HomeMy WebLinkAbout20021520 Technical
Bulletin
�r�M 10-00
Ensuring That Structures Built In or Near Special Flood
Hazard Areas Are Reasonably Safe From Flooding
in accordance with the
National Flood Insurance Program
Zone X
Zone X
Zone>AE
Filled Area /
Roadway•
_ if
Zone X
tr //
Limit of tII\
Roadway? Grade Elevated Lowest —
by
Base Flood by Fill Floor
Elevation
Stream Compacted
Channel Fill
`NCY M,q`y• II > FEDERAL EMERGENCY MANAGEMENT AGENCY
FIA-TB-10
MITIGATION DIRECTORATE
(8/00)
EXHIBIT
2002-1520 1 _AA_
Key Word/Subject Index
This index allows the user to locate key words and subjects in this Technical Bulletin.The
Technical Bulletin User's Guide (printed separately) provides references to key words and
subjects throughout the Technical Bulletins. For definitions of selected terms, refer to the
Glossary at the end of this bulletin.
Key Word/Subject Index . Page
Basement construction, engineered option 15
Basement construction, simplified approach 11
Basement foundation in fill, not recommended 7
Basement foundation in fill, vulnerability to subsurface flooding 1, 7
Basement foundation, in fill placed above BFE 8
Basement foundation, with lowest floor at or above BFE 7
Basement foundation, with lowest opening above BFE 9
Basement foundation, with lowest opening at BFE 9
Crawlspace foundation 5
Fill, placed to remove land from the SFHA 1
Fill, areas where prohibited 3
Fill, proper placement of 3
Foundation flood risk, summary table 10
Freeboard,recommendations 4
Insurance coverage for basement flooding, restrictions 7
"Reasonably safe from flooding," defined 2
"Reasonably safe from flooding," NFIP regulations concerning 2
Slab-on-grade foundation 6
Stem wall foundation 5
Sump pump, requirements for simplified basement construction 11
Any comments on the Technical Bulletins should be directed to:
Federal Emergency Management Agency
Mitigation Directorate
Program Policy and Assessment Branch
500 C Street, SW.
Washington, DC 20472
Wave design on cover based on the Japanese print The Great Wave Off Kanagawa, by Katsuchika
Hokussai (1760-1849),Asiatic Museum of Fine Arts, Boston.
TECHNICAL BULLETIN 10-00
Ensuring That Structures Built In or Near Special Flood Hazard Areas
Are Reasonably Safe From Flooding
in accordance with the National Flood Insurance Program
Introduction
For the purpose of administering the National Flood Insurance Program (NFIP), FEMA identifies
and maps flood hazard areas nationwide by conducting flood hazard studies and publishing
Flood Insurance Rate Maps (FIRMs). These flood hazard areas, referred to as Special Flood
Hazard Areas (SFHAs), are based on a flood having a 1-percent probability of being equaled or -
exceeded in any given year (also referred to as the 100-year flood or Base Flood).
Structures within the SFHA in a community participating in the NFIP are subject to floodplain
management regulations that impact building standards and are designed to minimize flood risk.
For example, Title 44, Part 60, Section 3(c)(2) of the Code of Federal Regulations—abbreviated
as 44 CFR 60.3(c)(2)—requires that the lowest floor of a structure, including basement, built
within the SFHA be at or above the Base Flood Elevation (BFE). In addition, flood insurance
must be purchased for these structures if they are used as collateral to secure a loan provided by a
federally regulated lender. All compliant structures within a participating community are eligible
for flood insurance coverage. Insurance rates differ for those structures located within the SFHA
and those located outside the SFHA.
When permitted under applicable Federal, state, and local laws, ordinances, and regulations,
earthen fill is sometimes placed in a SFHA to reduce flood risk to the filled area. Under certain
conditions, when engineered earthen fill is placed within an SFHA to raise the surface of the
ground to or above the Base Flood Elevation (BFE), a request may be submitted to FEMA to
revise the FIRM to indicate that the filled land is outside of the SFHA. When such revisions are
warranted, FEMA usually revises the FIRM by issuing a Letter of Map Revision based on fill
(LOMR-F).After FEMA has revised the FIRM to show that the filled land is outside the SFHA,
the community is no longer required to apply the minimum NFIP floodplain management
standards to any structures built on the land and the mandatory flood insurance purchase
requirements no longer apply. It is worth noting that states and communities may have floodplain
ordinances that are more restrictive than the minimum requirements of the NFIP and may continue
to enforce some or all of their floodplain management requirements in areas outside the SFHA.
Although a structure built on a site that has been elevated by the placement of fill may be removed
by FEMA from the SFHA, the structure may still be subject to damage during the Base Flood
and higher-magnitude floods. Constructing_the entire structure at or above the level of the BFE
will minimize the flood risk from the BaseFloQd and is therefore the most prudent annro�_n
constructing on fill. Conversely,a structure with a basement (subgrade area)_adjacent to or near
theme fllain may welLb_ impacted by subsurface flooding brought on by surface flooding.
- 1
This bulletin provides guidance on the construction of buildings on land elevated above the BFE
through the placement of fill. Several methods of construction are discussed, and the most
prudent—those that result in the entire building being above the BFE—are recommended.
