Development of Decision Making Tool (DMT) for Determining LID Stormwater Detention Requirements

Summary

To address localized flooding in the Lower Rio Grande Valley (LRGV), Texas region and U.S.- Mexico border water quality issues associated with non-point source pollution within the Arroyo Colorado watershed, the LRGV TPDES Stormwater Task Force, in partnership with the NADB BECC U.S. EPA Border 2020 program, supported the development of an innovative Decision Making Tool (DMT) for determining stormwater detention requirements while incorporating Green Infrastructure. TAMUK has partnered with the LRGV TPDES Stormwater Task Force (STF) in the planning and delivery of the DMT with this report.  In addition to this descriptive report, the Project Team is also delivering a copy of the developed Excel spreadsheet tool along with a User’s Manual document for the Tool.

Local governments in the LRGV control flooding and stormwater runoff by adopting strict drainage design policies. The flat terrain characteristic of the LRGV provides stormwater engineers with complicated flow, detention and flood design problems. According to an existing drainage policy, stormwater runoff generated from new commercial developments within the LRGV is generally required to be detained at on-site for a 50- year frequency storm event and released into the receiving system at a pre-developed rate for a 10-year frequency storm event.

One conventional approach to meet the discharge goal of retaining this huge volume of runoff is to design a detention pond with large footprints. Although traditional methods of designing detention ponds are somewhat cost-effective and calculation of storage is simple using excel spreadsheet, this approach may be a cause for lost land cover and aesthetic, safety, operating, and maintenance issues in the long run. With the use of innovated practices and green infrastructure strategies, the traditional design of a large detention pond footprint can be reduced by allowing the storage volume of Low Impact Development (LID) Best Management Practices (BMPs) to be incorporated into conventional design detention calculations. The challenge is to decide how to plan those BMPs design effectively within the development boundary to meet the 10 years frequency storm discharge goal.

The developed DMT tool and project achieves the two following principal objectives-

1. To provide a unique innovative calculator that can be used to determine stormwater detention requirements at urban and rural developments in the LRGV using LID

2. To conduct educational outreach activities in order to promote this tool to local school districts officials, colonies, institutes of higher education, city and county officials, water professionals, professional organizations and water-related organizations.