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Return type:                         (dictionary)

2.6    Groundwater

In the Urbanwb Model, underneath the Unsaturated Zone is the saturated zone, i.e. the Groundwater reservoir (GW). The Groundwater reservoir is modelled as an unconfined aquifer which consists of a pervious layer underlain by a (semi-)impervious layer, below which lies the deep groundwater, one of the boundary components that exchanges water with the Urbanwb model. Percolation from Unsaturated Zone and Open Paved recharges the groundwater, while downward seepage to deep groundwater and drainage to Open Water deplete the Groundwater reservoir. The inflow (percolation from Unsaturated Zone) and outflow (seepage and drainage) are driven by the head difference, so the value of these fluxes can both be positive or negative. Figure 11  shows the schematic overview of the Groundwater reservoir. The area of the Groundwater reservoir is calculated as the area of the total model minus the area of Open Water fraction that is not above the groundwater level and minus the area of Paved Roof fraction of which the basement is below groundwater. The maximum capillary rise and the storage coefficient for the current time step are determined by interpolation based on the groundwater level at the end of the previous time step.

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Figure   11      Schematic overview of groundwater reservoir (GW) in Urbanwb

The formula of groundwater level during current time step h(t) and its derivation are shown below (Figure 12 ). In this figure, P is percolation (assumed to be constant during a time step), q_s is downward seepage to deep groundwater, q_d is drainage to Open Water. All these water flows can get positive as well as negative values, negative meaning flow in the other direction. In Urbanwb all relevant levels are relative to the surface level, where the unit (m-SL) means meter below surface level.

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Figure 12   Groundwater level h(t) calculation

Seepage: Image Added            Drainage: Image Added       Continuity: Image Added

Substitution results in:

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Initial condition:     Image Added

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2.6.1       Assumptions

• The infiltration water from Open Paved flows directly to the Groundwater (percolation), thus passing the Unsaturated Zone.
• The area of the Groundwater reservoir is equal to the total area minus the part of the Open Water area that lies below the groundwater level minus the part of the Paved Roof area of which the basement is below groundwater level.
• Drainage and seepage are calculated based on the groundwater level at the end of previous time step. Drainage and seepage are reduced due to the changing groundwater level caused by the fluxes. It means that the larger the head difference between shallow groundwater and deep groundwater (or Open Water) is, the larger the driving force is, and thus the larger the water flow is. With water exchanging, the head differences get smaller, so the water flows get smaller.

2.6.2       Calculation order

• Percolation to groundwater is the sum of percolation from Open Paved and percolation from Unsaturated Zone converted with the area ratios.
• Calculate runoff from measure to groundwater reservoir if defined possible.
• Determine the groundwater storage coefficient μ by interpolation, based on the groundwater level at the previous time step. In a database, for a given soil type, information is stored on storage coefficient for different groundwater levels (from 0 to 10 m-SL). The storage coefficient of groundwater during the current time step is interpolated from a lookup table based on the groundwater level at previous time step.
• Determine the groundwater level at the end of the current time step, based on the calculation formula mentioned above.
• Determine the total seepage to the deep groundwater (positive: downward, negative: upward) during current time step based on predefined seepage conditions. Seepage to deep groundwater can be defined either as a constant flux (0 is fixed flux) or a dynamically-computed flux (1 is level based) which depends on predefined hydraulic head of the deep groundwater and a vertical drainage resistance between the Groundwater reservoir and the deep groundwater.
• Determine the drainage from the Groundwater reservoir to the Open Water during current time step, based on a water balance. Note that here the drainage is a resulting parameter based on the other fluxes and the differences in groundwater volume (groundwater level and storage coefficient).
• Determine the groundwater level below surface level and groundwater above surface level at the end of current time step. They are dependent on groundwater level at the end of previous time step, the calculated percolation, seepage, drainage flux and storage coefficient μ during current time step.