However, in some areas of the country, basements are a standard construction feature. Individuals
may wish to construct basements on land that has been removed from the floodplain. Therefore,
this technical bulletin also provides guidance to ensure that a building constructed on fill that has
its lowest floor elevation below the BFE is reasonably safe from flooding during the occurrence
of the Base Flood and larger floods. For the purposes of this bulletin, a basement will be
considered reasonably safe from flooding if the water associated with the occurrence of the Base
Flood is removed before infiltrating the basement, and the foundation is designed and
constructed for any increased loads that occur under the same Base Flood conditions.
NFIP Regulations
Part of a community's application to participate in the NFIP must include "a commitment to
recognize and duly evaluate flood hazards in all official actions in the areas having special flood
hazards and to take other such official actions reasonably necessary to carry out the objectives of
the program" [44 CFR 59.22 (a)(8)].
NFIP regulations at 44 CFR 60 include Subpart A: Requirements for Flood Plain Management
Regulations. Each community participating in the NFIP adopts a floodplain management
ordinance that meets or exceeds the minimum requirements listed in 44 CFR 60. Subpart A
establishes specific criteria for determining the adequacy of a community's floodplain
management regulations. The overriding purpose of the floodplain management regulations is to
ensure that participating communities take into account flood hazards, to the extent that they are
known, in all official actions relating to land management and use.
One of the minimum requirements established by the regulations is set forth at 44 CFR 60.3
(a)(3), which states that, for all proposed construction or other development within a participating
community, the community must "Review all permit applications to determine whether the
proposed building sites will be reasonably safe from flooding."44 CFR 59.1 defines
"development" as
"...any manmade change to improved or unimproved real estate, including but not
limited to buildings or other structures, mining, dredging,filling, grading, paving,
excavation or drilling operation or storage of equipment or materials,"
Construction of a building in an identified SFHA with a lowest floor below the BFE is a violation
of the floodplain management requirements set forth at 44 CFR 60.3(c)(2), unless the community
has approved exemption procedures in place. By issuance of this Technical Bulletin, FEMA is
noting that residual flood hazards may exist in areas removed from the SFHA by the placement
of engineered earthen fill. Residual risks in these areas include subsurface flood conditions and
flooding from events that exceed the base flood. This bulletin is intended to guide local
floodplain management officials in determining whether structures placed in filled areas are
reasonably safe from flooding. FEMA will require that a community determine that an area is
reasonably safe from flooding before removing it from the SFHA.
2
Floodways,V Zones,and Alluvial Fan Flood Hazard Areas
This bulletin does not apply to the following:
• Construction in the floodway. The NFIP prohibits encroachments into the floodway.
• Construction in SFHAs designated Zone V, VE, or V1-V30 on FIRMs. The NFIP prohib-
its the placement of structural fill beneath buildings in V zones. Buildings constructed in
a V zone must be constructed on an open foundation consisting of piles, piers, or posts
and must be elevated so that the bottom of thelowest horizontal structural member is at
or above the BFE. In addition, this bulletin strongly recommends that fill not be placed
to elevate buildings constructed in A zones in coastal areas. Detailed guidance concern-
ing proper construction methods for buildings in coastal areas is presented in FEMA's
Coastal Construction Manual (FEMA 55) and in NFIP Technical Bulletin 5, Free-of-
Obstruction Requirements.
• Construction in SFHAs subject to alluvial fan flooding (designated Zone AO with depths
and velocities shown on FIRMs). The NFIP prohibits the placement of fill in alluvial fan
flood hazard areas.
More Restrictive State and Local Requirements
NFIP Technical Bulletins provide guidance on the minimum requirements of the NFIP
regulations. State or local requirements that exceed those of the NFIP take precedence. Design
professionals should contact community officials to determine whether more restrictive state or
local regulations apply to the building or site in question.All applicable standards of the state or
local building code must be met for any building in a flood hazard area.
Placement of Fill
Properly placing fill requires an understanding of soil mechanics, local site conditions, the
specific characteristics of the soils being placed, the methods used to place and compact the fill,
and soil testing procedures. Standard engineering and soil mechanics texts cover these subjects in
detail. The performance of these filled areas should consider, but is not limited to, the following:
• the consolidation of the fill layers and any underlying layers
• the effect of this consolidation on either excessive settlement or differential settlement
• how the permeability of the soils affects water infiltration on any structures built on the site
3
Loss of Storage and Conveyance
The placement of fill in the SFHA can result in an increase in the BFE by reducing the
ability to convey and store flood waters. This can result in increased flood damage to both
upstream and downstream properties. To prevent these possible results, some communities
prohibit fill, require compensatory storage for filled areas, or identify zero-rise floodways.
Risk of Flood Damage in Areas Adjacent to the SFHA
Areas adjacent to the SFHA may have residual risks of flood damage similar to those in
areas removed from the SFHA through the placement of fill. Both areas are subject to
residual risk from subsurface water related to flooding and from floods greater than the
Base Flood.Methods of construction discussed in this bulletin should also be used in these areas.
Building on Land Removed From the SFHA by the Placement of Fill
The safest methods of constructing a building on filled land removed from the SFHA are those
that result in the entire structure being above the BFE. Methods that place the lowest floor of the
building at, rather than above, the BFE are at greater flood risk, and methods that result in the
lowest floor(including a basement floor) below the BFE, have the highest flood risk of all.
