Floodplain Mapping & Risk Analysis

Due to a complex interaction between Bay of Fundy tides, ice jamming and dyke constrictions, Truro – a low-lying urban development area in Nova Scotia – experiences widespread and frequent flooding from the Salmon River estuary and its river tributaries.

The Joint Flood Advisory Committee (JFAC) therefore decided to commission one of the most comprehensive flood risk studies ever undertaken in Atlantic Canada, which involved extensive 1D, 2D and 3D hydrodynamic modeling to simulate the various floodplain processes.

PCSWMM was used for its capacity to develop over 7,000 river and floodplain cross sections and a 17km² 2D mesh with over 14,000 cells. A radar-rainfall model of the calibration flood event was also developed using PCSWMM based on 1km² resolution historical radar data. The radar analysis found that rainfall amounts in upper regions of the watershed were twice as much as those recorded at the rain gauges located at the downstream end of the watershed.

The completed PCSWMM model not only demonstrated the importance of simulating spatial variations in rainfall for calibration purposes, but it was also used in developing floodplain mapping, running nearly 200 simulations and assessing over 40 large-scale potential flood mitigation solutions – providing the JFAC with numerical support for recommendations to alleviate flood risk.

An existing flood-prone neighborhood in Austin, Texas is putting the City at risk for water quality issues, erosion concerns, pollutant loads and significant runoff volumes. Faced with the need for system-wide storm drain additions and/or upsizing, cost prohibitions have encouraged the City of Austin Watershed Protection Department to investigate a retrofit plan for the neighborhood with a mixture of public and private decentralized Stormwater Control Measures (SCMs).

Using PCSWMM, SWMM models were developed and executed for existing conditions, maximum implementation of GI SCM and a hybrid scenario of decentralized GI hydrologic controls with strategically located grey infrastructure drainage improvements.

Project goals were to improve the current level of service (i.e. frequency and severity of flooding) and quantify the extent to which decentralized SCMs can reduce the frequency of flooding while also reducing long-term runoff volumes, pollutant loads, etc.

Design storm events were simulated using both 1D and 1D-2D PCSWMM hydrologic models, while continuous simulation of 23 years of 5-minute resolution precipitation records were simulated using the 1D model only.

Both the max GI scenario and the hybrid green/grey scenario decreased peak flow rates and met the goal of avoiding adverse impacts to the receiving water body. The hybrid green/grey scenario model shows further reduction in peak flow rates, peak water surface levels, inundated areas and inundation of structures in comparison to the level of service that the max GI scenario can provide while maintaining similar expected construction costs.

Originally designed to convey a 5-year storm event, a recent analysis estimated the capacity of the Don Mills Channel to be less than a 2-year storm. This insight led the City of Markham to retain The Municipal Infrastructure Group (TMIG) and Computational Hydraulics Int. (CHI) to build an integrated PCSWMM 1D-2D dual drainage model of the approximate 7.3 km² channel study area.

Discharging into the German Mills Creek, the Don Mills Channel is 3 km long and the study area is comprised mostly of commercial and light industrial development, with some residential areas. The completed model consists of approximately 120 subcatchments and 500 conduits representing almost 38 km of drainage network – including storm sewer pipes, major system flow paths, culverts, ditches and the channel itself.

Calibration of the model was based on observed high water marks from two historical flooding events: August 18th, 2005 and July 27th, 2014. An integrated 2D model component was added to better represent floodplain storage and conveyance throughout the study area.

Moving forward, the objectives remain to: estimate existing capacities of minor and major drainage system infrastructure under different design storms and recent major storm events; evaluate overland flow paths; estimate hydraulic grade lines in storm sewers; identify problematic areas due to capacity of the current drainage systems; and evaluate new developments and intensification on the capacity of major and minor drainage systems.

Downtown Fort McMurray is highly susceptible to flooding – especially under extreme ice jam conditions. To avoid riverbank erosion, washed out roads and threats to private property and public infrastructure, the Regional Municipality of Wood Buffalo utilized PCSWMM’s integrated 1D-2D modeling tools to simulate urban and riverine flooding compounded by ice jams in the Athabasca River.

A combination of ice-related and open water flood conditions were represented in the 2D hydraulic model. One extreme condition was an approximate 100-year ice-jam stage event concurrently with 100-year return period streamflow in the upstream rivers and watercourses.

In addition to the 2D model, PCSWMM’s Watershed Delineation and Transect Creator tools were used to set up a 1D model of an upstream watershed. Training on the development and application of integrated 1D-2D modeling was also delivered to help staff better visualize and depict flood inundation results that would be suitable for presentation to senior staff and elected officials.

As a result of this project, recommended enhancements to the model were identified and a series of videos were prepared to highlight flood inundation extents and velocity vectors throughout the 26.4 km² (10.2 mi²) study area.

Flooding and overflows are a recurring problem in São Paulo, Brazil due in part to increased impervious surface areas, inefficiency of drainage structures and channel obstructions. The Anhangabaú Watershed contains a major interconnection passage where heavy rains create a chaotic situation for the population as well as losses to the national economy.

In seeking a solution to flooding in the 540-ha basin, São Paulo City Hall hired the Hydraulics Technology Center Foundation (FCTH) to evaluate the performance of two traditional alternatives and to offer a new alternative based on modern concepts of water resources management.

A complex PCSWMM modeling network was developed, representing 110 km of roads, 50 km of drainage networks and 2,800 joint structures such as curb inlets and drainage grates. Subsequently, the PCSWMM 2D module was applied to the entire watershed to represent surface flooding while considering DTM and buildings as restrictions to the water flow.

Three alternatives intended to mitigate the flooding problem in the lower valley were evaluated: (A) 2 flood detention reservoirs and reinforcement of the main gallery system – designed to ensure safety against 25-year return period events; (B) reinforcement of the main gallery and flow derivation tunnel leading to the Tamanduateí River for a 100-year return period; and (C) distributed linear retention spread over the watershed in stages of return periods of 10-/25-/100-years.

By using a multi-criteria analysis technique – and considering the solution efficiency as well as prevented damages and factors such as cost, environmental impacts and public attention – the project pointed to alternative C as the most effective method to solve the drainage problems Brazil faces. The solution has since been implemented to help reduce flooding throughout São Paulo’s Anhangabaú Watershed.

The city of Newport News, Virginia is in the Hampton Roads region, a highly urbanized coastal area on the east coast of the USA. This area experiences multiple types of flooding – coastal, pluvial, and compound flooding. This project will explore different land use scenarios to estimate the flood risk reduction benefits of natural infrastructure in a 10 km2 watershed within the city of Newport News.

Using PCSWMM 2D for this ongoing project will allow us to produce realistic and high-resolution flood inundation scenarios that we can then pair with parcel level depth-damage estimates. Benefit cost analyses often focus on structural mitigation solutions to flooding at one particular site, but using scenario analysis and PCSWMM, we can estimate both the on-site risk reduction benefits of natural infrastructure as well as the effects on neighboring properties.