1. Introduction

2. Study Area

3. Necessity of Urban Flood Forecasting for Dhaka City

4. Approach, Methodology and Data Collection

4.2. Data Source:

Meteorological data including rainfall (3 hourly), evaporation (monthly historical), temperature (monthly historical) from Bangladesh Meteorology Department (BMD, https://live6.bmd.gov.bd ), and hydrological data such as water level and discharge from Bangladesh Water Development Board (BWDB, http://hims.bwdb.gov.bd/hydrology ) have been collected. Additionally, topographical information including land levels and cross-sections of drainage khals, sewerage network details, and information on drainage structures such as regulators and pumps, have been obtained from Dhaka Water Supply and Sewerage Authority (DWASA, https://dwasa.org.bd ). Infrastructure data including roadways, railway levels, and flood protection embankments have also been surveyed by Institute of Water Modelling (IWM, https://iwmbd.org ). All Collected data were meticulously checked using the standard procedure to ensure accuracy. Several field visits were conducted to oversee the quality control of survey procedures and verify the model. Under this study spot-level (Figure 4) data is collected for correcting the existing DEM data of Dhaka city.

4.2.1. Data Collection for Rainfall Runoff Model

Real time and Forecast Weather Data: Several organizations produce weather forecasts for Dhaka City using simulations of numerical weather models. The Bangladesh Meteorological Department (BMD) simulates the Weather Research and Forecasting (WRF) (https://live6.bmd.gov.bd/?/nwp-products-3km/&/wrf-model ) model to produce numerical weather predictions with a horizontal resolution of 3 km and a temporal resolution of 3 hours [43]. In contrast, other models provide forecasts at much coarser resolutions. BMD's weather forecast is generated for three days, though the accuracy decreases with increasing lead time. A 24-hour BMD forecast is typically used in our study model to predict the flood situation for the subsequent 24 hours. An example of a one-day forecast is illustrated in Figure 5 and 3 hourly real-time rainfall and temperature data is collected from BMD for model development.

4.2.2. Data Collection for 1D Hydrodynamic Model

Water Level and Discharge Data: Boundary condition time series for the peripheral rivers were obtained from multiple sources to accurately represent the hydrodynamic conditions of the river model of the study. Three upstream inflow (discharge) datasets and two downstream water level time series were collected from the Flood Forecasting and Warning Centre (FFWC). Additionally, intermediate water levels along the peripheral rivers, as well as water level measurements on both the city side and river side of the major pump stations, were collected for model verification from different sources. A detailed summary of these data sources and their use in the model is explained in Table 1.

Cross-Section Data: In the MIKE+ river and collection system, the computational domain is represented as a series of cross-sections perpendicular to the flow direction[44,45]. To develop a 1D hydrodynamic model for Dhaka City, cross-sectional surveys of rivers and canals in the region have been conducted. Depending on field conditions, these surveys were carried out using either leveling instruments combined with sounding, echo sounders, or a combination of both. A total of 174 cross-sections from 11 different rivers and canals in and around Dhaka City were collected through field surveys, while an additional 169 cross-sections were obtained from the IWM database and other sources, as detailed in Table 2 and Figure 6.

Canal Network: A total of 120.25 km canal network including dimension has been collected from the DWASA and incorporated into the model. Figure 6 and Figure 12 are showing the canal distribution in the model.

Pipe Network and Manholes: 160.1 kilometer pipe network and 1161 nos manhole data are collected from Dhaka WASA and Dhaka Noth (https://dncc.gov.bd ) and South City Corporation (https://dscc.gov.bd ). The Efficiency of pipe networks based on the field observations is adjusted in the models. All manholes with different diameters are connected to the 2D domain that are shown in Figure 6, Figure 7 and Figure 12.

Figure 7. Drainage zones, sluice gate, embankment and road network.

Figure 7. Drainage zones, sluice gate, embankment and road network.

Hydraulic Structure: Khals inside Dhaka city is controlled by the different types of hydraulic structures, embankment crest level (Figure 8), and pump stations. Various types of information about existing structures like invert level, gate height etc., have been collected from the field. The structural information incorporated into the model is shown in Figure 7 and Figure 12.

