MIKE 11 HD is mainly used for flood forecasting, dispatching measures, canal/irrigation system design, and dispatch and storm surge research. The basic aim of MIKE 11 HD is to provide hydrological factors, such as river water level and discharge process of each river section. It is solved by one-dimensional unsteady flow Saint-Venant equations based on vertical integration. Water level and discharge at each grid point are calculated alternately by the Abbott-Ionescu six points implicit scheme (Fig. 1), and the discrete equations are solved using the chasing method. 11B∂Q∂x+∂H∂t=qL2∂u∂t+∂u∂x+g∂H∂x+guuC2R=0 where H is the water level of the channel section, Q is discharge, u is average velocity, B is the width of the river cross-section, qL is lateral flow, R is hydraulic radius, C is the Chezy coefficient, and x and t are position and time coordinates.

MIKE 21 HD is mainly used to simulate two-dimensional free surface flow. This module can simulate changes in water level and discharge due to the impact of various forces in lakes, river mouths, and coastal areas. When the user provides terrain data, Manning coefficients, wind fields, and hydrodynamic boundary conditions, the model can simulate changes in water level and discharge in each grid. The model uses the finite difference method of alternating-difference implicit (ADI) second-term accuracy to solve the mass equation and momentum equation of dynamic flow. This model can describe various hydraulic phenomena, such as tidal exchange, tidal flow, storm surge, whirlpools, surface undulation, dam bursts, and tsunamis. The equations take into account many factors, such as Coriolis force (Ω), wind force (V), and atmospheric pressure (Pa). The variables ζ, d, h, p, and q represent water level, water depth, elevation, and discharge in the x- and y-directions respectively. 3∂ζ∂t+∂p∂x+∂q∂y=∂d∂t4∂p∂t+∂∂xp2h+∂∂ypqh+gh∂ζ∂x+gpp2+q2C2h2-1ρw∂∂xhτxx+∂∂yhτyy-Ωq-fVVx+hρx∂∂ypa=05∂q∂t+∂∂yq2h+∂∂xpqh+gh∂ζ∂y+gqp2+q2C2h2-1ρw∂∂yhτyy+∂∂xhτxx+Ωp-fVVy+hρx∂∂xypa=0

The MIKE Flood model dynamically couples the one-dimensional (MIKE 11 HD) and two-dimensional (MIKE 21 HD) models. The coupled model can take advantage of the benefits of both models, and can solve problems of spatial resolution and computation that often occur when the two models are used separately. In this study, the MIKE Flood model was used to simulate the two-dimensional flood evolution path, submerged area, and duration in urban areas as well as one-dimensional river flood overtopping and levee bursts.

MIKE Flood provides two different approaches to link one-dimensional and two-dimensional models, namely standard and lateral links, which are fitted for different occasions. An important part of MIKE Flood model application is link selection and creation.

Basic terrain data of Qiqihar are available on a 1:10000 scale, and are supplemented with road, house, and urban residence community datasets (Fig. 3). The datasets were converted into 20 m × 20 m grids using ArcGIS 9.3 software. The grid terrain data was input to the MIKE Flood model to simulate the urban submersion process in Qiqihar City.

Manning roughness coefficients (MRC) for urban terrain are discussed in DHI's global urban flood simulation projects in the past decades (Wang and Hartnack, 2006). Choice of different values of MRC will affect the flood peak time and to some extent the flow path within the urban area, but has little impact on the inundation area and flood depth (for a given design flood event, within the given flood duration the total breach volume could reduce as MRC increases). Roughness values are evaluated mainly on the basis of land use types. For instance, the roughness coefficient is set to 0.08 for developed areas, such as the leisure square with low buildings, gardens, and dense vegetation area; and in open space comprising grassland, sparse vegetation, streets, and pavements is set to 0.06. In similar projects in Germany, roughness coefficients in urban areas, streets, and gardens are set at 0.040–0.067 (considering the influence of pavements and vehicles parked on roads, the roughness coefficient of paths is larger); for vegetation are set at 0.05–0.2 (0.2 in dense young forest and 0.05 in open agricultural areas); and in other areas are set at 0.033–0.1, according to landscape features. Roughness coefficients in this study are not less than 0.033 (Fig. 4).

Based on the analysis above and field investigations, the principles of determining all-terrain roughness coefficients for Qiqihar are as follows:

Urban roads in the simulated city are generally wider, some main roads have central reservations, and some have green verges on both sides of the road. There are generally pavements on both sides and varying amounts of vehicles on the road at any one time. Therefore, the roughness coefficient of the main roads is set at 0.06.

Basic hydrological data were used for scenario simulation, including the once-in-a-century design flood process upstream of the Qiqihar hydrological station, the Q-H curve in the downstream section (Jiangqiao hydrological station) and design flood/peak/discharge in Nen River (Fig. 5).

Qiqihar City lies on the bank of the Nen River, which is upstream of Songhua River. The 1-D hydrodynamic model is established between CS31 and CS64, where the main stream of Nen River flows from 169 437 to 320 987 m (Fig. 6). The section position identification, such as 169 437 and 320 987 m, is a relative distance. The start point is at the dam of Nierji reservoir, which is a large and very important reservoir in the upper reaches of the Nenjiang River.

The upstream boundary condition of the model is the discharge of a 100-year return flood that occurred in August 1998. The downstream boundary condition of the model is the Q-H curve at Jiangqiao hydrological station in section CS64, which is more than 150 km from Qiqihar City and has little effect on the upstream water level.

After considering the topography and elevation of Qiqihar City, the real land elevation is defined as 165 m, which means regions where elevation is higher than 165 m are not included in the model. In order to satisfy the requirements of lateral links, the river is filled with land (because the river is calculated by a one-dimensional model and is not included in the two-dimensional simulation process).

The Nen River runs through the whole of Qiqihar City and divides the city into the main urban and Hulan Ergi regions. The model uses a 20 m × 20 m grid and the total calculated areas are 2081×1881 × 400 m2=1565.7 km2. The actual calculation areas cover about 430 km2, after removing the areas outside the boundary and inside the river, and urban areas that are not involved in simulation, as shown in Fig. 7. The boundary is set along the levee in the research area. The levee is surveyed and mapped by Songliao Water Conservancy Commission and provided in shape file of ArcGIS.

The complete design flood process over one month is simulated, with a time step of 3 s. In order to simulate the defence level of the Qiqihar City levee in resisting the Nen River flood and to analyse locations and times where dangerous overtopping in levees may occur, all urban levees are linked with the main stream of Nen River by lateral links.

The simulation of urban levee burst outfall is mainly used to analyse the urban inundation range, duration, and water depth distribution after the most dangerous levee section is broken. As there is no module in the MIKE Flood model designed especially for levee bursts, a water gate is used to simulate the burst process. When the water level in the river rises to the levee burst control threshold, the gate is opened, which corresponds to the levee having burst. The gate will not close until the flood is over. There are two water gates in this simulation, one to simulate the bursting of the west embankment in Qiqihar, and the other to simulate the flood diversion at Daang embankment (Fig. 8). MIKE 11 HD and MIKE 21 HD are linked by adding the virtual river at the two water gates.