# Dam Breach Size Comparison for Flood Simulations. A HEC-RAS Based, GIS Approach for Drăcșani Lake, Sitna River, Romania

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Study Area

^{2}[40]. Old historical sources state that the dam was built over the site of another earth dam at the beginning of the 20th century, and led to the formation of a lake with a length of 5 km and a surface of 13.6 km

^{2}[41]. Drăcșani Lake appears to be also mentioned in the historical writings from the 16th century, serving for storing irrigation water and flood attenuation.

^{2}and a river length of 78 km. The watershed is part of Jijia river basin. Relevant information concerning Sitna river basin highlight its hydrographic settings and include several calculated morphometrical coefficients, based on the digital elevation model data (Table 1).

#### 2.2. Chosen Area for Dam Breach Flood Simulations

^{3}of water, as stated in the official Flood Risk Management Plan, made by the Prut-Bârlad Water Basin Administration [43]. It holds an enormous volume of water, with high-risk potential for the villages located downstream.

#### 2.3. Data Aquisition

#### 2.3.1. Generating the Digital Elevation Model

#### 2.3.2. Hydrological Data

#### 2.4. Dam Breach Hydraulic Model

^{3}(which corresponds to an elevation of 81 m, along the dam section), introduced into the model based on the “Elevation vs. Volume curve” method. The chosen roughness values (Manning n values) were the default ones (0.06) since the land cover of Sitna river valley does not account for complex patterns, mainly consisting of pastures, agricultural land, and different grass formations. Prior to the simulations, testing was performed with Manning n values, in order to verify the difference in results. This implied performing several simulations, while leaving all parameters the same, except for the Manning values [66]. This revealed a calculated difference in flooded area of 0.97%, which is considered negligible, and this test was performed with the lowest Manning value possible, in accordance to the land cover characteristics of the study area (0.045 for the entire floodplain). It was verified if there are any relevant differences concerning velocity, flood extent or depth, and no such significant differences/results were discovered. This does not necessarily represent the real life scenario, but the intention was to compare any potential differences with a significantly lower Manning value. Previous studies have addressed the effects of mesh resolution on flood modeling, and have analyzed the topic of Manning values, which would be required to be very large, in order to have any significant impact on model predictions [62]. Furthermore, Manning values for the study area range from 0.045 to 0.07, therefore, an average value of 0.06 (the default value) is completely reasonable. In addition, because of the coarse resolution of the CLC land use layer, in conjunction with land use dynamics, too many uncertainty variables could have been introduced into the simulation. Regarding CLC layers, it is known that the methodology for their creation implies drawing polygons of a minimum area of 25 ha, which involves generalizing any fine cartographic details. This generalization is not recommended for high accuracy analyses. Therefore, in order to exclude potential calculation errors between all breach simulations, or induce unknown variables in the analysis, it was decided to standardize the Manning roughness coefficient for the entire floodplain, and leave it at default values.

^{3}, with a drainage basin of 943 km

^{2}), Belci dam was 415 m long, 4–8 m tall, storing a volume of 12.5 mil m

^{3}, with a drainage basin of 993 km

^{2}. Furthermore, both dams were covered with concrete slabs on the lake-side slope. Belci dam was breached by overtopping, due to exceptional rainfall, and maximum flow rate was recorded in approximately 2 h after the breach occurred. According to the official hydrological report of Siret Water Basin Administration, there was a total volume of 34.01 mil m

^{3}of water that flowed through the breach, during the flood. Considering this relevant comparative study, an empirically recommended breach formation time of half of the Belci event was included into the simulations (1 h). This information is backed up by previous studies, that mentioned failure times of earth dams between 15 min, up to 1 h [67]; or that 50% of earth dam breach formation times are under 1.5 h [68].

## 3. Results

^{3}/s, and was derived using the flood frequency curve. Based on historical hydrological data, this would be the predicted value of a yearly flood. Resulting times were rounded to 15 min increments, as this parameter is preferred to be considered as “estimated time”, rather than “exact”, as precise calculations would be impossible, due to the numerous factors that would influence them in a real-life scenario. These factors range from potentially larger flow-rates from upstream, sediment-related aspects of the bottom of the lake, residual water volume after the break, etc. In addition, the values corresponded to the expected results, when considering a comparison to the only similar dam break accident in Romania (Belci earth dam break by overtopping, 29 July 1991), which emptied out its 34.1 mil m

^{3}volume (not designed volume, but a real, upstream flood-generated, total water volume) in 5 h (according to an official hydrological report of Siret Water Basin Administration). Calculations revealed that this historical accident corresponded to a breach size of 17%, of the total dam size.

## 4. Discussion

#### 4.1. General Discussion

#### 4.2. Backwater Discussion

^{3}/s. This is followed by negative values, which represent the reversal of the flow, back towards downstream (Figure 8a).

