A Levee Policy for the National Flood Insurance Program (1982)

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9 FLOODPLAIN MAPPING APPROACHES IN LEVEE-PROTECTED AREAS PURPOSE AND DISCUSSION This chapter makes recommendations concerning the display on maps of flood risks and evacuation routes in levee-protected areas. The information requiring display is the set of lines bounding zones between the dimensions of levee recognition shown in Table 1. As a beginning example, the committee has recommended that FEMA recognize through reduced premiums to protected properties (1) existing levees that provide protection to a 25-year flood level (plus freeboard) and (2) areas protected by new levees that provide protection to a 100-year flood level (plus freeboard). Currently areas inundated by floods more frequent than the 100-year flood and areas protected by levees rated as providing protection against larger floods are not delineated on maps prepared by FEMA. The dimensions of levee recognition recommended in Table 1 would thus require that FEMA mapping practices be expanded in scope. It is important that people be aware when they live, work, or are considering the acquisition of property located in an area protected by a levee and that a levee can fail structurally or be overtopped during floods exceeding the levee height. Such awareness would work to reduce the false sense of security that levee systems tend to generate. However, flood insurance maps alone are not sufficient because the public seldom sees them. Even though the maps must be supplemented by other means (such as mailed annual notices, newspaper articles, and T.V. presentations) they serve as basic information documents. Flood insurance maps are read and studied by local government officials, realtors, insurance people, developers, and bankers whose businesses may be affected by floodplains. Without levee-protected areas being identified on flood insurance maps, there might not be an information base that would permit any other means of notification to be used. The committee's recommendations focus on credited levees providing 100-year flood protection and on crediting existing levees not providing 100-year flood protection. A credited levee is a levee recognized as meeting the minimum guidelines set forth in previous chapters. Before the recommendations are presented, however, a discussion of flood insurance maps is provided to lay a groundwork understanding on the types of flood insurance maps currently produced by FEMA and what they are intended to do. -47-

FLOOD INSURANCE MAPS PREPARED BY FEMA The Flood Hazard Boundary Map (FHBM) is prepared by FEMA for each community having special hazards. It shows the boundaries of the flood, mudslide, and/or related erosion hazard areas designated as Zone A, M, and/or E as illustrated on Figure 1. A FHBM is generally the first map prepared to meet the flood hazard area identification requirements of the National Flood Insurance Act of 1968, as amended. It can be issued before or after a community decides to participate. It is preliminary and contains little more information than the identification of special hazard areas, which in the case of flooding are A Zones as shown in Figure 1. The FHBM shows the location of any existing levee, but it does not delineate or identify the area that the levee protects. A Flood Insurance Rate Map (FIRM) is prepared following completion of a detailed flood insurance study. The FIRM is an official map of the community on which the special hazard areas and the risk premium zones applicable to the community are delineated. The FIRM is distributed upon completion to the community, lending institutions, insurance agents, and others who request the map and any subsequent revisions. The type of information shown on a FIRM is depicted in Figure 2, which includes a portion of the area shown in Figure 1. Note the location of Corrales Road crossing of the Rio Grande on both maps. It is significant that no reference is made on the FIRM to the levee that is located on the east side of the Rio Grande as shown in the FHBM. The levee-protected area is shown on the FIRM as Zone B (see lower right corner of the map). The primary purposes of the FIRM are to provide the information necessary to determine if insurance is needed on a property, to define the level of risk so a premium can be determined, and to provide flood elevation information for floodplain management purposes. If a property is located in Zone B or C, no insurance is required, and the property owner is more likely to assume there is little or no flood risk. Zone B can currently mean: (1) Areas between limits of the 100-year flood and 500-year flood; (2) Certain areas subject to 100-year flooding with average depths less than one foot or where the contributing drainage area is less than one square mile; or (3) Areas protected by levees from the base flood (Medium shading). Thus, a property in Zone B may be located in a levee-protected area, but there is no information on FIRM maps to distinguish this fact from the other two possibilities. The third basic map prepared by FEMA is the Flood Boundary and Floodway Map (Floodway Map). Figure 3 shows the same area covered by Figure 2, but the information presented is slightly different. A Flood Insurance Study report is prepared in conjunction with the Flood Boundary and Floodway Map and describes the area studied, the engineering methods used, a summary of discharges, floodway data, flood insurance zone data, -48-

