<ul><li>Human needs and a healthy environment are not opposing claims that must be balanced; instead, they are inexorably linked by chains of cause and effect. We need a healthy environment because we need clean water, clean air, wood, and food. </li></ul><ul><li>-Jared Diamond, biologist and author </li></ul>A Low Impact Development in Katy, Texas Ventana Lakes: Team: 19186
LID design allows for 3 decisions unavailable to traditional land designer. Since a great amount of stormwater is either captured or significantly delayed in getting to the stormwater detention facility, significantly less detention storage area is required for LID. The decision, what to do with the leftover land, allows the developer to increase lot yield, increase green space, or implement less intrusive detention facilities. Selected for this project were the later two. The water surface in the detention area was raised, the detention area was reduced by 10%, and, the side slopes were increased to 6:1 to reduce the need for maintenance berms. The resulting land was distributed as green space within the neighborhood so that every home has close access to green space, parkland, and walking trails. LID development in Maryland and Washington suggests that Houston traditional neighborhoods with blocks of 50 foot, 60 foot, and 70 foot lots will not be common, although required in this contest. LID developments when implemented effectively, has shown that sections with mixes of 50, 60 and 70 foot lots, and the variation in the homes that it causes, are more popular than non-mixed sections all of the same size. Developer’s Non-LID Plan LID Plan Traditional vs. LID Land Plans Click to Zoom
LID Methods: Dry Swales (Bio-Swales) <ul><li>Reduces run-off by encouraging infiltration </li></ul><ul><li>Only effective when designed as integral part of overall stormwater management system; surface drainage must be directed to swales </li></ul><ul><li>Encourages the planting of visually appealing, native vegetation along roadways </li></ul><ul><li>Allows for increased vegetation diversity; plants that can withstand periodic inundation are very different from “typical” landscape plants </li></ul><ul><li>Placed along streets, vegetated bioswales can actually make the residential lots “feel” bigger by providing visual depth </li></ul><ul><li>Increased curb appeal </li></ul><ul><li>Both storm sewer and gravel bedding must be graded to promote flow </li></ul>Pearl Brewery San Antonio, Texas Sustainable Sites Initiative. 2009. The Case for Sustainable Landscapes . Available at http://www.sustainablesites.org/report.
LID Methods: Rain Gardens <ul><li>Increase infiltration and reduce surface run-off </li></ul><ul><li>Terraced gardens decrease surface water velocity and hold water on site longer </li></ul><ul><li>Less expensive to install than catch basins and associated underground pipe </li></ul><ul><li>More efficient use of land; a visually pleasing and effective stormwater measure </li></ul><ul><li>Increases water quality by removing pollutants and suspended solids from run-off </li></ul>Malolepsy/Battershell residence garden Portland, Oregon Sustainable Sites Initiative. 2009. The Case for Sustainable Landscapes . Available at http://www.sustainablesites.org/report.
