MEMORANDUM
FROM: Reid Bandeen, Truchas
Hydrologic Assoc. Inc.
DATE: 11/25/05, added example runoff calculations
12/1/2005, updated 12/9/2005 with additional verbage from Mr. Bandeen
clarifying assumptions in example calculations.
TO: Southeast Neighborhood Association
RE: Hydrologic and Environmental Considerations of Road
Surfacing
In response to a request from Dan Baker, President
of SeNA, I have prepared responses to a few common questions regarding
attributes, advantages and disadvantages of a paved, curb & gutter type
street design, vs. dirt/gravel surfacing with bar ditch drainages.
1. Given the permeability of dirt/gravel road
surfacing vs. paved streets, does a dirt/gravel road enhance infiltration and
recharge of stormwater runoff relative to a paved street?
Response: No. Actually it's the other way around. Dirt and gravel do indeed have much
higher permeability to water than paved streets. The permeability of a dirt road/bar ditch system allows shallow
infiltration of stormwater flows. The
rougher surface of the dirt and gravel also slows the runoff flow, which further
enhances infiltration. However, the
infiltration realized from this system tends to be spread out over the entire
area of the road/ditch system, and generally only infiltrates a few feet from
the surface. As this moisture is
infiltrated under roads and ditches, it is stored bound up in the shallow
sediments in a location where it can't feed vegetation except near the road
margins. The advantage of a paved
street, curb/gutter system is that it comes as part of an integrated engineered
system that is specifically designed to recharge the stormwater into the
subsurface in a highly localized and concentrated manner. The result of using a paved street system
draining to an engineered infiltration detention pond that overflows to a large
arroyo (in this case Arroyo Chamiso) is that the water tends to infiltrate as a
concentrated slug in the pond or over a short reach of arroyo, and therefore
recharges deeper. As the sediments
beneath the pond and the arroyo get saturated more deeply, these soils tend to
conduct water more readily during infiltration events, further enhancing the
recharge effect. This water is much
more likely to provide useable recharge to the groundwater aquifer than a
dirt/gravel system, which largely leaves the water bound up in shallow
sediments beneath the roads and ditches.
2. Does a dirt/gravel road, allowing for local
infiltration, keep stormwater runoff more "local" than a paved
curb/gutter street?
Response: In a sense, yes, but the locally infiltrated
water doesn't provide much beneficial use.
For reasons discussed in the previous
response, infiltrated runoff from a dirt/gravel road system is largely shallow
and inaccessible for other uses. The
engineered paved street system with detention pond and arroyo overflow will
provide some useful recharge of the groundwater aquifer, representing a
"local" benefit to the community of Santa Fe.
The best way to keep stormwater runoff
"local" in the sense of immediate benefit for subdivision residents
is through landscaping controls.
Harvesting of rooftop runoff of rainwater, and use of berms and swales
aid retention of local rainfall and shall stormwater infiltration so as to feed
on-site vegetation. This aspect of
on-site water retention and harvesting for landscaping use is addressed in
other sections of the Santa Fe Development Code (see Section 14-8.4(B),
Landscape and Site Design Standards). A more attractive, vegetated landscape is
generally preferable to residents and neighbors, provides some small animal and
bird habitat, and represents a tangible local benefit of the retained water.
3. Does a dirt/gravel road system represent a
more economical street design?
Response: In the short run, yes. In the long run, no. Engineers preparing project cost estimates look at two major
project cost components: 1) Capital outlay (up-front construction and
materials costs) and 2) Operations and
Maintenance (long-term costs associated
with keeping the system operational).
With a paved street system, capital outlay is large and operations &
maintenance (O&M) costs are relatively low. With a dirt/gravel road system, capital outlay is less, but
O&M costs are higher. Over the long
haul, the properly figured O&M costs will probably meet or exceed the
initial capital outlay of the paved, curb & gutter street system. The dirt/gravel option is more attractive to
the developer, as their up-front costs are lower, and they can often pass on
the "marginal" O&M costs subsequently to residents. As Dan Baker has pointed out, a paved street
system obligates the City for some maintenance activities. Unfortunately, in many cases of subdivisions
opting for a dirt/gravel road system, O&M is neglected, resulting in a
dysfunctional drainage system long-term.
Without proper maintenance, bar ditches and check dams within the bar
ditches become clogged with sediment, causing overflow and erosion of the
neighboring terrain (including the road itself). A strong storm may flush large quantities of sediment into the detention
pond or Arroyo, contributing to a larger problem of regional sediment build-up
in drainage courses, and in-fill of the site detention pond. Without regular maintenance in a dirt/gravel
road system, the site detention pond will fill in with sediment, decreasing its
detention capacity and increasing the risk of an overflow. In the end, the paved, curb/gutter system is
probably more economical, safer and more ecologically sound due to lower
O&M costs long-term, controlling sedimentation, and providing some useful
groundwater recharge.
4. Is a dirt/gravel street system better than a
paved, curb/gutter system?