and flood profiles. The map and report provide the information needed to regulate the floodplain. Distribution of the Floodway Map and Flood Insurance Study is limited, being sent basically to the community, FEMA regional office, state coordinators, and other agencies with a direct involvement in floodplain management. The location of the levee is not noted on the Floodway Map of Figure 3, nor is the levee-protected area identified as such. The area protected by the levee is shaded as referenced in the "Key to Map" as between the 100-year flood boundary and the 500-year flood boundary. The three maps discussed and presented as Figures l, 2, and 3 are of the same area in Albuquerque, New Mexico. The comments contained herein are not intended as a review of these particular maps, and the mention of these maps is strictly for the purpose of illustrating map content. MAPPING FOR CREDITED LEVEES PROVIDING 100-YEAR PROTECTION Recommendations are made in this section on how best to add information recommended in this report to each of the three categories of maps previously discussed. FHBM The "Special Flood Hazard Area" is shown on a FHBM as a "Zone A" (see Figure 1), and no attempt is made to delineate or designate the levee-protected area. Most communities with identified flood hazards have been provided with FHBMs, and the FHBMs of many communities have been replaced by FIRMs. Consequently, very few new FHBMs will be prepared, and any recommendation pertaining to FHBMs will have limited effect. Also, a FEMA emphasis at this time is to convert communities from the emergency program to the regular program, which means the replacement of FHBMs with FIRMs. In the context of the progress being made in converting communities from the emergency program to the regular program, it is not desirable to revise policies on the various dimensions of levee recognition while communities remain with the emergency program. Major efforts spent in revising the emergency program would only detract from the more important effort of converting communities to the regular program. This policy, however, should be revised if communities are unduly extending the duration of their participation through the emergency program, particularly if such extension avoids compliance with the policies for dealing with levees in the regular program. In this spirit, FEMA should not revise FHBMs solely for the purpose of defining levee-protected zones; but, if for any reason a new or revised FHBM is prepared that involves a levee-protected area, the committee recommends: -49-

NATIONAL FLOOD INSURANCE PROGRAM FHBM FLOOD HAZARD BOUNDARY MAP BERNALILLO COUNTY, NEW MEXICO UNINCORPORATED AREA PAGE 3 OF 15 (SEE MAP lN0EX FOR PAGES NOT PRINTED! COMMUNITY-PANEL NUMBER 350001 0003 B MAP REVISED: JULY 10, 1979 U.S. DEPARTMENT OF HOUSING AND URBAN DEVELOPMENT FEDERAL lNSURANCE ADMlNlSTRATlON LEGEND SPEClAL FLOOD HAZARD AREA ZONE A NOTES TO USE* Boundiries oT the floodwiyl ver* i.omputed ii crow wcliont ind lnterpolited between crate Mcllom. Tht Tloodwiys were bi»ed on hydiulk contldentlom with re|ird to requirement) oT the Ftderil lniurwKc Admlnhlfitran. Thb mip w«i prepired to TitUiliu Ttoodplitn m*n«gemenl ictlvlttel only, rt miv not *o« ill lotcUl flood lwird tfiM In the community or *ll plinlmetrk feituret ouulde oT the flood- pleen. ReTer to the Ule« oTficiil Flood lrtwrince Riu Mip Tor iny idditionil ireit oT ipeciil flood hi/ird. To dtttrmln* iT flood Imunnc* tt inJUbto ln thb community, contict your lntunnca igwit, or call the Nitionil Flood lntwiance it IIDOI 6.IH.6JO, or (100l 4244172. SCALE: 0 t" « t333' I333 I000 FIGURE 1 Representative Flood Hazard Boundary Map (Bernalillo County, New Mexico)

