Overview: The Functions & Values of Rivers
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This document discusses the functions and values of rivers. It defines a river and explains that a river's characteristics are influenced by its watershed area, surficial geology, soils, land use, vegetation, and stormwater management. Urbanization can negatively impact rivers by increasing impervious surfaces and altering natural hydrologic responses. Proper stormwater management and maintaining floodplains and natural corridors are important for preserving river ecosystems and habitats. The document stresses using accurate precipitation data to properly design infrastructure like culverts that affect river flows.
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Overview: The Functions & Values of Rivers
- 1. 2012 Overview: The Functions & Values of Rivers Patrick C. Garner, Patrick C. Garner Co., Inc.
- 2. How Do We Define A River? A river is “any natural flowing body of water that empties into any ocean, lake, pond, or other river and which flows throughout the year.” (310 CMR 10.04) That’s only a start. Let’s dive down a bit…
- 3. 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
- 7. Surficial Geology On a broad basis, watershed soils are defined as tills and/or as “stratified drift.” Stratified drift is sand and gravel deposits that have been layered and sorted by glacial meltwater streams. Tills are mixed and often highly compressed silts, sands & clay.
- 8. Glacial Impacts in New England Most of New England’s geology was formed during the last ice age which retreated ~12- 14,000 years ago, and by subsequent wind and water processes. Our landscape features--surficial patterns of till and stratified drift deposits--were created by the repeated movement of glaciers across the landscape. Ice melt patterns during glacial retreat created much of the topography that now underlies our rivers
- 10. Land Use Considerations Urbanization--which rarely balances flows and provides minimal drainage mitigation-- has changed the character and functions of all small and large rivers. Urbanization typically increases impervious area, decreases vegetative cover and-- through manmade drainage systems-- transports stormwater to rivers far faster than under pre-development conditions.
- 11. Hydrologic Responses to Urbanization Increased Discharge Increased Peak Discharge Increased Velocities Shorter time to peak flow Increased bankfull events Increased flooding Lower base flow Less groundwater discharge
- 13. Stream Flooding As A Characteristic of Urbanization
- 15. Typical Urbanization “Armored” Stream in Chelmsford
- 17. Vegetative Changes Increase in impervious area over a 30-year period, 1965- 1995, 100-acre site in Tioga County, Penn. 1965 1995
- 18. Vegetative Changes Without mitigation, decreased vegeta- tion = increased stormwater. 1982 2007
- 19. Management of Drainage Contemporary design attempts to balance pre- and post-development peak flows Increased impervious area increases volume of stormwater Sensitive design attempts to recharge and retain the increased volume On a watershed basis, post-development stream dynamics should mimic pre- development (or natural) stream dynamics.
- 20. Common Management Techniques Use of various stormwater basins Wet basin on the left, detention basin right
- 21. Basin EffectsControlled release of stormwater Decrease of peak rates
- 22. Maintaining Base Flows Basins alone do not mimic natural recharge, so infiltration techniques become critical to maintain long-term stream flow. Perforated pipe Leaching “pits”
- 23. Improper Design May Cause Massive Structural Failure Improperly sized basins collapse Roadways and embankments too often end up in downstream rivers
- 24. Stream Quality Is Often Not the Only Victim of Poor Design 2006 flooding in New Hampshire Results of improper culvert design.
- 25. Floodplains Are Critical Decreased floodplain storage alters stream dynamics Stream base flow decreases in direct proportion to decreases in floodplain Stream velocities increase as floodplains decrease Wildlife habitat decreases dramatically as floodplain disappears
- 26. 26 Using Correct Precipitation Data Extreme precipitation data for our region and state has been updated in the last year through a collaborative effort between Cornell Univ & NRCS. An extreme precip website has just come out of beta testing. See http://precip.eas.cornell.edu/ 26
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- 29. 29 100-year storm, Worcester County, Massachusetts
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- 31. 31 Correct Rainfall Data = Correct Culvert Sizing For over a half century regional engineers have used a document called TP-40, a federal extreme precip atlas that predicts 2-100-year rainfall events. TP-40 itself was based on rainfall data from the 1940s and 1950s that experts now recognize as representing a period of cyclical drought. 31
- 32. 32 Course Corrections By the late 1990s climatologists became aware that real world storm events were not matching those actually occurring. Storms that were supposed to occur once every 50 and 100 years (based on TP-40) were happening as frequently as every 5 years. 32
- 33. 33 Increasely Large & More Frequent Storms Are Projected Cornell University, through the NE Regional Climate Center (NRCC), and Natural Resources Conservation Service (NRCS) have collaborated to create a more accurate database of precipitation data. Revised storm events for 100-year events are 25-30% larger than what was projected in TP-40. 33
- 34. 34 Ignore Reality At Public Peril When reviewing and approving new roads, and replacement culverts, it is essential to verify that correct rainfall data has been used to size stormwater structures. Real world, larger storm events require larger culverts, larger storm basins and more robust mitigation. 34
- 35. 35 Verdict? Rivers are almost always the final recipients of storm water discharge. Use of incorrect rainfall data guarantees that balancing pre- and post-develop-ment flows will not succeed. Increased stormwater flows will acerbate swollen rivers and heighten floodplain impacts. 35
- 36. 36 Good planning encourages... Preserving the entirety of a river, that is, it’s channel and the historic horizontal extent of its natural floodplain. Natural corridors protect both public infrastructure, and protect wildlife. 36
- 38. 38 Unnatural Corridors The Chickley River, Hawley (June 2012) The Chickley River, Hawley (June 2012)
- 39. 39 Unnatural Corridors The Chickley River, Hawley (June 2012) The Chickley River, Hawley (June 2012)
- 40. 40 Chickley River before 2011 Work Dec. 2010 (photo by Little Bang Theory) Dec. 2010 (photo by Little Bang Theory) Dec. 2010 (photo by Little Bang Theory)
- 41. Ecological Relationship of River Floodplains & Wildlife
- 42. Summary… Rivers are a deceptively complex ecosystem Hydrologically, to maintain “natural” conditions, rivers are watershed dependent Watersheds are sensitive to % of impervious area, floodplain storage & changes in vegetative cover Use of contemporary rainfall data is essential
- 43. Acknowledgments Images by Patrick Garner; Gardner Bent, USGS; Heidi Davis, Mass DEP; NRCS; USF&W; MassGIS & others.