Site resources have intrinsic and other values for habitat and biodiversity regardless of their stormwater benefits.

Transcription

1 Chapter 3 Site Resurces Lw Impact Develpment Intrductin Site resurces have intrinsic and ther values fr habitat and bidiversity regardless f their strmwater benefits. The site resurces referred t here are thse natural features r site characteristics which, t a large extent, prvide a benefit t receiving systems thrugh their existence. They prvide a benefit t the general public by their cntinued functin t reduce peak rates and vlumes f strmwater runff, prvide fr water quality treatment, and prevent damage t imprved r natural lands either n site where the site resurces exist, r dwnstream f thse resurces. Site resurces have intrinsic and ther values fr habitat and bidiversity regardless f their strmwater functins. They can include a wide variety f items, but thse discussed here are cnsidered primary resurces which shuld be recgnised and cnsidered in site develpment and use. In terms f this Chapter, the fllwing site resurces are imprtant due primarily fr their strmwater management benefits. Sme f the benefits are less bvius than thers, but all prvide a benefit. Terrestrial eclgy and landscape frm Wetlands Fldplains Riparian buffers Vegetatin Sils Slpes/tpgraphy Other natural features Linkage with site develpment Site resurces ften verlap. Fr example, a riparian buffer may lie within a fldplain r a frested area frm part f a riparian buffer. In this Chapter, they are discussed individually althugh their benefits may be, and generally are, cumulative. Terrestrial Eclgy and Landscape Frm It is ften said that the three principal ecnmic factrs that drive real estate prices are: lcatin, lcatin, lcatin. The same is true f natural resurces and site resurces. Where natural features are lcated n a site is just as imprtant as the characteristics f the natural features themselves. The imprtance f the psitin f eclgical features in the landscape has spawned an entire field f study called landscape eclgy. There are several basic principles f eclgy that can be used t imprve the quality f receiving envirnments. These principles detailed in Table 3-1 apply t all site resurces. 3-1

2 Chapter 3 - Site Resurces Table 3-1 Principles t Imprve the Quality f Receiving Envirnments Retain and prtect native vegetatin (native frest, regenerating native scrub and frest, wetlands, castal frest/scrub) - these ecsystem types have imprtant intrinsic values, and prvide different habitats fr native flra and fauna and different eclgical functins. Allw natural regeneratin prcesses t ccur (e.g. pasture => scrub => frest; wet pasture => wetland) Undertake weed and pest cntrl => t imprve the naturalness f native vegetatin, allw natural prcesses and seed dispersal mechanisms t ccur. Replant and restre with native plants t prvide vegetatin cver which is characteristic f what wuld nce have been there and/r which reflects ther lcal remnants in the area. Restre linkages with ther natural areas r ecsystems (e.g. using waterways and riparian areas, linking fragmented frest remnants, linking wetland ecsystems and freshwater ecsystems t terrestrial frest/scrub remnants). Native species need extensive areas f vegetatin t survive. Our knwledge is limited (need fr a factr f safety) Our knwledge is limited (need fr a factr f safety) The Eclgical Values f Site Resurces It is imprtant t retain natural areas (including scrub, frest, and wetlands) n a site fr their bilgical diversity and intrinsic values which include the fllwing. They are imprtant fr their values as characteristic examples f bidiversity in a regin r district, The diversity f species r ecsystem types that they cntain, Cntaining rare r special features r unusual ecsystem types, Their value as habitats fr indigenus species and the level f naturalness, Their ability t sustain themselves ver time (e.g. available seed surces, active regeneratin, brid dispersal prcesses active, level f weeds and pests and utside influrences cntrlled), Being f adequate size and shape t be viable, If they are buffered r they prvide a buffer t habitats r natural areas, frm utside influences (e.g. scrub n edges f native bush, intact sequences frm esturarine t terrestrial, frm freshwater t terrestrial, frm gully bttm t ridge tp); and prvide linkages with ther natural areas in an area (crridrs fr native birds, invertebrates). The strmwater benefits prvided by native vegetatin have been detailed in Chapter 2. The fllwing criteria fr the evaluatin f eclgical significance f native vegetatin prvide a set f basic principles fr the determinatin f ecsystem significance. These are paraphrased frm the Prtected Natural Areas Prgramme survey methdlgy (Myers et al, 1987). Representativeness It is imprtant t prtect what is cmmn and characteristic f the eclgy f an area. 3-2

3 Lw Impact Develpment Natural areas that are representative f the eclgical cmmunities nce frmerly present in a given area (e.g. an Eclgical District) are significant. It is nt nly rare and unusual features that are imprtant. Mst natural areas have been reduced dramatically frm their frmer extent, s remaining representative examples f each different type f eclgical cmmunity are valuable. There has been a mve away frm prtectin f nly rare species and their habitats t prtecting ecsystems which are gd examples f the character f a district r regin. Prtectin f substantial parts f ecsystems are usually needed t assure the survival f their cnstituent parts, such as individual species. It is easy t ignre r place less imprtance n elements f ecsystem functining which are nt bvius. Many evaluatins are based n visual assessment e.g. a cmparisn f pasture t mature frest. But there are many ther imprtant elements f ecsystem integrity that are nt s readily apparent: including water cycle, chemical factrs, energy flw, and bitic interactins. Rarity and Naturalness It is easy t underestimate the value f rare species. Rarity is an indicatr f the scarcity f numbers f a species r ther element f bidiversity. The presence f a rare r special r unusual feature in a natural area adds t its eclgical value. Rare species reflect the highest degree f ecsystem cmplexity and functin and are the mst sensitive t impct. Unfrtunately, their rarity makes them impractical fr use with mst assessment studies dne as part f develpment prjects. Naturalness is imprtant t the survival f species, cmmunities and ther cmpnents f bidiversity, many f which will nt survive utside a natural envirnment. Naturalness in ecsystems is inversely prprtinal t the degree f disturbance by humans r intrduced species (e.g. weeds). Generally, the eclgical value f a natural area increases with its diversity and the cmplexity f its eclgical patterns Diversity and Pattern A fundamental aim f nature cnservatin is t prtect natural bilgical diversity. The diversity f a natural area refers t the species f plants and animals present as well as its cmmunities, ecsystems, and physical features. Generally the eclgical value f a natural area increases with its diversity and the cmplexity f its eclgical patterns. Wetlands, fldplains, and mature frests are key resurces in lw impact design because they are generally the ldest and least disturbed site resurces. Ecsystem functin increases ver time. The develpment f ecsystem cmplexity and functin ver time can be seen in the fllwing example f transitin frm a pasture t a mature frested area. The initial land use f pasture, and a severely degraded wetland is shwn in the first picture and is typical f land use cnditins fr much f New Zealand. The secnd picture shws an example f regeneratin where grasses are replaced by native shrub/ scrub vegetatin. The secnd picture shws this prcess after apprximately five years. This first transitin stage tends t be t a fairly simple ecsystem. Changes ccur rapidly and little rganisatin exists. Clnising native plants are typically suited t sil that is lw in rganic cntent and fertility, able t withstand harsher cnditins, and require direct sunlight t survive. 3-3

