Low Impact Development Design Competition Green Roadway Design Challenge Team 07181 Independence Parkway, a Harris County / Precinct 2 Roadway: Discover a new path to history Independence Parkway to the San Jacinto Battlefield
Road design in the Houston area, in fact throughout the entire State of Texas, has a very traditional and very similar character: straight and narrow with a crowned driving surface and roadside ditches straddling each side of the pavement. This farm to market road design has been an effective model for as long as anyone can remember. It is time to change the model. By incorporating new drainage methods, altering the straight and narrow appearance and providing attractive native landscaping, the traditional major thoroughfare can be transformed. This low impact roadway design solution uses two simple techniques to achieve dramatic differences in performance: the paving section meanders within a 120-foot right of way and standard drainage ditches and culverts have been replaced with engineered bioretention cells. Effective low impact drainage bioretention cells require a wide and shallow profile versus the standard roadside ditches. This can be achieved by shifting the pavement to one side of the right of way, thus combining two ditches into one. To combat the usual straight roadway alignment, this design uses 5000 foot centerline radii to introduce gentle curves thus causing the bioretention cells to switch sides of the pavement periodically. This meandering has several positive effects: first, the driver’s visual corridor changes allowing views of the bio retention cells landscaping and second, the bioretention cells can more effectively manage segments of roadway drainage by being segmented themselves. Instead of standard stormwater interceptors and bar ditches, the low impact drainage system uses native grasses and plants to slow the stormwater as well as cleanse the stormwater of impurities. The addition of groupings of trees creates a very attractive driving experience when compared to standard roadway design. This design solution revealed that creating a meandering street pattern and consistently reversing the roadway slope into the bio retention cells effectively drained appropriate paving segments while preserving safety and maintainability. The predominant grasses that make up the filter strip adjacent to the paving are chosen because of their twice-yearly mowing schedule while the taller grasses and shrubs were selected because of their ability to allow water to permeate into the soils. GENERAL SYNOPSIS
Interconnected Channel and Pond Routing (“ICPR”) software, Version 3.10, created by Streamline Technologies, Inc. was used to model the proposed hydrologic and hydraulic conditions for this project. The software enabled us to accurately represent the Low Impact Design hydrologic techniques including the SCS Unit Hydrograph Method. The main advantage of using the SCS Method is it allows the properties of the multiple soil types to be accounted for in the hydrograph calculations. The benefits of the engineered soils were reflected by adjusting the Curve Number to reflect a Type A soil in the improved areas. The hydraulic benefits of the Low Impact Design elements were modeled using a stage-area component which accounted for the total storage volume provided in the bioretention cells. Screen capture of ICPR drainage modeling software at right. HYDRAULIC MODELING
BIORETENTION DESIGN CRITERIA The bioretention cells are positioned on each side of the right of way and provide storage capacity for the major thoroughfare. The bioretention cells both slow and cleanse the rainfall before delivering the stormwater to the drainage outfall point.
