Landscape Characterization / Impervious Surfaces / What are the effects? / Water quantity
What are the effects?
Quantity Impacts
The hydrological cycle describes the continuous movement
and exchange of water in all its phases (gas, liquid, and solid) between
subsurface, land, ocean, and atmospheric locations. It operates on a
global scale and consists of a multitude of interconnected "sub-cycles"
operating at different geographic scales. These "sub-cycles" operate
on continental (North America), large watershed (Chesapeake Bay), and
local (low-order streams and basins) scales. Urbanization significantly
alters local hydrological cycles. Increased imperviousness of
watershed surfaces affects the volume of water available at local locations
and the rates of water exchanges between locations.
The Hydrologic Cycle
1. Evapotranspiration
2. Condensation
3. Precipitation
4. Runoff
The Hydrologic Cycle - After
Urbanization
The hydrologic cycle is modified substantially once parking
lots and other impermeable surfaces are built. Because the paved surfaces
slow infiltration, there tends to be more surface runoff, which means
more pollutants are washed into our rivers, and more erosion takes place.
Source: CGIS
|
Increases in impervious cover generates more
stormwater runoff and higher peak stream discharges, decreases
soil moisture and groundwater recharge, and reduces the amount
of moisture evaporating from urban areas.
|
How Development Impacts the Hydrological Cycle
The impacts of urbanization upon peak stream discharges is even more pronounced
in areas serviced by storm sewers since the rapid conveyance of stormwaters
to nearby streams dramatically increases mean annual floods.
Effects of Urbanization on Mean Annual Flood
(For a 1 mile drainage area)
Source: CGIS after Leopold, 1968
Another change to local hydrological cycles is a decrease in annual
evapotranspiration. This is due to the removal of vegetation and pervious
covers and their replacement with surfaces that shed water rapidly and
which store little water.
|
Vegetated surfaces capture and store considerable
amounts of rainfall and thus delay runoff. This water either runs
off slowly, sinks into the ground, or returns to the atmosphere
via evaporation and transpiration.
|
Peak Storm-Water Discharges and Annual Runoff Estimations Before
and After Increased Imperviousness Due to Suburban Development: A Demonstration
The following demonstration offers two very useful models
that illustrate the impact of imperviousness on watershed dynamics. Imperviousness
increases the amount of precipitation that flows off the land as runoff
rather than infiltrating the ground. Even small increases in imperviousness
due to urban and suburban development lead to significant changes in peak
storm-water discharges and annual runoff. These changes can have drastic
consequences for natural systems.
Purpose
The purpose of this paper is to demonstrate development-induced
changes in (1) peak discharges under 10-year storm conditions (2.4inches/hr)
for north central Maryland using the Rational Runoff Method, and
(2) the amounts of average annual precipitation available as runoff
using a desktop version of the USACE STORM Model. Both models were
selected for their simplicity and relative ease of use.
Baseline Conditions
- 25 acres wooded
- sandy and gravelly soils
- 2% slope
- 600 feet from site boundary to stream channel origin
- 5% to 10% of the site is hydrologically active (generating
runoff)
- Small meandering stream - length is1040 feet over the given
25 acres
- High frictional resistance in stream channel
Scenario 1
- 4 acres of 25 suburban residential with 40% imperviousness
- 21 acres wooded
- Stream channel unaltered
Scenario 2
- 25 acres suburban residential housing with 40% imperviousness
- Stream channel unaltered
Scenario 3
- 25 acres suburban residential with 40% imperviousness
- Stream channel is straightened, shortened to 885 feet, and
paved with concrete.
Scenario 4
- 15 acres suburban residential with 40% imperviousness
- 5 acres high density residential with 65% imperviousness
- 5 acres commercial with 70% imperviousness
- Stream channel is straightened, shortened to 885 feet, and
paved with concrete.
|
This hypothetical site is a square with a stream exiting from the
lower right corner.
4 acres developed on
25 acres of land |
100% development on
25 acres of land. |
 |
 |
|
Results and Discussion
The two preceding graphs demonstrate how development can
dramatically influence peak storm-water discharges. In scenario 1, the
change from 100% wooded to 84% wooded (21 of 25 acres) and 16% suburban
residential (4 acres) resulted in a 59% increase in the estimated peak
discharge. A limited amount of development clearly results in a disproportionate
increase in peak runoff. Scenarios 2 and 3 generate even more profound
increases in peak discharges compared with base conditions. The peak discharge
estimate for Scenario 4, which represents the greatest amount of impervious
coverage and also introduces stream channelization, is 550% higher than
the base peak discharge. Modeling average annual runoff provides yet another
perspective on imperviousness, as the following graphs demonstrate.
These estimates of annual runoff for baseline conditions
and for the four scenarios also clearly demonstrate the environmental
impacts of development. Annual runoff increases significantly with even
modest levels of development. Under completely forested conditions, 13.5%
of mean annual precipitation is available for runoff, whereas 49% of mean
annual precipitation is available for runoff under full site development
with a mix of land uses (scenario 4). The previous models are based on
the assumption of a relatively small area (25 acres) in isolation from
neighboring lands. The following model considers the 25-acre site as part
of a larger development. As with the previous examples, peak discharges
are modeled using the Rational Method.
Runoff from a 100 acre site under different levels of development.
The following graphics provide a generalized representation of the developing
site:
| 25% development |
50% development |
 |
 |
| 75% development |
100% development |
 |
 |
|
Try to imagine the environmental impacts of developing a
100-acre site with little or no attention to imperviousness. Now consider
the cumulative environmental impacts of many such development projects,
both large and small, spread throughout your local watersheds and jurisdictions.
| 
