About the model

StormTac model description (including required input data)

StormTac web application: http://app.stormtac.com/

StormTac is a storm water and recipient (receiving water) software model. It is today, after over 15 years of operation and continuous updates, a fully functioning Web and Excel application, using input data forms and a flowchart.

The model is used as a tool for action planning in urban water management, and is suitable for water quantity and quality calculations within watersheds (catchment areas). It integrates processes of runoff, transport, recipient, treatment and flow detention.

This model description covers areas of implementation, unique properties, methods, required input data and license costs.

Water quantity

An increased urbanization and climate effects may cause an increasing number of floods. StormTac can calculate the capacity of the transport system (e.g. sewers and ditches) and required detention volumes for the design rain return time and rain duration, including the implementation of climate factors.

StormTac quantity calculations include:

• The quantification of yearly average water flows (yearly runoff volumes of storm water, base flow and groundwater), runoff flows during average rain events etc.

• Calculation of design flows for different return times, including climate factors and taking consideration to smaller contributed area at shorter rain durations.

• Design of storm water transport systems, e.g. sewers, ditches and channels.

• Calculation of the flow capacity for new designed transport systems, as well as for existing systems.

• Design of storm water flow detention facilities, e.g. dry/wet ponds and detention basins.

Water quality

Metals and nutrients are examples of pollutants in storm water that may cause toxic and eutrophic effects in the receiving waters. StormTac is the tool that can be of great use in the development of a more sustainable storm water management.

StormTac can be used as a simple-to-use forecast tool (and as such requiring little input data) for water quality and Action plans for storm water and surface water, e.g. to be used within the EU Water Framework Directive (WFD). It includes a large number (>70) of substances, of which several are included in the WFD.

StormTac quality calculations include:

• The quantification of yearly average pollutant concentrations and loads in the discharge points and from different land uses.

• A possibility to compare measured concentration data to calculated values.

• Identification of the largest pollutant sources and discharge locations to a recipient, presenting loads from different land uses and, as option, loads from different materials (such as copper roofs) if these specific areas have been set up as input.

• Presentation of data from up to 150 sub watershed areas in each file, to be used in e.g. Action plans for storm water management in a whole municipality or for different lake systems.

• Design of storm water treatment facilities (e.g. areas and volumes of wet ponds, filter strips, swales, ditches, constructed wetlands and underground filter basins) regarding used criteria for storm water concentrations in the discharges and/or allowable loads and surface water quality criteria.

• Calculation of treatment reduction efficiencies (% or in- and outlet concentrations and loads) for designed or existing facilities, for different shares of permanent pool area to reduced watershed area (area x runoff coefficient) or for different shares of permanent pool volume to average runoff volume. There are however other parameters that are included for a more site specific and more reliable calculations, such as the effects of inlet and outlet concentrations, the share of water vegetation, flow detention, hydraulic efficiency (length:width ratio etc.) and temperature.

• Setting up water and mass balances for receiving waters (lakes, sea bays and water courses), including calculation of net internal loading from the sediments (kg/year) or net sedimentation load to the sediments (kg/year).

• Calculation of required treatment load to reach allowable (acceptable) loads to the receiving waters, considering water quality criteria in the receiving waters (μg/l), as those stated in the WFD.

• Calculation of the new concentrations in the receiving water after reduced load after a designed treatment facility or after changed land use in the watershed area, e.g. after a planned residential area on an existing woodland area. Comparison to water quality criteria, presenting need for more treatment (larger facility, different facilities or more facilities in other sub watershed areas).

• Calculation of costs for monitoring and facility (standard calculation and a specific calculation from user input of used materials, linked to the calculated design). Presentation of cost-benefit (facility cost per reduced yearly load)

Unique model properties

The unique properties of the model especially refer to the aspects below:

• It is simple to use and consists of an overall system presentation from a flowchart of the entire watershed system (using clickable boxes and input forms in the flowchart). A changed input from the flowchart results in presented changed outputs within the flowchart (watershed system).

• The parameters and methods are continuously being updated with more input data.

• It requires little input data and no manual (help notes are included in the file), however a short manual that let you begin quickly (“within one hour”) with the calculations is included.

• It integrates watershed and runoff properties with treatment/ detention facilities and impacts on receiving waters; all in one model.

