CBRM Floodwater Containment and Mitigation Project
The purpose of this interview is to objectively summarize the Phase 1 report submitted to the CBRM entitled 'Wash Brook Floodwater Containment and Intensity Mitigation Project'. Lead Engineer for the Project, Alexander Wilson of CBCL answers questions regarding scope and capacity of the project.
Intro/Question 1: What are the goals of the project, and boundaries of the watershed?
Question 2: What is the Flood Risk Investment Program for Nova Scotia?
Question 3: How do engineers measure how much water moves though the watershed and infrastructure?
Question 4: Did the study consider seasonal impacts such as snow melt, frozen ground, etc.?
Question 5: How do native soil and geology conditions in the watershed impact flood reduction initiatives?
Question 6: How does climate change impact flooding in the Wash brook watershed?
Question 7: Can you explain what 'return frequency storm' classifications mean?
Question 8: Why were 1 in 20 and 1 in 100 return period of storm used in this study?
Question 9: What was the October 'Thanksgiving Storm' classified as on the 'return period storm scale'?
Question 10: What were the causes of the 'Thanksgiving Storm'?
Question 11: How much of the surface flooding can be attributed to runoff from impermeable surfaces?
Question 12: Which areas of the Wash brook were determined to be the most vulnerable to flooding?
Question 13: What are the differences between structural and non-structural floodwater mitigation measures?
Question 14: What structural mitigation measures did you determine to be the most effective?
Question 15: Can you explain the levels of effectiveness compared to stakeholder acceptance of options 5A and 5B?
Question 16: Will the Baille Ard trail system still be accessible to the public if Pond 5 is constructed?
Question 17: In general, how do berms operate and are they difficult to maintain?
Question 18: Could measures such as bioswales, rain gardens, detention tanks and trees help in the Washbrook?
Question 19: What non-structural flood risk mitigation measures were recommended for CBRM?
Question 20: What are some things home owners can do to flood proof their homes?
Question 21: What will Phase 2 of the project consist of?
Question 22: What approvals and assessments are required prior to construction?
Question 23: How does natural water retention from mature forest compare to man-made structures?
What are the goals of the project, and boundaries of the watershed?
Jen Cooper: Hello, this is Jen from ACAP Cape Breton. I'm here speaking to Alexander Wilson. Alexander is an engineer with CBCL Limited. Hi, Alexander, can you tell us a little bit about yourself and your role at CBCL?
Alexander Wilson: Hi Jen, thank you for having me. So I'm, as you mentioned, a water resources engineer, I have 21 years experience through Europe, Africa, and mostly Atlantic Canada, and have been working mainly on flood mapping, flood modelling, flood mitigation work, and I work for CBCL Limited, with the consulting engineering company. So, this is a company that is essentially made up of a team of engineers, scientists, and technologists, which specialize in a range of engineering and environmental services which include municipal water, wastewater, buildings, industry, energy, oil and gas, marine, transportation, bridges, environments, and geotechnical engineers. So we look at a wide range of engineering disciplines, which makes it really interesting for the project.
JC (ACAP): Yeah, great! So, the purpose of this interview is to objectively summarize the Phase One Report that you submitted to the CBRM, entitled ‘Washbrook Floodwater Containment and Intensity Mitigation Project’. So these questions are designed to clarify things from the report, and to neutrally pluck out some information and some details from that report. So if you are ready, we'll get started! My first question for you is that CBCL Limited was contracted to conduct an analysis of the Washbrook watershed. Can you tell us about where the boundaries of the Washbrook watershed lie, and what were the goals of this project?
AW (CBCL): Certainly. So the Washbrook watershed is fairly large, maybe larger than a lot of people can intuitively imagine. So it starts or, [rather], it outlets in the former Tar Ponds, then runs through downtown Sydney, will go up towards the Brookland Elementary School, right by Sherwood Park School, then it'll split to Highway 125. And at that point, we've only reached half of the watershed and the other half will go further up and will include one branch, a drain, Gilholmes Lake, and the other one that drains Mud Lake. So it’s a fairly large area, most of the development is in the downstream half, which is the one that’s vulnerable to flooding.
JC (ACAP): Right. And the goals of the project?
