This study examines the usability of unmanned aerial vehicle (UAV) data surveyed just after an agricultural reservoir collapse by comparing the survey results with the simulation results of the HEC-RAS (Hydrologic Engineering Centers River Analysis System) flood wave propagation to the downstream areas. A 61,400 m 3 storage dam broken by 89.0 mm (over 30.0 mm/hr rainfall intensity) of rainfall on August 21 st, 2014 was considered. The reservoir water capacity curve and downstream damaged areas were estimated by drone surveying 3 days after the dam break. The flood wave by the overtopped dam break was propagated using the HEC-HMS (Hydrologic Engineering Centers Hydrological Modeling System) reservoir inflow from the watershed. The model results showed flood inundation depths of 0.1 to 2.2 m, mainly in rice paddy areas along the stream, and the overtopped dam-break scenario exhibited 59% correspondence with the drone-surveyed areas.
Dam break is a serious devastating disaster that occurs around the world and causes large amounts of damage to people worldwide [
Recently, many studies on dam break have been performed, with the resulting damaged areas estimated using many approaches. Carling et al. [
The prediction of flood magnitude at ungauged reaches is an important task in designing river engineering and hydraulic structures and remains a fundamental challenge for hydrologists [
A high-resolution DTM or DSM is critical to extract the details of the channel topography, obtain the water surface elevations at the cross sections, and simulate flood inundation and depth [
The overall goal of this study was to analyze the flood-damaged area by dam break with typical hydrologic and hydraulic modeling in ungagged watershed and identify collapse type in actual broken reservoir. The specific objectives of the study were as follows: 1) to generate DSM and reference inundation map from UAV data within reservoir and downstream topographies of actual damaged areas for a 61,400 m3 storage dam that was broken by 92.6 mm rainfall event with rainfall intensity over 30.0 mm/hr on August 21st, 2014 for calibration of watershed parameters in the modelling, 2) to simulate two scenarios (overtopping and piping) using HEC-HMS (Hydrologic Engineering Centers Hydrological Modeling System) model, 3) to simulate inundation area using HEC-RAS model from estimated flood discharge, and 4) to identify collapse type compared with inundation map between two scenarios.
The broken dam is located in the southeastern part of South Korea, with a latitude and longitude of 35.895˚N and 128.96˚E, respectively. The Goeyeon reservoir dam was built in 1945 for rice paddy irrigation water supply to the downstream area. The dam was broken by the 92.6 mm rainfall event with over 30 mm/hr rainfall intensity in August 21st, 2014 after reaching the maximum water level to the bank top. The cause of reservoir collapse was predicted as overtopping by the rain storm, but was not certain with piping possibility.
A rotary-wing drone of 3.5 kg weight carried with the Sony NEX-5T sensors was used to acquire the image data (
Divisions | Detail |
---|---|
Model | GEO X-10S/E |
Dimension | 1300 × 1300 × 140 mm |
Weight | 3.5 kg |
Propeller | 8 propellers |
Image resolution | (4 cm × 4 cm)/pixel |
Sensor | Sony NEX-5T |
Post processing program | Terra 3D program (Ortho-image and Digital Surface Model production) Virtual Surveyor Tools (DSM points convert to CAD file) Civil Pro (Converted points extract contour line) |
Observation period | August 17 To 21, 2014 |
3D program. The DSM points converted to CAD file using the Virtual Surveyor Tools and then extracted contour line by using the Civil Pro program.
In order to orient and relate the drone images to the ground, twenty GCPs (Ground Control Points) were investigated and four checking points were arranged for uniform horizontal and vertical accuracy. With 60% end laps as well as 60% side laps, the drone routed two times for 0.43 km2 areas with total 338 images of first 153 images and second 185 images respectively. In this study, calibrated and validated UVA geospatial data obtained from the Korea Rural Development Administration (KRDA). Details regarding the estimation process of the UVA geospatial data can be found in Park and Park [
HEC-HMS and HEC-RAS were used to develop the dam break and inundation modeling. Numerous previous studies have shown these models to provide accurate and useful results in flood-related studies [
The watershed runoff to reservoir was modeled using HEC-HMS. HEC-HMS was developed by the US Army Corps of Engineers and designed to simulate the rainfall-runoff processes of dendritic watershed systems. During the runoff process, the infiltration capacity (Ia) is quantified by the Soil Conservation Service-Curve Number (SCS-CN) based on land use and hydrologic soil group. The runoff is calculated from the following set of empirical equations:
Q = ( P − I a ) 2 / ( ( P − I a ) + S ) (1)
I a = 0.2 S (2)
S = ( 1000 / CN ) − 10 (3)
Q = ( P − 0.2 S ) 2 / ( P + 0.8 S ) (4)
where Q is the runoff, P is the rainfall, S is the maximum potential retention, I a is the initial abstraction, and CN is the runoff curve number. The flood wave propagation by dam breakage was modeled using HEC-RAS. HEC-RAS calculates one-dimensional steady and unsteady flow, and the model equations are described by Horritt and Bates [
Q = V 1 A 1 = V 2 A 2 (5)
Q = C S ¯ f (6)
C = 1 n A R 2 / 3 (7)
K = T c 1.46 − 0.0867 L 2 A (8)
T c = 0.444 L S 0.515 (9)
where Q is the flow discharge (m3/s), V i is the average velocity at cross-section i (m/s), A i is the area at cross-section i (m2), R is the hydraulic radius (m), n is Manning’s roughness coefficient, C is the conveyance (m5/3), S ¯ f is the average friction slope between adjacent cross-sections, A is the watershed area (km2), L is the river length (km), S is the watershed slope, K is the storage constant (hr) from Kraven equation, and T c is the concentration time (hr) from Sabol equation.