^{3}/s, for P1, and 1730 m

^{3}/s, for P2), as well as the average flow values. Secondly, flood duration was larger for the downstream cross-section. The backwater flooding simulations for all 12 breach sizes reached peak extent in a time interval that ranged from a minimum of 55 min (100% breach size) to a maximum time of 1 h and 15 min (1% breach size). Afterwards, it started to flow backwards, in a regular, downstream-oriented manner.

^{3}). The lowest influence was of 0.19% (42,500 m

^{3}), for the 1% breach size, while the largest was of approximately 3.9% (86,600 m

^{3}), for the 100% breach, with intermediate values of 1.72%, 2.78%, 3.44%, and 3.76%, for the 5%, 10%, 20%, and 50% respectively. As previously mentioned, all of these values aid in moderating the high peak flow rates for the given simulations.

#### 4.3. Temporal Aspects

- The first class (the 1% scenario) corresponded to a delay of 7 h and 50 min. It posed little to no risk to the local population. Additionally, the flood extent would be limited enough, that it would only reach a reduced number of buildings.
- Second class was comprised of the 5% and 10% simulations, with an average delay of 6 h and 15 min. This was still considered to have low risk, during which warnings could reach the population early enough, that potential casualties would be completely preventable.
- The last, and most dangerous class, grouped the 20% to 100% scenarios, with an estimated delay ranging from 2 h and 30 min (100% breach) to 3 h and 30 min (20% breach), posing significantly less warning time, in addition to the high flow rate values, which would cause considerably more potential damages and casualties.

^{3}/s, which we considered to be a critical, reference value for this particular case. Taking into account the fact that there was a significant difference in flow rates between scenarios lower than 10%, and greater than 10%, all modeled simulations were virtually divided into two classes: critical and non-critical. Therefore, flow rates surpassed the 1000 m

^{3}/s threshold, for different time spans, as follows: 55 min for the 100% flow rate, 65 min for 50%, 75 min for 20%, 40 min for 10%, while at 5%, registering an estimated 600 m

^{3}/s.

#### 4.4. Flood Mitigation Aspects

#### 4.5. Validation

^{2}drainage basin, 24.8 mil m

^{3}of storage capacity, and comparable downstream slope) [52]. For this particular case, average velocities ranged from 0.5 to 1 m/s and flow rates between 1450 and 2748 m

^{3}/s were generated, offering similar results with the current study (maximum of 1730 m

^{3}/s, for P2). Furthermore, flood hydrographs revealed similarly shaped attenuation curves. This comparison is even more relevant, when accounting for the fact that this particular reference study was validated by comparison to real life dam failure data, also confirming the highly similar results of different Manning coefficients for the modeling. Furthermore, breach sizes were chosen according to literature references centralizing tens of real earth dam break cases, and covered a wide range of scenarios, with case studies concluding that breach sizes could get significantly large 26%/60% [64] regularly 2–5 times wider than the the dam height [65].

^{3}vs. 22.22 mil m

^{3}), dam dimensions and characteristics (415 m long, 4–8 m high vs. 610 m long, 5.85 m high), drainage basins (993 km

^{2}vs. 943 km

^{2}), and large breach size, Belci dam was used as a reference for the current outputs. This correlation was reasonably high, regarding reservoir evacuation time (Belci had an average evacuation rate of 6.82 mil m

^{3}/h, while Sulița reached very similar values), peak flow-rate values (1730 m

^{3}/s for Sulița vs. 2100 m

^{3}/s for Belci), and breach size dimensions (which in the case of the Belci dam was 17%). Furthermore, result values for all simulations fell in line with the expected results [22].

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Geographical location of Drăcșani Lake and the downstream, corresponding study area a. Location at county-level; b. Location of Sitna watershed in Botoșani county; c. Location of Sitna river, its main tributary (Burla stream), and the three hydrometrical stations from Sitna watershed; and d. Location of Drăcșani Lake, and the river sector for which the simulations were performed (between Drăcșani Lake and the confluence with Jijia river).

**Figure 3.**Cross-section sketch of Sulița Dam [50].

**Figure 5.**Water velocity and time travel values for representative breach sizes (1%, 10%, and 100%).

**Figure 6.**Worst (left) and best (right) scenarios, concerning the location of built-up areas (100% breach size).

**Figure 8.**Flow rate hydrographs of upstream (

**a**), and downstream (

**c**) cross-sections, with the associated cross-section depth graphs ((

**b**), and (

**d**) respectively).

Relief Ratio | Drainage Density | Form Factor | Circularity Ratio | Elongation Ratio |
---|---|---|---|---|

5.28 | 1.31 | 0.21 | 0.14 | 0.52 |

**Table 2.**Parameter comparison results for the given flood simulations, corresponding to each breach size.

Breach Size | NFB | TFA (ha) | MWD (m) | MWV (m/s) | FPT(h) | RET (min) | AWV (m/s) |
---|---|---|---|---|---|---|---|

