Duval County:Using the power of SWMM unsteady modeling for CLOMR applications

May 17 2011, Louisville KY

José Maria Guzmán, P.E. D.WREGaston Cabanilla, P.E., CFMGreg McGrath

Agenda

• Project Technical Background• Flood Insurance Study Documents based on SWMM• LOMC Process using SWMM• Conclusions/Discussion

Acknowledgments: Tom Nye, Michael F. Schmidt, Sandeep Gulati, Seungho Song, Erin Hardin, Katie Lytle.

Project Technical Background

• Community opted to use SWMM for several reasons– Previous models with successful model

calibrations – Complex system with multiple culverts,

ditches and closed conduits– Hydraulics are driven by tail water

conditions– Timing between different tributaries is

relevant– Agreement with local regulations driven

by 24 hour duration storms– Concurrent TMDL studies– Public domain software

• By having one single model, the City can keep updated one tool that is benefiting from continuous work in the LOMC, TMDL, and ongoing activities

Some experienced CFMs might need to get familiar with results from SWMM

• Model data are structured differently in SWMM than in other more traditional software:

• Hydrology and Hydraulics are solved simultaneously in SWMM• No need to solve tributaries separately, and account for backwater

curve with a dynamic stable simulation

Variable HEC RAS SWMM

Discharge Cross Section Link

WSE, Stage Cross Section Node

Velocity Cross Section Link

Floodway Width Cross Section Link

Floodway Surcharge Cross Section Node

Little Cedar Creek

• Hydrology– Tributary Area: 6

Square Miles – 14 Subcatchments

• Hydraulics– 74 Nodes– 83 Links

2010 DFIRM Panel Generated with SWMM Output

Stream Profile using RASPLOT and showing SWMM Nodes

FIS Discharge Table

Floodway Data Table in staggered format

Example of Map Revision Application using SWMM

Proposed 4-Lane Causeway and Bridge

Proposed Urban DevelopmentWithout Floodplain Encroachment

LOMC Process, What to submit

• Completed application forms.• Narrative on project and submittal (optional but very helpful). • Hydrologic Computations (if applicable) along with digital files of

computer models used.• Hydraulic Computations (if applicable) along with digital files of

computer models used.• Certified topographic map with floodplain and floodway (if

applicable) delineations.• Annotated FIRM and/or FBFM to reflect changes due to project • Items required to satisfy any NFIP regulatory requirements.• Fee payment if applicable.

The LOMC process requires the use of several forms

• SUMMARY OF FORMS• There are six forms plus a payment form that needs to be prepared, • Form 1 - Overview & Concurrence (All revisions)

– Requester , community official, and engineer signatures• Form 2 - Riverine Hydrology & Hydraulics

– Scope and methodology of hydrologic and/or hydraulic analyses • Form 3 - Riverine Structures

– Hydraulic structures in the stream channel or floodplain.• Form 4 - Coastal Analysis

– Scope and methodology of coastal analyses

LOMC Process

• Form 5 - Coastal Structures – hydraulic structures constructed along the coast

• Form 6 - Alluvial Fan Flooding – information for analyses of alluvial fans

• Payment Information– Information regarding any fees paid for a CLOMR, LOMR, or External

Data Request.

CLOMR Process for this case

• Form 1 - Overview & Concurrence (All revisions) – Identify that it is CLOMR or LOMR– Community Number, Name, State, Map Number, Panel Number,

and Effective Date– Flooding source name , type of flooding (Riverine)

For this example FORM 2 and 3 are required

• Form 2 - Riverine Hydrology & Hydraulics– Duplicate Effective Model

• Is a copy of the hydraulic analysis used in the effective FIS

– Corrected Effective Model • is the model that corrects any errors that occur in the Duplicate Effective

Model

– Existing or Pre-Project Conditions Model • Modifications to produce the Existing or Pre-Project Conditions Model to

reflect any modifications that have occurred within the floodplain since the date of the Effective model but prior to the construction of the project