AW (CBCL): Right, so the goals of the project. Really, what started the need for this project was that  Thanksgiving Day storm event, it was such an extreme event of such magnitude and it washed away a brand new culvert, and flooded a lot of people. It was a dramatic impact, very severe damage done to the downtown area. It really prompted a strong reaction from the municipality and also the province to really do something about that: Start to be more prepared, start to start planning, really understand what the causes of flooding were, see what maybe could be done to try to reduce potential impacts of flooding in the future. So this is essentially what the scope of our work has been, to really understand and see what can be done to help reduce those risks in the future.
What is the Flood Risk Investment Program for Nova Scotia?
JC (ACAP): Can you tell us a little bit about the Flood Risk Infrastructure Investment Program established by the province of Nova Scotia?
AW (CBCL): Certainly, so this is essentially a funding program that is designed to support municipalities with conducting studies to understand flood risks and then map them, and then look at potential mitigation options. So, it provides both a funding source as well as a bit of a structure for how to go step by step through that process to get the municipalities the tools to develop a plan for flood protection.
How do engineers measure how much water moves though the watershed and infrastructure?
JC (ACAP): Part of this study was to determine how water moves through the Washbrook watershed, both through municipal infrastructure and over land, and also to identify the areas that are most vulnerable to flooding. How did the engineers at CBCL determine this?
AW (CBCL): Yes, that's a great question. So, there's different ways that we can try to understand how water moves, how it comes together in those brooks, and builds up to eventually lead to flooding and flood damage, so it can be a pretty tricky thing. What we had done already, years past, when we were looking at the Tar Ponds studies, is we installed a number of water level gauges and that gave us great information, because we tried to put together a representation on the computer of the drainage area as well as the river system. So, we put together all the characteristics of the watershed, which include the surface roughness, the slopes, the type of soil, and that's going to allow the rainfall to develop into flows as it falls down. Some of it will be infiltrated, not much in this case, and then it’s going to build up over land to reach those streams and then the water will accumulate, have to go through each of the structures, and if they're fairly small structures, the water level will backup and potentially flood people, but it will make its way downstream. So the way that we have confidence in this estimate is by taking measurements of water level flows and trying to reproduce those with the models. So we went through this process where we have confidence that we're able to design rainfall events, and then we're able to estimate how much flooding might happen. Of course, the Thanksgiving Day event was very helpful because, in terms of checking the model, we're able to check in and refine the model. With this data we had, you know, good rainfall data and very clear information on where it flooded. So we're able to use that to really confirm the model quality.
Did the study consider seasonal impacts such as snow melt, frozen ground, etc.?
JC (ACAP): Flooding generally occurs when the ground is already saturated or in the winter when the ground is frozen. Did the study consider seasonal impacts on flooding such as frozen grounds, snowmelt, things like this?
AW (CBCL): In general, what we try to do to make sure that our model is representative is we are studying risks that have been experienced, and that's really the key is what do people experience? So we try to gather as much data as possible and what flooding events were, and then we see when they happened, and we try to reproduce that for those conditions. Often, it's wintertime that creates the worst conditions for flooding, but not all the time. For example, in this case, the Thanksgiving Day event wasn't wintertime. So it really depends. It turns out that in this watershed, because the soils are extremely impermeable, there's a lot of clay, which really doesn't allow much infiltration. It's almost as if the watershed had frozen ground for the entire year. So it allows a lot of runoff throughout the year. So we just essentially focus on the experience of flood events and we try to reproduce those.
How do native soil and geology conditions in the watershed impact flood reduction initiatives?
JC (ACAP): So, you’ve sort of gone a little bit to my next question, which was about how does the native soil and geology conditions in the watershed impact flooding? How does it impact flood reduction initiatives? How do you work with a ‘clay bowl’, essentially?
