Low Impact Development Design Competition Suburban Residential Design Challenge Team 19189 Ventana Lakes, a Mischer Investments project: Developer friendly, Environmentally sound Explorations in ecologically sustainable drainage while promoting customer excitement
Houston has developed as a city primarily in response to market forces which have been guided by land developers who choose tracts based on location and relative development costs. The Master Planned Community model has served the city well by providing amenities such as activity centers, greenbelts and trails, visual appeal and a sense of community for residents. Home builders have in turn chosen to build products which respond to consumer wants and needs including everything from fenced back yards to granite counter tops. And, consumers have, as a result, had extraordinary choices in location, features and pricepoint as Houston has continued to grow. One might argue that the development of Houston communities has established standards that every developer, home builder and home owner have come to expect. Once again responding to market forces, the Houston suburban model community is changing again by incorporating low impact development standards such as EnergyStar, LEED and a litany of other programs. The challenge of a low impact development in Houston is to meet all of the previous expectations from developers, builders and consumers while still meeting the goals of a low impact development. The new community design solution presented here addresses this challenge with only one key concept: simplicity . Put simply, this low impact development design solution uses a kind of “community rain garden” concept to handle all stormwater runoff. No burden is placed on individual homesites to control runoff, no detention basins are used, and no underground storage solutions have been proposed. No unusual construction methods are proposed and no special products or materials are required. This low impact design solution is simply efficient, inexpensive, attractive, maintainable and marketable. The first thing a developer will notice when studying this land plan is that it does not look different than any other subdivision. Sure, there appears to be more green areas, but that is balanced by the lack of detention basin sites which is the current drainage solution in Houston suburbs. Also, the street patterns, greenbelts, and lot locations all have a look and feel comparable to any subdivision in the Houston area. As in any land development project, the bottom line for developers is the bottom line. Using a $50,000 per acre development cost target, this low impact development solution must achieve all the same access, utility availability, and stormwater drainage expectations as a traditionally designed subdivision. The bottom line is that the development costs for this low impact solution do not look any different than any other subdivision, coming in under the $50,000 per acre target. The neighborhood trunk facilities all have the same design and costs as a traditional design. Sure, a few of the line items in the construction cost estimate are different, but the bottom line is the same. GENERAL SYNOPSIS
There is a noticeably different outcome in the drainage calculations achieved in this low impact design solution. While the developer does not typically see these calculations, the approving government entity, i.e. Harris County Flood Control District, does. In every storm event, the developed peak run-off is lower than existing predevelopment conditions. The necessary volume to achieve this is significantly lower than traditional design. Standard design uses a volume storage rate of 0.55 acre feet per acre of development. This low impact design results in a 0.25 acre feet per acre storage volume. This low impact development design mimics existing conditions and the results are better than traditional suburban design. When developers market their lots to home builders, having already chosen to address important factors like location and amenities, once again the bottom line becomes the most critical consideration. In this low impact design solution, the home builder will not notice any difference. Compared to standard developed lots throughout the Houston area, these lots have nothing different about them. The same standard easements, the same standard building lines and the same standard dimensions are all present on the lots. Utilizing the community rain garden concept does not require the builder to take any special precautions during construction nor does it not put any burden of actually constructing low impact drainage systems on the builder. Nothing is unusual to the home builder, so they can build any home product they desire, just like they would in any other subdivision. This fact allows them to easily compete in the Houston home market versus traditionally designed subdivisions. The home owner does notice a difference between this subdivision and a traditional Houston model. The intent of this low impact design solution is to provide a superior amount of open space and landscaped areas as amenities instead of traditional neighborhood detention facilities either used as water features or part-time parks. Landscaped areas, greenbelts, parks and trails weave seamlessly throughout this design, indicating to home owners that this is not your standard, cramped subdivision. Residents have room to move; they have room to live . The goal of this low impact design solution is to make simple design changes to the subdivision infrastructure without changing what each development group expects from a standard Houston development. This result is a better, more efficient drainage design, which is typically the largest obstacle in Houston development. While the developer and home builder see a standard development with standard development costs, the end-user, i.e. the home owner, sees a beautiful community. GENERAL SYNOPSIS
