Managing Prince George s Rising Flood Waters

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Georgia Nichols
6 years ago
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1 Managing Prince George s Rising Flood Waters N e c h a k o R i v e r s e r a r F

2 CONSULTING ENGINEERS OF BRITISH COLUMBIA 2011 AWARDS FOR ENGINEERING EXCELLENCE MANAGING PRINCE GEORGE S RISING FLOOD WATERS Owner/Client: Consultant: Contact: City of Prince George 1100 Patricia Boulevard Prince George, BC V2L 3V9 Northwest Hydraulic Consultants Ltd. 30 Gostick Place North Vancouver, BC V7M 3G3 p: f: Bruce Walsh, bwalsh@nhc-van.com Monica Mannerström, mmannerstrom@nhc-van.com Sub-consultants: McElhanney Consulting Services Ltd First Ave, Prince George, BC V2L 2Y8 EDI Environmental Dynamics Inc PG Pulpmill Road, Prince George, BC V2L 4R9 M. Miles and Associates Ltd. 645 Island Rd, Victoria, BC V8S 2T7 Kevin Brown Communications 2963 Dundee Drive, Prince George, BC V2K 3R5 Location: Category: City of Prince George, British Columbia Soft Engineering Summary Description of Project: The project assessed winter and spring flood hazards from the Nechako and Fraser Rivers at Prince George and developed solutions to reduce flood risks to the community, while protecting aquatic and riparian habitat values of the rivers.

3 EXECUTIVE SUMMARY The City of Prince George is partly located on the floodplain of the Fraser and Nechako Rivers and is subject to an unusual flooding situation. On Fraser River, floods occur in the spring, caused by melting snow. On Nechako River, floods occur during the fall or winter and are caused by ice freeze-up. In the winter of , the City experienced severe icerelated flooding which highlighted the vulnerability of flood prone areas along the river. Just six months earlier, flooding occurred along the Fraser. When people develop and settle floodplain areas, protection from flooding becomes a necessity. The traditional engineering approach to flood protection focuses on physical, technical, and economic challenges, but has not always adequately addressed social, cultural, and ecological constraints. The City selected Northwest Hydraulic Consultants to lead a flood hazard and solutions study to better understand the complex river and ice behavior and develop a plan for flood protection. The study was unique in many aspects, using a multidisciplinary approach to flood management that incorporates all the constraints and maximizes the net benefits while reducing the vulnerability and risks due to flooding. On a technical level, the study made use of sophisticated numerical modeling tools, GIS and an innovative statistical approach for estimating ice-related flood levels. Impacts of climate change and the Mountain Pine Beetle devastation on river flows were considered. Consultations, presentations and meetings with the community and all levels of government were key to developing, evaluating and recommending a flood protection plan to the City. CEBC 2011 Awards for Engineering Excellence Managing Prince George s Rising Flood Waters

