Title
Review and Discuss Draft Revisions to the Stormwater Management Ordinance
Summary
COUNCIL GOAL: Operational Excellence
MEETING DATE: March 20, 2017
DEPARTMENT: Development Services / Engineering
CONTACT: Mark Hines, PE, Director of Engineering
RECOMMENDED CITY COUNCIL ACTION:
• Review and Discuss Draft Revisions to the Stormwater Management Ordinance.
ITEM SUMMARY:
• City staff has several recommended changes to the Stormwater Management Ordinance to better address the stormwater management needs of the City.
• One significant change is rounding all floodplain elevations to the nearest 0.1 feet (just over an inch).
• Another significant change is requiring detention ponds in new developments to limit post-project peak discharge to the pre-project level for both the 5-year storm and the 100-year storm.
BACKGROUND INFORMATION:
• Regarding the rounding of all floodplain elevations to the nearest 0.1 feet:
o Hydrology, by its very nature, involves a high degree of uncertainty.
o Assumptions are used in hydrology calculations for rainfall amounts, rainfall distributions, times of concentration, impervious cover percentages, topography, Manning’s n values, and so on. Because they are based on assumptions, it must be acknowledged that results obtained from hydrology calculations are not extremely precise.
o Because of all that, it is suggested that all floodplain elevations be rounded to the nearest 0.1 feet.
• Regarding detention of the 5-year storm event:
o From 2006 to 2014, new detention ponds in McKinney were required to limit both the 10-year and 100-year peak discharges to pre-project levels and release the 1-year storm over a 24-hour period.
o With the intention of simplifying requirements and making the development process more predictable, the 2014 revision to the ordinance required that detention ponds only need to limit the 100-year peak discharge to pre-project levels.
o City staff now recommends that detention ponds in new developments must also limit the 5-year peak discharge to the pre-project levels.
o This change is recommended only to reduce the risk of minor flooding that may occur by the free release of smaller storms.
o This change is not anticipated to create an undue amount of additional work for developers.
FINANCIAL SUMMARY:
• N/A
BOARD OR COMMISSION RECOMMENDATION:
• N/A