UFMPs for Natural Disaster Prone Cities
September 2, 2016
I remember watching the devastation when Hurricane Katrina hit. Destruction and disaster filled the streets, families were displaced, lives lost, and neighborhoods destroyed. It was devastating. Over 1,200 people were killed and $125 billion in damages caused. Just a few short years later, Hurricane Sandy killed over 200 people and causing more than $75 billion in damages.
I thought: could anything have been done to prevent this? Sure, we could have been a little more prepared. There’s no way we could have prevented the storms from happening, however, we as a society could have taken more preventative measures and one of those preventative measures includes planning urban forests. This study found that in the locations that Hurricane Katrina hit, 23% of all the urban forests were lost. Lowering this number by even a few percentage points could potentially save lives and damage costs.
As we’ve talked about in previous posts, the goal of an urban forestry management plan (UFMP), is to create and maintain a healthy urban forest that is composed of trees to maximize ecosystem benefits and withstand natural and anthropogenic stresses and disturbances, such as wind from hurricanes and tropical storms, flooding, or pollution.
But not all surrounding environments are built the same, right? Not all tree species can withstand droughts as well as others, nor high winds or ice storms. So when we take this variability into account and plan for natural disasters in a city, a planner needs to think about both prevention and post-disaster plans. In this post, I will briefly cover some of the ways we can plan for prevention. In a later post, I’ll focus on post-disaster planning.
Prevention: factoring in where to plant different types of trees, how closely to plant them, in what types of groupings to plant them, etc. For example, understanding not to plant a tree that grows to a large size too close to a curb, sidewalk, foundation, or pavement is core of understanding prevention. Other factors include making sure roots on mature trees are not being deflected, decayed or cut too close to a trunk.
The number one factor in toppling trees is hurricane-force winds. The best way to protect against this is to allow for trees to grow a strong root system. This is done by encouraging good root growth and minimizing the amount of root cutting.
Ensuring roots can grow without being deflected by curbs, sidewalks, or pavement increases its chances of a strong root system. By protecting trees during construction to make sure their roots aren’t deflected or cut, the risk of failure decreases significantly.
Trees growing in groups also have a higher rate of survival than trees that stand individually. Ideally, this planning starts from scratch and you’re able to design your urban forest and community from step one. More often than not, however, we must deal with existing design situations, because almost always there’s already stuff there!
It can’t be stressed enough that trees that lack their main support roots are hazards in a landscape and need to be taken seriously. Ideally, we do not want to prune roots at all but when root pruning is absolutely necessary, the rule of thumb is to preserve all roots within an area about five times the trunk diameter. For example, if the tree’s trunk diameter is two feet, do not prune roots within ten feet of the trunk. This is not a steadfast rule but is a good start to get a gauge of what works best. There are quite a few techniques that can be used as solutions that do not interfere with the root system.
Install different surface material
Using crushed granite, gravel, wood decking, brick-in-sand or asphalt as a replacement for concrete may be a good solution for reducing root cutting. These porous materials provide some aeration to the soil beneath whereas concrete can trap moisture and can encourage roots to grow directly under and break the pavement. These alternative materials are usually more flexible so they are less likely to crack and are typically less expensive to repair.
A popular choice to use as a surface material is stone dust. The surface is easily repairable as roots continue to expand in diameter. Crushed rock is inexpensive, easy to install, and the surface is porous. The slight downside to stone dust is that it generally can only be used on fairly flat surfaces because rain can cause erosion on sloping ground and displaced stones will need to be replaced occasionally.
Sometimes it’s necessary to install interlocking concrete pavers, wood decking, rubber sidewalks, or metal bridges over roots to connect the walkway.
A great way to solve this problem is to just redirect a sidewalk. We there is space, it’s usually recommended to design the sidewalk to go around the roots.
Soil has two main components: 1) the total soil volume needed to sustain a tree for a reasonable period of time, and 2) the open soil area needed immediately surrounding the trunk to accommodate trunk flare growth. For details on soil requirements for trees based on their size at maturity, visit page 4 of this document.
Providing more rootable soil as root paths is a good idea for areas with little open space. Root paths are narrow channels of loose soil that provide a small path for air to encourage root growth under the pavement. This is done with a trenching machine to cut trenches through compacted soil and aeration mats are then placed in the trenches and are backfilled with loose soil.
Preventative pruning: Young and Mature Trees
The most common defects in young trees are codominant stems and aggressive low branches that either split from the tree or result in large pruning cuts upon removal. These kinds of problems can reduce the lifespan of the tree and place people and property at risk.
How do we determine what a good tree looks like? Well a good tree structure is characterized by a single dominant leader, and branches that are spaced and not touching throughout the canopy.
Generally, the issues that cause young trees to fall are: codominant stems, included bark, unbalanced canopy, lions-tailing or over-lifting, or large lower limbs. Check out the University of Florida’s Developing a Preventative Pruning Program: Young Trees for a detailed explanation on how to fix all five of these issues.
For a mature tree, one of the most common defects is the formation of large, low limbs. With mature trees it is important to minimize hazards such as branch failure.
The seven main objectives for pruning of mature trees are: reduce risk of failure, promote human safety, allow for safe passage, increase sun penetration to the ground, maintain health, influence flowering or fruit production, and improve aesthetics. For a detailed explanation on these check out the University of Florida’s Developing a Preventive Pruning Program: Mature Trees.
This was an attempt to give a ‘brief’ overview of how prevention can go a long way to prevent injuries and lessen the costs of when natural disasters are on the brink. Next week will be our final week exploring UFMPs and I will cover post-natural disaster, how best to clean up and repair the damage.