4 Chapter 3 - Site Resurces Typical pasture Early reversal perid t native vegetatin The secnd pair f pictures prvides anther time perid with the first picture being later in the prcess than the secnd picture in the first series. Where the time perid in the secnd picture is apprximately five years, the third picture represents apprximately a 15 year time perid t achieve that level f grwth. The furth picture shws a mature frested area and may take a hundred r mre years t achieve the level f grwth shwn. Later successinal stage Mature wdland With the passage f time, distinct clumps r patterns appear. Grups r assemblages f species appear that partitin the envirnment int layers r strata that begin t increase the cmplexity f the system. The increased cmplexity is mst easily seen by the displacement f grasses and herbaceus plants by wdy shrubs and small trees. The rganic cntent f the sil increases as leaf litter builds up and as the vegetatin decays. Lng-term eclgical viability is the ability f natural areas t retain their inherent natural values ver time. This includes the ability f a natural area t resist disturbance and ther adverse effects and fr its cmpnent plant and animal species t regenerate and reprduce successfully. Cmplex ecsystems ften have a messy r wild appearance t them as their cmplexity increases. A mature frest can take hundreds f years t develp s seeing ne indicates a lack f recent disturbance. Size and Shape Size and shape f the area affect the lng term viability f a natural area s eclgical cmpnents and functins. With increase in size, the diversity and resistance t disturbance f an area generally increases. The shape f a natural area influences its resistance t external effects (e.g. a cmpact shaped area is less vulnerable t edge effects than a cmplex ne). A mature frest can take hundreds f years t develp s seeing ne indicates a lack f recent disturbance. 3-4

5 Lw Impact Develpment Ecsystem functin increases as the size f the natural area gets larger. The inverse is als true that ecsystem functin is reduced when natural systems are fragmented by rads and urban develpment. But small fragments and patches f native vegetatin are still imprtant and may be the nly remnants left f a certain type in an area. They may prvide habitat fr relict ppulatin, r rare species may prvide seed surce fr lcal revegetatin. The smaller an area f bush is, the greater the edge effects, the lack f micrclimates fr certain species, and the mre likely weed invasin will be. Much f the Auckland Regin was cvered by frest prir t human settlement. This frest had maximum ecsystem functin due t its age, size, and cmplexity. Human influence n the land has shrunk this netwrk f cnnected wdlands t a fractin f its frmer size. The effect f area size n ecsystem functin is, t sme degree, a matter f gemetry; the varius dimensins f the tract change in prprtin t the area f the tract. A tract reduced in area by a factr f ne hundred reduces by ne-tenth the distance t the centre f the tract and increases ten times the dminance f the perimeter habitat (edge/area rati) as shwn in Figure 3-1. Tract size has imprtant implicatins fr species that require interir habitat. The tract can becme s small that the interir habitat and the species that depend n it are eliminated. Tract B Figure 3-1 Effect f Edge versus Area Tract C Interir 10 Tract Size A B C area In lw impact design, it is f great imprtance t cnsider the degree t which the landscape is permanently changed as a result f urban develpment. Tract A Edge As discussed in Chapter 2, urbanisatin causes a shift in the aquatic cmmunity frm ne dminated by pllutin sensitive species twards ne dminated by pllutin tlerant species. This eclgical principle als applies t the terrestrial envirnment where the adverse impacts tend t be mre subtle in nature and mre variable frm site t site. Hidden elements and scientific uncertainty length f side distance t centre perimetre length edge/area rati Obviusly, we, as stewards fr the envirnment, dn t have all the answers. In lw impact design, it is f great imprtance t cnsider the degree t which the landscape is permanently changed as a result f urban develpment. Safety factrs are used in

