HomeMy WebLinkAboutStorm Water Proposal - PLN General - 4/26/2007 Storm Water Proposal
In 4-26-07, I talked to John Sliva, Mason County Public Works, and told him I wished to do a Low Impact
Development (LID) stormwater system. I sent him various materials describing the proposal (attached)
The particular system that my engineer (Tom Holz, Olympia, WA, who helped author many of the LID
documents currently in use) and I wish to use is (Best Management Practices) BMP T5.30 Full Dispersal,
found in the latest version of Washington State Department of Ecology's Stormwater Management
Manual (February 2005, Publication No. 05-10-33). This is the lowest impact system in the publication,
and suitable only where over 65% of a property's forest can be left in its natural state --a condition that is
very rarely possible with most properties, but fits perfectly with my plans.
The issue is that Mason County uses an older DOE document for reference and does not yet have LID
regulations enacted, though it is in the process of developing them. On the surface it seems reasonable that
Mason County would want to use the latest BMPs, even ahead of official enactment of new LID
regulations, so Mr. Sliva said he would talk to Planning, et al, to see what the various departments think
about my proposal.
I haven't yet heard back on the matter.
Gregg
Entire 50.24 acres (4 parcels: 35 + 5 + 5 + 5.24 acres), showing 2007
construction phase for Camp compound (1 Kitchen / shower, 2 bunkhouses).
Also shows primitive campsite locations for RV Park Permit application.
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Camp compound showing rain garden locations Scale: V =40
Parcel 1 (3 5 acres): impervious surfaces created(roofs) I 1,400sf = .7%of total parcel. Area
ar . Ar
thinned (excluding building footprints): 26, 000sf. Area restored using native vegetation: 16,000sf.
Balance (I 0,000sf): lawns and pervious walking surfaces. No impervious roads on this parcel
Undisturbed forest outside of red line
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(July 2007)
(2008)
40 x 96 (3800st) 40 x 96 (3800sf)
BUnkhouse
Bunkhouse tn
35'
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35'
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TMW
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(July 2007)
35' F 3 5' (2008)40 x 96 (3800sf) IN.
40 x 96 (3800sf)
Bunkhouse
Undisturbed forest outside of red line
Parcel 2 (5.24 acres): impervious surfaces created (roofs) 10,800sf =4.7% of total parcel.
35�
Area thinned (excluding building footprints): 28,650sf. Area restored using native
vegetation: 17,000sf Balance (I 1,650sf): lawns and pervious walking surfaces. 1,100' of
20' wide road=22,000sf= 9.65% of parcel. No fines in crushed rock, so road is pervious,
but even if it were impervious, the total is still less then the 10% limit noted in BMP T5.3 0.
How we we intend to combine rain harvesting with rain gardens
x
A bunkhouse is shown, but kitchen and dining _
hall structures will use the same system:
selected native
(4)40 x 96 x 14 buildings - Bunkhouses filter strip plants and
Sides:01)B'x 8'windows.0'6'door hardy cultivars
Ends:no w�indorws.W 36'doors. water flow
north-t,vaticn, tcck thrcu6 mirrcr imatt:: Impervious ,ntinq
surface retFntion dnd Sail mix
filtration Zone
71 FT] FFI F7
rcr I�n��r ,)r
f:rter fabric
Rain garden cross-section from "Low Impact Development"2005.Hinman.
'F ba`' Our rain gardens will be sized and designed for suitable
infiltration characteristics (see BMP T5.13 et al for spec) so
_x that 100% of roof runoff can be adequately infiltrated. i.e.
Tanks are non-essential for proper functioning of the
stormwater dispersal system and therefore need only be sized
to meet actual needs for water. This is necessary in order for
system to keep function properly when tanks are full.
south elevation
Non-potable uses now. Potable uses
m m m m m m (perhaps with treatment) later when
regulations permit.