Response: It depends. Based on the above commentary, I believe the paved, curb/gutter
system to be superior from the standpoint of engineering design, cost to
residents, safety, water resource management and ecological integrity
considerations. However, if residents
have a sufficiently strong preference for the rural feel of a dirt/gravel road,
this system can work if properly
designed and maintained. In either
case, on-site stormwater retention achieved via enhanced landscape management
standards will reduce overall runoff, and provide a "greener"
subdivision landscape.
5.
Does Gravel or asphalt
generate more runoff?
Response: Asphalt would generate about 12.3% less
runoff if considering only the width of the street and gutterpans, or up to
6.5% more runoff if using a constant system width of 36’ given the same width
drivable surface and typical drainage design requirements, but it’s less
sediment-laden and perhaps more useful for local aquifer recharge. While Gravel is more
permeable, the added width of the barditches with gravel versus narrower
gutterpans required for asphalt essentially offset each other. It is important
to note however that the runoff that is generated in a gravel system carries
significantly more sediment, which impacts maintenance of drainage catchment
and control systems and as noted above, is less effective in recharge of the groundwater
aquifer.
Example
Calculations and discussion:
The
following analysis examines runoff volumes resulting from typical design width
road surfaces for both A) Gravel road
and bar ditch drainageway; and B) paved
street curb & gutter construction.
A
100 linear foot length of each representative road construction with 26’ wide
drivable surface is assumed, and a 1-inch, 1 hour rainstorm.
Walker
Engineering design drawings utilize the U.S. Soil Conservation Service TR-55
model for computing runoff volumes. For
road surfaces with no vegetation cover, the runoff Curve Number (CN) represents
a percentage of incident rainfall that runs off of the designated area. Walker provides the following CN values for
the following surfaces, which I believe to be reasonable:
Pavement/gutter: CN = 98
Compacted
dirt and gravel composite road surface:
CN=89
Undisturbed
ground = 74
Road runoff area = 26 feet x 100 feet = 2600 ft**2 (square feet)
Incident
rainfall = 1 inch = 1/12 feet = 0.08333 feet
TR-55
CN = 89 = (89% incident rainfall occurs as runoff)
Road
surface runoff
= 2600 ft**2 x 0.08333 ft x 0.89 = 192.82
cubic feet
Bar
ditch width (total, including both sides) = 10 feet
Bar Ditch runoff area = 10 feet x 100 feet = 1000
ft**2
Incident
rainfall = 1 inch = 0.08333 feet
Assume
TR-55 CN is average of compacted gravel road and undisturbed ground = (89+74)/2
= 81.5
Bar
ditch runoff
= 1000 ft**2 x 0.08333 feet x 0.815 = 67.92
cubic feet
Total Runoff of Gravel
Road/Bar Ditch System = 260.74 cubic feet
Per 100 linear feet with
total ‘disturbed width’ of 26+10= 36’ wide gravel/ditch system
Gutterpan width (total, including both sides) = 2
feet
Road/gutter runoff area = [26 + 2] feet x 100 feet =
2800 ft**2 (square feet)
Incident rainfall = 1 inch = 1/12 feet = 0.08333
feet
TR-55 CN = 98 = 98% runoff
Road/gutter
runoff =
2800 ft**2 x 0.08333 ft x 0.98 = 228.67 cubic feet
Per 100 linear feet with total ‘disturbed width’ of
26+2= 28’ wide asphalt+gutter system
Note: To
compare surface runoff of the asphalt+gutter street system to an equivalent
area footprint of the gravel+ditch system, one may add runoff from 8’ of
undisturbed margin to bring the total width to 36’. Whether this runoff would
enter the street drainage system would depend on the slope of the margin, and
whether it was sloped toward or away from the top of the curb. This "equivalent footprint"
scenario assumes that the 8' of undisturbed margin contributes its runoff to
the street system.
Undisturbed margin width = 8 feet
Road margin area = 100 ft x 8 ft = 800 ft**2
TR-55 CN = 74
Margin area
runoff =
800 ft**2 x 0.0833 ft x 0.74 = 49.33 cubic feet
Total Runoff of 28’ wide Paved road/gutter PLUS 8’ of undisturbed margin (equivalent total to gravel system bar ditch width) = 278.0 cubic feet.
Combining all of the above design assumptions, the
paved road system in the above example produces 32 cubic feet (about 12.3%)
less runoff when considering the street+gutter system width; and only about 17
cubic feet (or approximately 6.5%) more runoff than the gravel road and bar
ditch system if adding in 8’ of undisturbed width. The latter case assumes a constant 36’ wide footprint with
landscaping adjacent to the curbs draining into the street over the curbs on
both sides. Note also that the runoff from roads, paths and driveways generally
produce less than ten per cent of the total runoff produced by the total
surface of a typical subdivision. The difference in infiltration of using a
gravel road drainage system rather than the paved option represents only about
six-tenths of one per cent possible reduction in the overall subdivision
runoff, and could result in about 1% more runoff depending on margin slope
conditions.