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
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.
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
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
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.