1% | 36 | 1179.70 | 8.44 | 8.78 | 10 h 13 min | 54 h | 0.35 |

5% | 214 | 1876.74 | 9.95 | 8.80 | 7 h 59 min | 32 h | 0.57 |

10% | 233 | 1903.52 | 9.97 | 8.82 | 7 h 48 min | 23 h | 0.59 |

20% | 235 | 1908.71 | 10.01 | 8.83 | 5 h 41 min | 20 h | 0.60 |

30% | 238 | 1909.71 | 10.02 | 8.84 | 5 h 37 min | 14 h | 0.62 |

40% | 240 | 1913.38 | 10.05 | 9.19 | 5 h 32 min | 11 h | 0.62 |

50% | 241 | 1918.36 | 10.06 | 9.28 | 5 h 26 min | 8 h 15 min | 0.63 |

60% | 241 | 1921.03 | 10.07 | 10.13 | 5 h 21 min | 7 h 15 min | 0.63 |

70% | 242 | 1922.40 | 10.07 | 11.65 | 5 h 20 min | 6 h 30 min | 0.63 |

80% | 242 | 1926.84 | 10.08 | 12.91 | 5 h 19 min | 5 h 30 min | 0.63 |

90% | 242 | 1929.75 | 10.10 | 13.55 | 5 h 19 min | 4 h 15 min | 0.63 |

100% | 376 | 2068.34 | 11.76 | 21.78 | 5 h 18 min | 3 h 15 min | 0.68 |

**Table 3.**Flooded areas, by land use category, according to Corine Land Cover 2018, for all breach sizes.

CLC Code | Flooded Areas (ha) | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

1% | 5% | 10% | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% | |

112 ^{1} | 40.3 | 85.8 | 87.2 | 86.3 | 87.7 | 82.9 | 85.3 | 86.9 | 87.2 | 87.1 | 86.4 | 110.6 |

211 ^{2} | 204.7 | 339.8 | 343.9 | 340.5 | 347.1 | 329.0 | 337.6 | 343.1 | 344.4 | 343.9 | 341.6 | 398.7 |

221 ^{3} | - | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 |

222 ^{4} | 3.6 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 5.3 | 6.5 |

231 ^{5} | 771.1 | 1095.8 | 1106.3 | 1094.0 | 1107.0 | 1092.1 | 1093.3 | 1099.0 | 1101.4 | 1100.6 | 1096.7 | 1163.3 |

242 ^{6} | 25.5 | 108.1 | 110.0 | 109.4 | 107.7 | 94.7 | 108.4 | 109.6 | 109.4 | 109.5 | 109.0 | 105.8 |

243 ^{7} | 117.3 | 164.5 | 164.6 | 164.7 | 165.3 | 164.8 | 164.2 | 164.9 | 165.0 | 165.0 | 164.8 | 173.2 |

^{1}Discontinuous urban fabric;

^{2}Non-irrigated arable land;

^{3}Vineyards;

^{4}Fruit trees and berry plantations;

^{5}Pastures;

^{6}Complex cultivation patterns;

^{7}Land principally occupied by agriculture, with significant areas of natural vegetation.

Study Results (10% Breach Size) | 2D Unsteady (10% Breach Flow Rate) | Correlation | |
---|---|---|---|

Area (m^{2}) | 20,683,352.5 | 20,403,841.6 | 98.70% |

Official 0.1% flood extent | 2D Unsteady flow—0.1% recurrence | Correlation | |

Area (m^{2}) | 9,466,001.6 | 8,537,745.3 | 90.20% |

Study results (50% breach size) | Full momentum (50% breach size) | Correlation | |

Area (m^{2}) | 19,183,623.66 | 17,987,918.28 | 93.80% |

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Albu, L.-M.; Enea, A.; Iosub, M.; Breabăn, I.-G.
Dam Breach Size Comparison for Flood Simulations. A HEC-RAS Based, GIS Approach for Drăcșani Lake, Sitna River, Romania. *Water* **2020**, *12*, 1090.
https://doi.org/10.3390/w12041090

**AMA Style**

Albu L-M, Enea A, Iosub M, Breabăn I-G.
Dam Breach Size Comparison for Flood Simulations. A HEC-RAS Based, GIS Approach for Drăcșani Lake, Sitna River, Romania. *Water*. 2020; 12(4):1090.
https://doi.org/10.3390/w12041090

**Chicago/Turabian Style**

Albu, Liviu-Marian, Andrei Enea, Marina Iosub, and Iuliana-Gabriela Breabăn.
2020. "Dam Breach Size Comparison for Flood Simulations. A HEC-RAS Based, GIS Approach for Drăcșani Lake, Sitna River, Romania" *Water* 12, no. 4: 1090.
https://doi.org/10.3390/w12041090