The dam break was established using HEC-HMS, developed by the US Army Corps of Engineers. HEC-HMS allows the modeler to choose two different methods for computing outflow through two dam-breach options: overtopping and piping.
Two cases were applied in this study because the break cause was unknown. The ‘overtopping dam break’ is designed to represent failures caused by overtopping of the dam. This failure is most common in earthen dams but may also occur in concrete arch, concrete gravity, or roller-compacted dams as well. The failure begins when the appreciable amount of water begins flowing over or around the dam face. Another method is designed to represent failures caused by piping inside of the dam, called “piping dam-break”. This failure typically occurs only in earthen dams. The failure begins when water naturally seeping through the dam core increases in velocity and a sufficient quantity of fine sediment moves out of the soil matrix.
If a sufficient amount of material erodes, then a direct piping connection may be established from the reservoir water to the dam face. Once such a piping connection is formed, it is almost impossible to stop the dam from failing. The method begins the failure at a point in the dam face and expands it as a circular opening. When the opening reaches the top of the dam, it continues expanding as a trapezoidal shape. The flow through the circular opening is modeled as orifice flow, whereas it is modeled as weir flow in the second stage [
To apply dam break model, we need information on the collapse width, slope, and shape for collapse duration, cause of collapse, inflow hydrograph, and downstream cross section. The Goeyeon reservoir is a small reservoir and has no specifications related to the reservoir.
Collapse information | Reservoir information | ||||||
---|---|---|---|---|---|---|---|
Height (m) | Width (m) | Slope | Dam Type | Dam crest (EL. m) | Storage capacity (m3) | Reservoir length (m) | Drainage area (ha) |
5.5 | 20.0 | 1:1 | Fill dam | 183.7 | 61,000 | 160.0 | 125.0 |
The one-dimensional hydraulic model of HEC-RAS is widely and commonly used to analyze the water surface profile, flow characteristics, and hydraulic structure. HEC-RAS computes the water surface elevation and velocity at successive cross-sections by solving continuity, energy, and flow resistance equations in a scheme called the standard step-backwater method [
Using the UAV, the DSM (Digital Surface Model) was obtained on August 21st, 2014 under the broken dam with almost empty reservoir and downstream damaged area exposed condition. The DSM data were collected from a UAV to extract the detailed topography of the channel and inside the reservoir to ensure reliable hydraulic modeling results (
UAV images taken by Park and Park [
V o l u m e m = ∑ ( C e l l m × V o l u m e c e l l ) (10)
V o l u m e a = V o l u m e m ( = m i n ) + ⋯ + V o l u m e m ( = s p e c ) (11)
where Volumem is the volume of water at each unit elevation (meter), Volumecell is the volume of each unit cell, Volumea is the accumulated volume from the minimum to a specific elevation, Volumem(=min) is the volume when the water elevation is 1, and Volumem(=spec) is the volume when the water reaches to a specific elevation. Because the volume of each unit cell is 1 m3 (1 m length × 1 m
width × 1 m depth), the depth value of cell is equal to its volume.
The reported irrigation area of the reservoir is 125 ha and the effective storage capacity is 61,000 m3. Because of the lack of basic data such as dam minimum and maximum water levels in reservoir, we estimated water level in reservoir using accurate topographic map. The estimates of the topographic maps are overestimated. In order to calibrate this, we have derived the water level-water surface relation by setting the reported effective storage capacity of 61,000 m3 to the maximum water level. As a result, the minimum water level is EL. 176.77 m. The area and the storage capacity of each water level are shown in
Elevation (EL. m) | Storage Area (m2) | Storage Capacity (m3) |
---|---|---|
178.2 | 1370 | 1100 |
178.4 | 2750 | 3000 |
178.6 | 5500 | 6000 |
178.8 | 8100 | 10,000 |
179.0 | 14,000 | 19,000 |
179.2 | 24,750 | 35,000 |
179.4 | 35,750 | 50,000 |
179.6 | 38,500 | 61,000 |
Calibration of the model with appropriate data is a crucial step in the creation of a reliable basin representation. Watershed parameters such as infiltration coefficients, time of concentration, and baseflow may need modification to produce a best fit between model and observations. However, this study watershed as ungagged watershed has not directly measured runoff at reach or reservoir inflow. So, watershed parameters such as CN, K, TC were calculated from Equations ((3), (8), and (9)) without optimization process with observed runoff. Nevertheless, because the model calibration process is necessary, the watershed parameters were adjusted using actual flood map from UAV images.