4 PROJECT OBJECTIVES Floods are the most destructive, most frequent and most costly natural disaster that affects humans around the world, and the people in British Columbia are not immune. The residents of the City of Prince George (the City) are particularly aware of this fact. The City lies at the confluence of the Fraser and Nechako Rivers (Figure 1) with large areas of the City located within the floodplain. The nature of river hydraulics at Prince George creates a complex situation where flooding on the Fraser is governed by spring conditions while winter freeze-up conditions govern flooding on the Nechako. Over the years, the City has experienced numerous ice-related and spring flood events. The rivers have undergone many man-made changes over the last century (Figure 2). Development of the floodplain, filling of several side channels, and construction of many bridges has reduced the ability of the rivers to pass flood flows, which increases flood levels. The Nechako River is regulated by an upstream dam which affects the cycle of the natural flows. On a positive note, community parks have been developed along some sections of the rivers and the rivers continue to provide highly productive fish habitat. In the winter of , the City experienced severe ice-related flooding estimated to have an occurrence of once in 90 years (Photos 1 and 2). The event made news across Canada. Several different approaches were attempted to remove the ice to reduce the extent of flooding, including using an amphibious excavator and locally melting the ice using hot water from the nearby pulp mill; however, these had limited success. Just six months earlier, flooding occurred along the Fraser (Photo 3). Following these events, the City initiated a study with three key objectives: 1. To reduce the flood risk to all communities on the floodplain, while protecting aquatic and riparian habitat. 2. To promote innovative methods of flood hazard management, to minimise the shortand long-term economic, environmental and social costs, and where possible, increase the environmental and social value of the region. 3. To develop tools and recommendations that are achievable and sustainable in the long-term and have the support of project stakeholders and the community at large. PROJECT SOLUTIONS To address the flood risk, a thorough understanding of flows and ice flooding was developed using historical information on discharge, temperature, and ice observations. The investigations looked at the effects of climate change, the devastation of forests in northern British Columbia by the Mountain Pine Beetle, and regulation of the Nechako River. Temperatures and flows prior to and during past ice-related floods were identified and used for the ice analysis. Historical airphotos and surveys were used to assess how the Fraser and Nechako channel widths, depths and locations have changed, which was then related back to observed variations in flows. This information was used to predict what future conditions can be expected, since this could affect some flood management solutions. An in-depth environmental assessment was made which identified the area as spawning and critical rearing habitat for white sturgeon, a key species at risk. This work significantly increased the knowledge base on white sturgeon and other species in the region. CEBC 2011 Awards for Engineering Excellence Managing Prince George s Rising Flood Waters

5 Numerical models of the rivers and the confluence area were developed and used to predict water levels and flow paths for a range of flows, and to assess the impact of some of the flood control solutions. Using this information, risk areas or areas subject to flooding, were located along both rivers and flood hazards documented. For each risk area, a suite of solutions were developed and initially evaluated using a matrix ranking approach, taking into account hydraulic, infrastructure, environmental, geomorphic, economic and social considerations. Public and agency consultations occurred throughout the duration of the study, which provided updates on the status of the study and allowed a platform for public discussion. A World Café was used as a stage for collaborative dialogue. This innovative yet simple approach lets people move between discussion groups and taps into the collective intelligence of a community as they discuss issues that matter. Discussions were first focused on specific management options and then on specific geographical areas. Using the feed-back received from the public, the initial matrix ranking of flood solutions was revised to reflect the insights and preferences expressed by the public. Some of the key solutions identified included: 1) changing land-use along the rivers, to remove existing building and other infrastructure and to prevent future development, which returns the floodplain to a more natural state; 2) raising or floodproofing existing buildings to limit future damage; 3) providing dykes that are set-back from the main river channel, which protects infrastructure while maintaining the function of the floodplain; 4) improving internal drainage behind dykes or roads; and, 5) activating historical side channels which improves habitat and provides relief from ice-related flooding. In some cases, no permanent protection was recommended and emergency protection would instead be provided when necessary. This combination of solutions, along with detailed floodplain mapping was provided to the City to allow the engineers and planners to make informed decisions to manage flooding at Prince George. PROJECT ACHIEVEMENTS The primary achievement of the study was providing the City with a plan, which the community helped develop, to reduce flood risks to people and infrastructure. The plan identified 14 different risk areas within the City, and provided a prioritized list of flood management solutions and descriptions of associated financial, environmental and social benefits and costs. The winter flood exceeded previously established design levels. This study assessed in more detail the mechanism of ice formation and the role of temperature and flow, resulting in considerably higher design levels. As with many catastrophic floods, public and political pressure commonly builds after the event, quite often triggered by an emotional response from people who have lost homes and businesses. In the case of Prince George, extreme pressure was applied to have the City support gravel removal from the rivers, particularly at the confluence area, to solve the problem before the next spring or winter flood. At quick glance this solution seems obvious lowering the bed of the river will lower the water level during spring floods, or prevent ice from grounding during icerelated floods. However, the technical analyses showed this not to be the case, and it was necessary to develop and present a clear and understandable explanation of the physics of flooding. Gravel removal was eventually accepted by City officials as not being a viable solution. It was clear at the beginning of the study that the level of public and political interest was high, and a decision was made to include a communications expert on the team. Over the course of CEBC 2011 Awards for Engineering Excellence Managing Prince George s Rising Flood Waters