Placement of the lowest floor of these structures below the BFE, even through they are outside.4 the SFHA, will result in an increased threat from subsurface flooding and magnified damages
from flooding that exceeds the BFE.
Freeboard
When fill is used to protect buildings from the Base Flood, the community should consider
whether freeboard should be required. This consideration should include whether better
information exists or conditions have changed (from when the BFE was originally
established) that indicate that the BFE may be higher than originally expected. Even when
the BFE is not expected to be higher, freeboard may be appropriate to provide increased
protection from flood events less frequent than the Base Flood or to account for future
changes that may increase the BFE.
The foundation types for buildings outside the SFHA described in the following sections are
listed in order of their increasing risk of flood damage.
4
Non-Basement Foundations
Non-basement foundations consist primarily of stem wall,crawlspace, and slab-on-grade foundations.
Stem Wall Foundation
Placing fill to the level of the BFE does not provide freeboard.A stem wall foundation, however,
can be used to raise the lowest floor above the surrounding grade. After the stem walls have been
constructed and extended to the desired elevation, the area enclosed by the stem walls is filled
with engineered compacted fill and a slab is poured on top (see Figure 1). Through the placement
of additional fill, the site may be elevated above the BFE. This approach provides freeboard—an
additional amount of elevation that helps protect against subsurface flooding and floods that
exceed the Base Flood. Constructing a stem wall foundation and placing additional fill on the site
provide the highest level of flood protection.
Limit of ��\
Floodway Lowest Floor�
Above BFE ' V I! (—Slab Floor
Base Flood
Elevation
Engineered Compacted Fill
Stream Compacted Fill Placed Stem Wall Placed in Area Below Lowest
Channel to an Elevation Above Floor, Inside Stem Walls
the BFE (To Provide
Freeboard)
Figure 1 Structure on a stem wall foundation. The lowest floor is raised above the BFE.
The space enclosed by the stem walls is filled with engineered compacted fill.
Crawlspace Foundation
Placing fill to the level of the BFE does not provide freeboard. Constructing a crawlspace beneath
the first floor, however, will raise the lowest floor of"the structure above the surrounding grade (see
Figure 2). Openings in the foundation walls are recommended, If flooding reaches the building,
the openings allow flood waters to enter the area below the lowest floor and equalize the
hydrostatic pressure on the foundation walls (see NFIP Technical Bulletin 1, Openings In
Foundation Walls). The crawlspace alternative is less preferable than stem wall construction,which
does not result in an enclosed area under the first floor and therefore requires no flood openings.
Placing fill to a level above the BFE provides freeboard that helps protect against subsurface
flooding and floods that exceed the Base Flood. Constructing a crawlspace foundation and placing
additional fill on the site provide a higher level of freeboard and therefore increased flood protection.
5
Limit of /II\
Floodway7 Lowest Floor
Above BFE L
Base Flood
Elevation
Craw'space Openings for,
Stream Compacted Fill Placed Foundation the Entry of
Channel to an Elevation Above Flood Water
the BFE (To Provide
Freeboard)
Figure 2 Structure on a crawlspace foundation. The lowest floor is raised above the BFE.
Openings in the foundation walls allow water from floods higher than the fill
elevation to enter the crawlspace and equalize the pressure on foundation walls.
Slab-On-Grade Foundation
This method normally provides less flood protection than crawlspace construction because it
does not elevate the house above the adjacent grade (see Figure 3).As a result, the lowest floor of
the house can be as low as the BFE and would be inundated by any flood greater than the BFE.
Placing additional engineered fill beneath the building to a level above the BFE would provide
freeboard and therefore increased flood protection.
Limit of /II\
Floodway7
Base Flood Lowest Floor
at Grade
Elevation
Slab-on-Grade
Stream Compacted Fill Placed Foundation
Channel to an Elevation Above
the BFE (To Provide
Freeboard)
Figure 3 Structure on a slab-on-grade foundation. The lowest floor is at the same elevation
as the surrounding grade.
6
Basement Foundations
As noted previously, basements are a desired housing feature in some areas of the United States.
As a result, builders and property owners who build on land removed from the SFHA sometimes
elect to install basements, which are at a higher risk of flood damage than the foundation types
j described previously. Constructing a basement on such land is not recommended, because the
basement (i.e., lowest) floor and portions of the basement walls may well be subjected to
subsurface flooding. The basement may therefore be subject to seepage and lateral hydrostatic
and uplift pressure caused by high groundwater levels associated with flooding in surrounding
areas.Additionally, when flooding exceeds the BFE, the basement area may be totally inundated
with floodwater. When builders and homeowners decide to accept the additional risk associated
with basement construction on filled land, they need to ensure that the basement and the rest of
the house are reasonably safe from flooding.
Warning: In filled areas adjacent to floodplains, floods can still greatly influence the
groundwater at the filled site. High groundwater at a site with a basement can result in
water infiltrating the basement or greatly increased hydrostatic pressures on the walls and
basement slab that can cause failure or permanent deformation. Even when floods have not
reached houses with basements, FEMA has seen numerous examples of flooded basements,
bowed basement floors, and collapsed basement walls that have resulted from the effects of
high groundwater caused by flooding. In addition, the collapse of flooded basements has
also occurred when water is rapidly pumped from basements surrounded by saturated soils
whose pressure exceeds the capacity of the basement walls.