NATIONAL FLOOD INSURANCE PROGRAM FIRM FLOOD INSURANCE RATE MAP CITY OF ALBUQUERQUE, NEW MEXICO BERNALILLO COUNTY PANEL 2 OF 50 (SEE MAP lNDEX FOR PANELS NOT PRlNTEDl COMMUNITY-PANEL NUMBER 350002 0002 EFFECTIVE DATE: federal emergency management agency federal insurance administration FIGURE 2 Representative Flood Insurance Rate Map (Albuquerque, New Mexico) KEY TO MAP S00-Vw Flood Moundiry - - — l00 VeM Flood toundirt l-n* Onliniuom• l00-Viw Flood S00-ViM F toad feM Flood i .•,<!,.. ln FMl **',< Umlorm Vtthln ' .- • " itttiuMirh RM7X •MJ to UN Nmonil GndMK VwtKU Otlum ol l92* vEXPLANATlON OF ZONE 0ESlGNATlONS ONE EXPLANATlON ArtM ol l00-tur tdii'n. ' ...j .-| vhe'e drptt ir t belvmn one (il ind lhret ( l) lMi. t.i'nr depth )i mundillon in thown. but no ltood «i/*rd tuior M ol l00-T«r tfiillov tloodmi vftrrt aeplrii Mivien ont (ll ind thrtt ()} 1tet. e•e Hood tliont nf sr.o•n. bul no flood "'*1* J 'i t.. i dtUrmined ArtA belvmn limin of lht l0O-ttw llood ind 100- 'ti' tlood. ot ititiir irrii iHblCtT l0 ' JO-.tii UrtOC m§ - r j.. .,- depldi tnt thin ont lll tool or vlttri ltii tonirihulini dn.mgt *.ti 'l !e» lhtn oni wuirt (Mtd'um -,'',, Artit ol minmid rlood«i|. (No v »j--ti Artit ot undCta'itiKwd. BUT poH.bk tlood huw«v AnM ol l00-ritr ioMtil tlood v'tr' •elociiy |.m iciponl. bMt llood tit.irmni ind tlood huvd ltclort "oi diHrmmtd Mills TOUSIN Ctrlun wtn nol in tht tpiciil HooJ huird ceil donei A ind Vl mty bt proletled b> tlood control Urutruin Thii mip n lor Hood iniuuiKt purpotn only . 'l don not nicli utilt inoB ill trt« wbixt to tlooding in rht communily or til plinimeirk ttituni ovtvdt nwtiil llood hund ITML For ij|....i.n| mip peneil, iM wpirndy pi.nlM lndti To Mip lNlTlAL lMNTlFlCATtON. FLOOD HAZARD •OUMOAMV MAC dfl FLOOO lNVUHANCI HAT! MAF I 'f FLOOO lNIUMANCB RATt MAP Hl V Riler lD i'- FLOOO lNSUKANCE KATE MAF EFFECTlVE di« thovn on ttid nip to dMirmlne vhen iriuwUl rttn iprU to ttrucium ln tnt lonet vhore elniitont or dtptht h4M botn To dmrmlne ll Hood lnturtnci b ietittble in ihh cominunllt. contict your lnturtnct t|t">. °i fll l•it Nitionil F load lnturince Froittm it (B00I fill-M30. M (100I 4344873. SCALE: I" = 2666' 0 I333 2666 I000