6 Chapter 3 - Site Resurces engineering t accunt fr uncertainty and ensure that the bridge desn t cllapse. This cncept is even mre applicable t natural resurces that are cnsiderably mre cmplex and less well understd. Examples f safety factrs that might be applicable t lw impact design might include the requirement fr larger riparian buffer strips r native revegetatin adjacent t existing indigenus frest. Much f the large scale alteratin f natural resurces has bviusly already taken place in the Auckland Regin. Thus, urban develpment prjects will have much less verall impact. The basic principles f eclgy and landscape eclgy still apply t minimise the impacts f future prjects. Much f ur knwledge f the functins and values f natural resurces has develped in just the last 50 years. In a few years it is likely that we will lk back n hw little we knew in While it can be seen that terrestrial eclgy is imprtant fr prtecting intrinsic values f a given area it is als critical that we d nt lse sight f the majr benefits that result frm retentin f these areas frm a hydrlgical standpint. Wetlands Wetlands, as defined in the Resurce Management Act, include permanently r intermittently wet areas, shallw water, and land water margins that supprt a natural ecsystem f plants and animals that are adapted t wet cnditins. They ccur n land-water margins, r n land that is temprarily r permanently wet. Wetlands are a majr habitat fr at least eight species f indigenus freshwater fish as well as frgs, birds, and invertebrates. Wetlands have unique hydrlgical characteristics that can be irreversibly mdified by activities such as drainage. There can be few ther vegetatin classes which have suffered s severely during human times than have wetlands. The reasns fr this are many, but can be attributed largely t their psitin n flat land, suited t agriculture, and t the generally lw esteem in which such vegetatin has been held by the average laypersn. These changes have ccurred despite the manifest value f wetlands as wildlife habitats, as regulatrs f flding, their intrinsic values, fr recreatin, and fr scientific research. Nevertheless a far larger area than remains tday has been lst thrugh drainage, fire, tpdressing, and flding. Wetlands have unique hydrlgical characteristics that can be irreversibly mdified by activities such as drainage. An Example f a Wetland in the Auckland Regin 3-6

7 Lw Impact Develpment The prblem with wetlands is that they are rarely seen as being a valuable resurce. The prblem with wetlands is that they are rarely seen as being a valuable resurce. They are usually difficult t access, and therefre are rarely visited. Their wildlife is usually secretive and their plants are seldm spectacular r flambyant. Their values as a surce f mined material r as pastral land r fr hrticulture are nly realised after the wetland has been destryed. Their ability t assist in water cntrl is ften nly recgnised after bth flds and water shrtages have ccurred fllwing their destructin. Natinwide, freshwater wetlands cvered at least 670,000 hectares befre Eurpean settlement, but have nw been reduced by drainage fr pasture t arund 100,000 hectares. Althugh several thusand wetlands still survive, mst are very small and have been mdified by human activities and invasive species. It is likely that sme characteristic wetland types have been lst cmpletely, while very few examples are left f thers, such as kahikatea swamp frest and sme kinds f flax swamp. New Zealand s wetlands are as varied as the terrain that shapes them. The Auckland Reginal Cuncil, as detailed in Figure 3-2 have dne a brad inventry f wetlands in the Regin which prvides infrmatin n hw many larger wetland areas are in the Regin (Myers, January, 1997). The term larger is imprtant here as the survey nly identified wetlands whse areal extent exceeds ne hectare. There are many wetlands smaller than this size which are nt identified. The survey identified 152 wetlands with a ttal surface area f hectares. The vast majrity f wetlands are in the 1-5 hectare size. It is reasnable t expect that there are many mre wetlands in the less than ne hectare categry, but they have nt been reprted. This is especially true in headwater areas f catchments where wetlands may be present which are very small in extent. It is imprtant t recgnise that even withut the presence f humans, wetlands systems are mdified and eliminated by a natural eclgical ageing prcess referred t as successin. The filling and cnversin f wetlands int mre terrestrial type ecsystems ccurs naturally at a relatively slw rate. The interventin f man int the prcess vastly accelerates the cnversin prcess Figure 3-2 Wetlands in the Auckland Regin and Their Size Breakdwn (Myers, 1997) Number >50 Size (hectares) 3-7

8 Chapter 3 - Site Resurces In their natural cnditin, wetlands prvide many imprtant functins t man and the envirnment. Table 3-3 summarises the majr functins and values f wetlands. Table 3-3 Summary f Wetland Functins and Values Functin/Value fld cntrl Flw augmentatin Ersin Cntrl Water qualitymaintenance Habitat fr wildlife Fisheries habitat Fd chain supprt Recreatin/aesthetics Educatin Descriptin attenuatin f peak flws strage f water absrptin by rganic sils infiltratin t grundwater maintenance f stream flw during drught increased channel frictin reductin in stream velcity reductin in stream scur channel stability by vegetative rts dissipatin f stream energy sedimentatin burial f pllutants in sediments adsrptin f cntaminants t slids uptake by plants aerbic decmpsitin by bacteria anaerbic decmpsitin by bacteria fd shelter/prtectin frm weather and predatrs nursery area fr early life stages galaxids, eels, freshwater mussels, crayfish (kura) fd prductin frm sun (primary prductin) enjyment f nature hiking, bating, bird watching teaching, research In additin t the listed beneficial values, the water quality benefits f wetlands can be expanded. Natural systems have cmplex mechanisms and the fllwing listing f benefits describes the majr prcesses ccurring in wetlands that allw them t prvide water quality enhancement functins. These functins include: settling/burial in sediments uptake f cntaminants in plant bimass filtratin thrugh vegetatin adsrptin n rganic material bacterial decmpsitin temperature benefits vlatilisatin Fldplains Fldplains ccupy thse areas adjacent t stream channels which becme inundated 3-8

