Written By: Karlee May, Digital Media Coordinator and Cory Harris, Water Resources Engineer, with significant contributions from Katie Jane Harris, Environmental Planner
In the aftermath of the Burlington flood on August 4th, staff from Conservation Halton were busy at work collecting data and observations to assess the magnitude of the event and plot out exactly where the flood hit and how. Several questions need answers. What kind of flood did Burlington experience? Where were the high water marks? Where are the debris lines? Where was the flooding the worst? The answers are critical to our investigation. To gather this information, staff visited creeks and stream sites around Burlington during the storm and after the flood event. After reviewing radar images of the storm it became very clear that the storm was very localized and intense. Staff focused their investigations to the four creeks that were impacted the most by the event; Tuck, Shoreacres, Appleby and Sheldon Creeks. At the site level, staff studied the vegetation and the creek beds and looked for high water marks and debris lines. Scourings (no light silt left behind), stream bank erosion and freshly exposed shale are the result of deep, fast, flowing water. The silt and sediment are moved and washed downstream because the rapid water flow literally raked the rock with intense force. In nature, floods gradually flow over a wide area ie. a flood plain where flow energy is spread out and not as concentrated Often in historic urban areas, the water flow is pinched due to encroachment on the flood plain, and, thus, flows fast and deep and can be highly erosive. After a flood event, vegetation can provide a lot of useful clues as to the high water levels during the peak of the storm. Vegetation is often flattened to the ground, and even some trees and shrubs are uprooted from the force of the flowing water.
This type of impact to vegetation is very pronounced in older urban areas where streams and creek valleys were pinched and narrowed by historic development.The impacts can be made worse if stormwater management measures were not implemented as part of the development. From assessing the impacts to vegetation at each of the sites, CH staff can get a clearer picture of where the deepest flooding occurred and get a sense of approximate water depths.
One of the most helpful sources of information in assessing high water marks after a flood is through the use of debris lines. During a flood, creek systems will move a significant amount of vegetation, leaf litter and woody debris. A lot of this material floats at or near the surface of the water. Surprisingly, debris lines are even seen, in trees and bushes: anywhere the debris settles once the water recedes.
Debris is caught on branches and on the grass as far as the water spread during the peak of the storm when flow rates were at their highest. At each of the observation sites CH staff measure the height of the debris line and record the location. The staff in the field use aerial maps generated by our Geographic Information Systems (GIS) department. The maps indicate where the creeks and streams flow through our watershed, their respective flood plains, and where the creeks are transected by road crossings, culverts, pipes, etc. Besides looking for debris lines, staff also look upstream and downstream of each road crossing for possible blockages in urban and natural waterways as blockages can cause flood levels in excess of what has been predicted through CH’s flood plain models. When interviewed, Katie Jane Harris, Environmental Planner for Conservation Halton, said collecting data and feeding it into the flood models is similar to forensics, “it’s been and gone and we have to piece together what happened”. It is important to know when an area was developed. It wasn’t until the early nineties that storm water management systems began being incorporated into new development and construction.. Consequently, existing development in older established areas was permitted to be built in floodplains because the resources to accurately know where hazardous lands existed was not incorporated into the planning process.. It’s important for future planning to incorporate floodplain management, use effective stormwater management controls for construction and to exercise good stewardship of the land. Sometimes it is best to direct development outside of areas that are prone to flooding. There are, of course, provincial regulations that guide development in such areas. It is also important to protect the forests and riparian vegetation around natural waterways to prevent or minimize erosion. Once the field work and data collection has been completed, the engineers and GIS staff at Conservation Halton feed the data into existing floodplain modelling to compare the observed flood levels with predicted levels associated with storm events we use for planning and regulation purposes. The modelling will allow us to classify the storm accurately according to scale and frequency. When rainfall data is available, Water Resources Engineers use an IDF curve (Intensity, Duration, and Frequency) to compare the rainfall to historic trends. Drainage infrastructure is typically designed for the lower end of the historic curve: common storms such as the 2 year and 5 year storms. When the storm sewers are flowing full, excess flow is normally conveyed to the creeks via overland flow routes through the use of curbs and strategic grading of the roads within a subdivision. The Burlington Flood was extremely localized and significant: it travelled in a narrow line, with intense rainfall, and then fell in on itself as it hovered in the same location for several hours. Conservation Halton is providing added value to the Community and our municipal partners through our collection, analyses, and planning with the data gathered after the August 4th storm. The data will also help us confirm the magnitude of the storm event and plan for similar events in the future. Preliminary estimates are indicating that the storm is greater than a 100 year event. It is important to assess these events in case we are experiencing a new kind of storm system that bucks the trend of traditional storm patterns, and whether these new storms are going to become the new norm. It could be that we will see more storms of the like that happened in Toronto and in Burlington as mentioned in previous articles. In that case, a different approach land use and infrastructure planning may be required: that discovery and future discussions begin with these investigations. We’re always planning for the future. Conservation Halton is preparing a summary report of the August 4th storm that caused the flooding in Burlington and will release the document in coming months. Read the first, and second articles in this series.