When tank is full or rain gardens need watering,water is gravity-fed Larger
to the rain gardens via an elevated sprinkler pipe or a spillover canal. pillow tanks
1 1 1 1 1 1 1 1 1 1 I I I I I I I I I I I I I I I I I I I I I I I l l
� trrrrttrrtrtrrl�rtrrrrtrrttt�r��tttttrtrtrtttttrrtrtr�ttrtrttr�ttrrttrrrrrtt In crop
rtirtrirttirrrirrttrt � iirtr � rt � it � itrr areas.
(GP note: these are the germane portions of Vol V, along with my notes inserted therein.
Stormwater Management Manual
for Western Washington
Volume V
Runoff Treatment BMPs
Prepared by:
Washington State Department of Ecology
Water Quality Program
February 2005
Publication No. 05-10-33
(A revision of Publication No. 99-15)
Chapter 5-On-Site Stormwater Management.................................................. 5-1
5.1 Purpose......................................................................................................................5-1
5.2 Application...................................................................................................................5-1
5.3 Best Management Practices for On-Site Stormwater Management...........................5-1
5.3.1 Dispersion and Soil Quality BMPs(Required for Manual Equivalency)............5-3
BMP T5.10 Downspout Dispersion ............................................................................5-3
BMW T5.11 Concentrated Flow Dispersion................................................................5-9
BMP T5.12 Sheet Flow Dispersion ..........................................................................5-11
BMP T5.13 Post-Construction Soil Quality and Depth ............................................5-13
5.3.2 Site Design BMPs.................................................................................................... 5-16
BMP T5.20 Preserving Natural Vegetation ..............................................................5-16
BMP T5.21 Better Site Design..................................................................................5-18
5.3.3 Other Practices ...................................................................................................5-22
BMP T5.30 Full Dispersion ......................................................................................5-22
Section 5.3.2 - Site Design BMPs
BMP T5.20 Preserving Natural Vegetation
BMP T5.21 Better Site Design
BMP T5.30 Full Dispersion
February 2005 Volume V—Runoff Treatment BMPs 5-1
Projects shall employ these BMPs to infiltrate,disperse, and retain
stormwater runoff on site to the maximum extent practicable without
causing flooding or erosion impacts. Sites that can fully infiltrate(see
Volume III, Chapter 3) or fully disperse (see BMP T5.30) are not required
to provide runoff treatment or flow control facilities. Full dispersion
credit is limited to sites with a maximum of 10% effective impervious area
that is dispersed through 65%of the site maintained in natural vegetation.
(GP note: Meeting BMP T5.30 (below) is the objective. Mr. Thomas Holz walked the
property with me yesterday and discussed the building design. He confirmed that the rain
gardens we envision will meet the downspout infiltration requirement of Vol III, Ch 3,
and therefore be "fully dispersed." Btw, We are also harvesting water for irrigation and
nonpotable uses --and perhaps future potable uses if regulations eventually permit.)
• For sites with septic systems, the discharge point must be downslope
of the primary and reserve drainfield areas. This requirement can be
waived by the Local Plan Approval Authority if site topography
clearly prohibits flows from intersecting the drainfield.
(GP note: We are using sub-surface drip tubing rather than conventional drainfields. It is
designed to run through forests with non-disturbing installation (small trench 1"wide.
12" deep") and Mr. Thomas Holz says that this doesn't impact the forest. Current plans
call for some of this tubing to run through the same forested, sloped area used for
dispersal. However, the dispersal area is large,( icall 200' deep and 500'+wide) and
according to the last section show below: "..
The drip
tubing will be more that 25' from dispersal points. Therefore stormwater will always be
dispersed through at least 25' of native vegetation before reaching sub-surface drip
tubing. In any case, the tubing generally carries only gray water since we are using vault
toilets and will eventually add composting toilets.)
BMP T5.13 Post-Construction Soil Quality and Depth
Purpose and Definition
• Implementation Options: The soil quality design guidelines listed
above can be met by using one of the methods listed below
1. Leave undisturbed native vegetation and soil, and protect from
compaction during construction
2. Amend existing site topsoil or subsoil either at default"preapproved"
rates, or at custom calculated rates based on specifiers
tests of the soil and amendment
3. Stockpile existing topsoil during grading, and replace it prior to
planting. Stockpiled topsoil must also be amended if needed to
meet the organic matter or depth requirements, either at a default
"pre-approved"rate or at a custom calculated rate.
