2012Overview: The Functions &Values of RiversPatrick C. Garner, Patrick C. Garner Co., Inc.
How Do We Define A River? A river is “any naturalflowing body of water thatempties into any ocean,lake, pond, or other riverand which flowsthroughout the year.”(310 CMR 10.04) That’s only a start. Let’sdive down a bit…
Watershed Characteristics• Area• Surficial Geology• Underlying Soils• Land Use Considerations• % of Impervious Cover• Vegetative Characteristics• Stormwater Management• Floodplain Functions• Maintaining Natural Conditions• Habitat Preservation• Preserving Human Communities
Surficial Geology On a broad basis, watershed soils aredefined as tills and/or as “stratified drift.” Stratified drift is sand and gravel depositsthat have been layered and sorted by glacialmeltwater streams. Tills are mixed and often highly compressedsilts, sands & clay.
Glacial Impacts in New England Most of New England’s geology was formedduring the last ice age which retreated ~12-14,000 years ago, and by subsequent windand water processes. Our landscape features--surficial patterns oftill and stratified drift deposits--were createdby the repeated movement of glaciers acrossthe landscape. Ice melt patterns during glacial retreat created muchof the topography that now underlies our rivers
Land UseConsiderations Urbanization--which rarely balances flowsand provides minimal drainage mitigation--has changed the character and functions ofall small and large rivers. Urbanization typically increases imperviousarea, decreases vegetative cover and--through manmade drainage systems--transports stormwater to rivers far faster thanunder pre-development conditions.
Hydrologic Responsesto Urbanization Increased Discharge Increased Peak Discharge Increased Velocities Shorter time to peak flow Increased bankfull events Increased flooding Lower base flow Less groundwater discharge
VegetativeChanges Increase inimpervious area over a30-year period, 1965-1995, 100-acre site inTioga County, Penn.19651995
Vegetative Changes Without mitigation,decreased vegeta-tion = increasedstormwater.19822007
Management of Drainage Contemporary design attempts to balancepre- and post-development peak flows Increased impervious area increases volumeof stormwater Sensitive design attempts to recharge andretain the increased volume On a watershed basis, post-developmentstream dynamics should mimic pre-development (or natural) stream dynamics.
Common ManagementTechniques Use of various stormwater basins Wet basin on the left, detention basin right
BasinEffectsControlled release of stormwaterDecrease of peak rates
Maintaining BaseFlows Basins alone do not mimic natural recharge,so infiltration techniques become critical tomaintain long-term stream flow.Perforated pipe Leaching “pits”
Improper Design May CauseMassive Structural Failure Improperly sized basins collapse Roadways and embankments too oftenend up in downstream rivers
Stream Quality Is Often Not theOnly Victim of Poor Design 2006 floodingin NewHampshireResults of improperculvert design.
Floodplains Are Critical Decreased floodplain storage altersstream dynamics Stream base flow decreases in directproportion to decreases in floodplain Stream velocities increase asfloodplains decrease Wildlife habitat decreases dramaticallyas floodplain disappears
26Using Correct Precipitation Data Extreme precipitation data for ourregion and state has been updated inthe last year through a collaborativeeffort between Cornell Univ & NRCS. An extreme precip website has justcome out of beta testing. Seehttp://precip.eas.cornell.edu/26
31Correct Rainfall Data = CorrectCulvert Sizing For over a half century regionalengineers have used a documentcalled TP-40, a federal extreme precipatlas that predicts 2-100-year rainfallevents. TP-40 itself was based on rainfall datafrom the 1940s and 1950s that expertsnow recognize as representing a periodof cyclical drought. 31
32Course Corrections By the late 1990s climatologistsbecame aware that real world stormevents were not matching thoseactually occurring. Storms that were supposed to occuronce every 50 and 100 years (based onTP-40) were happening as frequentlyas every 5 years.32
33Increasely Large & MoreFrequent Storms Are Projected Cornell University, through the NERegional Climate Center (NRCC), andNatural Resources ConservationService (NRCS) have collaborated tocreate a more accurate database ofprecipitation data. Revised storm events for 100-yearevents are 25-30% larger than whatwas projected in TP-40. 33
34Ignore Reality At Public Peril When reviewing and approving newroads, and replacement culverts, it isessential to verify that correct rainfalldata has been used to size stormwaterstructures. Real world, larger storm events requirelarger culverts, larger storm basins andmore robust mitigation.34
35Verdict? Rivers are almost always the finalrecipients of storm water discharge. Use of incorrect rainfall dataguarantees that balancing pre- andpost-develop-ment flows will notsucceed. Increased stormwater flowswill acerbate swollen rivers andheighten floodplain impacts.35
36Good planning encourages... Preserving the entirety of a river, that is,it’s channel and the historic horizontalextent of its natural floodplain. Natural corridors protect both publicinfrastructure, and protect wildlife.36
Summary… Rivers are a deceptivelycomplex ecosystem Hydrologically, tomaintain “natural”conditions, rivers arewatershed dependent Watersheds are sensitiveto % of impervious area,floodplain storage &changes in vegetativecover Use of contemporaryrainfall data is essential
Acknowledgments Images by Patrick Garner; Gardner Bent, USGS;Heidi Davis, Mass DEP; NRCS; USF&W; MassGIS &others.