Note: The lotting target of 1,188 lots was exceeded by 2 lots, which may not seem impressive except for the fact that the 8.5 acres of floodplain in the northwest corner of the tract was left untouched as a natural preserve. LOT COUNT & ACREAGE CHARTS
EPA Storm Water Management Model (SWMM) software was chosen to model this proposed subdivision development to represent the proposed Low Impact Development interconnected hydraulic design techniques. Since this program does not include the SCS Method within its hydrological analysis options, hydrographs were calculated using US Army Corps of Engineers HEC-HMS and then inserted into SWMM at their corresponding junctions. The SCS Method was key in developing hydrographs to best represent amended soils using Curve Numbers within the proposed Low Impact Development rain gardens throughout the subdivision. The improved infiltration capacity of these soil amendments ultimately benefits the overall hydraulic design by decreasing the overall runoff volume leaving the proposed development. The time of concentration was also lengthened by interconnecting the proposed rain gardens through an open ditch system where possible. This open ditch system is designed to increase the time of concentration by not only replacing most of the traditional storm sewer systems, but also by adding improved landscaping to increase the manning roughness coefficient. Additional Low Impact Design techniques were modeled using a stage-area component within the hydraulic model which accounted for the total storage volume provided in the rain gardens. Rain gardens were distributed throughout the subdivision to capture and detain storm water runoff at the source, rather than at the traditional “end of pipe” system. Screen capture of SWMM 5 drainage modeling software at right. HYDRAULIC MODELING
RAINGARDEN DESIGN CRITERIA The community rain gardens are positioned within a 160’ right of way and provide storage capacity for lots and street paving. Each development pod is populated with appropriate acreage of rain gardens for specific numbers of lots. The rain gardens both slow and cleanse the rainfall before delivering the stormwater to the greenbelts which in turn eventually transmit the stormwater to the drainage outfall point on Peek Road.
The low impact landscape concept brings natural areas into the neighborhood environment. Unlike standard subdivision designs where roads, drainage and landscaping are distinctly separate layers in the community, this low impact design solution includes richly planted community rain gardens dispersed generously throughout the subdivision. Responding to the rain garden locations throughout the subdivision which provide drainage capacity, the landscaping introduced in each rain garden provides vast amounts of green space within the community. In fact, 71% of all lots in this subdivision design have direct line of sight to either a community rain garden or greenbelt. The rain garden and greenbelt systems are designed to mimic a natural stream system both functionally and visually. Within each rain garden 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 rain gardens, 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. Some new communities in the Houston area have introduced so called “purple pipe” systems which provide captured rainwater for irrigation as well as re-use of wastewater effluent and are named because of the purple colored PVC pipes connecting the collection ponds to the irrigation heads. Such systems are advantageous in that the water source is free, however the infrastructure and required land area is not. This low impact subdivision design uses native grasses and specific mowing schedules which significantly reduce the need for turf grass irrigation. The rain gardens and greenbelts hold the necessary water within the soils after storm events, eliminating the need for supplemental irrigation. Individual lots within this community can choose to use rain capturing methods for irrigation. The rain garden drainage concept does not require individual lots to hold or increase time of concentration before the water flows into the bio retention cells throughout the community. Any individual lot rainwater capturing only increases the efficiency of the overall drainage system. LANDSCAPING & IRRIGATION
RAIN GARDEN PHOTO REALIZATION This image was created to illustrate a typical street section showing single family lots facing the community rain garden. Buffalo grass and trees populate the areas closest to the paving while a lowland native grass mix and other native plants are used in the center drainage area over the PVC pipe underground.
