; validation, Y.C.; formal analysis, Z.L. and Y.C.; investigation, S.W.; resources: X.W.; data curation, F.L.; writing-original draft preparation, Z.L. and Y.C.; writing-review and editing, X.W.; visualization, S.W.; supervision, X.W.; project administration, Z.L. and X.W.; funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was partially funded by the national key research and development project, Research and Demonstration of Key Technologies for Real-time Flood Control and Drainage Operation in Highly Urbanized Areas", and partially by Guangdong Water Conservancy Science and Technology Project and Guangzhou Water Science and Technology Project.

**Acknowledgments:** The authors wish to express their gratitude to all authors of the numerous technical reports used for this paper.

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **References**


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