AW (CBCL): Right, yes, that's an excellent question because typically our number one recommendation when we look at flood mitigation is to try to encourage as much as possible that rainfall to get back in the ground. So we really focus on infiltrating as much of the rainfall as possible because that directly takes away volume, the volume of water from the volume of flooding. In this case, however, to me, it has the best clays in the province which is great for certain purposes, but it's certainly not great to reduce flooding. So as much as we can try to infiltrate water, we're not really going to be able to make an impact in that way. So this really limits the available options for flood mitigation. We have to store water on the surface, that’s sort of the only way to slow it down. We're not able to reduce volumes, so we just have to slow it down, to hold it back as much as we can so that it’s able to drain on the downstream side, go through the downtown and developed area without flooding as much, and then as that flood wave recedes, then we can release the the water stored upstream.
How does climate change impact flooding in the Wash brook watershed?
JC (ACAP): How does climate change impact flooding conditions in the Wash brook watershed, and how do you consider those sorts of impacts in a study like this?
AW (CBCL): Right, so climate change is a vast subject that is the focus of a lot of research throughout the world, and in Canada we’re very fortunate to have eminent scientists dedicated to that effort. Talking with those scientists, we find it's very challenging to get any kind of recommendation on exactly what is climate change going to look like in terms of rainfall and sea levels rise. So, we looked at whatever information we can extract from existing climate change models, and as a range it goes from maybe minus 5% of a reduction in rainfall is possible, to an increase of more than 200%, and somewhere in there is likely what might happen. So what we did is look at a number of sources, and what the general trends seem to be as of today – every day it changes – but we settled on a value of about a 30% increase in terms of rainfall in the future, so that's a very significant increase in rainfall amounts. That's a lot of risk to face in the future.
Can you explain what 'return frequency storm' classifications mean?
JC (ACAP): Can you explain to us what ‘return frequency storm’ classifications mean?
AW (CBCL): So there's two ways to describe rainfall events. You often hear ‘well, this was a one in 20’, or ‘one in 2000 year’ storm, what does that mean? Probably that can be a bit misleading and a bit confusing. Maybe a better way to actually describe the frequency of storm events is in terms of probability. So the two main types of flood events that are typically used for design and for assessing these events are the 1% and the 5% probability. 1% essentially means every year, you'll get a 1% chance that this event will happen. And then the other one, the lesser event, is the one that's got a 5% chance of happening. They're more often referred to as the one in 100 year event, because over 100 years with a 1% chance of happening, that's what people tend to imagine more clearly. But, really, it's misleading because often people think, ‘well, if one happens, we won't get another one for another 100 years’, or something like that, which is not the case. Every year has the same probability of those events occurring, so frequency is probably a good, a better way to to look at this and these are the bases that is used for designing for flood protection or for the construction of bridges, culverts, all the drainage infrastructure.
Why were 1 in 20 and 1 in 100 return period of storm used in this study?
JC (ACAP): In the Phase One Report, a one in 20 and a one in 100 year return period storms were run through your model. Can you tell us about why those were the frequency storms that you chose?
AW (CBCL): Those events which correspond to 1% and 5% chance of happening. They're essentially, it's a national standard. It's pretty much an international standard, but in Canada, it's recognized as the standard for design, and also the Province of Nova Scotia has a statement of provincial interest, which essentially means that every municipality has to look at those specific events for designing and for flood mapping so that people can look at risks on a consistent basis.
What was the October 'Thanksgiving Storm' classified as on the 'return period storm scale'?
JC (ACAP): What was the October 2016 ‘Thanksgiving Storm’ classified as on the ‘return period scale’?
AW (CBCL): So, this is a very extreme event which took a lot of people by surprise. In fact, the municipality asked Environment Canada to analyze the data recorded on their rain gauges, and they took a while to respond and the first reason for this is we did our own analysis. And we found that if we take the existing analysis on Environment Canada and plot that point of rainfall data that was measured, you would come up with a one in 2 million year event. So what they did is they included that point in the analysis, they redid their analysis and then looked at that return period or frequency and they came up with this event that corresponds to more like a one in 2000 year event. Whether it’s one in 2000 or 2 million year, it’s a very extreme event by any account.
What were the causes of the 'Thanksgiving Storm'?
JC (ACAP): Are you able to talk about what the causes of the ‘Thanksgiving Storm’ flood were?