9 Lw Impact Develpment Fldplain Limits Example f a Fldplain Just After Being at Fld Stage with strmwater during large rainfall/runff events. Fr the mst part, in the Auckland Regin, rainfall is the main cause f flding althugh surges by wind driven currents can exacerbate the prblem, r in unique situatins, cause the flding prblem. Flding prblems result frm tw main cmpnents f precipitatin: the intensity and duratin f rainfall, and its areal extent and distributin. Flding has been the mst cmmn reasn fr declaratins f civil defence emergency in New Zealand. Flding has been the mst cmmn reasn fr declaratins f civil defence emergency in New Zealand. In the 19th century fld related drwnings were dubbed the New Zealand death. Flds can ccur in any seasn, and in all regins f New Zealand. The rate f flding increased years ag fllwing widespread replacement f frests and scrub with shallw rted pasture grasses. Despite extensive river and catchment cntrl schemes, damage frm flding is estimated t cst at least $125 millin a year. Even drught stricken Auckland was afflicted by fld damage in the midst f its 1994 water supply crisis. Despite legislative effrts and massive investment in preventive measures, fld lsses have cntinued t rise. The main part f the reasn in Auckland is increasing urbanisatin. Many studies have shwn that paving and drainage systems in urban areas increase flding, particularly as many urban areas are lcated alng fldplains and frmer wetlands. Flding in and f itself is nt a prblem. Flds have been arund since the beginning f time and are a natural part f the water cycle. Prblems are caused when man interacts with the fldplain. Thus, fld hazard ptential relating t human health, prperty damage, and scial disruptin are strngly influenced by human activity n the fldplain. There are several key catchment characteristics which impact n fld frequency and depths. Catchment size and slpe The abundance f rainfall in the Auckland Regin feeds ur many small, first and secnd rder streams. These streams and their assciated fldplains are the cnveyance means f getting water dwnstream, thrugh the catchment, and t sea level. Smaller catchments have a rapid respnse time t strm flws where larger catch- 3-9

10 Chapter 3 - Site Resurces ments have a lnger respnse time as strm flws take time t travel thrugh Surface cnditins and land use Until the nineteenth century, 75% f the cuntry was cvered in temperate rainfrest. Replacing tw-thirds f it with extic grasses has dramatically increased the rate at which rain reaches the grund surface and flws verland int the stream system. Urbanisatin, with its impervius surfaces has an even mre prfund effect n fld flws. Nt nly d fld flws increase in size and number, but their speed f nset is increased, particularly in the first 20 percent f change frm rural t impervius cver. This makes intensive, shrt-duratin rainfalls mre fld prne. In additin, time f year can impact n fld levels via intensity f rainfall and saturated cnditin f sils. Fldplain tpgraphy The channel frm and assciated fldplain in part determine the size f fld, particularly its depth and areal extent. A small catchment and wide fldplain will result in a shallw, but widespread fld. On the ther hand, a deep channel and steep slpes will result in deeper flding, but n a small areal extent. The many benefits that fldplains prvide are partly a functin f their size and lack f disturbance. But what makes them particularly valuable eclgically is their cnnectin t water and the natural drainage systems f wetlands, streams, and estuaries. The water quality and water quantity functins prvided by the fldplain verlap with the landscape functins f tract size and ecsystem cmplexity t make them exceptinally valuable natural resurces. Fldplains prvide a wide range f benefits t bth human and natural systems. These functins and values can be bradly placed in three categries; water resurces, living resurces, and scietal resurces. Taking each f these individually prvides the fllwing: Water resurces Fldplains prvide fr fld strage and cnveyance during perids when flw exceeds channel bundaries. In their natural state they reduce fld velcities and peak flw rates by ut f stream bank passage f strmwater thrugh dense vegetatin. They als prmte sedimentatin and filter pllutants frm runff. In additin, having a gd shade cver fr streams prvides temperature mderatin f stream flw. Maintaining natural fldplains will als prmte infiltratin and grundwater recharge, while increasing r maintaining the duratin f lw surface stream flw. Fldplains prvide fr the temprary strage f fldwaters. If fldplains are nt prtected, develpment wuld, thrugh placement f structures and fill material in the fldplain, reduce their ability t stre and cnvey strmwater when the need fr fldplain strage ccurs. This, in turn, wuld increase fld elevatins upstream f the filled area and increase the velcity f water travelling past the reduced flw area. Either f these cnditins culd cause safety prblems r cause significant damage t private prperty. The fllwing Table 3-4 prvides values f rughness cefficients that have been established fr fldplain areas fr the purpses f hydraulic cmputatins t determine flw velcities and elevatins. They indicate the value that vegetatin has n the mvement f fld flw and can be cnsidered in the cntext f retardance factrs. The higher the value, the greater the retardance t flw. If fldplains are nt prtected, develpment wuld, thrugh placement f structures and fill material in the fldplain, reduce their ability t stre and cnvey strmwater when the need fr fldplain strage ccurs. 3-10

11 Lw Impact Develpment Table 3-4 Values f the rughness cefficient n fldplains Type f Grund Cver Nrmal n a. Pasture, n brush 1. shrt grass High grass b. Cultivated areas 1. N crp Mature rw crps Mature field crps c. Brush 1. Scattered brush, heavy weeds Light brush and trees Medium t dense brush d. Trees 1. Heavy stand f timber, little undergrwth Heavy stand f timber, fld stage in branches As can clearly be seen, the denser and taller the vegetatin, the greater the frictinal resistance t stream flw. Living resurces Natural fldplains are fertile and supprt a high rate f plant grwth which supprts and maintains bilgical diversity. They prvide breeding and feeding grunds fr fish and wildlife. In additin, they prvide habitat fr rare and endangered species. Grund cver in natural wetlands tends t be cmpsed f leaf and dense rganic matter. Organic sils have a lwer density and higher water hlding capacity than d mineral sils. This is due t the high prsity f rganic sils r the percentage f pre spaces. This prsity allws fldplain sils generally t stre mre water than mineral sils wuld in upland areas. Fldplains prvide areas fr active and passive recreatinal use. They increase pen space areas, and prvide aesthetic pleasure. Scietal Resurces Fldplains prvide areas fr active and passive recreatinal use. They increase pen space areas, and prvide aesthetic pleasure. They als cntain cultural and archaelgical resurces and prvide pprtunities fr envirnmental and ther studies. Many walkways exist in reserves and thse walkways tend t be adjacent t stream channels. Riparian Buffers Althugh reductin f cntaminants is a widely recgnised functin f riparian buffers, they als cntribute significantly t ther aspects f water quality and physical habitat. Habitat alteratins, especially channel straightening and remval f riparian vegetatin, cntinue t impair the eclgical health f streams mre ften and fr lnger time perids than cntaminants. When cnsidering riparian buffers, it is helpful t detail the variety f benefits that are gained by their prtectin r implementatin. Riparian buffer systems prvide 3-11

