a simplified method of implementing no rise analysis in unnumbered a-zones

A Simplified Method of Implementing

No Rise Analysis in

Unnumbered A-Zones

Based Upon No Loss of Conveyance

Dwayne E. Culp, Ph. D., P.E., CFM

Second National Flood Workshop, Houston, TX

Session C4 - Public Awareness/Mitigation

A Simplified Method of Implementing No Rise Analysis in Unnumbered A-Zones Based Upon No Loss of Conveyance


CFR 60 Requirements

44 CFR 60.3(a)(3)

Review all permit applications to determine whether proposed building sites will be reasonably safe from flooding. If a proposed building site is in a flood-prone area, all new construction and substantial improvements shall (i) be designed (or modified) and adequately anchored to prevent floatation, collapse, or lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy, (ii) be constructed with materials resistant to flood damage, (iii) be constructed by methods and practices that minimize flood damages, and (iv) be constructed with electrical, heating, ventilation, plumbing, and air conditioning equipment and other service facilities that are designed and/or located so as to prevent water from entering or accumulating within the components during conditions of flooding.



CFR 60 Requirements

44 CFR 60.3(a)(4)

Review subdivision proposals and other proposed new development, including manufactured home parks or subdivisions, to determine whether such proposals will be reasonably safe from flooding. If a subdivision proposal or other proposed new development is in a flood-prone area, any such proposals shall be reviewed to assure that (i) all such proposals are consistent with the need to minimize flood damage within the flood-prone area, (ii) all public utilities and facilities, such as sewer, gas, electrical, and water systems are located and constructed to minimize or eliminate flood damage, and (iii) adequate drainage is provided to reduce exposure to flood hazards.



CFR 60 Requirements

44 CFR 60.3(b)(7)

Assure that the flood carrying capacity within the altered or relocated portion of any watercourse is maintained;

44 CFR 60.3(c)(10)

Require until a regulatory floodway is designated, that no new construction, substantial improvements, or other development (including fill) shall be permitted within Zones A1-30 and AE on the communities FIRM, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood more than one foot at any point within the community.



Basis of Method

Determine Base Flood Elevation – Use Maps and Profiles with BFEs if available.– Use the survey methods with local agreement if

BFEs are not available.• Demonstrate No Net Fill (no loss of storage)• Demonstrate No Loss of Conveyance

Capacity (no loss in carrying capacity of the system)



Determine Base Flood Elevation

• When there are elevations– From Profiles (preferred)– From FIRM Panel

• When there are no elevations– From FIRM Panel overlaid over a topographical

map– From a FIRM Panel with staked elevations of the

lines of the FIRM panel




BFE=57.0 feet

Brid

ge u

pstr

eam

Brid

ge d

owns

trea

mProperty



Determine Base Flood ElevationBFE=57.0 feet from profile

Bridge upstream

Bridge downstream



Determine Base Flood ElevationIf there are No Elevations

Property




Property




Property

54.1553.55

54.3555.15

55.1055.0054.1554.35

55.0554.75



Conveyance

Manning’s Equation

Conveyance

Where, Q is flow (cfs / cms), A is area of flow perpendicular to flow direction (ft2 / m2), R is hydraulic radius (ft / m), n is Manning’s n, K is a conversion factor (1.49 in English units, 1.00 in metric), and X is the conveyance.

2/13/2fSAR

n

KQ

3/2ARn

KX



Conveyance

Note:If the Manning’s n increases, conveyance decreases.If depth of flow increases, both A and R will also

increase, and therefore conveyance increases.If depth of flow increases but A decreases, it may be

possible to have conveyance increase or decrease.

3/2ARn

KX



Goal of the Conveyance Analysis

• Demonstrate for multiple cross sections that there is no decrease in conveyance. – Note: City of Houston requires that there is no

significant increase in Conveyance too. The City of Houston does allow the most restrictive cross section to demonstrate no significant increase.

– For jurisdictions with unnumbered A zones with unmodelled streams, it should be adequate to demonstrate that there is no loss of conveyance.