AW (CBCL): So, yes, there are many different ways to answer that question, but I would say because of the extreme rarity of such an event, it's really not expected to be something that's going to happen regularly. So I'm not quite sure what happened in the climate to generate this but it's certainly not something to be expected to happen regularly, not to that magnitude in any case. Now, flooding in Sydney is something that happens somewhat regularly and the causes, if you can call it that, for flooding are somewhat related to a number of factors. One of them, as I was mentioning, is that the soils don’t infiltrate much water, which allow a lot of water to drain towards the downtown area. The upstream area of the watershed is fairly steep, which allows that water to build up pretty fast, and then the downtown area is very flat and very low, so that water really accumulates very fast and then sort of fills up that low lying ‘bowl’ area. The other challenge with this area is the low lying downtown area lies very low and is very close to the high tide level. So the tide typically has a chance to come up during the flood and will prevent a lot of water from draining so that water tries to get out but meets the water level that's already high, the water has nowhere to go but really to flood that downtown area. So it's a real chance, a very natural process, I mean, it has likely flooded in that mechanism for many, many years. But, sea level rise is definitely making things worse, and the natural setting of the watershed is such that it is a challenge. You can't start changing the soil under your whole watershed to improve the situation, so it’s certainly a difficult situation in this case.
How much of the surface flooding can be attributed to runoff from impermeable surfaces?
JC (ACAP): The actual surface of the ground in that area is impermeable, it's clay, but there is also a very dense neighborhood, there are roofs and parking lots. Do these paved and hard surfaces that are man-made really make it very much more impermeable than the natural ground there. So, what’s the difference there?
AW (CBCL): Right, so often it is through that development that increases risks of flooding, and there's often, you know, points to add for being a main cause of flooding but in this case essentially, like you mentioned, because the clays are so impermeable, it doesn't really make much of a difference. In addition, all the development roads and roof areas are located in the downstream end where water would run off quite fast in any case, so even if you were to grow grass on every roof and every driveway and every road, it really wouldn't make much of a notable impact.
JC (ACAP): Okay, because beneath the grass is clay, anyway.
AW (CBCL): That’s right.
Which areas of the Wash brook were determined to be the most vulnerable to flooding?
JC (ACAP): Which areas in the Wash brook watershed were determined to be the most vulnerable to flooding?
AW (CBCL): In the downtown area, it's really mainly the Steel City area. The Sobeys and parking lot are quite affected. Townsend Street floods quite a bit, as well. As we move upstream, Brookland Elementary School and that area will flood quite a bit. There’s residences on Cabot Street that will get flooded through the McEachern ball field that's really completely full of water. And then further upstream, above Highway 125 there, we’ve got some residences along Mira Road that are also affected.
What are the differences between structural and non-structural floodwater mitigation measures?
JC (ACAP): In this study, you looked at both structural and non-structural floodwater mitigation measures. Can you explain the difference between the two?
AW (CBCL): Certainly! So, this may be different interpretations but most of the cases will see structural as being something you can build. Non-structural is more ‘how do you prepare yourself for something?’, emergency management plans, essentially more about keeping people safe with what you have, rather than starting to build flood protection infrastructure.
What structural mitigation measures did you determine to be the most effective?
JC (ACAP): What structural mitigation measures did you determine would be the most effective at mitigating flooding in the Wash brook watershed, and where will the most improvement be seen?
AW (CBCL): Right, so yeah, when we did this study and we mapped the flood extent, we had a model of how the water builds up and how it comes out into the banks. So, as we, you know, we've been talking about those clays that are a real challenge because we would have loved to encourage infiltration of stormwater, but it’s just not possible. So we're really limited to trying to hold back that water on the surface wherever an area is available. Unfortunately, a lot of the areas that are available are already naturally flooded, so this really limits the places. So, we try to find, wherever possible, whether it's a sports field or an open area, and try to identify any of those that might be available and try to get a very rough concept drawn up and planned so that we could then see if there's interest and, if so, go to the next step. So this study just really looks at a very high level, what are the potential options that might make a bit of a difference. In terms of what works better than the other, it's really the downtown area we're trying to protect the most and this is at the downstream end of the watershed, so what we would like to do is hold back the water just upstream where a lot of the water is just about to enter the downtown area and we want to hold it there. If you go further up, there’s less water available that has built up that we can hold. So, you can build – there's more room to build things, but it has a lot less effect. So really, the ideal place where you would want to hold water is just upstream, the downtown area which is just downstream and Highway 125.