2.6.3       Code and input arguments

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class urbanwb.groundwater.Groundwater(gwl_t0, gw_no_meas_area, gw_meas_area, seepage_define=0, w=100, vc=20000, head_deep_gw=21.5, down_seepage_flux=1, soiltype=2, croptype=1, **kwargs)       [source]

Bases: _object

creates an instance of Groundwater class with given initial states and properties, iterates sol() function to compute states and fluxes of groundwater at each time step.

Parameters:

• gwl_t0 (float) – initial groundwater level (at t=0) [m-SL]
• gw_no_meas_area (float) – area of groundwater without measure [m^2]
• gw_meas_area (float) – area of groundwater with measure [m^2]
• seepage_define (int) – seepage to deep groundwater defined as either constant downward flux or dynamic computed flux which is determined by head difference and resistance [0=flux; 1=level]
• w (float) – drainage resistance from groundwater to Open Water [d]
• vc (float) – vertical flow resistance from shallow groundwater to deep groundwater [d]
• head_deep_gw (float) – predefined hydraulic head of deep groundwater [m-SL]
• down_seepage_flux (float) – predefined constant downward flux from shallow groundwater to deep groundwater [mm/d]
• soiltype (int) – soil type
• croptype (int) – crop type

sol(p_uz_gw, uz_no_meas_area, p_op_gw, op_no_meas_area, tot_meas_area, meas_gw, owl_prevt, delta_t=0.041666666666666664)  [source]

Calculates states and fluxes in groundwater during current time step.

Parameters:

• p_uz_gw (float) – percolation from Unsaturated Zone to groundwater during current time step [mm]
• uz_no_meas_area (float) – area of Unsaturated Zone without measure [m^2]
• p_op_gw (float) – percolation from Open Paved to groundwater during current time step [mm]
• op_no_meas_area (float) – area of Open Paved without measure [m^2]
• tot_meas_area (float) – total area of measure [m^2]
• meas_gw (float) – inflow from measure to groundwater during current time step [mm]
• owl_prevt (float) – Open Water level at previous time step [m-SL]
• delta_t (float) – length of time step [d]

Returns:                               A dictionary of computed states and fluxes of groundwater during current time step:

• sum_p_gw – Sum of percolation from Unsaturated Zone and percolation from Open Paved to groundwater during current time step [mm]
• r_meas_gw – Inflow from measure (if applicable) to groundwater during current time step [mm]
• gwl_up – First value in predefined lookup table above groundwater level at the end of previous time step [m-SL]
• gwl_low – First value in predefined lookup table below groundwater level at the end of previous time step [m-SL]
• sc_gw – Storage coefficient of groundwater for current time step [-]
• h_gw – Groundwater level at the end of current time step [m-SL]
• s_gw_out – Downward seepage from shallow groundwater to deep groundwater during current time step [mm]
• d_gw_ow – Groundwater drainage to Open Water during current time step [mm]
• gwl – Groundwater level below surface level at the end of current time step [m-SL]
• gwl_sl – Groundwater level above surface level at the end of current time step [m-SL]

Return type:                         (dictionary)

2.7      Sewer system

The sewer system in Urbanwb model is the combination of a Storm Water Drainage System (SWDS) and a Combined or Mixed Sewer System (MSS). Mixed implies a combined discharge of urban drainage and urban waste water. In urban areas both systems can be applied in practice. Hence, in the Urbanwb model the total paved area (PR, CP and OP) is split proportionally in a SWDS applied area (0 – 100%) and a MSS applied area (the rest). The proportion and system capacity of these systems should be predefined by the user to match the local situation. In dry to normal flow conditions, all water in a combined sewer system is transferred to waste water treatment plant (WWTP) for further treatment. In wet flow conditions (e.g. heavy rainstorms), a relief structure, a so called CSO weir, allows a large part of the combined storm- and waste water to be discharged untreated to an adjacent water body (e.g. a city pond). Combined sewer systems have two phases in wet flow conditions. In phase one, a combined sewer system collects water from paved areas and discharges it together with the urban waste water to a waste water treatment plant and the storage in the sewer system gets filled before sewer overflow through a CSO weir to Open Water occurs (phase 2). Sewer overflow of a combined system may impose pollution problems to the receiving water body. If rainfall is so extreme that the overflow discharge capacity is exceeded, then the sewer overflow into the street will occur. Opposite to combined sewer systems, in separated systems only waste water flows to the waste water treatment plant and only rainfall runoff enters the storm water drainage system, which drains this water directly to the surface water. The storm water drainage system is limited by a predefined system discharge capacity above which the sewer overflow into the street will occur. Figure 1 , in the first section of this document, provides a clear schematic overview of the sewer systems in the Urbanwb model.