How to Select Cross Sections

• Representative– Existing conditions– Proposed conditions

• Perpendicular to direction of channel flow• Include both onsite and offsite areas if

necessary• More than one. There will be a need for

multiple cross sections. It is suggested that a minimum of two are needed per 500 feet of property width perpendicular to channel flow direction



Example



Conveyance

• Calculate Conveyance for Each Section• Wetted Perimeter

• In CADD do not use a distorted cross section• Calculate edge of buildings if the buildings act as restrictions

to flow• When flow is under buildings without restriction – be aware

that piers increase wetted perimeter• Area• Mannings n

• Use values for floodplain areas where possible. TxDOT tables or the tables from Chou work good.

3/2ARn

KX

WP

AR



ExampleExisting Cross Section 1

Description Sta.1(ft)

Sta. 2(ft)

Material Manning's n

Area (ft2)

WettedPerimeter

(ft)

Conveyance(cfs)

Open space 0+00.00 1+02.32 Grass 0.040 169.9 102.3 8851.2

Driveway 1+02.32 1+85.11 Gravel with Vegetation

0.024 148.5 82.8 13682.1

Open space 1+85.11 2+00.00 Grass 0.040 34.4 14.9 2227.0

Total: 24760.4

Proposed Cross Section 1

DescriptionSta.1(ft)

Sta. 2(ft) Material

Manning's n

Area (ft2)

WettedPerimeter

(ft)Conveyance

(cfs)Open space 0+00.00 0+64.88 Grass 0.023 112.4 66.7 10196.6

Garage 0+64.88 0+85.72 Brick 0.000 0.0 0.0 0.0

Open space 0+85.72 0+91.89 Grass 0.023 10.6 9.6 725.7

Garage 0+91.89 1+12.72 Brick 0.000 0.0 0.0 0.0

Open space 1+12.72 1+16.49 Grass 0.023 6.5 5.1 493.2

Driveway 1+16.49 1+36.57 Concrete 0.011 46.5 22.6 10166.0

Garage 1+36.57 1+60.53 Brick 0.000 0.0 0.0 0.0

House 1+60.53 1+68.53 Under House 0.000 0.0 0.0 0.0

Post 1+68.53 1+69.53 Wood 0.000 0.0 0.0 0.0


Post 1+76.53 1+77.53 Wood 0.000 0.0 0.0 0.0


Post 1+88.03 1+89.03 Wood 0.000 0.0 0.0 0.0

Open space 1+89.03 2+00.00 Grass 0.023 29.9 13.8 3211.4

Open space 0+00.00 0+64.88 Grass 0.023 112.4 66.7 10196.6

Total: 24792.9



No Storage Loss

• Demonstrate no storage loss– CADD– Cross Sections and area volume calculations– 3D surfaces



No Storage Loss



Why should Agencies Use a Method Like This?

• The method is simple to implement– It does not require that the reviewer have significant

mathematic ability, or knowledge of specific programs.

• It is reproducible• It is more accurate on a site basis than hydrologic or

hydraulic models that average conveyance and storage over very large areas

• It complies with 44 CFR 60.3(b)(7) as it demonstrates no loss of carrying capacity

• It complies with 44 CFR 60.3(c)(10) as it demonstrates that the cumulative effect of the proposed development, will not increase the water surface elevation of the base flood



References

Managing Floodplain Development in Approximate Zone A Areas, a guide for obtaining and developing Base (100-year) Flood Elevations, FEMA April 1995.

Rules and Regulations for Chapter 19, City of Houston, February 2009.

Hydraulic Design Manual, Texas Department of Transportation, October 2011.

Chou, Ven Te, Open-Channel Hydraulics, McGraw Hill, 1988.



Questions

Dwayne E. Culp, Ph.D., P.E., CFMJacobs Engineering Group Inc.

5995 Rogerdale Road, Suite B3014BHouston, TX 77072

713-898-1977

a simplified method of implementing no rise analysis in unnumbered a-zones

Documents

flood hazards

flood damages

floodprone area

rise analysis

proposed development

proposed new development

loss of conveyance dwayne

requirements44 cfr