Flood Insurance Coverage for Basements
It is extremely important to note that the NFIP offers only limited coverage for basement
flooding. First, in order for a claim to be paid, there must be a general condition of
overland flooding where floodwaters come in contact with the structure. Secondly, the
NFIP offers limited coverage for construction, utilities, and contents in basement areas.
Contents coverage is restricted to a limited number items listed in the flood insurance
policy. Contact a local insurance agent for more information.
Four basement construction methods are described below in increasing order of flood risk.
Basement Foundation With Lowest Floor At or Above BFE
Placing the lowest floor of the basement above the BFE has the effect of eliminating flood-induced
damage up to the BFE (see Figure 4). In general, the higher the basement floor is above the BFE
the lower the risk of damage from seepage and hydrostatic pressure caused by flood-related
groundwater. Where possible, the basement should be built with its floor at or above the BFE. An
added benefit is that floods that exceed the BFE will cause significantly less damage to a structure
with this type of basement than to structures with basements whose floors are at greater depths.
7
Limit of Basement Floor a ❑
Floodway) (Lowest Floor)
{ Above BFE ,❑ ❑
Base Flood q qi u�•
Elevation
Stream Compacted Fill Placed
Channel to an Elevation Above
the BFE (To Provide
Freeboard)
Figure 4 Basement foundation with lowest floor above the BFE. Damage from floods
below the BFE are eliminated.
Basement Foundation in Fill Placed Above BFE
Placing fill to a level above the BFE has the effect of reducing the depth of the basement floor
below the BFE (see Figure 5). It is recommended that fill be placed to a level at least 1 foot
above the BFE. In general, the higher the basement floor the lower the risk of damage from
seepage and hydrostatic pressure caused by flood-related groundwater. Where possible, enough
fill should be properly placed so that the lowest grade adjacent to the structure is above BFE. An
added benefit of fill placed above the BFE is that it helps protect the building from floods greater
than the Base Flood.
Limit of
Floodway7 Basement Floor
Base Flood ((Lowest Floor)
Elevation below BFE �❑ ❑
Stream Compacted Fill Placed
Channel to an Elevation Above
the BFE (To Provide
Freeboard)
Figure 5 Basement foundation in fill placed above the BFE. The depth of the basement
floor below the BFE is less than when no fill is placed.
•
Basement Foundation With Lowest Opening Above BFE
In the event that the lowest floor is not elevated to or above the BFE and fill is not placed to a
level above the BFE, the next best method of reducing flood risk is to place the lowest opening
into the basement(e.g., window well) above the BFE (see Figure 6). This will reduce the chances
that surface flooding will enter and inundate the basement. However, the basement walls and
floor slab will still be subjected to hydrostatic pressure with the potential for damage and seepage
into the basement. In addition, the above-grade basement walls will be exposed to water from
floods greater than the Base Flood. For this reason the lowest opening in the basement walls
should be above the BFE, as shown in Figure 6.
Limit of F
Floodway7 L
Base Flood 11 Basement Floor
Elevation Below BFE Lowest Floor)
�J 3
Openings in
Stream Compacted Fill Placed J Basement
al s Above
Channel the Elevation of the the BFE
BFE(No Freeboard)
•
Figure 6 Basement foundation with lowest opening above the BFE. Surface flooding is less
likely to enter and inundate the basement.
Basement Foundation With Lowest Opening at BFE
This is the least preferable condition of all because it results in the highest flood risk from a lack
of flood protection (see Figure 7). The lack of fill above the BFE, coupled with the lowest floor
being below BFE and lowest opening at the BFE, exposes the basement to flooding from both
subsurface flooding and any flood greater than the Base Flood.
Limit of —
Floodway)
Base Flood
Elevation .west Opening at BFE
Li
Compacted Fill Placed J Basement Floor
Stream to the Elevation of the (Lowest Floor)
Channel BFE(No Freeboard) Below BFE
Figure 7 Basement foundation with lowest opening at the BFE. The basement is exposed to
flooding from any flood greater than the Base Flood.
9
Table 1 summarizes the foundation construction methods described in this bulletin and ranks
them in order of increasing flood risk--the safest foundation types appear near the top; the less
safe foundation types appear near the bottom. The foundation construction methods that result in
a building that is reasonably safe from flooding are shown in the dark gray area of the table. The
basement construction methods that must meet the requirements described in the following
sections regarding basement construction guidance in order for the building to be considered
reasonably safe from flooding are shown in the light gray area.
Table 1 Flood Risk by Foundation Construction Method
r Foundation Flood Risk
Foundation Construction Method
IFill Stem Walls Crawlspace Slab-On- Basement
Grade Floor Level Openings
1:3
Li. co CO CO CO CO CO CO
u. I LIJ
> I ( > u. 3 > Lwi
o lb 0 LSl o L7] o 03 o CO O 0 CO
.0 < < < .O < < < < a m a a
• ■
■
• U
II :
■ ■■ ■ ■ ■
4) ■ ■ ■
U '
C ■ ■ ■
• ■ ■
■ • ■
■ ■ ■
Reasonably Safe From Flooding
Follow Guidance in This Bulletin To
Ensure That Building Is
Reasonably Safe From Flooding
10
Basement Construction Guidance
For those who have chosen to accept the additional risk associated with basement construction on
filled land, this bulletin provides technical guidance about measures that can be taken to protect
basements and meet the requirement that buildings be made reasonably safe from flooding.A
simplified approach, including the requirements that must be met for its use, is presented first.