2.7.1       Assumptions

• Discharge capacities of sewer systems cannot be directly defined in the configuration file. Since the Urbanwb model is originally developed based on study cases in the Netherlands, there are some localized settings for ease of use. Hence it is user’s responsibility to understand the model and tailor the input for more realistic modelling of their interest. The system discharge capacity of the SWDS and the MSS are derived based on rainfall intensities (T=2year, T=1/6year) and predefined sewer system storage capacities. In the Netherlands, a combined sewer system is designed to overflow approx. 6 times per year, so the MSS discharge capacity to the WWTP above which the sewer overflow occurs is calculated as a rainfall intensity with a return period T of 1/6 year. Sewer overflow through manholes into the street is designed to occur once every 2 year, so the sewer system discharge capacities to Open Water above which the sewer overflow into the street occurs, is calculated as a rainfall intensity with a return period T of 2 year. Storage capacities of sewer systems in the Netherlands are normally estimated at 2 mm (over the paved areas) for SWDS and 9 mm for MSS. Some detailed explanations can be found in the parameter estimation section.
• The total area of the two sewer systems together is equal to the total paved area (PR, CP and OP). The areas of SWDS and MSS are user defined, predefined ratios. The runoff from all three possible paved areas to the sewer systems is divided by these predefined ratios. It means, for example for an urban area that has 60% SWDS and 40% MSS, that 60% of the PR area runoff will be drained to SWDS, and 40% to the MSS. Same runoff division goes for the CP and OP areas. Note that applying adaptation measures, where (part of) the urban runoff flows to these measures, may change this. However, the ratio (in this example 60% : 40%) will be maintained for the part of the runoff that does not flow to these measures.

2.7.2       Calculation order

• Determine sum of runoff from paved areas (PR, CP, OP) and measure (if defined) to SWDS for the current time step. Determine outflow from SWDS to OW for the current time step based on storage in SWDS at previous time step, runoff from paved areas and measure. Outflow from SWDS to OW is limited by the discharge capacity of the SWDS.
• Determine sum of runoff from paved areas (PR, CP, OP) and measure (if defined) to MSS for the current time step. Determine outflow from MSS to WWTP for the current time step based on storage in MSS at previous time step, runoff from paved areas and measure. Outflow from MSS to WWTP is limited by the discharge capacity of the MSS.
• Determine outflow from MSS to OW for the current time step, which is limited by the discharge capacity of MSS to OW.
• Determine sewer overflow onto the street from SWDS for the current time step. This overflow water is drained at the same time step to Open Water by assumption.
• Determine sewer overflow onto the street from MSS for the current time step. This overflow water is drained the same time step to Open Water by assumption.
• Determine the remaining storage in the SWDS at the end of current time step. Storage is only used when the discharge capacity is exceeded by the inflow volume. Storage is limited to the storage capacity. All other excess water will result in overflow.
• Determine the remaining storage in the MSS at the end of current time step. Storage is only used when the discharge capacity is exceeded by the inflow volume. Storage is limited to the storage capacity. All other excess water will result in overflow.

2.7.3       Code and input arguments

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class urbanwb.sewersystem.SewerSystem(swds_no_meas_area, mss_no_meas_area, stor_swds_t0, so_swds_t0, stor_mss_t0, so_mss_t0, q_swds_ow_cap, q_mss_out_cap, q_mss_ow_cap, storcap_swds=2.0, storcap_mss=9.0, **kwargs)                [source]

Bases: _object

Creates an instance of Sewer System class with given initial states and properties, iterates sol() function to compute states and fluxes of sewer system at each time step.

