The Open Channel Summer 2006

• Year: 2006
• Title: The Open Channel
• Author: Colorado Association of Stormwater and Floodplain Managers
• Description: CASFM: Your Guide to stormwater and floodplain management in Colorado
• Publications - Doc Type: Other

However, the design of post con-
struction structures in mountain con-
struction is not continuing the
momentum of construction site water
quality protection. For example, people
in the construction industry know to
use more straw bales and rock check
dams in steep wwales. As one looks up
the drainage Swale, the top of the lower
structure must visually line up with the
bottom of the uphill structure. Thus
the steeper the slope, the more check
structures required. Yet a detention
pond at the base of the mountain may
be the only permanent detention struc-
ture. Natural mountain waterways and
gulches typically have a stair - stepped
surface hydrology. The concept of
using cascading structures during con-
struction must be carried over to per-
manent BMPs throughout the site.
Construction BMPs
This mountain subdivision will cluster
53 homes along 6.5 miles of private road on
a 900 -acre property, and allow 85 per cent
of the property to remain undeveloped.
During construction, typical land develop-
ment includes silt fences around disturbed
areas and a single sediment pond at the low
point of the project site. In fractured granite
and steep terrain, silt fence causes damage
when installed and is often ineffective. On
mountain sites, a dirt berm that is reseeded
on the down hill side is more effective and
lower cost than installing, maintaining and
removing silt fence. Using onsite material
where appropriate allows money to be spent
in other areas where more robust protection
is needed. For example, straw wattles or
brush barriers are excellent options for
mountain construction.
Permanent BMPs
Currently, there is still an emphasis on
collection and conveyance by both regula-
tors and site designers. Applying the prin-
ciples of collection and conveyance in the
mountains means that runoff water is con-
veyed several hundred feet down a moun-
tain, picking up erosive power and sedi-
ment along the way.
Stormwater is then detained at the base
of the mountain and discharged into a
mountain stream or gulch. The greatest ben-
efit to the watershed comes from a series of
properly maintained, permanent BMPs that
begin close to the source of runoff and
restore predevelopment hydrology. For
example, roof runoff could be collected and
conveyed to infiltration structures. One sys-
tem that has been used mimics the function
of a septic system— infiltrating water back
into the ground. A combination of seepage
cisterns and gravel trenches with perforated
pipe will provide approximately 2,000 gal-
lons of infiltration capacity on each site with
an estimated potential recharge of over
20,000 gallons per year on each lot and
cumulatively over 1 million gallons for the
entire subdivision. In larger storms, the grav-
el trench may become saturated. However,
the trench follows a contour and the surfac-
ing water will sheet flow through a natural
landscape.
Drainage culverts could also be better
adapted to mountain use. In older devel-
opments, culverts often lack adequate out-
let protection and free - falling stormwater
creates a plunge pool. A flow diffuser and
infiltration structure could be installed to
reduce erosion. This structure should also
be considered for new construction to pre-
vent excessive erosion and water loss from
the subdivision. A second option is a seep-
age pit ring, where an area around the cis-
tern rings is excavated and filled with large
rock. This structure keeps a drainage Swale
from further eroding into a drainage gulch.
A third option is a sump or seepage
area. For example, a meadow will receive
concentrated flow from a drainage culvert.
Outlet protection will be added to the
drainage culvert; however the meadow soil
beyond the outlet protection will erode,
turning a meadow into a channel. The dis-
placed dirt and rock will further scour the
channel and accelerate the sedimentation
in the detention pond at the base of this
mountain. To reduce the erosion potential,
rock that measures approximately three
feet in diameter, will be placed in a semi
circle in the meadow in a 30 -50 foot radius
from the culvert discharge. This area will
be planted with willow shrubs and aspen,
which will provide soil stabilization.
Colorado Water Law
Mimicking predevelopment hydrology
through runoff reduction and infiltration
structures complies with Colorado Water
Law, which states the property (owner) is
entitled to the "first use" as gravity delivers
water to the ground and rain and
snowmelt returns to a surface or ground-
water system. Conveyance by gravity flow
to a detention pond or to an infiltration
structure is legal because it returns the
water to the system. Containment systems
and structures such as irrigation ponds
and rain barrels that capture water and
require pumping or otherwise defy gravity
constitute a "second use ". This "second
use" is not legal without a water right.
Conclusion
Land development in the fractured and
decomposed granite areas of the Rocky
Mountains could include a series of influation
10 CASFM