Rule Curve of Pump Operation: There are four major pump stations in Dhaka city. All these pump stations have their own command area and specific rule curves. These rule curves are collected from the associate authority and incorporated into the model. The main purpose of these pump stations is to drain out the domestic water and runoff from the countryside to rivers. Figure 6 shows the pump locations, command area and the connection with peripheral rivers.

4.2.3. Data Collection for 2D Overland Flow Model

Topography Data: The topography for the 2D overland flow model was developed using multiple data sources to ensure accuracy and detail. A 5m by 5m high-resolution Digital Elevation Model (DEM) was constructed by integrating the Bangladesh national DEM from the Institute of Water Modelling (IWM, iwmbd.org), recent spot level survey data obtained from the Survey of Bangladesh (SoB, sob.gov.bd), and road spot level data specifically collected for this research project in 2022. All datasets were thoroughly evaluated through field verification to ensure consistency and reliability. Figure 4 illustrates the distribution of spot level data points used in the model, demonstrating how these various sources were merged to generate a refined DEM that accurately represents the topographical features of the study area.

Water Retention Pond: All pump stations have their own retention pond. These retention ponds carry stormwater runoff and domestic water of residential areas and support wastewater treatment and prevent flooding. These retention ponds are included in the 2D domain by interpolation of survey spot level inside the retention ponding area. Retention ponds are connected to the pump stations and corresponding drainage canals and internal khals using standard links in the model. All retention ponds are shown in Figure 6.

5. Model Development

5.4. Conceptual Background of 2D Overland Flow

MIKE+ 2D overland module uses DHI's 2D engine MIKE 21 FM. This engine solves the two-dimensional St. Venant (dynamic flow) equations, using a cell-centered finite volume method. The time integration is performed using an explicit scheme and the numerical solution uses a self-adapting time step for optimizing stability and simulation times. The spatial discretization can either be done through a rectangular grid or a flexible mesh. The 2D overland module can be used to simulate free-surface flows to describe detailed flows in channels or describe surface floods from e.g., surcharging collection system networks or rivers.

MIKE 21 Flow Model is a modeling system for 2-D free-surface flows. The two-dimensional model performs simulation on a two-dimensional grid. The two-dimensional grid can be a normal rectangular grid or a mesh. The modeling system is based on the numerical solution of the two/three-dimensional incompressible Reynolds averaged Navier-Stokes equations subject to the assumptions of Boussinesqand of hydrostatic pressure. Thus, the model consists of continuity, momentum, temperature, salinity and density equations and it is closed by a turbulent closure scheme.

The two-dimensional shallow water equations in conservative form can expressed as

$${\displaystyle \frac{\partial h}{\partial t}}+{\displaystyle \frac{\partial hu}{\partial x}}+{\displaystyle \frac{\partial hv}{\partial y}}=0$$

(1)

(2)

(3)

Here, t is the time; x and y are the Cartesian coordinates; ℎ = η + d is the total water depth, where η is the surface elevation and d is the still water depth; u and v are the depth averaged velocity components in the x and y direction; f = 2Ωsinϕ is the Coriolis parameter (Ω is the angular rate of revolution and ϕ the geographic latitude); g is the gravitational acceleration; pA is the atmospheric pressure at the free surface; ρ is the density of water; ρ0 is the reference density of water. (τfx, τfy) are the x- and y- components of the stresses due to bottom friction, surface friction and flow resistance. and (τsx, τsy) are the x- and y-components of the surface stresses due to the wind. Fv = (Fvx, Fvy) is the drag force due to vegetation. The lateral stresses, Txx, Txy and Tyy, include viscous friction, turbulent friction, and differential advection. They are estimated using an eddy viscosity formulation based on the depth averaged velocities.

5.6. Model Setup

MIKE+ consists of three main modules: (a) rivers, collection systems, and overland flows; (b) SWMM5 for collection systems and overland flows; and (c) Water Distribution. In this study, the first module has been utilized which can simulate surface runoff, river, canal and drainage systems along with overland flows to represent the complete flood dynamics. Under this module, there are four features:

• Catchment: A hydrological model for simulating surface runoff within the catchment using various methods of MIKE+.
• Collection system network: A 1D hydrodynamic model capable of simulating pressure flow and backwater effects in drainage systems with various structures, incorporating both stormwater and domestic water inputs.
• River Network: A pure 1D hydrodynamic model that operates simultaneously with the collection system and catchment runoff, enabling interaction between these systems.
• Overland Flows: A 2D hydrodynamic model for floodplains and urban areas that simulates surface flooding and dynamically interacts with the river network and collection system using different linking structures.