LID Methods: Appropriate Vegetation <ul><li>Vegetation for rain gardens and bio-swales should have dense, fibrous roots for maximum water intake and water velocity reduction </li></ul><ul><li>Native plants often fit this category, which also utilize less water in times of lower rainfall </li></ul><ul><li>Amended soil areas, bio-swales and rain gardens provide a diversity of soil types, which can support a larger diversity of plant species than a “conventional” neighborhood, adding to the visual appeal of the community </li></ul>Chestnut Creek Restoration Project Town of Neversink, New York Sustainable Sites Initiative. 2009. The Case for Sustainable Landscapes . Available at http://www.sustainablesites.org/report. www.phoenixportals.com/photo.html www.nativeturf.com/images/bowie-buffalograss.jpg Buffalo Grass Texas Wild Flowers Mixed Vegetation
Pavement Safety Edge Treatment Dry Swale with SC-310 system Rain Garden Amended Soils Pervious Sidewalk, Rain Barrels Drive Pavers 3000 SF 70x125’ Lot 1800 SF 50x115’ Lot 2200 SF 60x115’ Lot COST COMPARISON INDIVIDUAL LOTS The cost calculations for the individual lots is included in the description of individual practices. The assumptions utilized in those calculations include the following: the garage setback is 25 feet; the back yard setback is at least 25 feet; the side lot setback is 5 feet; drives are assumed to be 17 feet wide with 5 foot turning radius; sidewalks are 5 feet wide; pavement is 30 feet wide which includes 2 feet of safety edge treatment on each side; medians will be constructed as raingardens; and non-home right-of-way will be constructed as dry swales if they do not include storm sewers, or amended soils if they do. These calculations were used to determine costs for the entire subdivision as designed. Amended soils are to be constructed over all green spaces, and for all portions of the detention system above the grade break. All properties will have raingardens per their lot size.  ECONorthwest, The Economics of Low Impact Development: A Literature Review, 2007, p 18  Urban Forestry, “Southeast Watershed Forum Wetland Fact Sheet,” Per the literature review by ECONorthwest , implementation of LID methodologies has a positive impact on property values ranging from 0 to 5% in floodplains, and up to 15% in non-floodplain areas. For the types of homes recommended in the development report this represents about $10,000-$20,000 per home. The same report, states that additional trees could reduce Houston’s stormwater infrastructure costs by $1.3 billon. Per Urban Forestry , a medium sized tree can absorb up to 2,380 gallons per year in its canopy.
COST COMPARISON for the 85.60 Acre Demonstration Tracts The subdivision includes 130 50-foot lots of which 37 are contiguous with significant green space, 59 60-foot lots of which 39 are contiguous with significant green space, and 58 70-foot lots of which 53 are contiguous with significant green space. The main collector street which is 80-100 feet of right of way and contains a significant blvd. The subdivision designed includes three sections, one each of 70 foot, 60 foot, and 50 foot lots. The selected pods all highly resemble standard development from the perspective of how utilities would be constructed, and how land in a pod would be allocated. The additional green space added due to the LID would change the shape of the section, but not the utilities. The comparison made for this project is therefore only dealing with the drainage system. As can be seen from Figure A, conventional system has inlets located at a number of points. These inlets provide opportunities for pollutants to reach the storm drainage system. Based upon this design, the storm sewer system is estimated to cost $1,370,000. Detention is based upon HCFCD minimum criteria, for small watersheds which requires 0.65 ac-ft per acre, and does not allow for variance from that minimum for developments other than pure parks. Figure A: Conventional Design Figure B: LID Design Figure C: Street Grades Click to Figures to Zoom in Figure D: Hydrographs And Explanation
Representative Cross Sections – Two Lots LID development allows lower lots, and therefore steeper slopes from the back of the lot to the front of the lot, thereby promoting better drainage from back to front. With all the extra green space, and with regulations that permit lots to drain from the lot into the public green space, type B grading will become more feasible, further decreasing drainage issues in the back yards. Also LID subdivisions do not have the issue of curbs preventing proper sheet flow outflow. Click to Enlarge Click to Enlarge
Local Street Section Conventional streets consist of a curb and gutter section with 13 feet of pavement with a curb and gutter. All lot drainage and street drainage is directed to gutters, which flow along the street to the gutter. Wide and potentially deep ponding areas are prevalent near the inlets during storm events. LID streets consist of slightly lower pavement, with vegetated swales located at almost all points along the street. Given this design, there are no deep points near inlets required. Streets can be gently graded to their final outfall. The bioswales provide both storage and drainage for the street’s stormwater. Ponding should only occur when both the infiltration rate of the bioswale (designed to be between 10 and 25 in/hr), and the volume of storage (5 cubic feet/ linear foot of bioswale) is exceeded. This adds safety to the road system, increases access, and better drainage.