– Revised or Post-Project Conditions Model• Modified to reflect revised or post -project conditions

For this example FORM 2 and 3 are required

• Form 3 - Riverine Structures– request involves a new bridge– Indicate the reason for the new bridge– Indicate the model used to analyze the hydraulics at the bridge

(SWMM)– Attach plans of the structure certified by a registered

professional engineer

• Payment Information– Fees paid for a CLOMR

Effective Model is available in SWMM5

• 100 yr Storm: 12.3 inches in 24 hours• The model took less than a minute to run 30 hours for the entire

stream • Peak Flow at proposed project site: + 1,600 cfs , - 150 cfs

Effective Model Peak WSE Results

• Each node reports the peak HGL (WSE)

• Note that many conduits peak around hour 21 (9 hours after the storm peak)

Existing Pre-conditions Effective Model

• In anticipation of the proposedconditions, the user addscross sections at criticallocations

• This represents pre-conditions model• There could be a BFE change

due to new cross sections

Proposed Model – Bridge Addition in SWMM

• State/Local criteria: no WSE increase is allowed

• Federal criteria: no increase allowed in floodway areas

• Peak Upstream WSE : 5.8 ft at Hour 16

• No encroachment• Two rows of bridge piers• Lower Chord Elevation: 6.8 ft• Adding the bridge takes up to 60

Minutes

SWMM Reports model information differently than HECRAS and this has implications in the location of cross sections• In anticipation of the

proposed bridge, HECRAS users would add four cross sections

• In order to extract thesame information fromSWMM the user shouldadd nodes and linksas shown

NodeLinkCross Section

LC_Junc

Le_Junc

LC20008L2S

LC20008L3

Le_

Cha

nLc

_Cha

n

User estimates the length of contraction and expansion reaches

• Based on proposedbridge geometry theuser can determine the following using HEC methodology

• Lc = 20 ft• Le = 40 ft• Rounded to nearest 10 ft

NodeLinkCross Section

LC_Junc

Le_Junc

LC20008L2S

LC20008L3

Proposed Model needs to show no WSE increase

• Net 3 cfs decrease in discharge upstream of bridge.

User compares Peak WSE Results with effective model and compares floodway results• Floodway Peak Stages:

• Cannot increase 100 year storm stages per Florida regulations.• Cannot increase Floodway with or surcharge per FEMA regulations.• Florida regulations control for this case.• End result is a proposed bridge with no floodplain encroachment.

An Updated Floodway Table is generated using the SWMM results

• Floodway widths are also verified

NODES LINKSDISTANCE

(FT)FLOODWAY WIDTH (FT)

BASE PEAK FLOW (CFS)

BASE PEAK VELOCITY

(FPS)REGULATORY

(FT NAVD)

WITHOUT FLOODWAY (FT NAVD)

WITH FLOODWAY (FT NAVD)

SURCHARGE (FT)

LITTLE CEDAR CREEK

C20007L1 268 2026 2.920007L1 4796 5.1 5.1 5.4 0.3

C20007 85 2117 0.720007 5709 5.2 5.2 5.6 0.4

C20008L3 292 1646 1.9Le_Junc 6406 5.4 5.4 5.8 0.4

Le_Chan 306 7868 1.420008L3 6446 5.3 5.3 5.7 0.4

Lc_Chan 472 31271 1.4Lc_Junc 6466 5.4 5.4 5.8 0.4

C20008L2 200 1765 2.620008L2 7143 5.8 5.8 6.6 0.8

C20008L1 343 1615 1.0

BASE FLOOD WATER SURFACE ELEVATIONFLOODING LOCATION FLOODWAY

Conclusions

• SWMM users can find the required information for FEMA LOMC applications

• When the model for the entire watershed is used, there is no need to pre-establish the domain of influence of new developments

• The dynamic simulation requires attention to model results to verify timing, direction and magnitude of flows.

• Duval County has developed a tool that can assist in FEMA mapping, urban planning, permitting, and water quality evaluations