علي شيخ على لحمر بن جنيد

ملخصالدراسة

          يقع الحوض الأدنى لوادي أحور في محافظةابين مديرية احور , ويشغل مساحة قدرها حوالي 3700كم² , وتتراوح مناسيبالارتفاع بالحوض بين 700 _ 750م فوق سطح البحر ولذلك يظهر أثر الظروفالجيومورفولوجية السائدة للنحت المائي وفعالية النشاط الجيومورفولوجي للمياهالسطحية الجارية في الحوض .

          تهدف هذه الدراسة للكشف عن الظاهراتالجيومرفولوجية في هذا النطاق , وقد استخدم الباحث اساليب الدراسة الميدانيةوالتحليل الوصفي والكمي للظواهر الجيومورفولوجية الناتجة عن الجريان السطحي , وفيالمقدمة تناول الباحث الإطار النظري الذي حدد فيه اهمية دور المياه الجارية ,وأهداف ومبررات ومشكلة وفرضيات البحث وطرق واساليب البحث .

          وقد تم تقسيم الدراسة إلى أربعة فصول ,تناول الباحث في الفصل الأول الخصائص الطبيعية لمنطقة الدراسة من حيث البنية والتركيب الجيولوجي والخصائص التضاريسيةوالأوضاع المناخية والهيدرولوجية والتي كان لها تأثير على العملياتالجيومورفولوجية في منطقة الدراسة.

          أما الفصل الثاني فقد اختص بالتحليلالمورفومتري لمنطقة الدراسة باعتبارهاناتجا للعمليات الجيومورفولوجية منذ نشأت الحوض في العصر الثلاثي حتى الوقتالحاضر, وقد تناول الباحث بالتحليل الكرتوجرافي الخصائص الخطية (طول الحوض ,عرضه,محيطه) والخصائص المساحية (مساحة الحوض , نسبة تماسك المساحة , نسبة الاستطالة)والخصائص التضاريسية (نسبة التضرس , التضاريس النسبية , قيمة الوعورة) معتمدا علىأساليب التحليل المورفومتري الشائعة وعلى خرائط تفصيلية لمنطقة الدراسة .

          أماالفصل الثالث فقد اقتصر الباحث فيه على تحليل وتفسير نشأة وتوزيع الظواهرالجيومورفولوجية في منطقة الدراسة بدأ بالعمليات الجيومورفولوجية وظاهراتها علىالسفوح, ثم الظواهر الناتجة عن النحت والإرساب المائي بالإضافة إلى بعض الظواهرالجيومورفولوجية الأخرى الموجودة في منطقة الدراسة , وفي الفصل الرابع تم عرض أهمالمشكلات الناتجة عن المظاهر الجيومورفولوجية المختلفة , وآثارها على النشاط  الاقتصادي للإنسان ويتمثل ذلك في الفيضاناتواثرها في جرف التربة وغيرها من المشكلات ,كما أشار الباحث ايضا في هذا الفصل إلى الخصائص البشرية المختلفة السكان

          وقد ختمت هذه الدراسةبالعديد من الاستنتاجات وأهمها سيادة النحت الرأسي وزيادة الجرف للتربة واتساعمساحة المجرى المائي على حساب الأراضي الزراعية , وإن معظم الظواهرالجيومورفولوجية المختلفة للمياه السطحية الجارية إنما هي نتاج لمناخ رطب حصل في العصور القديمة .

المحتويات

Abstract

WadiAhwar is considered to be one of the well-known Wadis of Yemen, andthe lowest basin of WadiAhwar in the Abyan Governorate,Ahwar District. Itoccupies an area of about 3,700 KM². The high water levels in thebasin range between 700 – 750 M above the sea level, For this reason, appearsthe impact of Geo-morphological conditions prevailing of the watery cutting andthe activation of the Geo-morphological activity of the surface waters runningin the basin.

            This Study explores thephenomena of Geo-morphology within this scope.

            The Study has beendivided into four chapters, the researcher in the First Chapter dealt with thenatural specifications of the studying area with regard to its structure,geological construction and relief specifications, atmospheric andhydro-geological specifications, which were affecting the geo-morphologicaloperations in the Studying area. The researcher has as well indicated in thischapter the different human specifications of the population.

            The second chapter hasconcentrated on the morophometricanalysis of the studying area  as being a result of the geo-morphologicaloperations since the evolution of the basin in the triterial era until thepresent time. The researcher has also used the carto-graphy to analyse thelinear specifications ( the length of the basin, its width and itscircumference, etc.) area specifications (area of the basin, ratio of thecohesion of the area, ratio of the length, etc.) and relief specifications,(relief ratio, proportional reliefs and value of corrugation, etc.) dependingon the prevailing morphometric analysis and on detailed maps of the studyingarea.