Collector Street Section LID streets are designed such that they drain into the bioswale as their primary drainage system. The bioswale provides filtered drainage with complete solids removal prior to entering the outfall drainage system. With no inlets or manholes, the two pollutants of most concern in Harris County (floatables and solids) have no access to the drainage system. Per studies in Maryland and Washington, once the bioswales become dominated by a robust root system, there is virtually no bacteria exiting the swale. Per Coffman (2008) biological treatment trains are capable of removing 95% of the TSS, 99% of heavy metals, 95% of oil & grease, 80% of total phosphorus, 40% of total nitrogen, and 80% coliforms. Unlike traditional systems which have decreasing efficiency with time, bioswales only get better with time as root systems become more prevalent, and more established. Selection of robust plants for the bioswale result in a system that does not require irrigation, further saving a valuable, and limited resource, drinking water. Collector streets under the LID scenario work very similarly to local streets. The major difference is that both the median and the green space next to the sidewalk are designed as bioswales or rain gardens, with the rainwater flowing towards the vegetation. Irrigation should not be required for either biological system. Trees, with their constant irrigation by normal low intensity events, should grow robustly.
Gutters Outfall Onto Drive / Pavement Few Trees In “Sea Of Lawn” Drive Serves As A Stormwater Conveyance To Curb And Gutter Street Driveway Grade Is Below Yard For Drainage Shrub Planting At Base Of Home Curb And Gutter Traditional Neighborhood Lot From a stormwater perspective, conventional development does not provide any avenues for reducing water volume on-site in soils which are predominately clay. Leaves from trees and large plants can capture water. However, in Houston, conventional development does not encourage the implementation of numbers of large trees. Grass provides limited capacity to capture water. The soils are compressed prior to home construction and may take years to recover. Even after recovery, clay soils are not noted for their capacity to store water. Most lots are graded to reduce the probability of localized ponding. The increase in time of concentration due to flow time across a developed lot is relatively low. Street drainage fairs little better. It is directed to inlets and storm sewers through a very efficient system of smooth channels. The average distance between inlets is kept short, less than 300 feet, the slopes are relatively consistent at 0.3%, and in general, flows from streets reach the storm sewers in less than 5 minutes [TxDOT, Figure 5-4].  Texas Department of Transportation, TxDOT Hydraulics Manual , 2009.
Gutters Outfall Onto Landscape Swales Between Lots Convey Lot Drainage Rain Gardens Along Property Lines Collect Storm Water Driveway Grade Is Higher Than Yard No Curb And Gutter; Runoff Drains Into Bioswale Low Intensity L.I.D. Vegetated Bioswale Along Street Textured Pavement Along Street Edge For Safety From a stormwater perspective, lots designed with Low Impact Development in mind do consider stormwater storage and time of concentration reduction. Rather than direct gutters to the sidewalk, they are directed to the grass. The grassy areas have been amended to better facilitate transfer of water to the root zones of plants. The natural flow path is across grassy surfaces, which significantly increases time of concentration. The raingardens provide both changes in time of concentration, and add volume for temporary storage. The outfall system consisting of a small storm sewer draining to the vegetated bioswale acts as a restrictor. Road systems get some of the same benefits as do lots. Drainage from the road to the bioswale is relatively quick. Bioswale has above ground storage for the first 5 cf/LF of bioswale. In addition, the bioswale allows flow through the engineered soil at a high rate of storage. The SC-310 culvert provides a flow path to the detention system. The stone around the culvert provides storage and slower outfall rate. The first 2-inches of stormwater must travel through the gravel to the final
Porous Pavers Provide Easier Drainage To Bioswale And Rain Gardens Water-loving Vegetation Provides Filtration In Rain Gardens And Bioswales Medium Intensity L.I.D. outfall. The rate of travel through the gravel is very slow. Per Mulqueen , flow through medium grade sharp gravel follows a formula of K=1918 i -0.13 where K is hydraulic conductivity in m/day and i is hydraulic gradient. Based upon the hydraulic gradient of 0.25% for the stone grade, and converting K to ft/min yields K=4.370 i -0.13 , and a velocity equal to approximately 9.5 feet per minute. This is significantly different than the 72 feet per minute estimated by using TxDOT . Quantifying the impacts utilizing procedures already utilized by the City and the County requires consideration of storage, time of concentration variations, and the overall design of the system. Maryland recommends the use of a curve number approach in determining the impacts of LID. Harris County utilizes rational method for design of storm sewers, and uses site runoff curves to determine flow rates and volumes for 10 and 100 year events. For the purpose of our analysis, we are presenting LID in a framework of existing County methodologies to the maximum extent we can.  J. Mulqueen, “The flow of water through gravels,” I rish Journal of Agricultural and Food Research 44: 83–94, 2005
