The intent of this MCM is to lessen storm water quality impacts after construction has been completed on new and redeveloped sites of one (1) acre or greater of disturbed earth. Dallas County’s Systems-In-Place consisting of adopted Commissioners Court Orders, state regulations, County policy, and engineering / construction plan requirements presently provides Storm Water Phase 2 and MS4 compliance in the County Unincorporated Urbanized Areas (UUA’s).
“To the extent allowable under state and local law”, Dallas County will develop and implement a Post-Construction MCM for the County Unincorporated Urbanized Areas. Like most Texas Counties, under this MCM Dallas County presently allows the use of unlined or pervious drainage ditches in lieu of impervious concrete curb and gutter pavement sections in combination with underground storm sewer / drain pipe systems and / or concrete lined channels which Cities typically require and / or allow.
Compared to standard impervious underground storm sewer / drain pipe systems, at grade open / unlined grassy ditch systems allow more storm water runoff to soak or filtrate into the ground. This type of drainage system thereby reduces downstream runoff velocities and erosion, and provides a means of detaining and treating, to some degree, pollutants such as sediments, fertilizers, nutrients, detergents, etc.. It also allows illicit discharges to pond and collect making them easier to detect, locate, and consequently to investigate the probable source to expedite the IDDE’s elimination.
Such grassy drainage ditch systems are the least expensive to construct, are compatible with larger lots that are required for maintaining OSSF’s (greater that ½ acre in land area when supplied with an underground potable water supply, or greater that 1 acre land when supplied with a private potable water well). This type of rural land development is popular among residents of the County Unincorporated Areas providing an incentive for their continued installation beyond this permit period. Like most Texas Counties, Dallas County presently maintains these at grade open / unlined grassy drainage ditch systems located within the County Road Right-Of-Way (R.O.W.), easement, or prescriptive rights easement, and shall continue to do so throughout the permit period.
Dallas County regionally participates in and coordinates with the NCTCOG on municipal storm water issues through the Regional Storm Water Management Program. Program includes cooperative initiatives that are regional and development of the annual regional storm water work program. Construction Site Storm Water Runoff Control is achieved regionally by utilizing the NCTCOG website for informational support and access to Best Management Practices (BMP’s). Coordination meetings with Program updates including watershed roundtable discussions for the multiple municipalities, counties, and governmental agencies are facilitated by the NCTCOG quarterly. Dallas County’s annual participation costs are approximately $ 2,110. 21
a) BMP, Measurable Goal, and Date – Strategy: Allow Pervious Drainage Systems
In lieu of requiring impervious concrete curb and gutter pavement sections in combination with underground storm sewer / drain pipe systems and / or concrete lined channels, Dallas County will generally allow developers, home builders, and residents in the County Unincorporated Areas to use at grade open / unlined grassy drainage ditches adjacent to the privately or County owned and maintained roads. This is both cost effective to the construction community and environmentally based for the County’s compliance with the State storm water permit. Specific legal agreements with some adjacent municipalities may supersede this County policy, but overall this strategy and policy will continue for the duration of the permit period.
b) BMP, Measurable Goal, and Date – Tracking: Additional Pervious Drainage Systems
Dallas County Road and Bridge forces will maintain the open / unlined grassy drainage ditches located within the County Road Right-Of-Way (R.O.W.), easement, or prescriptive rights easement in the Unincorporated Areas. This will allow the drainage system to operate properly and produce storm water quality benefits as stated previously.
Dallas County Stormwater Management Program