Effects of Urban Development on Streams and Aquatic Life (3)


Headwater streams within the County�s coastal
plain are undergoing severe erosion as a result of historic changes in land use from forest
to urban/agricultural development. The Chesapeake Bay TMDL has now become a regulatory
driver for the County, requiring us to meet certain pollutant load reductions associated
with sediment, nitrogen, and phosphorus. One way that the County will achieve these
load reduction requirements is through rehabilitation of our degraded stream systems. In today�s
talk, I will introduce you to typical streams in Anne Arundel County and to our preferred
method of rehabilitating our degraded natural infrastructure.
The Physical Habitat of more than half of the 410 miles of perennial streams channels
assessed within the County are severely degraded Erosion and the subsequent contribution of
sediment in more than half of the 410 miles of perennial stream channels assessed within
the County is significant. The biological condition of seventy-two percent
of the samples from perennial streams across the County whether in the highly urbanized
North County or rural agricultural South County is Poor to Very Poor.
The influence of urbanization and changing land cover has resulted in the above referenced
stream conditions which apply to both ephemeral and perennial stream channels. To better understand
these conditions please go to the next slide. This is a storm drain outfall that discharged
to an ephemeral channel. This storm drain system was built in the early 1990s as part
of a residential development. The outfall you see here in the bottom center of the picture
is from a stormwater pond. As you can see, the discharge of stormwater
from this outfall has severely degraded the channel, both vertically and laterally. In
fact, the pile of rocks on the left side of the channel is where the storm drain pipe
originally discharged. The erosive force of the stormwater entering the channel eventually
undermined the outfall pipe and the pipe literally fell away from the rest of the storm drain.
The end result is what you see here. The above picture is a restored Ephemeral
Channel where water flows only when it rains sufficiently to result in surface stormwater
runoff. Most of
the stormwater runoff flows through the sand lens used to reconstruct the channel. During
large storm events water rises to the surface and flows through the rock weirs.
Grade control within the rock weirs is established with large rock boulders embedded into the
designed channel to manage stormwater runoff from large storm events and prevent erosion.
This is our preferred method for rehabilitation of degraded storm drain outfalls.
Designed Ephemeral Channels as displayed above are planted with native vegetation which are
allowed to evolve into a ecosystem. The above picture is an example of a Highly
Degraded Intermittent Channel. Rehabilitation of this degraded channel is revealed in the
next slide. As illustrated in the previous example, rehabilitation
of the degraded Intermittent Channel is achieved by filling it with sand and providing grade
control with rock boulders. The shallow groundwater resulting from an extended period of rainfall
can contribute to a longer extended period of surface flow throughout the year within
the intermittent channel. Note that this photo is during a larger storm
event, hence the appearance of continual flow. During dry periods the stream does not always
exhibit a surface flow. This is an example of a Highly Degraded Perennial
Stream Channel. Groundwater saturation of the forested floodplain is dramatically lowered.
Stream flow is significantly reduced due to the reduced groundwater hydrology. The stream
is disconnected from its floodplain. Rehabilitation of the severely eroded Perennial
Channel on the prior slide restores shallow groundwater which flows continuously throughout
the year. The Perennial Channel is reconnected to its
forested floodplain, allowing stormwater runoff to overflow into the floodplain providing
stormwater quantity and quality control by providing storage and removal of sediment,
nutrients and other pollutants within the wetland.
This is our preferred method for rehabilitation of our degraded
Perennial Channels. Severely degraded forested wetlands rehabilitated
with a similar unique approach of filling and capping the degraded wetland have resulted
with significant success in recovery of the aquatic ecosystems as witnessed in the following
slides. A series of sand berms, step pools, rock weirs,
riffles and seepage wetlands were constructed with native sand and stone rehabilitates the
historic forested wetland. Within one year of original construction the
native vegetation begins to mature. Within five years of original construction
the native vegetation evolves into a vibrant aquatic ecosystem.
Again, this preferred stream rehabilitation technique has resulted in water quality benefits
(pollutant load reduction), reconnection with the floodplain, establishment of forested
floodplain/wetland systems, and the provision of a more diverse aquatic ecosystem.
The County�s rehabilitation efforts can extend beyond stream systems to our unique
bogs. Here, an historic natural bog that was filled
with dredge material many years ago presented a challenge for rehabilitation.
Once again a series of sand berms, step pools, rock weirs, riffles and seepage wetlands constructed
with native sand and stone boulders rehabilitated the historic wetland.
In summary, Anne Arundel County strives to rehabilitate our degraded streams and non-tidal
aquatic ecosystems primarily through the techniques that have just been described.
Through this effort, we have begun a long and arduous process that we believe will achieve
the goals and objectives listed in the slide above.

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