Additional Trees And Vegetation In Yard, Rain Gardens, And Bioswales Provide Enhanced Filtration And Increased Aesthetics Rain Barrels In Back Yard Collect Water For Future Irrigation High Intensity L.I.D. Since access to the primary drainage system for both the dry swales an rain gardens requires infiltration, design of infiltration rates into the engineered soil is important. Based upon existing literature from Maryland and Washington, infiltration rates in engineered soils can range from 100-inches per hour to 4-inches per hour. A 25-inch per hour infiltration rate should be possible with a combination of rice hull compost (15%), sand (65%) and sandy loam (20%)
Neighborhood Perspective: Detention As An Amenity Lake With Trails Because a significant portion of the stormwater is captured and stays onsite, the facilities used for stormwater can be smaller, with shallower side slopes, and provide more opportunities for recreational use and varying vegetation regimes. We have proposed areas of wildflowers, buffalo grass, and urban forest so that people walking, running, or playing can experience the nature of Texas as it is supposed to be.
Amenity Lake And Trails Detail The other than recreation, the advantages of a wet pond include the possibility of groves, or perhaps even stands of trees within the high banks and flat land areas of the ponds. Appropriate broad-leaf, native, and local trees for each of these areas could result in extremely high levels of stormwater capture.
LOW IMPACT DEVELOPMENT, THE LOCAL PERSPECTIVE INTRODUCTION AND APPROACH In some form Low Impact Development must now be considered as a most likely scenario for Harris and surrounding counties. Historically the City of Houston and Harris County have been leaders in implementing procedures and strategies which influence the quality and cost of land development throughout the region. The "buy in" of these agencies is a critical component of any program of the type to be considered as are equally important issues such as Municipal Utility District funding, developer and builder costs, public sector and property owners association maintenance involvement and last and by no means least, homeowner/taxpayer endorsement based upon an obvious benefit. Low Impact Development (LID) methodologies can consist of multiple options and variables and no single program will provide answers for all situations to be considered. this presentation will address some basic requirements necessary to produce the results envisioned and place high emphasis on the critical issues associated with the funding and implementation of a significantly different approach to land development. These issues are the eligibility for public funding, construction sequencing and the operation and maintenance of a different form of drainage infrastructure. PUBLIC FUNDING Some design parameters for the placement of water and wastewater systems will be suggested however, no changes in these elements of public infrastructure are proposed. For cost considerations a modification to the location of wastewater systems, discussed later in this report, can be considered which would bring about cost savings which will offset additional costs which may be incurred for drainage systems. The drainage systems considered will be significantly different from conventional systems and, to qualify for public funding, must be of such character that they are eligible for acceptance by either the County or the Municipal Utility District. The concept of regional storm water detention will remain unchanged however, the volume of detention will be reduced which will result in the lowering of construction costs and an increase in overall lot yield occasioned by less land being required for detention purposes. The most significant change from conventional storm sewer and inlet type drainage will take the form of dual storm water storage/conveyance systems, one on each side of the street. Extreme event conditions will be handled as now by sheet flow management which directs such flow via the streets to a regional detention facility. The dual storm water storage/conveyance system along each side of the street will incorporate a shallow swale on it's surface in combination with a curbless crowned street. Other portions of this presentation will show details of the function and economics of this treatment, all of which may be eligible for public funding.