12 Chapter 3 - Site Resurces the fllwing benefits: Temperature and light The daily and seasnal patterns f water temperature are critical habitat features that directly and indirectly affect the ability f a given stream t maintain viable ppulatins f mst aquatic species. Cnsiderable evidence shws that the absence f riparian cver alng many streams has a prfund effect n the distributin f many species f macrinvertebrates and fish. In the absence f shading by a frest canpy, direct sunlight can increase stream temperatures significantly (up t 12 0 C), especially during perids f lw stream flw in summer. Riparian buffers have been shwn t prevent the disruptin f natural temperature patterns as well as t mitigate the increases in temperature fllwing upstream defrestatin. Habitat diversity and channel mrphlgy The bilgical diversity f streams depends n the diversity f habitats available. Wdy debris is ne f the majr factrs in habitat diversity. Wdy debris can benefit a stream by: Cnsiderable evidence shws that the absence f riparian cver alng many streams has a prfund effect n the distributin f many species f macrinvertebrates and fish. stabilising the stream envirnment by reducing the severity f the ersive influence f stream flw, increasing the diversity and amunt f habitat fr aquatic rganisms, prviding a surce f rganic carbn, and frming debris dams and slwing stream velcities. Lss f the riparian zne can lead t lss f habitat thrugh stream widening where n permanent vegetatin replaces frest, r thrugh stream narrwing where frest is replaced by grass. In the absence f perennial vegetatin, bank ersin and channel straightening can ccur. The accelerated streamflw velcity allwed by straight channels prmtes channel incisin as ersin f sediment frm the stream bttm exceeds the sediment lad entering the stream. Riparian Buffer Adjacent t Stream Bundaries 3-12

13 Lw Impact Develpment This prcess can eventually lead t the develpment f wide, shallw streams that supprt fewer species. Fd webs and species diversity The tw primary surces f natural fd energy input t streams are litterfall frm streamside vegetatin and algal prductin within the stream. Ttal annual fd energy inputs are similar under shaded and pen canpies but the presence r absence f a tree canpy has a majr influence n the balance between litter input and primary prductin f algae in the stream. Having a stream expsed t sunlight fr mst f the day prmtes algal grwth and prmtes prliferatin f algal grazing species. This prliferatin reduces species diversity. The diversity f the macrinvertebrate cmmunity in a stream prtected by a riparian buffer has a much greater diversity than des a stream nt having a riparian canpy. This diversity is imprtant in that it is in such a small area which ges frm lw land wetter sil cnditins t upland fairly rapidly and thus prmting very different vegetative types. Als, riparian buffer areas are adjacent t streams and therefre fldplains. By peridic ut f bank flw, fldplains are depsitinal znes fr fertile sediments. Cntaminant remval Riparian vegetatin remves, sequester, r transfrm nutrients, sediments, and ther cntaminants. The remval functin depends n tw key factrs: New Zealand studies have shwn that the majrity f nitrate remval in a pasture catchment takes place in the rganic riparian sils which receive large amunts f nitrate laden grundwater. The capability f a particular area t intercept surface and/r grundwater brne cntaminants, and The activity f specific cntaminant remval prcesses (filtratin, adsrptin, bilgical uptake, etc.). New Zealand studies have shwn that the majrity f nitrate remval in a pasture catchment takes place in the rganic riparian sils which receive large amunts f nitrate laden grundwater. The lcatin f the high rganic sils at the base f gullies caused a high prprtin f grundwater t flw thrugh the rganic sils althugh they ccupied nly 12 percent f the riparian zne area. Sediment trapping in riparian frest buffers is facilitated by physical interceptin f surface runff that causes flw t slw and sediment particles t be depsited. Channelised flw is nt cnducive t sediment depsitin and can, having higher velcities, cause ersin in the riparian buffer. Frm a sediment depsitin perspective, tw main prcesses ccur: The frest edge fsters large amunts f carse sediment depsitin within a few metres f the field/frest bundary, and Finer sediments are depsited further int the frest and nearer the stream, and The reverse ccurs during ut f bank stream flw where sediments carried frm upstream in the catchment are depsited in the riparian buffer. The lwest velcities are at the uter edge f the buffer and the finer sediments are depsited there. 3-13

