Notes

Slide Show

Outline

1	Hamilton County, Ohio Wet Weather Initiative Meeting City of Dublin, Ohio Stormwater Master Plan and Other Lessons Learned Presented by: Christopher T. Calpin, P.E. Principal Engineer Cincinnati, Ohio October 25, 2001
2	Problem Statement Increased frequency of large events Increased frequency of overbank flow Stormwater pre- and post development runoff calculations??? Appropriate runoff calculation methodologies Rational method, TR-55 or SCS runoff curve number, SWMM -RUNOFF)
3	Dublin, Ohio’s Stormwater Program Elements Stormwater Master Plan Stormwater Funding Evaluations Administrative Policy and Stormwater Ordinance Evaluate BMP and Master Plan Impacts
4	Why did Dublin elect to perform a Stormwater Master Plan? Located in northwest Franklin County, and in Union & Delaware Counties Land area - 21 square miles Watershed area – 35 square miles Population - 32,735 10 year growth - 200% Memorial Tournament
5	Rainfall data shows the 1% chance storm volume has increased 20% in the last 30 years.
6	Development increases the imperviousness which impacts the watershed hydrology and hydraulics.
7	What are the impacts of more frequent overbank flooding?
8	Erosion and undermining of trees, buildings, roads, culverts, etc. and adverse impact on aquatic life.
9	Are stormwater engineers accurately calculating pre-development runoff rates? Dublin has determined the pre-development runoff rates Based on a calibrated SWMM-RUNOFF model Dublin dictates allowable post development runoff release rates Critical storm criteria Developers typically use Graphical Peak Discharge methodology for post development runoff and detention requirements (e.g., TR-55 or SCS Runoff Curve Number)
10	Dublin Example: 16 acre watershed
11	What does the example tell us about Dublin’s Approach to stormwater management? Better predicts predevelopment runoff (calibrated model) Helps to control erosion and flooding Developers are over predicting predevelopment runoff for the smaller storms Exacerbates erosion Equivalent, or slightly more stringent, flood control requirements on larger storms
12	What does the example tell us about Dublin’s Approach to stormwater management? TR-55 generally results in more conservative flow estimates (pre- and post development) SWMM-RUNOFF results in less conservative (more realistic) flow estimates Result – Built In Factor of Safety in Detention Basin Sizing
13	What are the differences between SWMM RUNOFF and TR-55? Impervious area Directly connected impervious area Pervious area Initial & final soil infiltration capacities Maximum soil storage Runoff curve number Simulate impervious and pervious areas, and soils Initial abstractions All losses before runoff begins Soil storage capacity
14	How will Dublin’s master plan and new ordinance impact development? Release rates/water quality BMPs more stringent than MORPC stormwater criteria Dublin considered the impacts to be minimal and enacted new stormwater management ordinance Dublin continues to require these standards on all new developments
15	What does the future hold for stormwater management? Volume-Time Detention Control Control both peak flow and volume over a critical time period Better for erosion and flood control Continuous Simulation Modeling Stream bank restoration
16	Volume – Time Detention Control
17	Continuous Simulation Modeling