CONSTRUCTION SEQUENCING Significant changes in the scope and manner of the required development construction will be necessary to bring about the desired environmental impact. The systems will be extremely vulnerable to the concurrent construction of streets and utilities and especially vulnerable to the home building activities in the immediately adjacent area. for this reason the street drainage systems will require protection in the form of a reinforced filter fabric fencing and the construction of the residence driveways up to the right of way line to provide builder access to each home site. Construction of driveways integral with the street paving will require that consideration be given to incorporate an appropriate cost sharing between the builder and the developer however, if the logistical issues are addressed, cost savings should accrue to the builder. The sequencing and management of drainage, utility and street construction will be a major factor in determining the economic feasibility of the proposed design scheme however, the following sequence can be implemented with the cooperation of contractors willing to be involved in such an innovative program: <ul><li>Clear lots and streets followed by the construction of a series of swales to control surface drainage during the construction activities which must follow. Interim site drainage by swales of this type will be influenced by topography and the street plan. </li></ul><ul><li>Construct the regional storm water detention facility and dispose of the excavated material in areas compatible with the overall LID design. </li></ul><ul><li>Construct water mains and wastewater mains and service lines. </li></ul><ul><li>Excavate streets and roadside ditches with disposal of materials at locations coordinated with LID design requirements. These roadside ditches will be the first portion of the parallel drainage systems. </li></ul><ul><li>Construct drainage at street crossings. </li></ul><ul><li>Pave streets. </li></ul><ul><li>Complete construction of the conveyance/storage drainage system on each side of the street. Construct swales on the surface of the system. </li></ul><ul><li>Construct driveways to provide builder access to each home site. </li></ul><ul><li>Construct a reinforced filter fabric fence around each element of the drainage system between driveways to provide protection during home construction. </li></ul><ul><li>If sidewalks are required and not provided by the developer, the builder will construct the sidewalks and the finished grading for the green systems. </li></ul><ul><li>This construction sequencing procedure will require a coordinated effort between the constructors of the utility and drainage systems and the constructors of the paving. A third contractor should be engaged to construct the "Green Systems" under the coordinated control of the developer, the engineer and the builder. Though unconventional, this collaboration, essential to the satisfactory development of the program, can be accomplished. </li></ul>
OPERATIONS AND MAINTENANCE Four units are involved in the operation and maintenance of subdivision infrastructure. They are the County and particularly the County Precinct, the Municipal Utility District, the Harris County Flood Control District and the property owners association which can accept a variety of responsibilities for supplementing the maintenance activities of other entities. It is contemplated that few issues, if any, will confront these agencies in their acceptance of the systems proposed by the suggested program. The critical issue to be dealt with in this conversation will be the storm drainage system. Fortunately many examples of the use of the materials and methodologies suggested are available and if the public sector is open to this concept, and we believe they are, Low Impact Development of this type can be successfully implemented without significant cost upset, if any. OTHER FACTORS AFFECTING DEVELOPMENT COST Presently wastewater systems are constructed on one side of the street right of way with service lines extended to the opposite side of the street. In order to avoid conflict with the storm drainage systems these service lines are much deeper at their terminus than if they were constructed in a different location. If they are constructed at the rear of the lots they are to serve the entire wastewater system could be raised at least six feet with major cost savings resulting both from the construction of a shallower system of collection lines but also from the reduction in the number of sewage pumping stations which the deeper systems require. Another option to consider to bring about similar savings would be to construct shallow systems on each side of the street thereby avoiding and requirement to avoid collisions with the storm system. Duplication of lines may increase the length of the sewer system however the shallower depth of the system together with the absence of the requirement to use special backfill under the pavement will more than offset this additional cost.
Traditional Neighborhood Perspective When all is said and done, there remains only one true question.
High L.I.D. Neighborhood Perspective Wouldn’t you rather live here?