            In the third chapterthe researcher concentrated only on the analysis, interpretation of theevolution and the distribution of geo morphological phenomena in the studyingarea, starting with the geo-=morphological operations and their phenomenons onthe tops of the hills. Then the phenomenon resulting from water cutting andprecipitation, in addition to certain other geo-morphological phenomenonexisting in the studying area. In the fourth chapter, the most importantproblems resulting from the different geo-morphological phenomena werereviewed, and their impact on the human economical activity. This is reflectedin the floods and their impact on the soil scooping and other problems.

            The study of the lowestbasin of WadiAhwar, whose area reaches 3700 M² many of theparticular results and facts in geo-morphology, represented in the following:

            It appears from thegeological development that the Cretacian and quadruplicate eras are theprecipitation eras, while in the ternary era the confusion of the tectonicmovement has took place, along with the accompanying movements of upraisal,pressure and volcanic breakouts  whichhave led to the emergence of two scopes at the surface of the area, an old archsystem and a recent volcanic scope,

 in addition to thediversification of the rock constitutions from arch rocks which refer to preCambry and the precipitated rocks which refer to the Crestecian era, the thirdera, in addition to the volcanic rocks during the quadruple era.

 It appeared that there is a greatreserve of limestone rocks in addition to the volcanic ash which helps thesetting up of the manufacturing of cement in the studying area, and that thegeological conditions resulted in differently constituted relief and level ofwater, represented in three scopes, mountainous, hill scope and coastalplateaus and the wadies.

It has appeared from the hydrologicalspecifications that the volume of the rainfall is (631, 04) million M³  and the volume of the surfacial running is(69) million M³ .

 The rate of surfacial water in1996 have reached to a high rate, whereas the level of water table at Ahwar Damreached more than 5 meters and at a running velocity which surpassed 6kilos/sec., it appeared also the difference of the volumes of the seasonalfloods and their Geo-morphological impact from one year to another according tothe rainfall quantity and their strength.

The laboratory analysis of the soil of the studying area showed theprevalence of sandy soil. The Delta is distinguished for being of a light base,poor in organic material and on its surface prevails a diversified vegetativecover, whose intensity appears during the rain season.

The morphometric analysis of the lowest basin of WadiAwar has also shownthat the length of the main basin is about 75,5 km, its width 49km, and thelength of its circumference reached 249 km., and its numeric density reached1,2/km²  .The basin isinclining to take the shape of lengthness because of its structuralconstruction and the cracks. The ratio of turn-about  (0,7) indicates the remoteness of the basinfrom the circular shape, the rate of reliefing reached (9,9 M/kilometer). Thisratio indicates that the reliefing is moderate in the studying area and theaverage of texture reached 18, which indicates that the texture of the basin issoft. The value of  Hispo-metricco-efficient reached (4,9). This value indicates the youth stage by which thebasin is passing . It appeared from the Study the effectivity of the cuttingprocess through the studying of the relationship between the specifications ofwatery inclination and leveling, whereas such relationship showed that thevolume of the scooped soil from the area through the main wadi is increasingday by day, which means the increase in the area of the Wadi on account of thecultivated lands, whereas the floods in the year 1996 caused the widening of thearea of the wadi to reach an average width of more than 1250 Meters, while itwas not exceeding 750 meters before that year.

The Study has shown the existence of a number of geo-morphologicphenomena related to wadies, like reservoirs in Wadi Barak (Al-Koa), and thewatersheds, like those seen in WadiHafa, in addition to bottlenecks andslidings and the different rocky terraces.

There are a number of morphologic phenomenon which are linked to thevolcanic activity in the Studying area, represented by the volcanic cones, likethose present in Rahbah Mountain, and the rocky eruptions and volcanic islands,like Makateen, in addition to that there are other phenomena connected withwinds and their impact in cutting and precipitating the sands and the formationof different sceneries like the caves of the peaks or the mountainous islands,as is found in Ara'er or the phenomena of precipitations, represented by sanddunes with their different shapes and what accompanying sceneries of theintensified sands and their creeping to the cultivated lands as is existing inAl-boob area.

The coastal geo-morphological phenomena diversify, whereas we find someof the coasts being covered with sands, as is the case in Bandar, while we findrock blocks and heavy rocks covering the coast of Asheer area. HisnBaleed,however, has its coast covered with tiny and medium size stones, and throughstudying the Delta as well, it appeared the exposition of some of thecultivated areas to alkalinity, as a result of exhaustion of the land bycontinued irrigation and cultivation, in addition to the reduction of theunderground water as a result of the great depletion of huge amounts of waterdue to random and undisciplined drilling of wells, whereas the number ofartesian wells has reached in the Studying are about 482 wells, other than openwells, in addition to the reduction in the cultivated land as a result of the impact of floods in scooping thesoil and the creeping of the sand-dunes in addition to the constructionalcreeping on account of the cultivated lands and the negligence of cultivationworks and the looking forward by the popultion for governmental employmentwhich resulted in the desertification of certain cultivated areas.