NATIONAL FLOOD INSURANCE PROGRAM FLOODWAY FLOOD BOUNDARY AND FLOODWAY MAP CITY OF ALBUQUERQUE, NEW MEXICO BERNAULLO COUNTY PANEL 2 OF 50 (SEE MAP lNDEX FOH PANELS NOT PRlNTEDI COMMUNITY-PANEL NUMBER 350002 0002 EFFECTIVE DATE: federal emergency management agency federal insurance administration FIGURE 3 Representative Flood Boundary and Floodway Map (Albuquerque, New Mexico) ' •. i' Ritinnet Mirk RM7 , R'wrMUe *Ml 5 NOTES TO USER Btt*MiriM ol thi floodviyl vert lomputed *i crow wttiom tnd hTdMilK ^oniiJrt*iKirn with rt|»d to rtauirtmttiU ol ihe Thti mip *M ptiPtrtd w t„,t,, tttMitn onU. n mn not «iov itl it.tn»i rtood hwird VIA 'n the conrrtuniit'br iU pltntmeirit luiuin ouiudt ol thi lluud .'ii '. Kt'«.- to thi . - i >' ,,, Flood lnuumct Rtit Mtp lor iny lodkionil HIM ol ipicUl llood hvard. Foe tdfolnin| mip pineh. wt wturtult prtnud lntfu To »Up LEGEN0 lNOEX CONTOUR lNTERME0iATE CONTOUR OEPRESSiON CONTOUR WOT ELEVATlON HORlZONTAL CONTROL POlNT VERTlCAL CONTROL POlNT HORlZONTAL - VCRTlCAL CONTROL POlNT PHOTOCENTER MATCH LiNE TO AOiOlNiNG MAPS FLOOO iNIUNANCC RATI HAP RIV»K>N«: Ritw to MM FLOOO lNSURANCE RATE MAT EFFECTlVE To d«tm.mi lT flootf tawruiM m inlibto m thb c«nuci ,out lnwrmnct «WH, M u« MM Nittotul Flood « (100I t»4«M. or (WOI 434-U7?. SCALE: I" = 2666' 0 I333 2666 1 """I r^ ^ I0OO

• All levees, dikes, and floodwalls should be labeled as such on any new or revised FHBM. Also, the areas protected by levees providing 100-year or greater protection should be delineated on the new or revised FHBMs as Zone ALP. Areas protected by levees not meeting this standard would continue to be mapped as Zone A. The ALP zone could be delineated on the new or revised FHBM by cross-hatching. The legend would have to be expanded to include this additional designation. The primary purpose of including this information on any new or revised FHBM is to make it a matter of public record that an area is located behind a levee and is subject to flooding if the levee is overtopped or otherwise breeched. Therefore, the legend should include language similar to the following: Levee-Protected Area (Levees protecting ALP Zones are designed to provide protection against the 100-year flood or greater, but they are subject to structural failure during lesser floods and overtopping during larger floods.) (example) One way to map the ALP Zone would be to make its outer boundary the line where the ground level matches the elevation of the top of levee minus freeboard. Though there are more detailed approaches, it is felt that this would generally be adequate because of the approximate nature of the FHBM. Also, as previously discussed, there will probably be very few new or revised FHBMs prepared. FIRM The FIRM is for flood insurance purposes and is used to determine actuarial rates that apply to structures in the zones where flood elevations or depths have been established. It shows important physical features and should include the location of levees, dikes, or floodwalls. The FIRM illustrated in Figure 2 is based on orthophotography provided by local agencies. Most FIRMs do not have this type of base mapping, and a more typical mapping base is illustrated by Figure 4, a FIRM of a section of the city of St. Louis, Missouri. The location of an existing levee is not noted on the FIRM in Figure 2, but one is noted on the FIRM in Figure 4. The committee recommends: • The location of all levees, dikes, and floodwalls credited as providing 100-year -56-