4. Import topsoil mix of sufficient organic content and depth to meet
the requirements.
_ ill do 1, 3, &4. The soil base for restored vegetation in the rain gardens
will consist of stockpiled and restored topsoil plus imported amendments as needed to
meet spec.)
BMP T5.21 Better Site Design
Purpose and Definition
Fundamental hydrological concepts and stormwater management concepts
can be applied at the site design phase that are:
•more integrated with natural topography,
•reinforce the hydrologic cycle,
•more aesthetically pleasing, and
•often less expensive to build.
A few site planning principles help to locate development on the least
sensitive portions of a site and accommodate residential land use while
mitigating its impact on stormwater quality.
Design Guidelines (sectiori titles only.)
• Define Development Envelope and Protected Areas-
• Minimize Directly Connected Impervious Areas -
• Maximize Permeability-
• Build Narrower Streets
• Maximize Choices for Mobility—
• Use Drainage as a Design Element
(GP note: This section is a good checklist for optimal LID design, and our approach
meets everything relevant listed therein. But it is the more environmentally sensitive
BMP below we wish to adhere to. )
5.3.3 Other Practices
The BMPs described in this section are other general practices for on-site
treatment of stormwater.
BMP T5.30 Full Dispersion
Purpose and Definition
This BMP allows for "fully dispersing" runoff from impervious surfaces
and cleared areas of development sites that protect at least 65%of the site
(or a threshold discharge area on the site)in a forest or native condition.
(GP note: Yes, well over 65% of the each parcel site will be preserved in its undisturbed,
forest and native condition. The actual figure will be closer to 80% - 90%.)
Applications and Limitations
• Rural single family residential developments should use these
dispersion BMPs wherever possible to minimize effective impervious
surface to less than 10%of the development site.
• Other types of development that retain 65% of the site(or a threshold
discharge area on the site) in a forested or native condition may also
use these BMPs to avoid triggering the flow control facility
requirement.
(UP note: l'his is the category applicable to us.)
• The preserved area should be situated to minimize the clearing of
existing forest cover, to maximize the preservation of wetlands
(though the wetland area and any streams and lakes do not count
toward the 65%forest or native condition area), and to buffer stream
corridors.
(GP note: Yes, the 65%+preserved area will have no clearing of forest cover. There are
no nearby wetlands to affect calculations.
• The preserved area should be placed in a separate tract or protected
through recorded easements for individual lots.
(Gl' note: This requirement is apparently aimed at groups of related parcels with different
owners. Doesn't apply here, but there is no reason not to give myself an easement or
conservancy covenant if the County requires it.)
• The preserved area should be shown on all property maps and should
be clearly marked during clearing and construction on the site.
(GP note: This has been done on the master site plan.)
• All trees within the preserved area at the time of permit application
shall be retained, aside from approved timber harvest activities and the
removal of dangerous or diseased trees.
(GP note: Agreed.)
• The preserved area may be used for passive recreation and related
facilities, including pedestrian and bicycle trails, nature viewing areas,
fishing and camping areas, and other similar activities that do not
require permanent structures,provided that cleared areas and areas of
compacted soil associated with these areas and facilities do not exceed
eight percent of the preserved area.
(GP note: This is perfect for my plan, since the primitive camping occurs only on the 35
acre site where 8% is far more space than I will use.)
5-22 Volume V—Runoff Treatment BMPs February 2005
Design Guidelines
• Roof Downspouts
Roof surfaces that comply with the downspout infiltration
requirements in Volume III, Chapter 3, are considered to be "fully
dispersed" (i.e.,zero percent effective imperviousness). All other roof
surfaces are considered to be "fully dispersed" (i.e.,at or approaching
zero percent effective imperviousness) only if they are within a
threshold discharge area that is or will be more than 65% forested(or
native vegetative cover)and less than 10% impervious (total), AND if
they comply with the downspout dispersion requirements of BMP
T5.10, and have vegetated flow paths through native vegetation
exceeding 100 feet.
