PartPart 33 COMBATINGCOMBATING RILLSRILLS ANDAND GULLYGULLY EROSIONEROSION  One of the most vexing problems along cut slopes is differential erosion, like that shown here. The selective removal of friable beds can undermine drainage interceptor ditches and negate the intended system of drainage collection , conveyance, and discharge. RetrogressiveRetrogressive sliversliver slumpsslumps

 Concentrated runoff and debris removal from the toe of a cut slope can also trigger retrogressive slides of steep cuts in jointed rock, as shown here.  This is the major problem with interceptor ditches that are not, or cannot, be maintained  CutCut slopesslopes tendtend toto bebe moremore cohesivecohesive towardstowards theirtheir crown,crown, ifif excavatedexcavated throughthrough thethe normalnormal residualresidual soilsoil horizon.horizon. RillRill erosionerosion isis mostmost problematicproblematic inin lowlow cohesioncohesion materials,materials, likelike sand.sand.  OnceOnce aa rillrill isis established,established, itit maymay soonsoon formform aa deeplydeeply incisedincised erosionerosion gully,gully, likelike thatthat shownshown here.here. TheseThese cancan bebe veryvery difficultdifficult toto mitigate,mitigate, requiringrequiring aa lotlot ofof handhand work.work.  ManyMany peoplepeople don’tdon’t believebelieve inin drainagedrainage benches;benches; especiallyespecially ifif itit hasn’thasn’t rainedrained recently.recently. ThisThis showsshows thethe scalescale ofof erosionerosion thatthat cancan occuroccur onon aa 3:13:1 fillfill slopeslope inin cohesivecohesive materialsmaterials subjectsubject toto seasonalseasonal desiccation.desiccation.  ThisThis showsshows aa newnew v-ditchv-ditch atat thethe footfoot ofof aa 3:13:1 fillfill slopeslope withwith siltsilt fencesfences (at(at left);left); whichwhich waswas completelycompletely filledfilled byby ravelingraveling debris.debris.  DrainageDrainage terracesterraces areare intendedintended toto curtailcurtail surficialsurficial ravelingraveling andand gullygully erosionerosion ofof bothboth cutcut andand fillfill slopes,slopes, whichwhich cancan bebe severesevere priorprior toto establishmentestablishment ofof vegetationvegetation onon suchsuch slopesslopes..  StakedStaked hayhay balesbales areare oftenoften employedemployed asas temporarytemporary erosionerosion controlcontrol weirs,weirs, toto catchcatch suspendedsuspended sedimentsediment andand retardretard velocityvelocity ofof runoffrunoff onon steep,steep, exposedexposed slopesslopes  PolyethylenePolyethylene sheetingsheeting isis oftenoften usedused toto covercover slopes,slopes, afterafter aa landslide.landslide. ImpermeableImpermeable membranesmembranes restrictrestrict directdirect infiltration,infiltration, butbut seldomseldom preventprevent actualactual slopeslope movements,movements, whichwhich isis usuallyusually exacerbatedexacerbated byby shallowshallow groundwatergroundwater flow.flow.  BatterBatter boardsboards havehave beenbeen employedemployed onon steepsteep slopesslopes withwith mixedmixed results.results. TheyThey areare intendedintended toto provideprovide temporarytemporary supportsupport untiluntil woodywoody vegetationvegetation cancan taketake root.root.  BatterBatter boardsboards andand pavingpaving blocksblocks andand otherother precastprecast interlockinginterlocking systemssystems likelike thatthat picturedpictured aboveabove cancan serveserve asas usefuluseful facingfacing elementselements restrictingrestricting furtherfurther slopeslope raveling,raveling, butbut theirtheir successsuccess isis dependentdependent onon adequateadequate subdrainagesubdrainage beingbeing provided.provided. NorthNorth sideside ofof USUS HwyHwy 101101 inin downtowndowntown LosLos Angeles.Angeles.  AA widewide arrayarray ofof precastprecast masonrymasonry pavingpaving blocksblocks areare availableavailable forfor facingfacing slopes.slopes.  SomeSome systemssystems offeroffer interconnectinginterconnecting blocks,blocks, andand mostmost allowallow freefree drainagedrainage throughthrough somesome systemsystem ofof macromacro voids.voids. ThisThis isis aa channelchannel inin Japan.Japan.