protection or more should be clearly denoted on all future FIRMs. This is apparently done on some but not all FIRMs, so implementation of this recommendation would involve an effort by FEMA to be consistent and would not involve policy or procedural changes. The areas behind currently credited levees are now denoted as Zone B on the FIRMs. Behind levees where insurance purchase will be required (see Table 1) the levee protected areas must be so designated on the FIRM and supplemented with information to determine insurance rates. Where insurance is to be required, the committee recommends: • Areas behind recognized 100-year levees that would be flooded (assuming no levee) by a 100-year flood should be designated as Zone ALP. Areas between the natural 100-year flood boundary and 500-year flood boundary should be designated as Zone BLP. The level of risk in Zone ALP may be minimal because of the levee, and the insurance rate would be very likely some fraction of the existing subsidized rates. However, this may not be the case, and specific risk should be determined by the type analysis advanced in Chapter 8. Since no insurance would be required in Zone BLP, the purpose of the BLP designation is to document that an area is protected by a levee but above the level of the base flood (100-year). Section 64.3(b) of the FEMA regulations will have to be modified to make the purchase of insurance mandatory in Zone ALP. The current procedure for determining whether insurance is to be required involves locating a property on a FIRM. If it is in Zone B or C, no insurance is required, the matter is dropped, and the property owner is most likely not aware that the check was even made. If an area is reclassified as Zone ALP, then insurance on buildings would be required and the property owners would be made aware of the hazard. If the properties are in an area reclassified as Zone BLP, insurance would not be required (but the property owner should be made aware of the levee). Explanations of Zone ALP and Zone BLP would have to be added to the "Explanation of Zone Designation" on the FIRM. There is an apparent inconsistency between the FIRM in Figure 2 and the FIRM in Figure 4. In Figure 2 the area behind the levee, lower right-hand corner, is shown as Zone B. In Figure 4 the area behind the levee, southern or right reach of Mississippi on west side of river, is designated mostly as Zone C with some isolated spots as Zone B. The reason for this apparent discrepancy is not known. It could be because the levee on the Mississippi, Figure 4, contains a 500-year flood, or it could be because FEMA's approach has changed since the FIRM in Figure 4 was prepared in July 1979. This issue is not critical, however, as the policy now used by FEMA to define Zone B's behind levees could continue to be used. The important change would be the designation of these areas as levee-protected zones. -57-

NATIONAL FLOOD INSURANCE PROGRAM FIRM FLOOD INSURANCE RATE MAP CITY OF ST. LOUIS,MISSOURI INDEPENDENT CITY PANEL 5 OF 40 {SEE MAP lN0EX FOR PANELS NOT PRlNTE0I COMMUNITY-PANEL NUMBER 290385 0005 A EFFECTIVE DATE: JULY 16.1979 I U.S. DEPARTMENT OF HOUSING I AND URBAN DEVELOPMENT FEDERAL lNSURANCE ADMlNlSTRATION/ FIGURE 4 Representative Flood Insurance Rate Map (St. Louis, Missouri) KEY TO MAP 300-Vw Ftood l00-Vw Flood l00-yeir Flood 500-VMT F kwtf BovMiry BOM Flood CbjvMlon ln FMl VtMrl LMlerm »ithin I arte" .',...«> -Ml .5 '*Rtltrtrw<d to iht NHHXUl CtodMk VtrlKt. 0ttum ol )9Jt vEXPLANATlON OF ZONE 0ESlGNATIONS lOHl CXPIANAllO* iwtd. ATCM ol l00-»iti tfiiUov lloodini vtw" depllH ol inundtUon tn ihovn, bwl no llood nw*d r.citK. I llood hui'd Tfcion noI dt lt'mined i hmiu ol ltii l0O-ttii tlood ind 500- NOTES TOUSIR lhi» mip il lOT Hood unli thov ill irtii ill plinimtl-« ltilurti iiude tpeciil llood huird trtii mip pvwli, inr wpirilrlt ptimed lndu To Mdp Rito u itw FLOO0 lNSURANCE RATE MAP EFFECTlVE tfilt tfiown on lhk mie to dilirmln* vlitn *nuirUI ni« ipplr to To DM*mlM ll f)o*tf HMrwiM k -...-• ln tub -- r ;., COKUCl yout lntunnc* icini, or c*K lhi NtUoncl Flood lnturtnct Protram n (100I 69»-M». or (100I 4J4«T J, SCALE: I" = 2400' 0 600 I200 2400