The innovative design solution for Ventana Lakes illustrates the economic savings to developers when utilizing low impact techniques. The basic design concept of transitioning the stormwater drainage from traditional large, underground storm sewers and massive detention basins to rain gardens and greenbelts yields significant, measurable cost savings. The construction cost estimate comparison illustrates that a majority of the cost savings are realized in drainage channel and detention excavation. Analysis reveals a $4.25 million cost savings by forgoing the traditional ditch and detention basin construction needed for a development of equal size. Paving costs are reduced overall by reducing the linear feet of boulevard streets typically found in suburban development. The traffic volume generated by 1,190 lots is easily served by two-lane collector roads due to the internal loop street pattern and the ease of access to the surrounding major thoroughfares. Additionally, the streets bordering the community rain gardens throughout the subdivision are curbed only on the residential lot side of the street, thus allowing water to drain from the lots, across the paving and into the rain gardens without obstruction. This translates to a cost savings by only building half the linear footage of curbing. This low impact design will require additional landscape installation costs associated with the rain gardens versus a standard community design. However, the fact that 71% of the lots in this design have direct line of sight to a rain garden or greenbelt will allow the developer to recoup much of the landscaping cost by invoking amenity lot pricing, a standard practice in most Houston communities. Traditional subdivisions may label lots with rear yard views of detention ponds as amenity lots, yet only 25% - 35% of the overall lot total may be categorized as such. By comparison, the over 40% increase in amenity lots is a tremendous sales and marketing tool allowing this community to compete favorably with the finest Houston neighborhoods. The potential for Municipal Utility District reimbursables still exists, with infrastructure bonds used to reimburse any pipes, swales, excavation, engineered soils, sand and turf establishment costs. Park bonds may be used to reimburse landscaping costs in addition to trails and park amenities. COST ANALYSIS
As with any drainage system, the low impact rain gardens require minimal maintenance to operate at peak levels. Unlike traditional concrete drainage systems however, the rain gardens 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 rain gardens 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. RAIN GARDEN MAINTENANCE
Considering the sheer number and variation between the numerous suburban developments in the Houston area, creating a unique new community might seem problematic. On the contrary, the low impact design solution for Ventana Lakes demonstrates that a simple and innovative drainage design can yield an unprecedented amenity system which can be easily marketed as a unique place to live. Developers will find the following key calculations of special interest. The facts show that infrastructure construction costs are reduced by 22% to the developer, the rain gardens can be constructed and the costs reimbursed by the Municipal Utility District while the additional landscaping costs can be addressed by using park bonds, also through the MUD. 71% of the lots in Ventana Lakes can be categorized as amenity lots, either having direct front access adjacent a community rain garden or having an attractive greenbelt opposite their rear yard. Home builders will find no difference between these lots and any other subdivision in Houston and the requested lot size targets were all met. Typical land development in the Houston area relocates and mitigates any changes to the 100-year floodplain. This design chooses to leave the 8.5 acres untouched as a natural preserve and routes a trailhead close to the amenity. It is of special note that the lot targets were met without impacting this environmentally sensitive area. Residents of this community will benefit from an abundance of greenspace throughout. The rain gardens add tremendous visual appeal and the greenbelts provide neighborhood connectivity between development pods, school sites and shopping while performing the required stormwater drainage functions. These are both highly marketable aspects which compare favorably to any other new home community. Current sales trends indicate that lots with direct visual connection to green spaces are preferred by home buyers, sell more rapidly and sell for a higher price than non-amenity lots. The design of Ventana Lakes uses standard and accepted land planning principles and follows all governing agency regulations. The internal street design is organized around a loop collector which ties in all four cardinal directions to the major thoroughfare system. Individual pods are designed for flexibility in phase construction and disperse the three different lot sizes throughout the community. The rain gardens are located within public right of way and will be maintained by the local MUD through an interlocal agreement as is standard throughout the Houston area. No individual lot is burdened with maintenance of rain gardens. Any additional low impact techniques utilized by home builders or home owners only enhances the efficiency of the overall drainage system. SUMMATION