Can you explain the levels of effectiveness compared to stakeholder acceptance of options 5A and 5B?
JC (ACAP): Structural flood mitigation options include construction of stormwater retention ponds 5A and 5B, on the same property as the Baille Ard Trail system. These options have respectively high and medium effectiveness for floodwater mitigation, but these actions are expected to have only low to medium acceptance by stakeholders in the community. Can you explain why?
AW (CBCL): Right. So, as I was mentioning purely from a physical standpoint, we would like to be able to hold water just before it enters the downtown area which is on the downstream end of the watershed, this is the location where the Baille Ard Trail system is. So, we put together one option, and it’s completely conceptual, that point, which is looking at how much water can we physically store. So we thought, ‘okay, let's see what physically is possible’ and we'd have to build, essentially, a dam there that would hold a lot of water just to see what's the merit of the option, is it even worth thinking about? So that's the great thing about a computer model, you can change whatever you want. It doesn't mean that any of this is going to happen, but you can test it. So we tested that, holding as much water as possible in that area and find that that really has the most positive impact on the flooding risk downstream. Of course, recognizing that it’s, in practice, not realistic to think that we can build a big dam just upstream of the downtown area, we’re trying to think of a way that blends or that is more connected to the type of area that is there. So allowing only limited depth of water, about a couple of meters, and doing that in steps. So this is an attempt to try to be more connected to the land area that’s there. It would involve much smaller berms and would allow water to be stored in steps. There's less potential – less volume – that can be stored in there so it has less effects on flood reduction, but it also has less impact on the area. So we do recognize that the Baille Ard Trail system is an extremely valuable natural area, with a mature forest. So, this is why we ranked, we made it clear in the report that it would would face some challenges. The more water we store the more we change that area, the more resistance, the impact it would have on the on the natural system as well as the community
Will the Baille Ard trail system still be accessible to the public if Pond 5 is constructed?
JC (ACAP): So this question might not have a concrete answer right now as you’re sort of changing from conceptual, or, from the idea into the actual development, but will the Baille Ard Trail system be accessible to the public if Pond 5 is constructed?
AW (CBCL): Right, so, Pond 5 and I think we're talking about the option 5B which is more of a stepped approach. I guess in both cases even with the dam option, the idea is that there would still be access. These structures are structures that nobody sees being in use until there's an extreme flooding event. Otherwise, it just looks like a shape in the landscape, but it doesn't really affect anything in terms of low flows, medium flows. Suddenly, during extreme flows, and that's the only time where they start to store water, is when you get an extreme rainfall event and then that water will build up on those structures. So really, other than the new structures themselves, there wouldn't be any change to the land. Anything that's not on the footprint of anything new that's built would not have any kind of change of its normal functioning. So, looking in more detail at 5B, which is the number of steps of berms, the idea was there to try to have a series of lower berms that try to follow or mimic a system of trails and that maybe can help with connectivity but that really try to blend the uses. So, they would be essentially a raised – a system of raised trails, you could see it that way. And, in between those berms nothing would change, but when you would get an extreme rainfall event, which might happen to the scale of once in a lifetime type thing, then you would see water building up and protecting the downstream area from flooding.
In general, how do berms operate and are they difficult to maintain?
JC (ACAP): In general, how do earth berms operate and are they difficult to maintain?
AW (CBCL): Yeah, so the idea is that those berms really are made of, essentially, just raised ground. It would look a bit like a raised trail, sort of a raised pass. There's no mechanical component in there, it’s just raised ground, and what it does, really, is it blocks the floodplain. It only leaves a pathway for water to flow in the brook itself. So, when the floodwaters come and start to fill the floodplain, it’s not able to continue having that wide expanse to flow downstream, it’s essentially restricted or pinched to the width of the natural stream itself. So that'll allow that water to be held back, that's the mechanism. It doesn't need any gates, it doesn’t need any computerized system or any operation, anything like that. There's just a raised ground that can be made to look or follow a trail system. So, this is likely going to be the next step of investigation, is really to work with the Baille Ard trail community and see how we can modify this to make it blend in as best as possible.