For buildings that do not meet the criteria for the simplified approach, this bulletin provides
technical guidance for the development of an engineering design tailored to the site conditions.
Simplified Approach
Design Requirements
If,for a building and building site, all the requirements listed below are met (see Figure 8), the
building is reasonably safe from flooding. If all of these requirements are not met, the more
detailed analysis described under Engineered Basement Option, on page 15 of this bulletin, must
be performed to determine whether the building is reasonably safe from flooding.
LJ The ground surface around the building and within a defined setback distance from
the edge of the SFHA (see next item) must be at or above the BFE.
The setback is the distance from the edge of the SFHA to the nearest wall of the
basement. The minimum allowable setback distance is 20 feet. (However, if site
conditions are defined accurately enough to permit a reliable analysis, the use of a
shorter setback distance may be justified.)
The ground around the building must be compacted fill; the fill material—or soil of
similar classification and degree of permeability—must extend to at least 5 feet below
the bottom of the basement floor slab.
I The fill material must be compacted to at least 95 percent of Standard Laboratory
Maximum Dry Density (Standard Proctor), according to ASTM Standard D-698. Fill
soils must be fine-grained soils of low permeability, such as those classified as CH,
CL, SC, or ML according to ASTM Standard D-2487, Classification of Soils for
Engineering Purposes. See Table 1804.2 in the 2000 International Building Code
(IBC) for descriptions of these soil types.
The fill material must be homogeneous and isotropic; that is, the soil must be all of
one material, and the engineering properties must be the same in all directions.
The elevation of the basement floor must be no more than 5 feet below the BFE.
There must be a granular drainage layer beneath the floor slab, and a 'A-horsepower
sump pump with a backup power supply must be provided to remove the seepage
flow. The pump must discharge above the BFE and away from the building. This
arrangement is essential to prevent flooding of the basement or uplift of the floor
under the effect of the seepage pressure.
11
Model building codes (such as the 2000 International Residential Code) also address
foundation drainage (IRC Section R405) and foundation walls (IRC Section R404).
Model building codes generally allow foundation drains to discharge through either
mechanical means or gravity drains. In addition, there is often an exception to the
requirement for drainage systems in well-drained soils. However, in or near
floodplains, well-drained soils can, in fact, help convey groundwater towards the
building foundation. Therefore, this exception should not apply in or near floodplains.
U In some cases in or near floodplains, even with standard drainage systems, hydrostatic
pressures from groundwater against the basement can result. When a standard
drainage system is unable to eliminate hydrostatic pressure on the foundation, model
building codes, including the 2000 International Residential Code (IRC Section
R404.1.3), require that the foundation be designed in accordance with accepted
engineering practice. The simplified approach contained in this Technical Bulletin
assumes no hydrostatic pressure on the foundation and should be used only
when a standard drainage system, discharged by a sump pump that is equipped
with backup power and that discharges above BFE, is employed. For other
drainage systems, the designer should use the engineered basement option presented
on page 15 of this bulletin and other appropriate building code requirements.
20 Feet or Greater
BFE BFE) BFE
r [,� . Engineered Fill
5 Feet Basement //
or Less Floor 7
Granular Drainage Layer
Sump Pump (1/4 Horsepower
or Greater,With Emergency I 5 Feet or Greater
Backup
the e Power and Discharge
Above
BFE)
Compacted Fill or Soil
of Similar Character
Figure 8 Requirements for use of the simplified approach to basement construction.
12
w , L
Phreati` BFE
LineBase
Floor
+ K
a
Base of Seepage Flow Zone 2
(1) q = k(a2-b2)/2L
where: q =flow in cfs for a 1-foot width of seepage zone
k= soil permeability in feet per second
a= head at entry surface in feet
b= head at drain surface in feet
L= length of seepage zone (setback distance) in feet
(2) Q = Pq
where: Q=total seepage flow into drain in cfs
P = peripheral distance around the basement
(for a square basement, P = 4w)
(3) Required sump pump capacity =4Q for a safety factor of 4
Figure 9 Method for calculation of seepage flow.
The Dupuit equation for the quantity of seepage flow is:
q = k(a2— b2)/2L
where: q is the flow in cubic feet per second for a 1-foot width of seepage zone
k is the soil permeability in feet per second (fps) (maximum value of k is lx10-'fps)
a and b are hydraulic heads in feet (a < b + 5)
L is the length of the flow zone in feet (L> 20 feet)
14
To obtain Q, the total seepage flow, in cubic feet per second, q must be multiplied by the length
around the periphery of the four sides of the structure. This is a simplifying approach that
obviates the need for a three-dimensional flow net calculation and is reasonably conservative.
It should be noted that the soil permeability does not affect the geometry of the seepage zone or
the geometry of the phreatic line. The permeability does have a significant effect on the quantity
of seepage that must be collected and discharged by the drainage layer and the sump pump. The
calculation of the quantity Q provides a basis for the selection of a sump pump of adequate
capacity. To allow for possible errors in the estimation of the soil permeability, the pump should
have a capacity of at least four times the calculated value of Q. As noted in the requirements
section, a standard sump pump of 1/4 horsepower or greater will generally satisfy the requirements
of seepage removal for the conditions described above.
