Incorporating Biological and Green Technologies into The City of West Palm Beach Coastal Drainage System Prepared by Robert Taylor, CEP Environmental Policy and Management January 24, 2015 To reduce the incidents of coastal flooding and the associated costs to residents and the City of West Palm Beach, biological (green) infrastructure concepts should be utilized with traditional drainage design to alleviate the documented flooding along North Flagler Drive. This letter will attempt to document that by increasing tree canopy and vegetation this will allow stormwater to recharge through the highly permeable sand ridge and will alleviate future incidents of street flooding. The 2007 US Department of Agricultural and American Forest Urban Eco Analysis Report for Palm Beach County findings state City of West Palm Beach has less than 14 percent tree canopy. The current acceptable practice for an urban area tree canopy should be 45 percent. Urban areas such as City of Miami, Tampa, Saint Petersburg, Los Angelos, Chicago and New York are currently enhancing their urban forest to alleviate street flooding and improve air quality. Utilizing trees as biological machines will provide a cost effective means to limit street flooding while also improving air quality and reducing negative micro-climate conditions. While stormwater sewer lines and drains are the back bone of the drainage system, the use of trees and green infrastructure is a critical component to any effective drainage system. The benefits of trees and green infrastructure are easily quantifiable. The retention of rain water from the urban tree canopy will buffer the drainage system from the initial surge of water. This buffering effect will increase the time interval for the first surge of water to travel to existing sewer drains allowing the traditional drainage system to work more effectively. Green infrastructure projects include permeable interlocking blocks, permeable asphalt, shallow percolation swales and infiltration galleries that can effectively divert water from storm sewers. These green infrastructure systems direct water away from existing drainage sewer reducing the load to the system during peak rainfall. A second benefit of these green infrastructure systems allows stormwater to percolate to groundwater, replenish the shallow groundwater aquifer and limits salt water intrusion from over pumping of the shallow groundwater aquifer and sea level rise conditions. To solve the stormwater issue in low lying areas we must acknowledge the effect sea level rise is having on our drainage system. Unfortunately, the existing regional stormwater system which was designed close to fifty years ago did not adequately account for rising sea level. As the water level rises in the Lake Worth Lagoon the flow from sewers is becoming more restricted. The challenge for drainage engineers is predicting the level of sea level rise when designing the drainage system. The rise in sea level combined with perigean (spring tides) or storm surges can easily overwhelm the drainage system of the citys coastal neighborhoods. Simply put, during low tide our drainage system will work fine, mid tide less effectively and at high tide poorly and during a rain event that corresponds with a spring tide event we can anticipate street flooding along North Flagler Drive. It was only four years ago that the city experienced a significant drought. City leaders and residents should not be surprised if we experience a drought again in the next few years. According to the Intergovernmental Panel on Climate Change (IPCC) we can anticipate changes in rain patterns due to climate change of 20 percent + or -. Climate change will likely include more intense rain followed by periods of drought. If we acknowledge the value of water, the goal should be to manage the water and not dump it as quickly as possible into the Lake Worth Lagoon which is causing another list of environmental concerns. The city should focus on how to utilize natural resources to enhance stormwater drainage while considering water supply. The north end of the city is well suited for both tree planting and green infrastructure projects. The inadequate tree canopy in the north end of the city has a direct correlation to street flooding. Because there is inadequate tree canopy, improving the canopy would provide measurable effects to reduce street flooding. This section of the city is also located on a highly permeable sand ridge. This coastal sand ridge is a natural resource which could effectively be used to limit street flooding. The implementation of properly design infiltration galleries will provide relief from street flooding. Ignoring the benefits of biological processes and geologic conditions will increase the citys stormwater costs and provide no relief for future droughts. Summary To limit street flooding communities situated on the coastal sand ridge should be implementing green infrastructure systems to retain water and allow stormwater to recharge to the highly permeable sand ridge. The planting of street trees along the urban corridors would increase the retention time of rainfall moving to storm drains providing a buffer from surface water runoff. The placement of trees will decrease the flow of stormwater by capturing and retaining water. Trees can act as mini-surface water reservoirs by holding water on their leaves, branches and bark, increasing filtration through the root system and limiting soil erosion. This buffer provided by trees will allow the traditional drainage systems to work more effectively. The second sustainable design system is to install permeable asphalt or pavers along specific areas to allow stormwater to percolate through the asphalt or permeable paver to the sand ridge. The replacement of the existing non-preamble asphalt speed-bumps with permeable brick pavers on the east/west streets adjacent to North Flagler Dr is an obvious location to enhance drainage. [However, the use of the permeable pavers and permeable asphalt would be less effective on areas that were historically filled i.e. area along North Flagler Drive compared with locations just a few hundred feet west of the seawall] Utilizing biological resources (trees) and the sand ridge to assist with drainage; the city should consider setting a small budget aside to implement demonstration projects. Demonstration projects provide a valuable tool for decision makers when assessing the cost of new or innovative projects. The tree planting projects should consider the soil conditions, canopy type and the need for supplemental irrigation and agro-chemicals. Infiltration /percolation projects need to consider location, soil conditions and maintenance. It is important that we guard against those critics that have become well versed at minimizing the effects of climate change as well as minimizing biological and innovative technologies. To manage the city drainage system effectively our leaders must acknowledge that as we experience the effects of climate change we must adapt and begin now to mitigate for these effects.
Posted on: Sat, 24 Jan 2015 20:51:30 +0000
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