1-6 UNNAME0 STREET 6 SCRANTON AVENUE 7 CHA MBEMS STREET 8 VALLEY DRlVE 9 BLUFF DRlVE 10 A0RlAN DRIVE 11 PROSPECT 0RIVE V t \ ZONE C ' ZONE. . ZONE. * -

Floodway Map The third map prepared by FEMA, the "Flood Boundary and Floodway Map" or "Floodway Map," is prepared to facilitate floodplain management activities only. The Floodway Map shown in Figure 3 has a levee located on the southeast bank of the Rio Grande, lower right-hand corner of map, but it is not designated. Regardless of whether floodplain management actions will be required in areas protected by levees (see Table 1), a levee is a major and significant feature and should be shown on the map. This should not present any difficulties from a cost or policy standpoint. The area behind a credited levee is currently shown on Floodway Maps with medium shading, see lower right corner of Figure 3. This area is identified in the "Key to Map" as the area between the 100-year and 500-year flood boundary and is equivalent to the Zone B area of the FIRM in Figure 2. No special floodplain management measures are required under present FEMA regulations. If any floodplain management measures are to be required in areas protected by credited levees providing 100-year protection, then these levee-protected areas must be differentiated on the Floodway Map as well as on the associated FIRM. On the FIRM, it is recommended that the area behind the levee that would be flooded by a 100-year flood assuming no levee (unconfined 100-year flood) be called Zone ALP and that the area between the 100-year flood boundary and 500-year flood boundary be designated as Zone BLP. Since the entire area protected by a credited levee up to the 500-year flood boundary should be subject to some floodplain management measures, such as preparation of warning and evacuation plans, this entire area should be designated by a distinguishing symbol on the Floodway Map. The committee recommends: • The locations of all credited levees, dikes, and floodwalls should be clearly denoted on all Floodway Maps. • Areas behind credited levees providing 100-year protection that would be flooded during a 500-year flood should be shown on Floodway Maps. This recommendation will require another form of cross-hatching on the Floodway Map. In the Floodway Map of Figure 3, the medium-shaded area east of the Rio Grande would have to be illustrated in a different way. The manner of illustration would have to be added to the "Key to Map" on the Floodway Map. MAPPING FOR CREDITED EXISTING LEVEES NOT PROVIDING 100-YEAR FLOOD PROTECTION Current FEMA mapping does not recognize any levee not providing a 100-year level of protection as having any effect on reducing insurance premiums and determines the 100-year floodplain based on unconfined flows. Where 25- to 100-year levees are given recognition, -60-

FEMA will have to modify its mapping policy. The following discussion presents specifics. FHBM Little change would be required with regard to the FHBM. Currently, Zone A adequately designates areas where insurance is mandatory and certain floodplain management measures are required as set forth in paragraph 60.3(b) of the FEMA National Flood Insurance Program regulations. A false sense of security is not developed because property owners are required to purchase insurance, and new construction and substantial improvements located in A Zones must be elevated or flood-proofed. FIRM Since the FIRM is used for setting flood insurance premiums by indicating actuarial rates, flood risks behind all levees providing at least 25-year protection need to be indicated. The FIRM must carry information that will allow the rates to be determined for the properties affected. The committee recommends: • FEMA should create an AL(No.) Zone that would designate an area protected by a levee with a frequency of protection defined by the (No.). For example, if the levee had an elevation equal to the 40-year flood (plus required freeboard), then the Zone would be AL(40). A schematic illustration is shown in Figure 5. Zone A is subdivided into Zone AL(40) and Zone A13 (one of the current Zones Al to A30). The insurance rate charged in Zone AL(40) would reflect the fact that the area is "flood-free" for floods up to the 40-year event. Zone A13 is unaffected by the levee and the insurance rates in this zone would be the same as if the levee were not recognized, as currently is the case. An explanation of Zone AL(40) would have to be added to the "Explanation of Zone Designations" on the FIRM. Floodway Map Since the Floodway Map is to facilitate floodplain management activities it needs to indicate areas where changes in floodplain management policy are recommended. Specifically, changes would involve insurance purchase requirements behind levees up to a 500-year flood design and which would, without the levee, be flooded by a 100-year flood and areas behind all levees for critical facilities regulation and emergency measure planning. Current practice is to not -61-

