ID#6085 made on
SSc6.1 - Stormwater management - rate and quantity
LEED BD+C: New Construction
When construction is completed in January 2007, the Dell Children's Medical Center of Central Texas (DCMCCT) will consist of a 470,300 square foot full service hospital. The existing 32.2 acre s...
When construction is completed in January 2007, the Dell Children's Medical Center of Central Texas (DCMCCT) will consist of a 470,300 square foot full service hospital. The existing 32.2 acre site is part of an abandoned airport facility (RMMA). The hospital project is the first building to be constructed within what is designated as Phase 1A of a 708-acre Planned Unit Development (PUD) less than three miles from downtown Austin, Texas. The Phase 1A development site totals 82.2 acres. The intent of SS Credit 6.1 is for the site to have no increase in the rate and quantity of storm water runoff due to the proposed development. The storm water management system for the RMMA PUD Phase 1A development, which promotes a wide variety of sustainable practices, is designed with a sedimentation pond and dual extended detention ponds to create a system that minimizes run-off, increases infiltration, reduces the potential for downstream flooding and erosion and reduces contaminant loads into the watershed in order to fully comply with the City of Austiníªs and the Lower Colorado River Authorityíªs ordinances for storm water management for all new building sites and developments. The City of Austin encourages regional water quality and detention ponds for developments, preferring fewer larger ponds rather than many small ponds on each separate property in order to achieve better management and maintenance of the ponds. The DCMCCT project will help directly fund the capital and maintenance costs for the new RMMA PUD storm water management system infrastructure of ponds and greenspaces. The RMMA PUD is within what is designated as the Tannehill Branch Watershed. The outfall from the detention ponds connects to an existing Texas Department of Transportation (TxDOT) drainage system. TxDOT requirements for the RMMA PUD Phase 1A development are that storm water flow rates from the extended detention ponds to the point of connection at the TxDOT drainage system be maintained at equal to or less than the existing storm flow rate which is equivalent to 9.2 cfs for a 1.5 year storm. The Zoning Regulations for the RMMA PUD allow development of 80% impervious cover. The proposed DCMCCT site impervious cover as designed is 72%. Several methods have been utilized to minimize the imperviousness of the site including íºGrasspaveí¿ pervious pavers, green roof and interior courtyard areas, a 3 acre íºHealing Gardení¿ and grass vegetative strips and numerous tree islands throughout the surface parking areas and around the perimeter of the site. The existing DCMCCT site impervious cover is 35% consisting of asphalt airport runways. The remaining pervious cover on the site consists of turf grass and weeds surrounding the runways. The 1.5 year storm event runoff for the RMMA Phase 1A development site with existing conditions equates to 96 cubic feet per second (cfs). The contribution of the DCMCCT site to this total is 52.8 cfs. With Phase 1A of the RMMA PUD fully developed (including CMCCT) the proposed 1.5 year release to the TxDOT drainage system from the sedimentation pond and dual extended detention ponds is 7.7 cfs. This exceeds the LEED requirement for no net increase in storm water rate and quantity on the post-development site. The design team would like to confirm that the intent of the requirements for SS Credit 6.1 are met for this project. The combination of the measures designed for the DCMCCT site and the 82 acre RMMA PUD Phase 1A site mentioned above will far exceed the LEED Credit requirements for the DCMCCT project.
If the existing imperviousness is less than 50%, the requirements for SSc6.1 (v2.1, with March 9, 2004, erratum) are to "implement a stormwater management plan that prevents the post-development 1.5 year 24 hour peak discharge rate from exceeding the pre-development 1.5 year 24 hour peak discharge rate AND quantity." The purpose of the erratum was to correct an erroneous diversion from Version 2.0 performance levels (v2.1 revisions had to maintain the same levels). The justification for the appropriate release rate of stormwater run-off has been demonstrated. Regarding quantity; it was stated that; "The storm water management system for the RMMA PUD Phase 1A development, which promotes a wide variety of sustainable practices, is designed with a sedimentation pond and dual extended detention ponds to create a system that minimizes run-off, increases infiltration, reduces the potential for downstream flooding and.....". This implies that some volume of stormwater is allowed to infiltrate. However, in order to demonstrate that the volume required by the LEED criteria is met, calculations should be provided that show that for a 1.5 year 24 hour storm, the difference in volume of stormwater from pre-development to post-development conditions is retained on site, or in the detention pond and is allowed to infiltrate. The imperviousness of the site has been increased from 35% to 72%. This is assumed to be a composite impervious area and takes into account infiltration in the pervious areas of the site. This increase in the amount of impervious surface will generate additional stormwater run-off volume and flow rate. The additional volume must be allowed to infiltrate to meet the LEED criteria. This can be demonstrated if the infiltration rate of the bottom of the detention pond times the area of inundation will allow the volume required to infiltrate before the pond empties at the controlled rate of discharge. It can also be demonstrated if the required volume is retained indefinitely in the detention pond and is not released through the controlled release structure.
Related Addenda (Corrections & Interpretations)