14 Chapter 3 - Site Resurces Imprtance t wildlife The greater availability f water t plants, frequently in cmbinatin with deeper sils, increases plant prductin and prvides a suitable site fr plants that wuld nt ccur in areas with inadequate water. This increases plant diversity. The shape f many riparian areas, particularly their linear meandering nature alng streams, prvides a great deal f prductive edge. Riparian areas frequently prduce mre edge within a small area. In additin, alng streams there are many layers f vegetatin expsed in stair step structure. This structure prvides diverse nesting and feeding pprtunities fr wildlife. Riparian areas alng intermittent and perennial streams prvide travel rutes fr wildlife. Althugh vulnerable t negative edge effects, such as weeds, riparian vegetatin maintains habitat required fr life cycle cmpletin by riparian species and many instream species. Usually riparian margins are the remnants f mre extensive natural areas, which is smething t build upn fr restratin. Channel stability and fld flw prtectin Streams are dynamic systems that are characterised by change. Instream stability and streambank ersin at a given pint are heavily influenced by the land use and cnditin in the upstream catchment. Hwever, vegetatin is essential fr stabilising stream banks, especially wdy vegetatin. Frested buffer strips have an indirect effect n streambank stability by prviding deep rt systems which hld the sil in place mre effectively than grasses, and by prviding a degree f rughness capable f slwing runff velcities and spreading flws during large strm events. While slwing fld velcities may increase fld elevatins upstream and in the buffer, dwnstream fld crest and damage may be significantly reduced. These prcesses are als critical fr building fldplain sils. Vegetatin is essential fr stabilising stream banks, especially wdy vegetatin. Vegetatin Cver New Zealand s vegetatin cver has changed cnsiderably in the past years, with the mst dramatic changes ccurring in the past century. The indigenus vegetatin is predminantly rainfrest, but a variety f ther vegetatin types exist t, resulting in a diverse range f land-based ecsystems. Frests have a number f cmpnents whse characteristics determine its effectiveness in terms f water quantity and quality. These characteristics include: Strmwater runff reductin As discussed in Chapter 2, wdy vegetatin and frest flr litter have a significant impact n the ttal vlume f rainfall cnverted t runff. Runff vlumes frm frested areas are much less than vlumes frm ther land uses. This lesser vlume in runff acts t minimise dwnstream ersin and instability prblems. Sil structure Frest sils are generally regarded as effective nutrient traps. In New Zealand, mst nutrients are retained (and recycled) in the leaf litter and shallw sil layers. Rts 3-14

15 Lw Impact Develpment are usually quite shallw. The ability f a frest sil t functin in remving nutrients in surface and grundwater is partially dependent n sil depth, grund slpe, density f vegetatin, permeability, extent and duratin f shallw water table, and its functin as a grundwater discharge zne. Organic litter layer A mature frest can absrb as much as 14 times mre water than an equivalent area f grass. The rganic litter layer in a frest buffer prvides a physical barrier t sediment mvement, maintains surface prsity and higher infiltratin rates, increased ppulatins f sil mycrrhizae (a symbitic relatinship f plant rts and the mycellium f fungi - aids in decmpsitin f litter and translcatin f nutrients frm the sil int the rt tissue), and prvides a rich surce f carbn essential fr denitrificatin. The rganic sil prvides a reservir fr strage f nutrients t be later cnverted t wdy bimass. A mature frest can absrb as much as 14 times mre water than an equivalent area f grass. The absrptive ability f the frest flr develps and imprves ver time. Trees release stred misture t the atmsphere thrugh transpiratin while sluble nutrients are used fr grwth. Frested areas Trees have several advantages ver ther vegetatin in imprving water quality. They aggressively cnvert nutrients int bimass. They are nt easily smthered by sediment depsitin r inundatin during perids f high water level. Their spreading rt mats resist gullying and stimulate bilgical and chemical sil prcesses. They prduce high amunts f carbn needed as an energy surce fr bacteria invlved in the denitrificatin prcess. A frest s effectiveness in pllutin cntrl will vary with the age, structural attributes and species diversity f its trees, shrubs and understry vegetatin. T cnsider the invlvement f a frested area in water quality treatment, there are a number f functins that define that perfrmance. These functins can be bradly defined as physical and bilgical functins and include the fllwing: sediment filtering The frest flr is cmpsed f decaying leaves, twigs, and branches which frm highly permeable layers f rganic material. Large pre spaces in these layers catch, absrb, and stre large vlumes f water. Flw f strmwater thrugh the frest is slwed dwn by the many bstructins encuntered. Suspended sediment is further remved as runff flws int the vegetatin and litter f the frest flr. This sediment is readily incrprated int the frest sil. With a well develped litter layer, infiltratin capacities f frest sils generally exceed rainfall and can als absrb verland flws frm adjacent lands. Nutrient remval Frest ecsystems serve as filters, sinks, and transfrmers f suspended and disslved nutrients. The frest retains r remves nutrients by rapid incrpratin and lng term strage in bimass, imprvement f sil nutrient hlding capacity by adding rganic matter t the sil, reductin in leaching f disslved nutrients in subsurface flw frm uplands by evaptranspiratin, bacterial denitrificatin in sils and grundwater, and preventin frm ersin during heavy rains. 3-15

16 Chapter 3 - Site Resurces Sils New Zealand sils can be bradly categrised int three main grupings: Pumice sils which derived frm vlcanic rhylite and are widespread in the central plateau and gethermal zne f the Nrth Island. Ash sils which are derived frm vlcanic basalt and are cmmn in Taranaki, Waikat, parts f Nrthland and als western Suthland. Sedimentary sils which are derived frm sandstnes, siltstnes, and mudstnes, and are widespread n plains, rlling hill cuntry and castal areas. Despite their diversity, mst sils tend t be thin and prne t acidificatin, with mderate t high carbn levels and lw nutrient levels. This is especially true in the Auckland Regin, where kauri trees prduced deep layers f highly acidic litter, which is implicated in the pdzlisatin and gleying prcesses that have cntributed t the pr physical prperties f many f the Regin s sils. A number f ther factrs besides the warm, humid climate and kauri vegetatin cntribute t the high degree f chemical and physical weathering f the rcks f the regin. Bilgical factrs influencing sil develpment Sils pssess several utstanding characteristics as a medium fr life. It is relatively stable structurally and chemically. The undergrund climate is far less variable than abve-surface cnditins. The atmsphere remains saturated r nearly s, until sil misture drps belw a critical pint. Sil affrds a refuge frm high and lw extremes in temperature, wind, evapratin, light, and dryness. These cnditins allw sil fauna t make easy adjustments t the develpment f unfavurable cnditins. On the ther hand, sil hampers mvement. Except fr rganisms such as wrms, space is imprtant. It determines living space, humidity, and gases. A wide diversity f life is fund in the sil as shwn in figure 3-3. The number f species f bacteria, fungi, prtists, and representatives f nearly every invertebrate phylum fund in sil is enrmus. It has been estimated that apprximately 50% f Sils pssess several utstanding characteristics as a medium fr life. It is relatively stable structurally and chemically. Figure 3-3 Assciated Sil Life Frms 3-16

