Hydrology and Hydraulics

Joseph P. Wilson, PE, PH, D.FERolla, Missouri

Acknowledgements

A special thanks to:Chris Dunnaway, City of SpringfieldKevin Barnes, Greene County ResourceManagementRyan Stack, Missouri Department ofNatural Resources, Dam & ReservoirSafetyDr. Charles Patterson

Session ObjectivesWhat are we trying to accomplish?Fundamentals of hydrology andhydraulics.Some of the common issues.Review some examples andcomparisons with specific considerationfor the City’s Design Criteria.

Topics for DiscussionComputation of Runoff

Topics for DiscussionCulverts

Topics for DiscussionDetention

Topics for DiscussionStorm sewers

Topics for DiscussionInlets

Topics forDiscussion

Open Channels

Topics for DiscussionSinkholes

Topics for DiscussionRules of Thumb

Hydrology

The study of the occurrenceand movement of water(Chow 1959).Why are we interested in

stormwater?

Hydrologic Analysis andHydraulic Design

Estimate runoff:Design of improvements, bridges, lakes, stormsewers, etc.Water quality.Determine flood risk for a location.Determine where to locate improvements.Replicate an observed event.

Note: We are working with estimates only…Models, by definition, are approximations of

reality!

A Little Background…January 1978 – August 2016, Missouri had 46,073flood insurance claims totaling over $727 billion(Floodsmart.gov).Nationwide flood insurance claims exceeded $53trillion for the same time period.Flash flooding is the leading cause of weather-relateddeaths in the U.S.- approximately 200 deaths peryear (CDC).Over 50% of flood-related drownings are vehicle-related.

Stormwater CriteriaPrimary obligation to protect the public.Jurisdictions establish minimum designcriteria to provide improvements withan acceptable level of risk for thepublic.Keep in mind that an accumulation ofminimums will most likely result in asubstandard design.

Columbia, SC, Associated Press, Tyson Bevirt, 2010

We don’t want to have to appear here!

Missouri Water LawRule of reasonable use.“Reasonable” to be determined by courts ona case by case basis.Three basic tenets:

Was the alteration to the drainage required toutilize the property?Were reasonable efforts made to mitigate theimpacts?Does the utility of the alteration outweigh thepotential harm?

No caption needed!

Hydrology is probability based

What is probability?Statistics allows one to make decisions abouta population of items, in this caserainfall/runoff events, based upon arepresentative sample.Based on an acceptable level of risk. TheAnnual Exceedance Probability (AEP) is thelikelihood a flowrate will be equaled orexceeded in any given year, 1/RecurrenceInterval.

Factor of SafetyHydrology is risk based.We select AEP’s based on allowablerisk.Selection of difficult to measure analysisparameters allows one to add anadditional factor of safety.For example: time of concentration,curve numbers, impervious area.

What is conservative withrespect to hydrology?

Higher peak discharges?Higher volume?What about situations that involvestorage?

Photo credit: stolen fair and square from internet.

“Must Have” ReferencesOpen Channel Hydraulics, ChowHandbook of Hydraulics, Brater & KingApplied Hydrology, Chow, Maidment, MaysNEH-4, National Engineering Handbook, Chapter 4,Hydrology, NRCSHDS-5, Hydraulic Design of Highway Culverts, FHWAHEC-22, Urban Drainage Design Manual, FHWATR-55, Urban Hydrology for Small Watersheds, NRCSHEC-HMS and HEC-RAS Manuals, U.S. Army Corps ofEngineers

Suggested Public Domain SoftwareFederal Highway Administration:

Hydraulic ToolboxHY-8

U.S. Army Corps of Engineers:HEC-RASHEC-HMSHEC-1

U.S. Geological SurveyNational Stream Statistics