Could measures such as bioswales, rain gardens, detention tanks and trees help in the Washbrook?
JC (ACAP): So do you think capturing stormwater in suitable areas in the Washbrook watershed through the installation of low impact developments such as bioswales, rain gardens, detention banks, and tree wells within the neighborhood in people's homes within yards and schoolyards, do you think that that would reduce flooding at all in those vulnerable areas?
AW (CBCL): Well, being a water resources engineer and always ready to promote the development of ecosystems and protection enhancements of natural systems, I will never discourage infiltration of water and the creation of rain gardens and bioswales, those are all fantastic things and they will work to some extent, so I promote that as much as possible. If it's possible to do them, and they can be integrated in a plan, this is really, really fantastic. If you really look at the impact on those huge floodwaters coming down, the impact is going to be limited, unfortunately, you know, but I still will never discourage anyone from doing those things which are great for many different aspects, you know, treatment of stormwater, recharging groundwater, filtration, they're all really good things.
What non-structural flood risk mitigation measures were recommended for CBRM?
JC (ACAP): What non-structural flood risk mitigation measures were recommended for the CBRM?
AW (CBCL): This case we were looking at nothing you can build, that was the general idea. So, we're looking more at emergency preparedness, flood forecasting. So, ‘how do you prepare as a community and as an individual, when there's a big storm coming?’ So, the first step is really to know that there is something coming, so the flood forecasting part is pretty important. So, we have a forecast from Environment Canada, and that's really, essentially, the main tool that we currently have that can be enhanced to look at the projected rainfall and turn that into a potential flood line. The models are there, this is a tool that can be developed. Now, if it looks like there’s, you know, really a risk of flooding, then we start putting a plan together, and it's all about the planning, you know, how do you prepare the community? You need to inform people this is really, really important. People need to know that they need to start getting ready, form a plan, think about communications, access to emergency services, those are all extremely important aspects so that nobody finds themselves stranded, or really at risk during such an event.
What are some things home owners can do to flood proof their homes?
JC (ACAP): What are some things that you would recommend homeowners do to flood proof their home?
AW (CBCL): There's a number of things, in terms of the home, that you can do and there's a lot of documentation about that which includes turning off your power to make sure that there's no issues with your electrical systems, moving your valuables from the basement, shutting off valves to oil and gas systems. But, really, beyond protecting the home, first and foremost, it's really important to protect the people. So at some point, you know, unfortunately it was experienced several times that the homes were damaged to a level that the best flood proofing couldn't have really saved the homes. So first and foremost, emergency management is really, really important. So really, we recommend contacting the Nova Scotia Emergency Management Office, the municipal Emergency Management Office. Public Safety Canada has a great emergency preparedness guide. So, just to give a couple of examples, it really includes preparing a plan. So, transportation: How do you get out if you need to get out? Communication: How do you stay in touch with those you need to communicate with? Maybe pre-arranging a meeting place, having emergency kits if you're stranded, or whatever you need. Thinking about what you’re going to do with your pets, shutting off your utilities, and making copies of important documents, if you’re going to lose everything. So, those are tough things to think about, but if something is coming, those are the things to think about.
What will Phase 2 of the project consist of?
JC (ACAP): Will you please tell us a little bit about what the second phase of the study will consist of?
AW (CBCL): Right, so, this first study looked at a wide range of options. The next step is really to start investigating what's feasible, what's not feasible, and how much more in detail can we go and is there even potential to build something? So, this process has already started and, in fact, some work has already been done on Mud Lake, for example. A structure has already been built and it's already functional, so it's able to hold back some of the water in that little upstream lake and is already able to reduce the risk of flooding, which is great. This is something that was identified early with an opportunity. And then next on the list is Gilholmes Lake, which also has some interesting opportunities there, too, in terms of feasibility, because nobody lives there, and it's accessible. It really depends on land ownership and access, things like that. But, in terms of disruption to the community, it's really not so much an issue. The next step is, of course, the main area that we identified as having valuable effects on flood reduction, which is the Baille Ard Trail area. So, this is something we're only just starting to look at and this has to be done very carefully and, of course, nothing is done before there’s any kind of communication with the local groups, so that we can, together, think about that balance between, ‘okay everybody understands, we need to keep people safe from flooding’. At the same time, we don’t really want to deface some extremely valuable assets in the community to achieve that, so it's a careful balance and, I think, a balance to be arrived at together, as a group.