The processing of the data for simulation in the MIKE+ hydrodynamic model (rivers, collections system, and overland flows) involves, preparation of the river network, cross- section, collection network, catchment runoff model, boundary condition data, mesh generation (for overland flows) and coupling between 1D and 2D by different links and structures are described step by step below.

5.6.1. Surface Runoff Model

Surface Runoff Model has been developed using MIKE+ RR module. Basic steps are adopted in developing surface runoff model:

• Delineation of sub-catchments
• Choosing runoff model and parameters
• Connection with canal and river.

886 sub-catchments have been included in the surface runoff model (Figure 6).

5.6.2. River Network

The River Network file allows to 1) define the river network and reference cross-sections and hydraulic control structures to the network [50]; and 2) graphically obtain an overview of the model information in the current simulation [51]. Figure 12 shows the detailed river network.

Figure 12. 1D hydrodynamic network coupled with 2D hydrodynamic model: lateral, standard, and manhole couplings with the floodplain.

Figure 12. 1D hydrodynamic network coupled with 2D hydrodynamic model: lateral, standard, and manhole couplings with the floodplain.

5.6.3. Cross-Section

The Cross-Section file contains streambed and canal cross-sections at specified locations along a river network and canal. The geometry of cross sections is obtained from a field-surveyed or Digital Elevation Model and it provides detailed spatial representation that is essential for accurate hydraulic modeling [52]. Figure 6 is showing the detailed cross-section distribution in the 1D network system.

5.6.4. Collection System

The Collection (Pipe and canal) Network file allows the modeler to define the collection network and reference cross-sections or shapes of the canal and control structures to the collection network. Detailed collection network along with structure and river network for this study model is shown in Figure 12. Zone wise total length of the collection system and structure are available in the Figure 6 and Figure 7 those are used in this network model.

5.6.5. Mesh Generation

The MIKE 21 FM (another module of MIKE Zero platform) model features an integrated mesh generator that constructs meshes using both triangular and quadrilateral elements, facilitating flexible spatial resolution to accurately represent complex geometries, including retention pond, bathymetric features of river, road and open area that are room for storing rainwater. Flexible mesh is used in this study. High-resolution meshes are applied in critical areas with steep gradients, road and flood vulnerable areas, while coarser meshes are used in less sensitive regions to optimize computational resources that is illustrated in Figure 12. Each element represents one calculation node. The quality of the mesh, particularly in terms of element shape and smooth size transitions, is vital for ensuring model stability and accuracy [53]. After finalizing the mesh, developed dem data (section 4.2.3) is used to prepare the topographic surface of the 2D model.

5.6.6. Develop 1D/2D Linked Urban Flood Forecasting Model

Through MIKE+, the benefits of 2D and 1D modelling has been utilized in an integrated model where complex networks and channels represented in 1D are coupled to an overland surface representation in 2D. Models of collection systems and river networks have also been coupled to models of the overland surface as shown in Figure 6 and Figure 12.

After defining the couplings, an integrated model is done taking stormwater pipes, open channels and the overland surface in account. Following 1D model components are linked to the 2D surface:

• Manhole
• Basin Outlet
• Pump
• Weir
• Natural channel
• River bank
• River end

5.6.7. Hydrological Boundary:

Rainfall: Observed rainfall data in a 3-hour interval measured by BMD is used in the model as the hindcast period in the model.

Temperature: Historical monthly average temperature data is used in the model. This data is calculated from BMD observed data.

Forecast rainfall: BMD predicted 3 km by 3 km meter forecast data has been processed as catchment average time series in 3 hours interval for each basin by external python script.

5.6.8. Wastewater Boundary Conditions

Domestic and sewage water volumes are estimated based on the population within each catchment and the per capita water usage. These calculated wastewater volumes are subsequently incorporated into the nearest pipe network or canal, representing domestic water use in the model [54].

5.6.9. Peripheral River Boundary

There are five open boundaries in the 1D model: three inflow boundaries from upstream and two water level boundaries are located downstream of the model. These five boundaries are collected from the Bangladesh Super Model (this model is run every day as a flood forecasting model at FFWC) [55].

6. Forecast Products

7. Discussion

8. Conclusions

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