F = ( A o p A o + A p − A o p ) × 100 (12)
where A o is the UAV observed area of inundation, A p is the HEC-RAS-predicted area of inundation, and A o p is the overlapped area between A o and A p . The
Watershed characteristics | Reservoir information | ||||
---|---|---|---|---|---|
Area (km2) | L (km) | Slope (%) | CN | Tc (hr) | K (hr) |
4.9 | 3.2 | 34.9 | 86.7 | 2.44 | 1.91 |
F statistic varies from “100” when observed and predicted areas coincide perfectly to “0” when there is no overlap between the predicted and observed areas.
The inundation map indicated using UAV is the actual observation data, so it can be seen as flood trace. Two methods were used to evaluate flood inundation according to the collapse runoff of Goeyeon reservoir for the most suitable floodplain. The first is a simple comparison by area, and the second is the comparison of the flood trace using the Lee Sallee Shape Index (LSSI) method. The LSSI method is a method of measuring the degree of agreement between two data by overlapping the reference data with the measurement data [
Scenario | Flood area (m2) | F (%) | A - B (m2) | A∩B (m2) | A∪B (m2) | A∩B/A∪B |
---|---|---|---|---|---|---|
Piping (B) | 15,931.3 | 54.8 | 3084.9 | 8857.2 | 16,398.2 | 0.54 |
Overtopping (B) | 11,918.4 | 59.0 | 2077.9 | 9864.2 | 12,552.9 | 0.79 |
Observed UAV (A) | 11,941.9 |
A: Inundation trace map using UAV, B: Flood inundation maps by scenario.
11,941.9 m2 (Ao) and the HEC-RAS dam break simulated damaged areas for (a) overtopping with 11,918.4 m2 (Ap_over) and (b) piping with 15,931.3 m2 (Ap_pipe) respectively. The UAV overlapped areas for overtopping (Aop_over) and piping (Aop_pipe) simulations were 9864 m2 and 8857 m2 with F values of 59.0% and 54.8% respectively.
For overtopping dam break, the flood depth ranged from 0.1 to 2.2 m after the peak discharge of 15 - 20 mins. As seen in
In this study, the flood-damaged areas caused by a dam-break accident were investigated using a UAV, and the collected data were compared with the HEC-RAS simulation results. For a small agricultural reservoir of 61,400 m3 water storage broken by 89.0 mm rainfall event in August 21st, 2014, the 2 dam-break scenarios of overtopping and piping were tested. To estimate the HEC-HMS parameters for the elevation-storage relationship of the reservoir, the high-resolution DSM was collected from drone surveying. The dam-break results by HEC-HMS were transferred to the HEC-RAS as the boundary conditions, and the flood areas and depth were expressed by using the HEC-GeoRAS module over the DSM map.
The peak discharges for overtopping and piping failure were 107.9 m3/sec and 140.2 m3/sec, respectively, with flood inundation depths of 0.1 - 2.2 m in the downstream area. From the distribution of the inundation areas from each dam-break scenario, we could infer that the dam was broken by the overtopping phenomenon. The discordance between the areas increased when it goes to the downstream area. The paddy rice near the dam was lodged by the fast velocity of the flood wave, but the far-downstream rice was less affected by the attenuated flood wave and recovered after drainage. In addition, the UAV surveying proved to be a valuable tool for the identification of flood-damaged areas and for economic loss evaluation.
The result of the dam-break in this study showed that the reservoir collapsed by overtopping. Overall, we proposed the application method of UAV data in ungagged area such as small reservoir watershed. The result from this study can be used to predict the risk of collapse of old and small reservoirs. Therefore, small and old reservoirs must once again make a safety review in ungagged area where flooding has occurred in the past using UAV and modeling to prevent reservoir collapse. Then, action plan is provided for each reservoir level.
This research was supported by a grant (17AWMP-B079625-04) from the Water Management Research Program funded by the Ministry of Land, Infrastructure and Transport of the Korean government.
Jung, C.-G. and Kim, S.-J. (2017) Comparison of the Damaged Area Caused by an Agricultural Dam- Break Flood Wave Using HEC-RAS and UAV Surveying. Agricultural Sciences, 8, 1089-1104. https://doi.org/10.4236/as.2017.810079