• It includes databases with continuously updated precipitation data, runoff coefficients, concentration data and reduction efficiencies.

• It includes a fine specification of both urban and rural land uses (around 80 land uses, increasing with new updates) and a large set of nutrients and pollutants (around 80 pollutants, among them substances included in the WFD, the number of pollutants increasing with new updates)

See www.stormtac.com for further information.

Required input data

The model requires very little input data. Watershed area (ha) per land use (e.g. residential area, roads and woodland) is the only obligatory input data. Information on the traffic intensity (vehicles/day) is needed if studying the loads from larger roads within the catchment area. The area and volume of the receiving water are needed for estimating allowable loads. If the sub model “Source model” is used then added input data are road length and material areas. The included databases help to make more accurate analyses by letting you change other input data such as precipitation, runoff coefficients and water depths or slopes of facilities.

The model parameters can be calibrated to measured data to ensure that site specific conditions are being considered. In such cases further input data consist of measured flow, precipitation, rain intensity and sampled concentration (mg/l or μg/l) in storm water, base flow and/or the receiving water.


The main methodology has been reviewed internationally through scientific papers and a doctoral thesis. The methods are described in more details in these and other model documents and publications, available for downloads from the web page.

The model has been developed to automate the calculations by using land use specific standard values. It is best suited for long-term predictions. Updated rain data are used for Sweden and a number of cities around the Word. Site specific yearly precipitation data and rain intensities can alternatively be a user input.

Runoff water flow is calculated from precipitation data and land use specific runoff coefficients and areas. Pollutant load rate is quantified from calculated flow and from standard concentrations.

The standard concentrations are estimated empirically from a large set of flow proportional field sampling data, which contributes to their general applicability. These are tabled as standard, minimum and maximum values. The data can be downloaded from the web page.

Base flow and also base flow concentrations and loads are calculated using specific coefficients (infiltration rates and coefficients for leakage/connection into ditches, lakes and storm water sewers) and standard concentrations for base flow (different for different land uses, from measured base flow concentrations). As is the case with standard concentrations and runoff coefficients for runoff, the base flow coefficients and concentration data can also be changed by the user. These data can be downloaded from the web page.

StormTac includes a large amount of sub models and equations for the design of different storm water facilities. The user can choose between a relatively detailed and a quick, simple design. The resulted dimensions by using different methods and by changing parameter values can easily be reviewed and compared. Examples of included design parameters are runoff coefficients, land use areas, facility permanent water depth, water depth of detention volume, slope, design rain depth, outflow, emptying time and reduction efficiency.

The design methods have been employed for a large number of case studies from pre studies to final detailed construction drawings.

Standard concentrations

Standard concentration data can be downloaded and the corresponding file presents standard, minimum and maximum concentrations for different urban and rural land uses. The standard concentrations should only be used when the storm water pollutant load from the studied land use is considered to be of average quantity, else values closer to the presented min- or max-values should be used. The background colors indicate the level of uncertainty, based on the number of data values and their uncertainties.

Observe that when using runoff coefficients and standard concentrations for runoff, only the runoff (storm water) part is calculated. In StormTac the base flow part is also calculated, see method description above and a separate table for base flow concentrations in the same data file.

Case studies

StormTac has been used for example in the following case studies, where * indicates that calibration or comparison to measured data has been performed: Nybohov*, Stockholm (residential); Essingeleden*, Stockholm (road); Sätra*, Stockholm (residential); Lake Flaten*, Salem (residential); Flemingsbergsviken*, Huddinge (mixed); Tyresö municipality (mixed); Upplands Väsby municipality* (mixed); Lake Edsviken and Lake Norrviken, Sollentuna (mixed), Lidingö municipality* (mixed); Karlstad municipality (mixed); Fittja, Botkyrka (residential); Reykjavik, Iceland (residential); Kaliningrad, Russia (road) and Lake Titicaca, Peru and Bolivia (mixed).

Calibration to measured data has also been performed for a large number of roads and treatment facilities where StormTac has been used.

System requirements and user information

• The web application only requires Internet.Microsoft Excel 2000 or later is required to use the Excel application.

• English is the model language of the recent Excel model. In the Web application the user is also be able to choose Swedish as model language. Future versions of the model are likely to offer additional language options.