What approvals and assessments are required prior to construction?
JC (ACAP): What types of approvals or assessments will take place prior to construction of any structural flow mitigation measures, or what sort of approvals and assessments did you need also for Mud Lake and Gilholmes Lake?
AW (CBCL): Anytime we propose any kind of modification or any kind of construction, especially if it's related to any water course, everything goes right to Nova Scotia Environment. Now, they will decide which departments, provincial or federal, this has to be reviewed by. So, typically, we're looking at [the Departments of] Lands and Forestry, Natural Resources, but also Department of Fisheries and Oceans. So, those are all groups that we can expect will be involved but everything is managed by Nova Scotia Environment.
How does natural water retention from mature forest compare to man-made structures?
JC (ACAP): How does natural floodwater retention from a mature, wooded area like the Baille Ard Trail compare to man-made detention structures in terms of flow reduction in Washbrook watershed?
AW (CBCL): Right, so, yeah, being a big proponent of natural flood mitigation measures, we always recommend restoration of floodplain vegetation as well as infiltration measures to help reduce flooding risks. In this case, we already have a very well developed, mature, natural system that is able to achieve some of that. So, it's great in terms of slowing down some of the water, but as we saw and experienced, it's only able to slow down so much water. It's not able to hold any kind of significant volume of water. It's great for slowing it down, and we don't want to change that. But really, it's really in terms of how much volume of water can you remove from that flood that we need to include in that system to really make an impact on those risks. So, you know, the system proposed would maintain, would not touch any vegetation outside of the berms themselves, so that would still be able to slow down the water. But, in addition, if the rainfall amount is such that the water was to fill the floodplain, it would be held back and start filling up those areas. And that would be for, you know, we're talking hours, not days. And it would be, you know, on such an infrequent basis that the vegetation system would not be affected.
JC (ACAP): So that ecosystem would still be doing its job, it's just, this would be helping it if the water came so much that it couldn't hold anymore?
AW (CBCL): Exactly. It's only during those extreme events that the water accumulation would change. Otherwise, most of the time, outside of those extreme events, no difference would be seen. The ecosystem would function exactly the same and we don't expect any impacts to the system. I mean, it's already a fairly wet area, so it's used to seeing water. So, having been flooded, you know, once in many years, for a few hours, is not expected to cause any notable impacts to that system.
Will the Washbrook watershed flood again?
JC (ACAP): My very final question for you is another question on everybody's mind. Will the Wash brook watershed flood again?
AW (CBCL): [Laughs] Well, I don’t know, really, to answer the question but, I’ll say it's highly likely, unfortunately, that whatever we do, it can never really stop flooding in a system like that. It's always flooded and it will always flood, and all the options that we're looking at, our aim is to reduce flooding in the most vulnerable areas. But, flooding is a completely natural process and it can never be eliminated. The reality, as well, is with climate change, as we just discussed, we were expecting somewhere in the range of 30% increase in rainfall amounts over the next 100 years, and sea level rise is also expected to bring up the tides by, maybe, one meter, and according to the worst case models maybe three meters. This is extremely worrying to see those numbers and when we're thinking about keeping our communities safe, it's certainly a big challenge. So, the approach to flood protection that we see throughout Atlantic Canada is really mostly looking at retreats, looking at holding back development and, wherever possible, reconstructing further upstream where the flooding risks are not there. So, nature changes, and going against those trends is really a challenge, of course, in the case of climate change, arguably a man-made change, but on the local level, you can't hold back the sea and you can't hold the rain in the sky. So, really, there are forces that are really challenging to protect ourselves against.
JC (ACAP): Right, and the Washbrook area is a floodplain, after all.
AW (CBCL): That’s correct.
JC (ACAP): I really appreciate you taking the time to clarify these things for us. Alexander, thank you so much for your time.
AW (CBCL): Thank you very much, thank you.