(GP note: As noted earlier, Mr. Thomas Holz walked the property with me yesterday and
discussed the building design. He confirmed with me that the rain gardens we envision
will meet the downspout infiltration requirement of Vol ITI, Ch 3, and therefore be "fully
dispersed.")
• Driveway Dispersion
Driveway surfaces are considered to be "fully dispersed" if they are
within a threshold discharge area that is or will be more than 65%
forested(or native vegetative cover) and less than 10%impervious
(total),AND if they comply with the driveway dispersion BMPs—
BMP 5.11 and BMP T5.12 -and have flow paths through native
vegetation exceeding 100 feet. This also holds true for any driveway
surfaces that comply with the roadway dispersion BMPs described
below.
(GP note: To meet Fire Marshall apparatus requirements, we are enlarging/extending a
20' road into the Camp compound area. This will be an all weather road utilizing
crushed rock. However, to the Iargest'extent possible, we will use an pervious surface,
i.e. rock that excludes fines, e.g. 3"- 'w'.)
• Roadway Dispersion BMPs
(GP note: Probably not applicable, our roads would probably all be considered driveways
(above), not roadways, for two reasons: 1. They are not for general, through traffic --
they are dead-end driveways serving specific buildings. In any case, Avery Township
prohibits motor vehicles except for emergency and delivery vehicles, so vehicle traffic is
negligible, 2. Despite some being 20' to meet Fire Code, they are all rustic,pervious
roads.)
Roadway surfaces are considered to be "fully dispersed" if they are
within a threshold discharge area that is or will be more than 65%
forested(or native vegetative cover) and less than 10% impervious
(total),AND if they comply with the following dispersion
requirements:
1. Roadway runoff dispersion is allowed only on rural neighborhood
collectors and local access streets. To the extent feasible,
driveways should be dispersed to the same standards as roadways
to ensure adequate water quality protection of downstream
resources.
2. The road section shall be designed to minimize collection and
concentration of roadway runoff. Sheet flow over roadway fill
slopes(i.e.,where roadway subgrade is above adjacent right-ofway)
should be used wherever possible to avoid concentration.
3. When it is necessary to collect and concentrate runoff from the
roadway and adjacent upstream areas (e.g., in a ditch on a cut
slope), concentrated flows shall be incrementally discharged from
the ditch via cross culverts or at the ends of cut sections. These
incremental discharges of newly concentrated flows shall not
exceed 0.5 cfs at any one discharge point from a ditch for the 100-
year runoff event. Where flows at a particular ditch discharge
point were already concentrated under existing site conditions
(e.g., in a natural channel that crosses the roadway alignment),the
0.5-cfs limit would be in addition to the existing concentrated peak
flows.
4. Ditch discharge points with up to 0.2 cfs discharge for the peak
100-year flow shall use rock pads or dispersion trenches to
disperse flows. Ditch discharge points with between 0.2 and 0.5
cfs discharge for the 100-year peak flow shall use only dispersion
trenches to disperse flows.
5. Dispersion trenches shall be designed to accept surface flows (free
discharge) from a pipe, culvert, or ditch end, shall be aligned
perpendicular to the flowpath, and shall be minimum 2 feet by 2
feet in section, 50 feet in length, filled with 3/4-inch to 1%2-inch
washed rock, and provided with a level notched grade board(see
Figure 5.2). Manifolds may be used to split flows up to 2 cfs
discharge for the 100-year peak flow between up to 4 trenches.
Dispersion trenches shall have a minimum spacing of 50 feet.
6. After being dispersed with rock pads or trenches, flows from ditch
discharge points must traverse a minimum of 100 feet of
undisturbed native vegetation before leaving the project site, or
entering an existing onsite channel carrying existing concentrated
flows across the road alignment.
Note: In order to provide the 100 foot flowpath length to an
existing channel, some roadway runoffmay unavoidably enter the
channel undispersed. Also note that water quality treatment may
be waived for roadway runoff dispersed through 100 feet of
undisturbed native vegetation.
7. Flowpaths from adjacent discharge points must not intersect within
the 100-foot flowpath lengths, and dispersed flow from a discharge