Engineered Basement Option
If the requirements specified for the simplified approach are not met, a licensed soils engineer or
geologist must perform a detailed engineering analysis to determine whether the structure will be
reasonably safe from flooding. The analysis should consider, but is not limited to, the issues
described in the following sections.
Depth, Soil Type, and Stratification of Subsurface Soils
The depth, soil type, and stratification of the subsurface soils may be complex. Four potential
generalized scenarios are shown in Figures 10 and 11. Figure 10 shows two cases of
homogeneous soil. The depth of penetration of the basement and the depth of the flow zone are
not limited to the assumptions on which the simplified approach is based. Case I represents a
foundation consisting of clayey soils, either fill or natural deposits or a combination, which are
more or less homogeneous because they have similar engineering properties. If an adequate
setback distance is provided, the seepage quantity would be relatively low, and uplift pressure
beneath the slab could be controlled by an appropriately sized sump pump.
Case II represents a foundation consisting of sandy soils, either fill or natural soil deposits or a
combination, which are more or less homogeneous because they have similar engineering
properties. The seepage quantity would be fairly large, and more attention would have to be given
to the setback distance and to the provision of an adequately sized sump pump to prevent
excessive uplift pressure beneath the floor slab.
Figure 11 shows two simple cases of stratified soils, with impervious clays overlying pervious
sands. This is a common occurrence in natural floodplain deposits. In Case III, the contact
between the two soil strata is at some distance below the basement floor. This case would involve
a moderate quantity of seepage, depending on the thickness, d, of the impervious stratum below
the basement floor. There is also a potential for excessive uplift pressure beneath the floor, at the
level of the bottom of the clay stratum. If d is equal to h, the net hydraulic head between the
flood level and the floor level, the safety factor against uplift would be approximately 1.0. If d is
less than h, there would be excessive uplift, with a safety factor equal to less than 1.0.
15
"---
CASEI
Low q
/ BFEI
I Impervious J !
i - I /
CASE II
. High q
Potential Uplift
BFE Th
i Pervious
Figure 10 Case I and Case II—homogeneous soil.
Case IV shows impervious soils overlying pervious soils, with the contact between the soil strata
at some distance above the basement floor. This case would involve a large quantity of seepage
and potential for excessive uplift beneath the basement floor.
Geotechnical Investigations
Geotechnical investigations must be made for cases that do not conform with the assumptions on
which the simplified approach is based. Information that is needed to permit an adequate
engineering analysis includes the following:
• The BFE, which is to be used as the design flood water surface for calculating expected seepage.
16
CASE III
Moderate q
Potential Uplift
BFEI
Impervious J h. i
1
d
Pervious J
/ . - --/ CASE IV
High q
Potential Uplift
� BFE
Impervious
Pervious
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Figure 11 Case III and Case IV—stratified soils.
• The elevation of the bottom of the basement floor. This can be adjusted as needed to achieve
more suitable conditions.
• The setback distance of the basement wall from the edge of the flood water. This can be
adjusted to achieve more suitable seepage control or to accommodate available space restraints.
• The elevation of the groundwater table and its seasonal variations. A high water table would
cause problems with groundwater control during construction of a basement, even without a
flood event.
• The stratification of the subsurface materials, for both natural and fill soils. In general, borings
should be drilled to a depth below the bottom of the floor slab that is at least two times as great
as the depth of the bottom of the floor slab below the BEE.
17
• The engineering classification of the soils, for both natural and fill soils. This must be done in
accordance with ASTM D2487, Classification of Soils for Engineering Purposes. This is the
Unified Soil Classification System that is universally used throughout the United States. Local
or county agricultural soil survey maps should not be used, because they do not give specific
information about location and depth of soils, and their designations are not pertinent to civil
engineering use.
• Subsurface conditions landward from the structure. This includes information about the
location of the water table, whether it is higher or lower than the flood level, and information
about any penetrations of the soil, such as ponds. Attention should be given to the possibility
that higher ground, such as valley walls, could contribute to the groundwater level in the
floodplain, either perennially or during periods of heavy rain.
• Information about any penetrations through the basement walls below the BFE, such as utility
lines and other openings.
• Analysis of seepage quantity. The analysis can be made by the conservative simplified method
described in Item 5 in the section titled Technical Background for the Simplified Approach
(illustrated in Figure 9), or by the construction of a flow net that takes into account all of the
boundary conditions more rigorously. A flow net may be required to permit analysis of uplift
pressures. Uplift pressures may be more significant in laminated or stratified soil deposits.
Buildings in Existing Filled Areas
In evaluating buildings in existing filled areas, the two approaches already described—the
"simplified approach"or the"engineered basement option"—can be used.If the simplified approach
is used, all the requirements for the use of this approach must be met. Some possible means for
evaluating whether these requirements are met include soil tests and investigations, including soil
borings and hand augers; field records from the time the fill was placed; and soil surveys. If the
requirements for the simplified approach are not met, a licensed soils engineer or geologist must
perform a more detailed engineering analysis as described under the "engineered basement
option." More extensive soil investigations and testing may be required to complete the analysis.
The NFIP
The NFIP was created by Congress in 1968 to provide federally backed flood insurance
coverage, because flood coverage was generally unavailable from private insurance companies.