Parameters:

• swds_no_meas_area (float) – area of storm water drainage system (SWDS) without measure [m^2]
• mss_no_meas_area (float) – area of combined sewer system (MSS) without measure [m^2]
• stor_swds_t0 (float) – initial storage in storm water drainage system (SWDS) (at t=0) [mm]
• so_swds_t0 (float) – initial sewer overflow from storm water drainage system (SWDS) (at t=0) [mm]
• stor_mss_t0 (float) – initial storage in combined sewer system (MSS) (at t=0) [mm]
• so_mss_t0 (float) – initial sewer overflow from combined sewer system (MSS) (at t=0) [mm]
• q_swds_ow_cap (float) – discharge capacity of storm water drainage system (SWDS) to Open Water [mm/ timestep]
• q_mss_out_cap (float) – discharge capacity of combined sewer system (MSS) to waste water treatment plant (WWTP) [mm/ timestep]
• q_mss_ow_cap (float) – discharge capacity of combined sewer system (MSS) to Open Water [mm/ timestep]
• stor_swds_cap (float) – predefined storage capacity of storm water drainage system (SWDS) [mm]
• stor_mss_cap (float) – predefined storage capacity of combined sewer system (MSS) [mm]

sol(pr_no_meas_area, cp_no_meas_area, op_no_meas_area, r_pr_swds, r_cp_swds, r_op_swds, r_pr_mss, r_cp_mss, r_op_mss, meas_swds, meas_mss, ow_no_meas_area, tot_meas_area)                                           [source]

Calculates states and fluxes of sewer system during current time step.

Parameters:

• pr_no_meas_area (float) – area of Paved Roof without measure [m^2]
• cp_no_meas_area (float) – area of Closed Paved without measure [m^2]
• op_no_meas_area (float) – area of Open Paved without measure [m^2]
• r_pr_swds (float) – runoff from Paved Roof to storm water drainage system during current time step [mm]
• r_cp_swds (float) – runoff from Closed Paved to storm water drainage system during current time step [mm]
• r_op_swds (float) – runoff from Open Paved to storm water drainage system during current time step [mm]
• r_pr_mss (float) – runoff from Paved Roof to combined sewer system during current time step [mm]
• r_cp_mss (float) – runoff from Closed Paved to combined sewer system during current time step [mm]
• r_op_mss (float) – runoff from Open Paved to combined sewer system during current time step [mm]
• meas_swds (float) – inflow from measure (if applicable) to storm water drainage system during current time step [mm]
• meas_mss (float) – inflow from measure (if applicable) to combined sewer system during current time step [mm]
• ow_no_meas_area (float) – area of Open Water without measure [m^2]
• tot_meas_area (float) – total area of measure [m^2]

Returns:                               A dictionary of computed states and fluxes of sewer system during current time step:

• sum_r_swds – Sum of runoff from paved area to storm water drainage system during current time step [mm]
• r_meas_swds – Inflow from measure (if applicable) to storm water drainage system during current time step [mm]
• sum_r_mss – Sum of runoff from pave area to combined sewer system during current time step [mm]
• r_meas_mss – Inflow from measure (if applicable) to combined sewer system during current time step [mm]
• q_swds_ow – Outflow from storm water drainage system to Open Water during current time step [mm]
• q_mss_out – Discharge from combined sewer system to Waste Water Treatment Plant (WWTP) during current time step [mm]
• q_mss_ow – Outflow from combined sewer system to Open Water during current time step [mm]
• so_swds – Sewer overflow from storm water drainage system during current time step [mm]
• so_mss – Sewer overflow from combined sewer system during current time step [mm]
• stor_swds – Storage in storm water drainage system at the end of current time step [mm]
• stor_mss – Storage in combined sewer system at the end of current time step [mm]

Return type:                         (dictionary)