17 Lw Impact Develpment the earth s bidiversity ccurs in sil. Dminant amng the sil rganisms are bacteria, fungi, prtzans, and nematdes. Prminent amng the larger sil fauna are the earthwrms. Earthwrm activity cnsists f burrwing thrugh the sil. Burrwing invlves ingestin f sil, the ingestin and partial digestin f fresh litter, and the subsequent egestin f bth mixed with intestinal secretins. Egested matter is defecated as aggregated castings n r near the surface f the sil r as a semiliquid in intersil spaces alng the burrw. These aggregates prduce a mre pen structure in heavy sil and bind light sil tgether. In this manner earthwrms imprve the sil envirnment fr ther sil rganisms by creating larger pre spaces and by mixing rganic matter with the mineral sil. Bilgical prcesses in sil develpment are the mst cmplex sil frming factrs. Lichens secrete rganic acids that disslve rck surfaces, successins f plants add nitrgen frm the atmsphere. Dead rts, stems and leaves decmpse and the prducts are absrbed back int the sil. The vegetative cmmunity als has a very strng impact n sil develpment. The micrclimate f a frest is very different frm that f grassland. Tree rts penetrate further int the grund than d grass rts and bring up minerals frm deeper areas and thus incrprate them int the rganic layer. As there is such a strng relatinship between the type f vegetatin and sils, examples can be given f the type f native vegetatin and its assciated sil cmplex. Kauri frest tends t be n deeply weathered, clay-rich sils, Manuka scrubland n infertile frmer kauri frest sites Kahikatea frest is n fertile wet sils in fldplains Bradleaf (taraire/puriri) frests n relatively fertile sils The ttal amunt f sil rganic material depends nt nly upn vegetatin, but als upn tpgraphic and climatic influences. Peat frmatin can ccur in basin situatins where the water table is high. High levels f rganic matter are als fund in sils in cl, wet climates. Withut deeper rted plants hlding a slpe, in situatins where native vegetatin has been replaced by grassed lawn, slpes in excess f 33 % (18 ) may start t creep Slpes/Tpgraphy The presence f shallw rt depths des nt resist slpe slippage n steeper slpes. Sil slippage is als directly related t the steepness f the slpe, the type f sil and the underlying gelgy. Withut deeper rted plants hlding a slpe, in situatins where native vegetatin has been replaced by grassed lawn, slpes in excess f 33% (18 0 ) may start t creep. Slpes greater than 45% (24 0 ) may see the nset f mass mvement. In the case f Onerahi Chas Breccia, slpes as flat as 1:8 can be unstable. Due t the shallw nature f the sils, mst mvement tends t ccur in the first 1.5 metres. Leaving native vegetatin n these steeper slpe areas is very imprtant t maintain slpe stability. Recent studies in New Zealand have assessed the susceptibility f different vegetatin types t landslides during rainstrms. In a study nrth f Gisbrne landslide densities were 16 times greater under pasture than indigenus frest and 4 times greater under pasture than regenerating scrub. A survey f strm damage in Tertiary sandstne/siltstne hill cuntry reprted that landslides in pasture were 3-4 times greater than in indigenus frest. Finally, in a detailed study f the relatinship between slpe mrphlgy, reglith depth, and landslide incidence in eastern Taranaki 3-17

18 Chapter 3 - Site Resurces hill cuntry, identified a 10 times increase in ersin rate fr mdal slpes f degrees fllwing defrestatin. Other Natural Features There are ther natural cnditins which exist n sites beynd thse discussed t this pint. Thse discussed earlier are the primary nes in terms f verall imprtance but there are thers and cnsideratin f their imprtance is in rder. Depressin strage Of the rainfall that strikes rfs, rads, pathways, and pervius surfaces, sme is trapped in the many shallw depressins f varying size and depth present n practically all grund surfaces. The specific magnitude f depressin strage varies frm site t site. Depressin strage cmmnly ranges frm 3 t 19 mm fr flat areas and frm 12 t 30 mm n grasslands f frests. Significant depressin strage can als exist n mderate r gentle slpes with sme estimatins fr pervius surfaces being between 6 t 12 mm f water and even mre n frest land. Typical depths n mderate slpes can be 1 t 2 mm fr impervius surfaces, 2 t 4 mm fr lawns, 4 mm fr pastures and 6 mm fr frest litter. Steeper slpes wuld bviusly have smaller values. When using traditinal hydrlgic prcedures, depressin strage is cntained in an initial abstractin term. The term includes all lsses befre runff begins. It includes water retained by vegetatin, evapratin, and infiltratin. It is highly variable, but generally is crrelated with sil and cver parameters. Prir t urbanisatin, catchments have a significant depressinal strage factr. Passing thrugh agricultural r wded areas after significant rainfall clearly demnstrates the existence f depressinal strage. The urbanisatin prcess generally reduces that strage in additin t significantly mdifying the land s surface. The cmbinatin f site cmpactin, site imperviusness, and reduced depressin strage causes dramatic increases in dwnstream fld ptential and channel ersin. Infrmatin frm the Mahurangi catchment indicates that lng term average annual predicted runff varied frm less than 300 mm (18% f rainfall) t greater than 600 mm (greater than 35% f rainfall). The 300 mm cincided with subcatchments under permanent frest cver. The 600 mm cincided with subcatchments in predminantly pastral land use and n lw infiltratin sils. There is a clear statement in these statistics that significant vlume reductins in runff exist in frested catchments as ppsed t vlumes f runff frm pastral land cver. Natural drainage systems The cmbinatin f site cmpactin, site imperviusness, and reduced depressin strage causes dramatic increases in dwnstream fld ptential and channel ersin. Natural site drainage features exist n every site. The mst cmmn f these features is having an existing flw path fr strmwater runff. Water desn t travel dwn a hill in a straight line. Straight lines are smething that humans have develped t accelerate the passage f water dwnstream as quickly as pssible. During site develpment, the tendency is t place water in cnveyance systems, pen and enclsed, which fllw the shrtest distance t site utfalls. Shrtening the flw distance effectively increases the slpe that water travels n, accelerates the flw f water, and increases the ability f water t scur dwnstream receiving systems. When water travels ver a meandering flw path, energy is dissipated which reduces the ersin ptential. Shrtening flw lengths reduces energy 3-18