The NFIP is also intended to reduce future flood losses by identifying floodprone areas and
ensuring that new development in these areas is adequately protected from flood damage. The
NFIP is based on an agreement between the Federal government and participating communities
that have been identified as floodprone. FEMA, through the Federal Insurance Administration
(FIA), makes flood insurance available to the residents of a participating community, provided
the community adopts and enforces adequate floodplain management regulations that meet the
minimum NFIP requirements. The NFIP encourages communities to adopt floodplain
management ordinances that exceed the minimum NFIP criteria set forth in Part 60 of the NFIP
Floodplain Management Regulations (44 CFR 60). Included in the NFIP requirements, found
under Title 44 of the U.S. Code of Federal Regulations, are minimum building design and
18
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Weld County Code Compliance
• Method of Subdividing Agriculturally-
Zoned Land
• Limit removal of Prime Agricultural Land
• Reduce impact to service providers
• Enhance the natural beauty of the County
Bonus
• Reuse and Recycling of housing stock
already in existence.
• Remove opportunity for existing
improvements to become a derelict
development and compliance problem.
5
Item 1A
Amendment
1A.Prior to scheduling the Board of County Commissioners hearing:
Section 223-100.8.1 of the Weld County Code states"new planned unit
developments or subdivisions should be planned to take Into account current and
future oil and gas drilling activity to the extent oil and gas development can
reasonably be anticipated." [The applicant shall either submit a copy of an
agreement with the properties mineral owners stipulating that the oil and gas
activities have been adequately Incorporated into the design of the site or show
evidence that an adequate attempt has been made to mitigate the concerns of the
mineral owners.] The plat shall be amended to include any possible future drilling
sites.[Evidence of approval shall be submitted to the Weld County Department of
Planning Services.] (Dept.of Planning Services).
Item 2G
Amendment
The right-of-way for the internal roadway
right-of-way shall be fifty (50) [sixty (60)]
feet in width including cul-de-sacs with a
sixty-five (65) foot radius at the dead ends,
and dedicated to the public.
6
Item 3B
Delete
[The PUD must show evidence that the plan
review process has been initiated with the
Drinking Water section of the Water
Quality Control Division of the Colorado
Department of Public Health and
Environment]
Item 3G
Delete
[The applicant shall address the concerns of
the Platte Valley Fire Protection District.
Written evidence of approval shall be
submitted to the Department of Planning
Services.]
Item 6A
Delete
[The applicant shall provide a pavement
design prepared by a professional engineer
along with the final plat submission.]
Thank you .
Todd Hodges Design , LLC
8
H. Section 27-6-120.6.h- Consistency exists between the proposed zone district(s), uses,
the specific or conceptual development guide.
The submitted Specific Development Guide does accurately reflect the performance
standards and allowed uses described in the proposed zone district, as described
previously. The applicant is requesting that the Final Plan be administratively
reviewed. The Department of Planning Services' staff concurs with this request.
This Approval recommendation is based upon compliance with Chapter 27 requirements.
The Change of Zone from A (Agricultural) to PUD with Estate Zone District uses is conditional
upon the following:
1. Prior to scheduling the Board of County Commissioners hearing:
A. Section 22-5-100.B.1 of the Weld County Code states "new planned unit
developments or subdivisions should be planned to take into account current and future
oil and gas drilling activity to the extent oil and gas development can reasonably be
anticipated." The applicant shall either submit a copy of an agreement with the properties
mineral owners stipulating that the oil and gas activitiec have been adequately
incorporated into the design of the site or chow evidence that an adequate attempt has
been made to mitigate the concerns of the mineral owners. The plat shall be amended to
include any possible future drilling sites. Evidence of approval shall be submitted to the
Weld County Department of Planning Servicec. (Dept. of Planning Services)
2. The plat shall be amended to include the following:
A. Include the location of the existing septic system, replacement envelopes
(meeting setbacks to the well and other pertinent physical features, and the location of
the existing well and a potential replacement well for Lot 13. (Department of Public
Health and Environment)
B. Correct the approximate absorption field/well setback on Lot 7 from 60 feet to 144 feet.
(Department of Public Health and Environment)
C. Relocate the primary and secondary envelopes on Lots 1, 5 and 11. (Department of
Public Health and Environment)
D. The right-of-way for Weld County Road 69. (Department of Public Works)
E. Verify and delineate the ditch location. If the ditch is outside the existing right-of-way, an
easement shall be provided. (Department of Public Works)
F. The road cross-section for all internal roads as approved by Weld County Department of
Public Works. (Department of Public Works)
G. The right-of-way for the internal roadway right-of-way shall be fifty (50) sixty(60)feet in
width including cul-de-sacs with a sixty-five(65)foot radius at the dead ends, and
dedicated to the public. (Department of Public Works)
H. The applicant shall delineate a Plant Material List specifying the Botanical and Common
names of all plant material to be installed if new, or plant material already installed on
site; the size of the plant material at installation and whether the plant material is to be
containerized or B&B. (Department of Planning Services)
EXHIBIT
I
KY G__
The location of the monument sign, plant material around the monument sign, the
location of the mailbox pedestal and mail boxes,the location of the picnic shelter, the bus
stop location and the proposed basketball/ sport courts. (Department of Planning
Services)
J. The location of all on-site light standards. (Department of Planning Services)
3. Prior to recording the Change of Zone plat:
A. The Change of Zone plat shall meet all requirements of Section 27-9-20 of the Weld County
Code. (Department of Planning Services)
B. The PUD must chow evidence that the plan review process has been initiated with tho Drinking
Water section of the Water Quality Control Divicion of the Colorado Department of Public Health
and Environment. (Department of Public Health and Environment)
C. All septic systems located on the property shall have appropriate permits from the Weld
CountyDepartment of Public Health& Environment. The Environmental Health Division of the
Weld County Department of Public Health & Environment was unable to locate a septic permit for
the septic system located on the proposed Lot 13 of this proposed PUD. This existing septic
system which is not currently permitted through the Weld County Department of Public Health&
Environment, will require an I.S.D.S. evaluation prior to the issuance of the required septic permit.