6 the work, the study was covered by print, radio, television and web media, and it became apparent that additional effort would be necessary to address the many issues and concerns identified during the public consultation process. Appropriate adjustments were made to the schedule and budget to meet these requirements. In the end, the public and political scrutiny applied to the project helped to produce a valuable and useful plan. Project management was fundamental to completing the project. The nature of the project required a diverse team of experts not normally available in a single company and continuous communication was necessary to ensure all of the work was integrated and focused to address the project objectives. Close client liaison ensured that the City had the information it needed to address the public attention to the study. Local and provincial government and community requests needed to be addressed in a timely manner, and City Council meeting schedules required prior submission of critical information. TECHNICAL EXCELLENCE AND INNOVATION Typically, future possible ice-related water levels are predicted based on hydraulic modelling of winter flows. However, ice levels on the Nechako River are not only a function of flow, but also dependent on other factors such as temperature, ice supply and the freeze-up conditions at the confluence. As a result, a new advanced probability based approach was adopted. The analysis concluded that the ice-related design flood profile was substantially higher than previously thought. Usually, floodplain maps for rivers in British Columbia only show flood boundaries and water levels for design events. A Severe Flood Hazard Area was added on the City maps showing where the water depth would be greater than 1 m. In this area, wading would be difficult even if flow velocities are very low and some structural damage to buildings may occur, particularly during icerelated flooding (Figure 3). It was recommended that no development be allowed in the Severe Flood Hazard Area and that any existing development be relocated and land-use change introduced. During the ice-related flooding, areas within the floodplain at considerable distance from the river were flooded by groundwater seepage flows. Investigations suggested that the infilled former side-channels in the area may provide concentrated seepage routes. Groundwater problems have also been experienced during Fraser River floods outside of the floodplain boundary. A High Groundwater Area was shown on the maps, useful for maintenance of existing buildings or design of new structures (Figure 3) A range of resources and expertise was used to successfully complete the project. Internally, NHC relied on its project management and hydraulic modelling expertise in Vancouver, supported by its ice engineering expertise in Edmonton. The three key people involved have a combined experience of 90 years working on rivers in British Columbia and Alberta and this greatly assisted meeting the challenges of the study. ENVIRONMENTAL, ECONOMIC AND SOCIAL SUSTAINABILITY The study developed a prioritized list of flood management solutions that the community helped to develop for identified risk areas. The solutions incorporate environmental, economic and social considerations and along with the floodplain maps provide an important tool for community planning. CEBC 2011 Awards for Engineering Excellence Managing Prince George s Rising Flood Waters

7 Overall, the solutions return the floodplain to a more natural and sustainable state, while still allowing development on some portions, which recognizes the economic value of development on the floodplain. The winter flooding assessment identified the flow and temperature combinations that are conducive to ice flooding. This will allow the City to develop a monitoring program to identify conditions that could lead to ice-related flooding, providing valuable lead-time to prepare, rather than quickly react to an emergency situation. This project clearly shows that engineers must be innovative and creative in their planning and design. By communicating with and engaging the public in meaningful ways many diverse interests were accommodated and combined into a set of practical and sustainable planning tools. CEBC 2011 Awards for Engineering Excellence Managing Prince George s Rising Flood Waters

Photo 1 Downstream view of the Nechako River during winter 2007-08 showing

8 Photo 1 Downstream view of the Nechako River during winter showing the ice-related flooding of the industrial area (dashed line) on the floodplain.

9 Photo 2 Damage to infrastructure and property during the winter ice-related flooding of the Nechako River.

10 Photo 3 Flooding of the Fraser River during spring flooding in June 2007 showing damage to private property.

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