19 Lw Impact Develpment expended and increases the available ersin prducing energy. Stream channels will meander regardless f the degree f human alteratin. Replicating existing flw paths and lengths, t the extent pssible, prmtes channel stability and increases functin and value. The additinal functins prvided by meandering channels ver straight channels is als simply related t the length f the aquatic resurce and the time that the water is in cntact with the varius bitic and abitic prcessing mechanisms. The additinal length f meandering channels prvides a greater ttal quantity f aquatic resurce, and the assciated functins and values they prvide. Uncmpacted vegetated areas A cmmn apprach t site develpment is t clear mst, if nt all, f the site being develped. Existing vegetated areas f the site are ften cleared even when in nn essential lcatins. Clearing and grading f areas which will remain pervius results in significant cmpactin f thse pervius areas. This cmpactin reduces expected infiltratin rates and increases verland flw. A key issue with respect t urban develpment is the issue f significant sil cmpactin. The activity f heavy earth mving equipment n a cnstructin site causes significant cmpactin f sils whse surface is designed t remain pervius. There are three ptins t address this cncern. The nly way that site develpment can ccur in a manner which integrates existing site resurces is t identify thse site resurces present n the site prir t initiatin f site design. 1. Where cuts r fills f at least ne metre are intended t facilitate site develpment, the expected permeability f the sil may be reduced. Strmwater management cmputatins which detail pst cnstructin hydrlgy shuld use a mdified apprach t sil classificatins. 2. In areas f significant site disturbance, and where there is less than ne metre f cut r fill, sil classificatins are nt mdified, but the apprved cnsent shuld cntain a cnstructin requirement that significantly disturbed sils in areas where thse sils remain pervius shuld be chisel plwed. Chisel plwing will break the surface crust f the disturbed sil and allw fr a greater infiltratin rate. This wuld then prvide a gd fundatin fr the placement f tp sil and prevent slippage f the tp sil when n slpes that becme saturated. 3. Avid cmpactin altgether by keeping equipment ut f areas preserved fr pen space. Linkage with Site Develpment The nly way that site develpment can ccur in a manner which integrates existing site resurces is t identify thse site resurces present n the site prir t initiatin f site design. The first step in site resurce integratin is in cnducting an inventry f site resurces and detailing them n a plan. A simple checklist can be develped which is based n the items presented here. The checklist culd include the fllwing items which have been discussed thrughut the Chapter. Wetlands Fldplains Riparian buffers Vegetative cver Sils Steep slpes Other natural features 3-19

20 Chapter 3 - Site Resurces A checklist shuld als include: archaelgical sites Iwi sites This plan shuld be included as a part f the strmwater management plan submitted which is prvided t the apprpriate territrial authrity r ARC. A narrative shuld als be submitted t detail what steps have been cnsidered and/r prvided t integrate existing resurces int the strmwater management plan. Plan designers and develpers shuld als be aware f territrial authrity specific criteria which may verlap r cnflict with the natural site features items listed abve. Natural Mechanisms fr Strmwater Pllutin Remval Althugh many strmwater related cntaminates can be reduced if nt eliminated thrugh preventive design appraches driven by water quantity reductin bjectives, nt all pllutin cntaminants can be eliminated. In such situatins, an array f natural pllutant remval prcesses are available fr use and shuld be explited t the maximum. Because these prcesses tend t be assciated with, even reliant upn bth vegetatin and sil prcesses, they can be readily incrprated int ther lw impact design appraches. Such natural cntaminant reductin/eliminatin prcesses include: Settling/depsitin The kinetic energy f strmwater washes all types f matter, particulate frm and ther, frm land cver surfaces. Particulates remain suspended in strmwater flws as lng as the energy level is maintained. Heavier particulates require mre kinetic energy in rder t remain in suspensin. As the energy level declines- as the strm flw slws, these suspended particulates begin t settle ut by gravity, with larger, heavier particulates settling ut mst quickly and the smallest cllidal particulates requiring cnsiderably mre time fr settling. T the extent that time can be maximised, mre settling can be expected t ccur, hlding all ther factrs cnstant. Therefre, appraches which delay strmwater mvement r appraches which reduce kinetic energy in sme manner (e.g., energy dissipaters) serve t maximise settling and depsitin. Filtering Anther natural prcess is physical filtratin. As cntaminants pass thrugh the surface vegetative layer and then dwn thrugh the sil, larger particulates are physically filtered frm strmwater. Vegetatin n the surface ranging frm grass blades t underbrush remves larger cntaminant particulates. Strmwater sheet flw thrugh a relatively narrw natural riparian buffer f trees and undergrwth has been demnstrated t physically filter surprisingly large prprtins f larger particulates. Bth filter strips and grass swales rely very much n this filtratin prcess. Filtratin may als ccur as strmwater infiltrates int the tpsil strata. T the extent that time can be maximised, mre settling can be expected t ccur, hlding all ther factrs cnstant. Bilgical transfrmatin and uptake/utilisatin Althugh gruped as ne type, this categry includes a cmplex array f different prcesses that reflect the remarkable cmplexity f different vegetative types, their varying rt systems, and their different needs and rates f uptake f different cn- 3-20