In the event the system is found to be inadequate, the system must be brought into compliance
with current I.S.D.S. regulations. (Department of Public Health and Environment)
D. Update the'Real Estate Purchase Agreement', Water Lease Agreement and'Bargain and Sale
Deed and Assignment' documents to include the 3rd well. Documents shall be reviewed and
approved by the County Attorney's office. (Department of Planning Services and Department
of Public Health and Environment)
E. Revise covenant item C-14 to specify the management of the Public Water System requirements,
the funding source, etc, and enforcement provisions. (Department of Public Health and
Environment)
F. The applicant shall submit a proposed estimate for time of construction of the PUD. (Department
of Planning Services)
G. The applicant shall address the concerns of the Platte Valley Fire Protection District. Written
evidence of approval shall bo submitted to the Department of Planning Services.
(Department of Planning Services, Platte Valley Protoction District)
H. The applicant shall address the concerns of the Weld County Sheriffs Department, in a memo
dated April 10, 2002 and incorporate remedies for these concems. Written evidence of a solution
shall be provided to the Department of Planning Services. (Weld County Sheriff)
The applicant shall apply for and be approved for a building permit for the construction of the
monument sign on-site. Further, the applicant shall submit an application for a Flood Hazard
Development Permit for the monument sign. (Department of Planning Services)
J. The applicant shall submit two(2) paper copies of the plat for preliminary approval to the Weld
County Department of Planning Services. Upon approval of the paper copies the applicant shall
submit a Mylar plat along with all other documentation required as conditions of approval. The
Mylar plat shall be recorded in the office of the Weld County Clerk and Recorder by Department
of Planning Services' Staff. The plat shall be prepared in accordance with the requirements of
Section 27-9-20 of the Weld County Code. The Mylar plat and additional requirements shall be
submitted within sixty (60) days from the date of the Board of County Commissioners resolution.
The applicant shall be responsible for paying the recording fee. (Department of Planning
W. The applicant shall comply with Section 27-8-50 Weld County Code, as follows: Failure
to submit a Planned Unit Development Final Plan - If a PUD Final Plan application is not
submitted within two (2) years of the date of the approval of the PUD Zone District, the
Board of County Commissioners shall require the landowner to appear before it and
present evidence substantiating that the PUD project has not been abandoned and that
the applicant possesses the willingness and ability to continue with the submission of the
PUD Final Plan. The Board may extend the date for the submission of the PUD Final
Plan application and shall annually require the applicant to demonstrate that the PUD has
not been abandoned. If the Board determines that conditions or statements made
supporting the original approval of the PUD Zone District have changed or that the
landowner cannot implement the PUD Final Plan, the Board of County Commissioners
may, at a public hearing revoke the PUD Zone District and order the recorded PUD Zone
District reverted to the original Zone District. (Department of Planning Services)
X. The PUD Final Plan shall comply with all regulations and requirements of Section 27 of
the Weld County Code. (Department of Planning Services)
Y. The site shall maintain compliance at all times with the requirements of the Weld County
Departments of Public Works, Public Health and the Environment and Planning Services,
and adopted Weld County Code and Policies. (Department of Planning Services)
5. The Change of Zone plat map shall be submitted to the Department of Planning Services' for
recording within thirty (30) days of approval by the Board of County Commissioners.
(Department of Planning Services)
6. At the time of Final Plan submission:
A. The applicant shall provide a pavemont design prepared by a profocsional engineer along
with the final plat submission. (Department of Public Works)
B. Roadway and grading plans along with construction details will be required for the final
plat. (Department of Public Works)
C. A final drainage report stamped, signed and dated by a professional engineer licensed in
the State of Colorado shall be submitted with the final plat application. The 5-year storm
and 100-year storm drainage studies shall take into consideration off-site flows both
entering and leaving the development. Increased runoff due to development will require
detention of the 100-year storm developed condition while releasing the 5-year storm
existing condition. (Department of Public Works)
D. Final drainage construction plans, conforming to the drainage report, shall be submitted
with the final plat application to the Department of Public Works for their review and
approval. (Department of Public Works)
E. The applicant shall submit final road layout and alignment plans with associated
documents to the Department of Public Works for their review and approval.(Department
of Public Works)
F. The applicant shall submit an on-site (Private) Improvements Agreement that addresses
all improvements associated with this development, per compliance with Section 24-9-10
of the Weld County Code. (Department of Planning Services)
G. The applicant shall submit evidence to the Department of Planning Services that approval
was received from the Department of Public Works of an Improvements Agreement
Regarding Collateral for the Transportation portion of the PUD. (Departments of Public
Hello