ZONEC FIGURE 5 Flood Insurance Rate Map Showing—Levee Protected Areas -62-

recognize the existence of levees with levels of protection less than 100 years, and 100-year floodplain areas are shown on Floodway Maps as heavily shaded areas ("Floodway Fringe"). It is recommended that the FEMA practice of requiring floodplain management measures as set forth in paragraph 60.3 of their regulations be continued for areas in the 100-year floodplain behind less than 100-year levees. Therefore, no change is recommended for portraying the hazard areas on the Floodway Map. EVACUATION ROUTES Consideration was given to showing evacuation routes on FEMA maps. One problem is that the maps are prepared on a community basis. In urban areas where there are multiple jurisdictions, the maps stop at community boundaries; it would not be very useful to show evacuation routes just up to corporate or county limits that fall short of reaching safety. Even where community boundaries are not involved FEMA maps cover relatively small areas. Often, several maps must be pieced together to show the way out of the hazard area. A new map showing complete evacuation routes would be needed. Evacuation routes must be coordinated with flood warning and evacuation plans. Both plans and routes must be prepared by the local communities, where the greatest expertise and familiarity with traffic and emergency personnel planning is likely to exist. If the routes are known when the FIRM and Floodway Map are being prepared and if an entire evacuation route can be shown on a simple map, there is a direct advantage to showing it on the FIRM or Floodway Map. An advantage of having evacuation routes on the FEMA maps would be to make the FEMA maps complete as basic reference documents. Citizens, companies, or citizen groups could piece the information together and transfer it to maps of their choosing. Since the FEMA maps are generally available, completeness would make the information more accessible. The committee recommends: • FEMA should not make a mandatory requirement to include evacuation routes on maps. If the information is available when the maps are being prepared and if it makes sense from a community-to-comraunity mapping standpoint, then the evacuation routes could be included on the maps. In other cases, it may be expeditious for communities to develop their own special evacuation route maps. INTERIOR DRAINAGE Areas behind levees subject to flooding by interior drainage need to be delineated. It would appear from examination of present FEMA maps that the current intent is to map such areas. For example, the FIRM in Figure 2 includes the two following Zone designations, which may not necessarily have been developed for levee-protected areas but which seem to be appropriate: -63-

AO—Areas of 100-year shallow flooding when depths are between one (1) and three (3) feet; average depths of inundation are shown, but no flood hazard factors are determined. AH—Areas of 100-year shallow flooding where depths are between one (l) and three (3) feet; base flood elevations are shown, but no flood hazard factors are determined. Depths greater than 3 feet are included in the A1-A30 Zone designation. These areas of shallow flooding are already being mapped and include areas inundated by interior drainage. Special attention should be placed on linking operation of the interior drainage system with stages on the river side of the levee. An interior drainage situation may not be a problem when the river is below flood stage and consequently may not be included on a FEMA map, but it may become a problem when the river is in flood stage. For this reason FEMA should make sure that interior drainage evaluation is coordinated with evaluation of river-related flooding. The committee recommends: • Interior drainage situations on the landside of levees should be analyzed in conjunction with the riverine flood analysis, and the areas flooded by interior drainage should be shown on FEMA maps in accordance with current practice. Appropriate consideration should be given to correlation of the event on the river and the event causing the interior drainage problem. If an interior drainage situation is a problem only when the river is in flood stage, then a joint probability appraisal can be made. If there is reasonable correlation between the events that cause the river to flood and the events that cause the interior drainage problem, then the area should be mapped as one of the A Zones. This certainly is the case during a hurricane, when heavy rainfall usually occurs at the same time as high stages. Also, it is usually possible to have heavy local storms at the same time river stages are high. -64-