The low impact landscape concept used in this roadway solution returns natural areas to the right of way. Unlike standard roadways where drainage ditches parallel each side of the driving surface, this low impact design solution includes richly planted bioretention cells dispersed generously along the roadway. The meandering pattern of the paving allows landscaped areas to be placed strategically to not only intercept stormwater drainage, but also to be positioned within drivers’ vision cone. Put another way, drivers will repeatedly see attractive landscaped areas within their sight distance while gently curving from side to side. The bioretention cells are designed to mimic a natural stream system both functionally and visually. Within bio retention cell there three water zones. Simply put, there are lower areas which receive typical storm events where the selected species must tolerate standing water. The middle area which is somewhat drier requires species to be submerged for short amounts of time and must tolerate fluctuating water levels. Finally, the uppermost area of the rain garden contains plant species that prefer drier soils but can handle the occasional heavy rain. Such a plant layout occurs naturally in the environment and also includes tree species having similar water needs. In these bioretention cells, the shrubs and trees are placed away from the center and lowest drainage area to limit root penetration into the underground perforated drainage pipe. The irrigation system for the project would be limited to establishing the proposed vegetation only; meaning that once the plant materials have rooted, supplemental irrigation would not be used. The irrigation system is a combination of drip irrigation and surface spray heads. The selected landscape plantings are regionally native to the area and therefore are accustomed to the rainfall amounts for the region as well as readily available from regional growers as researched by the Lady Bird Johnson Wildflower Center. This low impact design solution uses native grasses and specific mowing schedules which significantly reduce the need for turf grass irrigation. The bioretention cells hold the necessary water within the soils after storm events, eliminating the need for supplemental irrigation. LANDSCAPING & IRRIGATION
The design solution for Independence Parkway illustrates the economic advantages of utilizing low impact techniques. The standard drainage design of a major thoroughfare utilizes large reinforced concrete storm sewer, numerous manholes and inlets, as well as deep roadside ditches with potentially dangerous driveway culverts. When all costs are considered, standard thoroughfare improvements cost approximately $5,000,000 which does not include the costs of any detention volume or expensive off-site land for detention, should any be required. This innovative low impact drainage design solution results in savings of over $500,000 to Harris County and its taxpayers. The low impact design removes the large pipe and steep ditches, replacing them with landscaped bioretention cells and a small diameter perforated pipe which yields significant, measurable cost savings. By keeping the basic design of the concrete pavement and cross-section, including standard concrete paving and lane widths, the County can still employ typical construction contractors and be confident that the result will be a quality product that meets the intended design of the road. Additional upfront landscaping costs will be required with this low impact design compared to a typical major thoroughfare; however, the infrastructure costs savings will more than cover these increased costs. COST ANALYSIS
As with any drainage system, the low impact bioretention cells require minimal maintenance to operate at peak levels. Unlike traditional concrete drainage systems however, the bioretention cells require landscape and soil care which can be addressed by trained landscape maintenance crews according to a schedule as exemplified below. The plants chosen for the bioretention areas are native to the region and thus are more tolerant of typical growing conditions. Simple plant care including pruning, thinning, mulching and soil repair will satisfy most of the needs of the drainage system. Plant replacement must be performed as needed and soils must be continually inspected to assure that the hydraulic system is functioning as designed and is transmitting water through the engineered layers to the underground drainage pipe , a process called phytoremediation. Such minimal maintenance will allow the system to perform as designed, reducing pollutants and storm event flows while being an attractive neighborhood feature. BIORETENTION MAINTENANCE
The combination of innovative drainage solutions and subtle road design cues yield a new model for major thoroughfare design. The meandering roadway paving creates a different driving experience versus standard straight line roads. This is achieved using standard accepted roadway design and construction practices including pavement specifications and cross slopes, thus there is no difference between this and any other major thoroughfare from a utilization standpoint. The difference occurs in the techniques used to direct and manage stormwater. The introduction of bioretention cells to replace underground storm sewers achieves both hydrologic and aesthetic improvements. The piping, soils and plants combine to yield a 60% reduction in the 100-year peak discharge rate while simultaneously creating an attractive roadside landscape. The latter point is especially poignant when considering that the Independence Parkway is the gateway to one of the State of Texas’ most treasured landmarks. The gently curving road design allows for larger bioretention cell areas on each side of the thoroughfare so that it provides sufficient area to handle all design storm events as well as providing ease of maintenance for County personnel. By removing the drainage cell areas from the median of a heavily traveled thoroughfare, the maintenance crews have one less traffic direction that must be monitored to ensure their safety. Many areas in Houston utilize a Management District to improve property values in a specific area. Improvements often take the form of landscaping and signage enhancements which add visual appeal thus increasing values. By providing bioretention cells and associated landscaping in the right-of-way, adjacent property owners gain a similar visual benefit which creates a sense of community identity and pride in the vicinity of the thoroughfare, which can also result in higher property values. The previous points are even more dramatic when considering that the construction cost estimates indicate that this low impact roadway design results in a 10% cost savings to Harris County and its taxpayers. SUMMATION